A report of the field operations and early results of the South China Sea Monsoon Experiment (SCSMEX)
|Lau KM, Ding YH, Wang JT, Johnson R, Keenan T, Cifelli R, Gerlach J, Thiele O, Rickenbach T, Tsay SC, Lin PH BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 81: (6) 1261-1270 JUN 2000|
|Abstract: The South China Sea Monsoon Experiment (SCSMEX) is an international field experiment with the objective to better understand the key physical processes for the onset and evolution of the summer monsoon over Southeast Asia and southern China aiming at improving monsoon predictions. In this article, a description of the major meteorological observation platforms during the intensive observing periods of SCSMEX is presented. In addition, highlights of early results and discussions of the role of SCSMEX in providing valuable in situ data for calibration of satellite rainfall estimates from the Tropical Rainfall Measuring Mission are provided. Preliminary results indicate that there are distinctive stages in the onset of the South China Sea monsoon including possibly strong influences from extratropical systems as well as from convection over the Indian Ocean and the Bay of Bengal. There is some tantalizing evidence of complex interactions between the supercloud cluster development over the Indian Ocean, advancing southwest monsoon flow over the South China Sea, midlatitude disturbances, and the western Pacific subtropical high, possibly contributing to the disastrous flood of the Yangtze River Basin in China during June 1998.|
|Low frequency modes and their link with summer monsoon activity described by wavelet analysis throughout India|
|Ouergli A IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING 38: (3) 1277-1281 MAY 2000|
|Abstract: The purpose of this paper is to propose a method and its application to attempt investigating temporal-frequency characteristics of low frequency modes throughout India. This diagnostic method combines wavelet analysis (WT) interlaced by a factor D and empirical orthogonal function (EOF), It is shown that the stability related to WT is ensured by the adequate choice of factor D and the sampling frequency. Thus, the reconstruction of a time series from its wavelet coefficients becomes possible (i.e., "wavelet synthesis," which can constitute a narrow bandpass), This method has been applied to daily pressure during summer monsoon activity for the year 1977. The results of analysis indicate a link between summer monsoon activity and both a 10-20-day and 25-50-day band. This link is expressed by the simultaneous occurrence of both modes throughout India with a different amplitude from one region to another expressed by the following factors. During summer monsoon onset, it is the 10-20-day mode that is predominant, while during the break, both modes have nearly the same amplitude, These modes have a maximum activity situated nearly at 23 degrees N, 83 degrees E.|
|On the dynamics of easterly waves, monsoon depressions, and tropical depression type disturbances|
|Sobel AH, Horinouchi T JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 78: (2) 167-173 APR 2000|
|Abstract: The authors argue that certain aspects of the rotational, synoptic-scale disturbances of the wind field that are observed in ITCZ or monsoon trough regions can be understood by considering the linear response of a dry, initially resting atmosphere to a pulse of heating whose amplitude and spatial and temporal scales are characteristic of a large mesoscale convective system. The key points are that short Rossby waves have small intrinsic group and phase velocities, and that a heating pulse projects much more energy on the Rossby modes if it is located slightly off rather than on the equator. It follows that synoptic-scale Rossby waves, with characteristics broadly similar to those of observed disturbances, should be present in off-equatorial regions of persistent deep convection, since large mesoscale convective systems tend to develop in such regions.|
|Diurnal variation of GPS precipitable water at Lhasa in premonsoon and monsoon periods|
|Takagi T, Kimura F, Kono S JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 78: (2) 175-180 APR 2000|
|Abstract: In this study, precipitable water is estimated from GPS data with high time resolution at Lhasa. Mean precipitable water in the premonsoon and the monsoon periods are about 10 mm and 25 mm, respectively, while precipitable water increases in short term at around the onset. Prominentdiurnal variation of precipitable water can be observed in both periods. The minima of the diurnal variation appear around 1800 LST and 1500 LST in the premonsoon and the monsoon periods, respectively. These characteristics indicate that the diurnal Variation of water vapor is strongly affected by the local circulation over the deep valley.|
|Meso-scale distribution of summer monsoon rainfall near the Western Ghats (India)|
|Patwardhan SK, Asnani GC INTERNATIONAL JOURNAL OF CLIMATOLOGY 20: (5) 575-581 APR 2000|
|Abstract: The spatial distribution of southwest monsoon rainfall is studied over Maharashtra State (India), which includes part of the well-known Western Ghats mountain range, near its western boundary, running almost from north to south, perpendicular to the summer monsoon current in the lower troposphere. Meso-scale analysis of daily rainfall is performed for Maharashtra State, including the Western Ghats, for the two mid-monsoon months of July and August, during the 10-year period of 1971-1980. Strong and weak monsoon days were identified for the 5-year period of 1976-1980. The meso-scale pattern of average daily rainfall is obtained separately for strong and for weak monsoon conditions. All these average patterns show the following features: (i) the rainfall increases rapidly from the Arabian Sea coast close to the line of maximum height of the Western Ghats; (ii) there are two rainfall maxima corresponding to the two mountain peaks parallel to the coast line; (iii) between the two mountain peaks, there is a valley which is narrow at the western end (upwind end), broadening towards the east (on the downwind side). Ground contour height of the valley rises eastwards and ends as a part of the Deccan Plateau east of the Ghats. Here the valley opens out like a funnel with higher mountains flanking its two sides. In the valley, the rainfall increases from the coast up to the line of maximum height of the Ghats, and then decreases eastwards towards the plateau. The rainfall isopleths also take a funnel-shaped configuration.
An interesting feature is that near the wider section of the valley funnel, there is a rainfall minimum and then the rainfall increases further eastwards on the downwind side. This feature of rainfall minimum is somewhat similar to the rainfall minimum reported by Asnani and Kinuthia (personal communication); Asnani (Asnani GC. 1993. Tropical Meteorology, Vol. I. Prof. G.C. Asnani: Pune, India; 603) attributed the rainfall minimum to the Bernoulli effect. A somewhat similar phenomenon is assumed in the present study area.
|All India summer monsoon rainfall prediction using an artificial neural network|
|Sahai AK, Soman MK, Satyan V CLIMATE DYNAMICS 16: (4) 291-302 APR 2000|
|Abstract: The prediction of Indian summer monsoon rainfall (ISMR) on a seasonal time scales has been attempted by various research groups using different techniques including artificial neural networks. The predict on of ISMR on monthly and seasonal time scales is not only scientifically challenging but is also important for planning and devising agricultural strategies. This article describes the artificial neural network (ANN) technique with error- back-propagation algorithm to provide prediction (hindcast) of ISMR on monthly and seasonal time scales. The ANN technique is applied to the five time series of June, July, August, September monthly means and seasonal mean (June + July + August + September) rainfall from 1871 to 1994 based on Parthasarathy data set. The previous five years values from all the five time-series were used to train the ANN to predict for the next year. The details of the models used are discussed. Various statistics are calculated to examine the performance of the models and it is found that the models could be used as a forecasting tool on seasonal and monthly time scales. It is observed by various researchers that with the passage of time the relationships between various predictors and Indian monsoon are changing, leading to changes in monsoon predictability. This issue is discussed and it is found that the monsoon system inherently has a decadal scale variation in predictability.|
|Nonaxisymmetric thermally driven circulations and upper-tropospheric monsoon dynamics|
|Hsu CJ, Plumb RA JOURNAL OF THE ATMOSPHERIC SCIENCES 57: (9) 1255-1276 MAY 1 2000|
|Abstract: The authors investigate the nonlinear dynamics of
almost inviscid, thermally forced, divergent circulations in situations
that are not axisymmetric. In shallow-water numerical calculations, asymmetry
is imposed on a locally forced anticyclone by imposition of a mean wind,
or a planetary vorticity gradient. Behavior is similar in the two cases.
With weak asymmetry, the forced anticyclone is distorted but remains intact
and is qualitatively unchanged from the symmetric response. For sufficiently
large asymmetry, however, the elongated anticyclone becomes unstable and
periodically sheds eddies. This behavior shows how the circulation constraint
can be satisfied, even when the time-mean absolute vorticity remains finite
in the divergent region, and provides a continuous evolution between the
nonlinear (symmetric) and linear (highly asymmetric) limits.
Westward shedding of anticyclones from the Tibetan anticyclone is indeed evident in NCEP reanalysis data. These eddies are trapped near the tropopause. Cutoff potential vorticity features are confined to within about 20 K of the tropopause; in geopotential, they extend somewhat further, but not below about 400 hPa.
|Dynamics of breaks in the Indian summer monsoon|
|Krishnan K, Zhang C, Sugi M JOURNAL OF THE ATMOSPHERIC SCIENCES 57: (9) 1354-1372 MAY 1 2000|
|Abstract: In this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of "breaks" in the Indian summer monsoon (ISM). Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over the Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian Ocean. A major intensification of the convectively stable anomalies over the Bay of Bengal is seen about 2-3 days prior to commencement of a monsoon break. Both observations and modeling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from the Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from the central Bay of Bengal to northwest India in about 2-3 days' time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves the way for the emergence of a convectively unstable anomaly over the equatorial Indian Ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from the central Bay of Bengal toward northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the ISM and also help maintain the mutual competition between convection over the Indian subcontinent and that over the equatorial Indian Ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic how, which is driven by a steady north-south differential hearing, is a primary physical mechanism that controls the so-called monsoon breaks.|
|Interaction between the summer monsoons in East Asia and the South China Sea: Intraseasonal monsoon modes|
|Chen TC, Yen MC, Weng SP JOURNAL OF THE ATMOSPHERIC SCIENCES 57: (9) 1373-1392 MAY 1 2000|
|Abstract: The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu tin eastern China)-Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120 degrees E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30-60-day monsoon trough/ridge from the equator to 20 degrees N and with the westward-propagating 12-24-day monsoon low-high along the latitude of similar to 15 degrees-20 degrees N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north-south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.|
|Numerical simulation of monsoon depressions over India with a high-resolution nested regional model|
|Potty KVJ, Mohanty UC, Raman S METEOROLOGICAL APPLICATIONS 7: (1) 45-60 MAR 2000|
|Abstract: The structure and track of monsoon depressions over India during the summer monsoon have been simulated using a double-nested limited-area numerical weather prediction model. Four distinct cases of monsoon depressions that formed over the Bay of Bengal and adjoining areas have been studied Initial conditions for the simulations are from either the European Centre for Medium Range Weather Forecasts, Reading, UK or the National Centre for Medium Range Weather Forecasting, New, Delhi, India. The model is integrated for up to 48 hours for each case and the results are compared with verification fields. Forecasts of mean sea level pressure and low-level wind indicate that the location of the centres of the depressions and their track could be predicted satisfactorily even though the magnitude of the central pressure is slightly too high. Temperature forecasts show close agreement with the verification analyses and the distribution of precipitation is well simulated. The vertical cross-sections of temperature and wind forecasts show, the correct vertical structure. RMS errors of the mean sea level pressure, wind and temperature indicate that the model could simulate the large-scale fields reasonably well. RMS errors of the tracks of the depressions confirm the fact that the high-resolution nested grid model can predict the tracks of the depressions with reasonable accuracy.|
|IS-supported modelling of areal rainfall in a mountainous river basin with monsoon climate in southern India|
|Wilk J, Andersson L HYDROLOGICAL SCIENCES JOURNAL-JOURNAL DES SCIENCES HYDROLOGIQUES 45: (2) 185-202 APR 2000|
|Abstract: Spatial rainfall patterns and seasonal variability were assessed for a mountainous river basin with monsoon climate. Factors were identified that could explain this variability, and a GIS-supported method to determine the areal distribution of precipitation was developed. To find acceptable regression equations, a division had to be made between rainfall stations dominated by the southwest-monsoons and the northeast-monsoons, respectively. Distance to the southwestern border was the main explaining factor for precipitation at southwest-monsoon dominated stations. For northeast-monsoon dominated stations, altitude and slope ere the most important factors. The basin was divided into pixels with characteristics typical for northeast- or southwest-monsoon dominated rainfall stations to allow calculation of spatial rainfall. The difference when comparing regression-based estimates with Thiessen-based estimates was small when considering the annual estimates for the whole basin. However, when analysing seasonal rainfall or sub-catchments, the differences between Thiessen-based and regression-based estimates were significant.|
|Comments on "Choice of south Asian summer monsoon indices"|
|Goswami BN BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 81: (4) 821-822 APR 2000|
|Comments on "Choice of south Asian summer monsoon indices" - Reply|
|Wang B BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 81: (4) 822-824 APR 2000|
|Dominant factors responsible for interannual variability of the summer monsoon in the southwestern United States|
|Higgins RW, Shi W JOURNAL OF CLIMATE 13: (4) 759-776 FEB 15 2000|
|Abstract: Interannual variability of the summer monsoon in the southwestern
United States is controlled by various ocean- and land-based conditions
(e.g., SST, soil moisture, and snow cover) that provide sources of memory
of antecedent climate anomalies such as ENSO. It is hypothesized that this
interannual variability is also modulated by decade-scale fluctuations
in the North Pacific SSTs.
The following observations have been made in support of this hypothesis. First, the summer precipitation regime is dominated by a continental-scale precipitation pattern characterized by an our-of-phase relationship between precipitation in the southwestern United States and that in the Great Plains of the United States. Second, interannual fluctuations in the onset date of the monsoon in the southwestern United States are significantly correlated with interannual fluctuations in the intensity of summer rainfall in this region such that early monsoons are often very wet and late monsoons tend to be dry. Third, wet (dry) monsoons in the southwestern United States often follow winters characterized by dry (wet) conditions in the southwestern United States and wet (dry) conditions in the northwestern United States. Finally, interannual variability of the summer monsoon in the southwestern United States is modulated by long term (decade scale) fluctuations in the North Pacific SSTs. The mechanism relating the North Pacific SST pattern to interannual variability in the summer monsoon appears to be via the impact of variations in the Pacific jet on West Coast precipitation regimes during the preceding winter Multiyear fluctuations in the North Pacific SST pattern are consistent with multiyear fluctuations in the atmospheric circulation and in the West Coast precipitation regimes during Northern Hemisphere (NH) winter. hence with multiyear variability in the summer monsoon state. Influences on the summer monsoon during the preceding winter and spring are tied together using appropriate SST indices that capture decade-scale variability in the North Pacific during NH winter and interannual variability in the eastern tropical Pacific during NH spring. The results suggest that decadal variability in the North Pacific SSTs may be an important factor in determining long-term periods of summertime drought or rainy conditions both in the southwestern United States and in the Great Plains of the United States.
|Indian monsoon-ENSO relationship on interdecadal timescale|
|Krishnamurthy V, Goswami BN JOURNAL OF CLIMATE 13: (3) 579-595 FEB 1 2000|
|Abstract: Empirical evidence is presented to support a hypothesis that
the interdecadal variation of the Indian summer monsoon and that of the
tropical SST are parts of a tropical coupled ocean-atmosphere mode. The
interdecadal variation of the Indian monsoon rainfall (IMR) is strongly
correlated with the interdecadal variations of various indices of El Nino-Southern
Oscillation (ENSO). It is also shown that the interannual Variances of
both IMR and ENSO indices vary in phase and follow a common interdecadal
variation. However, the correlation between IMR and eastern Pacific SST
or between IMR and Southern Oscillation index (SOI) on the interannual
timescale does not follow the interdecadal oscillation. The spatial patterns
of SST and sea level pressure (SLP) associated with the interdecadal variation
of IMR are nearly identical to those associated with the interdecadal variations
of ENSO indices. As has been shown earlier in the case of ENSO, the global
patterns associated with the interdecadal and interannual variability of
the Indian monsoon are quite similar.
The physical link through which ENSO is related to decreased monsoon rainfall on both interannual and interdecadal timescales has been investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis products. The decrease in the Indian monsoon rainfall associated with the warm phases of ENSO is due to an anomalous regional Hadley circulation with descending motion over the Indian continent and ascending motion near the equator sustained by the ascending phase of the anomalous Walker circulation in the equatorial Indian Ocean. It is shown that, to a large extent, both the regional Hadley circulation anomalies and Walker circulation anomalies over the monsoon region associated with the strong (weak) phases of the interdecadal oscillation are similar to those associated with the strong (weak) phases of the interannual variability. However, within a particular phase of the interdecadal oscillation, there are several strong and weak phases of the interannual variation. During a warm eastern Pacific phase of the interdecadal variation, the regional Hadley circulation associated with El Nino reinforces the prevailing anomalous interdecadal Hadley circulation while that associated with La Nina opposes the prevailing interdecadal Hadley circulation. During the warm phase of the interdecadal oscillation, El Nino events are expected to be strongly related to monsoon droughts while La Nina events may not have significant relation: On the other hand, during the cold eastern Pacific phase of the interdecadal SST oscillation, La Nina events are more likely tobe strongly related to monsoon floods while El Nino events are unlikely to have a significant relation with the Indian monsoon. This picture explains the observation that the correlations between IMR and ENSO indices on the interannual timescale do not follow the interdecadal oscillation as neither phase of the interdecadal oscillation favors a stronger (or weaker) correlation between monsoon and ENSO indices.
|Net cloud radiative forcing at the top of the atmosphere in the Asian monsoon region|
|Rajeevan M, Srinivasan J JOURNAL OF CLIMATE 13: (3) 650-657 FEB 1 2000|
|Abstract: Based on the data from Earth Radiation Budget Experiment (ERBE), many investigators have concluded that the net cloud radiative forcing at the top of the atmosphere is small in the deep convective region of the Tropics. This conclusion has been shown to be invalid for the Asian monsoon region during the period June-September. The ERBE data have been used to show that in the Asian monsoon region the net cloud radiative forcing at the top of the atmosphere is negative and its magnitude exceeds 30 W m(-2) in 25% of the grids in this region. The large negative net cloud radiative forcing in the Asian monsoon region during June-September has been shown to be on account of the presence of large amount of high clouds and the large optical depth of these clouds. This combination of high cloud amount and high optical depth occurs in the Asian monsoon region only, In the other deep convective regions of the Tropics, high clouds with large optical depths are present, but they do not cover a large area.|
|Multiple quasi equilibria of the ITCZ and the origin of monsoon onset|
|Chao WC JOURNAL OF THE ATMOSPHERIC SCIENCES 57: (5) 641-651 MAR 1 2000|
|Abstract: Supported by numerical experiment results, the abrupt change
of the location of the intertropical convergence zone (ITCZ), from the
equatorial trough how regime to the monsoon trough flow regime. or the
monsoon onset, is interpreted as a subcritical instability. There are two
balancing "forces" acting on the ITCZ. One toward the equator, or an equatorial
latitude depending on the convection scheme, due to the earth's rotation,
has a nonlinear latitudinal dependence; and the other toward a latitude
close to the sea surface temperature peak has a relatively linear latitudinal
dependence. The highly nonlinear latitudinal dependence of the first "force"
is crucial for the existence of the multiple equilibria. This work pivots
on the finding that the ITCZ and Hadley circulation can still exist without
the pole-to-equator gradient of radiative-convective equilibrium temperature.
The numerical experiments are done with an atmospheric general circulation model over an aquaplanet with zonally uniform sea surface temperature. The existence of the two flow regimes, the two "forces," and the abrupt transition are all demonstrated in the experiments. Experimental results show high dependence on the choice of cumulus parameterization scheme. especially during the equatorial trough circulation regime. Although the proposed interpretation is more suitable for explaining the monsoon trough onset in the western Pacific, it is hypothesized that the same basic mechanism is also at the core of monsoon onset in other parts of the Tropics.
|Quasi-cycles in monsoon rainfall by wavelet analysis|
|Kailas SV, Narasimha R CURRENT SCIENCE 78: (5) 592-595 MAR 10 2000|
|Abstract: Indian rainfall data over the period 1870 to 1990 have been analysed using continuous wavelet transforms. Results obtained are found to be particularly revealing with regard to the temporal structure of the time series if local scaling is used for the transform coefficients, normalizing their values at any wavelet scale with the maximum at that particular scale. Tracking the maxima in such normalized values leads to a picture of seven prominent modes, which are modulated in wavelet scale (and hence also frequency) and in amplitude. Because the modes wander over the parameters of the wavelet map, they cannot always he easily detected by classical spectral analysis. The quasi-cyclic modes so identified exhibit appreciable jitter, and have average periods varying from a little less than 2 years to nearly 80 years.|
|Vertical variation of Madden-Jullian Oscillations in the normal monsoon season as revealed through MST Radar wind data|
|Rao DN, Singh HR, Kulkarni JR, Chandrika AY, Rao SVB METEOROLOGY AND ATMOSPHERIC PHYSICS 73: (1-2) 55-59 2000|
|Abstract: Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke)in the normal monsoon season. The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT) has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There are three dominant modes viz., 50-70, 30-40 and 10-20 day periodicity, which contain considerable fraction of energy and show high degree of vertical variability. The peak amplitude of 50-70 day mode in u, 30-40 mode in v and eke were observed at 16-17 km just below the tropopause level. The peak amplitudes of 30-40 day mode in u and 50-70 day mode in v were found in the height region of 13-16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere, and a part of the energy leaks into the stratosphere.|
|Winter monsoon circulation of the northern Arabian Sea and Somali Current|
|Schott FA, Fischer J JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 105: (C3) 6359-6376 MAR 15 2000|
|Abstract: The winter monsoon circulation in the northern inflow region of the Somali Current is discussed on the basis of an array of moored acoustic Doppler current profiler and current meter stations deployed during 1995-1996 and a ship survey carried out in January 1998. It is found that the westward inflow into the Somali Current regime occurs essentially south of 11 degrees N and that this inflow bifurcates at the Somali coast, with the southward branch supplying the equatorward Somali Current and the northward one returning into the northwestern Arabian Sea. This northward branch partially supplies a shallow outflow through the Socotra Passage between the African continent and the banks of Socotra and partially feeds into eastward recirculation directly along the southern slopes of Socotra. Underneath this shallow surface flow, southwestward undercurrent flows are observed. Undercurrent inflow from the Gulf of Aden through the Socotra Passage occurs between 100 and 1000 m, with its current core at 700-800 m, and is clearly marked by the Red Sea Water (RSW) salinity maximum. The observations suggest that the maximum RSW inflow out of the Gulf of Aden occurs during the winter monsoon season and uses the Socotra Passage as its main route into the Indian Ocean. Westward undercurrent inflow into the Somali Current regime is also observed south of Socotra, but this flow lacks the RSW salinity maximum. Off the Arabian peninsula, eastward boundary flow is observed in the upper 800 m with a compensating westward flow to the south. The observed circulation pattern is qualitatively compared with recent high-resolution numerical model studies and is found to be in basic agreement.|
|Layered clouds in the Indian monsoon region|
|Rama MK, Raja V, Asnani GC, Salvekar PS, Jain AR, Rao DN, Rao SV, Kishore P, Hareesh M PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 108: (4) 287-295 DEC 1999|
|Abstract: Contrary to the prevalent belief that tropical region is characterized by convective clouds rather than by layer clouds, we have suggested that deep convective clouds occur on meso-scale, but layer clouds occur on larger synoptic-scale with a relatively small region of deep convective clouds. Sustenance of deep convective clouds is inhibited by the presence of inertio-gravity waves, which have alternating layers of upward and downward motion in the vertical. We have also shown that inertio-gravity waves generate regions of relatively strong horizontal velocity, vertically separated by layers of relatively weak horizontal velocity. Layers of strong horizontal velocity are created by inertio-gravity wave system through convergence of vertical flux of horizontal momentum. We have also suggested that horizontal convergence/divergence of moisture flux is generated by inertio-gravity waves, giving rise to vertically alternating layers of high/low humidity, and visible or sub-visible clouds. Layers of high humidity become layers of strong radar reflectivity at frequency of 53 MHz at which MST Radar at Gadanki, near Tirupati, India, operates. These observations, more than 2,50,000 in number, for vertical grid points, spread over all the months of the year, have helped us, among other observations, to arrive at these conclusions. Further, the analysis suggests that the main source of strong MST radar reflectivity is not mechanical turbulence as is commonly believed.|
|Global temperature and monsoon activity|
|Dugam SS, Kakade SB PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 108: (4) 305-307 DEC 1999|
|Abstract: In this paper an attempt has been made to search a new parameter for the prediction of the Indian summer monsoon rainfall. For this purpose the relationship of the global surface-air temperature of four standard seasons viz., Winter (December-January-February), Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November) with the Indian summer monsoon rainfall has been carried out. The same analysis is also carried out with surface-air temperature anomalies within the tropical belt (30 degrees S to 30 degrees N) and Indian summer monsoon rainfall. For the present study data for 30 years period from 1958 to 1988 have been used. The analysis reveals that there is a strong inverse relationship between the monsoon activity and the tropical belt temperature.|
|Monsoon rainfall and its variability in Godavari river basin|
|Rao GN PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 108: (4) 327-332 DEC 1999|
|Abstract: Rainfall variability over a river basin has greater impact on the water resource in that basin. With this in view, the variability of the monsoon rainfall over the Godavari river basin has been studied on different time scales. As expected, the monsoon rainfall in Godavari basin is more variable (17%) than the all-India monsoon rainfall (11%) during the period of study (1951-90). Similarly, inter-annual variability of the monsoon rainfall on smaller time scales is found to be still higher and increases while going on from seasonal to daily scales. An interesting observation is that the intra-seasonal variability of the monsoon rainfall has a significant negative relationship (CC = -0.53) with the total seasonal rainfall in the basin.|
|Interannual and decadal variations of snow cover over Qinghai-Xizang Plateau and their relationships to summer monsoon rainfall in China|
|Chen LT, Wu RG ADVANCES IN ATMOSPHERIC SCIENCES 17: (1) 18-30 2000|
|Abstract: Interannual and decadal variations of winter snow cover over
the Qinghai-Xizang Plateau (QXP) are analyzed by using monthly mean snow
depth data set of 60 stations over QXP for the period of 1958 through 1992.
It is found that the winter snow cover over QXP bears a pronounced quasi-biennial
oscillation, and it underwent an obvious decadal transition from a poor
snow cover period to a rich snow cover period in the late 1970's during
the last 40 years.
It is shown that the summer rainfall in the eastern China is closely associated with the winter snow cover over QXP not only in the interannual variation but also in the decadal variation. A clear relationship exists in the quasi-biennial oscillation between the summer rainfall in the northern part of North China and the southern China and the winter snow cover over QXP. Furthermore, the summer rainfall in the four climate divisions of Qinling-Daba Mountains, the Yangtze-Huaihe River Plain, the upper and lower reaches of the Yangtze River showed a remarkable transition from drought period to rainy period in the end of 1970's, in good correspondence with the decadal transition of the winter snow cover over QXP.
|The interannual variability of East Asian monsoon and its relationship with SST in a coupled atmosphere-ocean-land climate model|
|Wang HJ ADVANCES IN ATMOSPHERIC SCIENCES 17: (1) 31-47 2000|
|Abstract: Based on a 200 year simulation and reanalysis data (1980-1996),
the general characteristics of East Asian monsoon (EAM) were analyzed in
the first part of the paper. It is clear from this research that the South
Asian monsoon (SAM) defined by Webster and Yang (1992) is geographically
and dynamically different from the East Asian monsoon (EAM). The region
of the monsoon defined by Webster and Yang (1992) is located in the tropical
region of Asia (40-110 degrees E, 10-20 degrees N), including the Indian
monsoon and the Southeast Asian monsoon, while the EAM defined in this
paper is located in the subtropical region of East Asia (110-125 degrees
E, 20-40 degrees N). The components and the seasonal variations of the
SAM and EAM are different and they characterize the tropical and subtropical
Asian monsoon systems respectively. A suitable index (EAMI) for East Asian
monsoon was then defined to describe the strength of EAM in this paper.
In the second part of the paper, the interannual variability of EAM and its relationship with sea surface temperature (SST) in the 200 year simulation were studied by using the composite method, wavelet transformation, and the moving correlation coefficient method. The summer EAMI is negatively correlated with ENSO (El Nine and Southern Oscillation) cycle represented by the NINO3 sea surface temperature anomaly (SSTA) in the preceding April and January, while the winter EAM is closely correlated with the succeeding spring SST over the Pacific in the coupled model. The general differences of EAM between El Nine and La Nina cases were studied in the model through composite analysis. It was also revealed that the dominating timescales of EAM variability may change in the long-term variation and the strength may also change. The anomalous winter EAM may have some correlation with the succeeding summer EAM, but this relationship may disappear sometimes in the long-term climate variation. Such time-dependence was found in the relationship between EAM and SST in the long-term climate simulation as well.
|The interannual variability of East Asian winter monsoon and its relation to the summer monsoon|
|Chen W, Graf HF, Huang RH ADVANCES IN ATMOSPHERIC SCIENCES 17: (1) 48-60 2000|
|Abstract: Based on the NCEP/NCAR reanalysis data the interannual variability of the East Asian winter monsoon (EAWM) is studied with a newly defined EAWM intensity index. The marked features for a strong (weak) winter monsoon include strong (weak) northerly winds along coastal East Asia, cold (warm) East Asian continent and surrounding sea and warm (cold) ocean from the subtropical central Pacific to the tropical western Pacific, high (low) pressure in East Asian continent and low (high) pressure in the adjacent ocean and deep (weak) East Asian trough at 500 hPa. These interannual variations are shown to be closely connected to the SST anomaly in the tropical Pacific, both in the western and eastern Pacific. The results suggest that the strength of the EAWM is mainly influenced by the processes associated with the SST anomaly over the tropical Pacific. The EAWM generally becomes weak when there is a positive SST anomaly in the tropical eastern Pacific (El Nino), and it becomes strong when there is a negative SST anomaly (La Nina). Moreover, the SST anomaly in the South China Sea is found to be closely related to the EAWM and may persist to the following summer. Both the circulation at 850 hPa and the rainfall in China confirm the connection between the EAWM and the following East Asian summer monsoon, The possible reason for the recent 1998 summer flood in China is briefly discussed too.|
|Impact of global warming on the Asian winter monsoon in a coupled GCM|
|Hu ZZ, Bengtsson L, Arpe K JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 105: (D4) 4607-4624 FEB 27 2000|
|Abstract: The Asian winter monsoon (AWM) response to the global warming was investigated through a long-term integration of the transient greenhouse warming with the ECHAM4/OPYC3 CGCM. The physics of the response was studied through analyses of the impact of the global warming on the variations of the ocean and land contrast near the ground in the Asian and western Pacific region and the east Asian trough and jet stream in the middle and upper troposphere. Forcing of transient eddy activity on the zonal circulation over the Asian and western Pacific region was also analyzed. It is found that in the global warming scenario the winter northeasterlies along the Pacific coast of the Eurasian continent weaken systematically and significantly, and intensity of the AWM reduces evidently, but the AWM. variances on the interannual and interdecadal scales are not affected much by the global warming. It is suggested that the global warming makes the climate over the most part of Asia to be milder with enhanced moisture in winter. In the global warming scenario the contrasts of the sea level pressure and the near-surface temperature between the Asian continent and the Pacific Ocean become significantly smaller, northward and eastward shifts and weakening of the east Asian trough and jet stream in the middle and upper troposphere are found. As a consequence, the cold air in the AWM originating from the east Asian trough and high latitudes is less powerful. In addition, feedback of the transient activity also makes a considerable contribution to the higher-latitude shift of the jet stream over the North Pacific in the global warming scenario.|
|Gadgil S CURRENT SCIENCE 78: (3) 309-322 FEB 10 2000|
|Abstract: The Indian monsoon is maintained by propagation of convective systems of synoptic (lows, depressions, etc.) and planetary scale (tropical convergence zones) from the warm tropical oceans, onto the heated subcontinent. As a result, the monsoon variability on subseasonal scales (between wet and dry spells) and on interannual scales (good monsoons and droughts) is linked to variation of the convective systems over the ocean, where variability in turn depends on the sea surface temperature through complex relationship. The nature of this relationship and the mechanisms suggested for understanding it are discussed. It is suggested that for further insights into monsoon-ocean coupling detailed observations particularly over the Indian seas with new satellites and special observational experiments are essential.|
|Influence of the Indian summer monsoon on ENSO|
|Kirtman BP, Shukla J QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 126: (562) 213-239, Part A JAN 2000|
|Abstract: Historical records (approximately 100 years) of Indian summer
monsoon rainfall and El Nino/Southern Oscillation (ENSO) indices show a
strong negative correlation. This negative correlation is strongest for
east Pacific sea surface temperature anomalies (SSTA) that occur during
the months of December through to March, which is about three to six months
immediately following the monsoon season (June to September). Based on
this correlation, one is tempted to speculate that monsoon variability
affects ENSO variability. However, it is well known that ENSO is phase
locked to the annual cycle in that the largest SSTA occur at the end of
the calendar year. In other words, an ENSO which originated well before
the summer monsoon season will have its peak amplitude at the end of the
calendar year. The purpose of this study is to explain the impact of the
monsoon which has a strong seasonal preference on ENSO which has a life
cycle of about 4 years.
First, a 50-year atmospheric general-circulation model simulation with climatological SST is examined to determine the tropical Pacific wind-stress anomalies that are associated with a variable monsoon but that are also independent of SST variability in the tropical Pacific. Using simple statistical techniques, it is found that a weak (strong) monsoon results in a weakening (strengthening) of the trade winds over the tropical Pacific. To examine how these 'monsoon-forced wind-stress anomalies' in the tropical Pacific affect ENSO, simulations were made with a simple coupled model that does not include the effects of a variable monsoon. The effects of the monsoon are then added in the coupled model by either specifying the strength of the monsoon or by parametrizing the strength of the monsoon in terms of the coupled-model simulated SSTA in the east Pacific. Based on these coupled simulations, a variable monsoon enhances the ENSO variability, particularly three to six months after the monsoon ends, and can also serve as a trigger mechanism for ENSO. It is found that an ongoing warm (cold) ENSO event is made even warmer (colder) by a weak (strong) monsoon. Similarly, warm (cold) events are weakened by a strong (weak) monsoon. These results also reproduce the observed lag/lead ENSO-monsoon relation where the maximum negative correlation between the monsoon and the SSTA in the east Pacific occurs 3-6 months after the monsoon season.
|A pronounced continental-scale diurnal mode of the Asian summer monsoon|
|Krishnamurti TN, Kishtawal CM MONTHLY WEATHER REVIEW 128: (2) 462-473 FEB 2000|
|Abstract: A pronounced continental-scale diurnal mode of the Asian summer monsoon is mapped using data from recent satellites Meteosat-5 and TRMM. These datasets were available at high temporal resolutions. A result that stands out is the diurnal divergent circulation that in the afternoon hours has an ascending lobe over north-central India and has a descending lobe that reaches our radially toward central China, the southern parr of China, the equatorial Indian Ocean, and the western Arabian Sea. The reverse circulation is clearly seen during the early morning hours. This diurnal pulsation of continental-scale divergent circulation appears to be an integral part of the monsoon. Another finding relates to th diurnal slowing down and speeding up of the Tibetan high circulations, especially in the southern flanks where the tropical easterly jet resides and exhibits a pulsation of intensity. The amplitude of pulsation was found to reach up to 7 m s(-1). Thus this continental-scale change appears to be a pronounced feature. The phase and amplitude of various satellite datasets derived from the 90-min datasets are also displayed to confirm this major mode, that is, the diurnal oscillation of monsoon.|
|Wavelet analysis of the association between the Southern Oscillation and the Indian Summer Monsoon|
|Kulkarni JR INTERNATIONAL JOURNAL OF CLIMATOLOGY 20: (1) 89-104 JAN 2000|
|Abstract: A new aspect of the monsoon-Southern Oscillation (SO) link has been investigated. All India Summer Monsoon Rainfall (AISMR) and Southern Oscillation Index (SOI) data (for August-September-October months) for the period 1871-1998 have been processed for wavelet analysis. Using the Haar wavelet function, the data are decomposed into seven dyadic scales corresponding to periods of 2, 4, 8, 16, 32, 64 and 128 years. The time frequency localization in the wavelet analysis was used to study the temporal variability of modes in AISMR and SOI. The 2 and 8 year modes in both are found to exhibit low frequency modulation. The 4 year mode in both showed large intermittency. The periods of high/low activities of 2, 4 and 8 year modes were associated with a large/low number of deficient AISMR years. The SOI derived from 2, 4 and 8 year modes in the ENSO years, is found to be related to AISMR variability, at 1% level of significance. The 2, 4 and 8 year modes in AISMR and SOI are found to be correlated at a 5% level of significance. There is a large temporal variability in the correlations of these modes. The occurrences of maxima and minima in these correlations followed a sequence, first in the 8 year mode, then in the 4 year mode and in the end, in the 2 year mode. The reasons for de-association between AISMR activity and SOI in the last 8 years of the present decade have been attributed to (i) the negative contributions by 128, 64, 32 and 16 year modes, (ii) the low activity of 4 and 8 year modes and (iii) the weak correlation between AISMR and SOI in 4 and 8 year modes during this period.|
|Forecasting Indian summer monsoon rainfall by outgoing longwave radiation over the Indian Ocean|
|Prasad KD, Bansod SD, Sabade SS INTERNATIONAL JOURNAL OF CLIMATOLOGY 20: (1) 105-114 JAN 2000|
|Abstract: The satellite derived outgoing longwave radiation (OLR) over
the Indian Ocean (30 degrees N-30 degrees S and 40 degrees E-100 degrees
E) from 1974 to 1996 has been analysed for the relationship with the Indian
summer monsoon total (June-September) rainfall. The OLR of two regions
appears to be related to summer monsoon rainfall. One of the regions is
located over the Head Bay of Bengal (near 22.5 degrees N and 92.5 degrees
E) during May and the other one over the south Indian Ocean (near 30 degrees
S and 97.5 degrees E) during April. The average OLR (index) for these two
regions shows a strong and stable relationship with the Indian summer monsoon
rainfall and they are found to be independent.
A multiple linear regression equation is developed to predict the Indian summer monsoon rainfall using these indexes and the empirical relations are verified on independent data.
Good results were obtained in forecasting the summer monsoon rainfall for the whole of India. The forecast of summer monsoon rainfall for west-central India and all-India rainfall for July also appears to be encouraging. The indexes, thus, seem to be useful in long-range forecasting of the Indian summer monsoon rainfall.
|ENSO-monsoon relationship in the MRI coupled GCM|
|Kitoh A, Yukimoto S, Noda A JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (6) 1221-1245 DEC 1999|
|Abstract: Climatological features and interannual variability of the
Asian summer monsoon and its relationship with equatorial Pacific sea surface
temperature (SST) anomalies simulated by a global coupled atmosphere-ocean
GCM (CGCM) are investigated. The coupled model results are compared with
the observation as well as the simulations by an atmospheric general circulation
model (AGCM). Overall features of the model climatology and variability
of the Asian summer monsoon in the CGCM are as close to the observed one
as in the AGCM. The simulated SST and its variability in the CGCM shows
some bias compared to the observation. The monsoon region in the models,
as defined by the seasonal change of wind direction and convection proxy,
agrees with the observed. The models are less successful in simulating
the summertime wind system in the western Pacific region.
The CGCM reasonably well reproduces the observed ENSO-related interannual variability of the tropical circulation system. Its main deficit is associated with a westward displacement of simulated SST variability. There is an underestimation of precipitation around the Philippines. Differences are found between the CGCM and the AGCM in the variability over the Indian monsoon region. The AGCM responds well to the prescribed SST anomaly in the Pacific. It behaves erroneously over the Indian Ocean. This may be related to the fact that the AGCM is only responding to the prescribed SST fields, while the CGCM includes two-way atmosphere-ocean interactions.
The CGCM results show that the simulated Indian summer monsoon rainfall anomalies are negatively correlated with the equatorial Pacific SST anomalies. It is consistent with the observed one, i.e., good monsoon is associated with La Nina. As a precursory signal, ground temperature is significantly warmer in spring in central Asia preceding a good monsoon. It is noted that the snow cover anomalies are negative in the above region, but its significance is marginal.
|Tropospheric biennial oscillation of ENSO-monsoon system in the MRI coupled GCM|
|Ogasawara N, Kitoh A, Yasunari T, Noda A JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (6) 1247-1270 DEC 1999|
|Abstract: The mechanism of the tropospheric biennial oscillation (TBO)
of the ENSO-monsoon system is investigated by an MRI coupled atmosphere-ocean
general circulation model. In this mechanism, biennial variability of the
South Asian monsoon affects the global scale climate variability through
interactions with the air-sea coupled system over the Pacific and/or the
In the strong phase of the TBO, the area of relatively strong monsoon convective maximum over South Asia in the spring to summer season moves southeastward to Indonesia in the autumn to winter season. This movement superimposes on its climatological seasonal cycle. It suggests that the northern winter monsoon convection tends to be strong around Indonesia to northern Australia when the summer Asian monsoon is strong. The anomalous state of the air-sea coupled system in the Pacific sector which forms in the summer season, seems to dissipate from its eastern edge. This occurs by a local atmosphere-ocean coupling process through a large scale Walker circulation.
The convection anomalies persist during the entire monsoon season over Indonesia and northern Australia. As a response to this equatorial monsoon convection anomaly, a Matsuno-Gill type stationary Rossby wave is established over the South Asian region. The appearance of upper level anticyclonic circulation and lower level cyclonic circulation anomaly in a strong monsoon year is a favorable condition for bringing the cold air advection over the Eurasian continent. Cold air advection after the strong monsoon persists through the whole winter to spring season to form the cold tropospheric temperature around Central to South Asia. Then reduced land-sea, or north-south temperature contrasts sets up the following weak South Asian summer monsoon. The simulated TBO of the South Asian monsoon is tightly phase locked with a seasonal cycle. The phase of the TBO changes in northern spring, which suggests that the extratropical-tropical interaction be realized mainly during winter to spring through the onset of South Asian monsoon. Our results imply that the TBO is an inherent feature in the land-monsoon-ocean coupled system, and emphasize a more active role of monsoon-extratropical interaction in the Indian sector in winter to spring season for regulating the TBO cycle.
|Changes in South Asian monsoon: New high-resolution paleoclimatic records from Tibet, China|
|Tang LY, Shen CM, Liu KB, Overpeck JT CHINESE SCIENCE BULLETIN 45: (1) 87-91 JAN 2000|
|Abstract: High-resolution pollen records from 6 small lakes in the Tibetan Plateau provided the details of evolution of South Asian monsoon since the Last Glacial Maximum. Prior to 16 kaBP, the region was a desert-steppe characterized by cold and dry climates, the January temperature was 7-10 degrees C lower than that of present and the annual precipitation only accounted for 40% of the present. The temperature and precipitation increased gradually and trees began to live in the region after 12 kaBP, but during the interval from 9.2 to 6.3 kaBP, forest and forest-meadow appeared occasionally. From 8 to 5 kaBP, both January and July temperature was 2-3 degrees C higher and annual precipitation was also about 200 mm higher than that of the present. After 5 kaBP, temperature and precipitation decreased linearly and steppe vegetation began to degenerate.|
|Principal components analysis of rainfall and associated synoptic models of the southwest monsoon over India|
|De US, Mazumdar AB THEORETICAL AND APPLIED CLIMATOLOGY 64: (3-4) 213-228 1999|
|Abstract: The summer monsoon circulation shows various spatial and
temporal oscillations and often a combination of systems produces an integrated
effect. In this study phases of the southwest (SW) monsoon have been identified
in an objective manner with the help of T-mode principal component analysis
(PCA) of weekly rainfall anomalies. Mean composite charts have been prepared
utilising all available upper air data (1977-1986) for each category of
the SW monsoon epochs identified by the PCA. These sets of charts have
been constructed for both the strong and weak phases associated with the
first four significant principal components (PCs).
A well defined east-west oriented trough system, extending from about 28 degrees N Latitude/65 degrees E Longitude to 20 degrees N Latitude/90 degrees E Longitude, in the lower levels, has been the main feature associated with the strong phase of the monsoon corresponding to PC I. The trough in the lower levels is more marked in the eastern half compared to the western half in both the sets of charts associated with strong phases of the monsoon related to the PC II and PC III. With PC II, the position of the troughs in the lower levels is further north of its location in PC III. The east-west trough system associated with the strong phase of PC IV has a large southward tilt with height. The charts corresponding to the weak phases of these PCs have synoptic features, such as the position of the trough close to the foothills of the Himalayas, and the shifting of middle and upper tropospheric anticyclones to the south.
The study suggests an objective method of interpretation of principal components by utilising synoptic data. In addition, synoptic models and data sets corresponding to different phases of the monsoon can also be prepared in an objective manner by such PCA.
|Variations of the SO relationship with Summer and Winter monsoon rainfall over India: 1872-1993|
|Rao GN JOURNAL OF CLIMATE 12: (12) 3486-3495 DEC 1999|
|Abstract: The lag correlations of the monthly and seasonal pressures at Darwin with the rainfall over India during summer (June-September) and winter (October-December) monsoons and their variations over 29 meteorological subdivisions in India are examined by using long time data for 122 yr(1872-1993). It is found that the summer monsoon rainfall over India has significant (at 5% level) positive correlation with the Darwin pressures during the previous June-August period, and significant negative correlation with the pressures during the concurrent March-May period and with the seasonal pressure tendency from winter to spring (March-May minus December-February). Of these, the correlations with the concurrent May (-0.36) and the winter to spring seasonal pressure tendency (-0.30) are strong. Similarly, the all-India rainfall during winter monsoon is found to have significant negative correlation with the Darwin pressures during the concurrent July and August. In this case, the correlation with the concurrent July pressures is strong (-0.21). During the winter monsoon season, southern peninsular India receives dominant rainfall and this rainfall is found to have only a weak positive correlation (0.17) with the concurrent September pressures at Darwin. The consistency of these relations during the period of study is examined by evaluating the 30-yr sliding window correlation coefficients. The anomaly rainfall patterns of the summer and winter monsoons over India during the years of the low and high values of the corresponding Southern Oscillation index are further examined.|
|Influence of El Nino on the 1997 Indian Summer Monsoon|
|Shen XS, Kimoto M JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (5) 1023-1037 OCT 1999|
|Abstract: The 1997 Indian Summer Monsoon is investigated in the context
of interannual variations. It is evidenced that the 1997/98 El Nino, one
of the strongest in history, suppressed the large-scale monsoon circulations.
On the contrary, however, the Indian summer monsoon rainfall was not much
reduced. In fact, it slightly exceeded the climatological seasonal average.
An inquiry into circulation and precipitation data indicates that it was the intraseasonal variability of the monsoon system that brought the above-normal rainfall over India. Furthermore, it is shown that the 1997 El Nino not only suppressed the large-scale Asian monsoon circulations, but also produced a convectively unstable area off the east coast of Somalia through the modifications in sea surface and lower tropospheric conditions. Anomalous convection was triggered, amplified over this area, then intruded northward to the Indian subcontinent as it propagated eastward.
A moisture budget analysis using 41 years (1958-98) of the NCEP/NCAR reanalysis data set confirmed that the transient part of moisture transport anomaly in the 1997 summer was larger than the stationary one, contributing positively to the small positive rainfall anomalies over India. The 1997 summer appears to be one of the singular cases in the context of the general positive correlation between the monsoon circulation and precipitation indices over the recent 41 years.
|Impact of convective downdrafts in a GCM on the simulated mean Indian Summer Monsoon and its variability|
|Mandke SK, Soman MK, Satyan V JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (5) 1061-1082 OCT 1999|
|Abstract: The sensitivity of mean monsoon and its variability simulated
by Hadley Centre Climate Model to changes in convection scheme, has been
examined by integrating the model with and without parameterisation of
convective downdrafts. Summer monsoon seasons during 7 years 1982, 1983,
1984, 1987, 1988, 1991 and 1994 have been simulated. These years have been
selected for the study as the monsoon rainfall over India showed large
interannual variability in these years. Results are compared with NCEP
reanalysis circulation and Xie-Arkin precipitation data.
Analysis of the model output shows that some of the systematic errors in the model in the monsoon simulation are reduced with incorporation of downdrafts. The systematic error by which the monsoon low level westerlies extend deep into the west Pacific ocean is substantially reduced by the inclusion of downdrafts. The lower tropospheric monsoon flow over the north Indian ocean is strengthened with downdrafts. TEJ simulated with downdrafts is closer to NCEP reanalysis. Inclusion of downdraft improves the precipitation simulation over land areas of India, and also simulates the correct position of the monsoon trough. When downdrafts are not included there is excessive precipitation over the west equatorial Indian ocean and less precipitation over India. Surface latent heat Aux increases with the downdrafts. The systematic error of the model which simulates stronger low level circulation in El Nino years than in La Nina years is reduced, though not completely eliminated. The interannual variability of simulated monsoon precipitation is more realistic with downdrafts. Spells of active and 'break,' or weak monsoon situations, are more prominently simulated when downdrafts are included. Larger number of strong synoptic disturbances are simulated over India with the inclusion of downdrafts.
|Hydrography and circulation of the Bay of Bengal during withdrawal phase of the southwest monsoon|
|Sarma YVB, Rao EPR, Saji PK, Sarma VVSS OCEANOLOGICA ACTA 22: (5) 453-471 SEP-OCT 1999|
|Abstract: Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18 degrees N and the other at 90 degrees E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18 degrees N / 87 degrees E and as high as 34.79 at 8 degrees N / 90 degrees E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90 degrees E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18 degrees N / 87 degrees E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18 degrees N transect with velocity slightly in excess of 30 cm s(-1). Similar high velocity (> 40 cm s(-1)) pockets were also noticed to extend up to 30 m depths in the southern region of the 90 degrees E transect. However, the currents below 250 m were weak and in general < 5 cm s(-1). The net geostrophic volume transports were found to be of the order of 1.5 x 10(6) m(3) s(-1) towards the north and of 6 x 10(6) m(3) s(-1) towards west across the 18 degrees N and 90 degrees E transects respectively. The surface circulation-patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12 degrees N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data. (C) 1999|
|Ifremer / CNRS / IRD / Editions scientifiques et medicales Elsevier SAS.|
|Dynamic features and maintenance mechanism of Asian summer monsoon subsystem|
|Xu JJ, Wu GX ADVANCES IN ATMOSPHERIC SCIENCES 16: (4) 523-536 1999|
|Abstract: In the context of the dissociation of vertical shear flow and tropospheric mean flow based on 1980-1996 NCEP/ NCAR re-analysis, study is undertaken of the barotropic and baroclinic development characteristics and kinetic energy maintenance mechanisms of Asian summer monsoon. Evidence suggests that the monsoon activity is marked by noticeable baroclinicity in three active regions of different dynamical characteristics, located in India, East Asian tropics and subtropics, respectively, with greatly differing maintenance mechanisms of barotropic/baroclinic kinetic energy.|
|The study on the interannual variation and the mechanism of the South China Sea monsoon|
|Jiang J, Qian YF ADVANCES IN ATMOSPHERIC SCIENCES 16: (4) 544-558 1999|
|Abstract: By using the USA NCAR/NCEP reanalysis data, the characteristics of climatic elements and the temporal and spatial structures of precipitation in the strong and weak years of the SCS monsoon are analyzed, the mechanism of the interannual variation of the SCS monsoon is discussed. It is found that the climatic elements in SCS have great differences, and there are great differences in the spatial and temporal structures of the precipitation anomalies between the strong and weak monsoon years. The variation of climatic elements in the south of Indochina Peninsula. In April is a good index of the strength of the SS monsoon. There is a good connection between the SCS monsoon and the sea surface temperature. The SCS monsoon is weak in the EL Nino years, and strong in the La Nina years. The strength of the SCS monsoon depends on the local heating differences between the eastern continent of China and the western Pacific. It depends on the intensity and the position of the western Pacific Subtropical High. The western Pacific Subtropical High is weak and eastward in the strong monsoon years, and the case is reversed in the weak monsoon years.|
|Subtropical high anomalies over the western Pacific and its relations to the Asian monsoon and SST anomaly|
|Sun SQ, Ying M ADVANCES IN ATMOSPHERIC SCIENCES 16: (4) 559-568 1999|
|Abstract: Using the data of 500 hPa geopotential height from 1951 to
1995, SST roughly in the same period and OLR data from 1974 to 1994, the
relation between the anomalies of subtropical high (STH for short) and
the tropical circulations including the Asian monsoon as well as the convective
activity are studied. In order to study the physical process of the air-sea
interaction related to STH anomaly, the correlation of STH with SST at
various sea areas, lagged and simultaneous, has been calculated.
Comparing the difference of OLR, wind fields, vertical circulations and SST anomalies in the strong and weak STH, we investigate the characteristics of global circulations and the SST distributions related to the anomalous STH at the western Pacific both in winter and summer. Much attention has been paid to the study of the air-sea interaction and the relationship between the East Asian monsoon and the STH in the western Pacific. A special vertical circulation, related to the STH anomalies is found, which connects the monsoon current to the west and the vertical flow influenced by the SST anomaly in the tropical eastern Pacific.
|On the role of sea surface temperature variability over the tropical Indian Ocean in relation to summer monsoon using satellite data|
|Kumar MRR, Muraleedharan PM, Sathe PV REMOTE SENSING OF ENVIRONMENT 70: (2) 238-244 NOV 1999|
|Abstract: The sea surface temperature (SST) variability over the tropical Indian Ocean is studied for the period January 1988 to December 1992 using the multichannel sea surface temperature (MCSST) from the NOAA series of satellites. The MCSST values were found to agree reasonably well with the Reynolds and Smith (1994) data for the same period, but the anomalies were found to differ significantly. An interesting result is the influence of the Somalia coast zonal anomaly of SST (SCZASST) and central Indian Ocean zonal anomaly of SST (CIOZASST) with the monsoon rainfall over different meteorological subdivisions of the Indian subcontinent. The SCZASST is negatively and significantly correlated with the monsoon rainfall over the western and central parts of India, while CIOZASST is positively and significantly correlated. (C)Elsevier Science Inc., 1999|
|The interdecadal variation characteristics of arctic sea ice cover - ENSO-East Asian monsoon and their interrelationship at quasi-four years time scale|
|Zhu CW, Chen LX, Yamazaki N ADVANCES IN ATMOSPHERIC SCIENCES 16: (4) 641-652 1999|
|Abstract: Interdecadal and quasi-four years variation characterstics of Arctic sea ice cover, ENSO and East Asian monsoon index (EAMI) are analyzed based on Singular Spectrum Analys. (SSA), lead-lag correlation and EOF for the past four decades. Results show that the Arctic sea ice cover decreased in the early 1970s, several years earlier than that of global SSTA increase in the mid 1970s, which indicates that recent warming over the Northern Hemisphere firstly begins in the Arctic region in the 1970s. Great change of the East Asian monsoon intensity from stronger to weaker in summer (from weaker to stronger in winter) took place in the mid 1970s response to the abrupt modulation of SSTA particularly in the tropical eastern Pacific. Focus on the quasi-four years oscillation, close relationship is found among the sea ice cover, ENSO and EAMI based on lead-lag correlation. In which, the correlation coefficient reaches its maximum when the index of NINO3 SSTA variation takes 6 and 9 months lead of the western Pacific subtropical high and sea ice cover index in Section-III. Their interaction can be explained in the framework of asymmetric Walker circulation anomaly and Western Pacific Northern Pole (WPN) teleconnection pattern in the context of quasi-four years oscillation.|
|Tropical-wide teleconnection and oscillation. II: The ENSO-monsoon system|
|Miyakoda K, Navarra A, Ward MN QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 125: (560) 2937-2963, Part B OCT 1999|
|Abstract: Two teleconnection indices, discussed in Part I, i.e. the
Tropical-wide Oscillation Index (TOI) and the Walker circulation Index
(WAI), are applied to the analysis of the ENSO-monsoon (El Nino Southern
Oscillation-Asian monsoons) system. The first hypothesis presented in Part
I was that the TOI for July-August-September (JAS) is closely related to
the Indian summer monsoon index as well as the Southern Oscillation Index.
As a result, the TOI represents the lead-lag characteristics of the tropical
circulation variability over the eastern hemisphere (45 degrees E-170 degrees
E) and simultaneously its interaction with the ENSO over the equatorial
Pacific. The second hypothesis was that there are two types of connection
between the ENSO and Asian monsoons: type I with distinct connection in
space and time, and type II without connection. The WAI provides a measure
for this connection. This idea is supported by comparisons of observed
and model teleconnection structures in Part I.
Part II investigates these relations further. Time-lag correlations are calculated between the key indices and atmospheric variables over the equatorial Indo-Pacific Oceans. If type II years, derived by the WAI, are removed from the 34-year time series, correlations between the TOI and these variables increase appreciably, now showing clearly the biennial character. The analysis identifies a sequence of events involving biennial oscillation of the ENSO-monsoon system from approximately JAS(-1) to JAS(0), followed by intensification of the ENSO from JAS(0) to November-December-January(+1). The ENSO-monsoon oscillation system is not sinusoidal but skewed.
To show the geographical patterns associated with the above sequence of events, planar maps are presented of the lag correlation between the observed TOI(JAS) and (i) Vertical velocity at the 500 hPa level, (ii) precipitation, (iii) sea surface temperature (SST), and (iv) atmospheric sea level pressure. Distinct geographical distributions of the ENSO-monsoon oscillation emerge in both the observations and model data. One pattern is characterized by a horseshoe shape over the Pacific, which is generally symmetric around the equator, but with geographical differences depending on location in the lag sequence. The other pattern is a see-saw shape, primarily a standing oscillation located in the eastern South Pacific and the Indian Oceans, resembling the sea-level-pressure pattern found by Trenberth and Shea. Applying the lead-lag relationship, it is demonstrated that the SST over the central Pacific four months ahead can be projected, based on the TOI(JAS). Conversely, the intensity of the Indian monsoon rainfall for non-type II years can be projected 15 months ahead by the SST over the eastern Pacific Ocean. This indicates that the ENSO-monsoon oscillation system is quasi-periodic, as opposed to irregular, with a two-year cycle; this is clearly revealed with the removal of type II years.
|Rossby waves in May and the Indian summer monsoon rainfall|
|Joseph PV, Srinivasan J TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY 51: (5) 854-864 OCT 1999|
|Abstract: Large amplitude stationary Rossby wave trains with wavelength in the range 50 degrees to 60 degrees longitude have been identified in the upper troposphere during May, through the analysis of 200 hPa wind anomalies. The spatial phase of these waves has been shown to differ by about 20 degrees of longitude between the dry and wet Indian monsoon years. It has been shown empirically that the Rossby waves are induced by the heat sources in the ITCZ. These heat sources appear in the Bay of Bengal and adjoining regions in May just prior to the onset of the Indian summer monsoon. The inter-annual spatial phase shift of the Rossby waves has been shown to be related to the shift in the deep convection in the zonal direction.|
|Interannual variability of the convective activities associated with the East Asian summer monsoon obtained from TBB variability|
|Ren BH, Huang RH ADVANCES IN ATMOSPHERIC SCIENCES 16: (1) 77-90 1999|
|Abstract: In this paper, the interannual variability of the convective
activities associated with the East Asian summer monsoon and its association
with the thermal distribution of SST anomalies in the tropical Pacific
are analyzed by using the daily TBB (Temperature of Black Body at Cloud
Top) dataset from 1980 to 1991.
The results of composite and individual analyses of TBB anomalies show that the interannual variability of the convective activities associated with the summer monsoon in East Asia is large and has a close relation to the thermal distribution of SST anomalies in the tropical Pacific, especially in the western Pacific warm pool. In the sumner with ENSO-like distribution of SST anomalies in the tropical Pacific, the convective activities are weak around the Philippines, then the convective activities are intensified and the summer monsoon rainfall is strong in the area from the Yangtze River basin and the Huaihe River basin in China to Republic of Korea and Japan. On the contrary, in the summer with anti-ENSO-like distribution of SST anomalies in the tropical Pacific, the convective activities are strong around the Philippines, then the convective activities are weakened and the summer monsoon rainfall is weak in the area from the Yangtze River basin and the Huaihe River basin to Republic of Korea and Japan.
It may be also found either from the composite analysis or from the individual analysis of TBB anomalies that the convective activities associated with the summer monsoon in East Asia have a good negative relation to that around the Philippines and a positive relation to that over the equatorial central Pacific.
|Relation between Eurasian snow cover, snow depth, and the Indian summer monsoon: An observational study|
|Bamzai AS, Shukla J JOURNAL OF CLIMATE 12: (10) 3117-3132 OCT 1999|
|Abstract: Satellite-derived snow cover data for 22 yr and snow depth
data for 9 yr over Eurasia have been analyzed to reexamine the possible
relation of snow with the Indian summer monsoon. In contrast to the previous
studies that use snow cover averaged over all of Eurasia as a single number,
the frequency of occurrence of snow at each grid point over Eurasia is
correlated with the Indian summer monsoon rainfall. Thus specific geographical
regions over Eurasia that are responsible for the well-known inverse relationship
between Eurasian snow cover and Indian monsoon rainfall are delineated.
It is found, somewhat surprisingly, that western Eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall. However, composites for high and low snow cover over Eurasia show spatially homogeneous large-scale patterns of snow cover and surface temperature anomalies. Winters of high and low snow cover for Eurasia are found to be associated with colder and warmer than normal temperatures, respectively, for large regions of the Eurasian continent. The inverse snow-monsoon relationship holds especially in those years when snow is anomalously high or low for both the winter as well as the consecutive spring season. Contrary to previous findings, no significant relation is found between the Himalayan seasonal snow cover and subsequent monsoon rainfall.
|On the role of the cross equatorial flow on summer monsoon rainfall over India using NCEP/NCAR reanalysis data|
|Kumar MRR, Shenoi SSC, Schluessel P METEOROLOGY AND ATMOSPHERIC PHYSICS 70: (3-4) 201-213 1999|
|Abstract: The role of the cross equatorial flow from the southern Indian Ocean on the Indian Summer monsoon is examined using the National Centre for Environmental Prediction (NCEP)/National Centre for Atmospheric Research (NCAR) data for the period January 1982 to December 1994. A comparison of NCEP/NCAR data with the satellite data retrieved from the Special Sensor Microwave Imager (SSM/I) sensor onboard the Defense Meteorological Satellite Program (DMSP) exhibited a negative bias for the wind speeds greater than 4 m/s, whereas in the case of specific humidity, SSMI values exhibited a positive bias and the precipitable water derived from the satellite data exhibited a negative bias. The NCEP reanalysis is able to depict the mean annual cycle of both the cross equatorial flow and moisture flow into the Indian subcontinent during the monsoon season, but it fails to depict these differences during excess (1983, 1988, 1994) and deficit monsoon (1982, 1986, 1987) composites. Further, it is seen that inter hemispheric flow far exceeds the excess moisture available over the Arabian Sea indicating that it is the cross equatorial flow which decides the fate of the summer monsoon.|
|The East Asian summer monsoon circulation anomaly index and its interannual variations|
|Huang G, Yan ZW CHINESE SCIENCE BULLETIN 44: (14) 1325-1329 JUL 1999|
|Abstract: Based on the concept of East Asia-Pacific (EAP) teleconnection which influences East Asian summer monsoon, an index for East Asian summer monsoon circulation anomaly was defined and it was pointed out that this index can describle the interannual variation character of summer climate in East Asia, especially in the Yangtze River and Huaihe River Valley.|
|Interannual variability of Asian monsoon precipitation, 1953-1982, using instrumental records|
|Garfin-Woll GM IAWA JOURNAL 20: (3) 227-238 1999|
|Abstract: Instrumental records were used to assess the interannual variability of precipitation for the greater Asian monsoon region (50 degrees N-15 degrees S, 60 degrees E-150 degrees E). Correlation analysis shows intriguing teleconnections between subtropical and midlatitude precipitation regions. Principal components analyses show that ENSO (El Nino-Southern Oscillation) is the dominant factor associated with recent interannual variation of precipitation in-the region. The strongest relationships between ENSO and boreal summer precipitation were found in subtropical regions, as well as North Central China and southeastern Kazakhstan; boreal winter precipitation in the tropics and subtropics also exhibited strong relationships with ENSO. Scenarios for reconstructing spatial and temporal patterns of Asian monsoon precipitation variation were generated by selecting individual records based on 1) correlation with regional time series and 2) length of record. Spatial patterns were highly dependent on the type of record selected; however, temporal patterns were reasonably well reproduced regardless of station selection criteria. The implication of the latter result is that the dominant modes of boreal summer and winter precipitation for East Asia might be reconstructed using relatively few sites.|
|Moisture balance over China and the South China Sea during the summer monsoon in 1991 in relation to the intense rainfalls over China|
|Ninomiya K JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (3) 737-751 JUN 1999|
|Abstract: The moisture balance over the central part of China in May,
June and July (MJJ) of 1991 is studied by utilizing the 24-hour prediction
data of a global weather prediction model of JMA, in relation with the
heavy rainfalls over China.
The moisture balance over a domain bounded by 20-40 degrees N and 110-125 degrees E (Area-C, main part of China) indicates that the moisture flux convergence within this domain is mainly due to the southerly moisture inflow across the southern boundary. The precipitation in this domain increases in the early June with the abrupt increase of the southerly moisture inflow. The precipitation within this domain is highly correlated with the southerly moisture inflow crossing 20 degrees N.
To relate the southerly moisture flow into Area-C with the moisture balance in the tropical zone, the moisture balance in a domain bounded by 0-20 degrees N and 110=125 degrees E (Area-S, the South China Sea including the monsoon trough region) is further studied. The variation of the cross-equatorial moisture inflow in Area-S is significantly smaller than that of the moisture efflux across the northern boundary of Area-S. In Area-S, the strong correlation is found between the zonal moisture flux convergence and the southerly moisture flux across the 20 degrees N latitude circle. This zonal moisture flux convergence is seen within the monsoon trough region between the monsoon westerly and the easterly flux in the southern rim of the Pacific subtropical anticyclone. The monsoon trough region plays the role of the channel to transport the moisture from 0-20 degrees N zone toward north, although Area-S itself is not a moisture source region. The west-east intraseasonal oscillation of the monsoon trough, monsoon westerly and the North Pacific subtropical anticyclone have strong influence on the precipitation and westerly moisture transport over China.
The features of precipitation over China in early July are characterized by a narrow intense rainfall zone over the Yangtze River Valley. The moisture flux field during this period indicates the strong moisture flux convergence within the precipitation zone.
|Interdecadal changes in the ENSO-monsoon system|
|Torrence C, Webster PJ JOURNAL OF CLIMATE 12: (8) 2679-2690, Part 2 AUG 1999|
|Abstract: The El Nino-Southern Oscillation (ENSO) and Indian monsoon
are shown to have undergone significant interdecadal changes in variance
and coherency over the last 125 years. Wavelet analysis is applied to indexes
of equatorial Pacific sea surface temperature (Nino3 SST), the Southern
Oscillation index, and all-India rainfall. Time series of 2-7-yr variance
indicate intervals of high ENSO-monsoon variance (1875-1920 and 1960-90)
and an interval of low variance (1920-60). The ENSO-monsoon variance also
contains a modulation of ENSO-monsoon amplitudes on a 12-20-yr timescale.
The annual-cycle (1 yr) variance time series of Nino3 SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935-60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales.
The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Nino3 SST and Indian rainfall are found to be highly coherent, especially during intervals of high variance. The Nino3 SST and Indian rainfall are approximately 180 degrees out of phase and show a gradual increase in phase difference versus Fourier period. All of the results are shown to be robust with respect to different datasets and analysis methods.
|Role of low pressure areas in the absence of tropical disturbances during monsoon months in India|
|Dhar ON, Nandargi S INTERNATIONAL JOURNAL OF CLIMATOLOGY 19: (10) 1153-1159 AUG 1999|
|Abstract: In this study an effort has been made to find out the contribution of low pressure areas (or lows) towards the rainfall of northern and central India during the monsoon months of June to September in the absence of more intense cyclonic disturbances such as depressions, deep depressions and cyclonic storms. 15 years of data from 1983 to 1997 has been used to study this aspect. This study has shown that the occurrence of moderate to heavy rainfall mainly depends upon their frequency, life span, track followed and origin of these disturbances provided there are no inhibiting meteorological factors like 'break' monsoon situations.|
|Summer monsoon of 1998 and associated floods - A brief appraisal|
|Dhar ON, Nadargi S JOURNAL OF THE GEOLOGICAL SOCIETY OF INDIA 54: (2) 193-195 AUG 1999|
|The effect of land-surface feedbacks on the monsoon circulation|
|Ferranti L, Slingo JM, Palmer TN, Hoskins BJ QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 125: (557) 1527-1550, Part A JUL 1999|
|Abstract: The effects of feedbacks from land-surface forcing on intraseasonal
monsoon activity are studied by performing idealized sensitivity experiments
with a general circulation model.
In agreement with observations, the simulated intraseasonal monsoon activity is mainly described by irregular alternations of active spells and break spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between a continental and an oceanic regime. In the model, the spatial characteristic of the intraseasonal monsoon variability is a robust feature which is primarily related to an internal mode of variability of the system, rather than to a response to land-surface feedbacks. Experimentation indicates that the simulation of northward propagating events, related to transitions in the regime, does not require the inclusion of interactive surface hydrological processes. This suggests that the transitions are also mainly related to internal atmospheric dynamics.
The temporal characteristics of the fluctuations between the two TCZ regimes, however, are influenced by an interactive surface. The low-frequency intraseasonal monsoon variability is enhanced by hydrological surface feedbacks. When the surface interacts with the atmosphere, the active and break regimes of the monsoon are equally likely. In the absence of surface feedbacks, the probability distribution is modified and the changes depend on the land-surface conditions imposed. The results show that the probability of a monsoon break exceeds that of an active phase when the imposed land-surface conditions are based on climatological values for July. This asymmetry in the probability distribution affects intraseasonal monsoon variability. In turn, the time-mean monsoon circulation, depending as it does on the statistics of the intraseasonal oscillations (such as frequency of occurrence and mean amplitude), is also modified by the surface feedbacks. It follows that the surface conditions play a role in the interannual predictability of the time-mean monsoon.
|The development of the South Asian summer monsoon and the intraseasonal oscillation|
|Wu MLC, Schubert S, Huang NE JOURNAL OF CLIMATE 12: (7) 2054-2075 JUL 1999|
|Abstract: Fourteen years (1980-93) of National Aeronautics and Space
Administration reanalysis data are used to document and study the variability
in the development of the South Asian summer monsoon associated with the
Intraseasonal Oscillation (ISO). The focus is on the coupling of the large-scale
upper-level divergent circulation with the low-level southwesterlies and
the associated developing regions of moisture convergence and precipitation,
which serve to define the onset times of the various regions of the South
The impact of the ISO on the development of the low-level southwesterlies is both local and remote, and depends on the strength and phasing of the ISO with the seasonal cycle. Of the 14 yr examined here, 6 showed a strong contribution to the northeastward progression and onset of the monsoon rains over India. In these cases, the ISO is initially (about 2 weeks prior to onset of rains over India) out of phase with, and therefore suppresses, the seasonal development of the regions of large-scale rising and sinking motion. As the ISO moves to the northeast, the rising branch enters the Indian Ocean and acts to enhance the latent heating in the region of the emerging Somali jet. At low levels the response rakes the form of an anticyclonic circulation anomaly over the Arabian Sea, and a cyclonic circulation anomaly to the south, which nets to inhibit the eastward progression of the Somali jet. As the ISO moves in phase with and enhances the seasonal mean upper-level divergent circulation, there is an abrupt and intense development of the southwesterly winds leading to an unusually rapid northeast shift and intensification of the monsoon rains over India and the Bay of Bengal. The general northeast progression of the anomalies may be viewed a!, an initial suppression and then acceleration of the "normal" seasonal cycle of the monsoon.
|On the weakening relationship between the Indian monsoon and ENSO|
|Kumar KK, Rajagopalan B, Cane MA SCIENCE 284: (5423) 2156-2159 JUN 25 1999|
|Abstract: Analysis of the 140-year historical record suggests that the inverse relationship between the El Nino-Southern Oscillation (ENSO) and the Indian summer monsoon (weak monsoon arising from warm ENSO event) has broken down in recent decades. Two possible reasons emerge from the analyses. A southeastward shift in the Walker circulation anomalies associated with ENSO events may Lead to a reduced subsidence over the Indian region, thus favoring normal monsoon conditions. Additionally, increased surface temperatures over Eurasia in winter and spring, which area parr. of the midlatitude continental warming trend, may favor the enhanced land-ocean thermal gradient conducive to a strong monsoon. These observations raise the possibility that the Eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal Level despite strong ENSO events.|
|Assessment of the impacts of the 1997-98 El Nino on the Asian-Australia monsoon|
|Lau KM, Wu HT GEOPHYSICAL RESEARCH LETTERS 26: (12) 1747-1750 JUN 15 1999|
|Abstract: Singular Value Decomposition (SVD) analyses of rainfall and sea surface temperature (SST) are carried out globally over the entire tropics and regionally over the Asia-Australian (AA)-monsoon domain. Contributions to monsoon rainfall predictability by basin-scale SST forcing and regional SST coupling are evaluated from cumulative anomaly correlation with dominant regional SVD modes. The observed 1997-98 AA-monsoon anomalies are found to be very complex with approximately 34% of the anomalies of the Asian (boreal) summer monsoon and 74% of the Australia (austral) monsoon attributable to basin-scale SST influence associated with El Nino. For the boreal and austral monsoon respectively, regional coupled processes contribute an additional 19% and 10%, leaving about 47% and 16% due to other factors including high frequency transients. Results suggest that in order to improve seasonal-to-interannual predictability of the AA-monsoon, we need to investigate and exploit not only monsoon-Fl Nino relationship, but also intrinsic monsoon regional coupled processes.|
|The mean evolution and variability of the Asian summer monsoon: Comparison of ECMWF and NCEP-NCAR reanalyses|
|Annamalai H, Slingo JM, Sperber KR, Hodges K MONTHLY WEATHER REVIEW 127: (6) 1157-1186, Part 2 JUN 1999|
|Abstract: The behavior of the Asian summer monsoon is documented and compared using the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA) and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis. In terms of seasonal mean climatologies the results suggest that, in several respects, the ERA is superior to the NCEP-NCAR Reanalysis. The overall better simulation of the precipitation and hence the diabatic heating field over the monsoon domain in ERA means that the analyzed circulation is probably nearer reality. In terms of interannual variability, inconsistencies in the definition of weak and strong monsoon years based on typical monsoon indices such as All-India Rainfall (AIR) anomalies and the large-scale wind shear based dynamical monsoon index (DMI) still exist. Two dominant modes of interannual variability have been identified that together explain nearly 50% of the variance. Individually, they have many features in common with the composite flow patterns associated with weak and strong monsoons, when defined in terms of regional AIR anomalies and the large-scale DMI. The reanalyses also show a common dominant mode of intraseasonal variability that describes the latitudinal displacement of the tropical convergence zone from its oceanic-to-continental regime and essentially captures the low-frequency active/break cycles of the monsoon. The relationship between interannual and intraseasonal variability has been investigated by considering the probability density function (PDF) of the principal component of the dominant intraseasonal mode. Based on the DMI, there is an indication that in years with a weaker monsoon circulation, the PDF is skewed toward negative values (i,e., break conditions). Similarly, the PDFs for El Nino and La Nina years suggest that El Nino predisposes the system to more break spells, although the sample size may limit the statistical significance of the results.|
|Sensitivity of the simulated Asian summer monsoon to parameterized physical processes|
|Eitzen ZA, Randall DA JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 104: (D10) 12177-12191 MAY 27 1999|
|Abstract: A study of the sensitivity of the simulated Asian summer monsoon to changes in general circulation model formulation is reported. The baseline version of the model fails to realistically simulate the precipitation, wind, and temperature fields. In one experiment the stratiform cloud parameterization was changed from a simple large-scale saturation Scheme to a scheme that prognostically determines cloud water, cloud ice, and rain. In a second experiment a parameter that relates the cumulus mass flux to the cumulus kinetic energy was altered so as to increase the convective adjustment time. These changes in the stratiform and cumuliform cloud parameterizations significantly improve the simulations of the precipitation and upper level wind fields, respectively.|
|Interannual and intraseasonal variations in monsoon depressions and their westward-propagating predecessors|
|Chen TC, Weng SP MONTHLY WEATHER REVIEW 127: (6) 1005-1020, Part 1 JUN 1999|
|Abstract: The majority of monsoon depressions develop from the regenesis
of westward-propagating residual lows from the east. Most of these residual
lows can be traced to weather disturbances in the south China Sea, including
tropical cyclones and 12-24-day monsoon lows. Hypothetically, any mechanism
causing a variation in the occurrence frequency of these two types of weather
disturbances in the western tropical Pacific-south China Sea (WTP-SCS)
region may result in a corresponding change in the formation frequency
of monsoon depressions over the Bay of Bengal. Two such possible mechanisms
are interannual and intraseasonal variations of large-scale summer circulation
in the WTP-SCS region induced by 1) the interannual variation of the sea
surface temperature (SST) in the eastern tropical Pacific and 2) the northward
migration of the 30-60-day monsoon trough/ridge. The National Centers for
Environmental Prediction-National Center for Atmospheric Research reanalysis
data and the 6-hourly tropical cyclone track collected by the Japan Meteorological
Agency for the period of 1979-94 were analyzed to substantiate the aforementioned
hypothesis. The findings are as follows.
1) Interannual variation. Based upon the SST averaged over the National Oceanic and Atmospheric Administration NINO3 region (150 degrees-90 degrees W, 5 degrees S-5 degrees N), the summers of 1982, 1983, 1987, and 1991 and 1981, 1984, 1985, 1988, 1989, and 1994 are defined as warm and cold, respectively. A clear interannual variation can be seen in the frequency of monsoon depressions in the Bay of Bengal: an enhancement (reduction) of monsoon depression activity occurs during cold (warm) summers. This interannual variation of monsoon depression activity is traceable to the corresponding variation of the combined tropical cyclone and 12-24-day monsoon low frequency in the south China Sea. The latter interannual variation results from the development of an;anomalous anticyclonic (cyclonic) circulation between 15 degrees and 30 degrees N in the WTP-SCS region in response to the warm (cold) SST anomalies in the eastern tropical Pacific.
2) Intraseasonal variation. There is an intraseasonal variability in the occurrence of tropical cyclones and of 12-24-day monsoon lows over the south China Sea, which is followed by a corresponding variability of monsoon depressions over the Bay of Bengal. The formation frequency of these depressions is dependent on the penetration role of the residual lows of these two types of disturbances across Indochina. These residual lows lead to an intraseasonal change in monsoon depression formation in connection with a deepening/filling of the monsoon trough over northern India and the Bay of Bengal.
|Principal components of monsoon rainfall in normal, flood and drought years over India|
|Singh CV INTERNATIONAL JOURNAL OF CLIMATOLOGY 19: (6) 639-652 MAY 1999|
|Abstract: Daily precipitation data of the monsoon season (June-September)
for the period 1940-1980 from 50 stations is considered in this study.
The correlation of individual station rainfall with all India seasonal
rainfall is carried out to identify the homogeneity associated with the
nature of rainfall activity of individual stations with the rainfall character
of the all India seasonal rainfall, taking into account statistical significance.
Further, empirical orthogonal function (EOF) analysis is carried out on
these data to find out the nature of rainfall distribution in different
monsoon categories, namely normal, flood and drought years. The percentages
of variance explained in these different categories for normal, flood and
drought years are estimated. The technique thus serves to identity spatial
and temporal pattern characteristics of possible physical significance.
It has been found that during normal, flood and drought years, the first four (most dominated) principal component with 'significantly positive' correlated stations explains 73%, 77% and 100% of the variance, while the remainder 'weakly correlated' stations explains 58%, 66% and 52% of the variance with all India seasonal mean rainfall.
|On the possible mechanisms of the evolution of a mini-warm pool during the pre-summer monsoon season and the genesis of onset vortex in the south-eastern Arabian Sea|
|Rao RR, Sivakumar R QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 125: (555) 787-809, Part A APR 1999|
|Abstract: During the pre-summer monsoon season (February-May), the
near-surface waters in the Arabian Sea progressively warm up and a mini-warm
pool with a core >30 degrees C is manifested in the south-eastern region.
The possible mechanisms for the observed seasonal build-up of this mini-warm
pool are examined, utilizing all the available monthly mean climatologies
of surface wind field, surface heat fluxes, near-surface thermohaline fields,
near-surface circulation, and mean sea level as monitored by satellites
and by some of the recent model solutions on the Arabian Sea circulation.
During winter (November-February), the equatorward-flowing East India Coastal Current in the western Bay of Bengal and westward-flowing North Equatorial Current in the southern Bay bring low-saline waters into the south-eastern Arabian Sea, causing a haline stratification within the near-surface isothermal layer. During December-April, the positive surface-wind-stress curl and the associated Ekman divergence shoals the pycnocline. A south-westward propagating mode-2 Rossby wave from off south-west India seen in satellite-derived mean sea level and model solutions also modulates the underlying pycnocline. During the pre-summer monsoon season, under clear skies and light wind conditions, the radiative heat input overwhelms turbulent heat losses at the air-sea interface, and the net surplus heat energy is absorbed in a shallow haline stratified near-surface layer, resulting in the formation of the observed mini-warm pool.
An examination of historical data on the genesis of monsoon onset vortices reveals that on most occasions the genesis has occurred over this mini-warm pool region. Evidence for the geographic coincidence in the occurrence of the genesis of onset vortex and the sea surface temperature maxima during individual years of a three decade period (1961-90) is presented.
|Choice of south Asian summer monsoon indices|
|Wang B, Fan Z BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 80: (4) 629-638 APR 1999|
|Abstract: In the south Asian region, two of the major precipitation
maxima associated with areas of intensive convective activity are located
near the Bay of Bengal and in the vicinity of the Philippines. The variations
of monthly mean outgoing longwave radiation in the two regions are poorly
correlated, particularly in the decade of 1980s. The enhanced convection
over the Bay of Bengal and Indian subcontinents is coupled with reinforced
monsoon circulation west of 80 degrees E over India, the western Indian
Ocean, and the tropical northern Africa. In contrast, the enhanced convection
in the vicinity of the Philippines corresponds to intensified monsoon circulation
primarily east of 80 degrees E over southeast Asia including the Indochina
peninsula, South China Sea, Philippine Sea, and the Maritime Continent.
To better reflect regional monsoon characteristics, two convection indices
(or associated circulation indices that are dynamically coherent with the
convection indices) are suggested to measure the variability of the Indian
summer monsoon (ISM) and the southeast Asian summer monsoon, respectively.
The change in the Bay of Bengal convection (the ISM) has planetary-scale implications, whereas the change in Philippine convection has primarily a regional impact including a linkage with the east Asia subtropical monsoon. The equatorial western Pacific winds exhibit a considerably higher correlation with the ISM convection than with the Philippine convection. During the summers when a major Pacific warm episode occurs (e.g., 1982-83, 1986-87, 1991-92, and 1997), the convection and circulation indices describing the ISM often diverge considerably, causing inconsistency among various normally coherent monsoon indices. This poses a primary difficulty for using a single monsoon index to characterize the interannual variability of a regional monsoon. The cause of the breakdown of the coherence between various convection and circulation indices during ENSO warm phase needs to be understood.
|Precipitation and moisture balance of the Asian summer monsoon in 1991 Part II: Moisture transport and moisture balance|
|Ninomiya K, Kobayashi C JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77: (1) 77-99 FEB 1999|
|Abstract: Features of the moisture transport and the moisture balance
within the Asian summer monsoon region in 1991 are studied utilizing the
24-hour prediction data by a global weather prediction model in relation
to changes of precipitation and the major circulation systems (CSs) defined
in Part I of the present paper. The change during May, June and July is
characterized by a shift of the intense rainfall areas, with the strong
moisture sink from the equatorial zone over the Indian Ocean and Indonesia,
to the subtropical area of the Indian subcontinent and East Asia. The southwesterly
moisture transport in 30-80 degrees E is mainly due to CS-3 (the clockwise
circulation over the Indian Ocean), whereas the southwesterly moisture
transport in 80-110 degrees E and 110-140 degrees E is mainly due to CS-4
(clockwise circulation around Indonesia) and CS-5 (circulation around the
Pacific subtropical anticyclone), respectively. The confluence between
the adjacent CSs yields strong southerly moisture transport into the monsoon
rainfall areas and large moisture-flux convergence.
It is an important fact that the seasonal variation of the moisture flux crossing the boundary of a large domain, which is bounded by 10 degrees S, 40 degrees N, 35 degrees E and 140 degrees E, and the variation of cross equatorial moisture transport from the southern hemisphere, are significantly smaller than the variations of the moisture transport within the domain. The moisture balance calculation also indicates that the essential feature of the summer monsoon is the formation of a pair of moisture source region, and the adjacent moisture sink region, within the large domain. The moisture source regions form under the subsidence and convective stable stratification. The moisture sink regions appear under the ascent motion and convective unstable stratification. Several parameters such as "moisture influx ratio" and "rainfall production ratio" are defined to discern the characteristics of the moisture balance. The temporal and spatial variations of these parameters depict well the features of moisture balance within the Asia summer monsoon region.
|Intercomparison of regional climate simulations of the 1991 summer monsoon in eastern Asia|
|Leung LR, Ghan SJ, Zhao ZC, Luo Y, Wang WC, Wei HL JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 104: (D6) 6425-6454 MAR 27 1999|
|Abstract: Regional climate models have become a common research tool for downscaling global climate simulations. To further examine their usefulness for climate studies and the impacts that different physical parameterizations have on the simulations, an intercomparison experiment has been performed where three regional climate models are used to simulate an extreme flood event. Although the dynamical components of the models are almost identical, the physical parameterizations used to represent clouds, radiative transfer, turbulence transport, and surface processes are very different. The models were used to simulate the heavy precipitation during the 1991 summer which caused severe flooding over the Yangtze River in China. This extreme event is selected to highlight the differences among regional climate models. Results from the intercomparison show that all models simulated the gross flood conditions reasonably well, although each model reproduced the observed rainband to varying degrees, and significant differences are found in the simulated energy and hydrological cycles, especially over cloudy areas. Through detailed analyses of model simulations at different spatial and temporal scales, several reasons are found to cause the departure of model simulations from each other. These include the simulation of the amount and vertical distribution of clouds, the treatment of cloud-radiative feedbacks, and the representation of land surface processes. This study suggests that aspects other than surface temperature and precipitation of the regional climate simulations need to be more carefully evaluated. One specially important evaluation criterion is the radiation balance which has serious implications for long-term climate simulations.|
|A broad-scale circulation index for the interannual variability of the Indian summer monsoon|
|Goswami BN, Krishnamurthy V, Annamalai H QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 125: (554) 611-633, Part B JAN 1999|
|Abstract: A broad-scale circulation index representing the interannual
variability of the Indian summer monsoon is proposed and is shown to be
well correlated with the interannual variability of precipitation in the
Indian monsoon region. Using monthly precipitation analysis based on merging
rain-gauge data with satellite estimates of precipitation for the period
1979-96, it is shown that the variability of precipitation on seasonal
to interannual time-scales is coherent over a large region covering the
Indian continent as well as the north Bay of Bengal and parts of south
China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is
defined as the precipitation averaged over the region 70 degrees E-110
degrees E, 10 degrees N-30 degrees N. The EIMR index is expected to represent
the convective heating fluctuations associated with the Indian monsoon
better than the traditional all India Monsoon Rainfall (IMR) based only
on the precipitation over the Indian continent. It is shown that large
precipitation over the Bay of Bengal with significant interannual variability
cannot be ignored in the definition of Indian summer monsoon and its variability.
The June-to-September climatological mean EIMR is found to be larger than
that of the LMR even though the former is averaged over a larger area.
The dominant mode of interannual variability of the Indian summer monsoon
is associated with a dipole between the EIMR region and the north-western
Pacific region (110 degrees E-160 degrees E, 10 degrees N-30 degrees N)
and a meridional dipole between the EIMR region and the equatorial Indian
Ocean (70 degrees E-110 degrees E, 10 degrees S-5 degrees N).
It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. An index of the monsoon Hadley (MH) circulation is defined as the meridional wind-shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR. Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium-Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi-biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Nino and Southern Oscillation during the latest eight years.
|Evolution of large-scale circulation and heating during the first transition of Asian summer monsoon|
|Hsu HH, Terng CT, Chen CT JOURNAL OF CLIMATE 12: (3) 793-810 MAR 1999|
|Abstract: This study investigates the characteristics of large-scale
circulation and heating during the first transition of the Asian summer
monsoon by a compositing technique. The first transition is characterized
by a sudden change in large-scale atmospheric circulation and convective
activity in South and Southeast Asia. The most notable features include
1) the development of the low-level cyclonic circulation and the upper-level
anticyclone in South Asia, 2) the strong convection in the Bay of Bengal,
the Indochina peninsula, and the South China Sea, and 3) the warming and
the subsequent cooling of the SST in the Bay of Bengal.
Results show the close relationship between the fluctuations of atmospheric circulation, heating, and surface condition. It is suggested that the atmospheric circulation abruptly changes during the transition owing to the interaction between convection, large-scale circulation, and lower-boundary forcing that includes topographically lifting ocean and land surface heating.
|Principal modes of climatological seasonal and intraseasonal variations of the Asian summer monsoon|
|Kang IS, Ho CH, Lim YK, Lau KM MONTHLY WEATHER REVIEW 127: (3) 322-340 MAR 1999|
|Abstract: Principal modes of climatological variation of the Asian
summer monsoon are investigated. Data used in this study include the high
cloud fraction produced by the International Satellite Cloud Climatology
Project and sea level pressure, and 850- and 200-mb geopotential heights
from ECMWF analysis for the five summers of 1985-89. It Is Shown that the
seasonal evolution of me Asian summer monsoon is adequately described by
a few leading EOFs, These EOFs capture the variations of regional rainbands
over the East Asian and Indian regions.
The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cloud hands from northern India crossing the Korean peninsula to Japan, aad dryness over the oceans in the south of cloud bands. This mode is related to the mature phase of Changma rainy season in Korea associated with the northward movement of cloud bands and circulation systems from the subtropical western Pacific. This mode appears as a first principal mode of climatological intraseasonal oscillation (CISO) over the entire Asian monsoon region. The CISO mode has a timescale of about 2 months.
The northward moving CISO also appears in the 850- and 200-mb geopotential height fields as a first mode of each. dataset. Based on the height variations of the CISO mode, it is suggested that the extratropical CISO during summer is related to a regional index cycle associated with the variation of north-south temperature gradient in East Asia.
|The monsoon rainband over China and relationships with the Eurasian circulation|
|Samel AN, Wang WC, Liang XZ JOURNAL OF CLIMATE 12: (1) 115-131 JAN 1999|
|Abstract: Yearly variations in the observed initial and final dates
of heavy, persistent monsoon rainband precipitation across China are quantified.
The development of a semiobjective analysis that identifies these values
also makes it possible to calculate annual rainband duration and total
rainfall. Relationships between total rainband precipitation and the Eurasian
circulation are then determined. This research is designed such that observed
rainband characteristics can be used in future investigations to evaluate
Normalized daily precipitation time series are analyzed between 1951 and 1990 for 85 observation stations to develop criteria that describe general rainband characteristics throughout China. Rainfall is defined to be "heavy" if the daily value at a given location is greater than 1.5% of the annual mean total. Heavy precipitation is then shown to be "persistent" and is thus identified with the rainband when the 1.5% threshold is exceeded at least 6 times in a 25-day period. Finally, rainband initial (final) dates are defined to immediately follow (precede) a minimum period of 5 consecutive days with no measurable precipitation. A semiobjective analysis based on the above definitions and rainband climatology is then applied to the time series to determine annual initial and final dates.
Analysis application produces results that closely correspond to the systematic pattern observed across China, where the rainband arrives in the south during May, advances to the Yangtze River valley in June, and then to the north in July. Rainband duration (i.e., final - initial + 1)is approximately 30-40 days while total rainfall decreases from south to north. A significant positive correlation is found between total rainfall and duration interannual variability, where increased rainband precipitation corresponds to initial (final) dates that are anomalously early (late). No clear trends are identified except over north China, where both duration and total rainfall decrease substantially after 1967.
The Eurasian sea level pressure and 500-hPa height fields are then correlated with total rainfall over south China, the Yangtze River valley, and north China to identify statistically significant relationships. Results indicate that precipitation amount is influenced by the interaction of several circulation features. Total rainfall increases over south China when the surface Siberian high ridges to the south and is overrun by warm moist air aloft. Yangtze River valley precipitation intensifies when westward expansion of the subtropical high along with strengthening of the Siberian high and monsoon low cause moisture advection, upward motion, and the thermal gradient along the Mei-Yu front to increase. North China total rainfall increases in response to intense heating over the landmass, westward ridging of the subtropical high, and greater moisture transport over the region.
|Impact of localized sea surface temperature anomalies over the equatorial Indian ocean on the Indian summer monsoon|
|Chandrasekar A, Kitoh A JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (6) 841-853 DEC 1998|
|Abstract: Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) during the Indian summer monsoon, one over the continent and the other over the equatorial Indian Ocean. An active spell of one TCZ coincides with a weak spell of the other TCZ. Observations also show the presence of positive sea surface temperature (SST) anomalies south of the equator over the Indian Ocean during the weak Indian summer monsoon years. The impact of such SST anomalies on the Indian summer monsoon is investigated through general circulation model ensemble experiments. The results indicate significant response over the Indian region and this response is manifested as a decrease in the monsoon precipitation and the weakening of the mean monsoon circulation. A series of identical experiments with a negative SST anomaly prescribed over the same region with the same magnitude confirm these findings.|
|Precipitation and moisture balance of the Asian summer monsoon in 1991 - Part I: Precipitation and major circulation systems|
|Ninomiya K, Kobayashi C JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (6) 855-877 DEC 1998|
|Abstract: The large-scale features of the moisture transport and the
water balance of the Asian summer monsoon in 1991 were studied by utilizing
the 24-hour prediction by a T106 spectral global model. In part I of the
present study, we will describe the seasonal change of the precipitation
in relation to the change of the major circulation systems over the Asian
The most remarkable change in the onset period of the Asian summer rainy season over the Indian Ocean and the Southeastern Asia is the rapid displacement of the major precipitation areas from the equatorial zone to the Indochina Peninsula and Indian subcontinent. The notable change in the onset phase of the rainy season over East Asia is the formation of a precipitation belt, which corresponds to the Meiyu and Baiu frontal zone.
In order to realize the relation between the variations of the precipitation with the those of the low-level circulation, several major circulation systems (CSs) in the lower troposphere are defined. Among them, the variations of the clockwise circulation centered over the Indian Ocean (CS-3) and a cyclonic circulation over the northern part of the Indian subcontinent (CS-6) are closely related with variations of the Indian monsoon rainfalls, while the cross-equatorial flow from the Australia anticyclone (CS-4) and the circulation around the North Pacific subtropical anticyclone (CS-5) are the major systems related with the East Asia rainfalls.
The variation of the mixing ratio of water vapor, equivalent potential temperature, vertical stability and the vertical motion are also described in relation with the variations of the precipitation and CSs. During the Asian summer monsoon season, the gradual decrease of the mixing ratio and the increase of the vertical stability, in association with the subsidence, are significant features over the western tropical Indian Ocean, while the increase of the specific humidity and the vertical instability are the notable features over the Indian sub continent, Indochina Peninsula and the southern part of China.
|A possible mechanism of the Asian summer monsoon-ENSO coupling|
|Kawamura R JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (6) 1009-1027 DEC 1998|
|Abstract: A significant coupling of the Asian summer monsoon and ENSO
was examined using the NCEP/NCAR reanalysis for the period 1973-1995. Results
show that a monsoon index, which is defined as meridional gradient of summertime
upper-tropospheric thickness (200-500 hPa) anomalies across 20 degrees
N over the Indian subcontinent, is highly correlated with Nino-3 SST anomalies
in the preceding spring. This is strongly suggestive of the presence of
the indirect impact of anomalous SST forcing associated with ENSO on the
Asian summer monsoon.
Due to attenuated Walker circulation in response to a warm episode, convection is suppressed over the northern tropical Indian Ocean and the maritime continent from the preceding winter to spring. The suppressed tropical convection in the preceding spring generates anomalous cyclonic circulation to the west of the Tibetan Plateau as a result of the Rossby-type response to convective heating off the equator. The convection-induced anomalous cyclonic circulation accompanied by large-scale ascending atmospheric motion contributes substantially to increased rainfall and greater soil moisture, thus resulting in decreased land-surface temperature over central Asia to the northwest of the Indian subcontinent. On the other hand, warm SST anomalies are initially introduced over the tropical Indian Ocean in late spring prior to the onset of the monsoon due to the changes in the surface heat flux and/or dynamic response of the ocean to wind forcing, in intimately association with pronounced in situ low-level northeasterly wind anomalies and less cloud cover. Both these different physical processes in the land and ocean areas are crucially responsible for reduced land-ocean thermal contrast (or reduced meridional tropospheric temperature gradient), eventually bringing about the weakening of the Asian summer monsoon. The reverse situation is quite true for strong monsoon years. Once the summer monsoon becomes weak (strong) at its early stage due to these processes, the initially induced warm (cool) SST anomalies over the tropical Indian Ocean are further intensified.
The mechanism proposed here is valid during the period from the late 1970s to the early 1990s when weak and strong monsoon years are categorized. During that period, the unusual Nino-3 SST anomalies tend to persist from the preceding winter until summer, hence serving as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon. However, regardless of when the monsoon-ENSO coupling is prominent, both the springtime outgoing longwave radiation and low-level wind anomalies dominating over the tropical Indian Ocean, associated with anomalous Walker circulation, are still crucial factors in terms of the potential predictability of the Asian summer monsoon.
|JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (6) 1045-1063 DEC 1998|
|Abstract: The Asian summer monsoon circulation, especially its climatological
seasonal change, was studied as the model response to the prescribed zonal
mean field and the prescribed diabatic heat source from the observation.
The obtained results are summarized as follows.
(1) During the Asian summer monsoon season, the prescribed deep heat sources in the southern part of Asia form the Tibetan High, the monsoon trough, the low-level circulation over South Asia, and furthermore, the downward motion in the western part of the Eurasian Continent. The heat sources near the surface over central Asia also induce downward motions aloft.
(2) In early summer (June), the deep heat sources in the southern part of Asia tend to form southwesterly low-level flows and upward motion southeast of Japan. Those are considered to be the background for the Baiu formation in East Asia as well as heat lows produced in the southern part of Asia. The mid-latitude heat sources associated with the Baiu precipitation produce a low-level jet south of that.
(3) Climatological seasonal change from early summer (June) to mid-summer (July) is characterized by an air temperature increase in the whole Northern Hemisphere and a northward shift of a weakened westerly jet. When in the model a zonal mean field in June is replaced by that in July, the major characteristics of the seasonal change are obtained qualitatively; low-level jets and upward motion areas in South Asia and East Asia shift from the ocean side of the coasts toward the land side. This change of vertical motion is consistent with the seasonal change of deep heat sources from June to July.
(4) The climatological seasonal change from mid-summer (July) to late summer (August) is characterized by enhanced convective activity in the extended area of the subtropical western Pacific. When deep heat sources in July are replaced by those in August over the western Pacific only, the major characteristics of the seasonal change over the Pacific and the Indian Ocean are obtained. The expansion of the Tibetan High at the upper-level and the Pacific High at low-level over Japan is also simulated by the seasonal change of the western Pacific heat sources only.
(5) The model simulation with the combination of the diabatic heat source for August and the zonal mean field for June is compared with the climatological August simulation. It is indicated that the zonal mean field delayed from its seasonal migration could be related to weak monsoon circulation and the associated precipitation anomalies in the mid-latitudes and the subtropics.
|Associations between China monsoon rainfall and tropospheric jets|
|Liang XZ, Wang WC QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 124: (552) 2597-2623, Part B OCT 1998|
|Abstract: Strong associations, in both the annual cycle and interannual variations, between east China monsoon rainfall and tropospheric jets are established with the use of observations and general-circulation model simulations. Two distinct systems dominate regional rainfall: the east Asian jet stream (EAJ) in the north and the Hadley cell in the south. The EAJ is associated with Mei-Yu and polar fronts as well as vigorous jet-transverse circulations, whereas the Hadley cell is allied to tropical upper-level easterlies and intertropical convergence zone convection. An equatorward EAJ displacement causes precipitation to increase over south-central (south) China during June August (January-March). Conversely, a poleward shift of the summer (winter) EAJ brings heavier precipitation over north (central-north) China. On the other hand, over the South China Sea the Hadley cell influence prevails and, consequently, increased rainfalls concur with enhanced lower-level westerlies. Furthermore, the EAJ fluctuations are strongly coupled with southern oscillation variations. Their interactions tend to precede (follow) El Nino phenomena during October through May (summer). The EAJ related flow anomalies also have potential skill to predict China rainfall interannual variability. To conclude, a realistic China monsoon simulation requires accurate representation of the EAJ and Hadley cell. Both features link regional rainfall to tropical and extratropical planetary-scale circulations and, in turn, to global surface characteristics.|
|Possible teleconnections of winter rainfall in southern Brazil with Indian monsoon activity|
|Satyamurty P, Pezzi LP METEOROLOGY AND ATMOSPHERIC PHYSICS 68: (1-2) 53-56 1998|
|Abstract: Three homogeneous subregions of rainfall anomaly are identified in southern Brazil from the precipitation data for the months of June to September for the period 1960-1993. The area average monthly rainfall of these regions is correlated with the Indian monsoon rainfall index (MRI). The correlations are weak; however some significant negative correlation coefficients of the order of 0.3 or higher are found, indicating that more than normal monsoon activity in July has an effect of reducing the rainfall in southern Brazil in austral winter. The relevance of this result lies in the fact that any rainfall shortage in the midwinter and spring seasons can increase ambiental hazards such as forest fires.|
|Influences of sea surface temperature and ground wetness on Asian summer monsoon|
|Yang S, Lau KM JOURNAL OF CLIMATE 11: (12) 3230-3246 DEC 1998|
|Abstract: The authors have conducted a series of experiments with a
general circulation model to understand the influences of sea surface temperature
(SST) and ground wetness (GW) (measured by snow amount and soil moisture
content) on the Asian summer monsoon. The experiments are designed to illustrate
the dominant features of monsoon response to SST and GW forcings and to
delineate the relative importance of each forcing function in contributing
to the variability of the monsoon.
Results indicate that ocean basin-scale SST anomalies exert a stronger control on the interannual variability of the monsoon compared to GW anomalies. The impact of SST anomalies on the monsoon appears nonlinear with respect to warm and cold events. The monsoon is weakened during the warm events but changes less noticeably during the cold events. The diminution of monsoon circulation associated with the warm SST anomalies is accompanied by a broad-scale reduction in water vapor convergence and monsoon rainfall.
Results also indicate that, following wet land surface conditions (enhanced snow and soil moisture) in the Asian continent during previous cold seasons, the summer monsoon becomes moderately weaker. Antecedent land surface processes mainly influence the early part of the monsoon. Wetter and colder conditions occur in the Asian continent during warm SST events. This results in reduced land-sea thermal contrast, which reinforces the weak monsoon anomalies produced initially by warm SST forcing. These interactive forcings are also responsible for the changes in the winter-spring westerlies over subtropical Asia, which are key precursory signals for the subsequent summer monsoon.
It should be pointed out that this study is conducted for the climate decade of 1979-88 only. The general robustness of the results needs to be explored by further investigations. In addition, chaotic features may have affected the results because of sampling errors.
|Relationships between stability and monsoon convection|
|McBride JL, Frank WM JOURNAL OF THE ATMOSPHERIC SCIENCES 56: (1) 24-36 JAN 1 1999|
|Abstract: Rawinsonde data from the Australian Monsoon Experiment are
analyzed to determine the manner in which the atmospheric stratification
of density and moisture respond to large amounts of convective latent heat
release. The study focuses on time series of data from a ship located at
the northern end of the Gulf of Carpentaria during active and break periods
of the monsoon.
Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to mid-tropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection.
Convective activity is weakly but inversely related to convective available potential energy variations. Day-to-day variations in CAPE are dominated by variations in equivalent potential temperature of the source level (boundary layer) air. The physical effect is one of changing the moist adiabat along which the air parcel rises. In temperature-log pressure space, moist adiabats diverge in the upper half of the troposphere. Since CAPE variations are dominated by changes in the moist adiabat of the rising parcel, the day-to-day CAPE changes occur almost totally in positive area variations above the 600-hPa level.
The above results are discussed in the context of other studies in the literature. It is proposed that stabilization of the atmosphere in response to deep convection occurs almost entirely through the modification of CAPE through decreasing theta(e) of the source air in the boundary layer. This occurs over relatively small spatial scales, whereas variations in lapse rate through the deep troposphere are hypothesized to occur over the relatively large scales associated with monsoon active and break events.
|Numerical simulation of the sensitivity of summer monsoon circulation and rainfall over India to land surface processes|
|Raman S, Mohanty UC, Reddy NC, Alapaty K, Madala RV PURE AND APPLIED GEOPHYSICS 152: (4) 781-809 OCT 1998|
|Abstract: The influence of soil moisture and vegetation variation on
simulation of monsoon circulation and rainfall is investigated. For this
purpose a simple land surface parameterization scheme is incorporated in
a three-dimensional regional high resolution nested grid atmospheric model.
Based on the land surface parameterization scheme, latent heat and sensible
heat fluxes are explicitly estimated over the entire domain of the model.
Two sensitivity studies are conducted; one with bare dry soil conditions
(no latent heat flux from land surface) and the other with realistic representation
of the land surface parameters such as soil moisture, vegetation cover
and landuse patterns in the numerical simulation. The sensitivity of main
monsoon features such as Somali jet, monsoon trough and tropical easterly
jet to land surface processes are discussed.
Results suggest the necessity of including a detailed land surface parameterization in the realistic short-range weather numerical predictions. An enhanced short-range prediction of hydrological cycle including precipitation was produced by the model, with land surface processes parameterized. This parameterization appears to simulate all the main circulation features associated with the summer monsoon in a realistic manner.
|Comments on "space-time structure of monsoon interannual variability"|
|Khandekar ML JOURNAL OF CLIMATE 11: (11) 3057-3059 NOV 1998|
|Answer to comments on "space-time structure of monsoon interannual variability"|
|Terray P JOURNAL OF CLIMATE 11: (11) 3060-3061 NOV 1998|
|Long term estimation of monsoon rainfall using stochastic models|
|Singh CV INTERNATIONAL JOURNAL OF CLIMATOLOGY 18: (14) 1611-1624 NOV 30 1998|
|Abstract: To study the persistence structure causing interannual variability in monsoon rainfall, stochastic modelling of monsoon rainfall data at 50 different stations across India has been attempted. For this, correlograms and partial correlogram wherever necessary, have been developed from 41 years of data at each station. Due to the resulting non-existence of persistence structure in the data, statistical modelling of these data have been carried out instead for quantitative estimation of monsoon rainfall at specified recurrence intervals. To overcome the difficulties in choice of a statistical distribution, it has been proposed to use the method of power transformation, the results of which have been used for verification with the observed rainfall at all stations. Using the observed information, maps have been developed which can be used for quantitative estimation of monsoon rainfall at ungauged locations: (C) 1998 Royal Meteorological Society.|
|Organization of convection and monsoon forecasts|
|Krishnamurti TN, Bedi HS, Han W METEOROLOGY AND ATMOSPHERIC PHYSICS 67: (1-4) 117-134 1998|
|Abstract: In this paper we address the issue of monsoon forecasts in
relation to the organization of convection. Given a physical initialization
procedure, within a data assimilation, it is possible to use the detailed
distribution of rainfall from mesoconvective precipitating elements to
define the initial state of a global model. If that is carried out using
a very high resolution model then the initial state can carry within it
an organization of convection within the resolvable scales. Then the impact
of physical initialization on the maintenance and prediction of tropical
weather such as the monsoon can be determined. Lacking such an initialization,
one can expect the convectively driven energetics to be biased, and a slow
degradation of the forecasts can follow. Several examples of forecasts
at different resolutions are discussed here. The main findings of this
study are that improved forecast results are obtained when physical initialization
is invoked where the observed rain and the model resolution are comparable,
i.e. the footprint of the highest resolutions rainfall estimates obtained
from satellite based data sets (principally we use the SSM/I instrument
over the oceans). At this resolution, we note that the model is able to
carry an organization of convection in the initialization and in the forecasts
through the medium-range time scale.
We have compared our results of monsoon studies at a resolution T255 with those at resolution T62. The transform grid separation at the resolution T255 is approximately 50 km and at the resolution T62, it is approximately 200 km. We find that the model at the higher resolution (T255) performs better and has more realistic energy conversions for the convectively driven synoptic scale monsoon.
An organization of convection, at the synoptic scales, is not seen in the forecasts at lower resolutions, T62, where the rainfall patterns are generally much broader and tend to be more zonal. Such organization appears more realistic at the resolution T255. Variances of the energy conversion, calculated in the two-dimensional spectral space, from physically initialized short range forecasts at the higher resolution are seen to be lamest on the scales of the monsoon. Similar calculations for the reanalyzed fields at lower resolutions show the spectral distribution of variances to be biased towards local Hadley scale overturnings.
|Mechanisms of monsoon Southern Oscillation coupling: insights from GCM experiments|
|Lau KM, Bua W CLIMATE DYNAMICS 14: (11) 759-779 OCT 1998|
|Abstract: The relative roles of internal atmospheric dynamics, land surface evaporation and sea surface temperature (SST) forcings on the coupling between the Asian monsoon (AM) and the Southern Oscillation (SO) are investigated in a series of GCM experiments. Results confirm previous studies indicating that the characteristic large-scale pattern of the SO is due primarily to SST anomaly (SSTA) forcing. The AM circulation anomalies are coupled to the SO via a characteristic upper level circulation couplet over the equatorial central Pacific. This couplet acts as a radiating node for teleconnection signals originating from the AM region to the extratropics. Generally, a weak AM is associated with warm SST over the eastern equatorial Pacific, concomitant with the negative phase of the SO, i.e., low (high) surface pressure over Tahiti (Darwin). The reverse holds for strong AM. Two wavetrains associated with the AM fluctuation have been identified: one arcing over northeastern Asia via the Aleutians to North American, and another emanating from northwestern Europe, via Siberia to northern India. Internal dynamics appear to underpin the origin of these wavetrains, which are strongly tempered by SSTA forcing and to a lesser degree by interactive land processes. Regionally, land-atmosphere interaction seems to have the strongest impact over East Asia/Indochina and the adjacent oceanic region of the South China Sea. Here, land-atmosphere interaction is responsible for the enhancement of a subseasonal scale see-saw oscillation in precipitation between land and the adjacent oceans. A local land-atmosphere feedback mechanism involving strong coupling between the hydrologic and energy cycles is identified. It is suggested that the interaction among precipitation, moisture convergence and land surface turbulent heat fluxes and radiation processes play key roles in determining the fast (subseasonal and shorter scales) response of the AM. On these time scales, the occurrences of cool/wet and hot/dry states associated with the precipitation seesaw appear to be chaotic. However, the preferred occurrence of a given state and the abrupt transition between states are dependent on the large-scale circulation and radiation forcings induced by the SO. One of the more provocative findings here is that effects of land-atmosphere interaction do not seem to alter the basic planetary scale features of the AM-SO system. As a result, the interannual variability of the coupled AM-SO is relatively small in the absence of anomalous SST forcing. Yet, the local effect of land-atmosphere interaction on AM is quite pronounced and dependent upon the large-scale forcings related to SO.|
|Monsoon response of the sea around Sri Lanka: Generation of thermal domes and anticyclonic vortices|
|Vinayachandran PN, Yamagata T JOURNAL OF PHYSICAL OCEANOGRAPHY 28: (10) 1946-1960 OCT 1998|
|Abstract: Results from an ocean general circulation model are used to study the response of the oceanic region surrounding Sri Lanka to monsoonal winds. East of Sri Lanka, a cold dome (Sri Lanka dome, SLD) develops during the southwest monsoon (SWM) in response to cyclonic curl in the local wind field. The dome decays after September due to the arrival of a long Rossby wave, associated with the reflection of the spring Wyrtki jet at the eastern boundary of the ocean. East of the SLD an anticyclonic eddy exists that is in intermediate geostrophic (IG) balance. North of Sri Lanka a cold dome (Bay of Bengal dome) develops after the SWM associated with a cyclonic gyre forced by Ekman pumping. The source of cold water of the Bay of Bengal dome is traced back to the SLD and upwelling zone along the east coast of India. South of Sri Lanka it major pare of the Southwest Monsoon Current (SMC) turns northeastward and flows into the Bay of Bengal. The part that flows eastward terminates at progressively western longitudes as the season progresses. This termination and the shallowness of the SMC is due to a Rossby wave generated near the eastern boundary by weakening of the spring Wyrtki jet and anticyclonic wind stress curl. This Rossby wave follows the one associated with the spring Wyrtki jet and has dominant velocities toward southwest. A large anticyclonic vortex, embedded in the SMC, results from the geostrophic adjustment process for the surface water converged by the long Rossby wave and the eastward zonal current. Energy analysis of this anticyclonic vortex as well as the IG eddy east of the SLD shows direct conversion from mean kinetic energy to eddy kinetic energy suggesting that barotropic instability is the mechanism that leads to eddy generation. This study suggests two links that allow exchange between the Bay of Bengal and the rest of the Indian Ocean: The first is the SMC, which is an open ocean current, and the second is the equatorward East India Coastal Current during November-January, which is closely attached to the coast.|
|Hydrologic processes associated with the first transition of the Asian summer monsoon: A pilot satellite study|
|Lau KM, Wu HT, Yang S BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 79: (9) 1871-1882 SEP 1998|
|Abstract: Results of a pilot study of the evolution of large-scale hydrologic processes associated with the first transition of the Asian summer monsoon in conjunction with the launching of the South China Sea Monsoon Experiment (SCSMEX) in May 1998 are presented. SCSMEX is a major international field experiment to study the water and energy cycles of the Asian monsoon region, with the aim toward better understanding and improved prediction of the onset, maintenance, and variability of the monsoon of southern China, Southeast Asia, and the western pacific region. In this paper, the utility of reliable satellite data in revealing characteristics of the South China Sea (SCS) monsoon is emphasized. Using a combination of satellite-estimated rainfall, moisture, surface wind, and sea surface temperature, the authors present some interesting and hitherto unknown features in large-scale atmospheric and oceanic hydrologic processes associated with the fluctuation of the SCS monsoon. Results show that, climatologically, the SCS monsoon occurs during mid-May when a major convection zone shifts from the eastern Indian Ocean-southern Indochina to the SCS. Simultaneous with the SCS monsoon onset is the development of a moist tongue and frontal rainband emanating from the northern SCS, across southern China and the East China Sea to southern Japan, as well as the enhancement of equatorial convection in the western Pacific ITCZ. Analysis of the satellite-derived moisture and rainfall shows that the onset of the SCS monsoon during 1997 was preceded by the development of eastward-propagating supercloud clusters over the Indian Ocean. The satellite data also reveal a strong onset vortex over the SCS and large-scale cooling and warming patterns over the Indian Ocean and western Pacific. These features signal a major shift of the large-scale hydrologic cycle in the ocean-atmosphere system, which underpins the SCS monsoon onset. The paper concludes with a brief discussion of the observational platform of SCSMEX and a call for the use of satellite data, field observations, and models for comprehensive studies of the Asian monsoon.|
|Spatial and temporal relationships between global land surface air temperature anomalies and Indian summer monsoon rainfall|
|Rajeevan M, Pai DS, Thapliyal V METEOROLOGY AND ATMOSPHERIC PHYSICS 66: (3-4) 157-171 1998|
|Abstract: Using the 60 year period (1931-1990) gridded land surface
air temperature anomalies data, the spatial and temporal relationships
between Indian summer monsoon rainfall and temperature anomalies were examined.
Composite temperature anomalies were prepared in respect of 11 deficient
monsoon years and 9 excess monsoon years. Statistical tests were carried
out to examine the significance of the composites. In addition, correlation
coefficients between the temperature anomalies and Indian summer monsoon
rainfall were also calculated to examine the teleconnection patterns.
There were statistically significant differences in the composite of temperature anomaly patterns between excess and deficient monsoon years over north Europe, central Asia and north America during January and May, over NW India during May, over central parts of Africa during May and July and over Indian sub-continent and eastern parts of Asia during July. It has been also found that temperature anomalies over NW Europe, central parts of Africa and NW India during January and May were positively correlated with Indian summer monsoon rainfall. Similarly temperature anomalies over central Asia during January and temperature anomalies over central Africa and Indian region during July were negatively correlated. There were secular variations in the strength of relationships between temperature anomalies and Indian summer monsoon rainfall. In general, temperature anomalies over NW Europe and NW India showed stronger correlations during the recent years. It has been also found that during excess (deficient) monsoon years temperature gradient over Eurasian land mass from sub-tropics to higher latitudes was directed equatowards (polewards) indicating strong (weak) zonal flow. This temperature anomaly gradient index was found to be a useful predictor for long range forecasting of Indian summer monsoon rainfall.
|Air-sea interaction over the Indian Ocean during the two contrasting monsoon years 1987 and 1988 studied with satellite data|
|Kumar MRR, Schlussel P THEORETICAL AND APPLIED CLIMATOLOGY 60: (1-4) 219-231 1998|
|Abstract: The air-sea interaction processes over the tropical Indian Ocean region are studied using sea surface temperature data from the Advanced Very High Resolution Radiometer sensor onboard the NOAA series of satellites. The columnar water-vapour content, low-level atmospheric humidity, precipitation, wind speed, and back radiation from the Special Sensor Microwave Imager on board the U.S. Defense Meteorological Satellite Program are all examined for two contrasting monsoon years, namely 1987 (deficit rainfall) and 1988 (excess rainfall). From these parameters the longwave radiative net flux at the sea surface and the ocean-air moisture flux are derived for further analysis of the air-sea interaction in the Arabian Sea, the Bay of Bengal, the south China Sea and the southern Indian Ocean. An analysis of ten-day and monthly mean evaporation rates over the Arabian Sea and Bay of Bengal shows that the evaporation was higher in these areas during the low rainfall year (1987) indicating little or no influence of this parameter on the ensuing monsoon activity over the Indian subcontinent. On the other hand, the evaporation in the southern Indian Ocean was higher during July and September 1988 when compared with the same months of 1987. The evaporation rate over the south Indian Ocean and the low-level cross-equatorial moisture flux seem to play a major role on the ensuing monsoon activity over India while the evaporation over the Arabian Sea is less important. Since we have only analysed one deficit/excess monsoon cycle the results presented here are of preliminary nature.|
|A study of south Asian monsoon energetics|
|Krishnamurti TN, Sinha MC, Jha B, Mohanty UC JOURNAL OF THE ATMOSPHERIC SCIENCES 55: (15) 2530-2548 AUG 1 1998|
|Abstract: Monsoon forecasting is one of the most difficult components
of the global weather prediction problem. The operational forecasts over
the Asian monsoon region are known to have useful skill only for roughly
2-3 days. The rapid deterioration of monsoon forecasts can be attributed
to a number of factors such as data deficiencies, physical parameterization
in the forecast models, and representation of orography and surface boundary
conditions, such as details of sea surface temperature, snow cover, etc.
The study of energetics of the model output helps in understanding the
above problems. This study is aimed toward the examination of the monsoon
energetics. Here it is shown that the use of kinetic energies of the rotational
and divergent motions have a special advantage. In order to show these
features in the maintenance of a fully developed monsoon, the authors have
taken the results of a global model. Such a model is internally consistent
and given a reasonable forecast of the motion, thermal, and precipitation
fields, the authors believe that such a model-generated dataset can provide
useful insights. Direct use of observations and their analysis make it
difficult to perform such studies because of data voids and the data errors
and inconsistencies. Models tend to produce somewhat more consistent fields
during the course of short-range prediction. Although the results obtained
contain a model bias, the authors nevertheless performed short-range forecasts
with a high-resolution global model and rely only on these if the forecasts
appear to be quite reasonable.
The Florida State University Global Spectral Model at the resolution T170 (170 waves triangular truncation) was run to carry out several experiments to investigate the issue of the maintenance of the Asian monsoon. In this context the authors examined the issue of the maintenance of the monsoon over a south Asian domain. The computations show that differential heating (i.e., the covariance of heating and temperature) lends to the growth of available potential energy, which is next passed on to the divergent motions via the covariance of vertical velocity and temperature. The final link in this scenario is the transfer of energy from the divergent to the rotational part of the motion field that describes the monsoon. These are largely described by psi-chi interaction via the covariance of del psi and del chi (where psi is a streamfunction and chi is a velocity potential). It is noted that lateral boundary fluxes are also important for the maintenance of the monsoon. Lateral coupling with the monsoon of southeast Asia and with the Southern Hemispheric circulation are also some of the crucial elements of the overall energetics.
|Possible role of symmetric instability in the onset and abrupt transition of the Asian monsoon|
|Krishnakumar V, Lau KM JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (3) 363-383 JUN 1998|
|Abstract: The physical connections between dry/moist (conditional)
symmetric instability (SI/CSI) off equatorial diabatic thermal forcing,
and the onset of the Asian monsoon are investigated using the Goddard Laboratory
for Atmospheres general circulation model (GLA GCM). The objective of this
study is two-fold: to elucidate the causal relationship between SI/CSI
and monsoon onset, and to investigate the threshold behavior of asymmetric
monsoonal thermal forcing under condition of SI/CSI. This work corroborates
the authors' earlier linear instability analysis results, which show, that
SI/CSI in the boreal summer monsoon basic states may be a plausible explanation
for the abrupt monsoon transition.
Monsoon transitions in the model, as depicted by the abrupt meridional movement of the axis of maximum vertical motion from equator to northern latitudes, occur during 16-20 May for the East Asian Monsoon (EAM) and 1-5 June for the South Asian Monsoon (SAM) regions. The necessary stability criterion for dry (moist) SI over the EAM and SAM regions reveals a sudden cross equatorial advection of negative dry potential vorticity (DPV) and moist potential vorticity (MPV) into the summer hemisphere five to ten days preceding the model monsoon transition. This causes dry and moist SI. Maximum shift of the zero line of DPV and MPV (dry and moist symmetrically unstable regions) happens subsequent to monsoon transition. Simplified analysis of the potential vorticity (PV) budget equation reveals that the lower tropospheric negative PV advection into the summer hemisphere is largely governed by the dominance of vertical differential diabatic heating over horizontal differential diabatic heating.
The diabatic heating also shows an abrupt increase from 2-3 K day(-1) before the transition, to 12-14 K day(-1) at the time of monsoon transition. The genesis of pre-monsoon weak heat source arises primarily due to unstable SI and CSI of the pre-monsoon basic states, which consequently produce moderately large scale lower (upper) tropospheric convergence (divergence) patterns slightly poleward of the zero line of DPV and MPV. Lower tropospheric conditionally unstable tropical atmosphere, in the presence of off equatorial large scale lower (upper) tropospheric convergence (divergence), is conducive to exciting CISK-like processes, which may eventually release large amounts of latent heat and develop a strong heat source at the time of monsoon transition. We have noted that a fully established model meridional circulation originates only when the diabatic forcing magnitude exceeds some threshold value of around 5 K day(-1) at the time of monsoon transition. The model transition is more pronounced over the EAM region than over the SAM region. The linear steady-state dynamical response of a zonally symmetric atmosphere as a consequence of varying the location and magnitude of an idealized asymmetric thermal forcing reveals that the most intense meridional circulation (maximum efficiency of vertical motion) is accomplished when the thermal forcing is located around 10 degrees N. The interrelationship between the location of zero DPV/MPV contour, lower tropospheric maximum convergence versus maximum vertical velocity of the monsoonal circulation in the summer hemisphere, clearly suggests that SI (CSI) of zonal monsoon flows is a causal mechanism for the onset of monsoon transition.
|Monsoon precipitation in the AMIP runs|
|Gadgil S, Sajani S CLIMATE DYNAMICS 14: (9) 659-689 AUG 1998|
|Abstract: We present an analysis of the seasonal precipitation associated
with the African, Indian and the Australian-Indonesian monsoon and the
interannual variation of the Indian monsoon simulated by 30 atmospheric
general circulation models undertaken as a special diagnostic subproject
of the Atmospheric Model Intercomparison Project (AMIP). The seasonal migration
of the major rainbelt observed over the African region, is reasonably well
simulated by almost all the models. The Asia West Pacific region is more
complex because of the presence of warm oceans equatorward of heated continents.
Whereas some models simulate the observed seasonal migration of the primary
rainbelt, in several others this rainbelt remains over the equatorial oceans
in all seasons.
Thus, the models fall into two distinct classes on the basis of the seasonal variation of the major rainbelt over the Asia West Pacific sector, the first (class I) are models with a realistic simulation of the seasonal migration and the major rainbelt over the continent in the boreal summer; and the second (class II) are models with a smaller amplitude of seasonal migration than observed. The mean rainfall pattern over the Indian region for July-August (the peak monsoon months) is even more complex because, in addition to the primary rainbelt over the Indian monsoon zone (the monsoon rainbelt) and the secondary one over the equatorial Indian ocean, another zone with significant rainfall occurs over the foothills of Himalayas just north of the monsoon zone. Eleven models simulate the monsoon rainbelt reasonably realistically. Of these, in the simulations of five belonging to class I, the monsoon rainbelt over India in the summer is a manifestation of the seasonal migration of the planetary scale system. However in those belonging to class II it is associated with a more localised system. In several models, the oceanic rainbelt dominates the continental one. On the whole, the skill in simulation of excess/deficit summer monsoon rainfall over the Indian region is found to be much larger for models of class I than II, particularly for the ENSO associated seasons. Thus, the classification based on seasonal mean patterns is found to be useful for interpreting the simulation of interannual variation. The mean rainfall pattern of models of class I is closer to the observed and has a higher pattern correlation coefficient than that of class II. This supports Sperber and Palmer's (1996) result of the association of better simulation of interannual variability with better simulation of the mean rainfall pattern. The hypothesis, that the skill of simulation of the interannual variation of the all-India monsoon rainfall in association with ENSO depends upon the skill of simulation of the seasonal variation over the Asia West Pacific sector, is supported by a case in which we have two versions of the model where NCEP1 is in class II and NCEP2 is in class I. The simulation of the interannual variation of the local response over the central Pacific as well as the all-India monsoon rainfall are good for NCEP2 and poor for NCEP1. Our results suggest that when the model climatology is reasonably close to observations, to achieve a realistic simulation of the interannual variation of all-India monsoon rainfall associated with ENSO, the focus should be on improvement of the simulation of the seasonal variation over the Asia West Pacific sector rather than further improvement of the simulation of the mean rainfall pattern over the Indian region.
|West African monsoon dynamics and eastern equatorial Atlantic and Pacific SST anomalies (1970-88)|
|Janicot S, Harzallah A, Fontaine B, Moron V JOURNAL OF CLIMATE 11: (8) 1874-1882 AUG 1998|
|Abstract: The Laboratoire de Meteorologie Dynamique atmospheric GCM is used to investigate relationships between West African monsoon dynamics and SST anomalies in the eastern equatorial Atlantic and Pacific for the period 1970-88. Positive SST anomalies in the eastern equatorial Pacific, mainly associated with a larger east-west divergent circulation over the tropical Atlantic, are found to coincide with negative rainfall anomalies over West Africa. This is the case for the composite ENSO warm episodes of 1972, 1976, 1982, and 1983. By contrast, positive SST anomalies in the eastern equatorial Atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the Sahel and positive rainfall anomalies in the Guinean region. This was the case in 1987. The ENSO warm event during this year had apparently no significant impact on West African monsoon dynamics. A zonal atmospheric coupling associated with differences of SST anomalies between the eastern equatorial Pacific and the Atlantic is evident in the period 1970-88. Positive (negative) phases of this coupling could enhance the impact of ENSO warm (cold) events on West African monsoon dynamics.|
|Probabilities of excess and deficient southwest monsoon rainfall over different meteorological sub-divisions of India|
|Kothawale DR, Munot AA PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 107: (2) 107-119 JUN 1998|
|Abstract: Temporal distribution of southwest monsoon (June - September)
rainfall is very useful for the country's agriculture and food grain production.
It contributes more than 75% of India's annual rainfall. In view of this,
an attempt has been made here to understand the performance of the monthly
rainfall for June, July, August and September when the seasonal rainfall
is reported as excess, deficient or normal. To know the dependence of seasonal
rainfall on monthly rainfall, the probabilities of occurrence of excess,
deficient and normal monsoon when June, July, August and also June + July
and August + September rainfall is reported to be excess or deficient,
are worked out using the long homogenous series of 124 years (1871-1994)
data of monthly and seasonal rainfall of 29 meteorological sub-divisions
of the plain regions of India.
In excess monsoon years, the average percentage contribution of each monsoon month to the long term mean (1871-1994) seasonal rainfall (June - September) is more than that of the normal while in the deficient years it is less than normal. This is noticed in all 29 meteorological sub-divisions. From the probability analysis, it is seen that there, is a rare possibility of occurrence of seasonal rainfall to be excess/deficient when the monthly rainfall of any month is deficient/excess.
|Intra-seasonal variations of kinetic energy of lower tropospheric zonal waves during northern summer monsoon|
|Bawiskar SM, Chipade MD, Singh SS PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 107: (2) 121-126 JUN 1998|
|Abstract: Space spectral analysis of zonal (u) and meridional (v) components
of wind and time spectral analysis of kinetic energy of zonal waves at
850 hPa during monsoon 1991 (Ist June 1991 to 31st August 1991) for the
global belt between equator and 40 degrees N are investigated. Space spectral
analysis shows that long waves (wavenumbers 1 and 2) dominate the energetics
of Region 1 (equator to 20 degrees N) while over Region 2 (20 degrees N
to 40 degrees N) the kinetic energy of short waves (wavenumbers 3 to 10)
is more than kinetic energy of long waves. It has been found that kinetic
energy of long waves is dominated by zonal component while both (zonal
and meridional) the components of wind have almost equal contribution in
the kinetic energy of short waves.
Temporal variations of kinetic energy of wavenumber 2 over Region 1 and Region 2 are almost identical. The correlation matrix of different time series shows that (i) wavenumber 2 over Regions 1 and 2 might have the same energy source and (ii) there is a possibility of an exchange of kinetic energy between wavenumber 1 over Region 1 and short waves over Region 2. Wave to wave interactions indicate that short waves over Region 2 are the common source of kinetic energy to wavenumber 2 over Regions 1 and 2 and wavenumber 1 over Region i. Time spectral analysis of kinetic energy of zonal waves indicates that wavenumber 1 is dominated by 30-45 day and bi-weekly oscillations while short waves are dominated by weekly and bi-weekly oscillations.
The correlation matrix, wave to wave interaction and time spectral analysis together suggest that short period oscillations of kinetic energy of wavenumber I might be one of the factors causing dominant weekly (5-9 day) and bi-weekly (10-18 day) oscillations in the kinetic energy of short waves.
|Relationship of tropospheric temperature anomaly with Indian southwest monsoon rainfall|
|Singh GP, Chattopadhyay J INTERNATIONAL JOURNAL OF CLIMATOLOGY 18: (7) 759-763 JUN 15 1998|
|Abstract: The statistical relationship between the Indian southwest
monsoon rainfall over all India (AIR), northwest India (NWR) and peninsular
India (PIR) and tropospheric temperature (TT) anomaly over seven selected
Indian stations between 300 and 850 hPa has been examined for the period
1961-1990. The study is furthered by means of sliding 15 years correlation
The results indicate that the previous May's tropospheric temperature anomaly has a strong and direct relationship with southwest monsoon rainfall, suggesting that warmer/colder tropospheric temperature in May leads to good/bad southwest monsoon rainfall over India. The correlations are stronger for AIR and NWR followed by PIR. Its stability analysis displays best correlation during 1967-1981 and 1968-1982 and again during the recent years 1975-1989 and 1976-1990 for all the regions. Significant direct correlations are also obtained with the tropospheric temperature anomaly of the previous April and spring season. The result also suggests that antecedent May tropospheric temperature anomaly may be useful in the long range prediction of the following southwest monsoon rainfall over India. (C) 1998 Royal Meteorological Society.
|Extremes of the ENSO phenomenon and Indian summer monsoon rainfall|
|Kane RP INTERNATIONAL JOURNAL OF CLIMATOLOGY 18: (7) 775-791 JUN 15 1998|
|Abstract: Characterizing every year during the 120 year interval 1871-1990 as a year of El Nino (EN), or Southern Oscillation minimum (SO), or equatorial eastern Pacific sea-surface temperature (SST) warm (W) or cold (C) episode or none (non-events), the corresponding summer monsoon rainfall departures for all India and for the 29 meteorological subdivisions were examined. The best relationship for droughts was with unambiguous ENSOW (El Nino year with SO and W near the middle of the calendar year) and for floods with C (cold SST). The droughts were generally widespread, although Assam and Bengal might have had normal rainfall or even floods when other subdivisions had droughts. In some ENSOW years when all India rainfall was normal, the rainfall in subdivisions was either normal or mixed (droughts in some subdivisions, floods in others). However, droughts and floods occurred during other types of events also, and ENSOW or C were neither sufficient nor necessary. Some floods and droughts were associated with incorrect type (floods during El Nino, etc.), and some occurred during non-events, indicating that factors unrelated to EN, or SO, or W, or C may be more influential in some years. (C) 1998 Royal Meteorological Society.|
|Monsoon regions: the highest rate of precipitation changes observed from global data|
|Fu CB, Zheng ZM CHINESE SCIENCE BULLETIN 43: (8) 662-666 APR 1998|
|Abstract: The global distributions of the rate of precipitation change at seasonal, interannual and interdecadal scales are computed from the observed global data sets. The analysis has revealed that the monsoon regions in Asia and West Africa, and to lesser extent Australia, have the highest rate of precipitation change at all time scales in the world. These changes are manifested as seasonal jump, high interannual and interdecadal variability and abrupt changes between climate regimes.|
|The Asian-Australian monsoon and El Nino Southern Oscillation in the NCAR Climate System Model|
|Meehl GA, Arblaster JM JOURNAL OF CLIMATE 11: (6) 1356-1385 JUN 1998|
|Abstract: Features associated with the Asian-Australian monsoon system
and El Nino-Southern Oscillation (ENSO) are described in the National Center
for Atmospheric Research (NCAR) global coupled Climate System Model (CSM).
Simulation characteristics are compared with a version of the atmospheric
component of the CSM. the NCAR CCM3, run with time-evolving SSTs from 1950
to 1994, and with observations. The CSM is shown to represent most major
features of the monsoon system in terms of mean climatology, interannual
variability, and connections to the tropical Pacific. This includes a representation
of the Southern Oscillation links between strong Asian-Australian monsoons
and associated negative SST anomalies in the eastern equatorial Pacific.
The equatorial SST gradient across the Pacific in the CSM is shown to be
similar to the observed with somewhat cooler mean SSTs across the entire
Pacific by about 1 degrees 2 degrees C. The seasonal cycle of SSTs in the
eastern equatorial Pacific has the characteristic signature seen in the
observations of relatively warmer SSTs propagating westward in the first
half of the year followed by the reestablishment of the cold tongue with
relatively colder SSTs propagating westward in the second half of the year
Like other global coupled models, the propagation is similar to the observed
but with the establishment of the relatively warmer water in the first
half of the year occurring about 1-2 months later than observed. The seasonal
cycle of precipitation in the tropical eastern Pacific is also similar
to other global coupled models in that there is a tendency for a stronger-than-observed
double ITCZ year round, particularly in northern spring, but with a well-reproduced
annual maximum of ITCZ strength north of the equator in the second half
of the year.
Time series of area-averaged SSTs for the NINO3 region in the eastern equatorial Pacific show that the CSM is producing about 60% of the amplitude of the observed variability in that region, consistent with most other global coupled models. Global correlations between NINO3 rim; series, global surface temperatures, and sea level pressure (SLP) show that the CSM qualitatively reproduces the major spatial patterns associated with the Southern Oscillation (lower SLP in the central and eastern tropical Pacific when NINO3 SSTs are relatively warmer and higher SLP over the far western Pacific and Indian Oceans, with colder water in the northwest and southwest Pacific). Indices of Asian-Australian monsoon strength are negatively correlated with NINO3 SSTs as in the observations. Spectra of time series of Indian monsoon, Australian monsoon, and NINO3 SST indices from the CSM show amplitude peaks in the Southern Oscillation and tropospheric biennial oscillation frequencies (3-6 yr and about 2.3 yr, respectively) as observed. Lag correlations between the NINO3 SST index and upper-ocean heat content along the equator show eastward propagation of heat content anomalies with a phase speed of about 0.3 m s(-1), compared to observed values of roughly 0.7 m s(-1). Composites of El Nino (La Nina) events in the CSM show similar seasonal evolution to composites of observed events with warming (cooling) of greater than several tenths of a degree beginning early in northern spring of year 0 and diminishing around northern spring of year +1, but with a secondary resurgence in the CSM events later in northern spring of year +1. The CSM also shows the largest amplitude ENSO SST and low-level wind anomalies in the western tropical Pacific, with enhanced interannual variability of SSTs extending northeastward and southeastward toward the subtropics, compared to largest interannual SST variability in the central and eastern tropical Pacific in the observations.
|Does a monsoon climate exist over South America?|
|Zhou JY, Lau KM JOURNAL OF CLIMATE 11: (5) 1020-1040 MAY 1998|
|Abstract: The climatology and the basic state of the summertime circulation
and rainfall over South America are studied using assimilation products
from the data assimilation system of Goddard Earth Observing System-1 (GEOS-1)
and satellite-derived rainfall. Results indicate the existence of a regional
summer monsoon circulation regime induced by strong diabatic heating over
the subtropical South American highland centered at the Altiplano Plateau.
Sensitivity of the results to the assimilation scheme is tested by comparing
that with the National Centers for Environmental Prediction (NCEP) reanalysis
and with satellite rainfall estimates. Results show general agreement between
the model produced rainfall anomaly and the satellite estimates, as well
as consistency between the basic circulation features in the GEOS-1 and
the NCEP reanalyses.
A case study of 1989-90 South American summer monsoon (SASM) reveals the following characteristics.
1) In late spring, the onset of SASM is signaled by an abrupt merging of the upper-tropospheric double westerly jets, one in the subtropics and the other in the subpolar region, into a single jet in the midlatitudes. This is followed by the establishment of a vortex to the southeast of Altiplano and occurrence of heavy precipitation over subtropical eastern Brazil.
2) During the mature phase of SASM, the heavy rainfall zone moves over the Altiplano Plateau and the southernmost Brazilian highland. The fully established SASM features are the following: (a) an enhancement of equatorial North Atlantic trade wind, which emanates from the Sahara high and crosses the equator over the South American continent; (b) a buildup of strong northwesterlies along the eastern side of the tropical Andes; and (c) development of the South Atlantic convergence zone in the southernmost position with strong convective activity. Meanwhile, the upper-tropospheric return flow emerges from an anticyclone formed over the Altiplano Plateau, crosses the equator, and sinks over northwestern Africa.
3) The withdrawal of SASM in late summer is signaled by the resplitting of the midlatitude westerly jet. At the same time, the low-level northwest monsoon how diminishes, reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland.
Results also show that the above-mentioned characteristics of SASM are clearly linked to the tropospheric temperature changes over the central South American highland. Sensible versus latent heating over the highland are bound to play an important role in the evolution of SASM.
To provide further support of presence of a monsoon climate over South America, SASM is compared and contrasted to the "classic" east Asian summer monsoon (EASM). Many similar features, including evolution characteristics between the two systems, have been identified. Contrasting aspects of the SASM from the EASM are also discussed. It is pointed out that a number of monsoonal characteristics of the climate of South America, such as the seasonal reversal of the low-level wind, become apparent only when the strong annual mean wind is removed. Based on the characteristic features and their evolution, the authors conclude that a monsoon climate does exist over South America.
|Land-sea geometry and its effect on monsoon circulations|
|Dirmeyer PA JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 103: (D10) 11555-11572 MAY 27 1998|
|Abstract: The role of land-sea geometry in determining the existence and character of monsoon circulations is examined using an atmospheric general circulation model with land surface properties represented by the simplified simple biosphere model. Idealized land-sea distributions are used to examine the effects of the latitude of continental coastlines, the meridional and zonal extent of continents, the shape of continents, and the presence of orography on circulation in the tropics and subtropics. It is found that the latitude of the continent is critical to the establishment of a distinct seasonality of precipitation and a summer monsoon circulation. Low-latitude positions of the continent lead to landlocked precipitation maxima, and poleward positions lead to a Mediterranean (winter monsoon) climate. In every case, the subsiding branch of the Hadley cell at continental longitudes locks over some part of the land. Variations in the meridional extent of land affect the strength of the Hadley cell and the degree of seasonality in the climate over land. The extension of a subtropical continent into the tropics is critical for establishing heavy convective rainfall over land in the summer. When tropical peninsulas are added to a subtropical continent, the distribution of moisture and rain over semi-arid and arid regions of the original continent are affected. When a broad mountain of 1200 m maximum elevation is present on the idealized continent, there is a band of increased precipitation to the south and east of the mountain with a systematic decrease to the south, north, and west of the band.|
|Role of land surface processes associated with interannual variability of broad-scale Asian summer monsoon as simulated by the CCSR/NIES AGCM|
|Shen XH, Kimoto M, Sumi A JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (2) 217-236 APR 1998|
|Abstract: The interannual variability of the South Asian summer monsoon
and associated land-surface processes over the Eurasian continent in a
ten-year integration (1979-88) of an atmospheric general circulation model
(AGCM) forced by observed sea surface temperatures (SSTs) is examined.
The AGCM has been developed jointly by the Center for Climate System Research
(CCSR), the University of Tokyo, and the National Institute for Environmental
Studies (NIES). A monsoon intensity index, based on the magnitude of summer-mean
vertical shear of zonal wind over the south Asian monsoon region, is used
to classify weak and strong monsoon years. It is found that the simulated
interannual variability of broad-scale summer monsoon shows a good correlation
Furthermore, distinct precursory signals, including the Eurasian snow in winter and soil moisture anomalies in spring, have been found in the pre-monsoon seasons of weak and strong monsoon years. There is a sharp contrast between weak and strong monsoon years; excessive snow over Eurasia south of 50 degrees N in winter and the increased soil moisture in spring are found prior to weak summer monsoon. These results are consistent with evidence found in observational data analyses and some model experiments. A detailed analysis of surface heat budget shows that snow-albedo feedback dominates over the Tibetan Plateau. On the other hand, to its west in the central Asia, the relatively lower land, the effective cloud albedo anomalies due to excessive rainfall and surface evaporation influence the surface conditions.
A numerical experiment with the Eurasian land surface initial conditions in spring, interchanged between weak and strong monsoon years, indicates positive roles played by the land surface processes in influencing the subsequent summer monsoon circulations during the 10-year period. However, such land surface feedbacks are not strong enough to change the sign of the monsoon circulation anomalies. The direct influence of the El Nino/Southern Oscillation through the changes in Walker circulation appears to predominate.
|The physics of ENSO-monsoon connection|
|Goswami BN INDIAN JOURNAL OF MARINE SCIENCES 27: (1) 82-89 MAR 1998|
|Abstract: The physical mechanism through which Ei-Nino and Southern Oscillation (ENSO) tends to produce deficient precipitation over Indian continent is investigated using both observations as well as a general circulation model. Both analysis of observations and atmospheric general circulation model (AGCM) study show that the planetary scale response associated with ENSO primarily influences the equatorial Indian Ocean region. Through this interaction it tends to favour the equatorial heat source, enhance precipitation over the equatorial Indian Ocean and indirectly cause a decrease in continental precipitation through induced subsidence. This situation is further complicated by the fact the regional tropospheric quasi biennial oscillation (QBO) has a bimodal structure over this region with large amplitude over the Indian continent. While the ENSO response has a quasi-four year periodicity and tends peak during beginning of the calendar year, the QBO mode tends to peak during northern summer. Thus, the QBO mode exerts a stronger influence on the interannual variability of the monsoon. The strength of the Indian monsoon in a given year depends on the combined effect of the ENSO and the QBO mode. Sines the two oscillations have disparate time scales, exact phase information of the two modes during northern summer is important in determining the Indian summer monsoon. The physical mechanism of the interannual variations of the Indian monsoon precipitation associated with ENSO presented here is similar to the physical process that cause intraseasonal 'active', 'break' oscillations of the monsoon.|
|Role of seasonal cycle forcing in simulating the summer monsoon circulation and precipitation|
|Dash SK INDIAN JOURNAL OF MARINE SCIENCES 27: (1) 97-103 MAR 1998|
|Abstract: Using a General Circulation Model (GCM), the role of seasonal cycle surface boundary conditions in simulating some large scale features of Indian summer monsoon circulation, rainfall and some surface parameters during two contrasting years (1987 and 88) of monsoon have been examined. When the seasonal variations in surface conditions are incorporated in the GCM, some important fields such as the magnitudes of zonal wind over Arabian Sea at 850 hPa, meridional wind over Somalia at 850 hPa, strength of easterly jet at 200 hPa and the rainfall rate over land to the north of Bay of Bengal are reasonably well simulated compared to their respective climatological values. Components of surface wind stress show three distinct branches of monsoon located over the Arabian Sea, Bay of Bengal and South China Sea. The model simulated June,July,August (JJA) mean monsoon winds over India at 200 hPa and 850 hPa are weak and the interannual variations in simulated surface wind, net heat flux and rainfall are very small.|
|Early prediction of onset of south west monsoon from ERS-1 scatterometer winds|
|Rao UR, Desai PS, Joshi PC, Pandey PC, Gohil BS, Simon B PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 107: (1) 33-43 MAR 1998|
|Abstract: Detailed analysis of the surface winds over the Indian Ocean
derived from ERS-1 scatterometer data during the years 1993 and 1994 has
been used to understand and unambiguously identify the onset phase of south-west
monsoon. Five day (pentad) averaged wind vectors for the period April to
June during both years have been examined to study the exact reversal of
wind direction as well as the increase in wind speed over the Arabian Sea
in relation to the onset of monsoon over the Indian west coast (Kerala).
The related upper level humidity available from other satellites has also
The results of our analysis clearly shaw a consistent dramatic reversal in wind direction over the western Arabian Sea three weeks in advance of the onset of monsoon. The wind speed shows a large increase coinciding with the onset of monsoon. These findings together show the dominant role of sea surface winds in establishing the monsoon circulation. The study confirms that the cross equatorial current phenomenon becomes more important after the onset of monsoon.
|Intraseasonal oscillations and interannual variability of surface winds over the Indian monsoon region|
|Goswami BN, Sengupta D, Kumar GS PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 107: (1) 45-64 MAR 1998|
|Abstract: The role of intraseasonal oscillations (ISOs) in modulating
synoptic and interannual variations of surface winds over the Indian monsoon
region is studied using daily averaged National Centers for Environmental
Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalyses
for the period 1987-1996. Two dominant ISOs are found in all years, with
a period between 30-60 days and 10-20 days respectively. Although the ISOs
themselves explain only about 10-25% of the daily variance, the spatial
structure of variance of the ISOs is found to be nearly identical to that
of high frequency activity (synoptic disturbances), indicating a significant
control by the ISOs in determining the synoptic variations. Zonal and meridional
propagation characteristics of the two modes and their interannual variability
are studied in detail.
The synoptic structure of the 30-60 day mode is similar in all years and is shown to be intimately related to the strong ('active') or weak ('break') phases of the Indian summer monsoon circulation. The peak (trough) phase of the mode in the north Bay of Bengal corresponds to the 'active' ('break') phase of monsoon strengthening (weakening) the entire large scale monsoon circulation. The ISOs modulate synoptic activity through the intensification or weakening of the large scale monsoon flow (monsoon trough). The peak wind anomalies associated with these ISOs could be as large as 30% of the seasonal mean winds in many regions. The vorticity pattern associated with the 30-60 day mode has a bi-modal meridional structure similar to the one associated with the seasonal mean winds but with a smaller meridional scale. The spatial structure of the 30-60 day mode is consistent with fluctuations of the tropical convergence zone (TCZ) between one continental and an equatorial Indian Ocean position. The 10-20 day mode has maximum amplitude in the north Bay of Bengal, where it is comparable to that of the 30-60 day mode. Elsewhere in the Indian Ocean, this mode is almost always weaker than the 30-60 day mode. In the Bay of Bengal region, the wind curl anomalies associated with the peak phases of the ISOs could be as large as 50% of the seasonal mean wind curl. Hence, ISOs in this region could drive significant ISOs in the ocean and might influence the seasonal mean currents in the Bay.
On the interannual time scale, the NCEP/NCAR reanalysed wind stress is compared with the Florida State University monthly mean stress. The seasonal mean stress as well a interannual standard deviation of monthly stress from the two analyses agree well, indicating absence of any serious systematic bias in the NCEP/NCAR reanalysed winds. It is also found that the composite structure of the 30-60 day mode is strikingly similar to the dominant mode of interannual variability of the seasonal mean winds indicating a strong link between the ISOs and the seasonal mean. The ISO influences the seasonal mean and its interannual variability either through increased/decreased residence time of the TCZ in the continental position or through occurrence of stronger/weaker active/break spells. Thus, the ISOs seem to modulate all variability in this region from synoptic to interannual scales.
|An ocean-atmosphere index for ENSO and its relation to Indian monsoon rainfall|
|Munot AA, Pant GB PROCEEDINGS OF THE INDIAN ACADEMY OF SCIENCES-EARTH AND PLANETARY SCIENCES 107: (1) 91-95 MAR 1998|
|Abstract: An Ocean-Atmosphere Index (OAI) for ENSO is developed using data on Southern Oscillation Index (SOI) and sea surface temperature (SST) over eastern equatorial Pacific. Seasonal values of OAI, SOI and SST have been computed for the seasons September-October-November (SON), December-January-February (DJF), March-April-May (MAM) and June July-August (JJA). Similarly SON to DJF, DJF to MAM, MAM to JJA and JJA to SON tendencies have been worked out for SOI, SST and OAI. The relationships between Indian Monsoon Rainfall (IMR) and SOI/SST/OAI, (i) for the seasons SON, DJF and MAM before and after the monsoon and JJA concurrent with the monsoon and (ii) for SON to DJF and DJF to MAM tendencies before and after the monsoon, and MAM to JJA tendency concurrent with the monsoon have been explored. It is found that LMR is more influenced by SOI before the monsoon than it is influenced by SST before the monsoon and IMR affects SST after monsoon more strongly than it affects SOI after the monsoon. It is also observed that DJF to MAM tendencies for SOI, SST and OAI before monsoon are significantly related to IMR, among which the relationship between IMR and DJF to MAM tendency for OAI is the best.|
|The interannual variations of the summer monsoon onset over the South China Sea|
|Xie A, Chung YS, Liu X, Ye Q THEORETICAL AND APPLIED CLIMATOLOGY 59: (3-4) 201-213 1998|
|Abstract: Interannual variations of the summer monsoon onset over the
South China Sea (SCS) have been studied using data from over seventeen
years (1979-1995) of NMC global analysis and of Outgoing Longwave Radiation
(OLR) observed with NOAA polar-orbitting satellites. It was found that
the summer monsoon onset in the SCS occurs abruptly with a sudden change
of zonal wind direction from easterly to westerly and an exploding development
of deep convection in the whole SCS region in the middle of May.
Based on the criteria defined in this paper for the SCS summer monsoon onset, the average onset date over the SCS from 1979 to 1995 is around the fourth pentad of May. The airflow and general circulation over the SCS changes dramatically after the onset. The ridge of the subtropical high in the western Pacific in the lower troposphere weakens and retreats eastward from the SCS region with an establishment of westerly winds over the whole region. During the SCS monsoon onset, the most direct impact in the vicinity of the SCS are the equatorial westerlies in the Bay of Bengal through their eastward extension and northward movement. An indirect influence on the SCS onset is also caused by the enhancement of the Somali cross-equatorial flow and the vanishing Arabian High over the sea; the latter may be a signal for the SCS onset.
There are quite significant interannual variations in the SCS onset. In the years of a delayed onset, the most profound feature is that the easterly winds stay longer in the SCS than on average. Deep convection activities are suppressed. The direct cause is the abnormal existence of the western Pacific subtropical high over the SCS region. Moreover, compared to the average, the equatorial westerlies in the Bay of Bengal are also weaker in the years of a delayed onset. No significant changes for the cross-equatorial flow at 105 degrees E are observed for these years. It has also been found that the interannual variations of the SCS onset are closely related with the ENSO events. In the years of a delay, the Walker circulation is weaker, and the sea surface temperature (SST) anomalies in the western Pacific are negative.
|Interannual variations of Indian summer monsoon in a GCM: External conditions versus internal feedbacks|
|Goswami BN JOURNAL OF CLIMATE 11: (4) 501-522 APR 1998|
|Abstract: The potential predictability of the Indian summer monsoon
due to slowly varying sea surface temperature (SST) forcing is examined.
Factors responsible for limiting the predictability are also investigated.
Three multiyear simulations with the R30 version of the Geophysical Fluid
Dynamics Laboratory's climate model are carried out for this purpose, The
mean monsoon simulated by this model is realistic including the mean summer
precipitation over the Indian continent. The interannual variability of
the large-scale component of the monsoon such as the "monsoon shear index"
and its teleconnection with Pacific SST is well simulated by the model
in a 15-yr integration with observed SST as boundary condition. On regional
scales, the skill in simulating the interannual variability of precipitation
over the Indian continent by the model is rather modest and its simultaneous
correlation with eastern Pacific SST is negative but poor as observed.
The poor predictability of precipitation over the Indian region in the
model is related to the fact that contribution to the interannual variability
over this region due to slow SST variations [El Nino-Southern Oscillation
(ENSO) related] is comparable to those due to regional-scale fluctuations
unrelated to ENSO SST. The physical mechanism through which ENSO SST tend
to produce reduction in precipitation over the Indian continent is also
A measure of internal variability of the model summer monsoon is obtained from a 20-yr integration of the same model with fixed annual cycle SST as boundary conditions but with predicted soil moisture and snow cover. A comparison of summer monsoon indexes between this run and the observed SST run shows that the internal oscillations can account for a large fraction of the simulated monsoon variability. The regional-scale oscillations in the observed SST run seems to arise from these internal oscillations. It is discovered that most of the interannual internal variability is due to an internal quasi-biennial oscillation (QBO) of the model atmosphere. Such a QBO is also found in the author's third 18-yr simulation in which fixed annual cycle of SST as well as soil moisture and snow cover are prescribed. This shows that the model QBO is not due to land-surface-atmosphere interaction. It is proposed that the model QBO arises due to an interaction between nonlinear intraseasonal oscillations and the annual cycle. Spatial structure of the QBO and its role in limiting the predictability of the Indian summer monsoon is discussed.
|Interannual variations of monsoon rainfall in Godavari river basin - Connections with the Southern Oscillation|
|Rao GN JOURNAL OF CLIMATE 11: (4) 768-771 APR 1998|
|Abstract: Interannual variations of the monsoon rainfall in one of the central Indian river basins. Godavari, are studied for the 40-yr period 1951-90. It is observed that the departures from the normal of the monsoon rainfall in excess and deficient rainfall years are larger in the plain areas than in the hilly regions. The connections between the monsoon rainfall in this basin and the Southern Oscillation are also examined. The winter to spring Darwin pressure tendency is found to have a significant negative connection with the monsoon rainfall in the basin as a whole. However, this connection is weak in hilly regions, where the rainfall is influenced by the orography. It is also observed that this connection is consistent throughout the entire period of study and suggests the use of Darwin pressure tendency as a parameter for long-range forecasting of the monsoon rainfall in this basin.|
|Role of warming over the Tibetan Plateau in early onset of the summer monsoon over the Bay of Bengal and the South China Sea|
|Ueda H, Yasunari T JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 76: (1) 1-12 FEB 1998|
|Abstract: In this study we examine the mechanisms of the onset of the
Southeast Asian monsoon (SEAM) over the Bay of Bengal and the South China
Sea in terms of thermal contrast between the Tibetan Plateau and surrounding
ocean based upon 5-day mean ECMWF circulation field data (1980-89) and
5-day mean GMS equivalent black body temperature (T-BB) data. The early
onset of the SEAM is recognizable at Pentad 28 (May 16-20) with accelerated
low-level monsoon westerlies followed by second enhancement of the monsoon
activities in early June.
The warming over the Tibetan Plateau from spring to summer is found in the 200-500 hPa thickness data on about 15-day intervals. Of importance is the observational evidence that the warming phase over the Tibetan Plateau around mid-May is concurrent with the early onset of the SEAM. Thus, the thermal contrast between the Tibetan Plateau and the adjacent ocean is likely to induce the acceleration and eastward extension of the low-level monsoon flow, causing the abrupt commencement of the SEAM including onset of the South China Sea monsoon (SCSM). This relationship between low-level wind over the key region (10 degrees-20 degrees N, 80 degrees-120 degrees E) and 200-500 hPa thickness over the Tibetan Plateau is also confirmed based on the correlation analysis in the interannual variabilities.
An influence for the mid-latitude atmosphere, stationary Rossby waves are generated over the South China Sea and propagate in a northeastward direction toward Japan because of the cyclonic vorticity and the tropical heat source associated with the onset of the SCSM. As a result of this wave propagation, a high pressure anomaly appears over Japan, which is consistent with a singularity of clear skies around Japan in mid-May (Kawamura and Tian, 1992).
|Tibetan Plateau forcing and the timing of the monsoon onset over South Asia and the South China Sea|
|Wu GX, Zhang YS MONTHLY WEATHER REVIEW 126: (4) 913-927 APR 1998|
|Abstract: Observations were employed to study the thermal characteristics
of the Tibetan Plateau and its neighboring regions, and their impacts on
the onset of the Asian monsoon in 1989. Special attention was paid to the
diagnosis of the temporal and spatial distributions of surface sensible
and latent heat fluxes. Results show that the whole procedure of the outbreak
of the Asian monsoon onset is composed of three consequential stages. The
first is the monsoon onset over the eastern coast of the Bay of Bengal
(BOB) in early May. It is followed by the onset of the East Asian monsoon
over the South China Sea (SCS) by 20 May, then the onset of the South Asian
monsoon over India by 10 June. It was shown that the onset of the BOB monsoon
is directly linked to the thermal as well as mechanical forcing of the
Tibetan Plateau. It then generates a favorable environment for the SCS
monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts
westward, the onset of the South Asian monsoon occurs.
Finally, the timing of the onset of the Asian monsoon in 1989 was explored. It was shown that the onset of the Asian monsoon occurs when the warm or rising phase of different low-frequency oscillations reach the "East Asian monsoon area" (EAMA) concurrently. These include the warm phase of the eastward propagating two- to three-week oscillation (TTO) of the upper-layer temperature in middle latitudes, the rising phase of the northward propagating Madden-Julian oscillation of the southern tropical divergence, and the rising phase of the westward propagating TTO of the western Pacific divergence. It was concluded that the timing of the Asian monsoon onset is determined when the favorable phases of different low-frequency oscillations are locked over the EAMA.
|Sea surface temperature in the South China Sea - an index for the Asian monsoon and ENSO system|
|Ose T, Song YK, Kitoh A JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 75: (6) 1091-1107 DEC 1997|
|Abstract: Interannual variability of the sea surface temperature anomalies
(SSTA) over the South China Sea (SCS) is recognized as an index for the
Asian monsoon and ENSO system because of its special geographical location
for that system. The following results are obtained by the statistical
analysis of the observational data.
(1) In the northern winter, the SCS SSTA are quite sensitive to the longitudinal shift of global wind anomalies associated with the equatorial Pacific SSTA. This fact is related to that the SCS SSTA and the neighbor SSTA have strong biennial oscillation.
(2) When the global wind anomalies are shifted eastward in the winter (BO-type years), the tropical eastern Pacific SSTA tend to change in the following spring. On the other hand, when those wind anomalies are shifted westward (LF-type years), the eastern Pacific SSTA tend to be maintained through the year. The associated differences between the BO and LF-type years are found in the seasonal change of the low-level tropical wind anomalies from the preceding summer through winter.
(3) The northern summer SCS SSTA seem to be controlled by in-situ low-level wind anomalies. Further more, easterly anomalies over South Asia and the tropical western Pacific and westerly anomalies over East Asia are found in the lower atmosphere for the positive SCS SSTA. It is also shown that the summer SCS SSTA have a statistical relationship with the equatorial central Pacific SSTA in the preceding winter. This fact suggests a relationship between the summer Asian monsoon and the winter phase of ENSO.
|Impact of 1990-'95 ENSO/WEPO event on Indian monsoon rainfall|
|Gopinathan CK INDIAN JOURNAL OF MARINE SCIENCES 26: (3) 258-262 SEP 1997|
|Abstract: The negative phase of the 1990 - '95 El Nino-Southern Oscillation (ENSO) and the associated Warming of the Equatorial Pacific Ocean (WEPO) was the longest observed in the 113 years of its recorded history, compared to its normal duration of 1 to 2 years. In case of earlier 11 ENSO events, the Indian South West Monsoon Rainfall (ISWMR) was below normal in 73% cases and above normal only 9% of the cases during the starting year of the negative epoch. The ISWMR was slightly above normal in 1990, the first year of the current negative epoch, and below normal in 1991 and 1992, normal in 1993, above normal in 1994 and normal during 1995.|
|Breaks in the Asian monsoon: The influence of Southern Hemisphere weather systems|
|Rodwell MJ JOURNAL OF THE ATMOSPHERIC SCIENCES 54: (22) 2597-2611 NOV 15 1997|
|Abstract: Atmospheric model results suggest that chaotic weather systems in the Southern Hemisphere midlatitudes can trigger ''breaks'' in the Indian monsoon rainfall. Indeed, the mechanism may be able to trigger a more general break of the entire Asian monsoon. The mechanism proposed involves the injection of dry, high negative potential vorticity air from the Southern Hemisphere midlatitudes into the low-level monsoon inflow. Observations from the 1994 monsoon season tend to support this mechanism and, if true, it may imply some predictive skill for shorter-range forecasting. However, the mechanism proposed may also imply that an accurate seasonal forecast of monsoon rainfall is an impossible objective, with important consequences for the agricultural economies of the region. Results are presented from both an idealized model and a full general circulation model.|
|The Indian monsoon .2. The rains|
|OHare G GEOGRAPHY 82: (357) 335-352, Part 4 OCT 1997|
|Abstract: The aim of this article is to consider the main elements of Indian monsoon rainfall - its timing, intensity amount and distribution. The monsoon rains are examined using a range of different spatial and temporal scales. First, the Indian mean annual and seasonal rainfall distributions are described These distributions are explained in terms of surface elevation, tropical cyclone tracks, and summer monsoon duration period. A review is then presented of important shorter-term regional variations in the rainfall supply produced by (a) individual tropical weather disturbances including tropical cyclones and thunderstorms and (b) natural break periods in the summer monsoon. A detailed discussion of the rainfall generating powers of the four major types of tropical cyclone, i.e. tropical lows, depressions storms and hurricanes together with tropical thunderstorms, highlights their varying individual contribution to the monsoon rains. Inter-annual or year to year variations in the Indian monsoon rainfall between 1871-1990 are considered next. A spatial analysis of inter-annual rainfall distribution reveals major geographical variations between years of deficit, normal and excess rainfall A temporal analysis of inter-annual rainfall shows marked year to year variability, though longer-term alternating epochs with excess and deficient rain are observed when the Indian Summer Monsoon Rainfall (ISMR) index is used. In a review of external monsoon forcings, ENSO events including both warm phase Fl Nines and cold phase La Ninas are shown to be the most significant factor. Nevertheless, because of the great number of factors involved in monsoon development, accurate summer monsoon forecasting remains a difficult task.|
|Rainfall variability over South-east Asia - Connections with Indian monsoon and enso extremes: New perspectives|
|Kripalani RH, Kulkarni A INTERNATIONAL JOURNAL OF CLIMATOLOGY 17: (11) 1155-1168 SEP 1997|
|Abstract: Seasonal and annual rainfall data for 135 stations for periods
varying from 25 to 125 years are utilized to investigate and understand
the interannual and short-term (decadal) climate variability over the South-east
Asian domain. Contemporaneous relations during the summer monsoon period
(June to September) reveal that the rainfall variations over central India,
north China, northern parts of Thailand, central parts of Brunei and Borneo
and the Indonesian region east of 120 degrees E vary in phase. However,
the rainfall variations over the regions surrounding the South China Sea,
in particular the north-west Philippines, vary in the opposite phase. Possible
dynamic causes for the spatial correlation structure obtained are discussed.
Based on the instrumental data available and on an objective criteria, regional rainfall anomaly time series for contiguous regions over Thailand, Malaysia, Singapore, Brunei, Indonesia and Philippines are prepared. Results reveal that although there are year-to-year random fluctuations, there are certain epochs of the above-and below-normal rainfall over each region. These epochs are not forced by the El Nino/La Nina frequencies. Near the equatorial regions the epochs tend to last for about a decade, whereas over the tropical regions, away from the Equator, epochs last for about three decades. There is no systematic climate change or trend in any of the series. Further, the impact of El Nino (La Nina) on the rainfall regimes is more severe during the below (above) normal epochs than during the above (below) normal epochs. Extreme drought/flood situations tend to occur when the epochal behaviour and the El Nino/La Nina events are phase-locked. (C) 1997 by the Royal Meteorological Society.
|Observed lead-lag relationships between Indian summer monsoon and some meteorological variables|
|Harzallah A, Sadourny R CLIMATE DYNAMICS 13: (9) 635-648 SEP 1997|
|Abstract: Lagged relationships between the Indian summer monsoon and several climate variables are investigated. The Variables examined are gridded fields of snow cover (14 years), sea surface temperature (41 years) and 500 hPa geopotential height north of 20 degrees N (42 years). We also used series of global air temperature (108 years) and Southern Oscillation index (112 years). Precipitation over all India during June-September over a 112 year period are used as Indian monsoon index. Emphasis is put on early monsoon precursors. In agreement with the tendency for a low frequency oscillation in the ocean-atmosphere system, several precursor patterns are identified as early as the year preceding the monsoon. The most important key regions and seasons of largest correlations are selected and the corresponding series are used to perform a monsoon prediction. The prediction shows however a relatively moderate score mainly due to the not highly significant correlations. To improve the predictions we filtered the variables into their biennial (1.5-3.5 years) and low frequency (3.5-7.5 years) modes. Correlations between the monsoon and the filtered variables are higher than those obtained without filtering especially for the biennial mode. The two modes are out-of-phase before the monsoon and in-phase during and after. This phasing is found in all variables except for snow cover for which the two modes are in-phase before the monsoon and out-of-phase during and after. It is suggested that such phasing may be important for the formation of snow and could explain the higher correlations when variables are concomitant or are lagging the monsoon. Early predictions of the monsoon based on those two modes show improved scores with highly significant correlations with the actual monsoon.|
|Pre-monsoon maximum and minimum temperatures over India in relation to the summer monsoon rainfall|
|Kumar KK, Kumar KR, Pant GB INTERNATIONAL JOURNAL OF CLIMATOLOGY 17: (10) 1115-1127 AUG 1997|
|Abstract: The pre-monsoon thermal field over the Indian landmass has an important bearing on the land-sea heating contrast in the region, consequently influencing the establishment, advance and overall performance of the Indian summer monsoon rainfall. This paper examines the relationship between the pre-monsoon thermal field over India and the following summer monsoon rainfall, in order to identify possible predictors for long-range forecasting of Indian summer monsoon rainfall. Based on the spatial patterns of correlations of monsoon rainfall with maximum and minimum temperatures at 121 stations well distributed over India, during the recent period 1951-80 two predictors showing a significant contribution to the variance in monsoon rainfall have been identified. They are (i) March minimum temperature in east peninsular India and (ii) May minimum temperature in west central India. These two predictors have performed very well in terms of their significant contribution to the multiple regression models during 1951-1987, vis-a-vis several other known predictors. They have also shown a consistently significant relationship with the monsoon rainfall during the recent period, from the mid-1940s till the end of the data period. A stepwise regression model for long-range forecasting of all-India summer monsoon rainfall, involving three regional predictors, has been developed, and has shown a multiple correlation of 0.89. (C) 1997 by the Royal Meteorological Society. A diagnostic study on heat sources and moisture sinks in the monsoon trough area during active-break phases of the Indian summer monsoon, 1979|
|Bhide UV, Mujamdar VR, Ghanekar SP, Paul DK, Chen TC, Rao GV TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY 49: (4) 455-473 AUG 1997|
|Abstract: The diabatic heating over the Indian monsoon trough area, along with its thermal structure are studied for the 1979 summer monsoon based on FGGE level-IIIb upper air date of the European Centre for Medium Range Weather Forecasts. The apparent heat source and the apparent moisture sink over the trough area varied coherently with the rainfall over central India. The spatial and temporal variations of the vertically integrated apparent heat source and moisture sink were found to be coincident. These coincidences suggested that diabatic heating was largely contributed by the latent heat released by cumulus convection. During the active periods, the vertical structure of spatially-averaged heating and drying rates above the monsoon trough area showed higher values, as much as 8 K to 11 K day(-1), at the mid-tropospheric level (500 hPa), but much smaller and even negative values during break periods. Analyses of the heating and drying rates at 500 hPa level in a x-t diagram revealed that heat sources and moisture sinks propagated westward across the trough area with a period of 10-15 days (often called monsoon mode). The 30-50 day period of fluctuations showed a close link with the two major active/break phases of monsoon during the season, The monsoon mode became a part of the mid-season fluctuation of monsoon activity between the two major active phases. This paper discusses the spatial distribution of rainfall and heat source and moisture sink over the trough area, and the role of east-west differential heating in the development of weak/break phases of Indian summer monsoon 1979.|
|The south Asian monsoon and the tropospheric biennial oscillation|
|Meehl GA JOURNAL OF CLIMATE 10: (8) 1921-1943 AUG 1997|
|Abstract: A mechanism is described that involves the south Asian monsoon
as an active part of the tropospheric biennial oscillation (TBO) described
in previous studies. This mechanism depends on coupled land-atmosphere-ocean
interactions in the Indian sector, large-scale atmospheric east-west circulations
in the Tropics, convective heating anomalies over Africa and the Pacific,
and tropical-midlatitude interactions in the Northern Hemisphere. A key
element for the monsoon role in the TBO is land-sea or meridional tropospheric
temperature contrast, with area-averaged surface temperature anomalies
over south Asia that are able to persist on a l-yr timescale without the
heat storage characteristics that contribute to this memory mechanism in
the ocean. Results from a global coupled general circulation model show
that soil moisture anomalies contribute to land-surface temperature anomalies
(through latent heat flux anomalies) for only one season after the summer
A global atmospheric GCM in perpetual January mode is run with observed SSTs with specified convective heating anomalies to demonstrate that convective heating anomalies elsewhere in the Tropics associated with the coupled ocean-atmosphere biennial mechanism can contribute to altering seasonal midlatitude circulation. These changes in the midlatitude longwave pattern, forced by a combination of tropical convective heating anomalies over East Africa, Southeast Asia, and the western Pacific (in association with SST anomalies), are then able to maintain temperature anomalies over south Asia via advection through winter and spring to set up the land-sea meridional tropospheric temperature contrast for the subsequent monsoon. The role of the Indian Ocean, then, is to provide a moisture source and a low-amplitude coupled response component for meridional temperature contrast to help drive the south Asian monsoon. The role of the Pacific is to produce shifts in regionally coupled convection-SST anomalies. These regions are tied together and mutually interact via the large-scale east-west circulation in the atmosphere and contribute to altering midlatitude circulations as well. The coupled model results, and experiments with an atmospheric GCM that includes specified convective heating anomalies, suggest that the influence of south Asian snow cover in the monsoon is not a driving force by itself, but is symptomatic of the larger-scale shift in the midlatitude longwave pattern associated with tropical SST and convective heating anomalies.
|Interaction between the ENSO and the Asian monsoon in a coral record of tropical climate|
|Charles CD, Hunter DE, Fairbanks RG SCIENCE 277: (5328) 925-928 AUG 15 1997|
|Abstract: The oxygen isotopic composition of a banded coral from the western equatorial Indian Ocean provides a 150-year-long history of the relation between the El Nino-Southern Oscillation (ENSO) phenomenon and the Asian monsoon. Interannual cycles in the coral time series were found to correlate with Pacific coral and instrumental climate records, suggesting a consistent linkage across ocean basins, despite the changing frequency and amplitude of the ENSO. However, decadal variability that is characteristic of the monsoon system also dominates the coral record, which implies important interactions between tropical and midlatitude climate variability. One prominent manifestation of this interaction is the strong amplitude modulation of the quasi-biennial cycle.|
|The role of radiative transfer in maintaining the Indian summer monsoon circulation|
|Leach MJ, Raman S, Mohanty UC, Madala RV PURE AND APPLIED GEOPHYSICS 149: (3) 601-622 JUN 1997|
|Abstract: The radiative-convective feedback and land-sea thermal forcing play significant roles in maintenance of the summer monsoon circulation over the Indian sub-continent. In this study, the role of radiative transfer in maintaining the monsoon circulation is examined with numerical sensitivity experiments. For this purpose, a sixteen layer primitive equation limited area model is used to perform numerical simulations with and without atmospheric radiative transfer processes parameterized in the model. The initial values and boundary conditions for the numerical integrations of the model are derived from operational analyses of the ECMWF, UK. The results show that the radiative transfer is essential in maintaining the intensity of the low level Somali Jet as well as the upper level Tropical Easterly Jet (TEJ) over the Indian sub-continent and adjoining seas. The meridional circulation over the neon is also well simulated. As a result, enough moisture transports from the warm equatorial region to simulate more realistic orographic precipitation in the windward side of the mountains along the West coast of India. Without radiative transfer processes in the model atmosphere the simulated monsoon circulation weakens, moisture transport decreases and the precipitation lessens.|
|Northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation|
|Wang B, Xu XH JOURNAL OF CLIMATE 10: (5) 1071-1085 MAY 1997|
|Abstract: Using climatological pentad mean outgoing longwave radiation
(OLR) and European Centre for Medium-Range Weather Forecasts analysis winds,
the authors show that the Northern Hemisphere summer monsoon displays statistically
significant climatological intraseasonal oscillations (CISOs). The extreme
phases of CISO characterize monsoon singularities-monsoon events that occur
on a fixed pentad with usual regularity, whereas the transitional phases
of CISO represent the largest year-to-year monsoon variations.
The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May-July), and westward along 15 degrees N from 170 degrees E to the Bay of Bengal during August and September.
The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October.
The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.
|Chaos and predictability of the Indian summer monsoon|
|Goswami BN PRAMANA-JOURNAL OF PHYSICS 48: (2) 719-736 FEB 1997|
|Abstract: Predictability of the Indian summer monsoon is investigated by conducting three multiyear integrations with the Geophysical Fluid Dynamics Laboratory's climate model. The mean monsoon simulated by the model is realistic. It is shown that a significant fraction of the interannual variance of the simulated Indian summer monsoon may be due to internal dynamics. It is discovered that the tropical atmosphere is capable of sustaining a quasi-biennial oscillation (QBO) accounting for most of the internal low frequency variability. It is also shown that neither air-sea interaction nor surface hydrology feedback is essential for the QBO of the model atmosphere. That such a QBO can arise due to modulation of the nonlinear intraseasonal oscillations by the annual cycle is demonstrated using a simple nonlinear dynamical model. The phase and the amplitude of the internal mode is unpredictable and hence may be responsible for limiting the long range predictability of the monsoon.|
|Variability of radiative cooling during the Asian summer monsoon and its influence on intraseasonal waves|
|Mehta AV, Smith EA JOURNAL OF THE ATMOSPHERIC SCIENCES 54: (8) 941-966 APR 15 1997|
|Abstract: Infrared radiative cooling rates are calculated over the
Asian summer monsoon between 5 degrees S-20 degrees N and 40 degrees-135
degrees E at a spatial resolution of 5 degrees X 5 degrees for the summer
seasons of 1984 and 1987. A medium spectral resolution infrared radiative
transfer model with specified temperature, moisture, clouds, and trace
gas distributions is used to obtain the cooling rate profiles. Cloud distributions
for the two summers are obtained from Indian National Satellite measurements.
Seasonal mean and intraseasonal variations of clouds and radiative cooling
rates over a 21-76-day range of periods are examined.
The analysis identifies centers over the central and eastern Indian Ocean, and western Pacific Ocean, along the equator, and along 15 degrees N, where seasonal mean cloud amounts range from 40% to 80% with cloud tops mostly in the middle and upper troposphere. Intraseasonal variability of clouds is also large over these centers (% variances >25%). Consistently, seasonal mean cooling rates are at a maximum (3 degrees-5 degrees C day(-1)) in the upper troposphere between 300 and 400 mb, related to cloud-top cooling. The cooling rates below 400 mb are between 1 degrees and 3 degrees C day(-1). The cooling rates exhibit intraseasonal amplitudes of 1.0 degrees-1.5 degrees C day(-1). The largest amplitudes are found between 300 and 500 mb, indicating that cooling rate variability is directly related to intraseasonal variability of convective clouds. Spatial distributions of clouds and cooling rates remain similar during the 1984 and 1987 summer seasons. However, during 1987, intraseasonal amplitudes of deep convective cloud amount and cooling rate over the Indian Ocean are 10%-15% larger than in 1984.
It is shown that intraseasonal variability of cooling rates over the Indian Ocean can perturb convective heating by 10%-30% in the upper and lower troposphere. Based on a one-dimensional radiative-convective equilibrium model, it is estimated that the radiative damping timescale over the Indian Ocean region is similar to 3 days. Based on this damping timescale and in conjunction with a model of equatorial Kelvin waves with first baroclinic mode, it is hypothesized that the variable cloud-radiative cooling rates can alter phase speeds of Kelvin waves by up to 60%. This helps explain why the frequency range of intraseasonal oscillations is so broad.
|Relationships between Indian summer monsoon and Pacific SST/SOI tendency from winter to spring and their stability|
|Mooley DA, Munot AA THEORETICAL AND APPLIED CLIMATOLOGY 56: (3-4) 187-197 1997|
|Abstract: Relationships of Indian monsoon rainfall with Sea Surface
Temperature (SST) and Southern Oscillation Index (SOI) tendencies from
DJF to MAM and those between concurrent SST and SOI tendencies are important
in view of their large-scale character. Some of these have application
in the field of forecasting. Bias on these or any other relationships can
possibly arise from a few years of extreme data. Whether the bias results
in suppression of an existing relationship, in creating a relationship
when none exists, or strenghthening or weakening an existing relationship,
over any period, needs to be examined, and if found so, the bias should
be removed and bias-free relationships should be discussed and considered
for applications. This problem has been examined in respect of the forementioned
relationships by following an objective procedure for removing the bias.
Removal of the bias has made a notable difference in respect of the strength
as well as significance of the relationship over some periods, for some
The main features of the relationships Free from such bias are: (a) Indian monsoon rainfall and SST tendency from DJF to MAM before as well as after monsoon are significantly related except within 1904-1940 in respect of relationship with tendency before monsoon, (b) Indian monsoon rainfall and SOI tendency before and after monsoon are significantly related over some non-overlapping component periods only, (c) though the best SST-SOI tendency coupling is for DJF to MAM tendency, no coupling is observed between these tendencies within 1904-1940, (d) linkage of SST tendency from DJF to MAM with the preceding Indian monsoon rainfall appears to be stronger than that with the concurrent SOI tendency and continues even during the period of no coupling between the tendencies, thus bringing out the dominating active role played by the Indian monsoon.
|Mechanisms of low frequency intraseasonal oscillations of the Indian summer monsoon|
|Krishnan R, Venkatesan C METEOROLOGY AND ATMOSPHERIC PHYSICS 62: (1-2) 101-128 1997|
|Abstract: This work deals with idealized modelling experiments designed
to understand the dynamical evolution of low frequency intraseasonal monsoonal
oscillations that result from interactions between the large scale monsoon
Reverse Hadley Cell (RHC) and moist convective processes. The monsoon differential
heating, which primarily determines the low-level convergence of the large-scale
monsoon flow, is found to play a decisive role in affecting the northward
progression of the monsoonal modes. A strong north-south differential heating
leads to a robust generation and steady maintenance of northward propagating
monsoonal oscillations. A weaker land-ocean thermal contrast leads to feeble
low frequency monsoonal modes that have relatively longer periods in the
30-50 day band. This increase in the period of the monsoonal oscillations
due to weak north-south thermal contrast is in good agreement with the
observational findings of Yasunari (1980) and Kasture and Keshavamurty
(1987). It is speculated that such an increase in the oscillatory period
may be an outcome from an elongation in the meridional scale of the transient
Hadley type cells which act as resonating cavities for the monsoonal modes.
A Mobile Wave CISK (MWC) form of interaction between the large scale monsoon and the transient circulations associated with the Madden Julian Oscillation (MJO) is projected as a viable physical mechanism for the northward movement of low frequency modes. It is demonstrated that the effective low level convergence, following such an interaction, tends to shift northward relative to the site of interaction. This enables the heating perturbations to be displaced northward which in turn causes the secondary circulations and wind perturbations to follow. The essential criterion for the occurrence of a prolonged northward propagation of the low frequency modes is that the heating perturbations should phase lead the wind perturbations at all times.
An examination of the psi-chi interactions on the 30-50 day time scale reveals that the conversion from the transient divergent motions to rotational motions is quite intense (feeble) in the strong (weak) monsoon differential heating experiments. Because of the closer proximity to the monsoon heat source and also due to the latitudinal variation of earth's rotational effects, the psi-chi interactions tend to be more pronounced to the north of 15 degrees N while they are less robust in the near equatorial latitudes.
The regularity of the monsoonal modes is found to depend on the strength of the monsoon differential heating and also on the periodic behaviour of the equatorial intraseasonal oscillations. The monsoonal modes are quite steady and exhibit extreme regularity in the presence of a weak north-south differential heating provided the equatorial forcing due to the MJO varies in a periodic manner. This result supports the findings of Mehta and Krishnamurti (1988) who found greater regularity of the 30-50 day modes during bad monsoon years.
The low frequency monsoonal modes are found to be quite sensitive to the moisture availability factor (m) and the vertical profile of heating used in the MWC parameterization. A small increase in the value of (m) is found to significantly intensify the amplitude of the monsoonal oscillations while there is no considerable shift in the spectral frequency within the 30-50 day band as such. The 30-50 day motions show significant enhancement, with a relatively sharp spectral peak around 45 days, when the vertical profile of MWC heating has a maximum in the lower troposphere. However an upward displacement of the heating maximum tends to weaken the low frequency oscillations.
|Symmetric instability of monsoon flows|
|Krishnakumar V, Lau KM TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY 49: (2) 228-245 MAR 1997|
|Abstract: Using a zonally symmetric multi-level moist linear model, we have examined the possibility of symmetric instability in the monsoon region. Stability analyses with a zonally symmetric model using monthly ECMWF (Jan-Dec) zonal basic flows revealed bath unstable as well as neutral modes. In the absence of cumulus heating, the linear stability of the monsoon Bow changes dramatically with the emergence of many unstable modes in the month of May and lasting through August; whereas with the inclusion of cumulus heating unstable modes appear early in April with substantially enhanced growth rates. This onset of instability of the May basic state may signal the abrupt transition of the south Asian meridional monsoon circulation. The abrupt nature of the monsoon transitions was also clearly seen in the ECMWF 5-day mean meridional circulation in the South Asian monsoon region. The most unstable modes have doubling time of about 1 to 2 days. The amplitude structure of these unstable modes were mainly confined to the equatorial regions. The growth rates and the amplitude structure of the most unstable modes agree reasonably with the corresponding growth rate and structure of the meridional monsoon cell during the monsoon transition.|
|Sensitivity of the Asian summer monsoon to aspects of sea-surface-temperature anomalies in the tropical Pacific Ocean|
|Soman MK, Slingo J QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY 123: (538) 309-336, Part B JAN 1997|
|Abstract: The response of both the time of onset, and the strength
of the Asian summer monsoon, to regional aspects of the sea surface temperature
(SST) anomalies in the tropical Pacific Ocean associated with El Nino/Soufhem
Oscillation (ENSO) has been investigated through a series of sensitivity
experiments with the Universities' Global Atmospheric Modelling Programme
(UGAMP) General Circulation Model (UGCM). This paper builds on the work
of Ju and Slingo (1995) and on their hypothesis that the relationship between
the Asian summer monsoon and ENSO involved the latitudinal position and
strength of the tropical convective maximum (TCM) over Indonesia and the
west Pacific in the preceding spring. The inference from their results
was that the modulation of the TCM might be associated either with changes
in the Walker circulation through the influence of the east Pacific SST
anomalies, or with changes in the local Hadley circulation associated with
the in situ SST anomalies in the west Pacific. The investigation has focused
on the particular contrasting years of 1983 and 1984. The experiments described
in this paper are designed to isolate the effects of the principal SST
anomalies in the east and central Pacific, associated with El Nino/La Nina,
from those of opposite sign which develop in the west Pacific as a complementary
pattern during the mature phase of El Nino/La Nina.
The results of the experimentation suggest that, at least for the test cases of 1983 and 1984, the modulation of the Walker circulation, with implied additional subsidence over the eastern hemisphere, is the dominant mechanism whereby the Asian summer monsoon is weakened during El Nino years. However, the late onset during El Nino years may also be associated with the complementary cord SST anomalies in the west Pacific which delay the northwards transition of the TCM. During La Nina, the modulation of the Walker circulation appears not to be the controlling factor which determines the stronger monsoons. The UGCM results suggest that the complementary warm SST anomalies in the west Pacific enhance the TCM, and it is this in situ response by the TCM that leads to an early onset and stronger monsoon. The importance of warm anomalies in the west Pacific in the development of a strong monsoon has been investigated further through a case-study of the 1994 season. The year 1994 was an El Nino year in which the monsoon was unexpectedly active, but which was also marked by warmer than normal SSTs in the west Pacific.
The sensitivity experiments have also elucidated the role of El Nino in influencing the precursory signature of stronger subtropical westerlies over India and south-east Asia during the winter and spring preceding weak monsoons. The results suggested that the equatorwards shift of the subtropical jet is a remote response to the warm SST anomalies in the central and east Pacific associated with El Nino.