Summary of Talk |
Regarding | Assertion |
.... the Seasonal Cycle |
P-E is large and positive over India and the northern
Indian Ocean during the monsoon season (JJA).
P-E is large and negative over the Southern IO during JJA.
Interhemispheric atmospheric transports from the
southern hemisphere are
large to enable observed values of P-E in the mean
to occur. They are especially large near the Somali Jet
and cumulatively account for nearly 80% of the net
moisture flux into India.
|
.... intraseasonal variations in P and E over India |
Strong intraseasonal variations exist in rainfall and winds over
India.
The atmosphere is destabilized near India
prior to the onset of a monsoon active period by low-level
moistening. This creates an unstable environment in
which deep convection can occur. Why does the atmosphere
moisten ? |
Scientific Questions:
How is the high frequency variability
in P and P-E manifested in the planetary scale hydrologic
cycle ?
Are intraseasonal variations in the southern hemisphere
coherent with those in the northern hemisphere ?
Do active periods lead or lag anomalies in the large-scale
hydrologic cycle ?
Are the trends suggestive of a mechanism for regulating
convective activity in the northern hemisphere ? |
Conclusions |
- Intraseasonal variations exist in all components of the
seasonal mean hydrologic cycle and are important elements
of the intraseasonal active and break phases of the entire
Indian monsoon system.
- Variations in southern hemisphere hydrology are found to precede
the active phase of the monsoon over India. The cumulative
effect of the anomalies is to accumlate moisture in the
atmosphere's lower levels near India and destabilize it.
- The active phase of the monsoon appears to be a response to the
destabilized atmosphere as moisture accumulates on a large
scale 10 to 15 days prior to the peak in winds and rainfall
over the monsoon core.
|
Seasonal Mean |
Point Made | Graphical Demonstration |
The Seasonal Cycle and Intraseasonal Variations |
P-E :
The seasonal mean distribution requires a contribution
of significant latent heat by atmospheric transports to
the region of monsoonal deep convection.
Transports
The transports postulated above do exist and are
substantial, especially over the Somali Jet in the
western equatorial Indian Ocean.
Precip Over India
Precipitation nevertheless is highly variable
on intraseasonal time scales as are winds and evaporation.
How is this manifested in
the planetary scale transport of moisture to India ? |
Data and Method |
NCEP Reanalysis, NVAP PW, MSU Precip, AVHRR OLR
The NCEP evaporation appears to be pretty good. Click
here to view a
comparison with both ship (Moana Wave) and satellite estimates
of evaporation.
|
Why look at interhemispheric exchange ? |
The NCEP
Reanalysis suggest that the vast majority of moisture to enter
the monsoon originate from interhemispheric transports.
(approx 80%) Also, a strengthened Somali Jet has been
observed by previous authors prior to the onset of deep
convection over India (Findlater, 1969; van deBoogard and Rao, 1981).
A number of anomalies in moisture transport and sources (evap) and
sinks (precip) are apparrent through the active-break sequence
of the Indian monsoon. Here we summarize a few of them.
|
A QuickTime Movie of Composite Hydrologic Anomalies |
This movie demonstrates the evolution
of evaporation, moisture transport and OLR anomalies in our composite
monsoon active period. The movie shows that large scale coherent
evaporation anomalies occur some 25 days prior to Day 0 - the peak of the
active event over India. Convection near the equator intensifies in
the following week and moves northwards with time. Deep convection
and westerly low-levels winds cover India near Day 0 and the anomalies
decay rapidly with time after Day 0. By Day 4, most anomalies have vanished.
|
PW |
The variability in precipitable water is suggestive
of an initial moisture accumulation over the northern Indian Ocean
which is consequently deplenished after Day 0 |
Moisture Transports |
Anomalies in moisture transports
can be found here. It is
apparent that a number of transport anomalies act to moisten the
atmosphere above the north Indian Ocean prior to the onset of
active periods. Here their importance is displayed in quantitative
estimates.
|
P-E |
Precipitation minus evaporation is highly variable on
intraseasonal time scales. Here, we see that 2 inital focii of
negative P-E anomalies exist, one over the Arabian Sea and one
over the southern hemisphere Indian Ocean. |