Please reference this work if used elsewhere


Science Question : What processes are responsible for the intraseasonal modulation of synoptic scale convection over the deep convective regions of the Indian monsoon ?
Summary of Talk
RegardingAssertion
.... 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
    1. 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.
    2. 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.
    3. 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.

    .....now for some substantiation

    Seasonal Mean
    Point MadeGraphical 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.

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