Ocean Observations and Monsoon Variability

The summer monsoon governs the very pulse of life in the Asian countries affecting more than two billion people and diverse fauna and flora inhabiting this tropical region. The main objective of weather and short term climate predictions here is the amount of rainfall, and there is great need and demand for monsoon prediction on different time scales from daily to seasonal. Statistical models have completely failed to predict the major droughts in recent decades. Numerical models are not doing better either and there are many uncertainties. Atmospheric models have serious difficulties in simulating even the basic structure of seasonal rainfall distribution over the Indian subcontinent.

Majority of the monsoon cloud systems originate over the oceans that surround the landmass, and then move onto the land.  The spatio-temporal distribution of the monsoon rainfall on land depends on the pattern of cloud formation over the oceans surrounding the land. In particular the Bay of Bengal is the breeding ground of majority of monsoon system. Here copious rainfall, river run off and sea water interact constructively to create conditions where (SST) remains above 28^oC to support frequent formation of deep cloud systems for more than five months. The details of physical processes involved and their contributions are yet to be quantified.

For predicting monsoon rainfall on medium, intraseasonal and seasonal time scales, coupled models are considered to be appropriate and they hold the promise for better predictions in the future. The present coupled models have to overcome many issues. For example, these models have a cold SST bias over the Bay. This will influence the thermal structure of the atmosphere above as well through the boundary layer (BL) for the following reason. Observations reveal that the temperature variation in the troposphere closely follows that of a moist adiabat (a parcel of air that rises in the atmosphere adiabatically conserving its initial entropy) of the BL air parcel. Thus, any bias in BL properties will reflect in the entire troposphere and on on the rainfall.

 Surface fluxes have large influence on BL properties. Flux algorithms have not been validated for the monsoon conditions. There is also large north-south gradient in freshwater and ocean mixed layer depth in the Bay. Some existing theories attribute the thermal gradient to be responsible for the propagation of monsoon systems. Supporting observations are missing. Therefore, there are several good reasons for conducting detailed field studies using state of the art instruments over the Bay. Detailed observations of the surface fluxes and properties of the (entire) boundary layer and the atmosphere above are required in different parts of the bay. These observations will provide data to test model physics and insights for incorporating appropriate physical processes where there are lacking. We expect a model whose physics are more sound, to  lead to improvements in monsoon prediction.