SENSITIVITY OF MONSOON CIRCULATIONS TO CHANGES IN SEA-SURFACE TEMPERATURES

Sensitivity of a regional scale model to different sea surface tempera tures (SSTs) in the context of short-range prediction of monsoon rainf all is studied using a three dimensional regional scale model. For the month of July, over certain regions of Arabian Sea, Bay of Bengal and the Indian Ocean, observed SSTs are about 1 to 2 degrees C warmer tha n the climatological SSTs. Two numerical experiments are performed usi ng observed and climatological SSTs for an active monsoon period. It i s found that the evaporation increases over these surrounding oceans w hen the observed SSTs are used. As expected, warmer SSTs caused the su rface pressure to decrease by 2 to 3 hPa leading to local acceleration s of winds. As a result, stronger circulation patterns are induced. Ar ea-averaged evaporation is about 20% higher and the rainfall 10% highe r when observed SSTs are used. Effect of an uniform increase in SST by 2 degrees C over climatological values is also investigated. Comparis on of simulations with the climatic and the uniformly increased SSTs i ndicated that the rainfall predictions are qualitatively similar to th ose obtained with the observed SSTs. Area-averaged evaporation is abou t 40% higher and the rainfall 15% higher than those obtained with the climatic SSTs. Results from these three numerical experiments indicate that the short-range prediction of monsoon weather is sensitive to th e sea surface temperature distribution. This is mainly because of vari ations in mesoscale circulations caused by the local gradients in the SSTs.