Sensitivity of monsoon circulations to changes in sea surface temperatures

Sensitivity of a regional scale model to different sea surface temperatures (SSTs) in the context of short-range prediction of monsoon rainfall 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°C warmer than the climatological SSTs. Two numerical experiments are performed using observed and climatological SSTs for an active monsoon period. It is found that the evaporation increases over these surrounding oceans when the observed SSTs are used. As expected, warmer SSTs caused the surface pressure to decrease by 2 to 3 hPa leading to local accelerations of winds. As a result, stronger circulation patterns are induced. Area-averaged evaporation is about 20% higher and the rainfall 10% higher when observed SSTs are used. Effect of an uniform increase in SST by 2°C over climatological values is also investigated. Comparison of simulations with the climatic and the uniformly increased SSTs indicated that the rainfall predictions are qualitatively similar to those obtained with the observed SSTs. Area-averaged evaporation is about 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 the sea surface temperature distribution. This is mainly because of variations in mesoscale circulations caused by the local gradients in the SSTs.