Impact of changes in sea surface temperature over southern equatorial Indian ocean on Indian summer monsoon : A model study

The influence of Sea Surface Temperature (SST) anomalies on Indian Summer Monsoon Rainfall (ISMR), especially over coastal regions of India during Subtropical Indian Ocean Dipole (STIOD) events from 19802009 has been studied. 6 strong positive and 8 negative dipole events were recorded during the period of study. The observed relationship between the meridional SST gradient and deep convection in relation to Outgoing Longwave Radiation (OLR) in the subtropical Indian Ocean (IO) and their impact on ISMR has been analyzed. We have demonstrated the common characteristics of the subtropical OLR and wind field in association with SST variations in the southern Indian Ocean during the strongest SST dipoles. It has been observed that the dipole events in different phases induce warming over different regions of the Southern Equatorial Indian Ocean (SEIO) which produces significant impact over ISMR, particularly over the coastal regions. Positive Subtropical Indian Ocean Dipole (PSTIOD) causes warming over the South Central Equatorial Indian Ocean (SCEIO) resulting in remarkable reduction (enhancement) in rainfall over the west coast (upper east coast) of India whereas Negative Subtropical Indian Ocean Dipole (NSTIOD) causes warming over the South Western Equatorial Indian Ocean (SWEIO) resulting in enhancement (reduction) in rainfall over the west coast (upper east coast). The impact of dipole phases is also significant on rainfall over Peninsular and south Peninsular India. Increase (decrease) of peninsular rainfall during NSTIOD (PSTIOD) phases depends on the shifting of deep convection over the SEIO region. The Regional Climate Model (RegCM3), a recent version of National Center for Atmospheric Research (NCAR) has been used to simulate. the ISMR. RegCM3 successfully simulates the rainfall over Indian coastal regions and its surrounding sea areas. The influence of increase in SST over SEIO by 0.5 degrees C in RegCM3 on the summer monsoon circulation and ISMR has been studied. The SST (warm) experiment produces warming of SCEIO with increased deep convection over the region with increase in rainfall over Orissa (UR) and Gangetic West Bengal (GWB), but reduced monsoon precipitation over the west coast of India. OLR over the West Subtropical Indian Ocean (WSTIO) during PSTIOD years is significantly negatively correlated (-0.81) with the mean sea level pressure (MSLP) over West Southern Indian Ocean (WSIO, 0 - 20 degrees S, 50 degrees E - 60 degrees E). When OLR is enhanced (reduced) over WSTIO during PSTIOD years, pressure is reduced (enhanced) over the WSIO region.