Remote SST forcing on Indian summer monsoon extreme years in AGCM experiments

An ensemble of AMIP-type experiments with prescribed interannual varying sea surface temperature (SST)and different initial conditions is used to study the relationship between Indian summer monsoon extreme conditions andthe El Nino Southern Oscillation (ENSO). Based on the selection of extreme monsoon rainfall years ‘In Phase’ or ‘Out ofPhase’ with respect to the observations, this study identifies specific SST and atmospheric circulation patterns responsible forthe remote forcing on the monsoon. A clear common characteristic of externally forced extreme monsoon years is identifiedwith an ENSO pattern having summer SST anomalies of the same sign in the tropical Pacific Ocean but also in the Indian andAtlantic tropical sectors. This finding assumes that the SST pattern in summer is enough to modulate the Walker circulation andconsequently to suppress or enhance convection over South Asia, even if it does not evolve into an ENSO event. The analysisof the ‘Out of Phase’ cases (i.e. when the model reproduces a weak monsoon instead of a strong one, or the reverse) revealshow the model wrongly responds to the SST forcing, ignoring other processes like ocean–atmosphere coupling. Once ENSOis linearly removed the main source of remote forcing for strong (weak) monsoon characteristics over India is the tropicalAtlantic with negative (positive) anomalies, and with weak anomalies of the same sign located in the south Indian Ocean. Theresults of the role of the forcing from the tropical Atlantic are also confirmed by a set of atmospheric model experiments whereinterannually varying SST is prescribed only in the Atlantic Ocean, while the rest of the SST is climatological.