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 and the El Nino Southern Oscillation (ENSO). Based on the selection of extreme monsoon rainfall years 'In Phase' or 'Out of Phase' with respect to the observations, this study identifies specific SST and atmospheric circulation patterns responsible for the remote forcing on the monsoon. A clear common characteristic of externally forced extreme monsoon years is identified with an ENSO pattern having summer SST anomalies of the same sign in the tropical Pacific Ocean but also in the Indian and Atlantic tropical sectors. This finding assumes that the SST pattern in summer is enough to modulate the Walker circulation and consequently to suppress or enhance convection over South Asia, even if it does not evolve into an ENSO event. The analysis of the 'Out of Phase' cases (i.e. when the model reproduces a weak monsoon instead of a strong one, or the reverse) reveals how the model wrongly responds to the SST forcing, ignoring other processes like ocean-atmosphere coupling. Once ENSO is linearly removed the main source of remote forcing for strong (weak) monsoon characteristics over India is the tropical Atlantic with negative (positive) anomalies, and with weak anomalies of the same sign located in the south Indian Ocean. The results of the role of the forcing from the tropical Atlantic are also confirmed by a set of atmospheric model experiments where interannually varying SST is prescribed only in the Atlantic Ocean, while the rest of the SST is climatological.