Different role of spring season Atlantic SST anomalies in Indian Summer Monsoon Rainfall (ISMR) variability before and after early 2000

Abstract

Abstract The present study shows that the weak (out-of-phase) correlation between the boreal spring Atlantic Meridional Mode (AMM) index and Indian summer monsoon rainfall (ISMR) during the 1990–2002 periods turned into a significant positive relationship after that. During the first period, the major mode of spring season Atlantic SST has stronger cooling in the southeastern region and is intensified by summer. After 2002, the spring pattern has stronger north Atlantic warming extending to the equatorial region continuing to summer. The pattern change of the Atlantic SST is associated with a pre-existing El Nino during period 1 and the formation of La Nina by summer during period 2. During period 1, ENSO-induced ISMR anomalies dominated with weak divergence over India and the Atlantic region, while during period 2, the warm SST anomalies of north Atlantic-induced off-equatorial convection and associated circulation contributed to increased ISMR along with La Nina direct influence on monsoon. The study further showed that during period 2, the increased correlation of ENSO and ISMR is also contributed by the Atlantic SST anomalies with additional off-equatorial wind anomalies and circulation from the north Atlantic extending to the Indian Ocean and monsoon region. Thus, Atlantic SST anomalies play a significant role in ENSO phase reversal and ISMR during the recent period. Many of the seasonal prediction models that participated in the NMME project capture the phase reversal of AMM-ISMR correlation when initialized during February. But models have stronger equatorial SST patterns during the summer season, which is mainly contributed by co-occurring ENSO. The models with a weak AMM-ISMR relationship in the recent period also have a feeble AMM-ENSO association, resulting in weak North Atlantic SST anomalies associated with AMM. The study indicates that the spring season AMM index can provide a predictive signal for ISMR in seasonal prediction models, but they need to simulate proper interactions of the tropical Atlantic and Pacific Oceans.