Different role of spring season Atlantic SST anomalies in indian summer monsoon rainfall (ISMR) variability before and after early 2000

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 1990–2001 periods turned into a significant positive relationship after that. During the first period, boreal spring (MAM) season Atlantic SST variability is dominated by stronger cooling in the southeastern region. After 2002, the spring season SST pattern has north Atlantic warming extending to the equatorial region and continuing to summer (JJAS). The primary mode of the Atlantic SST co-occurred 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 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 extended to the Indian Ocean and Indian land region resulting in increased monsoon rainfall during JJAS. 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, MAM season 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 more robust equatorial SST patterns during the summer season, and the north Atlantic SST anomalies are vital in the absence of ENSO. The study indicates that the spring season AMM index can provide a predictive signal for ISMR in seasonal prediction models. Still, they need to improve the simulation of the Atlantic basic state and its teleconnection with the tropical east Pacific.