Potential role of the February-March Southern Annular Mode on the Indian summer monsoon rainfall: a new perspective

Relationship between the Southern Annular Mode (SAM) and the India summer monsoon rainfall (ISMR) has been examined based on the data period 1949-2013. While the entire data period indicates a significant increasing trend in SAM, recent decades 19832013 indicate no trend. The relationship between the two strengthened considerably since 1983. Results reveal that the February-March SAM is significantly related with the subsequent ISMR. A positive (negative) SAM during February- March is favorable (unfavorable) for the ensuing summer monsoon rainfall over the Indian sub-continent. The delayed response is relayed through the central Pacific Ocean. We propose a hypothesis that states: when a negative (positive) phase of February-March SAM occurs, it gives rise to an anomalous meridional circulation in a longitudinally locked air-sea coupled system over the central Pacific that persists up to the subsequent boreal summer and propagates from the sub-polar latitudes to the equatorial latitudes inducing a warming (cooling) effect over the central equatorial Pacific region. In turn, this effect concomitantly weakens (strengthens) the monsoon rainfall over the Indian sub-continent. Thus, the February-March SAM could possibly serve as a new precursor to foreshadow the subsequent behavior of the Indian summer monsoon.