On the weakening association between South Asian Monsoon and Atlantic Multidecadal Oscillation

Abstract

Long-term multi-decadal scale variability of South Asian Summer Monsoon (SASM) is known to be associated with the Atlantic Multidecadal Oscillation (AMO) with positive phase associated with above-normal rainfall and vice-versa over SASM region. Analysis of the 120-year historical record suggests that the relationship between the AMO and the SASM has weakened in recent decades. Two possible reasons emerge from the analyses of observational datasets and model experiments. Enhanced Indian Ocean warming and weakened North Atlantic Subtropical High (NASH) are the prime factors weakening AMO-SASM association in the recent decades. Indian Ocean warming co-occurred during the recent AMO warm phase weakens the meridional temperature gradient, weakening the regional Hadley cell, large-scale monsoon circulation and precipitation. The suppressed convection triggers a Rossby wave which manifests as a cyclonic circulation over west central Asia weakening the Tibetan anticyclone. In addition, weakening of NASH weakens the Circum-Global Teleconnection (CGT) connecting AMO to the SASM, leading to weakening of AMO-SASM association. Projected warming of the Indian Ocean and Atlantic demands better understanding of the AMO-SASM teleconnection and their projected changes for assessing future changes in SASM in a warming world.