Global Warming Shifts the Monsoon Circulation, Drying South Asia

Monsoon rainfall over South Asia has decreased during the last 5 to 6 decades according to several sets of observations. Although sea surface temperature (SST) has risen across the Indo-Pacific warm pool during this period, the expected accompanying increased rainfall has occurred only in the tropical western Pacific. The above changes noted in observations are also seen in a coupled climate model, but only when the model includes the recent increase in greenhouse gas concentration. The hypothesis that the robust rise in SST over the warm pool, perhaps anchored by an increase in greenhouse gas concentrations, is instrumental in the east-west shift in monsoon rainfall (enhanced rainfall over tropical western Pacific and decreased rainfall over South Asia) is proposed. A suite of controlled experiments with an atmospheric general circulation model has been performed to isolate the impact of regional SST warming trends on the dryness over South Asia. Model experiments support the hypothesis that the rising SST trend over the tropical western Pacific has changed the atmospheric circulation: over the Bay of Bengal more dry and cool air is advected from the northeast than previously. Moist static energy budget diagnostics on the model solutions identify the sources for this east-west shift. SST warming over the warm pool has accelerated in recent decades. Therefore, a close monitoring of that warming is important for long-term variations of monsoon rainfall. The inconsistency in the amplitude of drying over South Asia among the various land-based rainfall observations and lack of sustained rainfall observations over the open oceans, however, poses constraints in the results.