Anthropogenically Forced Decadal Change of South Asian Summer Monsoon Across the Mid‐1990s

Abstract

AbstractAnalysis of observations indicates that there was a significant decadal change in summer (June–August) mean rainfall over South Asia and Southeast Asia across the mid‐1990s, which is characterized by less rainfall over central‐northern India and northern Indo‐China Peninsula. This study investigates impacts of anthropogenic forcing on the observed decadal change across the mid‐1990s. A set of experiments using the coupled atmosphere‐ocean‐mixed‐layer model MetUM‐GOML2 has been performed to quantify the relative roles of changes in anthropogenic greenhouse gases and anthropogenic aerosols (AA). Results indicate a dominant role of anthropogenic changes in the observed decadal changes. Separately, the changes in greenhouse gases forcing play an important role in the reduction of rainfall over central‐northern India through the changes of atmospheric circulation (i.e., the local Hadley circulation and the Walker circulation), with additional contribution from changes in AA forcing. The changes in AA forcing dominate the reduction of rainfall over northern Indo‐China Peninsula due to high‐pressure anomalies over northern South Asia and the western subtropical Pacific. These high‐pressure anomalies are induced by the surface cooling, mainly via aerosol‐radiation interaction that decreases downward clear‐sky shortwave radiation over South Asia during summer and aerosol‐radiation interaction and aerosol‐cloud interaction that decrease downward shortwave radiation over the western subtropical Pacific during presummer seasons.