In this study, we examine response of South Asian Monsoon (SAM) climate to transient increase of greenhouse gases and sulfate aerosols in GFDL coupled atmosphere-ocean climate change ensemble simulations. When subjected to increasing greenhouse gases, the GFDL coupled model simulation projects greatly enhanced SAM rainfall, and weakened SAM circulation. The addition of direct effect of sulfate aerosols notably reduces the contribution by greenhouse gases, but it does not alter the whole picture. The enhanced SAM rainfall is contributed by dramatically increased moisture content in the warm atmosphere over South Asian region, whereas the weakened SAM circulation is maintained by the El Niņo like climate change in tropical Pacific, as revealed by stationary wave modeling diagnosis.

With results from AGCM experiments with various prescribed boundary conditions, we further examine the relative role of enhanced land-sea thermal contrast, and SST anomaly in local and remote oceanic basins in contributing to the SAM climate change. We find that, the enhanced land-sea thermal contrast acts to displace the precipitation pattern northward, leading to enhanced SAM rainfall over land and reduced rainfall further south. Warmer Sea Surface Temperature (SST) in northern Indian Ocean is crucial in contributing to the enhanced SAM rainfall. To obtain the climate change of SAM rainfall magnitude and circulation projected by the GFDL coupled simulation, however, the influences of SST Anomaly in other tropical oceanic basins must also be considered. A mechanism on how the El Niņo like climate change in tropical Pacific affects the SAM region has been proposed. Finally, given the dependence of climate change model simulations on representation of physical processes therein, we discuss the general application of our findings to climate change simulations provided by other coupled atmosphere-ocean models.