We assess the Asian summer monsoon responses to greenhouse warming using output from coupled general circulation models (GCMs) in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and idealized experiments from the Atmosphere Model Intercomparison Project (AMIP). The direct radiative response to rising CO2 is quantified by quadrupling the CO2 concentration with prescribed SST, whereas the indirect response to SST warming is quantified by a 4K uniform warming of SST with fixed CO2 concentration. We show that over the monsoon regions, the summertime precipitation response associated with direct radiative forcing and that with indirect SST effect largely opposes each other. This competing effect may contribute to an overall weak response and model discrepancy in the CMIP5 coupled simulations.
The physical mechanisms causing the monsoon rainfall changes are examined using the moisture budget analysis. The mean moisture flux convergence term is separated into thermodynamic and dynamic components, involving changes in moisture and circulation, respectively. The differing monsoon rainfall response is mainly due to the opposing effect of the dynamic contribution to the mean moisture convergence, thus related to atmospheric circulation change.