Understanding the Asian Summer Monsoon Response to Greenhouse Warming: The Relative Roles of Direct Radiative Forcing and SST Change

Future hydroclimate projections from state-of-the-art climate models show large uncertainty and model discrepancy, particularly over the monsoon regions. It is important to understand the different physical pathways by which greenhouse gases (GHGs) may impact regional hydroclimate and distinguish those from the uncertainty caused by low model skill. The response to rising GHGs can be through both direct radiative effect and indirect effect via sea surface temperature (SST) warming, the relative importance of the two may result in discrepancies in conclusions.

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.