A Model Study of the Air-Sea Interaction Associated with the Climatological Aspects and Interannual Variability of the South Asian Summer Monsoon Development

The climatological characteristics and interannual variations of the development of the South Asian summer monsoon (SASM) in early summer are studied using output from a 200-yr simulation of a coupled atmosphere-ocean general circulation model (CM2.1). Some of the model results are compared with corresponding observations. Climatological charts of the model and observational data at pentadal intervals indicate that both the precipitation and SST signals exhibit a tendency to migrate northward. Enhanced monsoonal precipitation at a given site is accompanied by a reduction in incoming shortwave radiation and intensification of upward latent heat flux, and by oceanic cooling. An extended empirical orthogonal function analysis is used to identify the dates for initiation of the northward march of SASM in individual summers. It is noted that early monsoon development prevails after the mature phase of La Nina events, whereas delayed development occurs after El Nino. Sensitivity experiments based on the atmospheric component of CM2.1 indicate that the effects of SST forcings in the tropical Pacific (TPAC) and Indian Ocean (IO) on monsoon development are opposite to each other. During El Nino events, the atmospheric response to remote TPAC forcing tends to suppress or postpone monsoon development over South Asia. Conversely, the warm SST anomalies in IO, which are generated by the "atmospheric bridge" mechanism in El Nino episodes, lead to accelerated monsoon development. The net result of these two competing effects is an evolution scenario with a timing that is intermediate between the response to TPAC forcing only and the response to IO forcing only.