South and East Asian Summer Monsoon Climate and Variation in the MRI Coupled Model (MRI-CGCM2)

Abstract

Simulations of the major characteristics of the summer monsoon climate over South Asia, East Asia, and the western North Pacific by the new version of the Meteorological Research Institute coupled GCM (MRI-CGCM2) are analyzed. In addition to assessing the simulation of mean summer monsoon rainfall and its association with SST and basic circulation parameters, the model's performance in reproducing the seasonal variation, climatological onset, peak, withdrawal, and subseasonal variation of the monsoon is also studied. The mean rainfall distribution and the maximum rainfall centers over South Asia and the western North Pacific, including the monsoon rainbelt over central India, though shifted southward of its observed position by about 5°, and the baiu rainband across Japan, are well simulated. However, the model underestimates the monsoon rainfall over southeast China. The biases in model mean rainfall are found to be associated with biases in wind, moisture convergence, and vertical stability of the atmosphere. The relationship between mean SST and organized convection in the model is close to the observations, with a propensity for deep convection increasing with an increase in SST above the threshold value. The simulated summer anomalies of SST and surface fluxes imply the dominance of the SST?wind?evaporation feedback system over most of the Indian and western Pacific Oceans on intraseasonal time scales. In addition, in the model, equatorial eastern Pacific SST anomalies strongly influence the Indian summer monsoon predominantly on biennial time scales. The model captures the basic monsoon seasonal cycle, onset, peak, and withdrawal over India and the western North Pacific across Japan. However, the simulation shows an early onset of the monsoon over mainland China and the early occurrence of peak rainfall over southeast China. The active?break spells in Indian monsoon rainfall and meridional propagations of rainbelts over India and the western North Pacific are realistically simulated. But, over southeastern China, the model is unable to simulate northward propagation of rainbelts, which contributes to the poor simulation of seasonal mean rainfall.