Assessing Future Changes in the East Asian Summer Monsoon Using CMIP3 Models: Results from the Best Model Ensemble

Future changes in the East Asian summer monsoon (EASM) have been estimated from the six best-performing models in phase 3 of the Coupled Model Intercomparison Project (CMIP3) included in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The composite mean rainband over East Asia during the summer season exhibits the characteristic EASM front relatively well with slightly less precipitation over east–central China and the baiu front aligned more steeply in the latitudinal direction. An ensemble from the coupled models that poorly simulated the EASM is shown to degrade the overall results, giving rise to an underestimation of the projected increase rate in precipitation. In a quantitative estimate, the 22-member ensemble-mean precipitation for the period 2079–99 is anticipated to increase by only 5%–10%, which is half the increase in the corresponding mean precipitation from the good models (10%–20%). Moisture budget analysis demonstrates that the good-model ensemble mean is in a much closer correspondence with observations than the poor- or all-model ensemble mean, supporting the reliable selection of the best-performing models. An increase in the vertically integrated moisture flux convergence is attributed to the enhanced precipitation in the future climate. In particular, a significant increase in atmospheric water vapor because of a warmer SST over the western Pacific plays a more critical role in the enhanced precipitation than a change in monsoonal low-level circulation does. In contrast to the apparent meridional shift of the primary location of the future EASM front presented in previous studies, the current analysis shows a negligible amount of meridional movement.