Future Projection and Associated Uncertainty of the East Asian Summer Monsoon

Abstract:

The regional climate change of the East Asian summer monsoon is investigated in the Coupled Model Inter-comparison Project – Phase 5 (CMIP5) archive in the context of the moist static energy budget. In the greenhouse gas forcing scenario, the reduction of radiative cooling and the increase of continental surface temperature occur much more rapidly than changes in sea surface temperatures (SSTs). Without changes in SSTs, the rainfall in the oceanic monsoon region decreases, despite an increase in the land-sea thermal contrast traditionally considered as a fundamental driver of monsoons. The reduction in precipitation is robust amongst all CMIP5 models and is primarily attributable to a weakening of the subtropical westerly jet. The weakening of the jet, in turn, can be explained by changes in upper-level eddy momentum flux convergence and thermal wind balance. On longer time scales, SSTs increase, as does monsoon rainfall. This delayed precipitation increase is primarily driven by the thermodynamic contribution to precipitation changes, by which wet regions get wetter and dry regions get drier. Dynamical changes due to changes in circulation play a secondary effect. These results clearly highlight deficits of commonly proposed geo-engineering schemes as climate mitigation strategies, which, by reducing the surface warming without sequestration of CO₂, might still result in dramatic changes in rainfall, especially in heavily populated monsoonal regions. Similar analyses will be applied to other subtropical convergence zones in the Earth’s atmosphere.