Understanding Future Change of Global Monsoons Projected by CMIP6 Models

Projecting future change of monsoon rainfall is essential for water resource management, food security, disaster mitigation, and infrastructure planning. Here we assess the future change and explore the causes of the changes using 15 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble projects that, under the shared socioeconomic pathway (SSP) 2-4.5, the total land monsoon rainfall will likely increase in the Northern Hemisphere (NH) by about 2.8% per one degree Celsius of global warming (2.8% degrees C-1) in contrast to little change in the Southern Hemisphere (SH; 20.3% degrees C-1). In addition, in the future the Asian-northern African monsoon likely becomes wetter while the North American monsoon becomes drier. Since the humidity increase is nearly uniform in all summer monsoon regions, the dynamic processes must play a fundamental role in shaping the spatial patterns of the global monsoon changes. Greenhouse gas (GHG) radiative forcing induces a NH-warmer-than-SH pattern, which favors increasing the NHmonsoon rainfall and prolonging the NH monsoon rainy season while reducing the SH monsoon rainfall and shortening the SH monsoon rainy season. The GHG forcing induces a land-warmer-than-ocean pattern, which enhances Asian monsoon low pressure and increases Asian and northern African monsoon rainfall, and an El Nino-like warming, which reduces North American monsoon rainfall. The uncertainties in the projected monsoon precipitation changes are significantly related to the models' projected hemispheric and land-ocean thermal contrasts as well as to the eastern Pacific Ocean warming. The CMIP6 models' common biases and the processes by which convective heating drives monsoon circulation are also discussed.