Global monsoon: Concept and dynamic response to anthropogenic warming

Monsoon has been studied for centuries, yet only recently have regional monsoons been recognized as a global system. This paper begins with a review of the concept of Global Monsoon and related debating issues. We argue that GM drives annual cycles of Hadley circulation, Intertropical Convergence Zone, and subtropical high and dry climate regions. Land monsoon rainfall (LMR) provides water resources for about 70% of the world’s population. Here we review the climate sensitivity of global and regional LMR to anthropogenic warming projected by models participating in phase six of the Coupled Model Intercomparison Project (CMIP6), focusing on critical physical processes responsible for projected changes. In theory, regional mean LMR changes can be approximated by the changes in the product of the mid-tropospheric ascent and 850-hPa specific humidity, plus moderate contribution from evaporation. The greenhouse gases (GHGs) forcing increases moisture content but stabilizes the atmosphere; the two thermodynamic effects offset each other, resulting in a moderate thermodynamic impact on LMR. The GHGs-induced horizontally differential warming results in robust ‘‘northern hemisphere (NH)-warmer than- southern hemisphere (SH)’’, ‘‘land-warmer-than-ocean’’, and an El Nino–like warming pattern. The enhanced NH–SH thermal contrast will increase NH monsoon rainfall and reduce SH monsoon rainfall. The enhanced land–ocean thermal contrast will increase monsoon rainfall over the Asian–northern African monsoon regions. The projected eastern Pacific warming will reduce the North American monsoon. The Inter-model spread analysis suggests that the GHGs-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The last section discusses conceivable ways forward.