Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.5.1.5 Global Monsoon Precipitation and Circulation ====

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The AR5 assessed changes of the global monsoon in the context of long-term trends across the 21st century and the change by 2081–2100. The AR5 showed growing evidence of improved skill of climate models in reproducing the climatological features of the global monsoon. Taken together with identified model agreement on future changes, the global monsoon precipitation, aggregated over all regional monsoon regions, is ''likely'' to strengthen in the 21st century with increases in its area and intensity, while the monsoon circulation weakens. In all RCP scenarios, the global monsoon area is ''very likely'' to increase, and the global monsoon precipitation intensity is ''likely'' to increase, resulting in a ''very likely'' increase in the global monsoon total precipitation, by 2081–2100 ( [[#Kitoh--2013|Kitoh et al., 2013]] ).

Since AR5, there has been progress in understanding physical mechanisms for the projected changes in global monsoon and quantifying the sources of uncertainty in projections. The increase in global monsoon precipitation under warming is primarily attributed to the increase of moisture convergence, which comes mainly from the thermodynamic effect due to increasing atmospheric moisture but is partly offset by reduced convergence (W. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ; [[#Chen--2020|Chen et al., 2020]] ). The dynamic effect, such as monsoon circulation changes, dominates regional differences in the projected monsoon precipitation changes ( [[#Chen--2020|Chen et al., 2020]] ). Specifically, NH monsoon precipitation will increase more strongly than its SH counterpart, due to an increase in hemispheric temperature difference between the NH and SH, enhancement of the Hadley circulation, and atmospheric moistening, countered by stabilization of the troposphere ( [[#Lee--2014|Lee and Wang, 2014]] ). The seasonality of global monsoon rainfall is projected to be enhanced in response to warming, featuring a greater wet–dry season contrast (Lee and Wang 2014; Zhang et al. 2019). In addition, the interannual variability of global monsoon rainfall is projected to intensify mainly over land, with a strengthened relationship between global monsoon and ENSO ( [[#Hsu--2013|Hsu et al., 2013]] ; [[#Wang--2020|Wang et al., 2020]] , 2021).

For the uncertainty in mean monsoon precipitation projections, the model uncertainty is the dominant contributor throughout the century and explains more than 70% of the inter-model variance during near term, mid-term, and long term. The contribution of internal variability is only important at the beginning in early decades, while scenario uncertainty becomes important at the end of the 21st century. The sources of uncertainty for the mean and extreme monsoon precipitation mainly differ in the long-term projection, when the contribution of scenario uncertainty is comparable to the model uncertainty for extreme precipitation ( [[#Zhou--2020|Zhou et al., 2020]] ). Although the magnitude of internal variability differs between CMIP5 models and single-model, initial-condition large ensembles, the impact is only evident in the beginning decades. For the mid- and long term, the magnitude difference does not alter that model uncertainty is the dominant source of uncertainty in the projections of global land monsoon precipitation ( [[#Zhou--2020|Zhou et al., 2020]] ).

Based on the projections of changes in precipitation from CMIP6 under the four SSPs, the global monsoon precipitation is ''likely'' to strengthen in the 21st century with increases in its intensity, while NH summer monsoon circulation weakens (Figure 4.14). Global land monsoon precipitation will ''likely'' increase by 1.3–2.4% per °C GSAT warming among the four scenarios considered here. In the long term, the multi-model mean change (5–95% range of the available 41 projections) of global land monsoon precipitation index is 2.9% (–0.8 to +7.8%), 3.7% (–2.5 to +8.6%), 3.77% (–3.2 to +8.1%), and 5.7% (–2.8 to +12.3%) under SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. This enhancement is caused by thermodynamic responses due to increased moisture, which is partly offset by dynamic responses due to a weakened circulation ( [[#Chen--2020|Chen et al., 2020]] ). The patterns of monsoon rainfall change in the mid- to long-term include a north–south asymmetry characterized by greater increase in the NH than the SH, and an East–West asymmetry characterized by enhanced Asian-African monsoons and weakened North American monsoon ( ''medium confidence'' ) ( [[#Lee--2014|Lee and Wang, 2014]] ; [[#Mohtadi--2016|Mohtadi et al., 2016]] ; [[#Pascale--2017|Pascale et al., 2017]] ; [[#Wang--2021|Wang et al., 2021]] ).

Based on the assessment of CMIP6 models, we conclude that it is ''likely'' that, in the mid- to long term, the global land monsoon precipitation will increase with GSAT rise despite a weakened monsoon circulation. The global land monsoon precipitation will ''likely'' increase by 1.3–2.4% per °C GSAT warming among the four scenarios. Monsoon precipitation responses depend on region and emissions scenario ( ''high confidence'' ).

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