Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.4.2.4.4 North American Monsoon =====

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The AR5 concluded that the North American monsoon (NAmerM) will ''likely'' intensify in the future, even though there is ''low agreement'' among models. The AR5 reported ''medium confidence'' that precipitation associated with the NAmerM will arrive later in the annual cycle and persist longer.

Since AR5, analyses of CMIP5 projections suggest little change in the overall amount of NAmerM precipitation in response to rising global surface temperature. However, significant declines are projected in the early monsoon season and increases in the late monsoon season, suggesting a shift in seasonality toward a delayed monsoon onset and demise ( [[#Cook--2013|Cook et al., 2013]] ). It is recognized that CMIP5 models are generally too coarsely-resolved to simulate the Gulf of California and the moisture surges associated with the NAmerM ( [[#Pascale--2017|Pascale et al., 2017]] ). Under different RCPs, CMIP5 models tend to project a reduction in NAmerM precipitation but an increase in extreme precipitation events (Torres-Alavez et al. , 2014; Bukovsky et al. , 2015; Pascale et al. , 2019) . The almost unchanged or slight decrease in NAmerM total precipitation amount under global warming projections is at odds with paleoclimate records that suggest increased monsoon precipitation under past warm conditions ( [[#D’Agostino--2019|D’Agostino et al., 2019]] ; [[#Seth--2019|Seth et al., 2019]] ). However, there is ''low agreement'' on how those changes and the mechanisms that drive them are affected under different RCPs since most simulations are model-dependant ( [[#Cook--2013|Cook and Seager, 2013]] ; [[#Geil--2013|Geil et al., 2013]] ; [[#Pascale--2019|Pascale et al., 2019]] ). Projections from six CMIP6 models show a shortening of the NAmerM under the SSP5-8.5 scenario due to earlier demises ( [[#Moon--2020|Moon and Ha, 2020]] ). In addition, CMIP6 projections show a decrease in NAmerM precipitation under SSP2-4.5 and SSP5-8.5 scenarios by the end of the 21st century with large inter-model spread (Figure 8.22). This result is also supported by the analysis of 31 CMIP6 models under the SSTP5-8.5 scenario for the 2080 – 2099 period ( [[#Almazroui--2021|Almazroui et al., 2021]] ). Non-linearities and uncertainties in the NAmerM projected changes are valid for many water cycle variables, like precipitation, runoff and P–E (Table 8.2).

In summary, there is ''low agreement'' on a projected decrease of NAmerM precipitation, however there is ''high confidence'' in delayed onsets and demises of the summer monsoon.

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