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

===== 8.3.2.4.4 North American Monsoon =====

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Since AR5, there have been updates on the observed long-term variations and changes in the North American monsoon (NAmerM). During the Last Glacial Maximum (LGM; 21,000 – 19,000 years ago), the NAmerM was substantially weaker due to cold, dry mid-latitude air associated with the Laurentide Ice Sheet ( T. Bhattacharya et al. , 2017, 2018 ). The NAmerM strengthened until the mid-Holocene period, in response to ice-emsheet retreat and rising summer insolation, but probably did not exceed the strength of the modern system ( ''low confidence'' ), as indicated by model simulations ( [[#Metcalfe--2015|Metcalfe et al., 2015]] ) and paleoclimatic reconstructions ( [[#Bhattacharya--2018|Bhattacharya et al., 2018]] ). Paleoclimatic evidence from proxy datasets and mid-Pliocene (PlioMIP1) simulations suggest a wetter south-western USA during that warmer period (A.M. [[#Haywood--2013|]] [[#Haywood--2013|Haywood et al., 2013]] ; [[#Pound--2014|Pound et al., 2014]] ; [[#Ibarra--2018|Ibarra et al., 2018]] ) but it is not clear whether this is due to increases of precipitation associated with the monsoon or occurring during the winter season.

During 1948 – 2010, trends of boreal summer precipitation amount were significantly positive over New Mexico and the core NAmerM region, but significantly negative over south-western Mexico ( [[#Hoell--2016|Hoell et al., 2016]] ). In addition, diverse datasets like CRU, CHIRPS and GPCP show significant decreases of precipitation in parts of the south-western USA and north-western Mexico, including the NAmerM region ( [[#Cavazos--2020|Cavazos et al., 2020]] ; [[#Ashfaq--2021|Ashfaq et al., 2021]] ). Other studies suggest a strengthening of the NAmerM upper level anticyclone since the mid-1970s, with a more frequent northward location ( [[#Diem--2013|Diem et al., 2013]] ). Between 1910 – 2010, the number of precipitation events increased across the northern Chihuahuan desert, within the NAmerM domain, despite a decrease in their magnitude, and the length of extreme dry and wet periods also increased ( [[#Petrie--2014|Petrie et al., 2014]] ).

An increase in intense rainfall and severe weather events has been observed in several locations, especially in south-western Arizona since 1991, resulting from increases in atmospheric moisture content and instability; a change that has been confirmed by convective-permitting model simulations ( [[#Luong--2017|Luong et al., 2017]] ; [[#Pascale--2019|Pascale et al., 2019]] ). A dense network of 59 rain gauges located in south-eastern Arizona suggests an intensification of monsoon sub-daily rainfall since the mid-1970s ( [[#Demaria--2019|Demaria et al., 2019]] ), as expected by a stronger global warming signature for sub-daily rather than daily or monthly precipitation accumulation ( [[IPCC:Wg1:Chapter:Chapter-11#11.4|Section 11.4]] ). [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.3|Section 10.4.2.3]] provides further details on changes in precipitation in south-western North America. Evidence from multiple reanalyses suggests that increases in NAmerM rainfall have contributed to the increasing trend of global monsoon precipitation ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.2|Section 2.3.1.4.2]] ; [[#Lin--2014|Lin et al., 2014]] ). In addition, more frequent occurrence of earlier retreats of the NAmerM since 1979 is documented ( [[#Arias--2012|Arias et al., 2012]] , 2015), in association with the positive phase of the Atlantic Multi-decadal Variability (AMV) and a westward expansion of the North Atlantic Subtropical High (W. [[#Li--2011|Li et al., 2011]] , 2012).

Analyses from a 50-km resolution GCM indicate that the NAmerM response to CO <sub>2</sub> is very sensitive to SST biases, showing reductions in summer NAmerM precipitation with increased CO <sub>2</sub> when the SST biases are small ( [[#Pascale--2017|Pascale et al., 2017]] ) in contrast to CMIP5 models ( [[#Cook--2013|Cook and Seager, 2013]] ; [[#Maloney--2014|Maloney et al., 2014]] ; [[#Torres-Alavez--2014|Torres-Alavez et al., 2014]] ; [[#Hoell--2016|Hoell et al., 2016]] ). The NAmerM has been shown to be also sensitive to sulphur dioxide (SO <sub>2</sub> ) emissions ( [[#García-Martínez--2020|García-Martínez et al., 2020]] ).

In summary, both paleoclimate evidence and observations indicate an intensification of the NAmerM in a warmer climate ( ''medium confidence'' ). The intensification recorded since about the 1970s has been partly driven by GHG emissions ( ''medium con'' ''fidence'' ).

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