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

==== 8.4.2.4 Monsoons ====

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In AR5, monsoon precipitation over land was projected to intensify by the end of the 21st century, due to thermodynamic increases in moisture convergence despite weakening of the tropical circulation (see [[#8.2.1.3|Section 8.2.1.3]] ). Following the definition of regional monsoons in [[IPCC:Wg1:Chapter:Annex-v|Annex V]] and Figure 8.11, and the assessment of the observed changes ( [[#8.3.2.4|Section 8.3.2.4]] ), here we provide an assessment of projected changes in regional monsoons. Assessment is provided either in terms of SSP and RCP scenarios and global warming levels available since AR5, or from the newly available CMIP6 projections (Figure 8.22 and Table 8.2). Table 8.2 provides projected changes across the five SSPs used in this Report for precipitation (mm day <sup>–1</sup> ), P–E (mm day <sup>–1</sup> ) and runoff (mm day <sup>–1</sup> ) over the regional monsoons for the mid (2041 – 2060) and long term (2081 – 2100).

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'''Table 8.2 |''' '''Monsoon mean water cycle projections in the mid-term''' ( '''2041–2060''' ''') and long term''' ( '''2081–2100''' ''') relative to present day''' ( '''1995–2014''' '''), showing present-day mean and 90% confidence range across CMIP6 models (historical experiment) and projected mean changes and the 90% confidence range across the same set of models and a range of Shared Socio-economic Pathway scenarios. All statistics are in units of mm day''' –1 '''.''' Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

{| class="wikitable"
|-
!
! colspan="5"| '''Mid-term: 2041–2061 Minus Reference Period'''

! colspan="5"| '''Long Term: 2081–2100 Minus Reference Period'''

|-
!
! '''1995''' – '''2014 Reference Period'''

! '''SSP1-1.9'''

! '''SSP1-2.6'''

! '''SSP2-4.5'''

! '''SSP3-7.0'''

! '''SSP5-8.5'''

! '''SSP1-1.9'''

! '''SSP1-2.6'''

! '''SSP2-4.5'''

! '''SSP3-7.0'''

! '''SSP5-8.5'''

|-
| colspan="12"| '''South and South East Asian Monsoon (June–July–August–September, JJAS)'''

|-
| Precipitation

| 8.42 [6.66 to 10.14]

| 0.44 [0.08 to 0.74]

| 0.47 [0.1 to 0.96]

| 0.42 [0.03 to 0.81]

| 0.32 [–0.08 to +0.94]

| 0.54 [0.11–1.18]

| 0.46 [0.16 to 0.7]

| 0.52 [0.13 to 1.09]

| 0.66 [0.16 to 1.1]

| 0.94 [0.3 to 1.78]

| 1.46 [0.66 to 2.49]

|-
| Runoff

| 3.75 [1.8 to 5.71]

| 0.23 [0.1 to 0.38]

| 0.29 [0.02 to 0.65]

| 0.29 [–0.0 to +0.66]

| 0.24 [–0.04 to +0.52]

| 0.38 [0.07–0.78]

| 0.19 [–0.02 to +0.35]

| 0.29 [–0.04 to +0.65]

| 0.42 [0.04 to 0.83]

| 0.7 [0.12 to 1.2]

| 1.14 [0.36 to 2.05]

|-
| P–E

| 5.19 [3.68 to 6.5]

| 0.28 [0.03 to 0.52]

| 0.36 [–0.0 to +0.76]

| 0.36 [0.02 to 0.69]

| 0.3 [–0.04 to +0.85]

| 0.45 [0.06–0.95]

| 0.27 [0.06 to 0.38]

| 0.38 [0.11 to 0.76]

| 0.51 [0.02 to 0.83]

| 0.81 [0.24 to 1.56]

| 1.15 [0.45 to 1.84]

|-
| colspan="12"| '''East Asian Monsoon (June–July–August, JJA)'''

|-
| Precipitation

| 5.59 [4.47 to 6.86]

| 0.37 [–0.09 to +0.93]

| 0.37 [–0.09 to +0.87]

| 0.34 [0.05 to 0.76]

| 0.22 [–0.16 to +0.88]

| 0.43 [0.03 to 1.1]

| 0.43 [0.07 to 1.02]

| 0.44 [–0.0 to +1.08]

| 0.51 [0.11 to 1.09]

| 0.59 [0.02 to 1.31]

| 0.84 [0.24 to 1.74]

|-
| Runoff

| 2.24 [1.28 to 3.41]

| 0.11 [–0.16 to +0.4]

| 0.13 [–0.19 to +0.42]

| 0.13 [–0.15 to +0.4]

| 0.15 [–0.29 to +0.76]

| 0.2 [–0.11 to +0.72]

| 0.16 [–0.08 to +0.49]

| 0.16 [–0.13 to +0.58]

| 0.22 [–0.13 to +0.64]

| 0.36 [–0.05 to +0.87]

| 0.51 [0.06 to 1.24]

|-
| P–E

| 2.41 [1.51 to 3.31]

| 0.1 [–0.31 to +0.51]

| 0.13 [–0.2 to +0.48]

| 0.17 [–0.04 to +0.53]

| 0.17 [–0.2 to +0.75]

| 0.23 [–0.09 to +0.86]

| 0.16 [–0.07 to +0.57]

| 0.18 [–0.18 to +0.65]

| 0.24 [–0.1 to +0.76]

| 0.4 [–0.08 to +0.93]

| 0.5 [–0.13 to +1.34]

|-
| colspan="12"| '''North American Monsoon (July–August–September, JAS)'''

|-
| Precipitation

| 3.05 [2.24 to 3.96]

| 0.13 [–0.08 to +0.43]

| 0.07 [–0.27 to +0.32]

| 0.02 [–0.32 to +0.41]

| –0.03 [–0.37 to +0.38]

| –0.03 [–0.43 to +0.52]

| 0.18 [–0.05 to +0.44]

| 0.04 [–0.35 to +0.39]

| –0.1 [–0.51 to +0.37]

| –0.19 [–0.76 to +0.44]

| –0.15 [–0.96 to +0.57]

|-
| Runoff

| 0.46 [0.09 to 0.87]

| 0.03 [–0.04 to +0.12]

| 0.03 [–0.07 to +0.16]

| 0.02 [–0.1 to +0.14]

| –0.0 [–0.1 to +0.14]

| –0.0 [–0.11 to +0.14]

| 0.04 [–0.03 to +0.15]

| –0.0 [–0.19 to +0.15]

| –0.03 [–0.22 to +0.14]

| –0.05 [–0.23 to +0.19]

| –0.06 [–0.29 to +0.23]

|-
| P–E

| 0.78 [–0.1 to +1.45]

| 0.06 [–0.1 to +0.2]

| 0.02 [–0.18 to +0.24]

| 0.0 [–0.22 to +0.23]

| –0.03 [–0.24 to +0.2]

| –0.04 [–0.31 to +0.27]

| 0.09 [–0.06 to +0.31]

| 0.01 [–0.22 to +0.25]

| –0.08 [–0.28 to +0.25]

| –0.17 [–0.68 to +0.25]

| –0.18 [–0.72 to +0.38]

|-
| colspan="12"| '''South American Monsoon (December–January–February, DJF)'''

|-
| Precipitation

| 8.44 [5.98 to 10.22]

| 0.09 [–0.2 to +0.3]

| 0.12 [–0.29 to +0.62]

| 0.09 [–0.47 to +0.62]

| 0.07 [–0.55 to +0.62]

| 0.07 [–0.5 to +0.71]

| 0.02 [–0.32 to +0.36]

| 0.09 [–0.33 to +0.58]

| 0.07 [–0.63 to +0.81]

| 0.05 [–1.17 to +0.82]

| –0.0 [–1.22 to +1.19]

|-
| Runoff

| 2.49 [1.11 to 4.38]

| –0.02 [–0.23 to +0.26]

| –0.01 [–0.43 to +0.53]

| 0.01 [–0.45 to +0.46]

| –0.03 [–0.49 to +0.36]

| –0.03 [–0.56 to +0.53]

| –0.04 [–0.27 to +0.28]

| –0.01 [–0.41 to +0.39]

| –0.01 [–0.58 to +0.55]

| –0.06 [–0.81 to +0.24]

| –0.04 [–0.85 to +0.93]

|-
| P–E

| 4.5 [2.83 to 6.01]

| 0.04 [–0.23 to +0.25]

| 0.08 [–0.26 to +0.47]

| 0.04 [–0.43 to +0.53]

| 0.04 [–0.5 to +0.61]

| 0.02 [–0.45 to +0.58]

| –0.01 [–0.32 to +0.29]

| 0.03 [–0.34 to +0.43]

| –0.02 [–0.63 to +0.62]

| –0.02 [–1.03 to +0.72]

| –0.09 [–1.11 to +0.98]

|-
| colspan="12"| '''Australian and Maritime Continent Monsoon (December–January–February, DJF)'''

|-
| Precipitation

| 8.63 [6.79 to 10.7]

| 0.26 [0.04 to 0.49]

| 0.22 [–0.23 to +0.53]

| 0.28 [–0.2 to +0.79]

| 0.25 [–0.14 to +0.73]

| 0.38 [0.0 to 0.84]

| 0.15 [–0.09 to +0.34]

| 0.24 [–0.36 to +0.74]

| 0.5 [–0.1 to +1.07]

| 0.65 [–0.08 to +1.33]

| 0.9 [0.09 to 1.76]

|-
| Runoff

| 3.82 [1.78 to 7.25]

| 0.2 [–0.01 to +0.48]

| 0.23 [–0.11 to +0.48]

| 0.29 [–0.11 to +0.7]

| 0.24 [–0.13 to +0.56]

| 0.35 [–0.03 to +0.87]

| 0.12 [–0.06 to +0.39]

| 0.29 [–0.08 to +0.88]

| 0.49 [0.09 to 1.25]

| 0.61 [–0.09 to +1.05]

| 0.92 [0.14 to 1.83]

|-
| P–E

| 4.8 [3.19 to 6.63]

| 0.22 [0.03 to 0.47]

| 0.13 [–0.23 to +0.42]

| 0.2 [–0.16 to +0.7]

| 0.2 [–0.14 to +0.62]

| 0.27 [–0.09 to +0.61]

| 0.12 [–0.1 to +0.31]

| 0.16 [–0.31 to +0.54]

| 0.38 [–0.05 to +0.75]

| 0.54 [–0.08 to +1.13]

| 0.69 [0.09 to 1.27]

|-
| colspan="12"| '''West African Monsoon (June–July–August–September, JJAS)'''

|-
| Precipitation

| 5.14 [3.62 to 7.18]

| 0.16 [–0.19 to +0.4]

| 0.14 [–0.22 to +0.56]

| 0.24 [–0.14 to +0.72]

| 0.3 [–0.1 to +0.85]

| 0.38 [–0.12 to +1.24]

| 0.06 [–0.25 to +0.52]

| 0.1 [–0.25 to +0.57]

| 0.25 [–0.32 to +0.91]

| 0.38 [–0.49 to +1.14]

| 0.49 [–0.55 to +1.56]

|-
| Runoff

| 1.43 [0.34 to 2.57]

| 0.06 [–0.07 to +0.22]

| 0.05 [–0.18 to +0.27]

| 0.14 [–0.13 to +0.54]

| 0.2 [–0.05 to +0.7]

| 0.24 [–0.1 to +0.8]

| –0.01 [–0.2 to +0.21]

| 0.03 [–0.25 to +0.35]

| 0.1 [–0.25 to +0.51]

| 0.25 [–0.28 to +0.85]

| 0.3 [–0.33 to +0.93]

|-
| P–E

| 2.41 [1.05 to 4.07]

| 0.08 [–0.2 to +0.35]

| 0.1 [–0.2 to +0.4]

| 0.2 [–0.11 to +0.63]

| 0.23 [–0.11 to +0.74]

| 0.36 [–0.06 to +1.11]

| –0.01 [–0.27 to +0.35]

| 0.07 [–0.2 to +0.44]

| 0.18 [–0.21 to +0.6]

| 0.28 [–0.38 to +0.95]

| 0.46 [–0.44 to +1.4]

|}

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[[File:3be7e22b5339e28a893382bf3347666c IPCC_AR6_WGI_Figure_8_22.png]]

'''Figure 8.22 |''' '''Projected regional monsoons precipitation changes.''' Percentage change in projected seasonal mean precipitation over regional monsoon domains (as defined in Figure 8.11, [[#8.3.2.4|Section 8.3.2.4]] and Annex V) for near term (2021–2040), mid-term (2041–2060), and long term (2081–2100) periods based on 24 CMIP6 models and three SSP scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5). Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

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<span id="south-and-south-east-asian-monsoon-1"></span>
===== 8.4.2.4.1 South and South East Asian Monsoon =====

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In AR5, South and South East Asian monsoon (SAsiaM) precipitation was projected to increase by the end of the 21st century but with a weakening of the circulation, with ''high agreement'' across the CMIP5 models (Kitoh et al. , 2013; Menon et al. , 2013; Sharmila et al. , 2015; Sooraj et al. , 2015; [[#Kitoh--2017|Kitoh, 2017]] ; Kulkarni et al. , 2020) . Since AR5, most studies have confirmed projected increases in South Asian monsoon precipitation ( ''high confidence'' ), while one high-resolution model (35 km in latitude/longitude) projects monsoon precipitation decreases during the 21st century following the RCP4.5 scenario ( [[#Krishnan--2016|Krishnan et al., 2016]] ).

Over South Asia, the moisture-bearing monsoon low-level jet is projected to shift northward in CMIP3 and CMIP5 models ( [[#Sandeep--2015|Sandeep and Ajayamohan, 2015]] ). Greater warming over the Asian land region compared to the ocean contributes to intensification of the monsoon low-level south-westerly winds and precipitation ( [[#Endo--2018|Endo et al., 2018]] ), even though the combined effect of upper and lower tropospheric warming makes the Asian monsoon circulation response rather complicated. A high resolution model projection, based on the RCP8.5 scenario, indicates that a northward shift of the low-level jet and associated weakening of the large-scale monsoon circulation can induce a large reduction in the genesis of monsoon low pressure systems by the late 21st century ( [[#Sandeep--2018|Sandeep et al., 2018]] ). Experiments with constant forcing indicate that at 1.5°C and 2°C global warming levels, mean precipitation and monsoon extremes are projected to intensify in summer over India and South Asia ( [[#Chevuturi--2018|Chevuturi et al., 2018]] ; D. [[#Lee--2018|]] [[#Lee--2018|Lee et al., 2018]] ) and that a 0.5°C difference would imply a 3% increase of precipitation ( [[#Chevuturi--2018|Chevuturi et al., 2018]] ). CMIP5 models project an increase in short intense active days and decrease in long active days, with no significant change in the number of break spells for India ( [[#Sudeepkumar--2018|Sudeepkumar et al., 2018]] ).

Future monsoon projections from CMIP6 models show an increase of SAsiaM precipitation across all the scenarios and across all the time frames (Figure 8.22) with the maximum increase at the end of the 21st century in SSP5-8.5 (Almazroui et al. , 2020c; Z. Chen et al. , 2020b; Ha et al. , 2020; Wang et al. , 2021) . Table 8.2 confirms that changes in runoff and P–E over SAsiaM region are positive and largest in the higher emissions scenarios considered, as in precipitation. On the other hand, changes in the ensemble mean for all the variables considered in the SSP1-1.9 scenario are negative for both mid and long-term periods (Table 8.2). This is also consistently reflected in the spatial map of future precipitation changes (Figure 8.15). Different near-term projections of the SAsiaM may result given the diversity in the future aerosol emissions pathways and policies for regulating air pollution ( [[#Wilcox--2020|Wilcox et al., 2020]] ). Additionally, near-term projections of SAsiaM precipitation are expected to be constrained by internal variability associated with the PDV (X. [[#Huang--2020|Huang et al., 2020]] a). CMIP6 models also indicate a lengthening of the summer monsoon over India by the end of the 21st century, at least in SSP2-4.5, with considerable inter-model spread in the projected late retreat ( [[#Ha--2020|Ha et al., 2020]] ).

In summary, consistent with AR5, there is ''high confidence'' that SAsiaM precipitation is projected to increase during the 21st century in response to continued global warming across the CMIP6 higher emissions scenarios, mostly in the mid- and long terms.

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<span id="east-asian-monsoon-1"></span>
===== 8.4.2.4.2 East Asian Monsoon =====

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In AR5, the East Asian monsoon (EAsiaM) was projected to intensify in terms of precipitation, with an earlier onset and longer duration of the summer season. Since AR5, there has been improved understanding of future projected changes in the EAsiaM.

CMIP5 projections indicated a possible intensification of the EAsiaM circulation during the 21st century, in addition to precipitation increase, although there is a lack of consensus on changes in the western North Pacific subtropical high, this is an important feature of the EAsiaM circulation ( [[#Kitoh--2017|Kitoh, 2017]] ). Furthermore, the EAsiaM precipitation enhancements in the CMIP5 projections are prominent over the southern part of the Baiu rainband by the late 21st century, with no significant changes in the Meiyu precipitation over central-eastern China ( [[#Horinouchi--2019|Horinouchi et al., 2019]] ). It was also shown that the Baiu precipitation response in CMIP5 projections is accompanied by a southward retreat of the western North Pacific subtropical high and a southward shift of the East Asian subtropical jet ( [[#Horinouchi--2019|Horinouchi et al., 2019]] ). According to the high-resolution MRI-AGCM global warming experiments, future summer precipitation could potentially increase on the southern side and decrease on the northern side of the present-day Baiu location in response to downward-motion tendencies which can offset the ‘wet-gets-wetter’ effect, but is subject to large model uncertainties ( [[#Ose--2019|Ose, 2019]] ). Future projections of land warming over the Eurasian continent ( [[#Endo--2018|Endo et al., 2018]] ) and intensified land – sea thermal contrast (Z. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ; [[#Tian--2019|Tian et al., 2019]] ) can potentially intensify the EAsiaM circulation during the 21st century. However, there are large uncertainties in projected water cycle changes over the region ( [[#Endo--2018|Endo et al., 2018]] ), mostly in the near-term because of uncertainties in future aerosol emissions scenarios ( [[#Wilcox--2020|Wilcox et al., 2020]] ), as well as due to the interplay between internal variability and anthropogenic external forcing ( [[#Wang--2021|Wang et al., 2021]] ).

Inter-hemispheric mass exchange can act as a bridge connecting SH circulation with EAsiaM rainfall, however this inter-hemispheric link is projected to weaken in a future warmer climate as seen from a CCSM4 projection using the RCP8.5 scenario ( [[#Yu--2018|Yu et al., 2018]] ). A comparison of 1.5°C and 2°C global warming levels reveals how a 0.5°C difference could result in precipitation enhancement over large areas of East Asia ( D. Lee et al. , 2018; J. Liu et al. , 2018; Chen et al. , 2019 ), with substantial increases in the frequency and intensity of extremes ( [[#Chevuturi--2018|Chevuturi et al., 2018]] ; D. [[#Li--2019|]] [[#Li--2019|Li et al., 2019]] ). Future monsoon projections from the CMIP6 models show increase of EAsiaM precipitation across all the scenarios (Z. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] b), though with a large model spread mostly on the long-term and in the higher emissions scenarios (Figure 8.22). Considering all the five scenarios used across the report, changes in precipitation, runoff and P–E over the EAsiaM are positive and become larger for highest emissions scenarios and for the long-term mean, except for the mid-term SSP1-1.9 scenario where the changes are close to zero or even negative (Table 8.2). Additionally, CMIP6 models confirm a projected increased length of the EAsiaM season due to early onset and late retreat ( [[#Ha--2020|Ha et al., 2020]] ).

In summary, despite the uncertainties in the monsoon circulation response in CMIP5 and CMIP6 models, there is ''high confidence'' that summer monsoon precipitation over East Asia will increase in the 21st century and ''medium confidence'' that the monsoon season will be longer.

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<span id="west-african-monsoon-1"></span>
===== 8.4.2.4.3 West African Monsoon =====

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The AR5 concluded that projections of West African monsoon (WAfriM) rainfall are highly uncertain in CMIP3 and CMIP5 models, but still suggest a small delay and intensification in late wet season rains. Studies published since AR5 are broadly consistent with this assessment. CMIP6 models agree on statistically significant projected increases in rainfall in eastern-central Sahel and a decrease in the west for the end of the 21st century ( [[#Roehrig--2013|Roehrig et al., 2013]] ; [[#Biasutti--2019|Biasutti, 2019]] ; [[#Monerie--2020|Monerie et al., 2020]] ). However, the magnitude of WAfriM projected precipitation depends on the convective parametrization used ( [[#Hill--2017|Hill et al., 2017]] ), and large uncertainties remain in WAfriM projections because of large inter-model spread, particularly over the western Sahel ( [[#Roehrig--2013|Roehrig et al., 2013]] ; [[#Biasutti--2019|Biasutti, 2019]] ; [[#Monerie--2020|Monerie et al., 2020]] ). CMIP6 models show a general increase of WAfriM precipitation across all future scenarios but with a substantial model spread for the SSP5-8.5 scenario (Figure 8.22). This sensitivity arises from the combined and contrasting influences of anthropogenic greenhouse gas and aerosol forcing that affect WAfriM precipitation (particularly over the Sahel) directly and also indirectly through subtropical North Atlantic SST changes ( [[#Giannini--2019|Giannini and Kaplan, 2019]] ) . The large model spread and associated uncertainties in projected precipitation changes is reflected also in runoff and P–E changes (Table 8.2). Regional climate models (RCMs) ensembles (e.g., [[#Klutse--2018|Klutse et al., 2018]] ) agree with CMIP5 projected rainfall trends but some individual models show rainfall declines (e.g., [[#Sylla--2015|Sylla et al., 2015]] ; [[#Akinsanola--2018|Akinsanola et al., 2018]] ), highlighting the existing large uncertainties in RCMs WAfriM rainfall projections.

Changes in seasonality (Box 8.2) are projected with a later monsoon onset ( ''high confidence'' ) over the Sahel and a late cessation ( ''medium confidence'' ), suggesting a delayed wet season as a regional response to global GHG forcing ( [[#Biasutti--2013|Biasutti, 2013]] ; [[#Dunning--2018|Dunning et al., 2018]] ; [[#Akinsanola--2019|Akinsanola and Zhou, 2019]] ). Rainfall distribution is projected to be highly variable with a decrease in the number of rainy days in the western Sahel, consistent with an increase in consecutive dry days and a reduction in the number of growing season days ( [[#Cook--2012|Cook and Vizy, 2012]] ; [[#Diallo--2016|Diallo et al., 2016]] ). A decrease in the frequency but an increase in the intensity of very wet events is projected to be more pronounced over the Sahel than over Guinean coast, and also under higher emissions scenarios (i.e., RCP8.5; [[#Sylla--2015|Sylla et al., 2015]] ; [[#Akinsanola--2018|Akinsanola et al., 2018]] ).

In summary, post-AR5 studies and newly available CMIP6 results indicate projected rainfall increases in the eastern-central WAfriM region but decreases in the west ( ''high confidence'' ), with a delayed wet season ( ''medium confidence'' ). Overall, WAfriM summer precipitation is projected to increase during the 21st century but with larger uncertainty noted under high-emissions scenarios ( ''medium co'' ''nfidence'' ).

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<span id="north-american-monsoon-1"></span>
===== 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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===== 8.4.2.4.5 South American Monsoon =====

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The AR5 reported ''medium confidence'' that the South American monsoon (SAmerM) overall precipitation will remain unchanged, and ''medium confidence'' in projections of extreme precipitation. The AR5 also stated ''high confidence'' in the spatial expansion of the SAmerM, resulting from increased temperature and humidity.

Since AR5, some studies indicate that the SAmerM would experience changes in its seasonal cycle, with delayed monsoon onsets under increasing GHG emissions associated to different RCPs (Fu et al. , 2013; Reboita et al. , 2014; Boisier et al. , 2015; Pascale et al. , 2016; Seth et al. , 2019; [[#Sena--2020|Sena and Magnusdottir, 2020]] ) . In contrast, other studies indicate projected earlier onsets and delayed retreats of the SAmerM under the RCP8.5 scenario based on six CMIP5 models ( [[#Jones--2013|Jones and Carvalho, 2013]] ). These differences have been linked to the methodology used to determine monsoon timing, and sensitivity to the monsoon domain considered ( [[#8.3.2.4.5|Section 8.3.2.4.5]] ; [[#Correa--2021|Correa et al., 2021]] ). Recent studies provide further evidence for the projection of delayed SAmerM onsets by the late 21st century ( [[#Sena--2020|Sena and Magnusdottir, 2020]] ). An analysis of six CMIP6 models under the SSP5-8.5 scenario confirm the projections of delayed SAmerM onsets by the end of the 21st century ( [[#Moon--2020|Moon and Ha, 2020]] ). In addition, projected changes in the intensity and length of the SAmerM season have been found to be model-dependent ( [[#Pascale--2019|Pascale et al., 2019]] ). The analysis of CMIP5 projections of total monsoon rainfall indicate mixed signals in the Amazon and SAmerM regions ( [[#Jones--2013|Jones and Carvalho, 2013]] ; [[#Marengo--2014|Marengo et al., 2014]] ), with some studies suggesting increased summer precipitation in the core SAmerM region ( [[#Kitoh--2013|Kitoh et al., 2013]] ; [[#Seth--2013|Seth et al., 2013]] ). Dynamical downscaling of CMIP5 projections under the RCP4.5 and RCP8.5 scenarios with the Eta RCM suggests reductions of austral summer precipitation over the SAmerM region throughout the 21st century ( [[#Chou--2014|Chou et al., 2014]] ). Further analysis using 15 different CMIP6 models for the SSP2-4.5 scenario suggest reductions in total SAmerM rainfall ( [[#Wang--2020|]] [[#Wang--2020|]] [[#Wang--2020|]] [[#Wang--2020|B. Wang et al., 2020]] ). However, other analyses of CMIP6 projections under different SSP scenarios do not report clear changes in the SAmerM precipitation throughout the 21st century (Figure 8.22; Z. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] b; [[#Jin--2020|Jin et al., 2020]] ). Similar uncertainties for all the SSP scenarios used across the report are found for other water cycle variables, including runoff and P–E (Table 8.2). Furthermore, there is disagreement in projected extreme precipitation in the region, with some CMIP5-based studies suggest reductions ( [[#Marengo--2014|Marengo et al., 2014]] ), while others indicate increases based on CMIP5 and CMIP6 models ( [[#Kitoh--2013|Kitoh et al., 2013]] ; [[#Sena--2020|Sena and Magnusdottir, 2020]] ).

In summary, there is ''high confidence'' that the SAmerM will experience delayed onsets in association with increases in GHG. However, there is ''low agreement'' on the projected changes in terms of total precipitation of the South American summer monsoon season.

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===== 8.4.2.4.6 Australian and Maritime Continent Monsoon =====

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The AR5 concluded that projected changes in Australian and Maritime Continent monsoon (AusMCM) rainfall and seasonality are uncertain in the CMIP5 models, with some projecting increases and others projecting decreases for the range of emissions scenarios. Models that perform better at simulating present day regional climate project little change or an increase in Australian monsoon rainfall ( [[#Jourdain--2013|Jourdain et al., 2013]] ; CSIRO and BoM, 2015; [[#Brown--2016b|Brown et al., 2016b]] ). CMIP6 models project increased AusMCM precipitation in the 21st century but with a more robust signal in SSP2-4.5 and SSP5-8.5 rather than in lower emissions scenarios (Figure 8.22). A reduced range of CMIP6 rainfall projections but continued disagreement on the sign of change is reported over Australia ( [[#Narsey--2020|Narsey et al., 2020]] ).

The northern and eastern parts of the Maritime Continent have projected increases in rainfall in CMIP5 models ( [[#Siew--2014|Siew et al., 2014]] ), while there are projected decreases over Java, Sulawesi and southern parts of Borneo and Sumatra. Rainfall changes are correlated with the extent of warming in the western tropical Pacific in CMIP5 models ( [[#Brown--2016b|Brown et al., 2016b]] ) but inter-model differences are also related to modelled large-scale zonal mean precipitation response in both CMIP5 and CMIP6 model ensembles ( [[#Narsey--2020|Narsey et al., 2020]] ). Decomposition of projected rainfall changes indicates that the largest source of model uncertainty is associated with shifts in the spatial pattern of convection ( [[#Chadwick--2013|Chadwick et al., 2013]] ; [[#Brown--2016b|Brown et al., 2016b]] ). Uncertainties in capturing the spatial and temporal features of the Maritime Continent monsoon depend also on the horizontal resolution of coupled climate models (e.g., [[#Jourdain--2013|Jourdain et al., 2013]] ).

The role of anthropogenic aerosol forcing in future projections of the Australian monsoon has been investigated for CMIP5 models ( [[#Dey--2019a|Dey et al., 2019a]] ); decreases in anthropogenic aerosol concentrations over the 21st century are expected to produce relatively greater warming in the NH than SH, favouring a northward shift of the tropical rain belt (e.g., [[#Rotstayn--2015|Rotstayn et al., 2015]] ).

There are some clear projected changes in the rainfall variability and extremes of the Australian monsoon. Rainfall variability in the Australian monsoon domain increases on time scales from daily to decadal in CMIP5 models ( [[#Brown--2017|Brown et al., 2017]] ), indicating either more intense wet days or more dry days or both. There is also a projected increase in the intensity of extreme rainfall but a reduction in the frequency of heavy rainfall days for the Australian monsoon (Dey et al. , 2019a) . This is consistent with [[#Moise--2020|Moise et al. (2020)]] , who found an increase in Australian monsoon active phase or ‘burst’ rainfall intensity but a reduction in the number of burst days and events.

H. [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al. (2013)]] examined changes in Australian monsoon onset and duration in CMIP3 models and found model agreement on a delay in onset and shortened duration to the north of Australia, but less agreement over the interior of the continent. An updated study of CMIP5 models found similar mean changes with delayed onset and shortened duration, but substantial model disagreement (H. [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ).

In summary, CMIP6 projections show an increase of AusMCM precipitation across all emissions scenarios. There is strong model agreement on an increase in monsoon precipitation over the Maritime Continent while there is ''low agreement'' on the direction of change over northern Australia. There is a projected increase in rainfall variability over northern Australia, with increased intensity of rainfall during the active or ‘burst’ phase ( ''medium con'' ''fidence'' ).

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