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

===== 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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