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

===== 8.3.2.4.3 West African Monsoon =====

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Since AR5, there has been improved understanding of the West African monsoon (WAfriM) response to natural and anthropogenic forcing. On paleoclimate time scales, enhanced summer insolation in the Northern Hemisphere (NH) intensified the WAfriM precipitation during the early-to-mid Holocene ( ''high confidence'' ), as seen in rainfall proxy records and climate model simulations (Masson-Delmotte et al. , 2013; Mohtadi et al. , 2016; Braconnot et al. , 2019). Despite improvements in model simulations of the present-day monsoons, CMIP5 and CMIP6 models underestimate mid-Holocene changes in the amount and spatial extent of the WAfriM precipitation ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.2|Section 3.3.3.2]] ; [[#Brierley--2020|Brierley et al., 2020]] ).

During the recent past, long-term rain gauge observations display substantial variability in the WAfriM precipitation over the 20th century ( [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.1|Section 10.4.2.1]] ). The WAfriM experienced the wettest decade of the 20th century during the 1950s and early 1960s ( ''high confidence'' ), over much of the western and central Sahel region, followed abruptly by the driest years during 1970 – 1989 ( [[#Ali--2009|Ali and Lebel, 2009]] ; [[#Nicholson--2013|Nicholson, 2013]] ; [[#Descroix--2015|Descroix et al., 2015]] ). The percentage deficit in the annual rainfall during 1970–1989, relative to the long-term mean, ranged from 60% in the north of Sahel to 25 – 30% in the south ( [[#Le%20Barbé--2002|Le Barbé et al., 2002]] ; [[#Lebel--2003|Lebel et al., 2003]] ). The long decline in annual rainfall is related to a decrease of rain occurrence over the Sahel ( [[#Le%20Barbé--1997|Le Barbé and Lebel, 1997]] ; [[#Frappart--2009|Frappart et al., 2009]] ; [[#Bodian--2016|Bodian et al., 2016]] ) and the Soudano-Guinean sub-region of West Africa ( [[#Le%20Barbé--2002|Le Barbé et al., 2002]] ), even though the interannual variability pattern is more complex ( [[#Balme--2006|Balme et al., 2006]] ). Decrease of rainfall occurrences resulted from decreases in large convective events in the core of the rainy season ( [[#Bell--2006|Bell et al., 2006]] ), that modulate interannual variability of the WAfriM ( [[#Panthou--2018|Panthou et al., 2018]] ).

Wetter conditions of the WAfriM prevailed later from the mid-to-late 1990s, although the positive trend in precipitation started since the late 1980s (see also [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.1|Section 10.4.2.1]] ) over the Sahel ( ''high confidence'' ) and in the Guinean coastal region ( ''medium confidence'' ), indicating the geographical variation in the wetting recovery (Descroix et al. , 2015; Sanogo et al. , 2015; Bodian et al. , 2016; Nicholson et al. , 2018). While the interannual and decadal variability of annual rainfall is not homogeneous over the entire Sahel, the rainfall recovery was stronger in the east than in the west of the region ( [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.1|Section 10.4.2.1]] ; [[#Nicholson--2018|Nicholson et al., 2018]] ). A shift in the seasonality of the Sahelian rainfall, including delayed cessation has also been reported ( [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.1|Section 10.4.2.1]] ; [[#Nicholson--2013|Nicholson, 2013]] ; [[#Dunning--2018|Dunning et al., 2018]] ).

In the Sahel region, the emergence of this new rainfall regime is reflected in increased number of heavy and extreme events, compared to the 1970s – 1980s, still not exceeding the values registered in the 1950s to 1960s ( [[#Descroix--2013|Descroix et al., 2013]] , 2015; [[#Panthou--2014|Panthou et al., 2014]] , 2018; [[#Sanogo--2015|Sanogo et al., 2015]] ), and in higher interannual variability (W. [[#Zhang--2017|Zhang et al., 2017]] b; [[#Akinsanola--2020|Akinsanola and Zhou, 2020]] ) associated with SST variations in the tropical Atlantic, Pacific and Mediterranean Sea ( [[#Rodríguez-Fonseca--2015|Rodríguez-Fonseca et al., 2015]] ; [[#Diakhaté--2019|Diakhaté et al., 2019]] ). Increased frequency of extreme rainfall events impacts high flow occurrences of the large Sahelian rivers as well as small to meso-scale catchments ( [[#Wilcox--2018|Wilcox et al., 2018]] ). Overall, extreme intense precipitation events are more frequent in the Sahel since the beginning of the 21st century (Giannini et al. , 2013; Panthou et al. , 2014, 2018; Sanogo et al. , 2015; Taylor et al. , 2017). Intensification of mesoscale convective systems associated with extreme rainfall in the WAfriM is favoured by enhancement of meridional temperature gradient by the warming of the Sahara desert ( [[#Taylor--2017|Taylor et al., 2017]] ) at a pace that is two to four times greater than that of the tropical-mean temperature (K.H. [[#Cook--2015|]] [[#Cook--2015|]] [[#Cook--2015|Cook et al., 2015]] ; [[#Vizy--2017|Vizy et al., 2017]] ). Periods of monsoon-breaks and the persistence of low rainfall events are still prominent, particularly after the onset, thus exposing West Africa simultaneously to the potential impacts of dry spells (W. [[#Zhang--2017|Zhang et al., 2017]] b) and also extreme localized rains and floods ( [[#Engel--2017|Engel et al., 2017]] ; [[#Lafore--2017|Lafore et al., 2017]] ). Occurrence of extreme events is compounded by land use and land cover changes leading to increased runoff ( [[#Bamba--2015|Bamba et al., 2015]] ; [[#Descroix--2018|Descroix et al., 2018]] ).

The Sahel drought from the 1970s until the early 1990s was related to anthropogenic emissions of sulphate aerosols in the Atlantic, which led to an inter-hemispheric pattern of SST anomalies and associated regional precipitation changes (Section 6.3.3.2 and Box 8.1). Also the combined effects of anthropogenic aerosols and GHG forcing appear to have contributed to the late twentieth century drying of the Sahel through their effect on SST, by cooling the North Atlantic and warming the tropical oceans ( [[#Giannini--2019|Giannini and Kaplan, 2019]] ; [[#Hirasawa--2020|Hirasawa et al., 2020]] ). Subsequent aerosol removal led to SST warming of the North Atlantic, shifting the ITCZ further northward and strengthening the WAfriM ( [[#Giannini--2019|Giannini and Kaplan, 2019]] ). The recent recovery has been ascribed to prevailing positive SST anomalies in the tropical North Atlantic potentially associated with a positive phase of the Atlantic Multi-decadal Oscillation ( [[#Diatta--2014|Diatta and Fink, 2014]] ; [[#Rodríguez-Fonseca--2015|Rodríguez-Fonseca et al., 2015]] ). The Sahel rainfall recovery has also been attributed to higher levels of GHG in the atmosphere and increases in atmospheric temperature ( [[#Dong--2015|Dong and Sutton, 2015]] ).

In summary, most regions of West Africa experienced a wet period in the mid-20th century followed by a very dry period in the 1970s and 1980s that is attributed to aerosol cooling of the NH ( ''high confidence'' ). Recent estimates provide evidence of a WAfriM recovery from the mid-to-late 1990s, with more intense extreme events partly due to the combined effects of increasing GHG and decreasing anthropogenic aerosols over Europe and North America ( ''high confidence'' ). On paleoclimate time scales, there is ''high confidence'' that the WAfriM strengthened during the early-to-mid Holocene in response to orbitally-forced enhancement of summer warming in the NH.

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