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Rainfall trends in the African Sahel: Characteristics, processes, and causes

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Abstract Sahel rainfall is dynamically linked to the global Hadley cell and to the regional monsoon circulation. It is therefore susceptible to forcings from remote oceans and regional land alike. Warming of the oceans enhances the stability of the tropical atmosphere and weakens deep ascent in the Hadley circulation. Warming of the Sahara and of the nearby oceans changes the structure and position of the regional shallow circulation and allows more of the intense convective systems that determine seasonal rain accumulation. These processes can explain the observed interannual to multidecadal variability. Sea surface temperature anomalies were the dominant forcing of the drought of the 1970s and 1980s. In most recent decades, seasonal rainfall amounts have partially recovered, but rainy season characteristics have changed: rainfall is more intense and intermittent and wetting is concentrated in the late rainy season and away from the west coast. Similar subseasonal and subregional differences in rainfall trends characterize the simulated response to increased greenhouse gases, suggesting an anthropogenic influence. While uncertainty in future projections remains, confidence in them is encouraged by the recognition that seasonal mean rainfall depends on large‐scale drivers of atmospheric circulations that are well resolved by current climate models. Nevertheless, observational and modeling efforts are needed to provide more refined projections of rainfall changes, expanding beyond total accumulation to metrics of intraseasonal characteristics and risk of extreme events, and coordination between climate scientists and stakeholders is needed to generate relevant information that is useful even under deep uncertainty. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change
The main features of the rainfall climatology in West Africa. (a) Mean May–October rainfall (color shading); location of the African Easterly Jet (AEJ, magenta contour) as indicated by the 9 m/s contour of the easterly wind at 600 hPa; maximum African Easterly Waves (AEW, orange color; contour retraced from Figure 5a in Thorncroft & Hodges, ) as indicated by the a track density scaled to number density per unit area (106 km2) per season (MJJASO) greater than 6; and near surface (925 hPa) wind (streamlines; ERA40, Uppala et al., ). All fields are for the 1979–1998 period, for consistency with Thorncroft and Hodges (). (b) The seasonal cycle of sector mean (20°W‐30°E) surface air temperature (warm shaded colors; CPC Monthly Global Surface Air Temperature, Fan & van den Dool, ), precipitation (contours, same colors as in (a); GPCP Huffman et al., ), and the intertropical discontinuity (ITD, indicated in blue by the zero contour of the 925 hPa meridional wind from ERA40 reanalysis). Note how the advance of the ITD is connected with the warming of the Sahara and how the rain band stays to the south of the ITD
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Evolution of (a) the mean number of rainy days and (b) the mean intensity of rainy days (mm/day) over the whole Sahel and subdomains (West, Central, and East Sahel). Error bars (resp. shaded area) delineate 80% confidence intervals for annual values (resp. 11 year rolling mean) (Reprinted with permission from Panthou et al. (). Copyright 2018 the Creative Commons Attribution 3.0 license)
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Top multimodel averaged, annual mean rainfall anomalies (in mm/day) between future and past (2031–2060 in the RCP8.5 scenario and 1960–1990 in the Historical scenario), averaged over eight climate models. Field values are the anomalies in the raw model output (regridded to 1° resolution). Filled circles are anomalies in the bias‐corrected output. The boxes indicate the sites of West Sahel and Central Sahel. Bottom: Smoothed daily anomalies in rainfall (mm/day, green) and temperature (°C, red) averaged over the sites of (left) West and (right) Central Sahel. The solid line is the multi‐model mean, the shading represents 1 SD scatter (Reprinted with permission from Guan, Sultan, Biasutti, Baron, and Lobell (). Copyright 2017 Elsevier)
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(a) Summer‐mean (July to September) standardized anomalies in rainfall totals (black solid line) and number of rainy days (red dashed line) averaged over the region 10°N–20°N and 20°W–30°E (from the University of East Anglia Climate Research Unit gridded data set, TS4.1). (b) Linear trend in summer‐mean African rainfall between 1940 and 1985 (brown and green hues) and in annual‐mean sea surface temperatures over the same period (blue and orange hues) (Reprinted with permission from Biasutti (). Copyright 2011 Nature)
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(a) Easterly waves can be observed in satellite imagery because of their associated convection. They appear as circular or banded clouds in satellite images. The source of this material is the COMET (textregistered) website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). Copyright 1997–2017 University Corporation for Atmospheric Research. (b) Composite anomalies for 3‐ to 5‐day AEWs for the ERA‐I and TRMM observations. Vectors are 700 hPa wind anomalies (m/s). Red and blue contours are positive and negative 700 hPa geopotential height anomalies (m) drawn every 2 m. Black contours show the 0 m anomalies. Rainfall anomalies (mm/day) are shaded. Only anomalies above |1| mm/day that are significant at the 95% level according to a two‐tailed Student's t test are shown. (Reprinted with permission from Crétat, Vizy, and Cook (), Copyright 2014 Springer Nature). (c) Hovmüller (longitude–time) diagram of 850 hPa unfiltered meridional wind (v) and cold cloud streaks axes (black lines) averaged between 5°N and 15°N for the period July 1–August 15, 2001. Colored shading shows v at 2 m/s intervals; northerly winds are gold to red and southerly winds are green to blue. White lines are objectively computed positive meridional wind streaks used to identify AEWs. Cold cloud streaks have Tb < 233 K. The blue rectangle identifies the convective systems and AEW from which Tropical Storm Alberto formed (Reprinted with permission from Laing, Carbone, Levizzani, and Tuttle (). Copyright 2008 the Royal Meteorological Society)
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Schematic cross‐section of the atmosphere between 10°W and 10°E in July. Shown are the positions of the ITD, upper‐level jet streams (African easterly jet [AEJ], tropical easterly jet [TEJ]), the monsoon layer (ML) (as defined by westerly, that is, positive, zonal winds), streamlines, clouds, the freezing level (0°C isotherm), isentropes (θ), minimum (Tn), maximum (Tx) and mean (T) and dew point temperatures (Td), atmospheric pressures (p), and mean monthly rainfall totals (RR) (Reprinted with permission from Parker and Diop‐Kane (). Copyright 2017 Wiley)
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Graphic courtesy of John Chiang. The clouds in the left of the schematic represent the dominant convection center in the tropics, coincident with the warm pool. The Sahel is represented as a lesser area of convection. The two regions are connected via the free troposphere, where temperature anomalies are quickly homogenized by transport
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(a) Schematic that describes how inter‐hemispheric extratropical thermal forcing is balanced by the adjustment of the Hadley circulation. Suppose the northern (southern) extratropics are warmed (cooled). The faded (dark) gray contours in the upper panel indicate the Hadley circulation in the reference (perturbed) state. The Hadley circulation transports moisture equatorward following its lower branch (blue arrow) and transports energy poleward following its upper branch (red arrow). (Reprinted with permission from Kang, Shin, and Xie (). Copyright 2018 Nature; under the creative commons license http://creativecommons.org/licenses/by/4.0/). (b) Temperature contrast between Northern and Southern hemisphere extratropics (poleward of 24°N and 24°S) based on instrumental data (black), and average daily rainfall over the Sahel (12°−18°N, 20°W–35°E) during June–October based on land station data (blue). All temperatures and temperature contrasts are given as anomalies relative to the 1960–1991 AD mean (Reprinted from Schneider, Bischoff, and Haug (). Copyright 2014 Springer‐Nature)
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