Water and Environmental Change in the Sahel, Africa

Given the extreme vulnerability of the Sahel to rainfall variability (discussed in the overview ), accurate future precipitation projection is crucial for the  Sahelian people to develop adaptation and mitigation strategies. As can be seen from the previous blogs, anthropogenic activities have large impacts on Sahel's rainfall variability. However, the uncertainties in their projection simulations are substantial. Results under the framework of phase 3 and 5 of the Coupled Model Intercomparison Project (CMIP 3-5) vary from model to model, with s ome models projecting an increase in precipitation by the end of the 21st century, while others projecting a decrease in 2100 over the Sahel ( Caminade & Terray, 2010 ; Monerie et al., 2017 ;  Yan et al. 2019 ).  Monerie et al. (2020a)  examine the reasons behind the inter-model difference of Sahel's rainfall projections using CMIP 5 and the newly developed CMIP 6. They classify the causes of change of precipitation into three terms

Greenhouse gases (GHGs) emissions-induced warming not only influences Sahel's rainfall variability indirectly through  sea surface temperature, as introduced in the previous blog   but also affects Sahel's rainfall variability directly.  An increasing number of studies links the partial recovery of Sahel's rainfall and the intensification of extreme precipitation in the Sahel with GHGs-induced warming (such as  Dong & Sutton, 2015 ;  Donat et al., 2016 ;  Giannini & Kaplan, 2019 ).  As mentioned in the last blog , aerosol reduction in the Northern Hemisphere removes the cooling effect and shift the tropical rain belt northward. This impact is also strengthened by the GHGs-induced warming over the Northern Hemisphere ( Giannini & Kaplan, 2018 ) . The warming over the African continent also increases Sahel's precipitation. B ecause  GHGs can absorb the longwave radiation reflected by the Earth's surface to space, trap the heat in the atmosphere and then re

The anthropogenic and volcanic aerosols emissions also play significant roles in Sahel's rainfall variability.  As  mentioned in the overview , the Intertropical Convergence Zone (ITCZ) location greatly controls Sahel's seasonal rainfall.  Hwang et al. (2013)  found that sulphate aerosol-induced cooling in the Northern Hemisphere during the late 20th century shifted the tropical rain belt southward  and contributed to the profound 1970s-1980s Sahel drought. As shown in Figure 1, results from different models all show that there was drying on the northern side of the tropical rain belt and wetting on the southern side in the late 20th century.  Figure 1. Time series of zonal mean precipitation anomaly relative to the 20th-century mean based on a) the Global Historical Climatology Network (GHCN) gridded products, b) the 20th-century reanalysis project (20CR), and c) the ensemble mean of the 20th-century climate simulation from 14 GCMs in CMIP3 and 12 GCMs in CMIP5 ( Hwang et al.,

Since the 1990s, with the refutation of vegetation-rainfall feedbacks as the primary driver of Sahel's rainfall variability and model developments, a consensus has been reached that  the changes in Sea Surface Temperatures (SSTs) are the main reason behind Sahel's rainfall variability, especially on interannual and decadal timescales and supported by l arge amounts of modelling studies  (such as  Giannini, Saravanan & Chang, 2003 ; Cook, 2008 ; Rodríguez-Fonseca et al., 2011 ; Nicholson, 2013 ; Hill et al., 2017 ).  Rodríguez-Fonseca et al.(2015)  also provide a very nice and comprehensive review of the relationships between Sahel's precipitation and SST anomalies. The following sections will discuss the influence of different ocean basins (the tropical Atlantic Ocean, Mediterranean Sea, Pacific Ocean and the Indian Ocean) on Sahel's rainfall and at two timescales, interannual and decadal.  Interannual time scales a. The tropical Atlantic Ocean  Studies have shown t

This blog will focus on the interactions between vegetation and rainfall, also known as the  vegetation-rainfall feedbacks. At first, it was widely believed that the profound 1970s-1980s Sahel drought was mainly driven by positive vegetation-rainfall feedback via an albedo-based mechanism.  Charney et al.(1975) proposed that  overgrazing and agricultural expansion in the 1950s-1960s rainy period caused irreversible desertification in the Sahel,  which led to an increase in surface albedo (surface albedo of land without vegetation is around 20% higher than the land covered by vegetation).  As a result,  more incoming solar radiation would be reflected back to space, and less heating would be received by the ground surface, which would reduce the convection, suppressing the formation of rainfall-generating clouds.  A decrease in precipitation would, in turn, exacerbate desertification and lead to more rainfall suppression, creating a positive feedback loop .  Although this mechanism is

Two in three people in the Sahel region live from rainfed agriculture and livestock ( Solidarités International, 2020 ), making it  extremely vulnerable to rainfall  variability.  M ost of Sahel's annual rainfall is confined to the boreal summer  due to the northward excursion of the intertropical convergence zone (ITCZ) that takes the form of the West Africa Monsoon in the west and a northward shift of continental convection in the east.  The  mean annual rainfall varies from 100-200 mm in the north to 600-700 mm in the south . Although there is a strong north-south gradient, the rainfall pattern generally shows a west-east uniformity, as shown in Figure 1  ( Nicholson , 2013 ;   Hill et al., 2017 ) .  Figure 1. 1979-2010 average annual precipitation (mm) in Africa ( Siebert, 2014) Figure  2 shows Sahel annual precipitation anomalies from the average in 1901-2017 by the  Joint Institute for the Study of the Atmosphere and Ocean (JISAO) at the University of Washington . It can be s