Water and Environmental Change in the Sahel, Africa

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  St...

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 l...

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 (JI...