Interdecadal Changes in Ocean Teleconnections with the Sahel

Abstract

The West African Sahel is the transition region between the wet equatorial zone and the dry Sahara desert. Year-to-year, the Sahel alternates an extremely dry season with a strong rainfall regime from July to September. The water resources available during the long dry season depend almost entirely on the intensity of rainfall during the rainy season, also known as the West African Monsoon (WAM). The WAM presents a marked variability at interannual time scales (e.g., Sultan et al. 2003; Sultan and Janicot 2003), being a major topic of study. The severe drought experienced in the Sahel from the 1970s to the 1990s, and the apparent recovery trend in the recent period, also reveals the pronounced interdecadal variability of the WAM (Hulme et al. 2001; Nicholson 2005; Lebel and Ali 2009). The WAM system is primarily determined by the northward shift of the Inter-Tropical convergence Zone (ITCZ) along with a thermal gradient between the Sahara desert to the north and the Guinean Gulf to the south (e.g., Sultan and Janicot 2000; Chiang et al. 2000, 2002; Kushnir et al. 2003; Nicholson 2009). Thus, although land surface processes and internal variability cannot be neglected, the oceanic forcing plays the leading role in the predictability of the WAM (e.g., Folland 1986; Palmer 1986; Fontaine et al. 1998; Skinner et al. 2012; Rodriguez-Fonseca et al. 2015). On the one hand it is presented as the main driver of the decadal variability (e.g., Janicot et al. 2001; Biasutti et al. 2008; Mohino et al. 2011a; Martin et al. 2013). On the other hand, several observational studies address the interannual oceanic teleconnections from the tropical Pacific (Janicot et al. 2001; Rowell 2001; Joly and Voldoire 2009), the tropical Atlantic (Giannini et al. 2003; Polo et al. 2008; Joly and Voldoire 2009; Nnamchi and Li 2011) and the Mediterranean (Rowell 2003; Gaetani et al. 2010; Fontaine et al. 2011a)...