Motivation

Abstract

The oceans have the capacity to accumulate heat in the surface layer. This energy is transferred to the atmosphere leading to changes in atmospheric circulation and consequent impacts in remote and nearby locations. Thus, the SST becomes a key variable to detect ocean sources of seasonal predictability from interannual to multidecadal time scales (e.g., Mohino et al. 2011a; Skinner et al. 2012; Rodríguez-Fonseca et al. 2015). Global spatial patterns of SST anomalies (SSTA) are organized in the so-called modes of variability, which are particular of each ocean basin and determine the principal directions in which the variability takes place. In this framework, the anomalous monsoonal rainfall is strongly linked to SST variability. Regarding the WAM, leading patterns of SSTA play a key role when tackling rainfall variability (Folland et al. 1986; Palmer 1986; Xue and Shukla 1998; Fontaine et al. 1998; Rodríguez-Fonseca et al. 2011, 2015; Losada et al. 2010a, b; Rowell 2013; Vera et al. 2013), becoming a key factor in the severe drought experienced in the Sahel from the early 1970s (Lu and Delworth 2005; Mohino et al. 2011a; Rodríguez-Fonseca et al. 2015). The West African Sahel is a narrow belt located around 15^oN between the Sahara desert to the north and the woody Sudanese savanna to the south, crossing Africa from the Atlantic Ocean to the Red Sea. Every year, the Sahel alternates a dry season, with total absence of precipitation, and a very wet season coinciding with the boreal summer months. Thus, the water resources available during the long dry season depend almost entirely on the intensity of precipitation during the rainy season.