Impacts of the Tropical Pacific/Indian Oceans on the Seasonal Cycle of the West African Monsoon

B Rodríguez-Fonseca,C R Mechoso,E Mohino,F Chauvin,P Ruti,S Gervois

doi:10.1175/2011jcli3988.1

Abstract

Abstract The current consensus is that drought has developed in the Sahel during the second half of the twentieth century as a result of remote effects of oceanic anomalies amplified by local land–atmosphere interactions. This paper focuses on the impacts of oceanic anomalies upon West African climate and specifically aims to identify those from SST anomalies in the Pacific/Indian Oceans during spring and summer seasons, when they were significant. Idealized sensitivity experiments are performed with four atmospheric general circulation models (AGCMs). The prescribed SST patterns used in the AGCMs are based on the leading mode of covariability between SST anomalies over the Pacific/Indian Oceans and summer rainfall over West Africa. The results show that such oceanic anomalies in the Pacific/Indian Ocean lead to a northward shift of an anomalous dry belt from the Gulf of Guinea to the Sahel as the season advances. In the Sahel, the magnitude of rainfall anomalies is comparable to that obtained by other authors using SST anomalies confined to the proximity of the Atlantic Ocean. The mechanism connecting the Pacific/Indian SST anomalies with West African rainfall has a strong seasonal cycle. In spring (May and June), anomalous subsidence develops over both the Maritime Continent and the equatorial Atlantic in response to the enhanced equatorial heating. Precipitation increases over continental West Africa in association with stronger zonal convergence of moisture. In addition, precipitation decreases over the Gulf of Guinea. During the monsoon peak (July and August), the SST anomalies move westward over the equatorial Pacific and the two regions where subsidence occurred earlier in the seasons merge over West Africa. The monsoon weakens and rainfall decreases over the Sahel, especially in August.

Highlights

The Sahel is the region in West Africa ranging from 108 to 188N
It captures the latitudinal shift of the intertropical convergence zone (ITCZ) that occurs from late spring to summer over the region (Fig. 2)
Previous works have established that sea surface temperature (SST) anomalies in the Atlantic, Pacific, and Indian basins can all influence Sahel rainfall at different time scales

Summary

Introduction

A prolonged drought developed in the region during the last three decades of the twentieth century. This drought, which was an outstanding climate anomaly event in recent history (Dai et al 2004; Trenberth et al 2007), has been attributed to anomalies in surface conditions leading to desertification through positive land–atmosphere feedback processes (Charney 1975). Recent studies have identified sea surface temperature (SST) anomalies as the most important factor in influencing variability in Sahel rainfall during the late twentieth century (Folland et al 1986; Palmer 1986; Rowell et al 1992, 1995; Giannini et al 2003; Mohino et al 2011a).

Objectives

Results

Conclusion