Role of the Gulf of Guinea in the inter‐annual variability of the West African monsoon: what do we learn from CMIP3 coupled simulations?

Abstract

AbstractThe surface ocean explains a significant part of the inter‐annual variability of the West African monsoon (WAM). The present paper explores the role of Gulf of Guinea sea surface temperatures (SST): how is the ocean–atmosphere observed relationship reproduced by state‐of‐the‐art coupled models? The ‘Atlantic Niño’ is the main mode of inter‐annual variability in the Gulf of Guinea. SST anomalies are maximum in June–July, and are associated with a convective anomaly in the marine Intertropical Convergence Zone (ITCZ), which spreads over the Guinean coast. In most of the studied CMIP3 simulations, the inter‐annual variability of SST is very weak in the Gulf of Guinea, especially along the Guinean Coast. As a consequence, the influence on the monsoon rainfall over the African continent is hardly reproduced. Interestingly, many models exhibit a dipolar response of the marine ITCZ to the Atlantic Niño. In the observations and reanalyses, the absence of any evident shift in the position of the monsoon rainbelt is associated with a collapse of the correlations between Gulf of Guinea SST and Sahel rainfall at the end of the twentieth century. It is suggested that this may be due to the counteracting effects of the Pacific and Atlantic basins over the last decades. In CMIP3 simulations, the Atlantic Niño is often correlated with the El Niño Southern Oscillation (ENSO). However, only one simulation catches the observed evolution of the Pacific–Atlantic relationship at the end of the century. Copyright © 2009 Royal Meteorological Society