The relationship of rainfall variability in West Central Africa to sea-surface temperature fluctuations

This study examines the interannual variability of rainfall in western equatorial Africa and its links to sea-surface temperatures (SSTs). Five geographical regions within the latitudes 10 degrees N-5 degrees S are delineated for the analysis. The links to SSTs in the tropical Atlantic, Pacific and Indian Oceans are examined via seasonal composites of wet and dry years and via linear correlations. The results show that interannual variability is extremely complex in this region and that several factors govern it. The most important include SST anomalies along the Benguela Coast, a general warming or cooling of the tropical oceans, Atlantic SSTs specifically, the contrast between the Atlantic and Indian Oceans, and the Pacific El Nino-Southern Oscillation (ENSO). These factors differ seasonally. In much of the region, rainfall variability is linked to the Pacific El Nino and the western Indian Ocean early in the year, but to the Atlantic during the boreal summer months. The Indian Ocean again becomes important in late summer/early fall. The role of the Atlantic appears to be the modulation of the north-south excursion of the Intertropical convergence Zone (ITCZ). Hence the polarity of the SST/rainfall association depends on location. The association between SSTs near the coast and rainfall is positive if the influence is direct but can be positive or negative for indirect influences. An opposition between the Indian and Atlantic Oceans appears to displace convection in an east-west direction. Our results suggest several generic conclusions concerning the link between SSTs and continental rainfall. One is that the influence of the three oceans is seasonally dependent. The impact of a specific SST anomaly is also seasonally dependent. The same SST pattern may enhance rainfall in one season, but reduce it in the following season. Finally, the SST/rainfall associations are generally not symmetric. That is, the factors producing wet conditions are not the reverse of those producing dry conditions. In order to understand these associations, the underlying mechanisms via the general atmospheric circulation must be determined. Copyright (C) 2007 Royal Meteorological Society.