Thursday, 13 January 2005: 1:45 PM
Rainfall variability in equatorial Africa
This paper focuses on western equatorial Africa, a region that has not been extensively studied in terms of meteorology of climate variability. The region lies between roughly 10 N and 7 S. Rainfall variability is very complex here, because it is a transition zone between the influences of circulation in both hemispheres and between the Atlantic to the west, the Rift Valley highlands and the Indian Ocean to the east. The northern margin is the southern extremity of the semi-arid Sahel zone of summer rainfall. To evaluate causes of variablity composites of the five wettest and five driest years were compiled for each of five regions in western equatorial Africa and correlations with SSTs were calculated on a seasonal basis. The results showed that the causes of variability differed greatly among the five regions and that the dominant causes were highly seasonal. During the main "summer" season of the northern hemisphere, the Atlantic generally played the greatest role. Shifts in the ITCZ, related to SSTs in the Atlantic, were quite evident. In other seasons, the combined influence of the Atlantic and Indian Oceans was most important. However, during the March-April-May season (the main rainy season in much of the region), the dominant influence of the Pacific El Nino was clearly seen. Predictability can be enhanced by considering the seasonality of the dominant factors and how these change over relatively small areas in the part of the world.