The Nature of Rainfall Fluctuations in Subtropical West Africa

Abstract

The nature of rainfall fluctuations in the West African subtropics is described on the basis of three analyses of rainfall departures. Areally averaged. normalized rainfall departure series for the years 1901–75 are derived for four semi-arid sub-Saharan climatic zones: the Sahelo-Saharan, the Sahel, the Soudan and the Soudano-Guincan. Similar series are derived for four or five regions within each zone and the regional series are intercorrelated. Finally, correlation between annual rainfall departure maps is used to determine frequently occurring rainfall anomaly types. Several characteristics of West African climate variation emerge from these investigations. A strong coherence of variation occurs throughout the region from∼20–10°N, a latitude which frequently marks a climatic discontinuity with large departures of opposite sign prevailing to the north and south. This coherence is very pronounced in both the north-south and east-west directions, although certain longitudinal sectors occasionally behave differently from the rest of the zone. Such a case results in a mixed configuration of drought, with negative anomalies prevailing only within certain sectors. The determined rainfall anomaly types represent several drought types for the Sahel, characterized by their various relations to conditions elsewhere on the continent. In general, rainfall fluctuations in the West African subtropics are more commonly characterized by a contraction or expansion of the desert belt, rather than by its north-south displacement.