A REEVALUATION OF RAINFALL VARIABILITY IN THE SAHEL .1. CHARACTERISTICS OF RAINFALL FLUCTUATIONS

This article deals with the characteristics of rainfall variability in West Africa. Time series of June-July, August-September and annual rainfall anomalies are derived for 19 regions, most of which are in the Sahel-Soudan zones of West Africa. The temporal characteristics of the series, such as trend, persistence, and low frequency variance are evaluated using regression, lag-1 autocorrelation, and spectral analysis. High-frequency-filtered series are also derived. The spatial teleconnections among the regions are examined using linear correlations and principal component analysis. The study shows that there are primarily three spatial modes of rainfall variability in the region and these differ with respect to the part of the rainy season in which they are most important and to what extent they describe high and low-frequency components of the variability. The study also defines three broad homogeneous sectors and demonstrates that the characteristics of rainfall variability in these sectors differ markedly. A strong contrast between the bulk of the Sahel-Soudan and its West Coast sector is apparent.