Variability of African Rainfall on Interannual and Decadal Time Scales

SHARON E. NICHOLSON1

ABSTRACT

This paper examines the rainfall fluctuations that have occurred over the continent of Africa during the past century. Major characteristics include anomaly patterns that are continental in scale, with marked teleconnections between particular sectors of the continent, and quasi-periodic fluctuations with dominant time scales of approximately 2.3, 3.5, and 5 to 6 years. In the Sahelian regions these higher-frequency fluctuations are not apparent; instead, most of the variance is found on time scales of 7 years or longer. Both dry and wet episodes tend to persist for one or two decades at a time; decadal means change by a factor of 2. These characteristics are also apparent in the Sahelian climate record for earlier centuries. Elsewhere on the continent, synchronous changes of rainfall occur, but the "wet" or "dry" decades are not as uniformly wet or dry as in the Sahel. Examples are the 1950s, in which above-average rainfall prevailed over most of the continent, and the 1980s, during which most of the continent was abnormally dry. It is likely that the factors producing higher-frequency interannual variability differ from those producing the decadal-scale fluctuations. Both sea surface temperature and land-surface feedback have been suggested as causes of the unusual persistence in the Sahel, but the latter cannot explain the change of decadal means.

INTRODUCTION

The continent of Africa is primarily tropical or subtropical, so the climatic changes that have taken place have been manifested principally in the rainfall regime. The rainfall fluctuations that occur are extreme; those in the semi-arid regions are probably unmatched anywhere in their magnitude and spatial extent. In the Sahel, for example, decadal averages change by a factor of 2 or more. Most major periods of anomalous rainfall, either individual years or decadal-scale episodes, affect most of the continent.

This paper examines the rainfall fluctuations that have occurred during the twentieth century and compares them with historical episodes. The time scales of the fluctuations are summarized, and the decadal-scale fluctuations described in greater detail. The intra-continental teleconnections, and teleconnections to tropical or global rainfall anomalies, are also considered. Finally, a brief review of causal mechanisms is provided.

Undoubtedly, changes in the thermal regime over Africa have occurred, especially along the continent's poleward

1  

Department of Meteorology, Florida State University, Tallahassee, Florida



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