RAINFALL CHANGES IN AFRICA - 1931-1960 TO 1961-1990

It remains common for 1941-1970, or even 1931-1960, climatological rainfall normals to be used in applied climate studies in Africa. Often this is due simply to the easier availability of station means or rainfall maps for these periods. Such rainfall statistics, however, are unrepresentative of recent decades, especially the new World Meteorological Organization standard period, 1961-1990. In this paper two independent 30-year rainfall climatologies for Africa are constructed from 572 quality controlled station time series of monthly rainfall. These climatologies are for the periods 1931-1960 and 1961-1990 and are constructed on a 5-degrees grid. Differences between these two 30-year periods are determined with respect to mean seasonal rainfall, interannual variability and rainfall seasonality. Latitudinal profiles of mean seasonal rainfall emphasize the dominance of reduced Sahelian rainfall in the rainfall changes occurring over this period. This decline in boreal summer rainfall is shown to be statistically significant using field comparison statistics. Annual time series of regional rainfall anomalies for the Sahel, East Africa and south-western Africa are constructed; these three time series possess quite different interannual rainfall characteristics. Possible explanations for these observed rainfall changes are discussed with respect to land cover changes, global sea-surface temperature patterns, and greenhouse gas forcing.