Rainfall Variability in West Africa during the Years 1950–90

The study presented here makes use of about 300 daily rain gauges covering a 1 700 000 km2 area in order to characterize the rainfall regimes of West Africa at hydrological scales. The rainfall regime is analyzed as a combination of two variables, the average number of events over a given period of time (nT) and the average cumulative rainfall per event (h). These two parameters are a measure of the occurrence rate and magnitude of the convective storms that generate most of the rainfall in this region. They define the average water input to the hydrological systems and the average time available for this water to be redistributed into the continental hydrological cycle before a new input occurs. By analyzing for a period of 40 yr (1951–90), the space and time variations of these two parameters, it is possible to better understand how the intraseasonal to decadal rainfall variability may impact on the hydrological cycle. The analysis is carried out in two steps. First, the annual cycle and migrations of the weather zones characterizing the climate of West Africa are considered. This leads to evidence of a sudden and synchronous rain onset between 9° and 13°N, which does not follow the classic scheme of a progressive migration of the rain zones, north and south with the sun. Second, the differences in the rainfall regimes between the two succeeding subperiods of 20 yr are obtained, the subperiod P1 (1951–70) being wet and the subperiod P2 (1971–90) being dry. The difference—averaged over the 16° by 12° study region—of the mean interannual rainfall between the wet and the dry periods is 180 mm yr−1. This difference is relatively evenly distributed in space, with no clear meridional gradient. Between these two periods, the parameter nT displays a systematic decrease, which appears well correlated to the decrease of the mean interannual rainfall. The variations of h are, by contrast, smaller in amplitude and more erratically distributed in space. When looking at the intraseasonal scale, it appears that the rainfall deficit of the dry period is primarily linked to a deficit of the number of events occurring during the core of the rainy season over the Sahel, and during the first rainy season for the region extending south to 9°–10°N. It is also shown that, in the south, the dry period is characterized by a shift in time of the second rainy season. All these characteristics have strong implications in term of agricultural and water resources management. They also raise questions about the traditional scheme used to characterize the dynamics of the West African monsoon.