Recent trends in the Central and Western Sahel rainfall regime (1990–2007)
Résumé
One motivation for setting up the CATCH (Couplage de l'Atmosphère Tropicale et du Cycle Hydrologique) project at the end of the 1990s, was to contribute to documenting the Sahelian rainfall variability at the interannual scale and to provide a fine monitoring of possible long-term trends of the rainfall regime. This paper is a first attempt at characterising the Sahelian rainfall regime of the two last decades (1990– 2007) by comparison to the rainfall regime of the previous decades, namely the 20-year wet period 1950– 1969 and the 20-year dry period 1970–1989. While the rainfall deficit remained unabated in the Western Sahel (1990–2007 mean equal to the 1970–1989 mean, both being lower than the 1950–1969 mean), the Central Sahel progressively recorded wetter years from the end of the 1990s, but this recovery is limited (1990–2007 average larger by 10% than the 1970–1989 average, but still lower than the 1950–1989 average). There are also significant differences between the Western Sahel and the Central Sahel when looking at the interannual variability pattern and at the seasonal cycle. The low-frequency rainfall patterns are similar between the Western Sahel and the Central Sahel, but the interannual year-to-year variability is weakly related to each other. In the Central Sahel, the major modification of the seasonal cycle in the most recent decades was the disappearance of the well marked August peak observed during the wet period. In the Western Sahel the rainfall deficit is more or less evenly distributed all along the rainy season. The second part of the paper makes use of the CATCH-Niger recording rain gauge network in order to compare several ways of defining rainy events. The statistical properties of these various populations of rainy events are compared. It is shown that a simple CPP model allows for retrieving the statistical characteristics of point rainy events from daily rainfall series. It is also confirmed that in this area, the interannual rainfall variability is primarily linked to the year-to-year fluctuation of the number of large mesoscale rainfall events.