Recent Trends in the Regime of Extreme Rainfall in the West African Sahel

Abstract:

West Africa is known for having experienced an extreme drought starting at the end of the 1960s that is recognized to be the greatest climatic signal at regional scale since the beginning of meteorological measurements. Despite a moderate recovery of the annual precipitations since the 1990s in the Central and Eastern Sahel, rainfall over the last two decades remains lower by 15% than during the period 1950-1970. Paradoxically these persisting dry conditions have been accompanied by a dramatic increase of flood fatalities especially over the recent 10 years.

Using a homogeneous dataset of 41 daily rainfall series covering the period 1950-2010, an integrated regional approach based on the statistical extreme value theory was then used to reduce the local sampling effects and to provide robust estimates of intense rainfall distributions to be analyzed in conjunction with the annual rainfall series. This led to identify some key rainfall regime characteristics related to the decadal scale rainfall variability over the region.

The main factor of the rainfall deficit during the great 1970-2000 drought was a lower occurrence of rainy days, extreme rainy days being the most affected. Over the last ten years, the Sahelian rainfall regime is characterized by a lasting deficit of the number of rainy days, while at the same time the extreme rainfall occurrence is on the rise. As a consequence the proportion of annual rainfall associated with extreme rainfall has increased from 17% in the 1970-1990 years to 19% in the 1991-2000 years and to 22% in the 2001-2010 years. This tends to support the idea that a more extreme climate has been observed over the last 10 years, with a persisting deficit of the occurrence of rainfall associated with an increase of the occurrence of extreme daily rainfall. Our results also suggest that the intensification of the precipitation regime has likely contributed to the aggravation of the hydrological risks in the Sahel.