Precipitation Extremes in the West African Sahel: Recent Evolution and Physical Mechanisms

Abstract The West AfricanMonsoon (WAM) has undergone drastic changes in the recent past causing dramatic impacts on populations. After 30 years of devastating drought, the last two decades experienced a growing number of damaging floods concurrent with ongoing episodes of water shortages and famines. This raised the issue of a more extreme climate in the Sahel with more intense rainfall and dry spells. This chapter gives an overview of progress on documenting the rainfall extremes in the Sahel, a subject that has long been ignored in the literature. It focuses on statistical characteristics of extremes, on their trends in the context of WAM decadal variability and on the physical mechanisms involved in their occurrence. Based on recent literature and original analyses of daily rain gaugerecords in the central Sahel, a significant intensification of the Sahelian rainfall regime is confirmed over the last 15 years: more extreme events, larger size storms, accompanied by a deficit in the occurrence of less intense events. From a composite of 20 extreme storms in Ouagadougou, some key atmospheric conditions are shown to drive extreme precipitation, including an intense southerly monsoon flux coincident with strong African easterly waves and a large-scale moist anomaly at the regional scale.