Relationship Between Recent Rainfall Regime Characteristics over Southern West Africa and Associated Mesoscale Convective Systems

Abstract

Extreme climate events, either being linked to dry spells or extreme precipitation, are of major concern in Africa, a region in which the economy and population are highly vulnerable to climate hazards. However, recent trends in climate events are not often documented in this poorly surveyed continent. Changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. This analysis was extended to Southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. This study makes use of a large set of daily rain gauge data covering Southern West Africa (extending from 10° W to 10° E and from 4° N to 12° N) from 1950 to 2014. The evolution of the number and the intensity of daily rainfall events, especially the most extremes, were analyzed at the annual and seasonal scales. The north-south seasonal changes exhibit an increase in mean annual rainfall over the last decade during the second rainy season (September-November) linked by both an increase in the frequency of occurrence of rainy days as well as an increase in the mean intensity and extreme events over the last decade. Over the south-north gradient of West Africa, mesoscale convective systems (MCSs) tend to be responsible for the majority -of annual rainfall. Trends in intense MCS correlate well with intense rainfall in both rainfall seasons over Southern West Africa. Coldest cloud tops (intense MCSs) tend to be modulated by positive trends in wind shear during the first rainy season and by the availability of moisture in the second rainy season, although both rainy seasons occur mainly under similar conditions.