Return times and return levels of July-September extreme rainfall over the major climatic sub-regions in Sahel

Return times (RTs) and return levels (RLs) of July-September (JAS) extreme rainfall are estimated using stationary peaks-over-threshold approach through generalized pareto distribution (GPD) over the three climatic sub-regions in Sahel. Monthly rainfall data from the Climatic Research Unit were used. The period of analysis goes from 1901 to 2009. Results computed for both accumulated JAS rainfall and fractional area of extreme rainfall, indicate that JAS extreme rainfall in semiarid and sub-humid sub-regions were more frequent in the postchange period of 1971-2009 than in the prechange period of 1901-1970. The RLs of these extremes have greatly decreased after 1970. The changes at JAS extreme tails of the distribution have significantly contributed to the decrease in mean of JAS rainfall after 1970. Predicted 40-year RL shows that over each sub-region, JAS extreme rainfall are expected to be more severe after the postchange period. For the postchange period, 1999 has been detected as the most extreme year in arid and semiarid sub-regions, and 2008 in sub-humid sub-region. These events were found occurred during the manifestation of moderate La Niña. The spatial distributions of accumulated JAS rainfall associated with these years show that highest values were concentrated along the coastline of semiarid and sub-humid sub-regions, and the lowest values in the northern part of the arid sub-region. Using the GPD, the JAS extreme rainfall (fractional area) associated with 1999 and 2008 are expected at least once every 144 (40), 99 (17) and 197 (67) yr in arid, semi-arid and sub-humid sub-regions, respectively. These results highlight the importance of an analysis of extreme rainfall in Sahel based on climatic sub-regions.