Meteorological characteristics and potential causes of the 2007 flood in sub-Saharan Africa

In August and September 2007 media and aid organizations reported on a severe flood in large parts of sub-Saharan Africa caused by a sequence of extreme rain events. However, the reporting was partly controversial and much was speculated about how unusual the flood event was. Here, we analyse various observational data sets in order to assess and to compare the spatio-temporal characteristics and intensity of the flood and the associated rain events. The return times are estimated from daily precipitation time series at fine spatial resolution, and the potential causes are disclosed in a global and regional context. Satellite data reveal that the flood was not large scale but confined to the main river basins in sub-Saharan West Africa. Nonetheless, abundant rainfall prevailed over large parts of Western Africa extending north into the Western Sahara, particularly during the second half of August and the beginning of September 2007. In detail, various precipitation data sets differ considerably in terms of the monthly anomalies, demonstrating the difficulty to delineate meteorological extreme events even at a sub-continental scale and during the most recent past. Return times typically range between 1 and 50 years with high spatial heterogeneity but amount to 1200 years in the regional mean over the Upper Volta Basin. Among the potential causes we identify a La Nina event in the tropical Pacific, anomalous heating in the tropical Atlantic coming along with a greater depth of the monsoonal westerlies, and enhanced activity of African easterly waves, both evident in 2-6 day zonal wind and outgoing longwave radiation variance anomalies. Copyright. (C) 2010 Royal Meteorological Societ