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

Abstract

ABSTRACT: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 Niña 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.

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…This increase in flood related fatalities and economic losses may be due to an increase in inundations, but also potentially to an increase in exposure and vulnerability, as suggested by Aich et al (2016) [38]. The results of Paeth et al [39] and Lafore et al [40] also suggest that many factors, from synoptic-scale to large-scale circulations, including regional features and teleconnection patterns, contribute to the extremeness of the rainfall events that lead to flooding.…”

“…In sharp spatial scale contrast, large areas from four countries (the southern part of Burkina Faso, the northern parts of Benin, Togo and Ghana) provided stream flows to tributaries of the Volta River in August and September 2007 during a flood event that was part of a very active monsoon. Paeth et al [39] suggested that remote influences of anomalously high Atlantic SST (Sea Surface Temperature) and of a La Niña event in the Tropical Pacific played a role in this event. However, it is notable that the extreme rainfall event that occurred at Ouagadougou in 2009 was associated with very distinct large-scale and regional patterns [40].…”

“…The 2nd Sahelian paradox (increase in runoff and increase in vegetation cover) is explained by both rainfall intensification and a reduction in soil WHC, probably linked to extension in cropping areas, as well as vegetation degradation and soil erosion over shallow soils [20,26,38,39,53,76,77,80].…”

Evaluation of CMIP5 Global Climate Models over the Volta Basin: Precipitation

“…e selection of the GPCC data as reference data for this evaluation study is based on the fact that they have been applied in similar model evaluation studies (e.g., [31,32]) over the West African region. Other studies including Gruber et al [39], Nicholson et al [37], Paeth et al [38], and Nikulin et al [29] have demonstrated the robustness of GPCC data in reproducing observed precipitation in the West African region. ese studies expressed confidence in this particular data set.…”

“…Due to the highly varying spatial distribution of precipitation, spatial density of observed precipitation data is crucial over West Africa. Studies (e.g., [37,38]) have indicated that the availability of high-quality data set especially in the case of precipitation over West Africa is problematic. Consequently, gridded observational open-source data sets become the practical option for model validation purposes within the region.…”