Potential causes of 2007 flooding over West Africa and the Sahel

Abstract

The 2007 wide-spread flooding in West Africa and the Sahel regions raised a compelling argument in the scientific community as to its intensity, complexity, causes and vulnerability assessment. The paucity of observed data, lack of regional coupled models and the improper scaling of impact has aggravated the understanding of this event. However, it is generally known that these floods result from extreme rainfall of the West African Monsoon that is modulated by Sea Surface Temperatures (SSTs). Here, the influence of SST in the Gulf of Guinea (GG) along the path of the monsoon winds is examined to highlight its role on rainfall in those regions. The non-seasonal GG index and the spatial SST shows one month lead to non-seasonal cycle of rainfall. There was interannual variability of SST anomalies peaking in some years, notably summer of 2007 and that concurs well with the rainfall. The plots show stronger correlation with West Africa rainfall than with the Sahel. The SSTs lead Sahel rainfall with three months, signifying the local positioning of the Intertropical Discontinuity (ITD). The correlations are significantly higher with seasonal cycle, indicating more influence of seasonality than interannual variability akin to non-seasonal signal. When the winds weaken, the cold tongue and TIWs are damped, leading to enhanced SSTs and subsequently stronger convection and rain convergence over land. The opposite occurs when winds are stronger.