Understanding the Regionality of Extreme Rainfall in the Sahel: The Mali Hotspot

Abstract

West African Sahel extreme rainfall events cause flooding and property damage, and some areas are more prone to their occurrence. One favorable region is southwestern Mali. NASA IMERG precipitation and ERA5 reanalysis data are used to examine the most extreme boreal summer rainfall events from 2000–2019 over southwestern Mali to understand why they form, and to explain why this region is a hotspot. Events are sorted into 4 types based on the timing of the peak rainfall (before or after 00Z) and the associated mid-tropospheric circulation pattern (coastal low or ridge). The coastal low types are associated not with an increase of the low-level inflow of moisture into southwestern Mali, but a weakening of the mid-level easterly transport of moisture out of the region. The timing and longevity of the event depends on whether there is a second low to the east in the southern storm track. The coastal ridge types are associated with a build-up of warm, dry air over the western Sahara that leads to a stronger temperature inversion cap over southwestern Mali, allowing instability to build beneath the cap. How fast the cap dissipates and whether there is synoptic activity to the east in the southern or northern storm track determines when convective activity occurs. Thus, southwestern Mali is exposed to coastal lows and ridges in addition to the Saharan heat low and the summer southern storm track for African easterly wave disturbances. The confluence of these factors makes southwestern Mali a hotspot for extreme rainfall.