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

West African extreme rainfall events can be impactful as heavy rains often lead to flooding causing property damage and the loss of life. While extreme events can happen anywhere, they are more likely in some regions than others. One “hotspot” for extreme 24-h rainfall totals is Southwestern Mali. We utilize NASA IMERG satellite-derived rainfall estimates and ERA5 atmospheric reanalysis data to better understand southwestern Mali extreme rainfall events by identifying the common factors among them that are most influential for storm development. Twenty-two boreal summer extreme rainfall events between 2000 – 2019 are analyzed. Events are categorized based on their peak rainfall rate timing - before or after 00Z - and associated mid-tropospheric synoptic patterns - a coastal ridge or a low between 15°N - 20°N preceding maximum rainfall - yielding four composite types.

Events associated with coastal lows are accompanied by a weakening of the mid-level easterly moisture transport out of the region rather than with an increase of the low-level inflow of moisture into the region. The exact timing and duration of this type of event depends on whether there is a second low to the east in the southern storm track. Extreme events associated with coastal ridging are characterized by the build-up of warm, dry air over the western Sahara that leads to a stronger temperature inversion cap over southwestern Mali, allowing instability to increase below the cap. The timing and intensity of convective activity are determined by how fast the cap dissipates and whether there is synoptic activity to the east.

These case studies indicate that several factors make southwestern Mali favorable for boreal summer extreme rainfall events. The proximity to the Atlantic Ocean is important because it serves as a primary source of low-level moisture. The Sahara Desert – including the summer thermal low - to the north influences the regional circulation, mid-level moisture transport, and the development of temperature inversion caps. Additionally, southwestern Mali lies within the boreal summer southern storm track of African easterly wave disturbances which help to better organize convective activity. The confluence of these factors makes southwestern Mali an extreme rainfall “hotspot” during the boreal summer.