Tropical Waves Are Key Drivers of Extreme Precipitation Events in the Central Sahel

Extreme precipitation events (EPE) are often associated with severe floods and significant damages in Central Sahel. To better understand their formation and improve their forecasts, we investigate the sub-seasonal drivers of EPEs. A composite analysis reveals that moist, cyclonic and upper-level divergence anomalies are found on average as a result of several tropical waves. The equatorial Rossby wave (ER) dominates at large scale providing a moist and convectively-active anomaly over the northern Sahel together with a smaller-scale African Easterly Wave (AEW). The Madden-Julian Oscillation provides upper-level divergence anomalies and a Kelvin wave increases convection during the EPE. Statistics show the prevalence of AEW and emphasize ER as a key driver of EPE. The co-occurrences of several tropical waves, especially those involving AEW, ER, and Kelvin waves, increase the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. The drivers of extreme precipitation events (EPE) over the Central Sahel are studied at subseasonal scales. A statistical approach is adopted to build an average extreme rainfall event. EPEs occur within a large-scale moist anomaly, an upper-level divergence, and at shorter scales an intense vortex. These features are provided by multiple tropical meteorological systems, called tropical waves, that have typical spatio-temporal scales greater than that of a convective storm. The four tropical waves studied here contribute to this favorable environment. However two of them-an Equatorial Rossby wave, rather slow, and an African Easterly Wave (AEW) (more rapid)-explain the largest part of the favorable conditions leading to an EPE. Statistics show that a single intense AEW or the combination of multiple tropical waves increases the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts. Tropical waves are key to build the atmospheric environment conducive to an extreme precipitation eventEquatorial Rossby waves appear as a new driver of extreme rainfall in the Central Sahel when combined with African Easterly WavesThe combination of African Easterly Wave with an equatorial Rossby wave and/or Kelvin wave, increase the probability of extreme rainfall