Dynamic and thermodynamic contribution to the October 2019 exceptional rainfall in West Central Africa

Abstract. Exceptional rainfall hit West Central Africa in October 2019. To understand the underlying mechanisms, we diagnosed the regional moisture and Moist Static Energy (MSE) budgets with a view to highlighting the importance of the dynamic and thermodynamic effects associated with this historic event. Analysis of the moisture budget reveals that the precipitation anomalies in October were mainly controlled by dynamic effects (72.5 % of the sum of dynamic and thermodynamic contributions). Horizontal moisture advection induced by horizontal wind anomalies controls extreme precipitation north of West Central Africa, while vertical moisture advection induced by vertical velocity anomalies controls extreme precipitation south of West Central Africa. Changes in the thermodynamic effect, although not the key factor responsible for the events of October 2019, contribute up to 27.5 % of the total effect. Diagnosis of the MSE balance shows that the anomalous vertical motion is dominated by the dynamic effect, i.e. the wet enthalpy advection induced by the horizontal wind anomalies. The horizontal advection of the MSE induced by the variation of the wet enthalpy and the vertical advection of the MSE induced by the variation of the MSE seem less important. The variations in the MSE balance are linked to its meridional component, in particular the meridional wind anomalies in the dynamic effect and the meridional variations in latent heat in the thermodynamic effect. This is due to the increase in sea surface temperatures in the equatorial Atlantic, associated with the anomalous thermal depression over the Sahara, which has increased rainfall over West Central Africa. Our results suggest that dynamic and thermodynamic effects should be jointly considered for adequately anticipating this kind of extreme event. Understanding the associated mechanisms could help us improve our projections and increase the region's population resilience to these extreme weather events.