The Role of Tropical Waves in the May 2023 Extreme Rainfall Event in the Lake Kivu Region of East Africa

In early May 2023, extreme rainfall in the Lake Kivu region of East Africa led to catastrophic flooding and landslides killing about 135 (600) people in Rwanda (the Democratic Republic of Congo). Two automated weather stations in northwestern Rwanda recorded a 24-hour rainfall of 182 mm and 177mm on 2^nd May 2023, respectively, leading to extreme flooding in the Sebeya catchment flowing into Lake Kivu near the Rwandan town of Gisenyi. Overall, according to the “May 2023 Monthly Bulletin” of the Rwanda Meteorological Agency (RMA), the extreme rains caused the worst floods and landslides ever recorded in the Northern and Western Provinces of Rwanda, resulting in deaths, infrastructure destruction, and crop damage. This event was captured by the Kigali rain radar which shows long-lasting convective rainfall which was likely to have been orographically triggered at the western flank of the Albertine Rift mountains of Rwanda on that day. A second event on 05 May 2023 at the end of a multi-day wet spell affected the western shores of Lake Kivu with devastating landslides in the Kalehe territory of South Kivu province (Democratic Republic of Congo). The present contribution studies the large-scale precursors of these extreme rainfall events which includes an analysis of the role of tropical waves. It will also shed light on its predictability in the operational ECWMF Ensemble prediction (ECMWF Ens) system.

Wetter than normal conditions, as evidenced by rain gauge and GPM IMERG data, in eastern equatorial Africa commenced in the last decade of April 2023 and were associated with enhanced southeasterly (westerly) moisture fluxes from the Indian Ocean (Congo Basin). According to a recent study, the positive decadal rainfall anomalies in the last decade of April and first decade of May 2023 which stretched from Rwanda northeastwards across Uganda and Kenya to southern Ethiopia are reminiscent of a wet climatological circulation pattern at the peak of the March-May long rains. The enhanced mid- to-lower-tropospheric westerlies over the Congo that appear instrumental for one of the highest ever estimated amount of total column precipitable water (TCWV) over Rwanda and vicinities, were related to a passage of the wet phase of an Madden-Julian Oscillations in which two Kelvin waves were embedded in. The ECMWF Ens forecast at different lead times did indicate an increased likelihood of extreme rainfall for the 02 May 2023 Rwanda case when compared to the forecast model’s climatology, although locations varied with initialization time.

It is concluded that the convergence of lower tropospheric southeasterlies originating from the Indian Ocean and the tropical wave-related enhancement of westerlies over the Congo lead to extremely high TCWV values over the region, setting the larger-scale conditions for the extreme event. While orography played a role, the detailed causes for the exact location and time of the Rwanda event could not be explained nor forecast by ECWMF Ens.