On the Equatorial East Africa Floods during the Spring 2018 Rainfall Season

Equatorial East Africa (EEA) receives the bulk of its annual rainfall during two distinct seasons, the spring rains in MAM, and the fall rains in OND. While the latter seasonal rainfall can be strongly modulated by ENSO, leading to more predictability on the seasonal timescale, organized convective systems are more random during the spring. However, convection is also known to be partly modulated by the MJO. In March and April 2018, several consecutive days of extremely heavy rains led to severe flooding in Kenya and neighboring countries, causing several casualties and the displacement of hundreds of thousands of people. Areas in Kenya received the heaviest rains in 70 years according to the CPC land precipitation (PREC/L) data with totals exceeding 200% of the 30-year base period of 1981-2010. A diagnosis of the atmospheric circulation that contributed to the severity of the precipitation is performed to describe the climate context of these extreme events. The Madden Julian Oscillation (MJO) played an important role. Based on the Wheeler-Hendon diagram, a prominent MJO straddled the globe between January and June 2018, with a strong signal in Africa and the Indian Ocean in April and May 2018. MJO rainfall anomaly composites reveal that EEA tend to register moisture surpluses during phases 2 to 4 of the MJO, while rainfall is suppressed during phases 5 to 8. The intra-seasonal variability of EEA rainfall and the role of the MJO in modulating rainfall variability at this time scale are discussed. We further discuss the mid-latitude synoptic waves that sometimes interact with the MJO to sustain the extreme precipitation events. Finally, the performance of the NCEP’s Global Ensemble Forecast System (GEFS) and Climate Forecast System version 2 (CFSv2) in predicting these extreme events is presented.