The modulation of the MJO and tropical cyclone impact on East African rainfall by the Mascarene high during the 2018 “long rains”

The ‘long rains’ (March to May) in 2018 caused some of the most devastating flooding events in East Africa. It is important to understand the factors that lead to heavy rainfall in order to improve the forecast accuracy of similar events in the future. This will help to minimize the associated loss of lives and destruction property. The 2018 long rains were significantly influenced by the Madden-Julian Oscillation (MJO) and tropical cyclones Dumazile and Eliakim over the southwest Indian Ocean (SWIO). Generally, the prediction of long rains and the associated extremes remains a challenge due to several knowledge gaps. This study looks at how the Mascarene High (MH) acted as an intermittent driver in regulating anomalous atmospheric circulation related to MJO and tropical cyclones Dumazile and Eliakim during the 2018 long rains. We investigate the transitions of the MH from east to west over the SWIO during the months of March to May. Additionally, we analyze how these transitions affect cyclone trajectories and moisture fluxes in the East African region. Composite analysis is utilized to diagnose atmospheric circulation anomalies using a dataset that includes rainfall, vertically integrated water vapour flux, mean sea level pressure, relative vorticity, sea surface temperature, and zonal and meridional winds. Results show a weakening and an eastward shift of the MH, which drives the northeasterly flow that led to the southwestward drifting of tropical cyclones Dumazile and Eliakim over the SWIO in March. A weak MH, MJO phases 2–3, cyclones Dumazile and Eliakim, all contributed to the formation of westerly moisture flow throughout East Africa, resulting in significant rainfall in March and April. Despite the MJO being active in phases 2 and 3 in late May, the MH shifted westward and became strong between early and mid-May, steering southeasterly trade winds from the SWIO that blocked/recurved westerly (climatological mean easterly) moisture flux from the Congo Basin (Indian Ocean) towards East Africa. This resulted in the subsidence of rainfall in May. This knowledge on how the MH influences the characteristics of MJO, and tropical cyclones can help local weather forecasters improve the accuracy of their predictions for heavy rainfall events in East Africa on a sub-seasonal timescale.