Drivers and impacts of westerly moisture transport events in East Africa

Abstract. Equatorial East Africa (EEA) experiences strong intraseasonal precipitation variations. A better understanding of the processes that drive these variations can improve predictability and help local populations better prepare for extremes. Previous research has highlighted anomalous westerly moisture transport from the Congo basin as an important driver of enhanced precipitation in EEA. Here, we have developed the first spatially unconstrained, objective framework to detect what we refer to as westerly moisture transport events (WMTEs) in ERA5 reanalysis data from 1980 to 2022, revealing new insights into potential drivers of these westerlies and their impact on EEA precipitation. We show that over EEA, WMTEs were most common in January and February between about 5 and 15° S, where there were typically 4 to 6 WMTE days per month, with each individual event persisting in the region for around 2 to 4 d. During the main wet seasons in EEA, March to May and October to December, there was on average around 1 WMTE day per month. Using an established precipitation attribution algorithm, we estimate that WMTEs were associated with up to 60 % of precipitation during January and February in Tanzania, and up to 20 % of precipitation during March–May to the east of Lake Victoria. WMTEs over EEA with higher integrated moisture transport were associated with more intense precipitation. Consistent with previous work, we found that WMTEs were more likely during phases 2–4 of the Madden-Julian Oscillation (MJO). However, we extend previous case study investigations, by showing that WMTEs tend to have longer lifetimes when close to a tropical cyclone (TC). We also found that the presence of a TC anywhere in the south-west Indian Ocean made WMTEs in EEA up to three times more likely, even during inactive or unfavourable phases of the MJO. This work builds on previous studies of the westerly wind feature by developing an objective framework to describe them and providing new insights into the complex nature of the interactions between different features of tropical meteorology that drive these intraseasonal phenomena.