Abstract
AbstractVariability of rainfall in East Africa has major impacts on lives and livelihoods. From floods to droughts, this variability is important on short daily time‐scales to longer decadal time‐scales, as is apparent from the devastating effects of droughts in East Africa over recent decades. Past studies have highlighted the Congo airmass in enhancing East African rainfall. Our detailed analysis of the feature shows that days with a westerly moisture flow, bringing the Congo airmass, enhance rainfall by up to 100% above the daily mean, depending on the time of year. Conversely, there is a suppression of rainfall on days with a strong easterly flow. Days with a westerly moisture flux are in a minority in all seasons but we show that long rains with more westerly days are wetter, and that during the most‐recent decade which has had more frequent droughts (associated with the “Eastern African climate paradox”), there has been few days with such westerlies. We also investigate the influence of the Madden–Julian Oscillation (MJO) and tropical cyclones, and their interaction with the westerly flow. We show that days of westerly moisture flux are more likely during phases 3 and 4 of the MJO and when there are one or more tropical cyclones present. In addition, tropical cyclones are more likely to form during these phases of the MJO, and more likely to be coincident with westerlies when forming to the east of Madagascar. Overall, our analysis brings together many different processes that have been discussed in the literature but not yet considered in complete combination. The results demonstrate the importance of the Congo airmass on daily to climate time‐scales, and in doing so offers useful angles of investigation for future studies into prediction of East African rainfall.
Highlights
The severe societal impacts of droughts and floods in East Africa are well documented in the media and scientific literature (Kilavi et al, 2018)
Before exploring the interaction of the Congo Airmass, tropical cyclones and the Madden–Julian Oscillation (MJO) further, we first characterise the relationship between the moisture flux and East African rainfall using a multi-decade time series of observed rainfall from the CHIRPS and Global Precipitation Climatology Project (GPCP) products, and mean westerly column moisture flux from ERA-Interim
Having demonstrated the dependence of East African rainfall on the Congo and Indian Ocean airmasses on shorter time-scales, our analysis considers whether, despite being a minority of days, interannual variability of westerlies and weak easterlies have impacts on longer-time-scale variability of East African rainfall
Summary
The severe societal impacts of droughts and floods in East Africa are well documented in the media and scientific literature (Kilavi et al, 2018). Wind anomalies have been associated with enhanced rainfall over East Africa (the highland region) in studies of flooding events (Anyamba, 1983), seasonal variability (Camberlin and Wairoto, 1997), the Madden–Julian Oscillation (MJO; Pohl and Camberlin, 2006; Berhane and Zaitchik, 2014; Hogan et al, 2015), and climate change (Giannini et al, 2018). Following the data and methods sections (Sections 2 and 3), we use a case-study of the 2018 long rains (Section 4) to provide context of the interacting influence of westerlies, the MJO and tropical cyclones on East African rainfall. For the Indian Ocean these centres are: La Réunion, New Delhi and the Australian Bureau of Meteorology
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