The role of Mediterranean Troughs on Boreal Winter Dry Season Rainfall over Eastern Africa

Unexpected rainfall events during the January-February dry season over Eastern Africa have significant impact upon society, particularly when they lead to, or exacerbate, ongoing flooding (as in Kenya in 2020 and 2022). Populations across Eastern Africa do not expect rainfall to occur during the January-February dry season, and a lack of preparedness can exacerbate impacts when heavy rainfall does occur. Whilst recent dry season rainfall across Eastern Africa has severely impacted livelihoods and communities, the mechanisms controlling such rainfall are poorly understood, since the majority of previous research has focussed upon the climatological wet seasons.  Here, we aim to further explore the drivers of these boreal winter dry season rainfall events.

Recent research suggests that boreal winter precipitation and temperature anomalies over tropical central Africa are influenced by large-scale atmospheric variability originating in the Mediterranean region. Building on these findings, this study investigates the role of Mediterranean troughs in driving January–February dry season rainfall over Eastern Africa.

Our results show that dry season rainfall over Eastern Africa is linked to an upper-level ridge-trough pattern over the Mediterranean. The presence of a ridge in the central Mediterranean and trough in the Eastern Mediterranean leads to westerly wind anomalies across Central Africa, and anomalous westerly moisture transport that enhances moisture over Eastern Africa and a region extending north-east from Eastern Africa into the Arabian Peninsula and Asia. This enhanced moisture leads to enhanced rainfall over Eastern Africa, during the climatologically dry January-February season.

These findings will improve future forecasts of dry season rainfall over Eastern Africa, which will enhance preparedness for future rainfall events.  Furthermore, climate projections from CMIP5 and CMIP6 models indicate enhanced dry season rainfall over Eastern Africa under future climate change. Improving our understanding of drivers of present-day dry season rainfall will support our understanding of future rainfall changes.