Rainfall variability over the eastern Congo Air Boundary Region of Southern Africa

Abstract Changes in late summer (January to March) rainfall over the eastern Congo Air Boundary (CAB) region in southern Africa are examined. Using ERA5 data for the 1979–2024 period, a significant increasing trend in rainfall totals is found over most of the region which results from an increase in the number of heavy rain days (10 mm or more). On the other hand, there has been a significant decreasing trend in light rain day numbers (1–9 mm) over most of the eastern CAB. During the 2023/2024 El Niño, a particularly severe drought occurred in tropical southern Africa with the eastern CAB experiencing its driest late summer since 1979. The Niño 3.4 index was found to be significantly correlated with both rainfall totals and rain day counts (particularly heavy days) over the eastern CAB (as well as over many other parts of southern Africa). While all of the five driest late summers correspond to the mature phase of an El Niño event, only three of the five wettest summers occur during La Niña. Significant correlations of these rainfall characteristics are also found with indices of the Botswana High and the Angola Low. A composite analysis for these five driest and wettest late summers indicated that the rainfall anomalies were associated with large changes in relative uplift over tropical southern Africa, particularly the eastern CAB region, as well as in low-level moisture fluxes from the western Congo Basin, and the tropical South East Atlantic and subtropical South West Indian Oceans.