Dry-lines in Southern Africa: Rediscovering the Congo Air Boundary

Projected rainfall decline in southern Africa is likely to be highly sensitive to subtleties in the local atmospheric circulation. In an effort to understand the regional circulation complexities, a novel algorithm is developed to identify the Congo Air Boundary (CAB) in ERA-5, a high resolution reanalysis dataset. The CAB, a forgotten feature of the circulation, is defined in the Austral spring and early summer, using surface humidity gradients and near-surface wind convergence lines, and is found to be an indicator of the location of the southern edge of the African rain-belt. A related convergence and dry-line, described in this paper as the Trans-Kalahari Discontinuity (TKD), is also identified. It is established that either a dry-line CAB or TKD is present in southern Africa for over 95% of days between August and December, with arc-lengths typically exceeding 10 degrees. The seasonal and diurnal cycles of the CAB and the TKD are presented and their prevalence in station observational data is confirmed. The inter-annual variability of the CAB latitude and detection frequency is found to explain at least 55% of inter-annual spring rainfall variability in southern Africa between 15 to 25S. Links are established with the Angola and Kalahari heat lows, and tropical temperate trough events.