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 25◦S. Links are established with the Angola and Kalahari heat lows, and tropical temperate trough events.