Abstract
AbstractProjected 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 it is found to be an indicator of the location of the southern edge of the African rain belt. A related convergence-line and dryline feature, described in this paper as the Kalahari discontinuity (KD), is also identified. It is established that either a dryline CAB or KD is present in southern Africa for over 95% of days between August and December, with arc lengths typically exceeding 10°. The seasonal and diurnal cycles of the CAB and the KD are presented, and their prevalence in station observational data is confirmed. The interannual variability of the CAB latitude and detection frequency is found to explain at least 55% of interannual 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.
Highlights
Southern Africa is one of the few inhabited regions of the world in which CMIP5 models project a robust decline in annual rainfall under future warming scenarios (IPCC 2013, p. 1061)
Since both the level westerlies (LLW) and the Indian Ocean trades originated in the Southern Hemisphere, their confluence was not considered to be an intertropical convergence zone (ITCZ), and instead was labeled the ‘‘Congo air boundary.’’ the ITCZ was defined as the convergence of the LLW with the harmattans in North Africa, of the LLW with the northeasterly monsoon winds in East Africa, and of the southeasterly trades with the northeasterly monsoon in Tanzania and northern Mozambique
Surface drylines located along convergence lines at the edges of the easterly trade winds are persistent and important features of the southern African climatology
Summary
Southern Africa is one of the few inhabited regions of the world in which CMIP5 models project a robust decline in annual rainfall under future warming scenarios (IPCC 2013, p. 1061). The dynamics of the southern shift in the rain belt in spring and early summer are not fully understood (Nicholson 2018), and further investigation of the present day seasonal cycle is required before the mechanisms of future change may be fully characterized. We show that the Southern Hemisphere spring wind convergence and humidity gradients are the climatological aggregation of the Congo air boundary (CAB). Taljaard (1986) listed the placement of the interocean trough as one of the 10 factors that play a key role in the weather over southern Africa Both Torrance (1979) and Leroux (2001) point out that the converging air masses differ strongly in humidity, the LLW being close to saturation after their journey over the Congo rain forest, and the easterly trades having been ‘‘continentalized’’ and dried out over the great escarpment.
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