One of the major misconceptions about precipitation drivers over Africa is that they are basically associated with regions of localized convection that are embedded in the Intertropical Convergence Zone (ITCZ). However, in a very neatly presented analysis, Sharon Nicholson showed that this is a rather oversimplified explanation and that the ITCZ paradigm fails to account for wind patterns that are zonally asymmetric (Nicholson, 2018). Nicholson (2009) proposes the term “tropical rain belt”, instead of ITCZ (with an exception for coastal regions, where the impact of the prevailing trade winds is important).
One such case where the ITCZ paradigm does not apply, is the surface discontinuity of specific humidity that originates from the convergence of low-level westerly winds and easterly trade winds originating from the Indian ocean, known as the “Congo Air Boundary” (CAB) (Howard and Washington, 2019). The seasonal progression of CAB has been identified as an important driver for the southward movement of the African rain belt (Howard and Washington, 2019) that controls precipitation over southern Africa (SAF). According to (Howard and Washington, 2019), CAB displays high specific humidity gradients during August - October, but during November and December it weakens, due to the fact that the converging air masses mix and the humidity gradient becomes less steep. A distinct example of CAB can be found in Fig. 1, displaying the convergence of the humid air masses with the dry air masses. Convective activity is observed north of CAB (Fig. 2).
Figure 1: Relative humidity at 900 hPa at 15/10/2020.
Figure 2: CAPE (Convective Available Potential Energy) index at 15/10/2020.
*All maps are taken from Windy