Southern African synoptic climate dynamics and future change: tropical lows and the Congo Air Boundary

Southern Africa's development and economy is dependent on a rainfall resource that is projected to decline in coming decades as a result of anthropogenic climate change. This rainfall decline has been attributed to a half month delay in the onset of the wet season by the end of the \nth{21} century based on RCP8.5 projections, shortening the growing season and reducing the agricultural viability of the region. Yet many aspects of the basic climatological state of the southern African atmosphere remain chronically understudied. Rainfall onset and regional rainfall climate depend on the behaviour of day-to-day weather systems along the southern margin of the African rain belt. However, the dynamics of these systems are not well understood. This thesis addresses this problem by characterising the frequencies, spatial distributions, dynamics, interannual variability and future change of two classes of weather systems: tropical lows and the Congo Air Boundary (CAB). A vortex tracking algorithm is optimised to detect tropical lows in sub-daily reanalysis data and climate model output. It is found that tropical lows aggregate to form the synoptic expression of the summer Angola Low. Tropical lows are shown to exhibit a unique anchoring behaviour in Angola due to interactions with nocturnal low-level jets associated with differential surface heating. 70% of rainfall in south-east Angola and 31% of summer rainfall across the southern African tropical margin is attributed to tropical lows. Meanwhile, the majority of interannual rainfall variability in Angola, northern Namibia and western Zambia derives largely from tropical lows. Climate model projections indicate that tropical lows will become 15% less frequent by the end of the 21st century. Future change in the spatial distribution of tropical lows contributes to model disagreement over the spatial pattern of rainfall change in the summer season. Computational edge detection is applied to identify surface drylines and convergence lines associated with the CAB, a forgotten feature of the southern African climatology. The CAB is shown to be a sharp discontinuity which marks the southern edge of the African rain belt, and is detected on at least 80\% of days from August to October. Its seasonal southward progression over southern Africa is presented, and a close dynamic link between the CAB and the Angola Heat Low is demonstrated. Seasonal rainfall variability is closely linked to the CAB, since tropical rainfall does not occur to the south of the CAB, with CAB variability explaining at least 55% of local monthly rainfall variability. The projected delayed wet season onset is shown to be related to a projected 0.7° CAB latitude increase and a 13% CAB frequency increase from October to December, with inter-model CAB variability explaining 46% of inter-model variance in projected rainfall change.