Controls on Southern African Wet Season Timing under Climate Change

Southern Africa's economy, rural population and economic development are highly dependent on rainfall, which varies significantly on intraseasonal to multidecadal timescales (Cook and Reason, 2004; Tadross, Hewitson and Usman, 2005; Conway et al., 2015). The region is also very vulnerable to climate-related risks, such as drought and flooding, and climate change, yet has limited ability to prepare or react to these due to the socio-economic status of the subcontinent (Dunning, Black and Allan, 2018).

This research aims to understand why and how the Southern African rainfall season becomes later under climate change, and what role the CAB has in this process. The DPhil will be done through the paper route, so the objectives and methods are separated into three separate papers.
The first paper will focus on understanding what key process(es) cause CAB breakdown, both in reanalysis data, historical GCM simulations and higher resolution models. This is because a better understanding of the causation of CAB breakdown will enable better understanding of the influence it may have on rain belt movement under climate change. Firstly, three reanalysis datasets - ERA-5, ERA-Interim and MERRA2 - will be analysed to understand how CAB breakdown occurs and what atmospheric processes are involved. CAB locations will be identified using the method of Howard and Washington (2019), and the data fields of relative humidity, specific humidity, surface winds, temperature and atmospheric pressure will be used. The analysis will then be repeated in the historical simulations of a large group of CMIP6 models, to enable comparison with the reanalysis results to see how well the models are capturing CAB breakdown characteristics and the process of breakdown. Thirdly, the key process(es) will be evaluated in high resolution regional models, to see if they can reveal the characteristics of these processes in more detail.
The next research focus and paper of this DPhil will be an evaluation of how the key process(es) influencing CAB breakdown occur in climate change simulations of CMIP6 models, and what influence this has on rain belt timing. This will test in what way(s) the CAB breakdown is connected to the delayed movement of the rain belt under climate change. To begin, rain belt characteristics and their timing will be calculated for both historical and future CMIP6 model runs, in order to see how the timing of the rain belt changes. The next step is to separate the models into those with the greatest and smaller rain belt timing changes, and evaluate the key process(es) in CAB breakdown in each of them. This will likely require an evaluation of relative humidity, moisture, surface winds, pressure, and temperature data fields. This paper will use RCP8.5/SSP585 in the future climate simulations in to maximize any signal of change.
As work on the DPhil begins, the scope and focus of the third paper is likely to shift, depending on the interesting discoveries found in papers 1 and 2. However, two ideas for this paper include evaluating CAB breakdown key processes in idealized experiments, or assessing the sensitivity of rain belt timing changes to greenhouse gas emissions. It is likely to be difficult to find a clear causality in the second paper due to the great variety of models in the CMIP6 ensemble, so idealized experiments between the CAB breakdown key processes and rain belt timing under climate change could clarify the relationships between these variables. The latter idea would be a more policy-focused paper, investigating the climatic requirements of Southern African crops and when these are no longer met under different SSP emissions scenarios in CMIP6 models.