Controls on Southern African Wet Season Timing under Climate Change

南部非洲的经济、农村人口和经济发展高度依赖于降雨，而降雨在季节内到几十年的时间尺度上变化很大（Cook and Reason, 2004; Tadross， Hewitson和Usman, 2005; Conway et al., 2015）。该地区也非常容易受到气候相关风险的影响，如干旱和洪水以及气候变化，但由于次大陆的社会经济地位，该地区准备或应对这些风险的能力有限（Dunning， Black和Allan， 2018）。这项研究旨在了解在气候变化下南部非洲降雨季节为何以及如何变晚，以及CAB在这一过程中发挥了什么作用。哲学博士将通过论文路线完成，因此目标和方法分为三篇单独的论文。第一篇论文将重点了解在再分析数据、历史GCM模拟和更高分辨率模型中导致CAB崩溃的关键过程。这是因为更好地了解CAB破坏的原因将有助于更好地了解它在气候变化下可能对雨带运动的影响。首先，将分析三个再分析数据集——ERA-5、ERA-Interim和MERRA2，以了解CAB是如何发生的以及涉及哪些大气过程。CAB位置将使用Howard and Washington（2019）的方法确定，并使用相对湿度、比湿度、地面风、温度和大气压力等数据字段。然后将在一大批CMIP6模型的历史模拟中重复分析，以便与再分析结果进行比较，以了解模型如何很好地捕获CAB击穿特征和击穿过程。第三，在高分辨率区域模型中对关键过程进行评估，看看它们是否能更详细地揭示这些过程的特征。本博士的下一个研究重点和论文将是评估CMIP6模式气候变化模拟中影响CAB击穿的关键过程是如何发生的，以及这对雨带时序的影响。这将测试在气候变化下，CAB故障是如何与雨带延迟运动联系起来的。首先，将计算历史和未来CMIP6模式运行的雨带特征及其时序，以了解雨带时序如何变化。下一步是将模型划分为雨带时序变化最大和较小的模型，并评估每个模型中CAB击穿的关键过程。这可能需要对相对湿度、湿度、地面风、压力和温度数据场进行评估。本文将在未来的气候模拟中使用RCP8.5/SSP585来最大化任何变化信号。随着哲学博士的工作开始，第三篇论文的范围和重点可能会发生变化，这取决于论文1和2中发现的有趣发现。然而，本文的两个想法包括评估理想实验中的CAB击穿关键过程，或评估雨带时间变化对温室气体排放的敏感性。由于CMIP6集合中模型的多样性，在第二篇论文中可能很难找到明确的因果关系，因此气候变化下CAB击穿关键过程与雨带时序之间的理想化实验可以澄清这些变量之间的关系。后者的想法将是一篇更侧重于政策的论文，研究在CMIP6模型中不同的SSP排放情景下，南部非洲作物对气候的要求，以及这些要求何时不再得到满足。

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.