Tracing future spring and summer drying in southern Africa to tropical lows and the Congo Air Boundary

In southern Africa, models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) predict robust future drying associated with a delayed rainy-season onset in the austral spring and a range of wetting and drying patterns in the austral summer. This paper relates these rainfall changes to dynamical shifts in two classes of weather systems: the Congo Air Boundary (CAB) and tropical lows. Objective algorithms are used to track these features in CMIP5 model output. It is then established that the climatological locations and frequencies of these systems are reasonably well represented in the CMIP5 models. RCP8.5 end-of-twenty-first-century projections are compared with historical end-of-twentieth-century simulations. Future projections in tropical-low locations and frequencies diverge, but indicate an overall average decrease of 15% and in some cases a northward shift. The projected spatial change in the tropical-low frequency distribution is weakly positively correlated to the projected spatial change in the austral summer rainfall distribution. Meanwhile, future projections indicate a 13% increase in CAB frequency from October to December. This is associated with the gradual climatological CAB breakdown occurring half a month later on average in end-of-twenty-first-century RCP8.5 projections. A delay in the gradual seasonal decline of the CAB prevents rainfall to the south of the CAB’s mean position, most of which is shown to occur on CAB breakdown days, hence creating the austral spring drying signal and delayed wet-season onset. Intermodel variability in the magnitude of CAB frequency increase is able to explain intermodel variability in the projected drying.

在非洲南部地区，耦合模式比较计划第五阶段（CMIP5）的模式预测未来将出现显著的干旱化趋势，具体表现为南半球春季雨季起始延迟，且南半球夏季存在多样的干湿变化格局。本研究将上述降雨变化与两类天气系统的动力调整相关联：刚果气团边界（CAB）与热带低压。研究采用客观算法对CMIP5模式输出结果中的此类天气系统特征进行追踪，并证实CMIP5模式能够较为合理地再现这两类系统的气候态分布位置与发生频率。本研究对比了21世纪末典型浓度路径8.5（RCP8.5）情景下的未来预估结果与20世纪末历史模拟试验结果。热带低压的分布位置与发生频率的未来预估存在差异，但整体平均发生频率下降15%，部分区域的热带低压呈现北移特征。热带低压频率分布的预估空间变化与南半球夏季降雨分布的预估空间变化呈弱正相关。与此同时，未来预估结果显示，10月至12月期间CAB的发生频率将提升13%，这与21世纪末RCP8.5情景下刚果气团边界的气候学消散过程平均延迟半个月的现象密切相关。刚果气团边界季节性减弱过程的延迟，会阻碍其平均位置以南区域的降水——而该区域的多数降水均发生在刚果气团边界消散当日，由此形成了南半球春季干旱信号与雨季起始延迟的现象。不同模式间CAB发生频率增幅的差异，能够解释各模式预估干旱幅度的差异。