Future Changes in Winter-Time Extratropical Cyclones over South Africa from CORDEX-CORE Simulations

Key Points: 

• Reduction in extratropical cyclone frequency over South Africa, with an increase in storm tracks along the west coast
• Storm severity varies regionally over South Africa, with increases in the southern coast and northern regions
• Decrease in ETC rainfall over South Africa, especially Cape Town, but with notable increases in the South Indian Ocean

Abstract: 
Extratropical cyclones (ETCs) significantly impact mid-latitude weather patterns and are crucial for understanding the societal implications of regional climate variability, climate change, and associated extreme weather. In this study, we examine the projected future changes in winter-time ETCs over South Africa (SA) using simulations from CORDEX-CORE Africa. We utilized three regional climate models (RCMs), each driven by three different global climate models (GCMs) that simulate both the current climate and a future climate experiencing strong human-induced warming. From these, we assess changes in ETC frequency, track density, intensity, storm severity, and associated rainfall. The results indicate a significant reduction in the aggregate ETC frequency and track density, although track density is projected to increase prominently along the western coastal regions. Models show mixed trends in cyclone intensity projections, but overall results indicate weaker future cyclones, with reduced peak relative vorticity and increased minimum sea level pressure. Examining the Meteorological Storm Severity Index (METSSI) reveals notable regional variations in future storm severity. Average rainfall associated with ETCs is projected to decrease across SA, especially around Cape Town, highlighting a potential shift in the spatial distribution of rainfall with substantial consequences for water supply. We further investigated extreme ETCs (EETCs) and found that the trends for EETCs are generally similar to those for ETCs, with a notable decrease in frequency and regional variations in storm severity. These findings underscore the importance of developing targeted adaptation strategies to address the projected impacts of future ETCs on SA’s climate and communities. 

Plain Language Summary: 
This study investigates how winter-time extratropical cyclones (ETCs) over South Africa might change in the future due to human-forced climate change. These storms are vital to the region as they bring heavy rain and strong winds, which can significantly impact local communities by affecting daily life, economic activities, human safety, and the environment. Using advanced climate model simulations under a high greenhouse gas scenario (RCP 8.5), we explored future changes in the frequency, intensity, and rainfall of these storms. Our findings suggest a potential decrease in the overall number of ETCs, with an increase in storm occurrences along the western coastal regions. The storms are projected to be generally weaker in terms of wind speed and may bring less rain overall. However, areas like Cape Town could experience more severe storms, leading to changes in rainfall patterns that could have substantial impacts on water resources, agriculture, and infrastructure. Additionally, we examined extreme ETCs (EETCs) and found similar trends, with a decrease in their number and varying changes in severity across different regions. Understanding these potential changes is crucial for developing strategies to minimize the adverse impacts on South Africa’s environment and communities.