Abstract:
Despite robust warming trends in surface air temperatures over southern Africa, extreme low temperature (ELT) events still occur from time to time. A recent ELT event affected South Africa resulting in disruptions in socio-economic activities amid a coronavirus pandemic. At least 27 long-term low temperature records were broken during 22–24 July 2021, with snow falls observed mostly over high ground in subtropical districts. This study employs weather station data and European Centre for Medium-Range Weather Forecasts (ECMWF)'s ERA5 and ERA5-Land reanalyses to investigate dynamics of the ELT event focusing on the South African Highveld. Our approach employs multiscale analysis, with long term trends and climatologies of surface air temperatures, snow events and ground frost days presented as background to understanding the observed extreme weather anomalies. We found consistent and statistically significant warming trends in daytime and overnight temperatures, with corresponding decreases in ground frosts. The July 2021 ELT event resulted from a combination of complex circulation anomalies which included an intense offshore cut-off low (COL) that extended to the surface (and associated wave breaking), a cold front and a Type-S ridging anticyclone, all intensifying surface cold air advection from the Southern Ocean. A most significant finding is that COLs do not need to enter South Africa to cause severe weather over the country. Our study contributes to understanding the occurrence and dynamics of cold extremes in subtropical regions, against a robust warming trend.
