Granger causality of the local Hadley cell and large-scale cloud cover over South Africa

This study demonstrates that Hadley cell dynamics could be used as a proxy to determine cloud cover and thus solar energy potential over South Africa. Granger causality was used to investigate causal interactions between the Hadley cell and cloud cover for the period 1980–2015, and such links were established. Areas of strong causality are found over the northwestern parts of South Africa. Moreover, weak causality from cloud cover to the Hadley cell does exist, with vertical velocity being the main variable responsible for this causality, which hence indirectly links cloud cover to Hadley cell causality. Significance: Hadley cell dynamics may be used to identify regions of cloudlessness over South Africa. Hadley cell dynamics may further be used as a proxy for cloud cover towards understanding the solar energy potential in South Africa within the context of climate variability and change

Dynamics of an extreme low temperature event over South Africa amid a warming climate

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