Tropical Pacific Influence on Summertime South African High‐Frequency Temperature Variability and Heat Waves

Abstract

Day-to-day weather variability affecting Southern Africa is driven by a wide range of weather systems such as mid-latitude cyclones, cut-off lows, and tropical-temperate troughs (Lennard, 2019). The activity of many of these transient phenomena, as measured by their amplitude, duration, and/or frequency, exhibits notable year-to-year variations. For instance, cut-off lows (Favre et al., 2012;Singleton & Reason, 2007), tropical temperate troughs (Ratna et al., 2013), and cold fronts associated with mid-latitude cyclones (Picas & Grab, 2021) have been found to be more frequent during the cold phase of El Niño-Southern Oscillation (ENSO), that is, La Niña. As such, the likelihood of experiencing weather extremes associated with these systems may be predictable given ENSO's long-range predictability (Tang et al., 2018). Mulenga et al. (2003) suggested that ENSO, in the context of dry summers, impacts weather systems over Southern Africa through modulations of large-scale seasonal features of the atmospheric circulation. More specifically, the enhanced upper-level westerly winds over southwestern South Africa as part of El Niño's teleconnection could modulate baroclinic wave activity by steering frontal systems more toward southwestern South Africa, and less toward southeastern South Africa. The enhanced westerlies have been identified in the signature of El Niño in the zonal-mean circulation (L'Heureux & Thompson, 2006) and are accompanied, as expected from thermal wind balance, by dipolar temperature anomalies which reinforce the climatological equatorward gradient of temperature just south of South Africa. The associated enhancement of lower-tropospheric baroclinicity may contribute to the development of baroclinic eddies, but the detailed dynamical mechanisms and whether it has an impact on Southern African weather remain unknown.The primary goal of this work is thus to elucidate the dynamical processes regulating year-to-year variations in high-frequency atmospheric temperature variability that are associated with the aforementioned weather systems and to understand the role played by ENSO in these variations. Focus is placed on Austral summertime, when El Niño events are typically amplified (Rasmusson & Carpenter, 1982), have a greater impact on the Southern Hemisphere (SH) extratropics (L'Heureux & Thompson, 2006), and when weather variability has important socioeconomic impacts on agriculture and human health (e.g., Sazib et al., 2020).