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.

中文翻译：

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气候变暖背景下南非极端低温事件的动态


尽管南部非洲地表气温呈强劲变暖趋势，但极端低温（ELT）事件仍然时有发生。最近的一次 ELT 事件影响了南非，导致冠状病毒大流行期间的社会经济活动中断。 2021年7月22日至24日期间，至少打破了27项长期低温记录，降雪大多发生在亚热带地区的高地。本研究利用气象站数据和欧洲中期天气预报中心 (ECMWF) 的 ERA5 和 ERA5-Land 重新分析来研究以南非高地为重点的 ELT 事件动态。我们的方法采用多尺度分析，以地表气温、降雪事件和地面霜日的长期趋势和气候学为背景，以了解观察到的极端天气异常。我们发现白天和夜间温度呈现一致且具有统计显着性的变暖趋势，地面霜冻相应减少。 2021 年 7 月的 ELT 事件是复杂环流异常综合作用的结果，其中包括延伸至海面的强烈离岸截止低压 (COL)（以及相关的波浪破碎）、冷锋和 S 型脊状反气旋，所有这些都在加剧来自南大洋的表面冷空气平流。最重要的发现是，COL 不需要进入南非就能在该国造成恶劣天气。我们的研究有助于了解亚热带地区在强劲的变暖趋势下极端寒冷的发生和动态。