An unprecedented high temperature event in southern China in autumn 2021 and the essential role of the mid-latitude trough

Studies about high temperatures in China rarely touched the period beyond summer. But in September–early October 2021, a record-breaking high temperature event occurred in southern China, and affected agriculture in many provinces. Both the daily maximum temperature and the number of high temperature days were the highest in September since 1951. To better understand the different features and possible causes of the extreme event in autumn from those in summer, the analysis of atmospheric circulation patterns was conducted. The western Pacific subtropical high (WPSH) was quite stronger and extended much more westward in most of the days in September 2021 than its climatology, which directly caused this unprecedented autumn high temperature event. Besides, the mid-latitude trough over the east of Japan may also play an essential role in this high temperature event. Due to the southward extension of the trough, a cyclonic circulation anomaly appeared at 850 hPa, and its northerly wind component prevailed in the west of the trough and blocked the eastward retreat of the WPSH. As a result, southern China was stably controlled by the WPSH, which led to the persistence of this high temperature event. The conclusion is further confirmed by the statistical results from the analysis of the multi-year data and analogue analysis. Compared to the previous studies, this study reveals the essential role of the mid-latitude trough and highlights the joint impacts of the WPSH and the trough in causing the unprecedented autumn high temperature event.