South China experiences extreme heat (EH) most frequently in eastern China. This study specifically explores the large-scale circulation anomalies associated with long-lived EH events in south China. The results show that there is an anomalous cyclone (anticyclone) and active (inactive) convection over south China (the western Pacific) before the EH onset; then, an anticyclone develops and moves northwestward and dominates over south China on the onset day. The anomalous anticyclone maintains its strength over south China and then diminishes and is replaced by another cyclone migrating from the western Pacific after the final day of the EH event. Consequently, the temperature increases over south China around the onset day and is anomalously warm for approximately 10 days on average and then decreases shortly thereafter. The fluctuating anomalies over south China and the western Pacific are intimately related to two intraseasonal oscillation (ISO) modes, namely, the 5–25- and 30–90-day oscillations, which originate from the tropical western Pacific and propagate northwestward. The 5–25-day oscillation is vital to triggering and terminating EH, accounting for approximately half of the original temperature and circulation anomaly transitions. The 30–90-day oscillation favors the persistent warming during EH events, accounting for approximately one-third of the original prolonged warming and anticyclonic anomaly. This result suggests that different ISO modes play crucial roles at different stages of the events. Moreover, a higher annual frequency of long-lived EH days in south China is associated with the transition phase from El Niño to La Niña. It is suggested that both medium-range and interannual forecasting of long-lived EH in south China are possible.

Southern China, located in the tropical–subtropical East Asian monsoonal region, presents a unique anticyclonic–cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in southern China, the anticyclonic–cyclonic anomalies propagate northwestward over the Philippines and southern China. Before the EH onsets, the anticyclonic anomaly dominates southern China, resulting in stronger subsidence over southern China and stronger southerly (southwesterly) flow over the western (northern) margins of southern China. The southerly (southwesterly) flow transports more water vapor to the north of southern China, thus, together with the local stronger subsidence, resulting in drier air condition and accordingly favoring the occurrence of EH. Conversely, after the EH onsets, the cyclonic component approaches southern China and offsets the high temperature. The oscillations of temperature and circulation anomalies over southern China exhibit a periodicity of about 10 days and indicate the influence of a quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestward. Therefore, the 5–25-day-filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolution of the filtered circulation remarkably resembles the original anomalies with comparable amplitudes, indicating that the quasi-biweekly oscillation is critical for the occurrence of EH in southern China. The quasi-biweekly oscillation could explain more than 50% of the intraseasonal variance of daily maximum temperature Tmax and vorticity over southern China and 80% of the warming amplitude of EH onsets. The close relationship between the circulation of the quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecasting for high temperature in southern China.