Large‐scale circulation features responsible for different types of extreme high temperatures with extreme coverage over South China

Abstract

AbstractThis study classifies the summer extreme high temperatures (EHT) over South China into Day‐EHT, Night‐EHT and Mixed‐EHT (corresponding to EHT with extreme coverage only in daytime, only at night and both in daytime and at night) and compares their associated circulation patterns. All types of EHT are associated with the intensification and expansion of western Pacific subtropical high (WPSH). Concurrently, anomalous subsidence occurs over South China, which reduces the cloud cover and favours daytime warming. However, different types of EHT feature different circulation configurations. Day‐EHT are influenced by the northwestward‐extending WPSH and tropical cyclones, manifesting an abnormal anticyclonic‐cyclonic pair. Anomalous northeasterly winds dominate over South China, which cause lower humidity and faster heat dissipation and thus the EHT disappear at night. Night‐EHT are related to the westward‐extending WPSH and intensified East Asian westerly trough, presenting a subtropical anticyclonic anomaly and a mid‐latitude cyclonic anomaly. Anomalous southwesterly and northerly flows meet around South China, favouring more water vapour convergence in situ. The above‐normal humidity results in smaller amplitude of cloud cover reduction and weaker daytime warming compared to Day‐EHT, but favours heat maintenance and Night‐EHT. Moreover, the mid‐high‐latitude wave train preceding Day‐EHT (Night‐EHT) lies more northward (southward) with the upstream high‐pressure anomaly occurring around the Ural Mountains (Caspian Sea). Mixed‐EHT combine the anomalies for Day‐EHT and Night‐EHT and weak humidity anomaly occurs over South China, thus the daytime EHT could persist until night albeit cools down slightly. These distinguished features indicate that different types of EHT should be considered separately for better forecast and projection of EHT.