Is the western North Pacific subtropical high a necessary condition for heat waves over the mid-lower reaches of the Yangtze River?

Heat waves (HWs) can pose a serious threat to human health and natural ecosystems. Meanwhile, the mid-lower reaches of the Yangtze River are known as one of the most vulnerable regions suffering such disasters. At the mention of their potential drivers, the western North Pacific subtropical high (WNPSH) has been widely accepted as the key atmospheric circulation regulating these high-temperature extremes. Although considerable studies have proved the importance of the WNPSH, not enough work pays attention to the HWs which are not caused by the anomalous WNPSH. This study utilizes the instantaneous co-recurrence ratio α based on the Dynamical System (DS) method to quantify the forcing strength from the WNPSH to HWs, and classify HWs into four types according to their value of α and spatial pattern of the 500 hPa geopotential height, as high- α 1, 2 and low- α 1, 2. Results show that coverage by the WNPSH is not a necessary condition for the formation of HWs, which may also result from the anomalous South Asian high (SAH) and tropical cyclones (TCs), and the value of α can indicate whether the HWs are coupled with the WNPSH or not. Furthermore, the mechanisms behind these anomalous atmospheric systems are also different. The eastward-extended SAH in high- α HWs is associated with the latent heat of local condensation induced by the abnormal rainfall, while accompanied by an eastward-propagating wave train in low- α 1. The intensification of the WNPSH in high- α HWs are attributed to both meridional and zonal wave trains originating from the maritime Continent and Western Europe, respectively, and the anomalous TCs in low- α 2 are mainly due to the warmer sea surface temperature over the basin-wide equatorial Pacific, which may heat the bottom atmosphere and provide the convective instability.