Combined Influence of 10–30‐Day Tropical and Mid–High‐Latitude Intraseasonal Oscillations on the Rapid Increases of Humid Heatwaves in Southern China

As a typical hotspot of heatwaves, southern China experienced a marked shift from dry to humid heatwaves in the recent decade of 2013–2022. Significant 10–30‐day intraseasonal oscillation (ISO) in temperature and humidity is a major contributor to the humid heatwaves. Configuration of the 10–30‐day quasi‐barotropic wave train at mid–high latitudes with the dipolar pattern of tropical convection leads to the concurrence of strong moisture convergence and anomalous descents over southern China. The increased downward longwave radiation in association with moisture enhancement, shortwave radiation and adiabatic heating in association with descents bring about the persisting hot and humid conditions in humid heatwaves. Due to stronger barotropic energy conversion from mean flow to 10–30‐day circulation at mid–high latitudes, and stronger tropical convection activities in the recent decade, the occurrence of 10–30‐day circulation configuration induced humid heatwaves has increased by 13.3%, thus leading to the predominance of humid heatwaves. Plain Language Summary Southern China is a typical hotspot of heatwaves. In the recent decade of 2013–2022, the southern China experienced a marked transition from dry heatwaves to humid ones, with the occurrence frequency of humid heatwaves becoming more than 4 times that of dry counterpart. As the humid heatwaves usually lasting for 3–5 days are more directly associated with the intraseasonal (10–90‐day) variability, we have identified significant 10–30‐day intraseasonal oscillation (ISO) in temperature and humidity in southern China, and found their critical role in inducing the lethal humid heatwaves. This finding provides the scientific clues for extending the forecast lead time of humid heatwaves. The circulation configuration of the 10–30‐day atmospheric ISOs at mid–high latitudes and tropics leads to the concurrence of strong moisture convergence and anomalous descents over southern China. The increased downward longwave radiation in association with moisture enhancement, shortwave radiation and adiabatic heating in association with descents bring about the persisting hot and wet conditions in humid heatwaves. As more barotropic energy are transferred from mean flow to 10–30‐day circulation at mid–high latitudes, and tropical 10–30‐day convection activities get stronger, the occurrence of 10–30‐day circulation configuration induced humid heatwaves have increased from 39% in the period of 2003–2012 to 52.3% in the p