Rapid increase in warm‒wet compound extreme events with high health risks in southern China: Joint influence of ENSO and the Indian Ocean

High temperature and high humidity pose notable threats to the health and survival of humans, and the impact of compound extreme events involving multiple meteorological elements is usually greater than the sum of the impacts of individual extreme events. However, there is limited research on the compound extreme meteorological events, defined based on the extreme of health risks rather than just the extreme of meteorological elements, necessitating further investigation into their characteristics and underlying driving factors. Using daily records of emergency ambulance dispatches from selected Chinese cities, together with daily average temperature and relative humidity data, this study identified and quantified warm‒wet compound extreme events with high health risks (HRWWs) that occurred in southern China during 1979–2022. Results revealed that HRWWs were most prevalent in the coastal region of southern China. The annual frequency of HRWWs across China underwent a marked shift in 2014, with the rapid increase in recent years primarily contributed by the increase in summer HRWWs and advance and extension of the period of occurrence of HRWWs in summer. Further analysis indicated that the changes in HRWWs in southern China during summer were primarily regulated by the El Niño‒Southern Oscillation (ENSO) of the preceding winter and the springtime Indian Ocean basin-wide (IOBW) mode. El Niño and Indian Ocean warming force a reverse Walker circulation in the tropical Indian Ocean‒western Pacific, strengthening the Philippine anticyclone (PAC) in summer. This weakens convection in the northwestern Pacific and enables the western Pacific subtropical high to extend westward over coastal areas of southern China. The resulting abnormal atmospheric circulation causes prevailing subsidence over southern China, which is unfavorable for precipitation but conducive to maintaining high surface temperatures. Despite reduced precipitation, water vapor flux is increased owing to strengthened southwesterlies associated with the PAC that enhance regional humidity. Consequently, as temperature and humidity levels rise, HRWWs occur more frequently in southern China, as evidenced since 2014. Further analysis suggested that the contribution of intensification of IOBW warming to the sudden increase in HRWW occurrence over the past decade has been greater than that of ENSO.