Summer heatwaves in China during 1961-2021: The impact of humidity

In additon to temperature, heatwaves are also closely related to humidity. In this study, daily wet-bulb temperature was used to define heatwaves (WBHWs) with a unified threshold for more than 2000 meteorological stations throughout China. The climatic averages of summer heatwaves and changes from 1961 to 2021 were investigated. WBHWs primarily occur in three key subregions, namely North China, South China, and Yangtze River valley (YRV). Humidity makes heatwaves in South China more severe, with an average of 25 days per summer. Over the past decades, the frequency and intensity of WBHWs in China considerably increased accompanied by decreasing relative humidity (RH) and increasing temperature. South China experiences the strongest increase in WBHWs, with change rate fivefold higher than the national average. The occurrence of WBHWs in YRV is dominated by temperature, whereas in North and South China it is controlled by RH. The longterm increases in WBHWs for China and its three key subregions are primarily caused by temperature changes, but RH affects WBHWs at interdecadal time scales and generally exerts a negative contribution. The average contribution rates of temperature and RH to the increase in WBHWs in China for the period 1991 - 2021 are 2.3 days and - 0.6 days. Changes in WBHWs in North China are almost completely controlled by temperature, since the opposite phases of specific humidity and temperature result in a negligible effect of RH. Temperature and RH in other two subregions have substantial positive and negative effects, respectively. The negative effect of RH in South China is mainly attributed to the indirect effect of temperature, while that in YRV, particularly in the lower reaches, is a combination of specific humidity and temperature.