LAUSR

In this study, the total days, mean duration, and intensity of extreme hot events over southern China during the 1971–2020 warm seasons are analyzed, based on daily maximum and minimum temperatures, by comparing the newly proposed independent hot day (IHD), independent warm night (IWN), and compound extreme (CMPD, i.e., the continuous occurrences of hot days and hot nights) to the traditionally defined hot day and warm night. Relationships between the hot extremes and urbanization are explored with 1-km resolution population density data. Results show obvious differences in the spatial distributions between IHD, IWN, and CMPD over southern China. Positive correlations of 0.43, 0.41 and 0.37 are found between the population density and the total days, mean duration and mean intensity of CMPD, respectively, which are qualitatively similar to those using the traditional hot days and warm nights. In contrast, negative correlations between the IHD and IWN indices and the population density are found, as those indices are more apparent over rural areas. Moreover, total days, mean duration and mean intensity of CMPD increase significantly with trends of about 103% decade-1, 21% decade-1 and 38% decade-1, respectively, during the rapid urbanization period from mid-1990s to 2020, which are about 4.9, 2.1 and 2.4 times of their counterparts from 1970 to mid-1990s, while less significant and smaller differences between the two eras are found in IHD and IWN. These results will provide a new scientific basis for evaluating climate models of hot extremes in southern China and have important implications for the other urbanized regions as well.