Relative importance of driving factors for daytime and nighttime heatwaves in China

The increasing frequency and intensity of heatwaves (HWs) present significant threats to human safety and natural ecosystems while the relative importance of their driving factors remains poorly understood. In this study, we first investigated the spatiotemporal variabilities of daytime and nighttime HWs across four subregions of China from 1961 to 2022. The daytime and nighttime heatwave number (HWN) and the number of HW days (HWF) were found to increase significantly during this period in most areas of China. To explore the underlying mechanisms, we examined the relationship between HW indices and eight potential driving factors: surface air temperature (T2M), soil moisture (SM), sensible heat flux (SH), latent heat flux (LH), surface net solar radiation (SW) and long‐wave radiation (LW), El Niño‐Southern Oscillation (ENSO) and Atlantic Multidecadal Oscillation (AMO). The long‐duration linear trends of HWs and their driving factors were removed before investigating their relationships. The results showed that T2M, SH, SW, LW, and AMO were positively correlated with daytime HWs across all four subregions of China. The relative importance of these factors to HWs was then quantified using redundancy analysis and hierarchical partitioning. Among the eight factors, T2M was identified as the most significant effect on both daytime and nighttime HWs, that is, HWN, HWF and the length of the longest heatwave (HWD) over all subregions. The individual effects of T2M to daytime HWN were 40.08%, 63.24%, 19.17%, and 48.06% over NC, NW, SC, and TP regions, respectively. For nighttime HWN, its contributions were 64.22%, 69.98%, 28.61%, and 56.43% over the same regions, which were all significant at the 0.01 or 0.001 level. Additionally, SM or AMO was identified as another key factor influencing daytime or nighttime HWs in China. These findings highlight the critical drivers of HWs and provide valuable insights into the mechanisms underlying their occurrence in China.