Synergies or trade-offs between surface urban heat island and hot extreme: Distinct responses in urban environments

Hot extremes are among the most damaging climate extremes to human society, with urban areas being particularly vulnerable due to the surface urban heat island (SUHI). Despite the growing frequency and intensity of surface hot extreme (SHX), a comprehensive understanding of the patterns and drivers of SUHI responsiveness to SHX (difference in SUHI intensity [delta SUHII] between SHX and non-hot extreme conditions [NSHX]) remains obscure. This study aims to shed light on the spatial-temporal pattern of SHX and the distinct SUHI responses to SHX throughout climatically diverse Chinese urban clusters using the seamless daily land surface temperature (LST) dataset. Our results provided evidence that Chinese urban clusters have experienced increasingly frequent, prolonged, and intensive SHX events regardless of climatic context, with stronger occurrences in summer and fall. Spatially, urban clusters in the northern regions have been subjected to more intensified SHX episodes compared to their southern counterparts. Furthermore, our findings revealed distinct delta SUHII responses to SHX across China, demonstrating synergies between SUHI and SHX when SHX events constrained to urban areas whereas trade-offs when SHX isolated within rural settings. The magnitude of delta SUHII depended upon the relative intensity of SHX between urban and rural settings, and heightened delta SUHII generally associated with more intensified SHX events in urban than rural areas. Additionally, the explainable machine learning-based driver exploration showed that delta SUHII was largely controlled by disparities in evaporative cooling (delta ET) between SHX and NSHX during the daytime, whereas during nighttime, it was predominantly governed by changes in surface heat storage, including urban-rural disparities in surface albedo (delta ABD) and impervious surface fraction (ISF). Facing the intertwined challenges posed by climate change and urbanization, it is imperative for cities to develop effective cooling strategies that emphasize enhanced evaporative cooling and minimized heat storage. These strategies are essential for safeguarding urban residents from potential synergistic effects between SUHI and SHX towards sustainable cities and human settlements.