Diverse seasonal hysteresis of surface urban heat islands across Chinese cities: Patterns and drivers

Understanding the seasonality of, and contributors to, surface urban heat island (SUHI) will pave the way for developing more generalized urban cooling strategies. The direction and shape of SUHI seasonal hysteresis have been thought to be background climate dependent. However, there remains a relative paucity of knowledge about the patterns and driving mechanisms of such hysteretic phenomena across diverse climates. Here we show that the seasonal hysteretic cycles of SUHI in the daytime and nighttime across Chinese cities between 2003 and 2020 can be identified as eight and six primary patterns, characterized by diverse and complex behaviors including clockwise, anti-clockwise, twisted, concave, and convex directions/loops. The urban-rural differences in evapotranspiration (ΔET) and surface albedo (ΔABD) have been recognized as the primary contributors to SUHI in the daytime and nighttime and their seasonal hysteresis, respectively, as measured by structural equation modeling (SEM) and time lead/lag analysis. These biophysical mechanisms for diverse seasonal hysteretic cycles of SUHI in China re-emphasized climate context dependencies in SUHI terms. Furthermore, the observed divergence of SUHI hysteresis typologies within similar background climates highlighted the additional drivers of complex urban systems because SUHI is a combined result of rural background and urban characteristics. We conclude that the cost-effective UHI mitigation strategies creating synergies or avoiding trade-offs among seasons should be climate context-specific and tailored to local urbanicity.