Characterizing the Drivers and Spatiotemporal Evolution of Compound Extreme Heat Events in Subtropical Island Environments: A Multi-Scale Analysis of Taiwan

Under the accelerating pace of global warming, extreme heat events are becoming more frequent, intense, and prolonged, posing significant threats to public health and energy security. This study characterizes the evolution and physical mechanisms of "compound extreme heat", defined by the simultaneous occurrence of high ambient temperatures and high humidity, within the complex topographical and urbanized landscape of Taiwan.

By integrating high-resolution observational data with regional climate simulations, we identify the distinct spatiotemporal fingerprints of heatwaves across different geographical regions of the island. Our analysis reveals that the intensification of extreme heat is driven by a synergistic interaction between synoptic-scale circulation and local-scale surface processes. Specifically, the anomalous westward extension and intensification of the Western North Pacific Subtropical High (WNPSH) provide the necessary large-scale subsidence and clear-sky conditions. On a local scale, this is further exacerbated by the Urban Heat Island (UHI) effect and restricted sea-breeze penetration in basin terrains, leading to localized "hotspots" with significantly elevated wet-bulb temperatures.

Furthermore, this research assesses the changing risks associated with these compound events under various CMIP6 warming scenarios. We quantify the drivers of extreme heat through a budget analysis of the surface energy balance and atmospheric moisture, highlighting how land-atmosphere feedbacks amplify heat stress in rapidly growing metropolitan areas. The findings provide critical insights into the physical drivers of subtropical heat extremes and offer a scientific basis for developing region-specific adaptation strategies and early-warning systems for heat-related risks in a warming climate.