Concurrent Heat and Intense UV Extremes in North America: Divergent Dynamical Drivers and Exacerbated Ozone Pollution

Abstract. The co-occurrence of extreme heat and intense ultraviolet (UV) radiation poses severe yet insufficiently understood threats to terrestrial ecosystems and public health. Using reanalysis datasets from 1940 to 2022, we reveal the spatiotemporal characteristics and driving mechanisms of compound summer heat-UV events across North America. We identify two primary hotspots: the southwestern United States to northern Mexico, and central-western Canada to the northwestern United States. Dynamically, these events are anchored by anomalous anticyclonic circulations—specifically, the westward expansion of the North Atlantic Subtropical High in the south, and mid-latitude high-pressure ridges in the north. These persistent systems trigger severe soil desiccation, subsidence warming, and significant cloud reductions (up to ~13 %), synergistically amplifying surface temperature and solar UV flux. Furthermore, we demonstrate that this compound extreme substantially accelerates tropospheric photochemical processes, leading to a marked increase in surface ozone concentrations (up to 15.91 ppbv) and shifting the probability distribution toward extreme pollution episodes, primarily in the southern hotspot. Since 1980, the frequency of these compound events has increased significantly by 1.29 events per decade. The significant upward trend of these compound events highlights an intensifying climate-environmental hazard, underscoring the necessity of integrating combined heat, UV, and ozone risks into regional public health and pollution mitigation strategies.