Interactions of urban heat islands and heat waves in Swedish cities under present and future climates

The heightened awareness of heat wave impacts in high-latitude cities, particularly after the record-hot summer of 2018, highlights the need for improved understanding of heat waves and Urban Heat Island (UHI) effects. This study focuses on the interaction and future change of heat waves and UHI across southern Sweden under three specific warming levels: 0.9 °C (historical), 2 °C, and 3 °C. We utilize the HCLIM43-AROME convection-permitting regional climate model at 12 km and 3 km resolutions, and the SURFEX land surface model at 300 m resolution, employing a pseudo-global warming approach over target summers (2017, 2018 and 2022) representing different climates. Our results indicate that the UHI effects are well captured by model simulations. The nocturnal UHI weakens under climate change (assuming no changes in urbanization or greenhouse gas emissions) but intensifies (0.5 °C to 1 °C) during heat waves. During heat waves, higher sea level pressure, radiation and sensible heat flux contribute to enhanced urban warming due to higher thermal inertia. The nocturnal UHI is further accentuated by lower wind speeds and cloud fraction (indicative of weaker advection), lower moisture flux, and decreased soil moisture (associated with reduced evaporation) in urban areas during heat waves. These nuanced findings provide valuable insights for local heat stress adaptation strategies, with future research needed on the impacts of urban expansion. • Urban heat island effects are well modeled by the pseudo-global warming approach. • Urban heat island intensity amplifies during heat waves in high latitude cities. • Future urban heat island intensity shows a decreasing trend, assuming no changes in urbanization or greenhouse gas emissions.