Weakened sea breeze circulation driven by atmospheric heatwaves in the western Mediterranean

The Mediterranean region is undergoing a significant increase in the frequency and severity of atmospheric and marine heatwaves, a trend that has accelerated in recent decades and is projected to continue throughout the 21st century. These extreme heat events pose significant risks to human health and ecosystems, particularly in densely populated coastal urban areas.

Sea breezes, the dominant local summer wind circulation in the region, can mitigate heat stress by transporting cooler and more humid marine air inland during the daytime, potentially reducing coastal temperatures by up to 10 °C depending on urban conditions. However, recent studies have reported instances in which heat is aggravated in urban environments under sea breeze conditions, highlighting the complexity of their interactions with urban heat dynamics. In a warming climate, a weakening trend in sea breeze speeds has been observed at several sites worldwide, including in the Mediterranean region, a climate change hotspot. The reduction may be driven by a warmer Mediterranean, and preliminary results based on observations point to atmospheric heatwaves as the cause, potentially exacerbating heat stress in coastal cities by limiting natural ventilation.

In this study, we analyze nearly 30 atmospheric heatwave episodes from 1981 to 2021 occurring concurrently with sea breezes across the Mediterranean, using the Weather Research and Forecasting (WRF) model, and compare the simulated changes in sea-breeze intensity with meteorological observations to elucidate the dynamic mechanisms behind the observed weakening and to assess whether subsidence induced by overlying subtropical ridges contributes to a hypothesized flattening of the sea breeze circulation. Our preliminary results reveal a consistent reduction in sea-breeze intensity during heatwave conditions across the region, alongside a systematically shallower planetary boundary layer height, indicating a reduced vertical extent of the sea breeze circulation. These findings have important implications for coastal Mediterranean cities, as weaker sea breezes during heatwaves may exacerbate extreme heat, challenge adaptation measures, and increase the vulnerability of urban populations during prolonged warm periods.