Circulation pathways and surface drivers of extreme summer heat stress over Europe

Extreme summer heat stress presents increasing public health risks across Europe. These extremes are strongly influenced by large-scale atmospheric circulation, yet the specific pathways linking circulation evolution to surface heat stress amplification remain poorly understood. Using the simplified Wet Bulb Globe Temperature (sWBGT), which accounts for both temperature and humidity effects on heat stress, we analyze extreme summer (JJA) events during 1979–2023 based on ERA5 reanalysis and a seven-class European weather regime (WR) classification. We define extreme events as regional sWBGT exceeding the 95th percentile for at least three consecutive days. Extreme sWBGT events across Europe occur predominantly during blocking regimes, with European and Scandinavian blocking playing a dominant role in many regions. We then examine how blocking evolves prior to heat stress peaks. Results show that only Scandinavia exhibits a statistically robust tendency for blocking to develop shortly before the peak, suggesting a circulation transition preceding extreme heat stress. In contrast, most other European regions experience peak heat stress under blocking conditions that are already established several days in advance, highlighting the dominant role of persistent circulation patterns. The time interval between the onset of blocking and the heat stress peak typically ranges from 3 to 7 days. These contrasting circulation pathways are closely linked to different surface amplification processes. Circulation transitions maybe associated with rapid atmospheric adjustment and surface warming, whereas persistent blocking likely promotes the accumulation of radiative forcing and progressive soil moisture depletion. Understanding how these mechanisms vary across pathways can help explain regional differences in European heat stress extremes and may improve predictions of future events.