European Summer Wet-Bulb Temperature: Spatiotemporal Variations and Potential Drivers

Abstract

Abstract Heat stress is projected to intensify with global warming, causing significant socioeconomic impacts and threatening human health. Wet-bulb temperature (WBT), which combines temperature and humidity effects, is a useful indicator for assessing regional and global heat stress variability and trends. However, the variations of European WBT and their underlying mechanisms remain unclear. Using observations and reanalysis datasets, we demonstrate a remarkable warming of summer WBT during the period 1958–2021 over Europe. Specifically, the European summer WBT has increased by over 1.0°C in the past 64 years. We find that the increase in European summer WBT is driven by both near-surface warming temperatures and increasing atmospheric moisture content. We identify four dominant modes of European summer WBT variability and investigate their linkage with the large-scale atmospheric circulation and sea surface temperature anomalies. The first two leading modes of the European WBT variability exhibit prominent interdecadal to long-term variations, mainly driven by a circumglobal wave train and concurrent sea surface temperature variations. The last two leading modes of European WBT variability mainly show interannual variations, indicating a direct and rapid response to large-scale atmospheric dynamics and nearby sea surface temperature variations. Further analysis shows the role of global warming and changes in midlatitude circulations in the variations of summer WBT. Our findings can enhance the understanding of plausible drivers of heat stress in Europe and provide valuable insights for regional decision-makers and climate adaptation planning. Significance Statement Wet-bulb temperature, which takes into account the combined effect of temperature and humidity, is a good indicator for assessing heat stress. In the context of global warming, heat stress is anticipated to escalate, posing significant risks to human health and causing socioeconomic losses. However, variations in wet-bulb temperature and the associated physical mechanisms have received limited attention. This study aims to improve our understanding of the temporal and spatial variations and the potential driving mechanisms of summer wet-bulb temperature across Europe in recent decades. We have observed a noteworthy increase in summer wet-bulb temperature, indicating a regional intensification of heat stress, particularly within the last 10 years. We further investigate the connections between variations in summer wet-bulb temperature, large-scale atmospheric circulation, and sea surface temperature. Additionally, we explore their associations with global warming and changes in midlatitude atmospheric circulation. The outcomes of this study not only contribute to establishing a scientific basis for evaluating heat-related risks in Europe but also facilitate preparedness for future climate adaptation and mitigation at both regional and local scales.