Disentangling the role of trends in Atmospheric Circulation Patterns from Thermodynamic Effects for European Hydroclimate

In 2022, Europe experienced a severe and extensive drought with substantial ecological and economic impacts. The climatic hazard that led to these impacts can be attributed to two primary causes. First, thermodynamic warming due to climate change reduces water availability through increased evaporative demand. Second, an unusual atmospheric circulation pattern during the event compounded the situation. This was further exacerbated by strong decadal trends in atmospheric circulation. While thermodynamic changes are physically well understood, our understanding of the impact of circulation-driven trends on climate is largely limited to its impact on trends in surface temperature. To attribute the role of these different climatic drivers on the drought impacts, we use a storyline approach by nudging the Community Earth System Model Version 2 (CESM2) to atmospheric circulation patterns from the ERA5 reanalysis data at different forcing levels. Our findings indicate that the dynamical conditions leading to the 2022 drought were the most extreme in the observed period, following a long-term atmospheric circulation trend that explains around 50% of European drying. Moreover, the 2022 circulation patterns not only intensified the drought but also interacted with thermodynamic effects, exacerbating the hydroclimatic impacts. By distinguishing between circulation-induced trends and thermodynamic changes, we provide a nuanced understanding of the drivers of European hydroclimatic variability and their contribution to extreme events. We highlight the critical need to consider both atmospheric circulation changes and thermodynamic influences to evaluate accurately and project future hydroclimatic trends in Europe.