Regional storylines reveal evapotranspiration regime transitions as a key driver of amplified European heat extremes

Europe experienced exceptionally hot and dry summers during 2018-2022, with severe impacts from compound heat and drought. Storyline simulations have shown that, under similar large-scale circulation, summer heat extremes can be strongly amplified with increased global warming. However, the processes controlling the magnitude and spatial variability of this amplification are still not fully understood. Here, we analyse circulation-nudged regional climate storylines for the summers 2018-2022 over Europe produced with a global-to-regional model chain (AWI-CM-1.1-MR downscaled with ICON-CLM), spanning five climate states from pre-industrial conditions to +4 K global warming. Using the advantage of the nudged storylines to provide the local climate change signal on a daily timescale, we quantify the thermodynamic response at each grid point by computing the increase in daily maximum temperature per degree of global warming (warming amplification). We found that, for the most extreme events of the heatwave-drought series, the highest warming amplification (up to 3 K/K) tends to occur outside the heatwave core, whereas the warming amplification at the hottest locations is close to 2 K/K. To explain this behaviour, we investigate the relationship between soil moisture and evaporative fraction as a diagnostic of land-atmosphere coupling regimes. The strongest amplification occurs where warming shifts land surface conditions from an energy-limited to a moisture-limited evapotranspiration regime. In these transition regions, enhanced soil moisture depletion suppresses evaporative cooling and increases sensible heating, leading to amplified temperature increases. By contrast, amplification is smaller where conditions are already moisture-limited or remain energy-limited. Our results identify evapotranspiration regime transitions as a key mechanism controlling the amplification of future European heat extremes and highlight the importance of land-atmosphere coupling for understanding regional differences in heatwave intensification.