ERA5-Based Validation of Thermodynamic Extreme Heatwave Drivers of the Paris region in CMIP6 simulations.

Previous studies have argued that the upper bound of temperature extremes in mid-latitude regions is reached by minimizing near-surface moisture during high low-tropospheric temperatures. Here, we revisit these theories for the Île-de-France region using the ERA5 reanalysis and show that the highest annual temperatures occur within the moist-to-expected range of the summer (June–August) near-surface humidity distribution. However, during the most extreme events, relative humidity is minimized as soil moisture approaches the wilting point and the atmospheric boundary layer deepens. Using the statistical distributions of these indicators and their temporal evolution in ERA5, we evaluate the representation of thermodynamic drivers in selected CMIP6 large ensembles. Finally, we apply a recently published revised framework of dry convective instability to estimate maximum attainable temperatures in both ERA5 and CMIP6, highlighting how climate change may modify heatwave dynamics in the Paris region.