Large-scale atmospheric circulation as a source of uncertainty in western European heat extreme projections

Europe has been identified as a heatwave hotspot, where heatwave intensities have outpaced other mid-latitude regions in the Northern Hemisphere (Rousi et al. Nat. Comms. 2022). Accelerated European heatwave trends have been found to be associated with the increased persistence of Eurasian double jets, a specific set-up of the large-scale circulation in which the Northern hemisphere polar and subtropical jets occur as two clearly separated branches. However, if observed trends are projected to continue with anthropogenic warming and to what degree the present generation of climate models constitute useful tools to assess changes in the atmospheric circulation has not yet been ascertained.

In this study, we benchmark 11 CMIP6 climate models to evaluate their ability to reproduce the main characteristics of double jets and their relationship to heat extremes, aiming to identify the best-performing models for future projections. Our findings show that, on average, the models tend to underestimate the frequency of double jets by 80%. Moreover, half of the climate models underestimate the intensity of double-jet-associated heatwaves over Western Europe, with the remaining models even showing a negative anomaly in heatwave intensity during double jet events in the region. Furthermore, climate models fail to capture the growth rate of double jet persistence, with the model mean trend at -0.4 days per decade, while the observed rate is approximately 1.5 days per decade. The bias in the persistence trend of double jet in models is strongly correlated with the underestimation of the western European heat extreme trend, with an R² value of 0.42.

Despite this, some models show reasonable agreement with the observations, and these models are further analyzed to project circulation-driven changes in extreme heat. Using EC-Earth3-Veg-LR, we observe an increase in double jet frequency from 2020 to 2060, at a rate of 0.2 days per decade. Our work highlights the need for better representation of double jet characteristics and their relationship with heat extremes in climate models to enhance preparedness for future heat risks.