Double Jet circulation regimes and their association with Western European Heatwaves in present and future climates

Double Jets (DJ) refer to a specific configuration of the large-scale atmospheric circulation in which the Northern Hemisphere polar and subtropical jets occur as two clearly separated branches. European heatwave trends have been linked to an increased persistence of Eurasian DJs (Rousi et al. Nat. Comms. 2022). However, it remains unclear to what extent observed trends are anthropogenically forced or associated with internal variability. A central necessity to answer this question is the ability of climate models to reproduce central DJ properties and their association with surface anomalies.

Based on models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we provide first insights into model representation of DJ characteristics. Our findings show that most models qualitatively capture the structural configuration of the DJs, while systematically underestimating the magnitude of the polar jet branch by approximately 35%.We further demonstrate that this response is associated with an underestimation of the high-latitude (60°N–90°N) meridional temperature gradient across models, where models with weaker gradients exhibit weaker winds, in line with the thermal wind relation. Crucially, this underestimated polar jet intensity acts as a dynamical constraint, causing models to underestimate the cumulative heatwave intensity over Western Europe by approximately 30%.

Finally, by extending our analysis to future projections (2021–2100)  under the SSP3-7.0 scenario we reveal a transition toward a weakened DJ regime. Our work highlights the need for improved representation of DJ characteristics and their coupling with heat extremes in climate models to enhance our confidence in future heat risk projections.