Climate Models Do Not Capture Recent Trends in Double Jets over Eurasia

Summer heat extremes in Western Europe have been observed to be intensifying in magnitude and persistence more rapidly than the mean warming in the northern hemisphere midlatitudes. Previous work has linked the increased heat extremes to double jet trends over Eurasia, suggesting a significant dynamically driven component to the heatwave trend. However, the mechanism for the change in circulation is not known. Further investigation into the question requires accurate representation of the double jet, as well as its evolution under climate change, in climate models. Here we show that most state-of-the-art CMIP6 and AMIP6 models do not capture either the satellite-era climatology or observed trend of double jet occurrence. In both cases, reanalysis (ERA5, JRA55, and MERRA2) values fall near or outside the upper edge of the distribution of CMIP6 model values, and the multi-model mean trend is near zero. The inability of climate models to represent circulation changes linked to heat extremes implies that changes in heat extremes are also unlikely to be well-represented. Our results identify a roadblock in the way of attributing double jet trends (and by extension, extreme heat trends) to their driving mechanism, whether Arctic amplification, aerosols, or otherwise.