Climate models struggle to simulate observed North Pacific jet trends, even accounting for tropical Pacific sea surface temperature trends

We show that the wintertime (December-January-February) North Pacific jet in ERA5 has shifted northwards over the satellite-era (1979-2023) at a faster rate than any of the state-of-the-art CMIP6 coupled climate models used in this study. Differences in tropical sea surface temperature (SST) trends can only partially explain the discrepancy in jet trends between models and observations and a small minority of simulations forced with observed SSTs match the magnitude of the observed jet trend. However, analysis of longer-term jet variability in reanalysis suggests that the jet trend has not clearly emerged from multi-decadal internal climate variability. Consequently, it is unclear whether the difference in observed and modelled jet trends arises due to differing responses to anthropogenic forcing or overly weak long-term internal variability in models. These results have important implications for future climate projections for North America and motivate further research into the underlying causes of long-term jet trends.