The polar-to-subtropical temperature gradient in the free troposphere is a key driver of the mid-latitude jet stream response to climate change. Climate models tend to steepen this gradient in response to large greenhouse gas increases, due to very strong subtropical upper-level warming. This strengthens the simulated jets. However, multiple lines of observational evidence point to a slowing northern jet over the satellite era, driven by enhanced Arctic free-tropospheric warming and weakening of the gradient. Here, we reconcile these seemingly contradictory results by showing that the CMIP6 ensemble successfully simulates both the observed satellite-era slowdown/weakening, and the speedup/strengthening with strong global warming. Speci cally, the observed gradient weakening from 1980-1997 to 1997-2014 appears inconsistent (p < 0.05) with the simulated gradient changes for just 6 of 45 models using Microwave Satellite Unit observations, and for just 5 of 45 models using reanalysis estimates. The observed jet slowdown appears inconsistent with the simulated jet changes for just 1 of 45 models. Yet a strong majority of the models reverse course under a high-emissions future-type scenario, simulating gradient strengthening and jet speedup. Future work will seek to clarify the cause(s) of this unexpected difference between past and future atmospheric responses.

The polar-to-subtropical temperature gradient in the free troposphere is a key driver of the mid-latitude jet stream response to climate change. Climate models tend to steepen this gradient in response to large greenhouse gas increases, due to very strong subtropical upper-level warming. This strengthens the simulated jets. However, multiple lines of observational evidence point to a slowing northern jet over the satellite era, driven by enhanced Arctic free-tropospheric warming and weakening of the gradient. Here, we reconcile these seemingly contradictory results by showing that the CMIP6 ensemble successfully simulates both the observed satellite-era slowdown/weakening, and the speedup/strengthening with strong global warming. Specifically, the observed gradient weakening from 1980–1997 to 1997–2014 appears inconsistent (p < 0.05) with the simulated gradient changes for just 6 of 45 models using Microwave Satellite Unit observations, and for just 5 of 45 models using reanalysis estimates. The observed jet slowdown appears inconsistent with the simulated jet changes for just 1 of 45 models. Yet a strong majority of the models reverse course under a high-emissions future-type scenario, simulating gradient strengthening and jet speedup. Future work will seek to clarify the cause(s) of this unexpected difference between past and future atmospheric responses.