Fast-get-faster explains wavier upper-level jet stream under climate change

Earth's upper-level jet streams primarily flow in the eastward direction. They often exhibit a north-south component or waviness connected to extreme weather at the surface. Recently the upper-level eastward jet stream was found to exhibit a fast-get-faster response under climate change explained by the impact of the nonlinear Clausius-Clapeyron relation on the latitudinal density contrast. Here we show the fast-get-faster mechanism also applies to the upper-level north-south jet stream wind and the longitudinal density contrast, implying increased waviness under climate change. Arctic Sea ice loss, which has been proposed as a driver of increased waviness, cannot explain the response. It leads to a fast-get-slower waviness response at all vertical levels. We demonstrate the fast-get-faster waviness signal has emerged in reanalysis data in the Southern Hemisphere but not yet in the Northern Hemisphere. The results show the fast-get-faster mechanism explains upper-level waviness changes and highlights a tug of war between upper- and mid-level waviness under climate change.