Moving beyond the mean to understand circulation extremes under climate change

Much has been learned about the response of the mean circulation under climate change. In particular, the subtropical jet will accelerate, the eddy-driven jet will shift poleward, storminess in the Southern Hemisphere will increase whereas storminess in the Northern Hemisphere will be impacted by a tug of war between different factors. However, very little is known about how circulation extremes will respond to climate change beyond blocking. This is in stark contrast to our understanding of the response of extreme temperatures, which follow the mean via an additive increase, and the response of extreme precipitation, which does not follow the mean because of a multiplicative increase connected to the non-linear Clausius-Clapeyron relation. Here as a starting point, we investigate changes in upper-level circulation extremes defined using a daily distribution. We show fast upper-level jet stream (zonal) winds get faster under climate change. We also show extreme jet stream meandering or waviness (meridional wind) increases under climate change. These responses are geostrophic, robust across a climate model hierarchy (CMIP/AMIP/AQUA) and not connected to sea ice loss. The increase in upper-level circulation extremes is shown to be connected to a multiplicative response, whereby the extremes increase more than the mean, which follows from the non-linear Clausius-Clapeyron relation via its connection to moist thermal wind.