Changes in the Mid-Latitude Atmospheric Dynamics in the CESM Large Ensemble

Projected changes in the mid-latitude atmospheric circulation at the end of the 21^st century are investigated using coupled ocean-atmosphere simulations from the Community Earth System Model Large Ensemble (CESM-LENS). Different metrics are used to describe the response of the mid-latitude atmospheric dynamics in 40 ensemble members covering the 1920-2100 period. Different responses are identified depending on the season and longitudinal sector. In winter a slowdown of the zonal flow and an increase in waviness is found only over North America, while the European sector exhibits a reinforced westerly circulation and decreased waviness. Extreme temperature events in mid-latitudes are more sensitive to thermodynamic than dynamical changes, and a general decrease in the intensity of wintertime cold spells is found. Analysis of individual ensemble members reveals a large spread in circulation changes due to internal variability. Causes for this spread in the Northern Hemisphere are found to be tied to Arctic Amplification in the North Pacific/America sector, and to the polar lower stratosphere in the North Atlantic. A competition mechanism is discussed between the mid-latitude response to polar vs tropical changes. While the upper-tropospheric tropical warming pushes the jet stream poleward, in winter Arctic Amplification and the weaker polar vortex exert an opposite effect. This competition results in a narrowing of the jet path in the mid-latitudes, hence decreased/unchanged waviness/blockings. This signal is especially strong over the North Atlantic, but also over the Pacific basin.