Late 21st Century Changes in the Mid-latitude Atmospheric Circulation in the CESM Large Ensemble

Abstract

rojected changes in the mid-latitude atmospheric circulation at the end of the 21st 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 that is considered, different responses are identified. In winter, a slowdown of the zonal flow and an increase in waviness is found over North America, while the European sector exhibits a reinforced westerly flow and decreased waviness. Extreme temperature events in mid-latitudes are more sensitive to thermodynamical than dynamical changes, and a general decrease in the intensity of wintertime cold spells is found. Analyses of individual ensemble members reveal a large spread in circulation changes due to internal variability. Causes for this spread are found to be tied to Arctic Amplification in the North Pacific/America sector, and to the polar stratosphere in the North Atlantic. A competition mechanism is discussed between the mid-latitude response to polar vs tropical changes. While the upper-troposphere 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, leading to decreased/unchanged waviness/blockings. Our interpretation somewhat reconciles conflicting results between the hypothesized effect of Arctic Amplification and projected changes in mid-latitude flow characteristics. It also illustrates that further understanding of regional processes is critical for anticipating changes in the mid-latitude climate.