Title: Projected changes of the mid-latitude atmospheric dynamics at the end of the 21st century
Abstract: In this study we explore how state-of-the-art general circulation models project changes in the mid-latitude atmospheric circulation at the end of the 21st century. For this, we use an ensemble of RCP8.5 simulations from the Community Earth System Model Large Ensemble (CESM-LENS) and from the Coupled Model Intercomparison Project (CMIP5). Different metrics are used to describe the response of the mid-latitude atmospheric dynamics in terms of zonal mean flow, atmospheric blockings and jet stream waviness. We identify different responses depending on the season and longitudinal sector that is considered, as well as a large spread in circulation changes due to internal variability. Causes for this spread are discussed and found to be related to Arctic Amplification in the North Pacific/America sector, and to the polar stratosphere in the North Atlantic. We also identify a competition mechanism 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.