An evaluation of the large scale atmospheric circulation

17 The Community Earth System Model 2 (CESM2) is the latest Earth System Model de18 veloped by the National Center for Atmospheric Research in collaboration with the uni19 versity community and is significantly advanced in most components compared to its pre20 decessor (CESM1). Here, CESM2s representation of the large scale atmospheric circu21 lation and its variability is assessed. Further context is provided through comparison to 22 the CESM1 large ensemble and other models from the Coupled Model Intercomparison 23 Project (CMIP5 and CMIP6). This includes an assessment of the representation of jet 24 streams and storm tracks, stationary waves, the global divergent circulation, the annu25 lar modes, the North Atlantic Oscillation and blocking. Compared to CESM1, CESM2 26 is substantially improved in the representation of the storm tracks, Northern Hemisphere 27 (NH) stationary waves, NH winter blocking and the global divergent circulation. It ranks 28 within the top 10% of CMIP-class models in many of these features. Some features of 29 the Southern Hemisphere (SH) circulation have degraded, such as the SH jet strength, 30 stationary waves and blocking, although the SH jet stream is placed at approximately 31 the correct location. This analysis also highlights systematic deficiencies in these features 32 across the new CMIP6 archive, such as the continued tendency for the SH jet stream to 33 be placed too far equatorward, the North Atlantic westerlies to be too strong over Eu34 rope, the storm tracks as measured by low level meridional wind variance to be too weak 35 and a lack of blocking in the North Atlantic sector. 36

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