Improvements in Circumpolar Southern Hemisphere Extratropical Atmospheric Circulation in CMIP6 Compared to CMIP5

M Patterson,M Osman,J S Hosking,T J Bracegirdle,T Rackow,C R Holmes,G J Marshall

doi:10.1029/2019ea001065

Abstract

AbstractOne of the major globally relevant systematic biases in previous generations of climate models has been an equatorward bias in the latitude of the Southern Hemisphere (SH) mid‐latitude tropospheric eddy driven westerly jet. The far‐reaching implications of this for Southern Ocean heat and carbon uptake and Antarctic land and sea ice are key reasons why addressing this bias is a high priority. It is therefore of primary importance to evaluate the representation of the SH westerly jet in the latest generation of global climate and earth system models that comprise the Coupled Model Intercomparison Project Phase 6 (CMIP6). In this paper we assess the representation of major indices of SH extratropical atmospheric circulation in CMIP6 by comparison against both observations and the previous generation of CMIP5 models. Indices assessed are the latitude and speed of the westerly jet, variability of the Southern Annular Mode (SAM), and representation of the Amundsen Sea Low (ASL). These are calculated from the historical forcing simulations of both CMIP5 and CMIP6 for time periods matching available observational and reanalysis data sets. From the 39 CMIP6 models available at the time of writing there is an overall reduction in the equatorward bias of the annual mean westerly jet from 1.9° in CMIP5 to 0.4° in CMIP6 and from a seasonal perspective the reduction is clearest in austral spring and summer. This is accompanied by a halving of the bias of SAM decorrelation timescales compared to CMIP5. However, no such overall improvements are evident for the ASL.

Highlights

The circumpolar lower-tropospheric westerly winds over the Southern Ocean play a major role in the climate system both regionally and globally (Frölicher et al, 2015)
Comparisons against observations and re-analyses in Figure 1 indicate that the Coupled Model Intercomparison Project Phase 6 (CMIP6) models are broadly successful in reproducing the real-world strength of these summer SAMstn trends and their link to combined poleward shifting and strengthening of the tropospheric westerly jet
A key question highlighted in the Introduction is whether the equatorward jet bias is reduced in the CMIP6 ensemble compared to CMIP5

Summary

Introduction

The circumpolar lower-tropospheric westerly winds over the Southern Ocean play a major role in the climate system both regionally and globally (Frölicher et al, 2015). The most significant observed change has been a combined poleward shift and strengthening of the westerly jet caused primarily by stratospheric ozone depletion (Swart et al, 2015). This has been implicated in driving changes in Southern Ocean circulation, sea ice and Antarctic Peninsula temperatures (Marshall et al, 2006; Thompson et al, 2011). Model biases in historical climatology and projected change of the westerly jet are a major concern for many aspects of the regional and global climate system

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