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

Abstract One 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 c...

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Bibliographic Details
Published in:Earth and Space Science
Main Authors: T. J. Bracegirdle, C. R. Holmes, J. S. Hosking, G. J. Marshall, M. Osman, M. Patterson, T. Rackow
Format: Article in Journal/Newspaper
Language:English
Published: American Geophysical Union (AGU) 2020
Subjects:
CMIP6
CMIP5
Antarctic
Southern Ocean
westerly jet
Amundsen Sea Low
Astronomy
QB1-991
Geology
QE1-996.5
Amundsen Sea
Austral
Antarc*
Sea ice
Online Access:https://doi.org/10.1029/2019EA001065
https://doaj.org/article/dcc43c816582418a8aa884db988f4887
Description
Summary:Abstract One 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.

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