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

• Published in: Earth and Space Science
• Main Authors: Bracegirdle, T.J., Holmes, C.R., Hosking, J.S., Marshall, G.J., Osman, Marisol, Patterson, M., Rackow, T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley Blackwell Publishing, Inc
• Subjects: AMUNDSEN SEA LOW; ANTARCTIC; CMIP5; CMIP6; SOUTHERN OCEAN; WESTERLY JET; https://purl.org/becyt/ford/1.5; https://purl.org/becyt/ford/1; Antarctic; Southern Ocean; Austral; Amundsen Sea; Antarc*; British Antarctic Survey; Sea ice
• Online Access: http://hdl.handle.net/11336/144092

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. Fil: Bracegirdle, T.J. British Antarctic Survey; Reino Unido Fil: Holmes, C.R. British Artarctic Survey; Reino Unido Fil: Hosking, J.S. British Antarctic Survey; Reino Unido Fil: Marshall, G.J. British Antarctic Survey; Reino Unido Fil: Osman, Marisol. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. ...