Significantly reduced equatorward bias of Southern Hemisphere mid-latitude westerlies in CMIP6 compared to CMIP5

Abstract:

One of the major globally-relevant systematic biases in climate models has been an equatorward bias in the latitude of the Southern Hemisphere (SH) mid-latitude tropospheric eddy-driven westerly jet. 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. Far-reaching implications for Southern Ocean heat and carbon uptake and Antarctic land and sea ice are key reasons why addressing this bias has been identified as 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).

We will report on an evaluation of westerly jet characteristics calculated from output of standard-configuration historical simulations of 39 CMIP6 models. Comparisons both to atmospheric reanalyses and CMIP5 model output were conducted. A range of different atmospheric circulation indices were assessed to help provide dynamical insight into the results. Indices assessed were the latitude and speed of the westerly jet, decorrelation timescales of the Southern Annular Mode (SAM) and representation of the Amundsen Sea Low (ASL).

From the 39 CMIP6 models available at the time of abstract submission 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. From a seasonal perspective the reduction is clearest in austral spring and summer. This is accompanied by an overall approximate halving of the bias in SAM decorrelation timescales compared to CMIP5. However, no such overall improvements are evident for the ASL. Potential explanations for the reduced equatorward bias will be discussed along with implications for representation of the wider climate system.