Persistent Model Biases in the CMIP6 Representation of Stratospheric Polar Vortex Variability

Richard J. Hall, Daniel M. Mitchell, William J.M. Seviour, Corwin J. Wright

Research output: Contribution to journalArticlepeer-review

Abstract

Sudden stratospheric warmings (SSWs) can have major impact on surface wintertime weather, especially at mid-high latitudes. We do not yet have a complete understanding of why some of these events influence our weather more than others, but one factor may be the dynamical nature of the SSW; whether it involves a split or a displacement of the polar vortex, and one way to explore this is through comprehensive climate models. Here, we analyze the stratospheric dynamics of SSWs within models from the sixth Coupled Model Intercomparison Project (CMIP6). All CMIP6 models simulate SSWs to some degree, although we find a persistent bias in the relative underrepresentation of split vortex events. When comparing with CMIP5 models, large biases persist despite significant model improvements in resolution and in representing atmospheric processes. We show that the simulated displacement frequency is strongly related to climatological lower stratospheric eddy heat flux. The split frequency, on the other hand, is not related to lower stratospheric eddy heat flux, but is strongly related to both the vortex geometry (aspect ratio) and lower stratospheric zonal winds. This suggests that those models with a large positive bias in zonal winds may inhibit the propagation of zonal wavenumber 2 planetary waves from the troposphere, which are associated with split events. Our results suggest how future model development may address these longstanding biases.

Original languageEnglish
Article numbere2021JD034759
JournalJournal of Geophysical Research: Atmospheres
Volume126
Issue number12
Early online date8 Jun 2021
DOIs
Publication statusPublished - 27 Jun 2021

Keywords

  • CMIP6
  • displacements
  • model bias
  • splits
  • stratospheric polar vortex
  • sudden stratospheric warming

ASJC Scopus subject areas

  • Atmospheric Science
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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