11 Apr 2022
11 Apr 2022
Status: this preprint is currently under review for the journal ACP.

On the Robustness of the Surface Response to Austral Stratospheric Polar Vortex Extremes

Nora Bergner1,2, Marina Friedel1, Daniela I. V. Domeisen1,3, Darryn Waugh4, and Gabriel Chiodo1 Nora Bergner et al.
  • 1Institute of Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland
  • 2Extreme Environments Research Laboratory, EPFL Valais, Sion, Switzerland
  • 3Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland
  • 4Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA

Abstract. Extreme events in the stratospheric polar vortex can lead to changes in the tropospheric circulation and impact the surface climate on a wide range of timescales. The austral stratospheric vortex shows its largest variability in spring, and a weakened polar vortex is associated with changes in the spring to summer surface climate, including hot and dry extremes in Australia. However, the robustness and extent of the connection between polar vortex strength and surface climate on interannual timescales remain unclear. We assess this relationship by using reanalysis data and simulations from two independent chemistry-climate models (CCMs), building on previous work that is mainly based on observations. The CCMs show a similar downward propagation of polar vortex anomalies as the reanalysis data and weak (strong) polar vortex anomalies are on average followed by a negative (positive) tropospheric Southern Annular Mode (SAM) in spring to summer. The signature in the surface climate following polar vortex weakenings is characterized by high surface pressure and warm temperature anomalies over Antarctica, the region where surface signals are most robust across all model and observational datasets. However, the tropospheric SAM response in the models is inconsistent with observations. In one CCM, the SAM is more negative compared to the reanalysis after weak polar vortex events, whereas in the other CCM, it is less negative. In addition, both models do not reproduce all the regional changes in midlatitudes, such as the warm and dry anomalies over Australia. We find that these inconsistencies are linked to model biases in the basic state, such as the latitude of the eddy-driven jet and the persistence of the tropospheric SAM. Furthermore, bootstrapping of the data reveals sizable uncertainty in the magnitude of the surface signals in both models and observations due to internal variability. Our results demonstrate that anomalies of the austral stratospheric vortex have significant impacts on surface climate, although the ability of models in capturing regional effects across the Southern Hemisphere is limited by biases in their representation of the tropospheric circulation.

Nora Bergner et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-229', Anonymous Referee #1, 16 May 2022
  • RC2: 'Comment on acp-2022-229', Anonymous Referee #2, 16 May 2022

Nora Bergner et al.

Nora Bergner et al.


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Short summary
Polar vortex extremes, particularly situations with an unusually weak cyclonic circulation in the stratosphere, can influence the surface climate in the spring-summer time in the Southern Hemisphere. Using climate models and observations, we evaluate the robustness of the surface impacts. While models capture the general surface response, they do not show the observed climate patterns in midlatitude regions, which we trace back to biases in the models’ circulations.