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

Abstract

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.

Effects of Arctic ozone on the stratospheric spring onset and its surface response

“…To this end, we define the altitude-dependent FSW onset date on a given pressure level as the first day of the year when zonal mean zonal wind at 60°N has fallen below a threshold of 0 m/s for altitudes equal to or above 10 hPa, and 7 m/s for pressure levels lower than 10 hPa, and does not return above this threshold for more than 10 consecutive days until the following fall. Adjustment of the wind threshold to 7 m/s for lower stratospheric altitudes was necessary to allow the CCMs to generate a FSW every single year, correcting their vortex biases (Stenke et al, 2013;Butchart et al, 2011;Bergner et al, 2022). The polar vortex bias results in a delay of the FSW compared to reanalysis (in our models by 2-3 weeks).…”