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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-1016
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
17 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex
Jonathan Conway, Greg Bodeker, and Chris Cameron Bodeker Scientific, 42 Russell Street, Alexandra, 9320, New Zealand
Abstract. The winter-time stratospheric westerly winds circling the Antarctic continent, also known as the Southern Hemisphere polar vortex, create a barrier to mixing of air between middle and high latitudes. This dynamical isolation has important consequences for export of ozone-depleted air from the Antarctic stratosphere to lower latitudes. The prevailing view of this dynamical barrier has been an annulus compromising steep gradients of potential vorticity (PV) that create a single semi-permeable barrier to mixing. Analyses presented here show that this barrier often displays a bifurcated structure where a doubled-walled barrier exists. The bifurcated structure manifests as enhanced gradients of PV at two distinct latitudes – usually on the inside and outside flanks of the region of highest wind speed. Metrics that quantify the bifurcated nature of the vortex have been developed and their variation in space and time has been analysed. At most isentropic levels between 370 K and 850 K, bifurcation is strongest in winter and reduces dramatically during spring. From August onwards a distinct structure emerges, where elevated bifurcation remains between 475 K and 600 K, and a mostly single walled barrier occurs at other levels. While bifurcation at a given level evolves from month to month, and does not always persist through a season, inter-annual variations in the strength of bifurcation display coherence across multiple levels in any given month. Accounting for bifurcation allows the region of reduced mixing to be better characterized. These results suggest that improved understanding of cross-vortex mixing requires consideration of the polar vortex not as a single mixing barrier, but as a barrier with internal structure that is likely to manifest as more complex gradients in trace gas concentrations across the vortex barrier region.

Citation: Conway, J., Bodeker, G., and Cameron, C.: Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1016, in review, 2017.
Jonathan Conway et al.
Jonathan Conway et al.
Jonathan Conway et al.

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Short summary
The Southern Hemisphere stratospheric polar vortex limits the mixing of air between mid- and high-latitudes during winter and spring. This paper presents a new view of this stratospheric mixing barrier, revealing that a double-walled barrier with two distinct regions of weak mixing often occurs. This bifurcated structure is expected to alter the spatial and temporal variation of trace gas concentrations (e.g. ozone) across the polar vortex.
The Southern Hemisphere stratospheric polar vortex limits the mixing of air between mid- and...
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