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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 11 | Copyright
Atmos. Chem. Phys., 18, 8065-8077, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Jun 2018

Research article | 08 Jun 2018

Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex

Jonathan Conway, Greg Bodeker, and Chris Cameron Jonathan Conway et al.
  • Bodeker Scientific, 42 Russell Street, Alexandra, 9320, New Zealand

Abstract. The wintertime 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 double-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 395 and 850K, bifurcation is strongest in mid-winter and decreases dramatically during spring. From August onwards a distinct structure emerges, where elevated bifurcation remains between 475 and 600K, 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, interannual 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 characterised. 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.

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Strong westerly winds occur in the stratosphere during winter and spring. These winds, the polar vortex, limit how much air is mixed between mid- and high-latitudes. We present a new view of the polar vortex mixing barrier in the Southern Hemisphere, revealing a frequent double-walled barrier with two distinct regions of weak mixing. This double-walled structure is expected to alter the spatial and temporal variation of trace gas concentrations (e.g. ozone) across the polar vortex.
Strong westerly winds occur in the stratosphere during winter and spring. These winds, the polar...