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. The wintertime stratospheric westerly winds circling the Antarcticcontinent, also known as the Southern Hemisphere polar vortex, create abarrier to mixing of air between middle and high latitudes. This dynamicalisolation has important consequences for export of ozone-depleted air fromthe Antarctic stratosphere to lower latitudes. The prevailing view of thisdynamical barrier has been an annulus compromising steep gradients ofpotential vorticity (PV) that create a single semi-permeable barrier tomixing. Analyses presented here show that this barrier often displays abifurcated structure where a double-walled barrier exists. The bifurcatedstructure 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 developedand their variation in space and time has been analysed. At most isentropiclevels between 395 and 850 K, bifurcation is strongest in mid-winter anddecreases dramatically during spring. From August onwards a distinctstructure emerges, where elevated bifurcation remains between 475 and 600 K,and a mostly single-walled barrier occurs at other levels. While bifurcationat a given level evolves from month to month, and does not always persistthrough a season, interannual variations in the strength of bifurcationdisplay coherence across multiple levels in any given month. Accounting forbifurcation allows the region of reduced mixing to be better characterised.These results suggest that improved understanding of cross-vortex mixingrequires consideration of the polar vortex not as a single mixing barrierbut as a barrier with internal structure that is likely to manifest as morecomplex gradients in trace gas concentrations across the vortex barrierregion.

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