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

Abstract

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 850 K, 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 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, 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.

Reanalysis intercomparisons of stratospheric polar processing diagnostics

“…Given the combination of differences in MPVGs and SVA (and raw vortex area), the results shown here indicate that there are some inherent differences in the PV fields that lead to somewhat disparate equivalent latitude mappings, which in some cases could alter conclusions drawn about transport barriers and trace gases in equivalent latitude coordinates. It is also possible that results for the SH were contaminated by the presence of double-peaked (bifurcated) PV gradients (e.g., Conway et al, 2018) that could have different magnitudes or structures among the reanalyses.…”

“…Over approximately the past two decades, numerous studies have compared meteorological products from DASs in the polar stratosphere (e.g., Manney et al, 1996;Pawson et al, 1999;Davies et al, 2003;Feng et al, 2005) and/or compared such products with observations (e.g., Knudsen et al, 1996Knudsen et al, , 2001Knudsen et al, , 2002Gobiet et al, 2007;Boccara et al, 2008;Tomikawa et al, 2015); see, e.g., Lawrence et al (2015) and Lambert and Santee (2018) for further review.…”

responses to Simon Chabrillat's comments

“…Given the combination of differences in MPVG and SVA (and raw vortex area), the results shown here indicate that there are some inherent differences in the PV fields that lead to somewhat disparate equivalent 30 latitude mappings, which in some cases could alter conclusions drawn about transport barriers and trace gases in equivalent latitude coordinates. It is also possible that results for the SH were contaminated by the presence of double-peaked (bifurcated) PV gradients (e.g., Conway et al, 2018) that could have different magnitudes or structures among the reanalyses.…”

Unusual Enhancement of the Optical Depth on the Continental Shelf Depth Latitudinal Variation in the Stratospheric Polar Vortex

“…The effects of various variables on the vertical distribution of ozone must be continuously observed using limb satellites [9] in the context of changing atmospheric thermodynamics, dynamics, and chemistry [10]. The polar vortex acts as a barrier to ozone transport in polar regions where the climatic conditions differ from other regions [11]. Polar stratospheric cloud formation contributes to ozone depletion and the eventual formation of ozone holes [12,13].…”