Potential vorticity and mixing in the south polar vortex during spring

Abstract

A central part of the explanation of the Antarctic ozone hole is the dynamical isolation provided by the intense vortex present over the south polar region until late in the spring. In this paper some fluid dynamical aspects of the Antarctic ozone hole phenomena are investigated, using data collected by the ER‐2 aircraft during the Airborne Antarctic Ozone Experiment (AAOE). Analysis of high‐resolution potential vorticity and nitrous oxide sections determined from the aircraft data show relatively little evidence of irreversible lateral mixing associated with large‐scale disturbances. Less than 10% of the flight legs show evidence of reversed meridional gradients of potential vorticity and nitrous oxide for spatial scales of more than a few degrees of latitude. Correlation studies show that small‐scale static stability variations are correlated positively with wind speed and not with relative vorticity, which strongly suggests that these are gravity wave signatures. Smaller‐scale variations in nitrous oxide and wind velocity are thus believed to be associated with gravity waves and not to represent lateral mixing which would constitute a significant breach of the dynamical isolation implied by the large‐scale potential vorticity gradient. Compositing of wind, potential vorticity, and nitrous oxide relative to the edge of the chemically perturbed region shows an enhancement of the meridional gradient of potential vorticity and nitrous oxide near this boundary at the 450 K potential temperature level. It is argued that this may be caused by the developing ozone concentration gradient producing a strong gradient in solar heating, which in turn drives a steepened potential vorticity gradient, although the temporal sampling provided by the AAOE measurements is not sufficient to prove this hypothesis. Another possible explanation of the enhanced gradients would be gradient steepening through erosion, by lateral mixing on the inner edge of the region of the strong meridional gradients of conserved quantities that define the vortex.