On the magnitude of transport out of the Antarctic polar vortex

Abstract

The degree of isolation of the Antarctic stratospheric vortex in late winter and spring is investigated quantitatively by using a three‐dimensional global tracer transport model, in which the transport is computed from European Centre for Medium‐Range Weather Forecasts analyzed data. The evolution of the spatial distribution of passive tracers provides information about variations in the vortex structure, as well as about the magnitude of the transport out of the Antarctic vortex. The vortex structure revealed by tracers released inside the vortex at 72.5 hPa corresponds well with the satellite‐derived distribution of total ozone. The model computations indicate that in late winter and spring of the years 1990–1993, there is a quasi‐horizontal cross‐vortex transport of about 0.24% per day of the total tracer amount, while per day, 0.83% of the vortex mass descends into the troposphere. This indicates that roughly 65% of the vortex air is flushed out during August‐September‐October, the approximate lifetime of the Antarctic vortex. This number is insensitive to changes in model resolution, although the quasi‐horizontal outflow into the midlatitude stratosphere increases at the expense of the downward outflow if a coarser resolution is used. It is concluded that during late winter and early spring (i.e., the period of major ozone depletion), the vortex is a fairly well isolated air mass.