The evolution of the ozone 'collar' in the Antarctic lower stratosphere during early August 1994

The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5-10 August 1994 is analyzed. The analysis focuses on the ozone 'collar' (the band of maximum values in ozone mixing ratio around the Antarctic ozone 'hole' at these altitudes) and the development of 'collar filaments.' Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of 'contour advection with surgery' (CAS). The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the 'tongues' of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.