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The long-term evolution of total ozone column inside the Antarctic polar vortex is investigated over the 1980-2017 period. Trend analyses are performed using a multilinear regression (MLR) model based on various proxies for the evaluation of ozone interannual variability (heat flux, quasi-biennial oscillation, solar flux, Antarctic oscillation and aerosols). Annual total ozone column measurements corresponding to the mean monthly values inside the vortex in September and during the period of maximum ozone depletion from 15 September to 15 October are used. Total ozone columns from the Multi-Sensor Reanalysis version 2 (MSR-2) dataset and from a combined record based on TOMS and OMI satellite datasets with gaps filled by MSR-2 (1993-1995) are considered in the study. Ozone trends are computed by a piece-wise trend (PWT) proxy that includes two linear functions before and after the turnaround year in 2001 and a parabolic function to account for the saturation of the polar ozone destruction. In order to evaluate average total ozone within the vortex, two classification methods are used, based on the potential vorticity gradient as a function of equivalent latitude. The first standard one considers this gradient at a single isentropic level (475 or 550 K), while the second one uses a range of isentropic levels between 400 and 600 K. The regression model includes a new proxy (GRAD) linked to the gradient of potential vorticity as a function of equivalent latitude and representing the stability of the vortex during the studied month. The determination coefficient (R-2) between observations and modelled values increases by similar to 0.05 when this proxy is included in the MLR model. Highest R-2 (0.92-0.95) and minimum residuals are obtained for the second classification method for both datasets and months.

Trends in September over the 2001-2017 period are statistically significant at 2 sigma level with values ranging between 1.84 +/- 1.03 and 2.83 +/- 1.48DUyr(-1) depending on the methods and considered proxies. This result confirms the recent studies of Antarctic ozone healing during that month. Trends from 2001 are 2 to 3 times smaller than before the turnaround year, as expected from the response to the slowly ozonedepleting substances decrease in polar regions.

For the first time, significant trends are found for the period of maximum ozone depletion. Estimated trends from 2001 for the 15 September-15 October period over 2001-2017 vary from 1.21 +/- 0.83 to 1.96DU +/- 0.99 yr(-1) and are significant at 2 sigma level.

MLR analysis is also applied to the ozone mass deficit (OMD) metric for both periods, considering a threshold at 220DU and total ozone columns south of 60 degrees S. Significant trend values are observed for all cases and periods. A decrease of OMD of 0.86 +/- 0.36 and 0.65 +/- 0.33 Mt yr(-1) since 2001 is observed in September and 15 September-15 October, respectively.

Ozone recovery is also confirmed by a steady decrease of the relative area of total ozone values lower than 175DU within the vortex in the 15 September-15 October period since 2010 and a delay in the occurrence of ozone levels below 125 DU since 2005.