Onset of Stratospheric Ozone Recovery in the Antarctic Ozone Hole in Assimilated Daily Total Ozone Columns

In this paper we evaluate the long-term changes in ozone depletion within the Antarctic ozone hole using a 37 years (1979-2015) of daily Ozone Mass Deficits (OMD) derived from assimilated total ozone column data. For each year an ‘average daily OMD’ is calculated over a 60-day preferential time period DOY (Day of Year) 220-280). Excluding years with a reduced Polar Stratospheric Cloud (PSC) volume (the so-called PSC-limited years), the 1979-2015 time series of spatially-integrated average daily OMD correlates very well with long-term changes in Equivalent Effective Stratospheric Chlorine (EESC; R2 = 0.89). We find a corresponding statistically highly significant post year-2000 decrease in OMD of -0.77 ± 0.17 MegaTon (Mt; trend significance of 9.8σ), with an associated post year-2000 change in OMD of approximately -30%, consistent with the post year-2000 change in EESC relative to 1980 EESC levels of approximately -30%. The post year-2000 trend significance is robust to the choice of start year. The spatial distribution of the average daily OMD trends reveals a vortex-core region (approximately covering the region [90°W – 0 ° – 90°E / 75°S – 85°S]) largely unaffected by dynamics with a post year-2000 trend significance of > 8σ, and a vortex-edge region in which the trend is locally strongly affected by vortex dynamics though not spatially integrated over the whole vortex-edge region (trend significance > 9σ). For the trend significance we do not find consistent evidence for long-term changes in wave driving, vortex mixing, pre-ozone hole conditions, or the applied assimilation method, playing a role. Our observation/assimilation-based analysis provides robust evidence of a post year-2000 statistically highly significant decrease in the average daily OMD that is consistent with the long-term decrease in ozone depleting substances since 2000 following international emission regulations.