The Importance of Early Winter Antarctic Ozone Levels to Ozone Recovery Trends

Ozone depleting substances (ODSs) have the greatest impact on O₃ in the Antarctic lower stratosphere, and it is here that the earliest and largest recovery signature is expected. Using multiple linear regression and 5 different O₃ datasets, Weber et al. [2018] reported September ozone recovery of 8-10% per decade since 2000; however, they also noted that the trends were barely significant and could easily become insignificant with a change in regression model, proxies, or assumptions about data drift uncertainties. Solomon et al. [2016] determined a 2.6 DU/yr trend since 2000 using September satellite and sonde O₃ data and a model, but attributed only half that trend to ODS decline.

Strahan and Douglass [2018] found that Aura Microwave Limb Sounder (MLS) stratospheric column O₃ inside the September Antarctic vortex was affected by vortex O₃ levels in early winter prior to depletion. Taking early winter column O₃ into account, they identified the signature of reduced ozone depletion in response to declining chlorine. A GMI chemistry transport model simulation with MERRA2 meteorology produces June Antarctic vortex stratospheric column O₃ that agrees very well with MLS O₃ for 2005-2017. The simulation also agrees closely with observed September vortex total column O₃ from 1980-2017. Between 1994 and 2003, the simulated June Antarctic vortex column O₃ averaged about 8 DU lower than in the decades before or after. When low June vortex O₃ levels are not accounted for, seasonal depletion due to ODSs appears larger than it is. Ozone trend calculations typically choose a recovery start date in the range 1997-2000, and because of the low early winter O₃ levels in those years, they produce a trend from September column O₃ that is larger than the trend due to ODSs alone. We calculate the total O₃ depletion in each year of the GMI simulation by differencing it from a simulation with identical meteorology but without heterogeneous O₃ loss by halogens. When O₃ loss is estimated by the June-September (i.e., winter) difference instead of the September vortex mean, the winter O₃ loss trend and the model-calculated heterogeneous chemical loss trend are in much better agreement. Knowledge of early winter Antarctic O₃ levels is necessary to correctly attribute trends in Antarctic September column O₃.

Solomon, S., D. J. Ivy, D. Kinnison, M. J. Mills, R. R. Neely III, and A. Schmidt (2016), Emergence of healing in the Antarctic ozone layer, Science, 252(6296), 269–274, doi:10.1126/science.aae0061.

Strahan, S.E. and A.R. Douglass (2018), Decline in Antarctic ozone depletion and lower stratospheric chlorine determined from Aura Microwave Limb Sounder observations, Geophys. Res., Lett., 44, doi:10.1002/2017GL074830.

Weber, M., M. Coldewey-Egbers, V.E. Fioletov, S.M. Frith, J.D. Wild, J.P. Burrows, C.S. Long, and D. Loyola (2018), Total ozone trends from 1979 to 2016 derived from five merged observational datasets – the emergence into ozone recovery, Atmos. Chem. Phys., 18, 2097–2117.