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

Tuesday, 8 January 2019: 1:45 PM
West 212A (Phoenix Convention Center - West and North Buildings)
Susan E. Strahan, NASA GSFC, Greenbelt, MD; and A. R. Douglass, M. R. Damon, and L. D. Oman

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.

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