Attribution of recent ozone changes in the Southern Hemisphere mid-latitudes using statistical analysis and chemistry–climate model simulations

Abstract. Ozone (O₃) trends and variability from a 28-year (1987–2014) ozonesonde record at Lauder, New Zealand, have been analysed and interpreted using a statistical model and a global chemistry–climate model (CCM). Lauder is a clean rural measurement site often representative of the Southern Hemisphere (SH) mid-latitude background atmosphere. O₃ trends over this period at this location are characterised by a significant positive trend below 6 km, a significant negative trend in the tropopause region and the lower stratosphere between 9 and 15 km, and no significant trend in the free troposphere (6–9 km) and the stratosphere above 15 km. We find that significant positive trends in lower tropospheric ozone are correlated with increasing temperature and decreasing relative humidity at the surface over this period, whereas significant negative trends in the upper troposphere and the lower stratosphere appear to be strongly linked to an upward trend of the tropopause height. Relative humidity and the tropopause height also dominate O₃ variability at Lauder in the lower troposphere and the tropopause region, respectively. We perform an attribution of these trends to anthropogenic forcings including O₃ precursors, greenhouse gases (GHGs), and O₃-depleting substances (ODSs), using CCM simulations. Results indicate that changes in anthropogenic O₃ precursors contribute significantly to stratospheric O₃ reduction, changes in ODSs contribute significantly to tropospheric O₃ reduction, and increased GHGs contribute significantly to stratospheric O₃ increases at Lauder. Methane (CH₄) likely contributes positively to O₃ trends in both the troposphere and the stratosphere, but the contribution is not significant at the 95 % confidence level over this period. An extended analysis of CCM results covering 1960–2010 (i.e. starting well before the observations) reveals significant contributions from all forcings to O₃ trends at Lauder – i.e. increases in GHGs and the increase in CH₄ alone all contribute significantly to O₃ increases, net increases in ODSs lead to O₃ reduction, and increases in non-methane O₃ precursors cause O₃ increases in the troposphere and reductions in the stratosphere. This study suggests that a long-term ozonesonde record obtained at a SH mid-latitude background site (corroborated by a surface O₃ record at a nearby SH mid-latitude site, Baring Head, which also shows a significant positive trend) is a useful indicator for detecting atmospheric composition and climate change associated with human activities.