Abstract. Through the 21st century, anthropogenic emissions of the greenhouse gases N 2 O and CH 4 are projected to increase, thus increasing their atmospheric concentrations. Consequently, reactive nitrogen species produced from N 2 O and reactive hydrogen species produced from CH 4 are expected to play an increasingly important role in determining stratospheric ozone concentrations. Eight chemistry-climate model simulations were performed to assess the sensitivity of stratospheric ozone to different emissions scenarios for N 2 O and CH 4 . Global-mean total column ozone increases through the 21st century in all eight simulations as a result of CO 2 -induced stratospheric cooling and decreasing stratospheric halogen concentrations. Larger N 2 O concentrations were associated with smaller ozone increases, due to reactive nitrogen-mediated ozone destruction. In the simulation with the largest N 2 O increase, global-mean total column ozone increased by 4.3 DU through the 21st century, compared with 10.0 DU in the simulation with the smallest N 2 O increase. In contrast, larger CH 4 concentrations were associated with larger ozone increases; global-mean total column ozone increased by 16.7 DU through the 21st century in the simulation with the largest CH 4 concentrations and by 4.4 DU in the simulation with the lowest CH 4 concentrations. CH 4 leads to ozone loss in the upper and lower stratosphere by increasing the rate of reactive hydrogen-mediated ozone loss cycles, however in the lower stratosphere and troposphere, CH 4 leads to ozone increases due to photochemical smogtype chemistry. In addition to this mechanism, total column ozone increases due to H 2 O-induced cooling of the stratosphere, and slowing of the chlorine-catalyzed ozone loss cycles due to an increased rate of the CH 4 + Cl reaction. Stratospheric column ozone through the 21st century exhibits a near-linear response to changes in N 2 O and CH 4 surface concentrations, which provides a simple parameterization for the ozone response to changes in these gases.

The dependence of Antarctic ozone depletion on midlatitude planetary wave activity and South Pole temperatures was examined from 1979–2003 using NCEP/NCAR reanalyses and column ozone data. The annual severity of Antarctic ozone depletion was quantified using the seasonal mean of daily ozone mass deficit (OMD). The dependence of annual mean OMD on effective equivalent stratospheric chlorine (EESC) was removed to produce an anomaly time series (OMD′). Similar anomaly time series for 100 hPa South Pole temperatures (T′) and 20 hPa, 60°S midlatitude planetary wave activity (PWA′) were calculated. Regression of OMD′ against T′ and PWA′ shows that most of the interannual variability in Antarctic ozone depletion can be explained by variability in midlatitude planetary wave activity and South Pole temperatures. To estimate how future changes in South Pole temperatures, midlatitude wave activity and EESC will affect Antarctic ozone depletion, the regression model was applied to T′ and PWA′ values from a chemistry‐climate model run (1975–2019).

[1] Recently, it was shown that of the ozone-depleting substances currently emitted, N 2 O emissions (the primary source of stratospheric NO x ) dominate, and are likely to do so throughout the 21st century. To investigate the links between N 2 O and NO x concentrations, and the effects of NO x on ozone in a changing climate, the evolution of stratospheric ozone from 1960 to 2100 was simulated using the NIWA-SOCOL chemistry-climate model. The yield of NO x from N 2 O is reduced due to stratospheric cooling and a strengthening of the BrewerDobson circulation. After accounting for the reduced NO x yield, additional weakening of the primary NO x cycle is attributed to reduced availability of atomic oxygen, due to a) stratospheric cooling decreasing the atomic oxygen/ozone ratio, and b) enhanced rates of chlorine-catalyzed ozone loss cycles around 2000 and enhanced rates of HO x -induced ozone depletion. Our results suggest that the effects of N 2 O on ozone depend on both the radiative and chemical environment of the upper stratosphere, specifically CO 2 -induced cooling of the stratosphere and elevated CH 4 emissions which enhance HO x -induced ozone loss and remove the availability of atomic oxygen to participate in NO x ozone loss cycles.

Ozone mass deficit is a commonly used index to quantify Antarctic ozone depletion. However, as currently defined, this measure is not robust with respect to reflecting chemical ozone loss within the Antarctic vortex. Therefore, in this study, a new definition of ozone mass deficit (OMD) is developed. The 220 Dobson Unit based value currently used as the threshold for ozone depletion has been replaced with a new ozone background representative of pre‐ozone‐hole conditions. Second, the new OMD measure is based on ozone measurements within the dynamical vortex. A simpler method is also proposed whereby calculation of the vortex edge is avoided by using the average latitude of the vortex edge (62°S) as the spatial limiting contour. An indication of the errors in OMD introduced when using this simpler approach is provided. By comparing vortex average total ozone loss (defined using the new background and limiting contour) with partial column accumulated chemical ozone loss calculated with the tracer‐tracer correlation method for 1992–2004 and in more detail for 1996 and 2003, it is shown that the new OMD measure is representative of chemical ozone loss within the vortex. In addition the new criteria have been applied to the calculation of ozone hole area. The sensitivity of the new measures to uncertainties in the background have been quantified. The new ozone loss measures underestimate chemical ozone loss in highly dynamically disturbed years (2002 and 2004), and criteria for identifying these years are presented. The new measures should aid chemistry‐climate model intercomparisons since ozone biases in the models are avoided.

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