Review of “Interannual polar vortex-ozone co-variability” by Harzer et al.

This paper investigated the interannual co-variations between the zonal mean ozone distribution and the strength of the polar vortex during northern hemispheric winter using modern reanalyses. The authors consider the variability in both pressure and isentropic coordinates. Such a study is valuable to help us better understand current ozone recovery due to the phase-out of ozone-depleting substances. The topic is interesting and suitable for the journal. I have some minor comments and suggestions.

Major comments:

1. Why the paper only focuses on the northern hemispheric winter season? In the introduction, the authors only mentioned one sentence in line 26. Maybe the authors could provide more information and include some citations to justify their focus on the northern hemispheric winter.
2. It is related to question 1. Can the results presented in this paper apply to the southern hemisphere or other seasons? Those other results could be put in the supplementary or the next step. Also, supposed the paper only presents the results on the northern hemispheric winter, it may be a good idea to add “during northern hemispheric winter” in the title to narrow down the research area.
3. In Figure 2, as expected, the pattern of the anomaly of TEM vertical velocity is similar to the diabatic heating in most regions. But it is interesting to note that large differences exist in regions above 800 K (or 10 hPa) and poleward of 60N. Any explanations here? Also, the diabatic heating patterns are much closer to the temperature patterns in the regions above 800 K (or 10 hPa) and poleward of 60N, compared to the TEM vertical velocity.
4. The authors focus on diabatic vertical transport and horizontal eddy mixing to explain the anomalous ozone tendency. What about the role of changes in tropopause heights? Figure 4 and Figure 9c in Wang et al. (2020) showed that lower tropopause in cold climates (i.e., last glacial maximum) would lead to increases in ozone concentrations near the tropopause. The strength of the polar vortex may impact the tropopause heights and thus the ozone anomaly. This could be one of the reasons why the high ozone anomaly is gone in isentropic coordinates.

Wang, M., Fu, Q., Solomon, S., White, R. H., & Alexander, B. (2020). Stratospheric ozone in the last glacial maximum. Journal of Geophysical Research: Atmospheres, 125(21), e2020JD032929.

Minor comments:

1. In line 115, what about the stronger downward transport due to a stronger BDC (or stronger diabatic cooling) over the polar cap, as shown in Figure 2?
2. Figure 3 shows clearly that the results remain quite similar on different datasets. It could be helpful to add a figure to plot a similar figure as Figure 3 but on the isentropic levels.

This study investigates the co-variability at interannual timescales of the boreal polar vortex and polar cap stratospheric ozone during winter (DJF). An index based on polar cap temperature at 100 hPa (T100) is constructed to account for the polar vortex variability. The analysis is based on linear regressions of this index on different fields, including zonal-mean ozone and different components of the tracer continuity equation.

Overall, the results confirm that the interannual variability of the winter mean polar ozone is well connected to the dynamical variability of the polar vortex (see e.g., Fusco and Salby, 1999; Garcia and Hartmann, 1980; Garcia and Solomon, 1983; Hartmann and Garcia, 1979; Leovy et al., 1985; Ma et al., 2004; Plumb, 2002; Randel, 1993; 1994; 2002). This co-variability is controlled by the interplay between vertical advection by the residual circulation (which tends to accumulate ozone in polar regions) and horizontal eddy mixing (which reduces this accumulation by mixing with lower latitudes), both processes largely determined by large-scale wave activity of tropospheric origin.

Ozone anomalies, connected to T100 variability, just above the tropopause are shown to have opposite signs in pressure levels than in constant potential temperature surfaces. This is convincingly explained in terms of the vertical displacement of the tropopause in warm versus in cold winters, which is mostly adiabatic.

The paper is carefully written and the figures are clear. However, the goal of the paper is not very clear. In some parts, it seems that the main goal is to show that the T100 index is useful to understand polar ozone variability, but then the paper lacks of additional analysis to compare to existing metrics of stratospheric variability (such as eddy heat fluxes) and show the additional value of T100. In other parts, much of the analysis and discussion on the dynamical drivers of polar ozone variability does not seem to add much to the existing knowledge. I therefore suggest performing additional analyses and strengthening the arguments for what is new in this work and why it is important before meriting publication. I have recommended major revision, more detailed comments are listed below. 

Comments:

- The introduction is very general, and previous results on the topic are only briefly discussed in one short paragraph. I would suggest reviewing in more detail the established knowledge of the interannual variability of polar ozone in connection with extratropical stratospheric dynamics (see some suggested references in my general comment above).

- Lines 169-179 (Fig. 6). It is stated in the text that the contributions to the winter-mean ozone tendency of vertical mean advection and horizontal eddy mixing are about the same but with opposite signs, and hence the ozone tendency vanishes in the winter mean “as expected”. I have a few comments on this.

The first one is that the profiles in Fig. 6f do not show a compensation between both forcings, the negative tendencies from eddy mixing look larger than (at some isentropic levels, almost twice as large as) the positive tendencies from vertical advection. If indeed these are the main forcing terms, that would lead to negative ozone tendencies in the seasonal mean. It would be useful to add the seasonal mean profiles of both the ozone tendency and the ozone tendency derived from the tracer continuity equation to Fig. 6f. How do the winter-mean ozone tendencies in erai compare with the other datasets (particularly with observations)?

A second comment is that we should expect an accumulation of ozone in the lower stratospheric polar latitudes throughout the winter due to advection by the residual circulation. This accumulation is mainly driven by downward advection over the poles, which is larger than horizontal eddy mixing in the seasonal average (see e.g., Fig. 3 in De La Camara et al., 2018 for results with a chemistry climate model). This accumulation (late-minus-early winter ozone concentration) has been shown to correlate well with the winter-mean eddy heat flux at 100 hPa (e.g., Weber et al., 2011; Strahan et al., 2016; see also an update of Weber et al. 2011: Fig. 4-13 in Chipperfield and Santee, 2022). Based on this, I was expecting the regression on the T100 index to show larger ozone accumulation (i.e. winter mean positive ozone tendencies, and hence advection larger than eddy mixing) linked to warmer winters at 100 hPa. But Fig. 6f shows the opposite, at least in terms of the unbalance between vertical advection and eddy mixing. I am unsure of what the interpretation is here. Again, plotting the mean ozone tendency from the different datasets regressed on the T100 index may help elucidate this issue.

Related to this: If a goal of this paper is to show that the T100 index is valuable to understand polar ozone interannual variability, perhaps it’d be interesting to perform a thorough comparison between T100 and an index based on the eddy heat fluxes at 100 hPa (v’T’100), since the latter has been already proven to correlate well with ozone variability (e.g., Chipperfield and Santee, 2022). One way to do it would be to update Fig. 8 and Table 1 with a similar analysis based on v’T’100. Also, it could be useful to analyze not only the interannual variability of winter-mean ozone, but also of winter-mean ozone tendency, since the latter would more strongly depend on the dynamical evolution of the polar vortex and not that much on the initial ozone concentrations at the start of the season.

- The analysis derived from Fig. 7 is consistent with previous results showing that the initial changes of ozone in the lowermost stratosphere during SSWs are related to eddy fluxes, followed by a balance between vertical advection and mixing (De La Camara et al., 2018; Hong and Reichler, 2021). 

- One of the conclusions is that “interannual ozone variations are governed by similar dynamical processes as sub-seasonal ozone variability” (lines 218-222), referring to vertical advection and horizontal eddy mixing. However, these processes are already known to control polar ozone interannual variability (see e.g., Fusco and Salby, 1999; Garcia and Hartmann, 1980; Garcia and Solomon, 1983; Hartmann and Garcia, 1979; Leovy et al., 1985; Ma et al., 2004; Plumb, 2002; Randel, 1993; 1994; 2002). The authors should strengthen their case for what is new in their study.

References:

Chipperfield, M. P., and Michelle L. S. (Lead Authors), Alexander, S.P., de Laat, A. T. J., Kinnison, E., Kuttippurath, J., Langematz, U., and Wargan, K.: Polar Stratospheric Ozone: Past, Present, and Future, Chapter 4 in Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 509 pp., WMO, Geneva, 2022.

De la Camara, A., Abalos, M., Hitchcock, P., Calvo, N., and Garcia, R. R.: Response of Arctic ozone to sudden stratospheric warmings, Atmos. Chem. Phys., 18, 16 499–16 513, https://doi.org/10.5194/acp-18-16499-2018, 2018b.

Fusco, A.C. and M.L. Salby: Interannual variations of total ozone and their relationship to variations of planetary wave activity. J. Climate, 12, 1619–1629, 1999.

Garcia, R. R. and Hartmann, D. L.: The Role of Planetary Waves in the Maintenance of the Zonally Averaged Ozone Distribution of the Upper Stratosphere, J. Atmos. Sci., 37, 2248–2264, https://doi.org/10.1175/1520-0469(1980)037<2248:TROPWI>2.0.CO;2, 1980.

Garcia, R. R. and Solomon, S.: A numerical model of the zonally averaged dynamical and chemical structure of the middle atmosphere, J. Geophys. Res., 88, 1379, https://doi.org/10.1029/JC088iC02p01379, 1983.

Hartmann, D. L. and Garcia, R. R.: A Mechanistic Model of Ozone Transport by Planetary Waves in the Stratosphere, J. Atmos. Sci., 36, 350–364, https://doi.org/10.1175/1520-0469(1979)036<0350:AMMOOT>2.0.CO;2, 1979.

Hong, H.-J. and Reichler, T.: Local and remote response of ozone to Arctic stratospheric circulation extremes, Atmos. Chem. Phys., 21, 1159–1171, https://doi.org/10.5194/acp-21-1159-2021, 2021.

Leovy, C. B., Sun, C.-R., Hitchman, M. H., Remsberg, E. E., Russell, J. M., Gordley, L. L., Gille, J. C., and Lyjak, L. V.: Transport of ozone in the middle stratosphere: Evidence for planetary wave breaking, J. Atmos. Sci., 42, 230–244, https://doi.org/10.1175/1520-0469(1985)042<0230:TOOITM>2.0.CO;2, 1985. 

Ma, J., Waugh, D. W., Douglass, A. R., Kawa, S. R., Newman, P. A., Pawson, S., Stolarski, R., and Lin, S. J.: Interannual variability of stratospheric trace gases: The role of extratropical wave driving, Q. J. Roy. Meteor. Soc., 130, 2459–2474, https://doi.org/10.1256/qj.04.28, 2004.

Plumb, R. A.: Stratospheric Transport, J. Meteorol. Soc. Japan. Ser. II, 80, 793–809, https://doi.org/10.2151/jmsj.80.793, 2002.

Randel, W. J.: Global variations of zonal mean ozone during stratospheric warming events, J. Atmos. Sci., 50, 3308–3321, https://doi.org/10.1175/1520-0469(1993)050<3308:GVOZMO>2.0.CO;2, 1993.

Randel, W. J., Wu, F., and Stolarski, R.: Changes in column ozone correlated with the stratospheric EP flux, J. Meteorol. Soc. Jpn., 80, 849–862, https://doi.org/10.2151/jmsj.80.849, 2002. 

Randel, W.J., B.A. Boville, J.C. Gille, P.L. Bailey, S.T. Massie, J. Kumer, J. Mergenthaler and A. Roche: Simulation of stratospheric N2O in the NCAR CCM2: Comparisons with CLAES data and global budget analyses. J. Atmos. Sci., 51, 2834-2845, 1994.

Strahan, S. E., Douglass, A. R., and Steenrod, S. D.: Chemical and dynamical impacts of stratospheric sudden warmings on Arctic ozone variability, J. Geophys. Res., 121, 11836–11851, https://doi.org/10.1002/2016JD025128, 2016.

Weber, M., Dikty, S., Burrows, J. P., Garny, H., Dameris, M., Kubin, A., Abalichin, J., and Langematz, U.: The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales, Atmos. Chem. Phys., 11, 11 221–11 235, https://doi.org/10.5194/acp-11-11221-2011, 2011.