This is the first review of the paper “Global, regional and seasonal analysis of total ozone trends derived from the 1995–2020 GTO-ECV climate data record” by Melanie Coldewey-Egbers et al.

This paper is focused on updates of the trends derived from the extended GTO-ECV satellite total ozone record. The dataset was recently updated by including new satellite records (i.e. TROPOMI) and by extending previously used satellite records. The dataset is homogenized by using the same processing algorithm for all satellite records.

In the paper, the authors apply a statistical model to the GTO-ECV record to detect ozone changes over a wide range of latitudes (70S-70N). The regional and seasonal patterns in both Hemispheres are identified and linked to the chemical and dynamical processes that contribute to ozone recovery.

This paper is an extension to the previously published analyses by Coldewey-Egbers et al. (2014), however in this paper statistical model also includes additional explanatory parameters (AO and AAO), which improve the model fit to the data, especially at high latitudes, thus suggesting a significant contribution of the changes in the Brewer-Dobson circulation to the apparent ozone recovery in some regions.

Among new findings, authors report that positive trends in the 50-70 S region are highly significant in many regions. Since the AAO controls the transport of ozone to high latitudes, did you check if the AAO contribution to the ozone variability has significantly increased over the last 25 years?

This paper provides a strong contribution to the understanding of the drivers of ozone recovery.

 The figures are clear and support the discussion and main conclusion points of the paper.

Comments:

Line 30, “(though not statistically significant)” I recall that Ball et al. 2018 claimed a statistically significant ozone decrease in the lower stratosphere. He stated that the models did not support this finding. Please clarify if you are discussing total stratospheric ozone or low stratospheric ozone changes. I believe Ball et all (2019) has reassessed the significance of observed and modeled changes in the datasets extended to 2018, but still found high probability (90 % in NH mid-latitudes, 95 % in tropics, and 80 % in the SH middle latitudes) that the lower stratospheric ozone declined by 2018 as compared to the 1998 levels. I think it might be good to state in this paragraph that analyses were done using broad latitude bands and zonal averages, while your paper has assessed zonally resolved data for trends.

Line 35, “changes in the tropospheric amount do not indicate a clear global pattern” – this paper was published back in 2018. Should a more recent assessment of tropospheric ozone trends be used here (i.e. Ziemke et al. 2019) that found regional patterns in TOR trends?

Line 45, you mentioned “focusing on different questions”. Please indicate how this paper’s focus is different from other papers.

Line 64 “inter-relation” vs correlation:  please clarify what it means.

Lines 122-124. Do you introduce seasonal variability in the trend term only or also in coefficients of other proxies?

Line 167, you can also add Appenzeller et al, 2000 and Rieder et al (2010) papers to the references.

Line 175 “over Iceland” – the anomaly pattern in the figure is not centered on Iceland. Should the description be altered to describe the are (i.e. geographical coordinates of the region).

Lines 261-263. Have you considered the change in the position of the subtropical and polar jets that can contribute to regional changes in the total ozone (i.e. Seasonal and Regional Variations of Long-Term Changes in Upper-Tropospheric Jets from Reanalyses by Gloria L. Manney and Michaela I. Hegglin in J of Clim, 2018)?

Line 271-272, please explain what you mean by the “maximum values of the trends”.

Lines 286-289, Can the total ozone trend over Siberia be offset by an increase in tropospheric ozone due to wildfires?  It might be interesting to investigate seasonal differences in the tropopause height trends.

Figure 6. since the differences are hard to discern, would it make sense to make plots of the seasonal differences from the annual trends?

GENERAL

The paper is dedicated to the updated GTO-ECV total ozone climate data record, and to evaluation of global, regional and seasonal ozone trends.

The paper is well-organized and written, and it contains important information. Please find my minor comments below.

MAIN COMMENTS

1. For reporting trends with uncertainty interval (x +-y%), please make clear that uncertainties are 2-sigma (I believe, they are 2-sigma). It can be done with the first such trend reporting.
2. In addition to Figure 1, I would suggest adding a figure (maybe in appendix) showing the percent of variability explained by each proxy. For such analysis, 2 QBO components can be combined into one source. Such figure would be useful in visualization of relative contribution of each proxy to observed ozone variability.
3. The analyses presented in the paper show pronounced dependence of ozone trends on tropopause altitude. The tropopause altitude can be also used as a proxy in regression. It would be interesting to assess the influence of using tropopause height as a proxy on estimated ozone trends.

DETAILED COMMENTS

L. 30-31, Ball et al. (2018) reported statistical significance of the lower-stratospheric trends. It is worth to mention also recent studies - (Ball et al., 2019, 2020; Orbe et al., 2020; Dietmuller et al., 2021).

L.65 “inter-relation” – do you mean correlation?

L.88, 97: Please provide quantitative measures of “a very good quality”, “very good overall agreement”, “” excellent long-term stability”.

L. 100- 103: This sentence suits better for the discussion section.

L. 205-209: The comparison with height-resolved trends at different altitudes looks strange. In (Arosio et al., 2019) and (Sofieva et al., 2021), the longitudinal difference in trends between Scandinavia and Siberia are observed at all altitude levels in the stratosphere.

L. 237 – this paragraph. Compared to Coldewey-Egbers et al. (2014), this paper uses not only the updated dataset, but also a different MLR. This is worth to note.

L.265-270. Reanalyses data can be not optimal for trend analyses, since changing number of assimilated datasets with time can introduce artificial steps in data (for example, Simmons et al., 2014). Is it checked that the NCEP trends in tropopause height are in good agreement with those from  experimental data?

L.291. How many terms were used for characterization of seasonal dependence?

I think that Figure 8 does not bring essential information –   this is the summary of previous figures. Furthermore, the Hemispheres cannot be directly compared, as the latitude bands are different. I suggest placing this figure in Appendix.

L. 358-359. The last sentence of this paragraph contains technical information and can be omitted in  Summary.

L. 367 Please add “statistically” before “significant”

REFERENCES

Ball, W. T., Alsing, J., Staehelin, J., Davis, S. M., Froidevaux, L., and Peter, T.: Stratospheric ozone trends for 1985–2018: sensitivity to recent large variability, Atmos. Chem. Phys., 19, 12731–12748, https://doi.org/10.5194/acp-19-12731-2019

Ball, W. T., Chiodo, G., Abalos, M., Alsing, J., and Stenke, A.: Inconsistencies between chemistry–climate models and observed lower stratospheric ozone trends since 1998, Atmos. Chem. Phys., 20, 9737–9752, https://doi.org/10.5194/acp-20-9737-2020, 2020.

Dietmüller, S., Garny, H., Eichinger, R., and Ball, W. T.: Analysis of recent lower-stratospheric ozone trends in chemistry climate models, Atmos. Chem. Phys., 21, 6811–6837, https://doi.org/10.5194/acp-21-6811-2021, 2021

Orbe, C., Wargan, K., Pawson, S., and Oman, L. D.: Mechanisms linked to recent ozonedecreases in the northern hemisphere lower stratosphere, J. Geophys. Res., 125,https://doi.org/10.1029/2019jd031631, 2020

Simmons, A. J., Poli, P., Dee, D. P., Berrisford, P., Hersbach, H., Kobayashi, S., and Peubey, C.: Estimating low-frequency variability and trends in atmospheric temperature using ERAInterim, Quart. J. Roy. Meteor. Soc., 140, 329–353, https://doi.org/10.1002/qj.2317, 2014