Potential drivers of the recent large Antarctic ozone holes

Domain: earth_science
Published: 2023-11-21
Editorial outcome: major_revision

Abstract
The past three years (2020–2022) have witnessed the re-emergence of large, long-lived ozone holes over Antarctica. Understanding ozone variability remains of high importance due to the major role Antarctic stratospheric ozone plays in climate variability across the Southern Hemisphere. Climate change has already incited new sources of ozone depletion, and the atmospheric abundance of several chlorofluorocarbons has recently been on the rise. In this work, we take a comprehensive look at the monthly and daily ozone changes at different altitudes and latitudes within the Antarctic ozone hole. Following indications of early-spring recovery, the October middle stratosphere is dominated by continued, significant ozone reductions since 2004, amounting to 26% loss in the core of the ozone hole. We link the declines in mid-spring Antarctic ozone to dynamical changes in mesospheric descent within the polar vortex, highlighting the importance of continued monitoring of the state of the ozone layer.

Introduction
The Montreal Protocol is widely considered to be successful1, and observed levels of controlled ozone depleting substances (ODSs) in the stratosphere, quantified as equivalent effective stratospheric chlorine (EESC), have been slowly declining since the late 1990’s1–3. Recovery of the Antarctic ozone hole in response to declining EESC is of particular interest, as the hole’s dramatic development in the late 20th century has been directly linked to anthropogenic ODS emissions4–6. For as long as large ozone holes persist, climate variability across the Southern Hemisphere (SH) will continue to be impacted7–9. Part of assessing the recovery of the Antarctic ozone hole requires acknowledgement of other factors besides EESC which contribute to anomalously high or low ozone values. Springtime temperature and wind patterns greatly impact SH ozone hole development, along with aerosol loading from wildfires and volcanic eruptions, as well as changes in the solar cycle1,10–21. Trends in greenhouse gas emissions are also expected to impact ozone recovery22–24, and proposed climate change mitigation strategies such as stratospheric aerosol injection would further impact recovery dates, if implemented1. Taking ozone modulating factors into account, the 2022 Scientific Assessment of Ozone Depletion1 concluded that the Antarctic ozone hole should be on track to recover by 2065. Latest results25,26, however, indicate that recovery may be delayed due to previously unaccounted for chlorine release from wildfire aerosols and anthropogenic emissions.

Nevertheless, the 2022 Antarctic spring season saw yet another large ozone hole with an extent and duration remarkably similar to the large holes of 2020 and 20211,27. Considering that 2015 and 2018 were also similar record years (five of the past eight years overall have exhibited record ozone holes), our aim here is to better understand recent changes in Antarctic ozone.

In this work, we take a systematic approach to analyse the behaviour of SH polar ozone throughout the key springtime months of September to November. We begin by examining the recent record ozone hole years of 2020–2022 in the context of historical total column ozone (TCO) observations and onset/breakup metrics. We then focus on signs of ozone recovery in vertical and horizontal distributions of SH polar stratospheric ozone during the SH spring months. Finally, we investigate links between the ozone signals and dynamical changes in order to identify the likely drivers of the observed ozone changes.

Results
Total column ozone and onset/breakup dates
The Antarctic ozone hole emerges in the early austral spring, during the month of August1. It typically persists until the end of November, coinciding with the breakdown of the SH polar vortex1. Year-to-year trends in September SH ozone have been considered to be the most sensitive for assessing ozone recovery directly due to the reduction of EESC, as this month is largely dominated by chemical, rather than dynamical, variability1,12,28–32. While October and November are thought to be more influenced by dynamics, they are important for assessing the full extent of the ozone hole area and depth12,28,33 as well as its dynamical impacts34.

Considering the large Antarctic ozone holes of 2020–202227, we assess the overall changes in September, October, and November SH TCO from 2001 to 2022. Figure 1a presents the TOMS/OMI polar mean TCO (see Methods), with the years from 1979 included for context. The analysis is analogous to that previously reported for November by Zambri et al.34. We use a simple linear regression rather than a multivariate fit, as we focus on long term changes in ozone rather than attribution. Henceforth, we refer to the slope of the linear regression fit as the long term “change” in ozone instead of “trend”, analogous to Orbe et al.35.

Figure 1a highlights that the monthly mean TCO values have little year-to-year variability in 2020–2022. While the September TCO values are low in the most recent years, overall, September continues to display a slightly upwards slope (red line in Fig. 1a). This slope generally aligns with the expected recovery behaviour. However, this is not the case for October and November (blue and cyan lines in Fig. 1a), which show a negative change in TCO since 2001. Importantly for all months, the slope of the linear fit is statistically insignificant. As in previous studies34, we also find that the choice of turnaround/pivot year for the overall ozone trends (e.g., 2000 vs 2001) has little impact on the trajectory. This raises the question of how many years of observational records will be needed to solidify TCO recovery, particularly in light of emerging new sources of chlorine and the changing state of the Antarctic stratosphere due to climate change influences.

Additionally, we assess the ozone hole onset and breakup dates from 2005–2022, using a TCO depth threshold of <130 Dobson Units (DU) and an area threshold of 1 million km228 (see Methods). We choose the 130 DU threshold (as opposed to 175/220 DU) in order to focus on the deepest levels of ozone depletion, characteristic of recent years27. Similar to Stone et al.28, we identify the ozone hole onset date as the day when the ozone hole area exceeds the threshold for 3 days in a row. In addition to the onset date, we further identify the ozone hole breakup date as the day when the <130 DU ozone hole no longer surpasses the 1 million km2 area threshold. The ozone hole onset and breakup dates are presented in Fig. 1b. The trend in the onset date (green line in Fig. 1b) agrees with previous findings of a delayed ozone hole opening date1,12,27,28,33. This delay is likely an indication of early-spring recovery due to EESC reduction1,12. Our results for the breakup date trend (magenta line in Fig. 1b) show a similar delay in the breakup of the ozone hole, albeit much noisier and insignificant. Given that the deep ozone hole breakup typically occurs during October/November, a later breakup date is in agreement with the declines in October/November TCO in Fig. 1a. These results suggest that there may be new patterns emerging in the mid-spring evolution of the ozone hole, thus delaying the breakup date.

Long term change in vertical ozone distribution
Previous studies have found evidence of vertically and latitudinally resolved patterns in mid-latitude ozone trends that can oppose each other at different stratospheric layers and, as a result, can be indistinguishable in column measurements15,36–39. Figure 2 shows the vertically and latitudinally resolved SH ozone change [ppmv/year] from 2004 to 2022 using MLS/Aura observations (see Methods). The change is based on a linear regression with time as the sole predictor and is determined individually for September-November, as before. Areas with positive/negative concentration changes indicate atmospheric layers which have an increased/decreased amount of ozone at their given latitude through the 19 years of MLS observations. We find consistent, significant positive (red, hatched) ozone change of up to 0.03 ppmv/year in the upper stratosphere (between about 1–7 hPa, or 34–46 km) throughout the spring months. This is focused equatorward of 60°S and is consistent with previously reported increases in ozone near the edge of the polar vortex32. Our results also show a region of significant reduction in ozone (blue, hatched), with a change of up to −0.09 ppmv/year, in October. This takes place at lower altitudes, in the middle stratosphere between 7 and 60 hPa (~20–34 km). Unlike the recovery signal, the ozone reduction is focused on polar latitudes, poleward of 60°S.

To further determine the time evolution of the positive and negative changes found in the monthly analysis, we investigated a day-to-day progression of the high latitude polar ozone volume mixing ratios (see Methods), with results shown in Fig. 3. We find a primarily insignificant positive signal (red) starting above 10 hPa in late August and extending to around 100 hPa by the end of September. This finding of a positive change in middle stratospheric ozone during late August and early September is in agreement with the results presented in Fig. 1, which show an insignificantly increasing September TCO trend and a shift towards a later ozone hole onset date. A region of positive change (up to 0.04 ppmv/year) is also present in the upper stratosphere in September. This region extends into the first ten days October, after which it appears to descend to 3–26 hPa (~25–40 km) by the start of November. For the positive signals overall, the peak values are between 0.04 and 0.06 ppmv/year.

Contrary to the positive signals, the large area of negative (blue) change found during October (Fig. 2) is clearly present, and statistically significant, throughout the month of October. Most of the ozone loss signal in October is focused between 5 and 50 hPa (~21–37 km) and is linked to a feature that first appears in late August at around 1–2 hPa (~45 km). The regions of the highest negative ozone change reach upwards of −0.10 ppmv/year and are found to be statistically significant. Critically, this negative region overlaps with the main ozone layer, with highest ozone mixing ratios and therefore the core of the SH ozone layer, located between 4 and 20 hPa (~27–38 km)40. The finding of a negative change in middle stratospheric ozone during October is in agreement with the results presented in Fig. 1, which indicated decreasing October TCO and a shift towards a later breakup date of the deep ozone hole.

Ozone change in the upper vs middle stratosphere
The findings in Figs. 2 and 3 support the notion that analyses conducted purely with TCO measurements obscure key information regarding vertical changes within the SH springtime ozone layer. We demonstrate this further by using MLS/Aura observations to construct a partial column ozone (PCO) time series over the regions of interest found previously. Figure 4 presents the October PCO time series for four distinct regions: Middle stratosphere (5–50 hPa, ~20–37 km) (1) polar latitudes (60°S–82°S), and (2) high polar latitudes (75°S–82°S), as well as upper stratosphere (1–5 hPa, ~37–50 km) (3) polar latitudes (60°S–82°S), and (4) near the vortex edge region (45°S–60°S).

Both PCO time series in the middle stratosphere show a significant negative trajectory. Overall, the net change of the time period from 2004 to 2022 is approximately −26% in the area closest to the pole (magenta line, 75°S–82°S). The PCO values for the 5–50 hPa region are generally between 80 and 160 DU, indicating that this region is a large contributor by weight to the corresponding TCO measurements. The values from higher altitudes, 1–5 hPa, tell a different story. This linear fit is slightly positive and only significant when the region of interest is limited to 45°S–60°S (light blue line). While the slope is positive, this increase is equivalent to less than 2 DU, which does not represent a notable contribution to the change in the total ozone column.

Drivers of the changes in ozone
We now turn towards identifying potential drivers for the decline in October polar ozone in the 5–50 hPa vertical range, as identified in Figs. 2–4. While ozone depletion due to ODSs is known to begin in late August1, October marks the month with the largest ozone hole extent and depth1,12,28,33. The month of October is also when dynamical variability is thought to begin to play a larger role in the ozone hole progression, particularly in the form of mesospheric air transport into the polar vortex1,17,41,42.

We first look for a change in mesospheric descent inside the polar vortex by using carbon monoxide (CO) observations from MLS/Aura as an atmospheric tracer43. Similar to Fig. 3 for ozone, we perform a linear regression on the day-to-day progression of polar CO across the years 2004–2022. The results are presented in Fig. 5a. We find a tongue of increasing CO which reaches the same altitude (7–20 hPa, ~27–34 km) as the upper portion of the negative ozone change detected during the month of October (Fig. 3). This tongue sits above a region of negative change in CO (20–50 hPa, ~21–27 km), implying an upwards shift in altitude of the arriving mesospheric air mass.

To investigate the potential implications of this shift on the stratospheric chemical balance, we look at the relationship between CO and stratospheric nitrogen dioxide (NO2) column (from OMI/Aura). NO2 is a member of the NOx family of chemicals which are known to be transported into the polar vortex from the mesosphere, alongside CO, where NOx can in turn deplete stratospheric ozone43,44. Descending NOx is also known to curb chlorine-driven ozone loss through the Energetic Particle Precipitation (EPP) indirect effect when the NOx reaches the middle and lower stratosphere1,17,42. Figure 5b presents the correlation between the altitude of late-October CO descent and the average October polar stratospheric NO2 (see Methods). We find a significant negative correlation (R2 value of 0.78) between the CO altitude and stratospheric NO2, with recent years (2020–2022) consistently corresponding to higher altitudes of CO and lower NO2 column.

Finally, we correlate the altitude of late-October CO descent with the second PCO series from Fig. 4 (magenta line, 75°S–82°S, 5–50 hPa), with results presented in Fig. 5c. This too produces a significant negative correlation, with an R2 value of 0.85. These results imply that the EPP indirect effect (or lack of arriving NOx) due to changes in the timing and altitude of mesospheric descent could play a role in PCO during October. Further investigation is required to determine the exact dynamical origin of these effects.

Methods
Data and analysis for Fig. 1
Panel a: We use Total Ozone Mapping Spectrometer (TOMS) TCO data for the years 1979–2004 and the months of September, October, and November46,47. TOMS data is missing for Sept. and Oct. in 1993–1996 and for Nov. in 1993–1997, thus these months/years are excluded from the analysis. Gaps are shown in figures at the corresponding times. The latitude range 60°S–90°S is used to find monthly zonal mean TOMS TCO for each year present in the dataset34.

The TOMS dataset is extended to 2022 using the Ozone Monitoring Instrument (OMI, on board the Aura-satellite) TOMS-Like Ozone L3 daily data from 2005 to 202248. OMI data is missing for Sept. 28–29, 2008, thus these two days are excluded from the analysis. For each day of OMI measurements, the latitude range 60°S–90°S is used to find a zonal mean TCO34. The monthly mean TCO and standard deviation for each month are then calculated from the OMI daily mean TCO for each year present in the dataset. In the analysis, the years 2002 and 2019 are excluded, as they have been associated with sudden stratospheric warming events that dominate the formation and breakup of the ozone hole1,18,49,50. Linear regression with time as the sole predictor is performed from 2001 to 2022 to find the overall change in ozone for each month under investigation. We use 2001 as the starting point based on the theoretical pivot point in ozone depletion at the turn of the 21st century34, however, changing the pivot year does not influence the overall results. The statistical significance is determined using the 95% confidence interval on the coefficient estimates. In addition, the R2 value is reported.

Panel b: We again use the OMI TOMS-Like Ozone L3 daily data from 2005 to 202248 for best continuity with the TCO analysis in panel a. We first calculate a latitude-weighted area for all grid points from 50°S to 90°S. Using a 130 DU ozone depth, as in Stone et al.28, we find the total daily area (in km2) of grid points which are less than the threshold. If there is missing data over the southernmost grid points, we check the furthest south grid point. If this grid point goes below the threshold, we assume that all missing grid points south of that point are also below the threshold. We then identify the first day when the area surpasses the 1 million km2 area threshold and remains above for 3 days or more, again as in Stone et al.28.

We additionally calculate the ozone hole breakup dates from 2005 to 2022 by finding the last day in the season when the <130 DU ozone hole reaches an area above the 1 million km2 threshold. The day after this is considered the breakup date.

Data and analysis for Fig. 2
We use Microwave Limb Sounder (MLS, on-board the Aura satellite) Level 2 Ozone Mixing Ratio Version 5 data for 2004–2022 and for the months of September-November. MLS/Aura ozone data is known to have minimal drift over time when compared to ground-based detectors, so it is a reliable instrument to use when comparing ozone measurements over the 21st century51. The MLS ozone volume mixing ratio (VMR) data is limited to the recommended vertical range: 261 to 0.0215 hPa52. The data is interpolated to 1°×1° latitude-longitude coverage. Outlier days excluded due to anomalous data are (reported as day of year) 2004: days 252 and 253; 2005: day 293; 2006: day 307; 2007: day 310; 2009: day 300; 2012: day 328; 2016: day 292; 2018: day 298; 2022: day 277.

Monthly means for the interpolated MLS/Aura ozone data are found for Sept., Oct., and Nov. for each year from 2004 to 2022. Then, a zonal average is calculated for every latitude across 45°S–82°S for each month/year and a linear regression fit to the resulting data is determined. As previously, data from 2019 is excluded from the fit18. The slope of each point is reported as change in parts per million volume ozone per year (ppmv/year). The 95% confidence interval, reported in the figure, is found using bootstrapping with 1000 iterations (results are consistent when 10,000 iterations are used).

Analysis for Fig. 3
A daily zonal and latitudinal average from 75°S to 82°S is found in the daily MLS/Aura ozone profiles from August 21st through the end of November. A starting point of 75°S is used to reduce any interaction with the polar vortex53. Daily profiles are lined up sequentially from August 21st to November 30th to form a time series for each year. Data is stacked across years and a linear regression fit is found. Data from 2019 is excluded from the fit18, as before. The slope of each point is reported as the change in parts per million volume ozone per year (ppmv/year), shown by contour colouring. The 95% confidence interval, reported in the figure, is found using bootstrapping as in the previous section.

Analysis for Fig. 4
Four series are created from the October mean MLS/Aura data, each further isolated by a range of altitude and latitude levels: Pressure levels 5–50 hPa averaged from 60°S–82°S and from 75°S–82°S; Pressure levels 1–5 hPa averaged from 60°S–82°S and from 45°S–60°S.

MLS ozone concentration values are then averaged zonally and across the latitude range specified to produce a single profile for each year, which is used to calculate a PCO. The resulting PCO values are used to find a linear regression fit for each time series. Statistical significance is reported using 95% confidence interval on the coefficient estimates from the regression fit, along with the R2 value. An approximation of the net ozone loss through the range of years is calculated using the estimated slope of the linear fit.

Data and analysis for Fig. 5
Panel a: We use Microwave Limb Sounder (MLS, on-board the Aura satellite) Level 2 Carbon Monoxide (CO) Mixing Ratio Version 5 data for 2004–2022 and for the months of August-November. The MLS CO volume mixing ratio (VMR) data is limited to the recommended vertical range: 215 to 0.00564 hPa54. Outlier days excluded due to anomalous data are (reported as day of year) 2004: days 252 and 253; 2009: day 299 and 300; 2018: day 298; 2022: day 277.

A daily zonal and latitudinal average from 75°S to 82°S is found in the daily MLS/Aura CO profiles from August 21st through the end of November. A starting point of 75°S is used to match the MLS/Aura ozone selection in Fig. 3. Daily profiles are lined up sequentially from August 21st to November 30th to form a time series for each year. Data is stacked across years and a linear regression fit is found. Data from 2019 is excluded from the fit, as before in Fig. 318. The slope of each point is reported as the change in parts per billion volume CO per year (ppbv/year), shown by contour colouring. The 95% confidence interval, reported in the figure, is found using bootstrapping as in the previous sections.

Panel b: The MLS/Aura CO daily profiles are used for October 27th to October 31st (last 5 days of October) and are averaged zonally and latitudinally from 75°S to 82°S, as before in panel a. The altitude range is cropped to 5–50 hPa to match the coverage used for the PCO calculation for the 2nd series in Fig. 4. A Modified Akima Interpolation is performed to find the altitude (in kilometres) of the CO profile maximum for each day. The altitude of the CO maximum is averaged across the 5 days to find a single value for each year present in the dataset.

We use the Ozone Monitoring Instrument (OMI, on board the Aura-satellite) NO2 Stratospheric MINDS Daily L2 daily October data from 2005 to 202255. For each day of OMI measurements, the latitude range 75°S–90°S is used to find a zonal mean stratospheric NO2 concentration in mole/cm2 × 1015. October mean stratospheric NO2 is then calculated from the OMI daily mean NO2 for each year present in the dataset.

The October mean stratospheric NO2 versus the altitude of late-October CO maximum is used to find a linear regression fit across 2005–2022 (2004 is excluded from the CO data due to the starting point of 2005 for the OMI NO2 data). Statistical significance is reported using 95% confidence interval on the coefficient estimates from the regression fit, along with the R2 value.

Panel c: The MLS/Aura altitude of late-October CO maximum data is used again, as calculated in panel b.

We select the 2nd PCO series produced in Fig. 4, using the pressure level range of 5–50 hPa and averaged from 60°S–82°S, across 2004–2022.

The October PCO (from 5 to 50 hPa and 60°S–82°S) versus the altitude of late-October CO maximum is used to find a linear regression fit across 2004–2022. Statistical significance is reported using 95% confidence interval on the coefficient estimates from the regression fit, along with the R2 value.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of “Polar stratospheric ozone confinues on a decline: A complex story of the changing Antarcfic ozone hole” by Kessenich et al. (2023) submifted to Nature communicafions.\nBased on MLS satellite, Kessenich et al. (2023) analyzed the observed ozone trends in the middle and upper stratosphere in Southern Hemisphere (SH). The major findings are 1) with the recent two years’ data, the Antarcfic stratospheric total column ozone (TCO) trends are all negafive and insignificant; 2) the ozone trend is negafive in the Antarcfic middle stratosphere, and the ozone PCO decrease by 26% (or ~30 DU) from 2004 to 2020.\nBy including the 2020-2022’s data, the Antarcfic ozone trend from 2004 to 2020 become negafive. It is not surprising as we know at least wildfire and volcanic erupfions amid 2019-2022 contributed to the observed change. From the fitle of “A complex story of the changing Antarcfic ozone hole”, I expected the authors analyzing and quanfifying the causes of the observed ozone change in the Antarcfic stratosphere. Unfortunately, I failed to find sufficient discussions either from observafion or the modeling point of view. I also failed to find analysis how much the ozone recovery year can be affected. What caused the negafive trend in the middle stratosphere since 2004? Is it dynamics-driven or chemistry-driven?\nI do think the Antarcfic ozone trend is an important issue, and I don’t see any major technique problem of the analysis in the manuscript. However, I worry the analysis and conclusions present in the manuscript are superficial.\nMinor comments:\n1. It is helpful to add the Line number in the manuscript.\n2. “Our findings suggest that changes in the SH atmosphere are contribufing to a persistent Antarcfic ozone hole. ” This is an ambiguous statement. What are the “changes”?", "reviewer_2": "This study uses verfically-resolved observafions of ozone to focus on changes in the Antarcfic ozone hole over the period 2001 to 2022. The authors claim that their results support their conclusions that 1) Antarcfic ozone is not recovering significantly and 2) the region and month of strongest ozone decline occurs over the polar middle and lower stratosphere during October. As a result of 1) and 2), the authors conclude that dynamical factors are resulfing in a persistent SH ozone hole that is not showing any stafisfically significant signs of recovery. Given that this conclusion is quite bold (as it quesfions the efficacy of the Montreal Protocol) I would like to see how this result bears up when other metrics of the ozone hole are considered (major comment 1 below). I strongly recommend this as I suspect that a more exhausfive analysis will reveal subtlefies that may challenge the authors’ main conclusions and, if anything, may lead to the opposite conclusion. I also have concerns about the mechanism for confinued ozone decline that is (hasfily) proposed in the conclusions as no evidence for this mechanism (from either modeling or observafions) is presented. To this end, I suggest major revisions before I can begin consider recommending publicafion.\nMajor Comment 1: Ozone hole onset date: One of the references cited in the study, Kane et al. (2021), points out that, despite the recent large SH ozone holes of 2015, 2018, 2020 and 2021 (also noted in this study), there has been a delay in ozone hole onset since 2020 (see their Figure 2). The implicafion, therefore, is that “even though 2015, 2018, 2020, and 2021 had large ozone holes, there is a clear later onset of ozone deplefion in these years compared to 1999–2008 (the peak in stratospheric EESC loading) expressing that ozone recovery is sfill proceeding following the decline in polar EESC since 2000 (WMO, World Meteorological Organizafion,2018).” As a result, the authors interpret this later start date for the ∼ rapid onset of ozone deplefion as “a robust sign of ozone recovery post-2000.”\nRequest: Can the authors examine ozone hole onset date and corroborate the findings from this study? If so, I have to admit that I agree with the Kane et al. (2021) assessment that ozone loss must be considered in the context of the formafion date of the ozone hole and that, furthermore, a delayed ozone hole onset could be interpreted as a sign of ozone recovery. This should be discussed more in the present study and discussed with respect to how this may change the authors' major conclusiosn.\nStone, K.A., Solomon, S., Kinnison, D.E., Mills, M.J.: On Recent Large Antarcfic Ozone Holes and Ozone Recovery Metrics. Geophysical Research Lefters 48(22), 2021–095232 (2021). hftps://doi.org/10.1029/ 2021GL095232\nMajor Comment 2: The proposed mechanisms for October ozone decreases seems rather hasfily thrown in the conclusions and unsupported. In parficular, the authors suggest that the ozone reducfions are “linked to an air mass descending from late August from around 1-2 hPa.” There is no observafional or modeling evidence to support this claim. My request, therefore, is that the authors either 1) run modeling experiments integrafing an age-of-air tracer which would capture the transport phenomenon that they propose is occurring or 2) provide evidence from the observafions using observafionally-based transport metrics. If they cannot provide either 1) or 2), then the manuscript loses a significant part of its novelty and it’s not clear that the results presented substanfiate publicafion in a high-impact journal like Nature."}, "author_response": "We would like to thank both reviewers for providing their valuable comments on our manuscript. After reviewing their recommendations in detail, we have incorporated a number of changes into our new draft. The reviewers’ input has helped to strengthen the quality of our manuscript, particularly in terms of further evidence on drivers for the October ozone changes. Our detailed responses to all reviewer comments are included below. We note that some formatting changes were made in revision to meet the journal formatting guidelines.\nReviewer #1 (Remarks to the Author):\nOverall Comment\nBased on MLS satellite, Kessenich et al. (2023) analyzed the observed ozone trends in the middle and upper stratosphere in Southern Hemisphere (SH). The major findings are 1) with the recent two years’ data, the Antarctic stratospheric total column ozone (TCO) trends are all negative and insignificant; 2) the ozone trend is negative in the Antarctic middle stratosphere, and the ozone PCO decrease by 26% (or ~30 DU) from 2004 to 2020.\nBy including the 2020-2022’s data, the Antarctic ozone trend from 2004 to 2020 become negative. It is not surprising as we know at least wildfire and volcanic eruptions amid 2019-2022 contributed to the observed change. From the title of “A complex story of the changing Antarctic ozone hole”, I expected the authors analyzing and quantifying the causes of the observed ozone change in the Antarctic stratosphere. Unfortunately, I failed to find sufficient discussions either from observation or the modeling point of view. I also failed to find analysis how much the ozone recovery year can be affected. What caused the negative trend in the middle stratosphere since 2004? Is it dynamics-driven or chemistry-driven?\nI do think the Antarctic ozone trend is an important issue, and I don’t see any major technique problem of the analysis in the manuscript. However, I worry the analysis and conclusions present in the manuscript are superficial.\nResponse\nWe have revised the manuscript by adding more satellite data analysis to particularly address the differences between chemistry-driven and dynamics-driven ozone loss. For the latter, we include more observational evidence from the Aura and OMI instruments as well as discussion of dynamical drivers that influence the ozone loss patterns reported in the manuscript. More details on these are included in the responses to specific questions below.\nIn terms of the ozone recovery year, our aim was not to reassess the current prediction. Rather we are focused on the complex interplay of chemistry and dynamics of the Antarctic atmosphere, which itself is often less of a focus on large assessments looking at overall global ozone levels, or annual scale recovery. Our aim is to specifically focus on the spatiotemporal variability of Antarctic stratospheric ozone, as this is not captured for total ozone column assessment or many of the other ozone recovery metrics commonly used.\nWe now address the reviewer’s primary request to present evidence for the cause of the negative ozone trend in the middle stratosphere with the addition of a new section titled “Drivers of the changes in ozone” and Figure 5 to our manuscript. Further related details are also included in our responses to specific points raised by Reviewer #2, included below.\nMinor Comment One\nIt is helpful to add the Line number in the manuscript.\nResponse\nLine numbers have been added as requested.\nMinor Comment Two\n“Our findings suggest that changes in the SH atmosphere are contributing to a persistent Antarctic ozone hole. ” This is an ambiguous statement. What are the “changes”?\nResponse\nWe have rewritten the last conclusion paragraph to be more specific regarding the dynamic changes in mesospheric descent we see in the polar middle stratosphere. This arises from the new analysis added in the new Figure 5 and the section titled “Drivers of the changes in ozone” of our manuscript. With more specific findings that are now discussed, this previous ambiguity regarding the “changes” has been replaced by more details.\nFor example, we have added new text for the last paragraph of the conclusion which reads:\n“By taking a holistic look at the yearly progression of the Antarctic ozone hole over the last two decades, our results shed new light on the deep and long-lived ozone holes of recent years. We identify links between the mid-spring ozone hole evolution and the timing/depth of mesospheric air arrival into the polar vortex. This mechanism is of dynamical origin and could be operating independently from the volcanic and wildfire effects implicated for the heightened ODS-induced ozone loss during the early springs of recent years [1, 6, 20, 23]. More study is warranted to determine the precursors for the changes we identify in mesospheric descent and better understand the extent to which these effects will impact polar ozone in our changing climate.”\nReviewer #2 (Remarks to the Author):\nOverall Comment\nThis study uses vertically-resolved observations of ozone to focus on changes in the Antarctic ozone hole over the period 2001 to 2022. The authors claim that their results support their conclusions that 1) Antarctic ozone is not recovering significantly and 2) the region and month of strongest ozone decline occurs over the polar middle and lower stratosphere during October. As a result of 1) and 2), the authors conclude that dynamical factors are resulting in a persistent SH ozone hole that is not showing any statistically significant signs of recovery. Given that this conclusion is quite bold (as it questions the efficacy of the Montreal Protocol) I would like to see how this result bears up when other metrics of the ozone hole are considered (major comment 1 below). I strongly recommend this as I suspect that a more exhaustive analysis will reveal subtleties that may challenge the authors’ main conclusions and, if anything, may lead to the opposite conclusion. I also have concerns about the mechanism for continued ozone decline that is (hastily) proposed in the conclusions as no evidence for this mechanism (from either modeling or observations) is presented. To this end, I suggest major revisions before I can begin consider recommending publication.\nResponse\nOur intention is not to question the efficacy of the Montral Protocol, nor do we believe our results refute that. Indeed, we included analysis of the total column ozone that continues the long timeseries that has been published many times previously. As we show in our Figure 1, September total column ozone continues to indicate recovery. The effectiveness of the ban on CFCs and the impact on the early spring total column ozone is a positive development, and, as we also show, this recovery is mainly focused on the upper stratosphere and the poleward side of the polar vortex (Figure 2). We do highlight that this progress is yet to reach the core of the stratospheric ozone layer.\nOur original aim was to focus on the changes in Antarctic ozone regardless of whether these were caused by CFC related ozone depletion or other mechanisms. The recent large ozone holes have a significant impact on both UV levels and climate and dynamics of the Southern Hemisphere. We need to understand these extended periods of large ozone holes to be able to understand how well these processes are captured in regional variability as well as future model predictions for our part of the world (the authors reside in New Zealand). We very much agree that both reviewers raise a good point, and we acknowledge that identifying the likely cause behind the changes we find is an important next step and should be addressed in this manuscript. Thus, we have added more data and analysis to support our previous results – we detail the new additions in the following responses. This additional analysis has helped sharpen the conclusions of the work and we have also rewritten the text in the manuscript to try and make sure the manuscript is clear in that we do not refute the progress from the Montral Protocol.\nMajor Comment One\nOzone hole onset date: One of the references cited in the study, Kane et al. (2021), points out that, despite the recent large SH ozone holes of 2015, 2018, 2020 and 2021 (also noted in this study), there has been a delay in ozone hole onset since 2020 (see their Figure 2). The implication, therefore, is that “even though 2015, 2018, 2020, and 2021 had large ozone holes, there is a clear later onset of ozone depletion in these years compared to 1999–2008 (the peak in stratospheric EESC loading) expressing that ozone recovery is still proceeding following the decline in polar EESC since ∼2000 (WMO, World Meteorological Organization, 2018).” As a result, the authors interpret this later start date for the rapid onset of ozone depletion as “a robust sign of ozone recovery post-2000.”\nRequest: Can the authors examine ozone hole onset date and corroborate the findings from this study? If so, I have to admit that I agree with the Kane et al. (2021) assessment that ozone loss must be considered in the context of the formation date of the ozone hole and that, furthermore, a delayed ozone hole onset could be interpreted as a sign of ozone recovery. This should be discussed more in the present study and discussed with respect to how this may change the authors' major conclusions.\nStone, K.A., Solomon, S., Kinnison, D.E., Mills, M.J.: On Recent Large Antarctic Ozone Holes and Ozone Recovery Metrics. Geophysical Research Letters 48(22), 2021–095232 (2021). https://doi.org/10.1029/ 2021GL095232\nResponse\nThe ozone hole onset date is a beneficial metric to consider, and we agree with the reviewer that it would make a good addition to our analysis, thus we have added this information to the revised Figure 1. Stone et al. (2021) find that even large ozone holes of recent years (2015 and 2020) show a shift towards later onset dates, and hence support the continued recovery from CFC-induced ozone loss. We find that this metric goes hand-in-hand with the TCO analysis in Figure 1 of our manuscript.\nWe calculated the ozone hole onset date from 2005-2022 using a similar method as Stone et al. (2021) for the TCO thresholds of 130 DU, 175 DU, and 220 DU. As we are focused on the very deep ozone holes of recent years, we included the results for the 130 DU threshold trend in Figure 1b in the manuscript. The results for the 175 DU and 220 DU onset dates are included here. All robustly show a delay in the onset date, consistent with Stone et al. (2021).\nWe also calculated the deep ozone hole breakup dates for each of the thresholds (included in the figures below). This date was identified as the day after the last time the ozone hole surpasses the threshold during the season. While the breakup dates show more variability over time, the recent years in particular have presented a clear delay though all the thresholds we considered.\nFurthermore, assessing the onset date and breakup date together on one figure emphasizes that both metrics show a similar degree of delay, or shift, during 2005-2022. Recent years have also seen some of the largest “windows” of deep ozone holes throughout the period analyzed.\nBy adding this ozone hole onset and breakup date assessment to our new Figure 1b, this helps differentiate the signs of recovery we find in September from the declines in ozone during October and November.\nWe have revised Figure 1 to the version shown below and now write in the main manuscript text:\n“The trend in the onset date (green line in Fig. 1b) agrees with previous findings of a delayed ozone hole opening date [1, 2, 16, 29, 34]. This delay is likely an indication of early-spring recovery due to EESC reduction [1, 16]. Our results for the breakup date trend (magenta line in Fig.1b) show a similar delay in the breakup of the ozone hole, albeit much nosier and insignificant. Given that the deep ozone hole breakup typically occurs during October/November, a later breakup date is in agreement with the declines in October/November TCO in Figure 1a. These results suggest that there may be new patterns emerging in the mid-spring evolution of the ozone hole, thus delaying the breakup date.”\nNew manuscript Figure 1:\n175/220 DU onset date figures (not included in manuscript):\nMajor Comment Two\nThe proposed mechanisms for October ozone decreases seems rather hastily thrown in the conclusions and unsupported. In particular, the authors suggest that the ozone reductions are “linked to an air mass descending from late August from around 1-2 hPa.” There is no observational or modeling evidence to support this claim. My request, therefore, is that the authors either 1) run modeling experiments integrating an age-of-air tracer which would capture the transport phenomenon that they propose is occurring or 2) provide evidence from the observations using observationally-based transport metrics. If they cannot provide either 1) or 2), then the manuscript loses a significant part of its novelty and it’s not clear that the results presented substantiate publication in a high-impact journal like Nature.\nResponse\nThis is a very valuable point, and we would like to thank the reviewer for raising this. We have now addressed this by adding in satellite observations of an atmospheric tracer to assess the potential dynamical drivers. Overall, the new data has sharpened the picture of the role of dynamics, and changes in vertical descent that we propose to provide an explanation to the recent trends in October ozone levels. We discuss this and the new analysis in more detail below.\nAs we wrote above as a response to the overall comment, it was our original intention to focus on the changes in Antarctic ozone regardless of whether these were caused by chlorine/bromine ozone depletion or other mechanisms. The issue of CFC-related ozone recovery is often the only focus of published studies, while the large ozone holes of recent years are still impacting UV and circulation in the Southern Hemisphere. The reviewers raise a good point, and we do agree that identifying the likely cause behind the changes we find is an important next step and should be addressed in the manuscript. As a response we have added a new figure, Figure 5, as well as a section titled, “Drivers of the changes in ozone” in the revised manuscript. The new figure is also provided below for reference.\nPanel a in Figure 5 provides observational evidence of changes to descending air mass from higher altitudes. We choose to use observational evidence, in the form of the atmospheric tracer carbon monoxide (CO) from MLS/Aura, as this links directly to the MLS data we already used in the manuscript.\nThe MLS CO shows air descending from the mesosphere into the spring polar vortex. Our new figure identifies changes in the timing and altitude of the descending CO, with signs of a shift towards later/shallower mesospheric descent in recent years. The regions where this shift in CO is statistically significant line up with regions of significant negative changes in ozone in the October middle stratosphere that we identified earlier in Figures 3 and 4. This would indicate that there are changes aloft which are of dynamical origin occurring during October.\nWe also include a new correlation result in Figure 5, panel b, which compares the carbon monoxide altitude at the end of October to OMI/Aura observations of the stratospheric NO levels during October. Previous 2 work has shown that, in addition to transport from lower latitudes, within the polar vortex NO (or NO) has 2 x an important source in the mesosphere, which is known to descend into the polar stratosphere during the springtime. Once in the stratosphere, it can both destroy ozone and impact the timing of chlorine and bromine reservoir formation. We show that the two are highly correlated, indicating a relationship between the timing and depth of the descending air within the polar vortex and the amount of NO (and likely the 2 NO family of chemicals) arriving in the October stratosphere.\nx\nFinally, we include another new correlation result in Figure 5, panel c, comparing the carbon monoxide altitude at the end of October to the high latitude middle stratosphere partial column ozone from Figure 4 in our manuscript. This PCO window covers the region of declining ozone we identified in Figure 3 of our original manuscript. The two are highly correlated, indicating a relationship between the timing and depth of the descending air within the polar vortex and ozone concentrations at the core of the ozone hole.\nTogether these new results provide evidence that the October ozone changes are linked to changes in vertical transport of airmasses. These dynamical changes are taking place alongside of volcanic eruptions and wildfires and can now help explain the extend of ozone loss observed in the recent years.\nWe propose that this mechanism should be investigated further in a follow-on study due to its complexity. We hope that the links we identify in Figure 5 and the section titled, “Drivers of the changes in ozone” are sufficient to address the possible drivers of recent declines in ozone we present.\nWe do note here that careful considerations would need to go into any model studies looking into this as many models have been known to underestimate the descent of mesospheric air inside the polar vortex, and do not routinely capture the high altitude production of NO .\nx\nNew manuscript Figure 5:"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Kessenich et al. has done an excellent job in addressing my and reviewer2's comment. A new figure (Fig.5) and new secfion has been added in the manuscript discussing the potenfial causes of the ozone decline. I found the fig5 very interesfing and seems the dynamical variability of the descending of the mesospheric air is of quite important role in polar ozone. Although the effect on the PCO/TCO has sfill not been explicitly quanfified, and I agree with the authors that probably it is not that easy to model at this moment. I am happy to recommend this manuscript for publicafion.\nbtw, Figure5 legend: “and T = years since 2005.”\nI don’t see T in Fig5.", "reviewer_2": "The reviewers have done a good job incorporafing and responding to my major comments. In parficular, I appreciate the 1) new analysis of breakup and onset dates and 2) CO evolufion which, respecfively, befter place their results in the context of previous studies and suggest a possible dynamical origin to the observed O3 trends. Both contribufions strengthen the manuscript.\nOne lingering issue, however, regards the greater impacts of these findings, specifically on whether these results qualify for a higher-impact journal. In parficular, it is clear now that the authors corroborate the findings from the Kane et al. (2021) study showing a later ozone onset date, which some readers may choose to interpret as a sign of ozone recovery (not deplefion). The quesfion then becomes why do we care about such a localized (~10-50 hPa) anomaly? I believe the authors do have an answer to this, as they explain in their rebuftal that “The recent large ozone holes have a significant impact on both UV levels and climate and dynamics of the Southern Hemisphere. We need to understand these extended periods of large ozone holes to be able to understand how well these processes are captured in regional variability as well as future model predicfions for our part of the world (the authors reside in New Zealand).” My recommendafion, therefore, is that the authors include a new paragraph in the conclusions stafing something along these lines so that the readers clearly understand why the reported results mafter.\nOnce the addifional material addressing the impact of the results is added, I am inclined to recommend publicafion (pending, of course, that the comments from the other reviewer are addressed)."}, "author_response": "We would like to thank both reviewers once again for providing their valuable comments on the revised version of our manuscript. Our responses to all reviewer comments are included below.\nReviewer #1 (Remarks to the Author):\nOverall Comment\nKessenich et al. has done an excellent job in addressing my and reviewer2's comment. A new figure (Fig.5) and new section has been added in the manuscript discussing the potential causes of the ozone decline. I found the fig5 very interesting and seems the dynamical variability of the descending of the mesospheric air is of quite important role in polar ozone. Although the effect on the PCO/TCO has still not been explicitly quantified, and I agree with the authors that probably it is not that easy to model at this moment. I am happy to recommend this manuscript for publication.\nMinor Comment One\nbtw, Figure5 legend: “and T = years since 2005.”\nI don’t see T in Fig5.\nResponse\nWe appreciate the reviewer bringing this to our attention, as the “T = years since 2005” text in Fig. 5 was included in error. We have edited the caption to remove this text.\nReviewer #2 (Remarks to the Author):\nOverall Comment\nThe reviewers have done a good job incorporating and responding to my major comments. In particular, I appreciate the 1) new analysis of breakup and onset dates and 2) CO evolution which, respectively, better place their results in the context of previous studies and suggest a possible dynamical origin to the observed O3 trends. Both contributions strengthen the manuscript.\nMajor Comment One\nOne lingering issue, however, regards the greater impacts of these findings, specifically on whether these results qualify for a higher-impact journal. In particular, it is clear now that the authors corroborate the findings from the Kane et al. (2021) study showing a later ozone onset date, which some readers may choose to interpret as a sign of ozone recovery (not depletion). The question then becomes why do we care about such a localized (~10- 50 hPa) anomaly? I believe the authors do have an answer to this, as they explain in their rebuttal that “The recent large ozone holes have a significant impact on both UV levels and climate and dynamics of the Southern Hemisphere. We need to understand these extended periods of large ozone holes to be able to understand how well these processes are captured in regional variability as well as future model predictions for our part of the world (the authors reside in New Zealand).” My recommendation, therefore, is that the authors include a new paragraph in the conclusions stating something along these lines so that the readers clearly understand why the reported results matter.\nOnce the additional material addressing the impact of the results is added, I am inclined to recommend publication (pending, of course, that the comments from the other reviewer are addressed).\nResponse\nThe reviewer raises a great point, as the issue of the impact of our findings was an important topic to address when replying to the reviewer’s comment. Following that logic, we highly agree that addressing this question within the body of our manuscript as well is of similar importance. A new paragraph addressing the impacts of our results has been added within the “Discussion” section (previously “Conclusions”, the change was due to editorial requirements). See the text below for the new Discussion section, with the added text in bold:\n“The recent deep and long-lived ozone holes have already resulted in extreme UV levels over Antarctica [45]. Beyond local UV effects, Antarctic ozone is intrinsically linked to the climate and dynamics of the Southern Hemisphere; changes in stratospheric ozone levels drive circulation changes across the entire hemisphere, impacting the Hadley cell, the subtropical dry-zones, and the Southern Annular Mode [7–9]. We need to understand the changes taking place at the core of the ozone layer in order to determine how well these now ongoing processes are captured in future model predictions.\nBy taking a holistic look at the yearly progression of the Antarctic ozone hole over the last two decades, we find that:\n1. The addition of recent years to the Antarctic (60°S–90°S) total column ozone time series results in insignificant long term change since the early 2000’s, even where significant recovery has previously been reported. During this time, we find a delay in both the deep ozone hole onset date as well as the breakup date.\n2. The early springtime upper stratosphere near the polar vortex edge shows signs of ozone recovery. However, from late September, the middle and lower stratospheric ozone poleward of 60°S declines at a rate of over −0.03 ppmv/year from 2004 to 2022, with peak values in October exceeding −0.10 ppmv/year.\n3. At the core of the ozone hole (75°S–82°S, 5–50 hPa), the October partial column shows a total reduction of 26% from 2004 to 2022. This is equivalent to approximately 30 DU in the total column.\n4. Atmospheric tracer observations show evidence of a vertical shift in mesospheric air descending into the core of the ozone hole. This is highly correlated with October stratospheric NO2 and the ozone decline identified above.\nThe link we identify between the mid-spring ozone hole evolution and the timing/depth of mesospheric air arrival into the polar vortex sheds light on the potential drivers behind the recent large ozone holes. This mechanism is of dynamical origin and could be operating independently from the volcanic and wildfire effects implicated for the heightened ODS-induced ozone loss during the early springs of recent years [1, 16, 19, 25]. More study is warranted to determine the precursors for the changes we identify in mesospheric descent and better understand the extent to which these effects will impact polar ozone in our changing climate.”"}]

Decision letter