Signs of Slowing Recovery of Antarctic Ozone Hole in Recent Late Winter–Early Spring Seasons (2020–2023)

Round 1

Reviewer 1 Report

My comments are given in doc file.

Comments for author File: Comments.pdf

My comments are given in doc file.

Author Response

Response to Reviewer 1: (the authors’ answers are in italic)

“The paper deals with a quite interesting problem of slowing recovery of Antarctic ozone during the last years. The paper is well written and structured adequately. The results seems to be quite interesting and urgent. I would recommend to publish it after making several corrections.

The most important comment concerns the conclusions, which should include the specific results of the analysis in a more clear way. They should be formulated and added in the Section 4.”

Section 4 was rewritten. Redundant part of the section (not directly linked with the presented results) were removed. The results of the analysis are shown in clearer way. For example:

“A decrease in stratospheric temperature over the SH polar cap (Figure 2) and more water vapor resulted in larger volumes of PSC NAT and PSC ICE (Figure 1b), leading to an increase in the area of the hole and greater ozone loss within the hole between 2020 and 2023. If Australian wildfires and the Hunga Tonga eruption had not appeared, the ozone holes in recent years would likely be smaller.  (l. 315-319)

Minor comments:

row 16. “Statistically significant trends in the hole metrics, which were revealed in the short period (2000- 2019), disappeared in the longer period (2000-2023). The multiple regression of the metrics values on chemical and dynamical proxies shows stabilization of the metrics values since the early 2010s.” Hard to read, please, edit

In the revised manuscript, we have:

“Statistically significant trends in the hole metrics were revealed in the short period (2000-2019), but not in the longer period (2000-2023). The modelled time series of metrics from multiple regressions with standard chemical and dynamic explanatory variables indicate that the recovery has slowed down since around 2010.” (l.16-19).

row 56. “As many ODS have a long residence time in the stratosphere, even the current full reduction in their production will result in the persistence of the hole for many decades to come. “Better to begin with “Since”.

We follow this recommendation and start with “Since”. (l.53).

row 63. “This means that by this year almost 27.2% of the EESC has been removed from the Antarctic atmosphere since the EESC maximum” I would recommend to add the year ”the EESC maximum in 2001.

We add the year (l.64).

row 67-89. I would recommend to re-write this part of the text emphasizing the aim of the paper. The details of ozone changes during the last years could be described before setting the aim.

We follow this recommendation. The aim of the paper appeared in the last two paragraphs of Introduction, i.e., after details of ozone changes.

row 98. This is a repetition of the metrics shown in the Introduction.

In the revised manuscript, in the Introduction, we only briefly mention the metrics used, and more details about them can be found in section 2.1 Data.

Table 1 should include the sources of all datasets similar to the first two datasets.

As there was no space for sources in Table 1, they are listed in the main text (l.105-112)

“Polar_Cup_SATELIT is not good abbreviation, written with the errors in English ( word Satellite. I would recommend to use the following abbreviation: “Polar_Cup_Sat”.

We follow this recommendation and use ““Polar_Cup_Sat”.

row 105. The authors used the two datasets on total ozone content. It is necessary to discuss, why they decided to apply both of them. In addition, the gap in satellite ozone data in 1990s should be also mentioned and its influence on the results should be discussed in the text.

We add that “ There were no satellite observations in 1993 and 1995. Therefore, a comparison between statistical analyses of Polar_Cup_MERRA and Polar_Cup_SAT will allow a discussion whether the lack of observations affected the pattern of long-term changes in the latter metric.” (l.112-115).

and “Similar long-term patterns were found using original MERRA-2 and satellite data (blue curves in Figure 3a and Figure 3b). (l.206-207).

row 113. “NO3W: decoding of this abbreviation is needed here”.

Decoding appeared in previous paragraph (l. 111).

row 131. “These, will be used in Section 3 to discuss sources of the metrics variability in the period 1980-2023”. The latter or both of them? In this case it is better to write “These characteristics”.

According the reviewer’s recommendation, we use “These characteristics”. (l.135)

Table 2 and Table 3 should be divided in the text. Include one two sentences between them.

We add a few lines between these tables. (139-142).

row 150. “potential independent explaining variables, however, but their long-term variability is important for understanding the ozone trends and it will be discussed further in the text (Section 3). I recommend to clarify the text using the proposed variant.

We use the proposed variant (l.155-157).

“1 EESC” ? I do not understand. Does it mean a1=EESC?

It was misunderstanding a₁ EESC was a part of Equation (1) denoting chemical forcing (l.170).

Figures should appear after their mentioning in the text.

Figures have been moved to Section 3 and appeared after mentioning in the text.

Smoothing procedure should be described.

We add “Lowess filter is used for smoothing the time series [26]”. (l.175)

Row 208. ”The probability of 4 years in a row with a hole of >23 million km2 is 1/16=0.0625…”. I do not understand. How did you obtain “16”? Please, clarify. And, in addition, it is not clear, why this is important for the analysis? It is also should be clarified.

We add the explanation: “ the probability of 4 years in a row with a hole of > 23 million km² is (0.5)⁴= 0.0625, i.e., close to the 0.05 threshold commonly used in statistical significance calculations. So the appearance of this sequence by chance is unlikely, indicating unusual behavior of the ozone hole over the past four years.” (l. 214-217).

Table 4. – The title of the last column should be “Trend coefficient dimension”.

We use ““Trend coefficient dimension” in Table 4 (first line in Table just after l.240).

Row 220. Table 4. Please use the probability level instead of sigma.

We explain in added text:

“*  and ** denote statistically significant result at the 1 σ and 2 σ level, i.e., corresponding to the significance level of about 68% and 95% respectively, assuming a normal distribution.” (l.241-242).

row 225.” Adding data from another additional 2019-2023 four-year period”. Please edit following my recommendation or make additional editing.

We follow the reviewer recommendation (l. 222).

Table 5: The authors should use the standard math for writing the equations in the Table.

In the revised paper, standard math for writing the equations was used

row 237. Instead of “site should be “side”.

We change “site” to “left-hand side” (l.244).

row 239 Instead of “summed” better to use “total”.

In the revised text, “total” replaced “summed”. (l. 245)

row 250. This part should be rewritten. I see quite nice reproduction of observations by the regression model shown in Fig.3, while the authors wrote that this is not so (“cannot provide a second overturning”). Please, make this part more clear. Fig.5 is okay.

We add that the second overturning was found in Figure 3a.

“However, the model is able to reproduce the second overturning in the SH polar cup TCO₃ from the satellite data (Figure 3a)’’. (l.266-267)

row 263. The whole paragraph is too vague. The discussion should be focused on the obtained results and on possible comparisons with the results from other papers.

Section 4 has been rewritten according to the reviewer’s comments (see also the explanation to the general comment at the beginning of the response to the reviewer's comments).

row 291. “such a large hole”.

“a” was added (l. 278)

row 303. “Modelled time series of the metrics based on the multiple regression of the metrics…”

The sentence should be edited.

This sentence has been replaced by:

“The time series of metrics (listed in Table 1), which were derived from the multiple linear model (1) using various chemical and dynamic explaining variables as given in Table 5, showed that the recovery of SH polar ozone started around 2000 due to decreasing ODS concentrations in the lower stratosphere, but stopped around 2010 (Figure 3 and Figure 5).” (l.290-294).

Row 342 “for a while”. This therefore makes it difficult”. Please, edit.

New version is as follows:

“… The long-term patterns of all analyzed metrics of the SH polar ozone show another overturning around 2010 after the first one in 2001 related to the beginning of ODS removal from the stratosphere due to MP and its subsequent amendments. Particular atmospheric dynamics in recent years and extreme events such as the fires in Australia and the eruption of the Hunga-Tonga volcano mask the recovery of the hole for a while. Any possible estimate of the timing of full recovery of Antarctic ozone based on the predicted steady removal of ODS from the stratosphere, which began in the early 2000s, should be treated with caution, as the analyses presented here do not confirm the continuation of recovery trends in the commonly used SH polar ozone metrics since about 2010.” (l. 326-337).

Reviewer 2 Report

The manuscript deals with the analysis of different ozone physical quantities time series obtained from satellite borne observations from the beginning of ‘80 to almost nowadays for a limited period of the Antarctic springs (September). The choice of the analyzed timeframe is due to the strongest manifestation of the ozone hole in that season. In addition, the modelling of the same datasets are performed aiming to demonstrate the slowdown of the ozone recovery. The paper is interesting and well written; the introduction section give a clear view of the problem and it clarifies the different metrics used to assess the status of the ozone hole

When I read for the first time the work, I was very excited to find the sentences from line 79 to 84 related to the Hunga-Tonga volcano eruption and the Australian wildfires. I was thinking that along the paper some kind of correlation of the ozone behavior in the last 4 years with the above-mentioned events, would be presented. Unfortunately, I was disillusioned. The article only reports in the last paragraph the citations to the works of Evan, Carl and Millan related to the volcanic eruption.

Anyway, apart that the reading of the paper is fluently, the analysis is very consistent, and the results are clearly presented. I recommend the publication of the paper after only minor changes listed in the following:

Line 115 – please check ‘7 September 7’

Table 3 – replace ‘web link’ with ‘Data source link’

Table 3 – invert QBO50hPa with QBO30hPa

Table 3 – Remove ‘Data taken from the page’

Line 237 – please replace ‘site’ with ‘side’

Table 5 - first line - regression model - please check the formula

Author Response

Author Response File: Author Response.pdf