the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Apr 2025
Large Ozone Intrusions during Sudden Stratospheric Warmings Enhance Ozone Radiative Forcing over South Asia
Abstract. Tropospheric ozone pollution in South Asia is mainly blamed on anthropogenic emissions. However, this study highlights the contribution of stratospheric ozone intrusions into the troposphere associated with sudden stratospheric warming (SSW) events in enhancing tropospheric ozone over the South Asian region using ERA-5 reanalysis data. We report that specifically split-downward propagating SSWs (dSSWs) cause enormous ozone enhancement in the upper troposphere and lower stratosphere (UTLS) over South Asia around the dSSW-onset, with a maximum of ~290 % within ±30 days. The ozone intrusions propagate deep into the troposphere, causing near-surface maximum ozone increase by 43 % within ±30 days around the SSW-onset. The ozone enhancement increases ozone radiative forcing in the troposphere by 0.04±0.03 W.m-2 and UTLS by 0.08±0.06 W.m-2 over South Asia. Frequent SSW events in a warming climate will thus likely increase stratospheric ozone intrusions and ozone radiative forcing over South Asia, potentially exacerbating regional climate warming. The elevated tropospheric ozone amounts due to stratospheric intrusions are posing threat to humans and vegetation.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-1098', Anonymous Referee #1, 16 May 2025
Roy et al. analyze in their work ERA5 ozone data for different sudden stratospheric warmings in the northern hemisphere and connect these events with increased ozone concentrations at UTLS levels and near-surface. Finally, a brief analysis for the radiative impact is shown.
The manuscript addresses an important and interesting topic well within the scope of ACP, but I think this manuscript needs major revisions before considering publication. In particular, the main focus of this paper is not clear to me and also, it could be better structured. Another very important point to me is that the ERA5 data, which is the heart of this manuscript is not discussed if it is suitable for this kind of analysis. Finally, the year 2018 is in focus without motivation why, but then its outstanding role in the time series plots is not discussed in detail. I see room for improvement on this manuscript.
General comments:
- Some figures are rather small and of poor quality. Please increase the size, in particular the font size of axes, labels, etc. Some axes or color bars are even not labeled at all: please add labels for all axes and color bars!
- Many figures were produced using the COLA/GrADS software, but I did not find any information or reference to this tool. However, it is good that the authors mention how their figures are created.
- The analyses of this work are based on ERA5 ozone data. In order to estimate the findings of this paper, I need some background information about the reliability of this ERA5 ozone data set. What is the base for the ERA5 ozone? Are there measurements assimilated? Are there validation papers? In particular: How is the UTLS represented in ERA5 in comparison to independent measurements? How does that change over the long time range analyzed in this work?
- Does ERA5 only show stratospheric ozone, which is transported to the UTLS, or are contributions from ground sources (anthropogenic, biomass burning,...) also included? If the tropospheric sources are included, how can these be separated from the stratospheric sources?
- In the description of the materials and methods, already results (e.g. from Fig. 1) are discussed, but the Figure is reintroduced again in Section 3.1. This makes the structure inconsistent and should be reconsidered.
- The event in 2018 is discussed in detail in this manuscript. For that reason, I am missing some background information about this SSW. How was the Arctic winter 2017/18 in general prior to the SSW? Are there articles published dealing with this specific winter, or even this specific SSW? Further, Fig. 2c looks like that this 2018 event was an extreme outlier. Are there explanations for that outstanding behavior in the UTLS? Also, some motivation, why 2018 was chosen would be helpful.
- What is the focus of this paper with respect to ozone? Showing the contribution of stratospheric ozone to UTLS ozone and highlight its role for the radiative imapact, or showing the impact on near-surface ozone, which is relevant for human health. Both aspects are mentioned here and there (near-surface more in the introduction, UTLS more in the conclusions), but I am missing the focus.
- In the big picture, I am missing a comparison to different regions of similar latitudes than India: Are the stratosphere-troposphere exchange processes after SSWs similar here, or is the Indian region outstanding? If so, why?Specific points:
L72: I would not call "wildfires" or "sea ice melt" tropospheric weather phenomena, but rather consequences of these
L83: The word "displaced" is used to explain the "displaced case", which is a circular explanation.
L84: Is the term "baby vortices" really the scientific term, or rather slang?
L124: I think this is a typo: Typically ERA5 has 137 pressure levels, not just 37
L143: It seems like the software has also an acronym (at least some characters in the name are capitalized). Please also give the common acronym.
L250: "Figure 2c-d shows clear evidence of a substantial ozone increase...": I do not agree with this statement. I think the authors cannot write about "clear evidence of a substantial ozone increase" in general. Actually, I think that only at 850 hPa one can see an increase over time. At 150 hPa, I would argue that one can see a decrease over time with an extreme outlier for 2018. Further, I think that this outlier should be discussed in detail. Further, the language should be more specific here and it should be mentioned that the discussed ozone increase is an increase over time over several years.
Figure 4: Why is 200 hPa chosen as a UTLS level, while in Fig.2, 150 hPa have been shown? Are there good reasons for that, or would it be possible to have the selection of pressure layers shown here to be consistent?
L322: "Our analysis shows strong vertical coherence between 10 and 200 hPa levels": How can one observe this "strong vertical coherence" by comparing these plots? Please elaborate.
L324: Please explain "wave-1" and "wave-2".
Figure 4 nicely showed the region of interest with a box. I suggest to repeat that box for the panels of Figure 5. Further, the orientation of the maps are different between these figures. Also in panels a-b, it would be good to mark the boundaries of this box, since the text is mentioning that region.
Section 3.3: These radiative forcings highlight again the outstanding role of the 2018 event, which is in focus of this paper, but this outstanding role is not sufficiently commented here.
Figure 6: I am not sure about the relevance of this figure, since it basically shows 6 numbers, which are already given in the text.
Line 414: I think the 290% given here are linked to the outlying event in 2018 and should not be given here as a general trend.
L439: I do not understand the final sentence: please rephrase.
L444: The given link is not accessible to me.
Figure S1: Why is this figure given? It shows very, very similar results as Figure 1, just that the climatology is not subtracted from the values. Why do the authors want to show this figure in addition?
Caption Figure S1: Panel (b) is specifically mentioned, while panel (a) is not. I guess this was just missed to mention.Citation: https://doi.org/10.5194/egusphere-2025-1098-RC1 -
RC2: 'Comment on egusphere-2025-1098', Anonymous Referee #2, 18 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1098/egusphere-2025-1098-RC2-supplement.pdf
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RC3: 'Comment on egusphere-2025-1098', Anonymous Referee #3, 21 Jul 2025
The study addresses sudden stratospheric warmings, especially split-downward propagating ones, and their impact on ozone increment over South Asia from the upper troposphere-lower stratosphere to the near-surface. Using the ERA 5 dataset from 1962 to 2018, the authors classified SSW events into three groups: split-dSSW, displace-dSSW, and nSSW. The paper showed a significant increase in ozone over the UTLS and near-surface around the SSW onset. The paper further focuses on the 2018 SSW event among 12 split-dSSWs to understand the dynamics, and concluded that Rossby wave breaking intensification relevant to SSW is the cause of the ozone increment. The authors additionally calculated the radiative forcing of such ozone increment.
Roy et al. covers relevant scientific questions within the scope of ACP, but needs major revision. To be specific, the dynamic linkage with SSW and ozone increment is weak and needs further explanation. Also, the significance test and the year 2018 as a representative case are not convincing enough. Lastly, figures and writing could be improved.
Major comments
- How can ozone response prior to and close to the SSW event date be caused by SSW? It requires some time for the response to propagate to the UTLS midlatitudes and low latitudes.
- The novelty and motivation of the study should be emphasized more. Although not focused on South Asia, previous studies like William et al. (2024) and Lee et al. (2025) have already calculated the radiative impact and global ozone anomaly due to SSW. I think the authors showed an interesting result that only split-dSSW has a substantial impact, while other types of SSW do not. I recommend emphasizing this more and providing additional explanations for why.
- When introducing the previous studies and comparing, please match the units and consider the time scale. For example, % and ppb units are hard to compare and confuse their importance. Also, stratospheric intrusion (hourly to daily) should be carefully compared to the stratosphere-troposphere exchange (often in seasonal, interannual, and even climatology) or trend due to anthropogenic emissions.
- Is the Student t-test appropriate in this sample size? I do not understand how a significance test is being conducted on a single case (year 2018). Also, 12 cases of split-dSSWs are pretty small. I would recommend adding more detail on how the significance test is done and improving the statistical robustness of the result.
- Can the year 2018 represent the split-dSSWs? Figures 2 and 3 compare the year 2018 with the other split-dSSWs. However, the year 2018 seems to be anomalously strong, and the patterns look quite different. If there is a reason to choose the year 2018, please explain it. Also, as stated in the paper, due to anthropogenic emissions and global warming, tropospheric ozone has a positive tendency. If this is not detrended, could there be a bias in the impact of SSW in the year 2018?
- Authors emphasized the impact on the near-surface ozone level from SSW, but detailed explanations are missing. How can the signal in the UTLS propagate toward the near-surface? The propagation of geopotential height in the figure is difficult to recognize. Also, tropospheric ozone has multiple sources, such as local emissions and tropospheric long-range transport. What is the contribution of each driver to the ozone increment, and is the stratospheric intrusion truly the dominant driver?
- It seems the Rossby wave breaking is the suggested mechanism for the ozone intrusion. However, it is hard to see that RWB has been amplified or more frequent due to the split-dSSWs. It would be helpful to explicitly show that the frequency or amplitude of the RWB, whichever matters, differs by type of SSW. The current qualitative discussion with Fig. 5 is not convincing enough.
- Is the radiative forcing due to SSW significant over this region? If so, further information to compare and show the importance could be helpful. Also, what is the temperature change due to this radiative forcing?
Lee, J., Butler, A. H., Albers, J. R., Wu, Y., & Lee, S. H. (2025). Impact of sudden stratospheric warmings on the stratosphere‐to‐troposphere transport of ozone. Geophysical Research Letters, 52(2), e2024GL112588.
Minor comments
L47: What does this ‘increase’ mean? A trend or variability?
L48-50: Add reference to show the contribution of the stratosphere to the South Asian ozone
L98: Isn’t Lu et al. (2023) only focused on the stratospheric ozone variance?
L99-100: Willims et al. (2024) only focused on the PJO type of SSW when leading to that conclusion. Please clearly state it.
L111: Maybe split-dSSWs? Not all the downward-propagating SSWs?
L127-129: What does the corresponding daily mean of all the non-SSW days mean? Is every day except SSW onset days considered?
L152: UTLS definition should be included.
L159-163: These two sentences seem to be repeated.
L193-195: Hall et al. (2021) used ERA-Interim and ERA 40 for the classification. Is ERA5 consistent with these two datasets?
L210-212: Why is ± 30 days selected when the previous sentence mentioned up to 60 days after SSW onset?
L220: I think this is a typo. Maybe section 3.2, not 2.2?
L237-239: It is hard to see the downward propagation of GPH in the figure
L239-241: I can't see the lowering of the 380K. Also, why is the 380K isoline being used here?
L252-255: What leads to the two different peaks in 2018?
L282-283: I don’t think this has been discussed later in this section
L283-286: Where is the anomalous lowering of the tropopause? Figure 3 doesn’t show the anomalous tropopause
L323-326: It is hard to see the wave-1 and wave-2 pattern in Fig. 4. Also, please explain the connection with vertically coherent waves at 10 hPa and 200 hPa to the subtropical jet perturbation. Currently, there is no explanation why the first sentence leads to the conclusion in the next one.
L338-340: Which pattern is the persistent GPH anomaly over South Asia? A more detailed description would be helpful.
Figure 4: It is hard to follow the explanation without the latitude grid line
L368-369: Please explain in more detail about the synoptic wave pattern that only exists in Fig. 5c. It is hard to see in this figure. Also, the figure is rotated from Fig. 4, which is confusing.
L370-371: What does it mean to enhance RWB? Is it amplitude-wise or frequency-wise?
L432-435: A reference is needed for the future projection
L435-436: A reference is needed for the high-top model performance expectation
Citation: https://doi.org/10.5194/egusphere-2025-1098-RC3
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