the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Feb 2025
Exploring Ozone-climate Interactions in Idealized CMIP6 DECK Experiments
Abstract. Under climate change driven by increased carbon dioxide (CO2) concentrations, stratospheric ozone will respond to temperature and circulation changes, and lead to chemistry-climate feedback by modulating large-scale atmospheric circulation and Earth's energy budget. However, there is a significant model uncertainty since many processes are involved and few models have a detailed chemistry scheme. This work employs the latest data from Coupled Model Intercomparison Project Phase 6 (CMIP6), to investigate the ozone response to increased CO2. We find that in most models, ozone increases in the upper stratosphere (US) and extratropical lower stratosphere (LS), and decreases in the tropical LS, thus the total column ozone (TCO) response is small in the tropics. The ozone response is mainly driven by the slower chemical destruction cycles in the US and enhanced upwelling in the LS, with a highly model-dependent Arctic ozone response to polar vortex strength changes. We then explore the feedback exerted by ozone on climate, by combining offline calculations and comparisons between models with ("chem") and without ("no-chem") interactive chemistry. We find that the stratospheric temperature response is substantial, with a global negative radiative forcing by up to -0.2 W m-2. We find that chem models consistently simulate less tropospheric warming and strong weakening of the polar stratospheric vortex, which results in a larger increase of sudden stratospheric warming (SSW) frequency than in most no-chem models. Our findings show that ozone-climate feedback is essential for the climate system and should be considered in the development of Earth System Models.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. Besides this, we have no other competing interests to declare.
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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-340', Anonymous Referee #1, 03 Mar 2025
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Summary: The authors explore trends in interactively simulated ozone in 4xCO2 and 1pctCO2 simulations contributed to CMIP6. Models used in this part of the work simulate ozone interactively, either using a comprehensive chemistry scheme or some simplified approach. They find salient patterns of ozone change that are largely consistent (with limited inter-model variations), also consistent with prior work in this space, and also consistent between the two different experiments (that differ in which idealized CO2 increases are assumed). The authors find different regimes, with fast chemistry dominating the response in the upper stratosphere, dynamical feedbacks and transport playing a leading role in the lower stratosphere, and only small impacts on ozone in the troposphere. They use an offline radiative transfer model to work out how the changes in ozone alone would have contributed to the radiative forcing associated with these experiments, finding sizeable contributions. Finally they use pairs of models hat mainly differ in that one member of the pair has interactive ozone, the other does not. They find interesting differences in temperature trends in the stratosphere, zonal wind changes, and surface warming.
I don’t have any major criticism to make of the paper. To my understanding the 4xCO2 and 1pctCO2 experiments of CMIP6 have not previously been analyzed w.r.t. ozone changes and climate-ozone interactions. The authors conclude, and I agree, that ozone-climate feedbacks are important enough to be included in future Earth System models. The list of possible CMIP6 pairings of chemistry and no-chemistry models is incomplete: EC-Earth3 / EC-Earth3-AerChem could be added. My anticipation is that it would be worth adding this pair to the analysis.
Furthermore, the authors state that there are differences other than the treatment of chemistry between these pairs. That is true for half the pairs but not the other. Perhaps something more profound can be said about how these other differences (resolution of middle / upper atmosphere, height of the model top, and tuning of the non-orographic gravity wave drag scheme, that characterize the CESM2 and GFDL pairs) affect model behaviour. To my understanding there are no substantial differences in anything other than chemistry between the HadGEM3/UKESM1, SOCOL4/MPIESM, and GISS pairs.
It is clear to me that most of the large role of climate-ozone interactions is due to the fact that in no-chemistry models the prescribed ozone field is not changing with the changing state of the atmosphere in the experiments considered here, unlike e.g. in “historical” simulations where ozone is amongst the external-forcing fields varying with time. Maybe this can be discussed, and whether the results of this study could motivate changes to the experiment definitions of 4xCO2 and 1pctCO2, where for no-chemistry models ozone could be made to change consistently with the evolving CO2 forcing, much like in “historical” simulations in future iterations of CMIP.
The language in this publication is generally adequate, the number and level of detail in the graphics too, so I recommend publication subject to addressing my minor comments.
Minor comments:
Table 2: As noted, the EC-Earth3 /EC-Earth3-AerChem pair can be added here.
Figure 1: Similar patterns of change were found by Morgenstern et al., ACP, 2018 (their figure 10), using CCMI1 models. They also documented similar inter-model differences to those seen here. However the mechanism discussed in the text (NOx production changes under climate change) may not have been represented in the older CCMI models, hence the pronounced increases in tropical-tropospheric ozone were not simulated. Perhaps this is worth a mention.
Figure 2: I find this figure hard to parse. A suggestion might be to calculate dO3/dT as a function of latitude and pressure for the various models and display that. Where these two quantities do not highly correlate, this could be made NaN. Might that be a more intuitive way of displaying this information?
Figure 3: Indeed the relatively weak dependence of ozone on temperature is because of the low abundance of halogens in a PI world. There is no way the dots can be visually attributed to a particular model (not in my print-out, at least). Perhaps again a different way of displaying this can be considered?
Figure 5: This figure is also similar to Morgenstern et al., ACP, 2018, their figure 11, showing essentially the same: Unambiguous increases in TCO in the northern extratropics,model-dependent signs of the tropical TCO trends due to cancellations, and a large spread of the ozone change over Antarctica.
Citation: https://doi.org/10.5194/egusphere-2025-340-RC1 -
RC2: 'Comment on egusphere-2025-340', Anonymous Referee #2, 05 Mar 2025
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The manuscript provides a comprehensive investigation of ozone-climate interactions. Using CMIP6 models, the authors assessed the response of ozone to increasing carbon dioxide concentrations and explores the associated climate feedback mechanisms. It reveals different feedback mechanisms of ozone to increased CO2 at different altitudes (in the upper stratosphere and lower stratosphere). To explore ozone feedbacks to climate, this study compares models with (“chem”) and without (“no-chem”) interactive chemical effects. The authors conclude that ozone feedbacks with chem under increased CO2 lead to negative global radiative forcing. They also highlight that chem models lead to a significant increase in the frequency of sudden stratospheric warming (SSW) events. Distinguishing between chem and no-chem models provide valuable insights into the role of ozone-climate feedbacks.
Overall, the paper is well written and the results contribute to a better understanding of how ozone produces feedbacks to the climate as CO2 increases, with a detailed comparison between the effects of chem and no-chem models. However, I have some concerns with the results of the analyses and believe that significant revisions are necessary.
The authors pointed out that Arctic ozone increase when the Arctic stratospheric vortex weakens. The study uses multiple CMIP6 models to analyze the relationship between ozone and the polar vortex; however, the models differ significantly in their simulations of polar vortex strength, suggesting that there may be uncertainty in key processes within the models. The authors suggest that this relationship is stronger in winter but weaker in spring on the interannual timescale. But from the perspective of seasonal variation, a weakening of the barrier in early spring may lead to enhanced transport, so why is the response weaker in this period? Actually, the breakup of polar vortex associated with final warming during early spring is also closely related to the transport barrier effect. The authors shall investigate the connection of breakup time of polar vortex in early spring to ozone changes, instead of using March-April-May mean, which may mask this relationship. In a short, I think the sentence of ‘the transport barrier role of the polar vortex is generally weaker in spring than in winter’ is not appropriate. In addition, the Antarctic polar vortex is stronger and more stable than the Arctic polar vortex, why is there no discussion of how changes in the Antarctic polar vortex respond to ozone feedbacks?
Minor:
Line2: ‘…, and lead to’ -> ‘…, leading to’
Line6: ‘This work employs the latest data from Coupled Model Intercomparison Project Phase 6 (CMIP6), …’ The comma after “CMIP6” is unnecessary.
Line10: ‘We then explore the feedback exerted by ozone on climate’. This expression can be simplified as ‘We then explore the ozone-climate feedback’
Lines11-12 ‘We find that the stratospheric temperature response is substantial, with a global negative radiative forcing by up to −0.2 W m−2.’ The radiative forcing responses of the different models have large variations, and in the text analysis shows that the largest radiative forcing is −0.19 W m−2 and is derived from the UKESM1-0-LL that does not perform well in any of the other feedback processes (including ozone response to 4×CO2, ozone response to temperature change and SSW frequency change due to 4×CO2), and I think that a clear range of global mean net radiative forcing should be included in the abstract.
Line152: ‘against’ -> ‘with’
Line158: ‘year 135 to 145’ -> ‘years 135 to 145’
Line172: It should be 200-240 nm in this reference.
Line 209: Figure 3 only reflects the correlation between ozone and zonal wind. How did you know that the polar vortex is weakening from Figure 3? Is it through the average zonal wind of each model?
Line223: in most locations -> in most regions
Line243: Decoupling -> Decomposing
Line259: ‘during the last 80 years’ perhaps it could be changed to ‘over the subsequent 80 years’
Line354: Do you mean stratospheric ozone depletion or stratospheric ozone recovery?
There are different behaviors of the polar vortex and jet stream under these two scenarios. Please clarify it.
Line410: Expanded AMOC as “Atlantic Meridional Overturning Circulation” when first introduced in the sentence, then used the abbreviations consistently.
Although the authors mention statistical significance tests (e.g., t-tests), there is limited information on the exact methods used. It would be useful to provide more details about the statistical.
The manuscript provides a detailed assessment of the long-term (150-year) ozone response to increased CO2. Meantime, the authors mention that ozone changes in the early stages of CO2 increase are characterized by rapid adjustment. Does this fast-adjusting response exhibit nonlinearities or threshold points? Could this threshold point depends on whether the chem or no-chem model? Is there some consistency of threshold in the chem/no-chem models?
Figures:
The different colors in Fig.2 are hardly to see. Please redraw it.
Citation: https://doi.org/10.5194/egusphere-2025-340-RC2
Data sets
Data and script for research paper: Exploring Ozone-climate Interactions in Idealized CMIP6 DECK Experiments Jingyu Wang, Gabriel Chiodo, Timofei Sukhodolov, Blanca Ayarzagüena, William T. Ball, Mohamadou Diallo, Birgit Hassler, James Keeble, Peer Nowack, Clara Orbe, and Sandro Vattioni https://doi.org/10.5281/zenodo.14545386
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