Review of “Tropospheric ozone responses to the El Niño-Southern Oscillation (ENSO): quantification of individual processes and future projections from multiple chemical models” by Li et al.

Manuscript summary

This study investigates the response of tropospheric ozone to ENSO using a combination of satellite data, the GEOS-Chem chemical transport model, and CMIP6 chemistry-climate models (CCMs). The authors evaluate GEOS-Chem against OMI/MLS satellite observations, conduct sensitivity experiments to disentangle the roles of transport, chemistry, and biomass burning, and assess how well CMIP6 models capture the observed ozone-ENSO relationship. Finally, the study examines projections under the SSP3-7.0 scenario using selected CMIP6 models.

The key conclusions are:

• GEOS-Chem reproduces observed ozone-ENSO variability very well.
  
  
  
  
• ENSO-driven changes in transport (via the Walker Circulation) explain most of the ozone variability, though chemistry and biomass burning also contribute.
  
  
  
  
• CMIP6 models with interactive chemistry capture the ozone-ENSO response more realistically than those with prescribed chemistry.
  
  
  
  

This is an interesting and timely study that falls well within the scope of ACP. I recommend publication after the following concerns are addressed.

Major Comments

1. The manuscript would benefit from a deeper discussion of the limitations of the sensitivity experiment design. The assumption of linear additivity may not fully capture the interactions between transport, chemistry, and emissions. For example, transport changes also affect precursor distributions, which in turn influence ozone chemistry. Can the authors quantify how much of the total ozone response is not explained by the sum of the isolated processes (e.g., residuals)? This would help assess the robustness of the attribution.
2. The discussion of chemical contributions to the ozone-ENSO response is somewhat limited. It would be helpful if the authors could provide quantitative changes in lightning NOx and BVOC emissions under ENSO conditions from their simulations. Can these changes be linked to the observed or modelled ozone responses, particularly in the eastern Pacific?
3. While spatial correlation is an informative metric, the authors do not assess how well the models capture the magnitude of interannual variability in TCO. A model may simulate the correct spatial pattern but still underestimate variability. Consider including an evaluation of the standard deviation or amplitude of the TCO–ENSO relationship (e.g., variance in the regression residuals) for each model.
4. The manuscript lacks a clear explanation of how ENSO events are identified in the CMIP6 models under the SSP3-7.0 scenario. Since these models are free-running, ENSO phasing and intensity are not aligned with observations and may differ significantly between models.

Minor Comments

• The frequent use of opposing effects in parentheses (e.g., “increase (decrease)”) in the abstract and main text is hard to read. Consider rephrasing for clarity.
  
  
  
  
• The introduction would benefit from additional references, especially in lines 32, 33, 44, and 46. In particular, the discussion of BVOC and lightning NOx responses to ENSO could be expanded. Suggested references:
  
  
  
  
• https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgrd.50857
  
  
  
  
• https://bg.copernicus.org/articles/20/4391/2023/
  
  
  
  
• https://www.frontiersin.org/articles/10.3389/ffgc.2018.00012/full
  
  
  
  
• Lines 136–137 suggest that GEOS-Chem runs freely, but the model is in fact driven by nudged reanalysis meteorology. Please clarify this to avoid contradiction.
  
  
  
  
• The SST values used in the sensitivity simulations should be described more clearly.
  
  
  
  
• Line 205 – consider rephrasing to improve clarity.
  
  
  
  
• More explanation is needed on how r_TCO–Niño3.4 is calculated. Are the Niño3.4 index values spatially uniform?
  
  
  
  
• While the manuscript avoids using a p-value threshold, some discussion of statistical confidence is warranted. How confident are the authors that the reported correlations and sensitivities exceed internal variability?
  
  
  
  
• Line 274 – citation needed.
  
  
  
  
• Line 364 – “nudging” is more accurate than “imposing.”
  
  
  
  
• Line 375 – are these effects statistically significant?
  
  
  
  
• Line 399 – citation needed.
  
  
  
  
• Use the more established term Chemistry-Climate Models (CCMs) instead of “climate-chemistry models.”
  
  
  
  
• The manuscript would benefit from a brief overview of the SST and ocean components in the CMIP6 models.
  
  
  
  
• Line 374 – contains a typo.
  
  
  
  
• Lines 510–517: The explanation of future projections is unclear. How are you comparing responses under “the same SST anomalies” when SSTs are not synchronised across free-running models? Please clarify or rephrase.

General comments

Li et al. present an interesting analysis of tropospheric ozone responses to ENSO, including a quantification of the effects of transport, chemistry and biomass burning emissions on ozone in the tropical Pacific. The authors utilise the GEOS-Chem model alongside satellite observations for analysis of ‘present-day’ conditions, as well as CMIP6 models to study projections of future ENSO-ozone relationships.

The scientific questions addressed fall well within the scope of ACP and I recommend publication after the following comments, alongside concerns raised by reviewer 1, are addressed.

Specific comments

1. To make the research reproducible, more information on the methodology needs to be provided. Reviewer 1 has already raised the question of how future ENSO events are identified in the CMIP6 models. In addition, please specify what correlation and linear regression methods were used to calculate the respective coefficients.
2. The uncertainty in the OMI/MLS retrieval should be introduced in the methodology or discussed when evaluating GEOS-Chem.
3. Explain the choice of boundaries for the west and east Pacific regions in more detail (line 246-247). Were any particular thresholds used or, maybe, do these regions align with previous studies?
4. Three years of data are used for the El Niño and La Niña conditions to minimize potential impacts from other climate modes. This is not the case for the ‘normal’ conditions, which are based just on 2013. Please clarify how other climate modes might impact the ‘normal’ input data or why this is unlikely to be an issue.
5. The 1997/98 El Niño event is discussed in some detail on line 401. I think a brief introduction to the event should accompany the first mention of 1997 ozone levels on lines 53-54

Technical corrections

1. Adding an explanation to the figure captions of the boxes highlighting specific regions in some of the figures (e.g., Fig. 2 g) would provide more clarity.
2. On figure 7, from what I can see, not all the models feature on all the subplots. Please explain whether the missing model values are beyond the scales or whether a particular variable was not available. Additionally, a brief explanation of the Taylor diagram could be included in the figure caption to help the reader. For example, clarifying the axes.
3. There are missing ‘the’, ‘a’, and ‘an’ articles throughout the text, e.g. line 15: “Here, we evaluate the GEOS-Chem model…”. I assume these will be addressed during the copy-editing stages.
4. Similarly, occasionally there are mistakes associated with the plural or singular. For example, on line 59: “Mechanisms contributing to the ozone-ENSO response has been examined…”. I again assume these will be addressed during the copy-editing stages.
5. Line 47: “featured by” is unnecessary in the sentence starting “The key response is”
6. On line 182 you introduce the ensemble member for the CMIP6 models and identify a different one is used for UKESM1-0-LL. What is the potential impact of using a different ensemble member on the results? The explanation of ‘r1i1p1f1’ may provide too much detail if there are no substantial implications of using that particular ensemble members. If there are implications, please highlight them.
7. Line 186: should ‘forces’ be ‘forcings’ in this sentence?
8. Lines 190 and 192: rephrase awkward phrasing of “perform interactively tropospheric chemistry” and “perform interactively stratospheric ozone chemistry” to improve readability. For example, change to “simulate tropospheric chemistry interactively”.
9. Line 247: “showing a significant but contrasting ozone response”
10. Lines 288-290. I suggest splitting in two the sentence starting “The simulated regional mean” for better readability.
11. Line 324: Remove the unnecessary “by” in “estimated by from the corresponding sensitivity experiments” in the figure caption.
12. As you state you are not using a threshold for significance, I suggest changing the wording on line 375 from significant to substantial.
13. I suggest moving the text on limitations (Lines 591 – 597) to earlier in the Discussion and Conclusions section, so that you have a stronger ending focusing on the key results and their implications.