the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 May 2026
Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)
Abstract. We describe the protocol for coordinated experiments in phase-2 of the Quasi-Biennial Oscillation initiative (QBOi) and the participating models. The experiments involve nudging tropical stratospheric zonal-mean zonal winds in the models toward observations, enabling analysis of the impact of QBO biases on simulated teleconnections. Additionally, nudging the tropical winds allows investigation of the origins of QBO biases by examining the behaviour of resolved and parameterized equatorial waves under realistic equatorial wind shear conditions. We document here the scientific rationale and design of the nudging experiments, summarize the QBOi data request, and provide an overview of participating models. An initial evaluation is given of tropical stratospheric winds simulated by the multi-model ensemble for both nudged and free-running cases, and the overall impact of nudging on climatological aspects of the atmospheric circulation is examined.
Competing interests: JA serves as co-editor for the special issue to which this paper belongs.
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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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-1165', Anonymous Referee #1, 30 May 2026
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AC2: 'Reply on RC1', J.A. Anstey, 02 Nov 2026
review of "Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)" by Anstey et al
This study introduces the protocol for phase 2 of the QBOi project, with the main addition being two nudged experiments. In Exp1-ObsQBO, the tropical stratospheric zonal winds are nudged to observed reanalysis wind, and in Exp1-NoQBO they are nudged to the climatological wind over the 1979-2020 period. The paper also introduces key aspects of the participating models, and also describes the data request. The paper also demonstrates that the models respond to nudging as expected.
I have no major comments - this study is very appropriate for GMD - and I look forward to reading the subsequent papers that report results. To be explicit, I don't think the authors should be asked to include any additional results in the current paper. It should be promptly accepted subject to the technical edits described below.
The authors thank the reviewer for kindly reviewing the manuscript and recommending prompt publication.
technical edits:
line 59-62 this sentence could be reworded for clarity
We have rephrased this sentence as two sentences, which now reads (lines 60-63 in the revised manuscript):
"A brief description of the models is provided in Section 4.1. To demonstrate that the nudging protocol is effective in the models, Section 4.2 confirms that in the tropical stratosphere the expected QBO-bias correction of the zonal-mean zonal winds is achieved while in the extra-tropics the nudging has no significant direct or undesired impact on the model's climate (background state)."
line 87 "seasonal" -> "subseasonal"
Corrected.
caption of figure 1: give- > given
Corrected.
line 227 "Sensitivity of QBO simulations to vertical resolution is not well quantified," is confusing. Do you mean "mechanisms are poorly understood"?
We mean that it is difficult to quantify how much vertical resolution should be needed to accurately simulate the QBO, as this likely depends on modelling choices (numerical method, diffusion, etc). We have changed this part of the sentence to read (line 232):
"Sensitivity of QBO simulations to vertical resolution is model-dependent and not well quantified, …"
line 245, 502 Garfinkel et al 2025 seems appropriate in these two locations. Consider citing it.
We agree this reference is relevant for its results on the QBO’s response to the solar cycle and prescribed ozone forcing and have added it in both places in the revised manuscript (lines 250 & 516, respectively).
line 275: therefore **are** not
We think the original sentence is grammatically correct, since the “are” already occurs earlier in the sentence (“...are similar to…”).
line 302 nudged -> nudge
Corrected.
line 355 "responding" is superfluous
We have deleted “the response of” so that this part of the sentence now reads (lines 366-368):
"…, and may result from changes in the QBO-induced mean-meridional circulation as well as equatorward-propagating planetary waves responding to the effects of the nudging, …"
line 368 "and" is missing between Exp1-NoQBO and Exp1-ObsQBO
Corrected.
line 375 **the** simulated
Corrected.
line 396 consider a comma after "winds"
We have added a comma after “winds”.
line 465 GCMs
We have changed this to “AGCMs” (lower-case “s”). “AGCM” is consistent with the name convention used in the Introduction.
line 506 superfluous "is"
Corrected.
section A2: it might be worth stating explicitly that the profile for Exp1-noQBO is the climatological mean over 1979-2020 to match the period for obsQBO, though this is probably obvious. For SNAPSI, the specific wind and temperature profiles to be relaxed towards were provided to model developers. For SNAPSI this was done because the initial spinup in the first few days after initialization needed to be handled somewhat carefully to avoid shocks for the runs nudged to climatology due to the fast nudging timescale. Presumably this is less of a problem for the QBOi runs.
It is noted in Section 2 that the climatology used for nudging the NoQBO is the climatology of the nudging used in ObsQBO, but for additional clarity we have now stated this in the second sentence of Section A2 (lines 450-451).
Garfinkel, C. I., Avisar, D., Osprey, S., & Smith, D. (2025). The response of the QBO to external forcings: implications for disruption events. Journal of Geophysical Research: Atmospheres, 130(22), e2025JD044438.
Citation: https://doi.org/10.5194/egusphere-2026-1165-AC2
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AC2: 'Reply on RC1', J.A. Anstey, 02 Nov 2026
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CEC1: 'Comment on egusphere-2026-1165 - No compliance with the policy of the journal', Juan Antonio Añel, 20 Jun 2026
Dear authors,
Unfortunately, after checking your manuscript, it has come to our attention that it does not comply with our "Code and Data Policy".
https://www.geoscientific-model-development.net/policies/code_and_data_policy.html
First, in your manuscript you present some preliminary results obtained with the models contributing to the QBOi. As you present results, you must publish the code of the models involved and that you have used to obtain them in a repository acceptable according to the policy of the journal. Also, we would expect that you provide in a repository the data for the variables plotted in the manuscript.
Please, therefore, publish your code and data in one of the appropriate repositories and reply to this comment with the relevant information (link and a permanent identifier for it (e.g. DOI)) as soon as possible. We cannot have manuscripts under discussion that do not comply with our policy.
Later, if the Topical Editor decides to continue with the review or publication process of your manuscript and you are requested to upload a new version of it, then The 'Code and Data Availability’ section of your manuscript must also be modified to cite the new repository locations, and corresponding references added to the bibliography.
I must note that if you do not fix this problem, we cannot continue with the peer-review process or accept your manuscript for publication in GMD.
Juan A. Añel
Geosci. Model Dev. Executive EditorCitation: https://doi.org/10.5194/egusphere-2026-1165-CEC1 -
AC1: 'Reply on CEC1', J.A. Anstey, 30 Jun 2026
Dear Editor,
Thank you for your helpful comments. We have updated the data availability statement accordingly and will include the updates at the revision stage.
Our paper references the original model documentation papers for full details of the participating models, including information on source code availability if applicable. However please note that models participating in QBOi generally do not have public code repositories due to their institutes’ licensing restrictions. Our approach here is similar to that of the recent multi-model experiment protocol description paper in GMD, Zhu et al. 2025 (https://doi.org/10.5194/gmd-18-5487-2025), who like us reference model description papers for further information on the participating models. Also this manuscript does not specifically report any further (new) developments to the models we use.
The manuscript doesn’t mention “preliminary results” but presents results for variables plotted using data from the QBOi multi-model archive hosted at the JASMIN supercomputing infrastructure (https://jasmin.ac.uk) which is part of the UK’s Centre for Environmental Data Analysis (CEDA). Our Data Availability statement already includes the Zenodo record for this data (https://doi.org/10.5281/zenodo.19059510) and describes how to request access to the archive. The HTHH-MOC project described by Zhu et al. (2025) similarly used a JASMIN group workspace to host their multi-model data archive.
ERA5 monthly-mean data was retrieved from the Copernicus Climate Change Service, https://doi.org/10.24381/cds.6860a573. This is the DOI of the Hersbach et al. 2023 data citation that was included in our Data Availability section.
The input4MIPs boundary condition data in Appendix B was described as being available from the input4MIPs project as the URL https://esgf-node.llnl.gov/search/input4mips/. We will include this data in the data availability section and add a new citation:
Durack, Paul J.; Taylor, Karl E.; Ames, Sasha; Po-Chedley, Stephen; Mauzey, Christopher (2022). PCMDI AMIP SST and sea-ice boundary conditions version 1.1.7. Version 20220201.Earth System Grid Federation. https://doi.org/10.22033/ESGF/input4MIPs.16485
We will make these revisions to the manuscript when we respond to all the reviewers comments.
Sincerely,
James AnsteyCitation: https://doi.org/10.5194/egusphere-2026-1165-AC1 -
CEC2: 'Reply on AC1', Juan Antonio Añel, 30 Jun 2026
Thanks for the reply. Unfortunately, it does not address the outstanding issues, and additional work will be necessary.First, we understand that some of the models used in the work presented are not allowed to be distributed; however, several of them can be distributed, and we expect you to provide a valid repository for them. For those that cannot be distributed, the Code Availability section of the manuscript should state this, provide clear instructions for accessing them (contact form, etc.), and include information on how they are preserved. That is, the fact that the authors cannot distribute the model does not mean they should not take the necessary steps to preserve it internally. For example, Copernicus and GMD have an agreement with the Met Office to access the Met Office's model code, which is stored on the Met Office's supercomputers. For others, you could provide a Zenodo private repository.Regarding citation to other papers, although I have not checked them here case by case, it is often the case that citations to papers presenting the specific version of a model do not reflect the exact code and routines used in an experiment, which often are modified to include for example nudged data or specific parametrisations or small improvements since the original version of the model was released. Therefore, please, if you are going to cite papers, ensure that the repository linked in the mentioned paper: a) contains exactly the same code used for your experiments, b) is a repository that we can accept (not Git sites, webpages of research centers, etc.).We could accept JASMIN to deposit the QBOi data. We realize that it is a large dataset and that a decision was made to host it there; however, it is hard to move elsewhere. You need to provide specific information about how to access the data openly. If you cannot provide an open link, please provide a rationale and clear instructions for how readers can access the data.We usually do not accept the Copernicus Climate Data Store as a data repository. It does not offer the preservation period or the guarantees that the policy of the journal requests. For example, several old reanalyses that were available there are no longer accessible. This is because the Copernicus Climate Data Store is designed more as a service than as a large-scale storage repository. Therefore, the ideal approach is to store the monthly datasets in a repository. If the size is excessive (for example, over 150 GB), an exception can be made.We can accept that you cite data (input4MIPs) hosted on the ESGF; however, please provide specific information about the files used and the sources from which they were obtained. A link to the main webpage or a generic description is not enough to ensure access to the data used in your work.The Zenodo repository linked in your manuscript does not contain any assets, but only three links to external webpages, which we cannot accept.In your reply, you state, "We will include this data in the data availability section". The mentioned issues have to be solved at this point, not later. Therefore, please provide a new "Code and Data Availability" statement for your manuscript in your next replies. Also, it speeds up the evaluation of compliance with the code and data policy.Also, please, understand that while we try to maintain uniformity in requirements across papers submitted to the journal and in requests for code and data, we evaluate each paper individually. Perceived discrepancies in the requirements for different papers can have a variety of causes, from editorial oversights to other considerations taken into account when reviewing the manuscript. Therefore, a judgment made from a published paper can be biased.Juan A. Añel
Geosci. Model Dev. Executive EditorCitation: https://doi.org/10.5194/egusphere-2026-1165-CEC2 -
AC4: 'Reply on CEC2', J.A. Anstey, 02 Nov 2026
Thanks for the reply. Unfortunately, it does not address the outstanding issues, and additional work will be necessary.
Thank you for clarifying the changes needed to address the outstanding issues. Our responses below indicate how we have revised the manuscript to satisfy journal requirements for the “Code and data availability” section.
First, we understand that some of the models used in the work presented are not allowed to be distributed; however, several of them can be distributed, and we expect you to provide a valid repository for them. For those that cannot be distributed, the Code Availability section of the manuscript should state this, provide clear instructions for accessing them (contact form, etc.), and include information on how they are preserved. That is, the fact that the authors cannot distribute the model does not mean they should not take the necessary steps to preserve it internally. For example, Copernicus and GMD have an agreement with the Met Office to access the Met Office's model code, which is stored on the Met Office's supercomputers. For others, you could provide a Zenodo private repository.
We have added details of model code repositories to the “Code and data availability” section.
Regarding citation to other papers, although I have not checked them here case by case, it is often the case that citations to papers presenting the specific version of a model do not reflect the exact code and routines used in an experiment, which often are modified to include for example nudged data or specific parametrisations or small improvements since the original version of the model was released. Therefore, please, if you are going to cite papers, ensure that the repository linked in the mentioned paper: a) contains exactly the same code used for your experiments, b) is a repository that we can accept (not Git sites, webpages of research centers, etc.).
We could accept JASMIN to deposit the QBOi data. We realize that it is a large dataset and that a decision was made to host it there; however, it is hard to move elsewhere. You need to provide specific information about how to access the data openly. If you cannot provide an open link, please provide a rationale and clear instructions for how readers can access the data.
In the “Code and data availability” section we have expanded the description of how to access the QBOi data from JASMIN. The data is not fully open access, i.e. it is not freely available online as is, for example, CMIP or input4MIPs data. However the QBOi coordinators welcome any reasonable request to access the data for scientific analysis, and have now stated this in the “Code and data availability” section. We have also noted that access is ultimately granted at the discretion of JASMIN administrators (i.e., QBOi coordinators do not have complete control over data access).
We usually do not accept the Copernicus Climate Data Store as a data repository. It does not offer the preservation period or the guarantees that the policy of the journal requests. For example, several old reanalyses that were available there are no longer accessible. This is because the Copernicus Climate Data Store is designed more as a service than as a large-scale storage repository. Therefore, the ideal approach is to store the monthly datasets in a repository. If the size is excessive (for example, over 150 GB), an exception can be made.
We have created a Zenodo repository with the ERA5 1979-2020 monthly zonal-mean zonal wind data at https://doi.org/10.5281/zenodo.23845023.
We can accept that you cite data (input4MIPs) hosted on the ESGF; however, please provide specific information about the files used and the sources from which they were obtained. A link to the main webpage or a generic description is not enough to ensure access to the data used in your work.
We have added additional details on how to access input4MIPs data from ESGF in the “Code and data availability” section.
The Zenodo repository linked in your manuscript does not contain any assets, but only three links to external webpages, which we cannot accept.
In our original submission we were requested to create a placeholder Zenodo repository in order to provide a DOI for the QBOi phase-2 dataset (at ~30 TB, the actual dataset is too large for a Zenodo repository). We have now improved the description of how to access the data in the “Code and data availability” section, and removed the reference to this Zenodo repository.
In your reply, you state, "We will include this data in the data availability section". The mentioned issues have to be solved at this point, not later. Therefore, please provide a new "Code and Data Availability" statement for your manuscript in your next replies. Also, it speeds up the evaluation of compliance with the code and data policy.
Also, please, understand that while we try to maintain uniformity in requirements across papers submitted to the journal and in requests for code and data, we evaluate each paper individually. Perceived discrepancies in the requirements for different papers can have a variety of causes, from editorial oversights to other considerations taken into account when reviewing the manuscript. Therefore, a judgment made from a published paper can be biased.
Juan A. Añel
Geosci. Model Dev. Executive Editor
Citation: https://doi.org/10.5194/egusphere-2026-1165-CEC2
Citation: https://doi.org/10.5194/egusphere-2026-1165-AC4
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AC4: 'Reply on CEC2', J.A. Anstey, 02 Nov 2026
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CEC2: 'Reply on AC1', Juan Antonio Añel, 30 Jun 2026
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AC1: 'Reply on CEC1', J.A. Anstey, 30 Jun 2026
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RC2: 'Comment on egusphere-2026-1165', Anonymous Referee #2, 19 Jul 2026
Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)
Anstey et al.
This manuscript presents an experimental protocol for a second phase of the QBOi project. QBOi is a well-known and high profile model intercomparison project in this field; a description of the experiemntal protocol is clearly appropriate for publication in GMD.
The manuscript presents the protocol, discusses the formulation of the nudging technique that forms the core of the experimental design, and validates this approach in various useful ways, demonstrating that it is accomplishing its intended purpose.
It also describes participating models and the data request for the archive that is publicly available.
Commenting on the actual experimental design seems a bit besides the point given that the runs have been carried out, but the choice of experimental design and of the nudging profile seems very appropriate for the stated goals of the project. The manuscript is well written, and with only very minor exceptions detailed below, provides the details that are helpful and relevant to users of this valuable dataset. I recommend publication with minor revisions.
A few small comments:
1) It would be helpful to specify more details about the prescribed ozone field for those models running without interactive chemistry, particularly given the interests of the referenced QUOCA project. Are these zonal mean climatologies? Do they have a representation of trends?
2) I find the presentation in Fig. 4 to be quite busy - it took me a while to sort out what I was looking at, and then it takes effort to spot outliers given the choice to plot all three experiments on a single set of axes. The authors might consider splitting up the three experimental ensembles into different panels, perhaps duplicating the MMMs and the reanalysis results to aid comparison. This is only a presentation comment, though, so feel free to disregard.
3) The illustration in Fig. 5 of the bias in QBO width is interesting and useful to see documented. However, my eye is drawn to the large disagreement between the models in the subtropics in the `normalized' panel (b). If I understand the processing correctly, the curves have been normalized by the (near-equatorial) peak value of the amplitude. That seems reasonable for the core of the QBO, but it's less clear to me that it's the case in the subtropics where the QBO no longer contributes to the interannual variability. It seems to me that is overly favourable to those models with a weak amplitude; with apologies for picking on E3SM, I'm not sure that I believe that it's renormalized width is really comparable to ERA 5. Perhaps the normalization should be relative to the MMM `amplitude' at 25 degrees N/S where the variability will be dominated by other processes and the models are in better agreement with the renaalysis? (Use of Exp1-NoQBO might also be revealing, but seemed like it could introduce further issues).
4) Fig. 6: Just as a comment, this figure seems like a rich source of information about model biases.
5) Fig. 7: It seems like there is more to understand here, although a detailed analysis is likely beyond the scope of this paper. Do those models that show more of a response in the climatological state (E3SM, MIROC6.1, CAS-ESM, CESM2) also have larger mean state biases in the tropics that the nudging is correcting? Also perhaps out of scope, and may have been addressed in Andrews et al. (2026), but are the QBO teleconnections in these models also more sensitive to the nudging than other models?
Citation: https://doi.org/10.5194/egusphere-2026-1165-RC2 -
AC3: 'Reply on RC2', J.A. Anstey, 02 Nov 2026
Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)
Anstey et al.
This manuscript presents an experimental protocol for a second phase of the QBOi project. QBOi is a well-known and high profile model intercomparison project in this field; a description of the experimental protocol is clearly appropriate for publication in GMD.
The manuscript presents the protocol, discusses the formulation of the nudging technique that forms the core of the experimental design, and validates this approach in various useful ways, demonstrating that it is accomplishing its intended purpose.
It also describes participating models and the data request for the archive that is publicly available.
Commenting on the actual experimental design seems a bit besides the point given that the runs have been carried out, but the choice of experimental design and of the nudging profile seems very appropriate for the stated goals of the project. The manuscript is well written, and with only very minor exceptions detailed below, provides the details that are helpful and relevant to users of this valuable dataset. I recommend publication with minor revisions.
The authors thank the reviewer for kindly reviewing the manuscript.
A few small comments:
1) It would be helpful to specify more details about the prescribed ozone field for those models running without interactive chemistry, particularly given the interests of the referenced QUOCA project. Are these zonal mean climatologies? Do they have a representation of trends?
We prefer not to add further details on the ozone forcing because the protocol left a wide range for modellers to use their discretion in the choice of prescribed ozone forcing (Appendix B, lines 510-517). Robustly discerning the impacts of ozone on the QBO requires careful comparison between interactive and non-interactive experiments as discussed in the QUOCA protocol paper (Orbe et al. 2026, https://gmd.copernicus.org/articles/19/773/2026/). For models that use prescribed ozone (9 models here), given the number of other differences between models we believe it would be difficult with the present experiments to robustly attribute model spread to differences in their treatments of prescribed ozone (i.e., zonal-mean vs. zonally varying, with or without trends, etc).
2) I find the presentation in Fig. 4 to be quite busy - it took me a while to sort out what I was looking at, and then it takes effort to spot outliers given the choice to plot all three experiments on a single set of axes. The authors might consider splitting up the three experimental ensembles into different panels, perhaps duplicating the MMMs and the reanalysis results to aid comparison. This is only a presentation comment, though, so feel free to disregard.
We agree Fig 4 is quite busy and therefore we have moved the headline results (i.e., ensemble means) to separate panels (a,b). We hope this will then make it easier to interpret the busier panels that show individual model results (c,d). However, we have not split the busy panels by experiment as we believe readers may want to check how a single model performed across the three experiments. Also, we are conscious of leaving space for the more detailed analysis papers and not pre-empting those studies by going into too much detail in this paper.
3) The illustration in Fig. 5 of the bias in QBO width is interesting and useful to see documented. However, my eye is drawn to the large disagreement between the models in the subtropics in the `normalized' panel (b). If I understand the processing correctly, the curves have been normalized by the (near-equatorial) peak value of the amplitude. That seems reasonable for the core of the QBO, but it's less clear to me that it's the case in the subtropics where the QBO no longer contributes to the interannual variability. It seems to me that is overly favourable to those models with a weak amplitude; with apologies for picking on E3SM, I'm not sure that I believe that it's renormalized width is really comparable to ERA 5. Perhaps the normalization should be relative to the MMM `amplitude' at 25 degrees N/S where the variability will be dominated by other processes and the models are in better agreement with the reanalysis? (Use of Exp1-NoQBO might also be revealing, but seemed like it could introduce further issues).
As with Fig. 4 we have moved the ensemble-mean results to a separate panel in Fig. 5 and added the ensemble mean results for Exp1-NoQBO. This shows that the nonzero value in the subtropics (i.e., poleward of 20 degrees) of the amplitude diagnostic used in this figure is almost certainly not due to the QBO and must result from other sources of interannual variability. Furthermore all 3 experiments are in good agreement with ERA5 in the subtropics (Figure 5a) and hence the spread in the normalised Exp1 results in the subtropics (Figure 5c) merely reflects the spread in peak QBO amplitudes in Exp1 (i.e., the spread in the peak amplitude by which the curves are normalized). The reduced width of the normalized Exp1 profiles, in comparison to ERA5, indicates that the relative strength of interannual variability decays more rapidly with distance from the equator in the free-running (Exp1) models than in ERA5 or the nudged models (Exp1-ObsQBO). We have noted this in the revised manuscript (lines 330-332).
4) Fig. 6: Just as a comment, this figure seems like a rich source of information about model biases.
Thank you for this comment, and we agree. Further analysis of the behaviour seen in this figure may be interesting but we believe is out of scope of the present study since we do not want to pre-empt here the more detailed analysis papers already submitted or potential future studies using this dataset.
5) Fig. 7: It seems like there is more to understand here, although a detailed analysis is likely beyond the scope of this paper. Do those models that show more of a response in the climatological state (E3SM, MIROC6.1, CAS-ESM, CESM2) also have larger mean state biases in the tropics that the nudging is correcting? Also perhaps out of scope, and may have been addressed in Andrews et al. (2026), but are the QBO teleconnections in these models also more sensitive to the nudging than other models?
As noted above we were conscious when preparing this paper of striking a balance between presenting supporting results for the analysis studies while not pre-empting new results that would be included in them. It is interesting that CAS-ESM and ESM4 show a large mean-state tropical wind bias correction in Fig. 4c (Fig. 4a in the original submission) and large mean-state high-latitude wind differences in Fig. 7, but E3SM and CESM2 show high latitude differences in Fig. 7 but little mean-state bias correction in Fig. 4c. Hence it is not immediately clear to us that the tropical mean-state bias correction and extratropical mean-state changes are connected (although the connection is physically plausible) so we prefer to leave any further consideration of this question for future work. As the reviewer has noted, the Andrews et al. study examines the QBO teleconnections in these models. That paper is now published (https://wcd.copernicus.org/articles/7/1797/2026/) and a citation has been added in the paragraph describing Fig. 7 (line 362).
Citation: https://doi.org/10.5194/egusphere-2026-1165-AC3
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AC3: 'Reply on RC2', J.A. Anstey, 02 Nov 2026
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review of "Experiment design, nudging protocol, and models participating in Phase 2 of the APARC Quasi-Biennial Oscillation initiative (QBOi)" by Anstey et al
This study introduces the protocol for phase 2 of the QBOi project, with the main addition being two nudged experiments. In Exp1-ObsQBO, the tropical stratospheric zonal winds are nudged to observed reanalysis wind, and in Exp1-NoQBO they are nudged to the climatological wind over the 1979-2020 period. The paper also introduces key aspects of the participating models, and also describes the data request. The paper also demonstrates that the models respond to nudging as expected.
I have no major comments - this study is very appropriate for GMD - and I look forward to reading the subsequent papers that report results. To be explicit, I don't think the authors should be asked to include any additional results in the current paper. It should be promptly accepted subject to the technical edits described below.
technical edits:
line 59-62 this sentence could be reworded for clarity
line 87 "seasonal" -> "subseasonal"
caption of figure 1: give- > given
line 227 "Sensitivity of QBO simulations to vertical resolution is not well quantified," is confusing. Do you mean "mechanisms are poorly understood"?
line 245, 502 Garfinkel et al 2025 seems appropriate in these two locations. Consider citing it.
line 275: therefore **are** not
line 302 nudged -> nudge
line 355 "responding" is superfluous
line 368 "and" is missing between Exp1-NoQBO and Exp1-ObsQBO
line 375 **the** simulated
line 396 consider a comma after "winds"
line 465 GCMs
line 506 superfluous "is"
section A2: it might be worth stating explicitly that the profile for Exp1-noQBO is the climatological mean over 1979-2020 to match the period for obsQBO, though this is probably obvious. For SNAPSI, the specific wind and temperature profiles to be relaxed towards were provided to model developers. For SNAPSI this was done because the initial spinup in the first few days after initialization needed to be handled somewhat carefully to avoid shocks for the runs nudged to climatology due to the fast nudging timescale. Presumably this is less of a problem for the QBOi runs.
Garfinkel, C. I., Avisar, D., Osprey, S., & Smith, D. (2025). The response of the QBO to external forcings: implications for disruption events. Journal of Geophysical Research: Atmospheres, 130(22), e2025JD044438.