the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Oct 2025
Two-tier MOM6 Regional Modelling Suite of the East Australian Current System
Abstract. We present a new ultra-high resolution 1/30° (∼3 km) regional ocean model of the eastern Australian region and evaluate the performance of this model against two 1/10° (∼10 km) models as well as a suite of satellite and in situ observations. We evaluate model biases in the context of (i) submesoscale-permitting (∼3 km) vs. mesoscale-permitting (∼10 km) horizontal resolution and (ii) differences between version 5 and version 6 of the Modular Ocean Model (MOM5 vs. MOM6) to assess the added value in each case and determine the suitability of our higher resolution model for scientific research. There are some consistent biases shared by the two regional MOM6 configurations, and also in the higher resolution configuration that are not seen in the lower resolution models. These biases are further investigated with two sets of sensitivity experiments to understand the effect of submesoscale eddy parameterization and imposed dynamic viscosity at a submesoscale-permitting resolution. The high-resolution simulation has much higher variance compared to the lower resolution simulations across all evaluation metrics, indicating that the greater spectrum of length scales also manifests in more variability in the temporal domain. The two MOM6 regional configurations of differing resolution appear to be more aligned than the regional (MOM6) and global (MOM5) 1/10° configurations in most results, reflecting the substantial changes made to the MOM between version 5 and version 6. Importantly, we also show that higher resolution is not a panacea: in regions where key dynamics are quasi-linear and well-captured captured by coarser grids (e.g., the EAC jet), further refinement may offer limited benefit – and actually degrade the performance if parameterizations are not appropriately tuned.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-4226', Anonymous Referee #1, 06 Nov 2025
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AC3: 'Reply on RC1', John Reilly, 05 Jan 2026
Dear Reviewer 1,
Thank you for your detailed and constructive review. We have addressed each of the three points individually below;
- Fair Comparison: We agree that isolating the impact of a specific model setting requires changing only one factor at a time. This principle is applied in the second half of the manuscript via controlled sensitivity tests within the 3km configuration. The first half, however, is not intended to as a “fair” attribution-style intercomparison; rather it provides contextual evaluation across two broad axes – model framework (MOM5 vs MOM6) and resolution within MOM6 – with observations used as an external reference. We will revise the Methods to make these objectives explicit and add a comparison matrix summarising the key differences among configurations. We also agree that dependence of EAC03K on the parent boundary forcing should be quantified; we have therefore completed an additional 3km simulation forced by OM2-01 boundaries and will include a targeted assessment of sensitivity to boundary forcing in the revised manuscript.
- MLE Parameterisation: We would like to clarify that the MLE parameterisation is indeed active in the 3km configuration: when the parameterisation is switched off or its parameter value (frontal length scale) is varied, the model exhibits a clear change in behaviour, most notably in the long-term mean and variability of mixed-layer depth (consistent with the expected restratification effect). We agree that MLE parameterisations are typically evaluated in coarser, non-submesoscale-permitting models; however, the objective of this section is different – namely, to test whether the parameterisation remains necessary (or becomes largely redundant) at 3km resolution where submesoscale processes are partially resolved. We will revise the manuscript to make this objective explicit and to frame these experiments as a resolution-regime sensitivity test rather than a validation of MLE performance in a setting where its necessity is already established.
- Surface Momentum Flux: MOM6 computes the wind stress online using the relative winds (U10-us) which explicitly includes wind-current interaction. We thank the reviewer for highlighting this ambiguity in the manuscript, and will revise this to document the wind-stress formulation explicitly and briefly discuss its implications for interpreting SSH variance/EKE comparisons.
We thank the reviewer again for these constructive comments, and we believe that the clarifications, restructuring, and additional sensitivity analysis outlined above will substantially improve the transparency, interpretability, and overall robustness of the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-4226-AC3
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AC3: 'Reply on RC1', John Reilly, 05 Jan 2026
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CEC1: 'Comment on egusphere-2025-4226 - No compliance with the policy of the journal', Juan Antonio Añel, 05 Dec 2025
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
You have archived your code on GitHub. However, GitHub is not a suitable repository for scientific publication. GitHub itself instructs authors to use other long-term archival and publishing alternatives, such as Zenodo. Therefore, the current situation with your manuscript is irregular. Please, publish your code 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, as we can not accept manuscripts in Discussions that do not comply with our policy.Also, you must include a modified 'Code and Data Availability' section in a potentially reviewed manuscript, containing the information of the new repositories.
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 our journal.
Juan A. Añel
Geosci. Model Dev. Executive EditorCitation: https://doi.org/10.5194/egusphere-2025-4226-CEC1 -
AC1: 'Reply on CEC1', John Reilly, 05 Jan 2026
Dear Juan,
Thank you for flagging the Code and Data Policy issue, and apologies for the earlier non-compliance.
To resolve this, we have now archived the exact source code used in this study (MOM6, SIS2, FMS, and coupling components) in a long-term repository on Zenodo. The permanent DOI is: https://doi.org/10.5281/zenodo.18149344.
We will revise the manuscript to update the "Code and Data Availability" section to cite this Zenodo DOI as the archival record of the model source code. Any remaining GitHub links will be retained only as supplementary development/documentation references (not as the archive of record for this paper).
Kind regards,
John Reilly
Citation: https://doi.org/10.5194/egusphere-2025-4226-AC1 -
CEC2: 'Reply on AC1', Juan Antonio Añel, 05 Jan 2026
Dear authors,
Many thanks for addressing this issue. We can now consider the current version of your manuscript in compliance with the Code and Data policy of the journal.
Juan A. Añel
Geosci. Model Dev. Executive Editor
Citation: https://doi.org/10.5194/egusphere-2025-4226-CEC2
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CEC2: 'Reply on AC1', Juan Antonio Añel, 05 Jan 2026
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AC1: 'Reply on CEC1', John Reilly, 05 Jan 2026
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RC2: 'Comment on egusphere-2025-4226', Anonymous Referee #2, 12 Dec 2025
Two minor comments:
1) All these models are on a Mercator grid, which is typical but should be made explicit. For example add:
Line 75: (Kiss et al., 2020), which is Mercator (i.e. the meridional spacing scales as the cosine of latitude) across the domain used here.
2) The ALE discussion (line 100) overstates its advantages. The ALE scheme is accurate and robust to vertical motion and there is no need to compromise topography or resolution (unconditionally stable), but it does not necessarily reduce mixing across z-surfaces when configured for z-star coordinates. It does reduce spurious diapycnal mixing when using MOM6's HYCOM1 hybrid regridder, but that is no employed here.
A more serious issue is the used of relative wind forcing (i.e. U10-Uocean). This is the only available option in MOM6 and it is certainly the best choice when coupled to an active atmospheric model component as would be the case in a climate simulation. However in "prescribed atmosphere" cases, as here, direct winds (i.e. U10 alone) are typically preferred because they give more realistic kinetic energies. Relative winds are not wrong, but their implications for the strength of fronts and eddies should be noted and the paper would be improved with the addition of simulations forced by direct winds. It is hard to tell if the conclusions would have been different has direct winds been used.
This is a comprehensive model to model and model to observations comparison paper and the addition of direct winds cases could be deferred to a follow on paper.
Citation: https://doi.org/10.5194/egusphere-2025-4226-RC2 -
AC2: 'Reply on RC2', John Reilly, 05 Jan 2026
Dear Reviewer 2,
Thank you for these insightful and constructive comments - the following will address each main point specifically;
1. Mercator Grid: We will revise the manuscript to explictly state that all model configurations use a Mercator horizontal grid and add the suggested clarifying text in the methods section.
2. ALE Discussion: We agree that the current wording overstates the advantages of ALE in our z-star configurations. In the revised manuscript we will rephrase this section to more accurately reflect the use of ALE in our context.
3. Relative vs. direct wind forcing: We appreciate this important point. We will add a clear description of the wind-stress formulation used (relative winds) and include a dedicated discussion of its implications for mesoscale kinetic energy, frontal/eddy strength, and interpretation of SSH/EKE metrics in the data atmosphere context. We agree that additional simulations using direct winds would provide a valuable sensitivity test however given the already broad scope of the present manuscript, we will defer this to a follow-on study/experiment and note this as a limitation in the present work.
Thank you again for your careful review and for improving the clarity and robustness of the manuscript.
Kind regards,
John Reilly
Citation: https://doi.org/10.5194/egusphere-2025-4226-AC2
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AC2: 'Reply on RC2', John Reilly, 05 Jan 2026
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General comment:
It is very difficult to make fair comparisons among three models: OM2-01(MOM5 0.1 degree global model), STHPAC10K(MOM6 0.1 degree large regional model), and EAC03K (MOM6 1/30 degree small regional model) because the model settings differ much among the three models. To properly assess the impact of a specific model setting, only the setting should be changed. Also, most results from EAC03K with the small domain appear to be largely influenced by STHPAC10K. The comparisons between MOM5 and MOM6 by using OM2-01 with the global domain and STHPAC10K with the regional domain is also complicated. A large portion of the manuscript is devoted to the non-fair comparisons. If the authors would like to show how EAC03 simulated EAC region well, they should focus on the topic. Therefore, I recommend that the authors should rewrite the entire manuscript.
I am not convinced that the parameterization of mixed layer eddies is implemented into 3km model. The model partially resolves submesoscales including mixed layer eddies as the author mentioned in the manuscript. The effect of the parameterization should be tested in the model at the coarser resolution that cannot resolve mixed layer eddies. Previous studies implemented the parameterization into the coarser models that do not resolve mixed layer eddies. Therefore, the authors should remove the sensitivity experiment of mixed layer eddy parameterization of 3km model.
Please explain about the surface momentum flux. The eddy variability (SSH variance) in the model is much influenced by how the surface momentum flux is estimated. The eddy killing (e.g. Zhai and Greatbatch, 2007; Renautl et al. 2016) should be considered when the eddy variability in the model is verified. If the surface ocean current is not considered, the eddy variability becomes relatively too strong.
Renault, L., M. J. Molemaker, J. C. McWilliams, A. F. Shchepetkin, F. Lemarié, D. Chelton, S. Illig, and A. Hall, 2016: Modulation of Wind Work by Oceanic Current Interaction with the Atmosphere. J. Phys. Oceanogr., 46, 1685–1704, https://doi.org/10.1175/JPO-D-15-0232.1.
Zhai, X., and R. J. Greatbatch (2007), Wind work in a model of the northwest Atlantic Ocean, Geophys. Res. Lett., 34, L04606, doi:10.1029/2006GL028907.
Specific comment:
Data availability is insufficient. Please clarify how to get the data.
Figure 3, 7, 9: Please clarify the units.