the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Jul 2025
Assessing Vertical Coordinate System Performance in the Regional Modular Ocean Model 6 configuration for Northwest Pacific
Abstract. The Northwest Pacific (NWP) has a complex ocean circulation system and is among the regions most affected by climate change. To facilitate rapid responses to marine incidents and effectively address climate variability impacts, the Korea Institute of Ocean Science and Technology (KIOST) developed the Korea Operational Oceanographic System–Ocean Predictability Experiment for Marine Environment (KOOS-OPEM), a high-resolution regional ocean prediction system based on Modular Ocean Model version 5 (MOM5). In this study, the base model of KOOS-OPEM was upgraded to MOM6 to enhance its regional ocean modeling capabilities. A key advancement of MOM6 is its flexible vertical coordinate system enabled by a Lagrangian remapping system. Taking advantage of this feature, we evaluated the impact of vertical coordinate choices on model performance by comparing the HYBRID (z*-isopycnal) and ZSTAR (z*) configurations. Model outputs from the 2003–2012 period were assessed against multiple observational datasets and reanalysis products to determine their ability to reproduce key oceanographic features. The results indicated that HYBRID better preserved stratification and reduced spurious diapycnal mixing, significantly improving the representation of North Pacific Intermediate Water (NPIW). In contrast, ZSTAR exhibited excessive diapycnal mixing, resulting in a thicker isopycnal layer associated with NPIW and a salinity bias of approximately 0.2 psu. An idealized age tracer experiment further confirmed that ZSTAR facilitates excessive downward diffusion of younger surface waters, eroding the minimum salinity layer of the NPIW. For sea surface temperature (SST), both configurations captured seasonal patterns but exhibited biases. HYBRID produced a stronger warm bias in the Kuroshio-Oyashio Transition Zone. In the East/Japan Sea, ZSTAR displayed an exaggerated warm bias exceeding 3 °C, primarily due to an overshoot of the East Korea Warm Current (EKWC), which extended too far north without proper separation. In tidal simulations, HYBRID outperformed ZSTAR in reproducing M2 tidal amplitudes in the Yellow Sea, where stratification plays a key role. Conversely, ZSTAR underestimated these amplitudes due to its limitations in representing stratification. Despite its advantages, HYBRID underperformed in high-latitude regions, exhibiting larger temperature and salinity biases between 100 m and 600 m depth, with temperature biases reaching approximately -1 °C. This discrepancy arose because HYBRID maintained fewer active layers in weakly stratified regions, reducing vertical resolution and leading to errors in water mass representation. To mitigate these issues and improve HYBRID’s performance in high-latitude regions, adjustments to the maximum layer thickness and target density profiles are necessary. Beyond physical ocean modeling, integrating biogeochemical components is essential for advancing the understanding of ecosystem dynamics in the NWP.
- Preprint
(4423 KB) - Metadata XML
-
Supplement
(683 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
CC1: 'Comment on egusphere-2025-3211', Matthew Harrison, 15 Jul 2025
-
AC1: 'Reply on CC1', YoungHo Kim, 19 Jul 2025
Regarding model convergence with increased/decreased layers:
While our current study focuses on comparing two vertical coordinate systems (HYBRID and ZSTAR) under identical vertical degrees of freedom (75 layers), we agree that assessing model convergence by systematically increasing or decreasing the number of layers (e.g., 37-layer or 150-layer configurations) would provide additional insight. However, such convergence experiments were not included in this manuscript primarily due to computational cost and because our primary objective was to evaluate the sensitivity of ocean circulation and hydrography to the choice of vertical coordinate system. We acknowledge the importance of convergence testing and plan to address this aspect in future work.Regarding optimization of the hybrid vertical coordinate for the regional domain:
We appreciate your insightful comments regarding the applicability of the hybrid vertical coordinate system and the potential for further optimization of the target density layers for our regional model configuration.As noted, the current hybrid coordinate configuration employed in this study was adapted from a previously published global MOM6 setup. This choice was initially made to maintain consistency and comparability with established global modeling practices. However, we fully acknowledge that the range of density classes relevant to our Northwest Pacific regional domain is more restricted, and thus the default global configuration may not be optimal across all subregions.
To address this concern, we have conducted additional sensitivity experiments using modified sets of target densities in the hybrid coordinate. These experiments indicate that the target density range adopted from the global setup performs reasonably well in capturing key water mass structures in the Northwest Pacific. In particular, it effectively represents the Kuroshio and the North Pacific Intermediate Water, both in terms of core depth and vertical extent. These findings support the suitability of this target density configuration for representing the large-scale features within our domain.
Nonetheless, the results presented in the current manuscript also reveal regional limitations, especially in high-latitude marginal seas such as the East/Japan Sea and the Sea of Okhotsk. In these regions, where stratification is relatively weak, the fixed target density distribution results in a substantial loss of vertical resolution in the upper water column. This resolution loss likely contributes to the degraded performance in representing summer stratification and internal tide dynamics in these areas. We agree that a more regionally optimized hybrid coordinate—potentially with an adjusted density range could alleviate these deficiencies.
We are currently designing a follow-up experiment that incorporates a modified hybrid coordinate tailored to improve vertical resolution in weakly stratified environments. While such results are not yet available for inclusion in the present manuscript, we plan to report them in a future study. We will revise the discussion section to include this point as a future direction for improving the performance of hybrid coordinate systems in marginal seas.
Once again, we thank you for this valuable suggestion.
Citation: https://doi.org/10.5194/egusphere-2025-3211-AC1
-
AC1: 'Reply on CC1', YoungHo Kim, 19 Jul 2025
-
RC1: 'Comment on egusphere-2025-3211', Anonymous Referee #1, 23 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3211/egusphere-2025-3211-RC1-supplement.pdf
-
AC2: 'Reply on RC1', YoungHo Kim, 22 Oct 2025
Dear Reviewer,
We sincerely appreciate your thoughtful and constructive comments.
Please find attached our detailed responses to your remarks.Kind regards,
Inseong Chang and Young Ho Kim- AC5: 'Reply on AC2', YoungHo Kim, 22 Oct 2025
-
AC2: 'Reply on RC1', YoungHo Kim, 22 Oct 2025
-
CC2: 'RC on egusphere-2025-3211', Matthew Harrison, 16 Sep 2025
This manuscript documents the performance of a regional ocean model configuration based on observational metrics derived from altimetry, and hydrography for the North-West Pacific basin. It also documents model performance using two different vertical coordinate configurations. The authors highlight areas of good agreement and inconsistencies within these two configurations.
This manuscript is recommended for publication pending resolution of mostly minor comments below:
- Figure 1: Spelling correction. “East Chania Sea” should be “East China Sea”
- L110: Reference Ilicak, 2012, Ocean Modeling.
- L143: How does limiting the depth to 5000m reduce the model’s computational burden?
- The “background” vertical viscosities and diffusivities are less than molecular (1.e-6 m2 s-1) values. Are MOM6 configurations typically using such small values? This appears to be a departure from typical ocean configurations which would typically rely on background values an order of magnitude higher. Please discuss the rationale for using such a small value.
- Table 1 could be reformatted, since the third column is empty with the exception of the first row.
- L201: For clarification, are you applying discharge adjustments at the Yangtze river only?
- L237-241: This paragraph is redundant with the previous one.
- L352-366: The differences in wintertime MLD between HYBRID and ZSTAR are most prominent South of the Kuroshio extension between 25N-35N and in the Okhotsk Sea. In the first case, is this a result of enhanced stratification below the actively mixed layer prior to the onset of wintertime convection, for example? In the latter case, is this a result of poor representation (fewer active layers) due to the use of sigma2? It would be helpful to see the seasonal evolution of the stratification and/or the actual internal layer representation in the model for these regions. These questions could be addressed later in the text as well.
- Fig15: salinity differences on sigma2 can be mostly estimated from the temperature differences, so this figure and the previous one are redundant. Suggest removing this figure.
- L592. Did the authors evaluate simulations without explicit tides and using parameterized tides instead and did this reveal differences in YBCMW?
- L622. Can you provide additional comments on the role of the vertical coordinate in impacting the volume transport through straits?The connection is not immediately obvious.
Citation: https://doi.org/10.5194/egusphere-2025-3211-CC2 -
AC3: 'Reply on CC2', YoungHo Kim, 22 Oct 2025
Dear Reviewer,
We are grateful for your careful review and valuable suggestions.
Enclosed please find our detailed replies to your comments.Sincerely,
Young Ho Kim- AC6: 'Reply on AC3', YoungHo Kim, 22 Oct 2025
-
RC2: 'Comment on egusphere-2025-3211', Anonymous Referee #2, 10 Oct 2025
Review of “Assessing Vertical Coordinate System Performance in the Regional Modular Ocea Model 6 configuration for the Northwest Pacific” by Chang et al.
Chang et al. evaluate model performance sensitivity to the choice of vertical grid coordinates (hybrid vs. z-star) on a regional configuration of the Northwest Pacific. They examine 10 years of model output and compare to a suite of observational and reanalysis products. I find the manuscript generally well written and organized. The manuscript will make an important contribution to the growing body of literature describing the deployment of regional MOM6 for different applications and, in particular, the sensitivity to different configuration choices.
One main comment relates to the choice of target densities for the hybrid coordinates; I was surprised that the author’s choice for the hybrid grid was based on a global grid whose target densities are not optimized for the regional domain. I note that the same comment was made by another reviewer and in a public comment in the pre-print and acknowledge the authors’ response there.
Furthermore, the choice of vertical coordinate can be additionally influenced by computational costs . I think the manuscript would benefit from discussion on computational cost differences between the z-star and hybrid coordinates. I would recommend the manuscript to publication after these and the comments below are addressed by the authors.
Line 200-204: My interpretation is that the bias correction was applied only to the Yangtze River. If so, why was this only done for this river? The authors recognize in the results and conclusions the biases in salinity, I wonder why the authors did not apply the same correction to other rivers to potentially help with the SSS biases shown.
Line 270: “The K-KE regions…” as obvious as it may seem, you have not defined what this stands for.
Lines 249-254: It would be useful to define the period over which the average of the u and v velocities is calculated.
Figures 2, 3 and others: I found it hard to distinguish land and contours on areas with high contour line density in the panels showing differences between products and model output because the color used for land and contours is the same. I would encourage changing land color to black or setting the contour line color to a color other than gray.
Section 3.4: It would be useful to include here and/or in Section 4 a discussion on how the volume transport differences come about with the different vertical grids. It seems that, while not perfect, z-star was consistently better than the hybrid grid.
Lines 658-660: provide reference and/or examples.
Citation: https://doi.org/10.5194/egusphere-2025-3211-RC2 -
AC4: 'Reply on RC2', YoungHo Kim, 22 Oct 2025
Dear Reviewer,
Thank you for your helpful comments.
Please find our responses attached for your review.Sincerely,
Young Ho Kim- AC7: 'Reply on AC4', YoungHo Kim, 22 Oct 2025
-
AC4: 'Reply on RC2', YoungHo Kim, 22 Oct 2025
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 1,795 | 92 | 39 | 1,926 | 28 | 26 | 32 |
- HTML: 1,795
- PDF: 92
- XML: 39
- Total: 1,926
- Supplement: 28
- BibTeX: 26
- EndNote: 32
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
This manuscript demonstrates vertical grid sensitivity using 2 configurations with identical vertical degrees of freedom. An evaluation of model convergence with additional layers is missing. For instance, if the number of z* layers is increased (2x,4x?) or decreased (0.5x), do we see convergence towards the observations with increased resolution, or are there other sources of model bias to contend with? Additionally, the hybrid vertical coordinate selected was based on a previously published global configuration, which would have been optimized for the global ocean. Clearly, the range of densities under consideration for the regional domain is more restrictive, and a more optimized vertical grid could have been employed. As discussed in the preprint, high latitude degradation was attributed to loss of resolution in weakly stratified regions. An additional simulation with a modified hybrid coordinate designed to address this deficiency would be very helpful to the paper.