the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jul 2025
Modelled dynamics of floating and grounded icebergs, with application to the Amundsen Sea
Abstract. Icebergs that ground on the submarine Bear Ridge in the Amundsen Sea are known to block the drift of sea ice, playing a crucial role in maintaining shelf sea ocean conditions. This important iceberg—sea ice—ocean interaction is ubiquitous around the Antarctic shelf seas. To better represent the drift, grounding, and ungrounding of icebergs in the vicinity of such seabed ridges, we introduce new dynamics into the iceberg component of the Nucleus for European Modelling of the Ocean (NEMO) ocean general circulation model. The pre-existing iceberg capability in NEMO did not facilitate iceberg grounding, but here we implement a physically-motivated grounding scheme with parameter choices guided by observations from the Amundsen Sea. When the bergs are grounded, they now experience bottom sediment resistance, bedrock friction, and an acceleration due to gravity acting down topographic slopes. We also improve the representation of ocean turbulent drag and ocean pressure gradients, both for freely-floating and grounded icebergs, by incorporating the depth-dependence of these forces. We examine the diverse set of forces acting on simulated icebergs in the Amundsen Sea, and compare our simulations with iceberg observations near Bear Ridge. The new iceberg physics pave the way for future studies to explore the existence of possible feedback mechanisms between iceberg grounding, changing sea ice and ocean conditions, and iceberg calving from the ice shelves.
Competing interests: At least one of the authors is a member of the journal's editorial board.
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)
- RC1: 'Comment on egusphere-2025-2423', R. Marsh, 11 Aug 2025
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RC2: 'Comment on egusphere-2025-2423', Till Wagner, 14 Aug 2025
In this manuscript, Y Kostov and co-authors present an updated grounding representation for icebergs in the NEMO ocean model, as well as improvements to how iceberg drift is computed.
The paper is very well written and structured, clearly illustrated, and the subject matter is a natural fit for The Cryosphere. I believe this work presents substantive steps forward in the representation of icebergs in models and I am looking forward to seeing how these changes will improve future iceberg modeling efforts.
In light of this I recommend the paper for publication after revisions, with my comments detailed below. (Please note that some of these comments are musings rather than requests for edits, arising largely because I am fascinated by this topic. Relatedly, I am keenly aware that I refer to my own papers quite a lot in my comments - which is mostly just a consequence of being most familiar with those and not a request for citations).
General Comments:
1) My most substantial comment is that I do wonder whether the paper may benefit from being split into 2 separate articles: one on grounding and one on drift dynamics. My reasons for suggesting this are two-fold:
i) The paper is quite long and it is at points hard to keep track of all the different pieces (see also a similar comment by the editor).
ii) The paper consists of two fairly independent components: the grounding parameterization and the free drift analysis and improvements. While the grounding work is more developed in the manuscript as it stands I would argue that there is plenty of material to expand the drift analysis into its own paper (without too much extra work). Such a split could streamline the presentation in a number of ways, for example, you wouldn't have to bring in the MEDIUM icebergs at all for the grounding work. I do think a split would also help the impact of the work - other modeling groups may be more likely to pick up on the improvements in grounding when this is presented in a more focused way.Having made this case, I happily leave it to the authors and the editor what to do about it.
2) There are a few passages where I thought text could be shortened somewhat. I have highlighted those in the attached pdf.
3) It would be helpful to early on provide a short discussion of the types of icebergs that get stuck on Bear Ridge with typical sizes and approximate numbers. While reading the paper, I somehow assumed there would be only a handful of large tabular icebergs at a given time, until I got to Appendix A and realized you are mostly talking about ~hundreds of fairly small icebergs. Relatedly, I would recommend picking one of the images from the timelapse movie (ideally one with very clear sea ice differences on the two sides of the "wall of icebergs", e.g., timestamp 2:15 of the movie), annotate this, and combine it with figure 1, to provide the reader early on with a sense of the general setup. These images are rather striking.
4) The manuscript is largely focused on grounding, however, I'd argue that the subsequent ungrounding is also important. [As a side note: Reading the paper I was wondering whether ungrounding is primarily the result of melting (and potentially capsizing), or rather changes in ocean current/wind direction? This is not a focus of this work, but if you have any insight I'd be interested to hear it.]
While I agree with the authors' choice to focus on the novel representation of the grounding process, I do think it would be helpful to also discuss ungrounding and the role of melting. Two things came to mind:
- First, as far as I know the melt model in NEMO-ICB contains a dependence of basal melt on the relative velocity between the iceberg and the ocean current at the height of the iceberg base (Merino et al, 2016, eq 2). I wonder how this plays out for grounded icebergs in the latest version? Lines 871/872 make it sounds like this is not the case in the current model formulation? The dependence on the size of the iceberg is evident as well in the Merino et al. formulation. I'm likely missing something, but maybe this paragraph could be reworded and/or clarified?- Second, freely floating icebergs typically erode much faster on the side walls due to wave erosion (~1 m/d) than the base (~0.1 m/d) - see, e.g., Wagner & Eisenman (GRL, 2017, https://doi.org/10.1002/2016GL071645). In that case you might expect that icebergs shrink laterally until the aspect ratio becomes unstable and they become ungrounded by capsizing (as the authors mention). This may be particularly relevant for the smaller icebergs found all over Bear Ridge. However, since for grounded icebergs the relative basal velocity is higher, maybe the thinning is substantially faster than for freely floating ones, which might entail that capsizing isn't that important after all. Would it be easy to check how often ungrounding in the model coincides with capsizing? I appreciate that a detailed analysis of these processes is beyond the scope of this study, but I do think it would be helpful to comment on how melt is represented in the model, and to at least mention some of the considerations above.
5) It was my understanding that the original NEMO-ICB used the erroneous capsizing criterion of Bigg et al (2017). We published a correction to this in Wagner et al (Ocean Modeling, 2017, https://doi.org/10.1016/j.ocemod.2017.07.003) and I discussed this briefly with Bob Marsh back then but never followed up. I just want to make sure the capsizing errors have been fixed, if there ever were any.
Specific comments:A number of mostly minor and technical comments are provided as annotations to the attached pdf.
Dear authors - I am often wrong, and if you think that any of my comments are misguided please reach out to me and I'll be eager to amend my review.
Till Wagner
Model code and software
Kostov et al 2025. Updates to the NEMO iceberg dynamics and grounding Yavor Kostov et al. https://doi.org/10.5281/zenodo.15484879
Video supplement
Modelled dynamics of floating and grounded icebergs, with application to the Amundsen Sea. Mosaic of satellite images showing Copernicus Sentinel-1 SAR (Synthetic Aperture Radar) data. Yavor Kostov et al. https://doi.org/10.5446/70447
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- 1
The authors have developed the modelled dynamics of drifting and grounding/grounded icebergs, with close attention to realism, in particular the evidence from scouring. In the former instance, the pressure gradient force for drifting bergs is more correctly separated into barotropic and baroclinic parts. In the latter case, with a focus on the topographic obstacle that is Bear Ridge in the Amundsen Sea, more extensive improvements to the NEMO-ICB model configuration are outlined. The attention to dynamical detail is impressive, most notably representation of the force balance for a grounded (and ungrounded) berg. The authors outline in considerable detail the additional forces and accelerations, based on clear fundamental physics, with just a degree of uncertainty in the coefficients of Coulomb friction.
The manuscript is succinctly written throughout. The Introduction (Sect. 1) clearly motivates the model development presented here, with a view to the wider system ice-ocean-climate system. Sect. 2 provides thorough background information on the character of seafloor and sediments, or relevance to grounding. Sect. 3 provides a detailed outline of the existing model equations and developments thereof, model configuration and experimental design. In the Results (Sect. 4), well-crafted figures convey a rich level of information, in particular the wind roses that summarise the strength and relative orientation of accelerations and forces, and the summary force balances (given typically small net accelerations). Sect. 5 provides a brief summary and discussion, pointing towards new modelling possibilities now that the basis is provided for more realistic representation of tabular bergs near Antarctica, specifically the consequences of grounding for sea ice, hydrography and even feedback on the calving process. I close with the following technical comments:
Technical Comments: