the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Mar 2026
Ice dynamics in McMurdo Sound, Antarctica, based on precise synthetic aperture radar interferometry analysis
Abstract. The embayment at the end of Ross Island's Hut Point Peninsula is often covered by a patch of landfast sea ice (henceforth fast ice), which can remain attached to the McMurdo Ice Shelf (MIS) for a few years at a time, rather than breaking out every summer. Over recent years, later fast ice formation and increased fast ice breakout have been observed, but the influence on ice shelf stability is unclear. We examined historical MIS front positions, as well as the summer 2025 fast ice breakout and MIS calving, using Landsat optical imagery and historical USGS aerial photography.
A precise interferometric satellite analysis was conducted for spring 2024 based on ascending-descending combinations of TerraSAR-X image acquisitions. Two non-orthogonal horizontal component velocity fields were calculated, and ground-referenced with precise movement data acquired with three automated GNSS stations that had been placed on the ice. This allowed us to calibrate three simply connected subsets to each of the velocity field components: the MIS itself, the multi-year fast ice, and the first-year fast ice. These component fields then enabled us to calculate a near-complete 2D horizontal velocity field of ice motion in the area. This analysis was the basis for calculating the divergence field of the velocity as well as principal strain rate fields. The overall ice dynamics were then related to the areas where we expect a stabilizing effect of the fast ice on the ice shelf front, or where the ice shelf geometry suggests stabilizing effects.
Once the January–February 2025 fast ice breakout was complete, the MIS front began calving almost immediately, leading to a net retreat of the MIS front to a minimum beyond any other found in the Landsat 4–9 record. The dynamics of the fast ice were largely dependent on age, with the behavior of the multi-year fast ice resembling in most ways that of the adjoining MIS rather than the first-year fast ice, due to strong coupling at the MIS–multi-year fast ice interface. The divergence of the velocity field and the principal strain rates show convergence and compression of the fast ice in the embayment, providing evidence of a stabilizing effect and possible buttressing of the MIS by fast ice.
- Preprint
(10925 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-6300', Alexander Fraser, 06 Apr 2026
-
AC1: 'Reply on RC1', Kallie Bohn, 23 May 2026
We appreciate the thoughtful and constructive suggestions, and agree with almost all of them.
Our responses, referenced by line number or figure (excluding typos, which we appreciate the help finding):
Ln 5: Agreed, we will revise for clarity, eg “To provide context for our interferometric study of this interaction, we examined historical MIS front positions, as well as the summer 2025 fast ice breakout and MIS calving, using Landsat optical imagery and historical USGS aerial photography.”
Ln 6-7: Agreed, we will clarify this. Eg “based on ascending-descending combinations of
TerraSAR-X image acquisitions” → “using a pair of ascending-path and a pair of descending-path TerraSAR-X image acquisitions”
Ln 15, 70, & 81: Agreed, these additions will be made.
Ln 84: This will be corrected.
Ln 90: Agreed, this does feel a bit out-of-place, and will be removed.
Fig 2: Good points. Caption will be revised for clarity. “Blue lines: ice extend during fast ice breakout” → “Blue lines: fast ice extent during breakout” We think it would help to change the background to a pre-breakout image (such as in Fig. 3), to remove any chance of giving readers the impression of the stations having been on pack ice.
Ln 108: Agreed. Sentence will be revised to make it cleaner. “one on the first-year fast ice (ST1), one on the multi-year fast ice (ST2), and one on the MIS near its edge (ST3)(Fig. 2).” → “ST1 on the first-year fast ice, ST2 on the multi-year fast ice, and ST3 on the MIS near its edge (Fig. 2).”
Ln 124, 126: While this information is non-essential, we think it could be of interest to some readers, and think the small addition to word count is worth including it.
Ln 141: Agreed. “We used ascending-path and descending-path interferometric pairs from TSX which cover the study area” → “We used 8 total TSX images for the study area: 2 ascending-path pairs and 2 descending-path pairs”
Ln 169: We agree that deposition should definitely be mentioned as another possibility. Its omission was an oversight, and this will be corrected.
Around Fig 4: Agreed, this value (45.8°) should be stated; Ln 161 is probably the sentence it will be added to.
Fig 4:
Annotating the rows with dates is a good idea; this addition will be made.
The interferograms are rendered without background shading, but are scaled by phase coherence. Mention of this will be added to the caption.Ln 176: This abbreviation was an oversight. The reference is “Land Information New Zealand”. The in-line citation will be corrected.
Ln 177: Correct, m/m – this will be added.
Ln 178: We agree that the repeated “(an update to...)” is awkward, but this is the in-line citation requested by the CATS2008 team.
Fig 5: We like this idea, and are fairly confident it should be possible to make this addition while preserving clarity and readability of other features.
Ln 327: Yes, this should be defined. Since we only use the term once, we will probably simply replace the term with the definition. Eg “We inferred [...] a decelerating flow implies an adverse force” → “Conservation of momentum requires that if the ice is slowing down, it must be subject to an opposing/resistive force”
Paragraph beginning L352: Good idea, will do.
Ln 420: Point taken; we will host the scripts and include a link in the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-6300-AC1
-
AC1: 'Reply on RC1', Kallie Bohn, 23 May 2026
-
RC2: 'Comment on egusphere-2025-6300', Anonymous Referee #2, 08 Apr 2026
Review of: Ice dynamics in McMurdo Sound, Antarctica, based on precise synthetic aperture radar interferometry analysis
By: Bohn et al. for The Cryosphere
Manuscript No.: egusphere-2025-6300Summary & General Comments
This paper reports on the fast ice and ice shelf dynamics in McMurdo Sound (MIS) and their dynamical interconnections using high resolution SAR data from TerraSAR-X during the Austral summer season 2024/25. The authors apply InSAR using both ascending and descending repeat passes in combination with in-situ GNSS measurements to derive a full 2D velocity field of the fast ice and ice shelf. The velocity field is used to compute flow divergence and strain rates and, together with the interferograms, serves as a basis for the subsequent analysis and discussion. Further supporting material is provided by tide model data and Landsat data, the latter used to identify historic ice front positions and the timing of calving and fast ice break out. The discussion focusses on the coupling between the fast ice and ice shelf and some further noteworthy observations derived from the presented datasets. Based on this the authors conclude that the fast ice appears to have a significant stabilizing effect on the ice shelf along most of their boundary, except along the southern section. Possible future thinning of the fast ice is therefore expected to lead to enhanced calving.This is a well written, illustrated and referenced paper that in my opinion provides a valuable contribution to the Antarctic, sea ice and wider glaciology community. The authors processed a relative rare set of SAR observations of the fast ice/ice shelf in MIS to create a unique dataset that enabled the investigation of their coupling during a period of fast ice breakout. This topic is both interesting and highly relevant, as gaining a better insight into the interaction between fast ice and ice shelves or outlet glaciers is critical to assess the future stability of the floating and grounded ice upstream. The authors pay close attention to detail to produce a dataset that is both reliable and well-suited for the intended analysis. The discussion is interesting and to the point and makes clear that these types of datasets are of great relevance for exploring factors that are controlling changes on ice shelves in Antarctica and the future response of the ice sheet to climatic change.
I only found the introduction a bit weak and missing some crucial information. Adding some more background and context, focusing also on current knowledge gaps and how specifically the unique dataset derived in this paper contributes to that, could strengthen the paper (e.g. lack of high-resolution datasets capturing the coupling processes).
Below follow a few specific comments on text and figures, requiring some corrections, clarifications or additions, to help improve the manuscript.
Specific Comments
The line numbers refer to the numbers in the margin of the manuscript.• Ln 43: “where” → consider “whether”
• Ln 60 the McMurdo Ice Shelf (MIS) → I would use in this first sentence a more general wording, e.g. ice shelves, before focussing on MIS.
• Ln 62-68 A summary of InSAR applications for fast ice is provided, as this paper deals with both fast ice and ice shelves, this section would be more complete if ice shelves are also included in the overview.
• The introduction could be expanded a bit and include a short high-level description of the observations and methods that are being used in the study (TSX, GNSS etc), in fact I would move the intro of the Data and Methods section to the main introduction instead.
• It would be worth mentioning the rapid outbreak and subsequent calving in the introduction, as, together with the unique input dataset, they provide a valuable opportunity to investigate the coupling in greater detail. In this context, it would also be beneficial to refer to the Larsen B region, where recent fast ice breakout events appear to be linked to glacier speedup (e.g. Parsons et al., 2024; Surawy-Stepney et al., 2024; Sun et al., 2023; Ochwat et al., 2024)
• Ln 70: “We used a similar approach” → anything different or novel, apart from the region?
• Ln 77: “between fast ice and grounded ice“→ I assume between fast ice and ice shelves is meant here.
• Ln 78-80: The relevance of MIS for Antarctic fieldwork is noted; however, the connection to the work described here—while implied—is not explicitly stated, though it is mentioned in the Plain-Language Summary. (Ln 31-32).
• Ln 87: advancement → advance
• Ln 90: “The amount of calving and net retreat may be related to” → retreat or advance
• Ln 139: the the → remove duplication
• Ln 156: “However, as far as we know, this method has not yet been applied to fast ice.” → This is definitely interesting; it would be worthwhile to also highlight the complexity of obtaining results with this approach due to its stringent requirements.
• Ln 158: An an → remove duplication
• Ln 171: It would be nice to see the velocity field as an image in the figure, we only get to see the vector field as arrows in Fig. 5, probably losing a lot of the spatial details of the 15m map.
• Ln 174: “an update to the model described by Padman et al., 2002” → this is already mentioned in Ln 131-132
• Ln 175: “we calculated pairwise linear regressions of the timeseries for change in height.” → Since this paragraph describes a critical processing step, it would be beneficial to include a visualization of the time series and the regression results to provide supporting evidence.
• Ln 178: “an update to the model described by Padman et al., 2002” → see previous comment, this is already mentioned in Ln 131-132
• Ln 182: “the coast Hut Point Peninsula” → missing “of”
• Ln 184: “we were able to use our GNSS data to verify that the displacement is consistent between acquisition times” → see previous comment, it would be good to provide somewhere a plot of the time series.
• Ln 189: remove “for it”
• Ln 271: out → our
• Ln 274-275: “an update to the model described by Padman et al., 2002” → see previous comment, this is already mentioned in Ln 131-132
• Ln 317-316: “The ice velocity profile ...15 km” → I think a cross reference should be added here to Fig. 5b.
• Ln 322: Interferometric → no need to capitalize
• Ln 352: “Near the southern MIS edge, southeast of Station 1,” → would be good to point the pressure ridges out in the figure
• Ln 385: Out of curiosity, how far was station 3 from the front after the calving?
• Ln 401: Despite this → I would rather say “Because of this”, since there was little stabilizing effect from the fast ice, the break out of fast ice did not have major consequences in this section contrary to the other parts.
• Ln 417: before an after → before and after
• Ln 429: Soun → SoundGrammar, punctuation & style
Clearly written sentences, few typos. In some places one-sentence paragraphs interrupt the flow; consider combining them into more cohesive paragraphs (e.g. Ln 245-255).Figures
Overall, the figures are nice and very informative.
• Figure 2: The 03-06 line is not really visible, likely due to its overlap with 03-11, best to make it different colour as well. The dark blue lines do not really stand out very clear against the black ocean background.
• Figure 2: Most of what we see here is fast ice/sea ice moving away. Considering the amount of calving in the weeks before the image acquisition, I am surprised that there are barely any ice bergs visible. Have they all been transported out of view, can you perhaps explain?
• Figure 3a: There is no reference made to this figure in the text.
• Figure 3a & b: Probably best to merge these two figures into one and to enlarge it.
• Figure 5: formatting of units: use superscripts
• Figure 5: cm d-1 cm-1 → The more common notation is d⁻¹ rather than cm d⁻¹ cm⁻¹.
• Figure 6: Same as comments for figure 5.Further suggested references
Fraser, A. D., Wongpan, P., Langhorne, P. J., Klekociuk, A. R., Kusahara, K., Lannuzel, D., Massom, R. A., Meiners, K. M., Swadling, K. M., Atwater, D. P., Brett, G. M., Corkill, M., Dalman, L. A., Fiddes, S., Granata, A., Guglielmo, L., Heil, P., Leonard, G. H., Mahoney, A. R., . . . Wienecke, B. (2023). Antarctic Landfast Sea Ice: A Review of its physics, biogeochemistry and ecology. Reviews of Geophysics, 61(2). https://doi.org/10.1029/2022rg000770Ochwat, N. E., Scambos, T. A., Banwell, A. F., Anderson, R. S., Maclennan, M. L., Picard, G., Shates, J. A., Marinsek, S., Margonari, L., Truffer, M., and Pettit, E. C.: Triggers of the 2022 Larsen B multi-year landfast sea ice breakout and initial glacier response, The Cryosphere, 18, 1709–1731, https://doi.org/10.5194/tc-18-1709-2024, 2024.
Parsons, R., Sun, S., Gudmundsson, G. H., Wuite, J., and Nagler, T.: Quantifying the buttressing contribution of landfast sea ice and melange to Crane Glacier, Antarctic Peninsula, The Cryosphere, 18, 5789–5801, https://doi.org/10.5194/tc-18-5789-2024, 2024.
Sun, Y., Riel, B., & Minchew, B. (2023). Disintegration and buttressing effect of the landfast sea ice in the Larsen B Embayment, Antarctic Peninsula. Geophysical Research Letters, 50(16). https://doi.org/10.1029/2023gl104066
Surawy-Stepney, T., Hogg, A. E., Cornford, S. L., Wallis, B. J., Davison, B. J., Selley, H. L., Slater, R. A. W., Lie, E. K., Jakob, L., Ridout, A., Gourmelen, N., Freer, B. I. D., Wilson, S. F., and Shepherd, A.: The effect of landfast sea ice buttressing on ice dynamic speedup in the Larsen B embayment, Antarctica, The Cryosphere, 18, 977–993, https://doi.org/10.5194/tc-18-977-2024, 2024.
Citation: https://doi.org/10.5194/egusphere-2025-6300-RC2 -
AC2: 'Reply on RC2', Kallie Bohn, 23 May 2026
We are grateful for the thoughtful and constructive comments, almost all of which we agree will improve the quality of the manuscript.
Our responses, referenced by line number or figure (excluding typos, which we appreciate the help finding):
Specific Comments
Ln 43: Agreed, ‘whether’ works better here.
Ln 60: Agreed, this will be revised.
Ln 62-68: Agreed, we will add a few sentences and citations to cover InSAR applications for ice shelves in this overview.
We agree about expanding the introduction in this way by moving the first paragraph of Data and Methods to the introduction. The short second paragraph of Data and Methods will have its sentences moved to fitting places in the relevant Data and Methods subsections.
We agree that it is a good idea to mention the summer 2024-25 outbreak and calving and referring to the Larsen B region in the introduction, and are very grateful for the reference suggestions.
Ln 70: Good point, this should be more specific. Some elaboration will be added. In addition to our focus on an ice shelf—fast ice system instead of glacier tongue—fast ice, we are concerned with the changes in behavior over a single season rather than year-to-year, and lateral flexure and tidal flexure are not major factors for us. We also use higher-resolution data from TSX, and ground-reference data from the GNSS stations.
Ln 77: We use ‘grounded ice’ here as a general term for ice shelves and glacier tongues. To ensure this is clear, we will add a parenthetical earlier in the paragraph when the two are first mentioned collectively.
Ln 78-80: Good point. A similar statement to Ln 31-32 will be added here, eg “Therefore, better understanding the role of fast ice in stabilizing the MIS is highly valuable insight for the future of these operations.”
Ln 90: Agreed, the statement applies to both net retreat and advance, and will be revised accordingly. “The amount of calving and net retreat may be related to” → “The amount of net advance and net retreat may be related to”
Ln 156: Agreed, some elaboration will be added here. Studies of this kind are challenging due to the difficulty of validating the results with ground-reference; the necessity of overlapping asc and desc interferometric pairs; and the fact that the longer the difference between asc and desc acquisition times, the less accurate the results will be.
Ln 171: A map of the magnitude of velocity might be interesting, but we are unsure how valuable seeing the magnitude is without indication of direction. We will draft the figure and consider whether to include it in the main text or as a supplement.
Ln 174 (and 178, and 274-275): We agree that the repeated “(an update to...)” is awkward, but this is the in-line citation requested by the CATS2008 team.
Ln 175: While this step is important justification and we think including some visualization is a good idea, we do not consider it one of the main points of the paper, and therefore our plan is to include this as a supplementary figure.
Ln 184: Similar to Ln 175, we agree that this visualization would be valuable, but since this is not one of the main points of the paper, we might make this a supplementary figure rather than placing it in the main text.
Ln 317-319: Agreed, a Fig. 5b cross reference will be added.
Ln 352: Agreed, mention of the pressure ridges will be added. Eg “Near the southern MIS edge, southeast of Station 1, many pressure ridges are visible as a wave-like pattern in the divergence field. On the northwest side of this area of pressure ridges, and contacting the Southern MIS edge, was a line of strong convergence and noise”
Ln 385: Station 3 was recovered before the calving was complete, but the post-calving MIS front was approximately 20 m from the Station 3 cite.
Ln 401: Agreed, this revision will be made.
Grammar, punctuation & style
We will attempt to better integrate the one-sentence paragraphs.
Figures
Figure 2: We have decided to remove the 03-06 line, since the overlap with 03-11 makes it somewhat redundant, and this will reduce visual clutter. We think it would help to change the background to a pre-breakout image (such as in Fig. 3), which would make the blue fast ice extent lines clearly visible.
Good question on the lack of visible icebergs – I had fun looking into this. What we see in the background of Fig. 2 on 03-12 is actually new pack ice consolidating and moving into the embayment; the remains of the fast ice that broke out earlier in the season are long gone. I checked the sequence of Landsat images used to draw the lines, and as far as I can tell, the icebergs that calved from 02-02 to 03-11 were almost always transported out of frame within 2 or 3 days (or by the next clear image). Fig. 2 does show two shapes that are lighter than the nearby pack ice, which might be the final icebergs of the season, which are being kept in the embayment by the fresh pack ice that is consolidating (one is partially covered by the 01-30 line, directly to the right of the 01-30 label; another is partially covered by the first 0 in the 02-09 label; both are also visible more clearly in Fig. 1 on or near the 1963 line).Figure 3a: We will correct this oversight by adding least one reference, probably at least Ln 108 and/or Ln 120.
Figure 3a & b: Early versions of the figure did use a single panel, but we found that (in our opinion) zooming the background out far enough to clearly show the full lines between ice masses without the insets covering them, and/or making the insets smaller, led to a rather cluttered figure, and making the insets smaller also made them feel messy and difficult to read even when zoomed in closely on the image.
Figure 5 & 6: Units will be revised to use superscripts. Our rationale for using cm d-1 cm-1 was that we thought it was easier to understand the meaning as a unit of deformation, whereas d-1 is reminiscent of frequencies and waves.
Citation: https://doi.org/10.5194/egusphere-2025-6300-AC2
-
AC2: 'Reply on RC2', Kallie Bohn, 23 May 2026
Data sets
McMurdo Sound GNSS Files 2024/2025 D. Price et al. https://doi.org/10.26021/canterburynz.30768455
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 748 | 406 | 72 | 1,226 | 79 | 125 |
- HTML: 748
- PDF: 406
- XML: 72
- Total: 1,226
- BibTeX: 79
- EndNote: 125
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Review of Bohn et al - McMurdo Sound ice dynamics.
This manuscript is a well-written synopsis of the details of ice dynamics (both fast and ice shelf) in southern McMurdo Sound in late 2024. I was particularly interested to realise that this is the first InSAR fast ice paper to quantify the precise horizontal motion field of fast ice - thanks in part to fortuitous orbital placement of TerraSAR-X asc and desc tracks occurring within only a few hours here. This study maximises the information from TerraSAR-X using GPS stations and tidal models to constrain velocities to produce a very detailed interpretation of the velocity, divergence and strain of both the fast ice and the ice shelf.
I have only minor comments to report, and am otherwise very supportive of the publication of this manuscript at TC.
Minor comments, referenced by line number:
5: Abstract structure is a little unclear. It’s not clear why you used landsat etc to determines the MIS front position - it’s a little disconnected.
6-7: Although it became clear when reading the main text, originally I was wondering how InSAR was produced from ascending and descending data together, which is what is (unintentionally) suggested here. Reword for clarity (i.e. need to say that *pairs* of ascending and *pairs* of descending were used together to…).
15: Rather than telling us which Landsat platforms, it would be better to give dates.
70: Add a “Here,” before “We used” to clarify that you are referring to your present study.
81: Suggest to reword this section to “Study Area Description” since it goes beyond a simple study area definition.
84: Fraser et al., 2012 didn’t cover McMurdo Sound - suggest that you meant Fraser et al., 2021 instead?
90: I don’t think that this sentence belongs in this section.
Fig 2 caption: Typo “extend” -> “extent”. Do the blue lines represent the fast ice edge? Or the limits of the pack ice? Not clear. From this figure it looks like ST1 is in pack ice, so it would be good to let the reader know here that ST1, ST2 and ST3 are FYFI, MYFI and ice shelf respectively.
108: Brackets a bit messy.
124, 126: The precise reason for malfunction doesn’t seem so important here. Consider removing information.
141: Here would be a nice place to definitively say that you used 4 interferograms composed from 8 images, 4 asc and 4 desc.
169: An interesting comment on ablation. InSAR tests in other Antarctic regions have also yielded a loss of phase coherence in some limited regions, like what’s shown here. As far as I’m aware there is no published literature on the reason behind these coherence dropouts. I have always thought they were either ablation (e.g., wind scour of snow) or deposition of snow. You suggest here that ablation is the reason - do you have any evidence? It would be nice for you to comment on the possibility that such dropouts could be also from heavy snow, localised by nearby orography for example. Basically I’m not convinced that ablation is the only cause, and this could be a good opportunity to discuss a range of causes.
Around Fig 4: Could you state the angular separation here between an asc and desc pass? This would help quantify how complementary the two passes are here.
Fig 4: Could you annotate dates onto each row? You also don’t state that the background shading is phase coherence (I assume), which could be helpful to some readers.
176: What is LINZ? This one doesn’t appear in the reference list.
177: For 0.033, the units are m/m, right?
178: This is the 3rd (I later found there’s a fourth) mention of the CATS model update. I guess this is how your reference manager is adding the date, but it gets quite repetitive.
Fig 5a: Could you please annotate the ice boundaries like on Fig 6? I realise this might cover up some of the features you describe, so the authors may rightly push back on this!
271: “out” -> “our”
322: unnecessary capital i
327: I’m not familiar with this terminology “Adverse force” - could you perhaps define it here?
Paragraph beginning L352: Could you reference the figure number you’re discussing here?
420: “Soun” typo. Also it’s a little unusual now to only provide code upon request - consider hosting it?
Reference list: Meyer reference missing capital A in Arctic.