the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Feb 2025
On the seasonal variability of ocean heat transport and ice shelf melt around Antarctica
Abstract. The delivery of ocean heat to Antarctic ice shelves is due to intrusions of waters warmer than the local freezing point temperature. Changes in the supply of ocean heat will determine how rapidly ice shelves melt at their base, which affects Antarctic Ice Sheet mass loss and future global mean sea level rise. However, processes driving ice shelf basal melting are still poorly understood. Here we investigate the drivers of heat convergence along the Antarctic margins by performing an ocean heat budget analysis using a high-fidelity 4 km circum-Antarctic ocean–ice-shelf model. The simulation produces high basal melting in West Antarctica associated with sustained ocean heat convergence driven by advection of relatively warm deep water intrusions, with minimal seasonality in both heat supply and basal melting. For East Antarctica, ice shelves have substantial basal melt seasonality, driven by strong air-sea winter cooling over the continental shelf depressing shallow melting, while in summer, increased heat inflow towards the ice shelves is driven by surface-warmed waters that subduct under shallow regions of ice, increasing melt. The high seasonality of basal melting in East Antarctic ice shelves is responsive to interactions between the atmospheric forcing, the local icescape, and the activity of polynyas. Our results suggest that seasonal changes in future climate change scenarios are critical in determining the duration and intensity of air-sea fluxes with substantial impacts on ice shelf basal melting and ice sheet and sea level budgets.
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RC1: 'Comment on egusphere-2024-3905', Anonymous Referee #1, 17 Apr 2025
General comments
In this study the authors produce a pan-Antarctic heat budget using 4km ocean-ice shelf model. They focus on seasonality of this heat transport, with their main results being that ice-shelves in East Antarctica are subject to high seasonality, in contrast to those in West Antarctica. Overall I found this study interesting and easy to follow with clear enough conclusions. While I don’t have any particularly major comments, I do have a number of more minor, which I think require addressing before the manuscript be published.
Specific comments
- After equation 1, I suggest reminding readers that Q_sfc includes latent heat from imposed sea-ice melt/formation. Further, it was never quite clear to me if this also includes heat fluxes associated with ice-shelf melting. Stating whether or not this is case would be appreciated.
- I would suggest including, potentially just as a figure in the appendix, confirmation that the heat budget is closed/accurate. For the ice-shelf cavities, this would require the latent heat flux from melting of ice shelves. Has this been diagnosed in these simulations (from my comment 1, I wasn’t certain it’s part of Q_sfc)? Subtracting this from the timeseries in Fig 1c will lead to a measure of the temporal change in the heat content in the cavities, which can be used as a useful verification of the heat budget itself. If the latent heat flux from ice-shelf melting hasn’t been diagnosed, it could be estimated from the ice-shelf melting itself, but this would have small errors since this is also dependent on the local temperature field.
- L140: “The heat transport term integrated meridionally over the continental shelf describes the effect from the cross-slope heat transport”. There’s also a contribution coming from heat going into/out of the ice shelf cavities.
- L147. “The correlation between the annual mean heat convergence integrated meridionally over the continental shelf and within the ice shelf cavities is indeed low”. Is this the correlation between the black lines in Figs 1c,d? I would imagine advective timescales cause this correlation to be low. I would also imagine that considering lagged correlations wouldn’t help much since the lag would be location-dependent. I suggest adding a sentence describing such potential reasons for the low correlation, and how it doesn’t necessarily such a weak physical relationship between the timeseries.
- L154,155. Can it be clarified what this “spatial correlation” is referring to? My thinking is that it refers to the correlation between instantaneous maps of heat flux convergence in the ice-shelf cavity and ice-shelf melting, which is then averaged over time. Is this correct?
- L196. It’s stated that the grey sections in Fig 1b are based on high basal melt rates, but some areas with high basal melt are not included (e.g., near -20), and some grey regions have low basal melt (e.g., 1st and 8th grey section). Can the authors explain the reasoning for this?
- In section 4 there is some discussion surrounding the use of just one year of model data and associated limitations. I would suggest adding more discussion of how this can also limit confidence in the diagnosed seasonality.
Technical corrections
“Antarctic Ice Sheet” is capitalised in places, but not everywhere.
The phrase “ice-shelf” is hyphenated in instances of “ocean–ice-shelf”, but not elsewhere. I suggest sticking with a consistent choice.
L18. Change “impede” to “impedes”.
L23. Change “climate models outputs” to “climate model output”.
L30. Change “;neither some” to “, nor are some”.
L51. This paragraph repeats much the previous paragraph, e.g., that warm water ice shelves have mode 2 melting etc. This bit of the text could be made a bit briefer.
L79. Specify that these are ice-shelf thermodynamic interactions.
L101. Check the wording of this sentence.
L128. ‘Whereas…” This is not a full sentence.
Fig. 3e,f. Add to the caption the meaning of “<300m” and “>300m” in the legend.
Figs. 4,5. Can either panels b,e (or c,f) be edited to show basal melt anomaly in each season?
L280. Typo: “Totten ice Shelf”.
L289. Typo: “excerce”.
Citation: https://doi.org/10.5194/egusphere-2024-3905-RC1 - AC1: 'Reply on RC1', Fabio Boeira Dias, 26 Jun 2025
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AC2: 'Reply on RC1', Fabio Boeira Dias, 26 Jun 2025
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed on 27 June 2025.
Citation: https://doi.org/10.5194/egusphere-2024-3905-AC2
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RC2: 'Comment on egusphere-2024-3905', Anonymous Referee #2, 23 Apr 2025
Dias et al. present a study of continental shelf and ice shelf cavity ocean heat convergence around Antarctica, highlighting the seasonality of basal melt in East Antarctica in contrast to the more consistent high basal melt rates of West Antarctica. These findings are based on heat budget analysis in a high resolution circum-antarctic ocean-ice shelf model, WAOM.
Overall, the paper is well-written and clear and would make an interesting contribution to TC. However, I do have some concerns about the limitations of the model setup, including the lack of a sea ice model, that I think should be addressed with further discussion on the limitations/implications. I’ve listed general comments below followed by specific comments related to some interpretations and aspects of the text.
General comments:
- I have some concerns about the model representation that require further model evaluation/description of limitations of the model and how they may affect the results. For example, the T-S diagrams of Getz in Fig. 5 look quite strange and are not consistent with observations (such as in Dundas et al., 2022), so I worry about how representative the model results are. The T-S diagrams for Totten also look strange when compared with Khazendar et al. (2013).
- Interpretation of the surface heat fluxes is complicated by the lack of a sea ice model and the sea surface temperature relaxation applied. I’d appreciate some clarification on:
- The impact of the monthly relaxation to SOSE SSTs on the key results
- How realistic are the surface heat flux estimates given that the model runs with sea surface temperature correction/restoring? How do you distinguish the regions with strong seasonality from the regions that just require strong sea surface temperature restoring (possibly due to model drift)?
- Looking at the heat budget figures for each of the different ice shelf regions in the appendix, the main signal is essentially whether or not there is sea ice cover and I’m wondering to what extent that signal is present due to the way sea ice is treated in this model (mentioned a bit in line 218-222
- The link suggested between surface fluxes and bottom temperature changes (cooling effect of polynyas) is not very clearly demonstrated by the figures. Similarly, the links between surface fluxes and inferring what happens in the cavities mechanistically isn’t very solid. I think some further analysis or maybe clearer demonstration would help support this point.
- The splitting of the heat fluxes into which components dominate is an important portion of the study, but is occasionally challenging to follow because of all the acronyms and nuances to the interpretation due to the model setup. For example, in line 160-175 I find the descriptions of the different components a bit difficult to follow. I’m not sure what the best solution is, but you could consider adding either another equation that shows each of these components (or annotating Eqn. 1), or a table/diagram as a visual aid to follow the heat budget components.
Specific comments:
- Since the focus of the paper is on basal melt, I’d suggest specifying basal melt in the title.
Introduction section:
- Line 33: “A leading source of the uncertainty is due to the differences between ice sheet models” Please include specifics for the differences between ice sheet models you are referring to.
- Overall, the introduction is strong (especially the second paragraph!) but it would benefit from some links for the 3rd and 4th paragraphs with the rest as they sort of stand alone.
Methods section:
- Line 82-85: Consider rephrasing or re-ordering these sentences as it took a couple of re-reads to get the method for initialisation and that there is no interannual variation in forcing.
- Line 93: I suggest splitting the paragraph here to create a separate paragraph focussed on the key points of the model evaluation that you highlighted and expanding on it to provide further context for your findings (for example to address the general comment about water mass representation).
- Line 105: Heat convergence equation. Should this also include a term of artificial heat addition/removal due to the monthly relaxation to SOSE surface temperatures or is that already included in the net surface heat flux term? If it is included in the surface heat flux term, it would be good to clarify that here.
- Line 115:
- Why is only the last year of the simulations used instead of the monthly mean climatology over the full set? Are the earlier years still responding to the change in conditions, so effectively spin up? If that is the case, what evidence are you using to suggest that the results are no longer dependent on the initial state at the end of spin up?
- An additional thought, if the 10 year period is not spin up: an improvement to indicate uncertainty in the heat fluxes would be to edit the figures to include solid lines based on the model mean and shading to indicate the range of basal melt rates / heat convergences over the multiple years of running the model.
Results section
- Line 127: “A clear seasonally cycle is found in some but not all.” Suggest specifying that you are referring to a seasonal cycle in basal melt.
- Line 128-137: The Bellingshausen Sea is a notable exception that requires clear comment here (mentioned later on), otherwise it seems like the classification is fitted to match the east-west divide rather than coming directly out of the findings
- Figure 1: Great figure overall and I really appreciate the region definitions shown here! The caption says “green in panel b”, maybe this is my monitor, but I only see orange and yellow for the steady and seasonal regime; consider using a different color?
- Line 142: Possibly I’m misunderstanding something, but based on the heat convergence calculation, I don’t think you can distinguish offshore cooling from cavity melt related cooling. Is that true? If so, it would be good to add a comment here to help interpretation.
- Line 153: “heat convergence over the continental shelf does not exhibit a clear seasonality” Suggest replacing clear with consistent, since in some places there does seem to be a strong seasonal difference.
- Fig. 2:
- Panel (b) shows the strongest magnitude of seasonal difference between summer and winter on the continental shelf in West Antarctica, whereas the NET seasonal difference is smaller and varies less in East Antarctica. I think this requires some further interpretation/discussion in the text as it could contradict the argument of strongest seasonality in East Antarctica that you are presenting.
- Can you comment a bit more about what’s going on around Fimbul (0 longitude) in the ice shelf cavity transport in Fig. 2?
- In this figure and the following figures, it would be helpful to have the shading of named regions as in Fig. 1. I recognize it’s involved, but I think it would really assist interpretation.
- For the caption, please write out the TW definition as in Fig. 1.
- Line 206-208: Does the model actually capture/represent coastal waves? If not, this shouldn’t be listed as a driver of behaviour in the model. The other suggested processes are also lacking evidence to support their dominance, other than the high temporal variability, so it would be strengthened with more support.
- Line 214: The phrasing of advective heat convergence here is a bit misleading because the heat convergence can be both positive and negative (negative being essentially heat divergence). I agree that the advection term dominates the heat flux, but this sentence can be misinterpreted to mean that you see consistent heat convergence with a positive sign throughout the year, which isn’t supported by Fig. 3, so I’d suggest rephrasing.
- Line 226-228: What is the evidence for the proposed mechanism of spreading of wintertime cooled water? Fig. 4 is a very helpful figure overall, but it does not provide a very convincing link between the surface fluxes and the bottom temperature conditions and this is a key point in the results/discussion.
- Fig. 4: It is hard to see differences in spatial distribution and magnitude of melt rates in this figure. To more clearly demonstrate this point, I’d suggest visualising seasonal melt rate anomaly in panels b/c, e/f and the mean melt rate in panel a.
- Line 237-238: Is the dominance of mCDW below 300 m inconsistent with the overall argument of the difference between East and West Antarctic melt regimes? Please comment.
- Line 250-255: The flow in this region is also strongly influenced by the cross-shelf density gradient, which sea ice and seasonal surface conditions do play a role in. So, the general idea of correlating polynya location with bottom temperatures captures one aspect of changes, but is certainly not comprehensive. I’d suggest adding a comment on other factors that may contribute.
Discussion and conclusions
- Line 287-289: These sentences have some typos and reference formatting issues.
- Line 290-291: “giving confidence that the model captures relevant processes, such as the landfast ice effect on coastal polynya locations.” The model specifically can’t capture the actual processes, just the effect, because it does not have a sea ice model. Suggest removing or clarifying.
- Line 295: Does the seasonality of warm mCDW intrusions in Prydz Bay conflict with the argument of mode 3 melting suggested in this region? If so, I think it’s important to more clearly identify this difference and comment on the reasons.
- Line 300: “Most of this observed seasonality points to similar mechanisms as described”. Please specify the similar mechanisms you are referring to.
- Line 312-318: You might also want to consider the findings in Haigh & Holland (2024) for the role of sea ice on decadal variability in the Amundsen Sea in this portion of the discussion.
- Line 324: “The Bellingshausen Sea is the only region which holds substantial differences” Sudden transition from the previous paragraphs, consider linking to the previous paragraphs for continuity.
- Line 330: Need a link as to why you’re mentioning the differences in steady versus seasonal melting regimes between model resolution and what it means for the findings. Does it mean that it’s uncertain what the regime is or that high resolution scale processes dominate the seasonal behaviour etc.
- Line 331: “These results show how shallow”. Specify which results you are referring to.
- Line 353-354: Given the role of wind in CDW transport on the shelf in West Antarctica, could that be why this is the dominant mode in your study?
- Line 374: SAM introduction is a bit sudden since it isn’t mentioned in the discussion. I’d suggest either bringing it up earlier or removing the mention here.
References
Dundas, V., Darelius, E., Daae, K., Steiger, N., Nakayama, Y., & Kim, T. W. (2022). Hydrography, circulation, and response to atmospheric forcing in the vicinity of the central Getz Ice Shelf, Amundsen Sea, Antarctica. Ocean Science, 18(5), 1339-1359.
Haigh, M., & Holland, P. R. (2024). Decadal variability of ice‐shelf melting in the Amundsen Sea driven by sea‐ice freshwater fluxes. Geophysical Research Letters, 51(9), e2024GL108406.
Khazendar, A., Schodlok, M. P., Fenty, I., Ligtenberg, S. R. M., Rignot, E., & Van Den Broeke, M. R. (2013). Observed thinning of Totten Glacier is linked to coastal polynya variability. Nature Communications, 4(1), 2857.
Citation: https://doi.org/10.5194/egusphere-2024-3905-RC2 - AC3: 'Reply on RC2', Fabio Boeira Dias, 26 Jun 2025
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