the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
High-end projections of Southern Ocean warming and Antarctic ice shelf melting in conditions typical of the end of the 23rd century
Pierre Mathiot
Nicolas C. Jourdain
Abstract. How much Antarctic ice shelf basal melt rates can increase in response to global warming remains an open question. Here we describe the response of the Southern Ocean and Antarctic ice shelf cavities to an abrupt change to high-end atmospheric conditions typical of the late 23rd century under the SSP5-8.5 scenario. To achieve this objective, we first present and evaluate a new 0.25° global configuration of the NEMO ocean and sea ice model. Our present-day simulations demonstrate good agreement with observational data for key variables such as temperature, salinity, and ice shelf melt rates, despite remaining difficulties to simulate the thermocline and melt variability in the Amundsen Sea. The ocean response to the high-end atmospheric perturbation includes a strengthening and extension of the Ross and Weddell gyres and a quasi-disappearance of sea ice, with subsequent decrease in production of High Salinity Shelf Water and increased intrusion of warmer water onto the continental shelves. This induces a substantial increase in ice shelf basal melt rates, particularly in the coldest seas, with a total ice shelf basal mass loss rising from 1,180 to 15,700 Gt yr-1 and an Antarctica averaged ice shelf melt rate increasing from 0.80 m yr-1 to 10.64 m yr-1. In the perturbed simulation, most ice shelves around Antarctica experience conditions that are currently found in the Amundsen Sea, while the Amundsen Sea also warms by 2 °C. These projections can be used as a base to calibrate basal melt parameterisations used in long-term ice sheet projections.
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Pierre Mathiot and Nicolas C. Jourdain
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1606', Kaitlin Naughten, 21 Jul 2023
General comments
“High-end projections of Southern Ocean warming and Antarctic ice shelf melting in conditions typical of the end of the 23rd century” by Mathiot and Jourdain presents a sensitivity study of the Antarctic continental shelf under extreme climate change conditions. A global ocean-sea ice model with ice shelf cavities is forced with high-end projections of the 23rd century, and the response of Antarctic ice-ocean processes is analysed. The already warm regions of West Antarctica warm further, but a much larger contribution to total mass loss comes from the currently cold ice shelves which tip into a warm state.
I very much enjoyed reading this study and my suggested revisions are all minor. It has a nice balance between building on previous work (eg expansion of the Ross Gyre, tipping of the FRIS cavity into a warm state) and exploring uncharted territory by warming the entire continent outside the bounds of what has been tested before. The new configuration of NEMO is also a major advance, and in places the tuning choices need more explanation (see my specific comments below). The processes responsible for warming and ice shelf melting in each sector are only explored briefly, but this is probably appropriate given the circumpolar approach and the references to previous work. I hope that future work will build on these simulations by analysing the sector changes in more detail and using the results to drive ice sheet models.
I feel the paper could do more to position the simulation as an idealised change or hypothesis test, rather than an outcome which is plausible for the future. Between the fossil fuel scenario, the time frame, and the high sensitivity climate model used for forcing, this is an extreme upper bound for what we might expect in the real world. The uncoupled atmosphere and ice sheet also introduce substantial uncertainty, as well as the step-change nature of the forcing. This simulation is still very useful for our theoretical understanding of Antarctica, but I would hesitate to consider it a “projection”.
There is very little discussion of the Amery Ice Shelf, but from the figures it appears to experience the same mechanism of tipping as the Ross and FRIS. If this is the case, it is the first simulation of Amery tipping to my knowledge, and this warrants more attention in the text.
The paper needs more discussion of glaciological implications, perhaps at the very end. Which marine basins of Antarctica would be threatened by these changes (all of them?), and what combined sea level equivalent could be at risk from basal melting? Do we have any idea of the timescale of response? Of course the details cannot be answered by the current study, but some exploration of the implications would be welcome. A brief discussion of potential feedbacks between ice sheet geometry and the ocean state would also be suitable here, as a very retreated ice sheet would surely change the total melt flux.
Specific comments
Title: change “typical of” to “possible by”. How can we say what is “typical” of a time period that hasn’t happened yet?
Line 4 (abstract): change “typical of” to “projected by”, for the same reasons as above.
Lines 16-23: The first paragraph of the introduction needs a bit more fleshing out. How do ice sheet models infer basal melting from climate simulations (I understand there’s a few different approaches, eg nearest neighbour SST or averaging over the continental shelf), and why are these the wrong processes? The casual reader would probably not follow this as written.
Line 43: Can you summarise in 3 words what this bug related to? The current text sounds a bit alarming, and not all readers will go and track down the ticket.
Lines 52-54: Thinning the Getz is an unusual way to compensate for a high melt bias. Is the Getz draft poorly constrained by data, which could somewhat justify this choice?
Lines 60-67: What is the physical justification or reason for changing the slip condition and bottom friction around the Antarctic Peninsula?
Line 95: Add “currently” before “negligible” as surface runoff will surely not be negligible in the extreme scenarios considered later.
Lines 97-98: The freshwater flux correction needs a bit more explanation and justification for readers unfamiliar with the model configuration. Why was this necessary?
Line 107: How is the SSP5-8.5 scenario extended beyond 2100? I expect it has a sustained level of very high fossil fuel emissions - is this even possible given available fossil fuel reserves?
Line 110: Presumably there is a trend in simulated global climate over 1979-2018. How does repeating this period influence the simulation?
Figure 1: I struggled to interpret the zonal wind changes visualised in panel e), especially the negative values on the continent. Perhaps anomaly vectors, and/or plotting the reference state, would help.
Line 163: Change “requires” to “would require” to make it clear that this iceberg and fast ice physics does not exist in this version of NEMO.
Figure 4: Adding a third column of anomaly panels would make it easier to identify the model biases in temperature and salinity.
Lines 207-213: This short section should be expanded, to explore the possible reasons for underestimated variability. Does your bathymetry consider grounded icebergs on Bear Ridge (which Bett et al. 2020, doi:10.1029/2020JC016305 found was crucial to simulate colder conditions in the western Amundsen Sea)? Perhaps the polynya activity is insufficient, or the mixed layer salinity is biased low?
Lines 233-235: Summarise why an expanded Ross Gyre leads to a much warmer Amundsen Sea than local changes in onshore transport and modification, for those readers who are not familiar with the Gomez-Valdivia study.
Lines 266-268: Siahaan et al. had a much coarser resolution, which could explain their weaker response of Ross melt rates.
Lines 269-273: One key point this discussion is missing: the Amundsen sector ice shelves have much smaller area, so even with very high melt rates they cannot contribute much to total mass loss compared to the large cold-cavity ice shelves becoming warm.
Line 275: Does the refreezing weaken over time, with a view to eventually disappearing? Or does the refreezing increase as melt rates increase?
Lines 276-277: The results from Naughten et al. (2021) are more similar than the authors imply; both studies simulate a factor of ~20 increase in FRIS mass loss, although their absolute values (both initial and final) differ.
Line 288: Again, the word “typical” seems inappropriate here.
Technical comments
Line 62: typo in Northern
Line 211: typo in 2005
Line 213: typo in Dotson
Citation: https://doi.org/10.5194/egusphere-2023-1606-RC1 -
RC2: 'Comment on egusphere-2023-1606', Anonymous Referee #2, 26 Jul 2023
This paper, by Pierre Mathiot and Nicolas Jourdain, uses a ¼° version of NEMO, with various alterations compared with previous versions of the same model to improve the present-day climatology, forced by surface forcing from a high-end scenario in the late 23rd century. The goal of this study is to understand how ocean conditions change under such extreme forcing and the likely impact on ice shelf melt rate, which is likely to ultimately impact ice sheet mass loss and sea level rise. The model and experiments are thoroughly described and the results are interesting, although the authors correctly note that this is a highly idealised scenario and the model lacks key components of the earth system response (principally the lack of interactive ice sheets and ice shelves) that would be expected to alter the ocean conditions. I have the following suggestions to improve the manuscript:
Section 2.1: As noted on line 136, other configurations of NEMO and similar models at ¼° are unrealistic. Therefore, it would be useful to have a summary of the key changes made in this configuration that led to the improved climatology. I understand that the authors are unlikely to know the impact of every change, but it would be useful to have some discussion of the changes that are likely to have made a large improvement to the representation of the present-day conditions.
L52-54: Please justify the reasons for thinning Getz ice shelf: why this ice shelf and not others, and how could this approach be justified? How big a difference did this single change make compared with other tunable parameters in the model?
L62: What is the motivation for (and impact of) applying a no-slip condition around the islands near the Antarctic Peninsula?
L82-83: Please clarify this sentence about the calving pattern: I don’t understand what this means, nor its significance, and I expect many readers will also be confused here.
L95-96: Surface runoff from Antarctica could become regionally important in such a high-end future scenario, which should be noted here.
L96-98: Why is this freshwater flux correction needed? What other errors in the model is this compensating for? Might these issues undermine the realism of the future scenario?
L111-112: Although the anomaly method will correct a part of the model biases, I find it hard to believe that model biases will not affect the projected changes, especially under such an extreme scenario. Therefore, it would be useful to briefly summarise the performance of the IPSL-CM6A-LR model in this region, and consider whether there are processes that are poorly represented that may affect the realism of the surface forcing projections.
L190-191: Clarify this: are you suggesting that the updated Thwaites ice draft should improve the simulated melt rates? Or that it might have inadvertently caused larger biases?
L191-192: Please clarify this statement. The ice shelf draft was reduced by 200 m to counteract a longstanding bias that was producing excessive melt rates (section 2). Here, it would be useful to repeat that the ice shelf draft was reduced by 200 m. Furthermore, it would be useful to qualify the statement that this correction was too strong: I assume that the melt rates agree better with observations than before the correction was made? Is the implication that future studies using this model could apply a similar but smaller change to the Getz ice shelf draft?
Section 4.1: Could you give some indication of the likely reasons why the gyres intensify and increase in extent? Is this consistent with changes in ocean surface stress curl in the projected future forcing, as suggested by Gomez-Valdivia et al. (2023)? Many readers won’t have read that reference, so at least discuss this possibility.
L230-231: Can the slow trend really be “explained by slow changes in deep ocean properties at the global scale”? 100 years seems like a short time for such global deep ocean changes to be manifest. Instead, it seems more likely that the deep changes in all these locations are generated by changes in deep water source regions driven by changes in the deep circulation around Antarctica. Perhaps just replacing “global” with “circum-Antarctic” would be better?
L237-242: It is understandable that you can’t diagnose all the mechanisms, but this discussion still feels unsatisfying and like a list of possible mechanisms. Several of these mechanisms are very region-dependent, so it would be good to split this paragraph into coherent groups of regions (as done in the final paragraph of the conclusions). The relatively uniform warming might help to diagnose the most important mechanisms. For example, the currents along the shelf break are likely to be very different, and these currents are strongly linked to the supply or blocking of CDW onto the continental shelf. Figure 8 implies that these currents have changed, but it would be useful to plot the differences. It is not clear to me that the removal of sea ice and the subsequent freshening down to 400 m and deeper would lead to warming except in regions of HSSW production. Another mechanism to consider is whether the increased melt rates directly increase the overturning circulation on the continental shelf and thus help to bring more warm water onto the shelf?
L249-251: What reasons might explain this difference? Is this just an area of model uncertainty? Similar question for L 274-278
L261-262: Presumably the increase in melt rate at Getz is over-estimated due to the artificially-thinned ice shelf draft?
L295: While this is a very good summary overall, it would be useful to at least speculate on how these results will impact the retreat of the ice sheats and how this might in turn influence the ocean circulation.
Typos etc:
L35, L154, L197, L234: Check parentheses around citations. In some places they should be added, in others they should be removed.
L89: “equation” should be plural (equations)
L163: Fasten -> fastened?
L176: “to the exception of” -> “with the exception of”
Citation: https://doi.org/10.5194/egusphere-2023-1606-RC2 -
EC1: 'Comment on egusphere-2023-1606', Karen J. Heywood, 02 Aug 2023
You have two positive and constructive reviews that make helpful suggestions to further strengthen your manuscript. I encourage you to respond now to each of these reviews in the open discussion - the reviewers may respond if the paper is still in discussion. The discussion phase will end on 11th September; you may receive further comments from the community during this time. After the discussion closes, you will be invited to upload a revised manuscript and final responses (which may be updated from the responses you post online in the discussion phase). I look forward to reading your revised paper.
Citation: https://doi.org/10.5194/egusphere-2023-1606-EC1
Pierre Mathiot and Nicolas C. Jourdain
Pierre Mathiot and Nicolas C. Jourdain
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