the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Nov 2023
Oceanic gateways to Antarctic grounding lines – Impact of critical access depths on sub-shelf melt
Abstract. Melting underneath the floating ice shelves surrounding the Antarctic continent is a key process for the stability of the Antarctic Ice Sheet and therefore its current and future mass loss. Troughs and sills on the continental shelf play a crucial role in modulating sub-shelf melt rates, as they can allow or block the access of relatively warm, modified Circumpolar Deep Water to ice-shelf cavities. Here we identify potential oceanic gateways that could allow the access of warm water masses to Antarctic grounding lines based on critical access depths inferred from high-resolution bathymetry data. We analyse the properties of water masses that are currently present in front of the ice shelf and that might intrude into the respective ice-shelf cavities in the future. We use the ice-shelf cavity model PICO to estimate an upper limit of melt rate changes in case all warm water masses up to a certain depth level gain access to the cavities. We find that melt rates could increase in all regions at least by a factor of 2. Depending on the presence or absence of an oceanic gateway and the current ice-shelf melt conditions we find up to 200-fold larger melt rates. The identification of oceanic gateways is thus valuable for assessing the potential of ice-shelf cavities to switch from a 'cold' to a 'warm' state, which could result in widespread ice loss from Antarctica.
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RC1: 'Comment on egusphere-2023-2583', Anonymous Referee #1, 07 Jan 2024
Nicola et al. use the BedMachine bathymetry product and ISMIP6 dataset (observations + extrapolation of temperatures (T) and salinities (S) around Antarctica and its continental shelf) to discuss some features of these datasets, for example the relevance of different troughs in delivering heat to the grounding lines (for BedMachine) and the spatial differences in ocean temperatures and salinities (for ISMIP6 dataset).The authors then calculate ice shelf basal melt rates using the box model PICO, where the T/S input is a) conditions at the calving front and b) conditions at the continental shelf. The latter is presented as the "upper limit of melt rate changes".
General:
I have to admit that I have struggled with the aims and the novelty of this manuscript. The first half of the paper introduces new terminology of 'oceanic getaways' and 'critical access depths' but it is really just talking about bathymetric features, specifically troughs that have received a lot of attention in the past decades as the sea floor around Greenland and Antarctica have become better mapped. The same flood-fill algorithm that the authors use here was also used to produce and extrapolate the ISMIP6 dataset, so I don't think that part is anything new. The next bit of the paper that discusses ocean properties at the calving front vs the continental shelf doesn't do much more than stating the differences in the fields in ISMIP6 dataset and some generalities.
The section "Oceanic gateways to major Antarctic ice shelves" is a mixture of literature review, and speculation about potential impacts of high melt rates (as calculated by using shelf break instead of calving front temperatures) - but the impacts are not modeled here so just a brief mention in discussion would be enough for that part. For the literature review - a lot of this refers to studies about ocean circulation beneath the specific ice shelves, but all that is ignored in PICO, so I don't see why that is reviewed here, since the box model doesn't know anything about horizontal circulation. Other references serve to show that bulk present day PICO melt rates are reasonable, but that was already tuned elsewhere in previous publications, so not sure why that is necessary here again.I think that especially with the simple box model it is really easy to produce large increase in melt rates for any given ice shelf, all that is needed is a change in input temperature that is fed into box 0. Since there are no oceanic processes accounted for that would be allowed to mix or divert away this change, the model essentially by construction contains a tunnel that conducts outer T and S directly to the grounding line. So the result that large change in input (which is chosen here but not really physically justified) causes large change in output, which is the result of this manuscript, is definitely not a surprising one. The question is whether the numbers produced here for the increased melt rates are realistic or otherwise useful in some way. I don't think the authors have even tried to make a case for either usefulness or realism of these high melt rates. The only argument that was provided in the paper is that this approach here is "straight-forward and easy to run" but without it being realistic or useful, simplicity on its own, is not enough of an argument for publication.
A general characteristics of this paper is that the authors state assumptions but don't justify them. A good example of an unjustified assumption is the one that grounding lines are always accessed via 'prominent getaways' - that clearly doesn't hold in present day for the cold ice shelves Ross and Filchner-Ronne and others, yet this inconsistency is not at all addressed. Also there is some misuse of terminology. For example the temperature the authors have chosen for the CSB T and S is quite arbitrary, yet they call their perturbed melt rate result "upper limit". Surely not everywhere is this arbitrary point the temperature max along the shelf break, so even higher melt rates could be reached with PICO. I don't think the term warm-water intrusion is accurate for the use in the context of a long term, large scale and lasting change. Intrusion is an intermittent oceanographic feature. The sensitivity study here assumes that oceanographic conditions within the cavity change, that is warm water from the open ocean comes across the continental shelf break and stays and that is something very different and more difficult to establish than an intrusion which would largely mix in with other water masses on its way to the grounding line and become much cooler and fresher by the time it comes in contact with the ice.
The PICO model has some clear biases compared with the observations of Adusumilli et al. 2020, namely it overemphasizes a melt rate pattern of high melt at grounding line and low melt towards the front and does not take into account the 3D structure of the circulation, which results, for example, in omitting mode 3 melting features near ice shelf fronts. Accordingly, the melt rates 'assuming warm water intrusion' have the same biased melt rate pattern as the original PICO melt rates except now the melting is higher. Can you comment on the bias and its implications? For example in the context of Reese et al 2018 - if grounding line are most sensitive to melt rate change, overestimating melting there could be problematic, yet it is probably happening since the bulk melt rates are tuned to agree with observations and mode 3 is absent - resulting in freezing or low malt rates near the front - positive bias in grounding line melt rates is clearly visible in sectors 3-5.
Specific:
Access depth seems to be a key concept here but it is not clearly defined (I think the language is the problem). Figure 1 doesn't help - it is stated in the text that access depth is a field defined everywhere (and provided on a certain discrete grid) but the figure only points to a single point in the image, which is confusing. Further on Fig one - what is the -1800 m in the image showing horizontal distance? Shouldn't that be depth for the purposes of your continent definition?
Similarly g is not clearly defined. From the paper it is sort of clear what the authors mean from the context but that is relying on the reader being on board with the writers.
Sign convention of z vs depth needs to be consistent.
Fig 7 and similar - x axis needs to be labeled on each subplot to make clear what distance is meant for each case
other specific comments are in the attached pdf
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AC1: 'Initial reply to RC1', Lena Nicola, 02 Feb 2024
We thank the reviewer for carefully reviewing our manuscript. In the following, we provide a general comment to some of the key points raised by the reviewer. We will provide a more detailed (point-to-point) response to all comments and a revised manuscript once we have received all reviews.
One main point is raised on the aims and scope of the manuscript being not sufficiently clear. We will address this by:
1) highlighting the novel aspects, i.e., including ice sheet vulnerability into the flood-fill assessment, and
2) clarifying the assumptions of the study, e.g., for the upper limit assessment (now called upper bound to be more precise), and
3) streamlining the manuscript, e.g., avoiding to repeat previously published methodologies such as of the ISMIP6 ocean climatology, removing not directly relevant literature review, adjusting terminology (intrusions and others mentioned).We will include 3) in the revised manuscript. As one point, we want to note that we will rename the “upper limit” to “upper bound” to be more precise in our language.
On the novelty (1): our study extends the work by the ISMIP6 focus group to create the ocean climatology dataset, since our approach takes into account the depth of the grounding line. Access depths can serve as a measure for the degree of connectedness of the grounding lines to the open ocean and thus for the vulnerability of ice sheets to ocean waters off the continental shelf (for clarity, we added a new explanation of how we derived the access depths in the attached PDF and include two animations in this zenodo archive https://zenodo.org/records/10599774). To our knowledge, no study has so far assessed the deepest possible bathymetric access to the relevant grounding lines for every Antarctic basin. This approach can be updated whenever new data becomes available. As a next step, we assess an upper bound on sub-shelf melt changes, which is also a new approach to our knowledge.
Some methodological concerns (2) were raised with respect to the approach:
We here use the cavity module PICO, a simplified approach to calculate sub-shelf melt rates based on the ambient ocean properties, combining it with the oceanic gateways analysis. PICO has proved highly-useful to fill the gap between high-resolution ocean models that fully resolve the dynamics in the ice-shelf cavity and more simple parameterisations (see e.g. Beckmann and Goosse 2003, applied in Martin et al., 2011, Pollard and DeConto 2012), especially in longer simulations (Kingslake et al., 2018, Garbe et al., 2020, Albrecht et al., 2020a,b, Reese et al., 2023).
PICO does not include horizontal ocean circulation, modification of water masses on the continental shelf or blocking of water masses entering the continental shelf nor mode 3 melting, which might bias melt rates in cold cavities at the moment as mentioned by the reviewer. Furthermore, PICO smears out melting, melt rates are more distributed than in ocean circulation models, and PICO does not reach the very high melting found close to grounding lines (Dutrieux et al., 2014, Paolo et al, 2015). The relevance of this for ice sheet model studies needs to be further assessed (some first analyses were done in Reese et al., 2018b). A recent study suggested that bulk melting is more relevant than spatial patterns for the small, constrained Pine Island Glacier Ice Shelf (Joughin et al., 2021). The question if bulk melting or the melt pattern is more relevant, is not resolved yet, but our study does not aim to estimate this and we would refer hence to future work.Despite these limitations, we want to stress that melt parameterisations such as PICO are essential for large-ensemble studies or long-term studies that cavity-resolving ocean circulation models cannot cover due to computational costs; they will thus serve an important purpose also in future ice-sheet model simulations and projections.
One advantage of the approach presented in our manuscript is to improve the input for such models (as we show, using the access depth allows us to match present-day melt rates as well as estimated sensitivities of melt rates to ocean temperature changes (from Reese et al., 2023) without the need to adjust underlying input temperatures). This was a priori not clear, and is an important improvement that such a (unphysical) modification is no longer required.
For available observational data, e.g., by Schmidtko et al., 2014, previous studies (e.g. Reese et al., 2018a, Albrecht et al., 2020a.b) assigned a temperature and salinity estimate by averaging over water masses sitting just above the continental-shelf seafloor. The overturning strength and turbulent exchange velocity parameters were then fitted to obtain present-day averaged melt rates. Recognising the importance of the sensitivity of melting to ocean temperature changes for making sea level projections, the latest PICO tuning aimed at optimizing/tuning for melt sensitivity, which unfortunately required temperature corrections in some basins (Reese et al, 2023). Here we show that with the new input water mass specification, we can obtain both, sensible melt sensitivity to ocean temperature changes as well as avoiding an unphysical modification of the box 0 water masses. We hope that this approach proves useful also for other melt parameterisations in the future.Furthermore, we think that our approach to assess an upper bound of melt rate changes with PICO which assumes the direct “tunneling of ocean water masses through the oceanic gateways without any modification” is justified for our application here: ocean circulation models show that access of warm water masses offshore occurs through exactly these bathymetric features (e.g., through Filchner trough, Hellmer et al., 2012; Naughten et al, 2021). Furthermore, assuming water masses are not modified is in line with the upper bound approach as modification would act to reduce temperatures. We agree with the reviewer that the choice of maximum temperatures is a bit arbitrary. We will adopt their suggestion to use the maximum temperature, and consider calculating this not only on the continental shelf break, but generally the maximum water temperature we can find at the critical access depth within the basin.
We would be pleased to strengthen our manuscript based on the points above, and including of course also any further changes the reviewer(s) suggested or might still suggest.
References not yet included in the submitted manuscript:
- Albrecht, Torsten, Ricarda Winkelmann, and Anders Levermann. "Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM)–Part 1: Boundary conditions and climatic forcing." The Cryosphere 14.2 (2020): 599-632.
- Albrecht, Torsten, Ricarda Winkelmann, and Anders Levermann. "Glacial-cycle simulations of the Antarctic Ice Sheet with the Parallel Ice Sheet Model (PISM)–Part 2: parameter ensemble analysis." The Cryosphere 14.2 (2020): 633-656.
- Beckmann, A., and Hugues Goosse. "A parameterization of ice shelf–ocean interaction for climate models." Ocean modelling 5.2 (2003): 157-170.
- Garbe, Julius, et al. "The hysteresis of the Antarctic ice sheet." Nature 585.7826 (2020): 538-544.
- Joughin, Ian, et al. "Ocean-induced melt volume directly paces ice loss from Pine Island Glacier." Science advances 7.43 (2021): eabi5738.
- Kingslake, J., et al. "Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene." Nature 558.7710 (2018): 430-434.
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AC1: 'Initial reply to RC1', Lena Nicola, 02 Feb 2024
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RC2: 'Comment on egusphere-2023-2583', Erwin Lambert, 26 Feb 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2583/egusphere-2023-2583-RC2-supplement.pdf
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RC3: 'Comment on egusphere-2023-2583', Anonymous Referee #3, 29 Feb 2024
General comments:
Nicola and coauthors identify key oceanic pathways in the major Antarctic glacier basins and estimate current and future melt rates based on the present water properties, assuming that warm waters from the shelf break reach the grounding lines. I found it interesting to see the range of access depths and ocean water properties around the Antarctic Ice Sheet in one simple figure, and it is interesting to quantify the upper limit of future melting assuming warm water intrusion through these pathways. The data were clearly visualized, making it easy to synthesize the wealth of information presented. The analyses and profiles shown for the major ice shelves were especially nice to see and will make this study of interest to a range of groups who study various components of the Antarctic ice/ocean system.
I do not have any major concerns about the approach or the conclusions, as the caveats associated with this methodology are clearly noted in the discussion. I have several comments mostly relating to the presentation of the study, so I recommend this study for publication after minor revisions. I hope the authors find these comments helpful.
Specific comments:
- The motivation of the study could be more clearly laid out. Ocean gateways are important for ice shelf melting, but why does this analysis help the scientific community? Rather than giving a lengthy overview of why troughs and gateways are important and then explaining the approach, I suggest making a more concise overview of the importance, state (more clearly) what the critical uncertainties are, what the approach is, and what the hypothesis is.
- What are the criteria for classifying something as a ‘prominent gateway’? In L145 the authors state that “large portions” of the GLs need to be reached, but how was this determined?
- Section 3.3.4 (Amundsen Sea) – the main gateway is identified as the Abbot Cosgrove Trough. Is this distinct from the ‘Eastern Trough’ commonly referred to in studies about the Amundsen Sea (e.g., Dutrieux et al., 2014 already cited in this study)? Does it feed into the PIG-Thwaites Trough shown in Fig 9a, or is it separate? It would be helpful to clarify which of the oceanic gateways identified are/aren’t in agreement with the main pathways identified in previous studies.
- The mismatch in melt rates between the approach in this study and the melt rates of Adusumilli need to be addressed, as that is critical for gauging how reliable the estimates of future melt rates are. Some regions do better than others, perhaps due to different reasons, so I’d suggest explaining this for each of the major ice shelf regions.
- I found Section 3.3 rather hard to read. Rather than explaining the results and then reviewing various relevant literature, I’d suggest reviewing the findings from the literature and then presenting the new results within that context. It feels very scattered in its present state.
Other suggestions (to improve the presentation, not necessary for publication in my opinion):
- In Figure 2, it would be nice to see the temperature relative to the pressure melting point.
- Several figures likely need larger text to for readers to see the numbers in each of the small boxes – probably fine for PDFs, but not printing. Perhaps the numbers are not critical for understanding the figures, as the colors also reflect the values.
Citation: https://doi.org/10.5194/egusphere-2023-2583-RC3
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