the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jun 2024
Current reversal leads to regime change in Amery Ice Shelf cavity in the twenty-first century
Abstract. The Amery Ice Shelf (AmIS), the third largest ice shelf in Antarctica, has experienced relatively low rates of basal melt during the past decades. However, it is unclear how AmIS melting will respond to a future warming climate. Here, we use a regional ocean model forced by different climate scenarios to investigate AmIS melting by 2100. The areally-averaged melt rate is projected to increase from 0.7 m·yr−1 to 8 m·yr−1 in the low-emission scenario or 17 m·yr−1 in the high-emission scenario in 2100. An abrupt increase in melt rate happens in the 2060s in both scenarios. The redistribution of local salinity (hence density) in front of AmIS forms a new geostrophic balance, leading to the reversal of local currents. This transforms AmIS from a cold cavity to a warm cavity, and results in the jump in ice shelf melting. While the projections suggest that AmIS is unlikely to experience instability in the coming century, the high melting draws our attention to the role of oceanic processes in basal mass loss of Antarctic ice shelves in climate change.
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RC1: 'Comment on egusphere-2024-1287', Anonymous Referee #1, 24 Jul 2024
Overall, I find this to be a well written manuscript that looks at an interesting and topical case of whether, and how, the Amery ice shelf cavity could switch from a cold to a warm regime over the next 100 years. If so this would have implications for it's future stability and consequent contribution to sea level rise. I recommend publication with minor corrections provided my two main comments are addressed (one potentially not so minor).
1) My first comment is that the text in figures throughout the manuscript is far too small. Whether this is merely a formatting error in the cryosphere draft format or not I can not say, but it needs to be addressed before publication.
2) The second, and potentially larger comment, is the lack of discussion about how choices of ocean model parameters affect the conclusions. The runs as presented are just for two different climate scenarios, with no investigation into the effect of model parameters. Was sensitivity analysis carried out for viscosity and diffusivity, time step, grid resolution, (all properties known to have a large impact on modelled melt rates) etc? I do appreciate that ocean models are expensive to run and so all of this may not be feasible, but would like to see some discussion about what was done. I suspect that the qualitive conclusions will hold up, but would like to see some analysis backing this up.
Line comments below.
L17 - (and throughout) Bathymetry as a positive number reads oddly to me. This may be a glaciology bias, but I think there would be less ambiguity to any glaciologists reading if in cases like this it is referred to as negative from sea level, or otherwise made explicitly clear what it is referring to.
L 75 - How does the ice shelf draft from BEDMAP2 compare to a more recent data set such as BEDMACHINE (Morlighem, 2022)?
L90 - To clarify, by "top boundary" I am assuming you are averaging ice-ocean boundary conditions for melting over 30m. How is this weighted if this occurs over more than one cell?
L 136 - What time step is the model running at? Please include somewhere.
L 202 - What is the exact value of Cp used?
L 253 - /rho is used in equation 1 as a variable, not a constant. Suggest using /rho_ref or similar here to refer to reference density or the like and avoid confusion.
L 302 - To clarify, by net melt rate here are you referring to area averaged melt rate minus surface precipitation?
Fig 1- (and throughout) Is it possible to change the grey calving front lines to a more contrasting colour, as at present it blends into the contour lines?
Fig 2- The text is too small to make out at the current figure size.
Fig 3 - The text, particularly in the legend, is too small to easily make out.
Fig 4 - Text is too small. Could average cavity temperatures be included to show the lag between shelf water temperatures and cavity temperatures?
Fig 5 - Text by velocity arrow key too small.
Fig 8 - Legend text too small.
Figure A4 - I find this a potentially misleading figure. I am assuming this is just the running total of increased melting compared to initial geometry? Without any feedback on the glacial system included in the model I am not sure how useful this is. In reality, a thinner ice shelf would flow faster, thus a 10m increase in melt rate would not necessarily lead to a 10m reduction in thickness per year at a given point.
Ref.
Morlighem, M. (2022). MEaSUREs BedMachine Antarctica, Version 3 [Data Set]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. https://doi.org/10.5067/FPSU0V1MWUB6. Date Accessed 07-24-2024.
Citation: https://doi.org/10.5194/egusphere-2024-1287-RC1 -
AC1: 'Reply on RC1', Jing Jin, 25 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1287/egusphere-2024-1287-AC1-supplement.pdf
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AC3: 'Reply on RC1', Jing Jin, 25 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1287/egusphere-2024-1287-AC3-supplement.pdf
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AC1: 'Reply on RC1', Jing Jin, 25 Oct 2024
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RC2: 'Comment on egusphere-2024-1287', Anonymous Referee #2, 06 Aug 2024
This paper addresses the behavior of the Amery Ice Shelf under two climate change scenarios, SSP1-2.6 and SSP5-8.5. While the topic is indeed interesting, the paper falls short of meeting the minimum requirements for publication for several reasons, which I will detail below.
The goal of this study is to estimate the future melt rate of the ice shelf by 2100 and to understand the mechanisms driving this melt. The authors utilize a regional simulation, AME025, forced with UKESM outputs.
In my assessment, there are significant issues in every part of the paper, with the most critical problems found in section 3.2.
1) Section 2
There are several pieces of missing information:
- Which version of NEMO is used?
- I understand that the configuration is an extraction of GO7 with some modifications to account for the specifics of a regional study. However, these modifications are not detailed in the text. For instance, at line 78, the authors mention that slip conditions are modified, but it is unclear how and why. What parameters are important here? Why not change the number of vertical levels as in Mathiot et al. (2023)?
- The viscosity and diffusion coefficients are set to -1.08e-10 m^4/s and 135 m^2/s, respectively. First, I suspect there is an error with the viscosity value, and it should be -1.08e+11; otherwise, it is extremely small. Second, these coefficients seem to be directly taken from the global configuration GO7 without adjustment, which is not appropriate. However, this might only marginally affect the simulations.
- The authors prescribe a constant tidal velocity of 5 cm/s. Where does this value come from? I suspect its impact on the melt is significant.
- Is there any freshwater or salt restoring?
- Lines 117 to 124: I do not understand the discussion about imposed melt in UKESM while AME025 has open interactive cavities.
- The spin-up phase involves repeatedly simulating the year 1976, but how is the transition between December and January handled? Is there not a discontinuity?
2) Section 3.1.1
This section evaluates the melting rate by 2100. The authors focus on two periods: 2015-2055 and 2075-2100. However, it would be beneficial to explore the slow adjustment after the initial increase. For instance, there are fluctuations in the melt rate, especially in the 2090s for SSP1-2.6, that the authors did not explain. In my view, the most interesting aspect is the 10-year transition from a cold to a warm ice shelf around 2060-2070.
Additionally, there is no discussion of estimates from other studies, which, I believe, found much weaker sensitivity. For instance, Kusahara et al. (2023) and Naughten et al. (2018) reported increases in melt by factors of 3 and 2, respectively, compared to a factor of 10 in this study.
3) Section 3.1.2
This section evaluates the concomitant warming on the continental shelf. However, several issues and approximations need to be addressed.
The authors average temperatures in the 300-800 m depth range over the continental shelf without further explanation. It is necessary to specify the exact region considered and justify the choice of these depths.
This averaging obscures the identification of the specific water masses that are warming. It would be more effective to examine sections extending from the grounding line to the shelf break and beyond. This approach would allow for a discussion on the warming or displacement of interfaces and the emergence of new water masses.
The authors state that the temperature increase occurs several decades earlier than the increase in melt. However, based on Figure 4, the deviation from the steady state begins around 2040, approximately 15 years before the increase in melting, not decades. Additionally, the transition from the cold state to the warm state occurs clearly between 2060 and 2070 for both melting and warming, so the claim of a significant delay is questionable.
4) Section 3.2
The goal of this section is to explain the changes in the melt rate. To do this, it is necessary to accurately depict the changes in water masses inside and ahead of the cavity, and their link to changes in ocean circulation. The analysis presented here is insufficient to address the question. The missing analyses are:
- Temperature transects.
- Analysis of mCDW transport inside the cavity with a proper closed budget.
- Use of daily outputs instead of monthly, since (v⋅T)1mo≠v1mo⋅T1mo , especially given the strong variability in this area.
- Show the reversal of PGB with a streamfunction, rather than just two sections of meridional velocity.
The causes of PGB reversal are only conjectures in this analysis. For instance, the authors state that PGB is in geostrophic equilibrium due to the salinity difference between two boxes, S1 and S2. However, this is not a rigorous calculation. It merely suggests that it is possible for the gyre to be in geostrophic balance. To make a definitive statement, one would need to consider all the tendency terms of the forces driving the gyre. This approach is not rigorous.
From lines 290 to 298, the authors claim that the upstream systems influence the ice shelf downstream, but there is no proof provided, only hypotheses.
5) Conclusion
- From lines 314 to 323, I am unclear about the authors' intended message. Galton-Fenzi (2009) mentioned that the PBG depends on the relative strength of McKenzie and Barrier polynyas, and it is already known that PBG drives mCDW into the cavity. What new information is being presented here?
- Lines 324-331 are not informative.
- Lines 335-349 do not offer conclusions but rather present some reflections, without providing a perspective on future work.
- Lines 350-380 appropriately discuss the limitations of the work, but the authors overlook the issue of frazil ice refreezing at the ice shelf base, which the model does not account for.
FiguresThe quality of the figures is inadequate. For example, the color bar in Figure 1 does not provide a clear visualization of the bathymetry in the Amery cavity. Additionally, the labels in Figure 2 are so small that they are barely legible, among other issues.
Citation: https://doi.org/10.5194/egusphere-2024-1287-RC2 -
AC2: 'Reply on RC2', Jing Jin, 25 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1287/egusphere-2024-1287-AC2-supplement.pdf
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AC4: 'Reply on RC2', Jing Jin, 25 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1287/egusphere-2024-1287-AC4-supplement.pdf
Model code and software
AME025 configuration Jing Jin, Christopher Bull, and Antony Payne https://doi.org/10.5281/zenodo.10797900
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