the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2025
Modelling ocean melt of ice mélange at Greenland's marine-terminating glaciers
Abstract. Many of Greenland's marine-terminating glaciers have retreated and accelerated in recent decades, contributing significantly to sea level rise. Increased submarine melting of calving fronts is often cited as the dominant driver of this retreat. However, the presence of ice mélange and its associated buttressing force on a glacier terminus is also thought to significantly impact glacier advance and retreat. The buttressing force depends on the mélange thickness, and thickness will be modulated by ocean melt rate, but our understanding of the melting of ice mélange by the ocean remains limited, and it is not yet known how these melt rates vary across a range of glacial and environmental conditions. Here, we perform high-resolution numerical simulations using MITgcm to model the circulation of ocean waters through an ice mélange close to marine-terminating glaciers and estimate the resultant melt. We explore the sensitivity of mélange melt rate to environmental conditions, finding that melt rates increase sublinearly with subglacial discharge and approximately linearly with ocean temperature. In this sense, mélange melt rate appears to respond to environmental forcing in a similar manner to submarine melting of the calving front, and can be parameterised as such. This work is a step towards both a better understanding of ice mélange dynamics and a better parameterisation of its effects on glaciers and the ocean.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-4081', Erwin Lambert, 20 Feb 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-4081/egusphere-2024-4081-RC1-supplement.pdf
- AC1: 'Reply on RC1', Lokesh Jain, 17 Mar 2025
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RC2: 'Comment on egusphere-2024-4081', Anonymous Referee #2, 21 Feb 2025
Summary
The study modifies the MITgcm ocean model to simulate iceberg melting within the dense mélange that occupies many of Greenland’s fjords for a portion of the year. They perform sensitivity tests to solve for an empirical equation that describes mélange melting as a function of ocean thermal forcing and subglacial discharge and find that the relationship for their simulations is similar to that for estimates of glacier terminus melting. Their mélange size distributions are intended to approximate observed distributions and the model simulations explore the influence of differences in iceberg depth on melt rates. Although their model is constructed largely based on observations of icebergs, fjord geometry, and ocean conditions for Sermilik Fjord in SE Greenland, where mélange is particularly densely-packed and persistent, the authors expect that their results are likely to be applicable more generally.
I think this is a well thought-out modeling study and the topic is both relevant to understanding glacier change and largely overlooked. The figures are high quality and the writing is easy to follow for the most-part. I have a few relatively minor comments below that should be addressed in a revised version of the manuscript.
Major Comments
- I do not expect the authors to run more simulations but I think that they should make it clear in the methods that because they use the same aspect ratio and areal coverage of icebergs in their three thickness simulations, that they inherently include more icebergs in the thin simulations. The authors discuss how the larger number of icebergs in the thin simulations increases the surface area of the icebergs in the fjord and that brings up the meltwater flux even though the average melt rate is reduced. Does the difference in iceberg abundance also influence the magnitude of horizontal velocities in the simulations? It is not clear if the velocities are approximately the same for all thickness simulations because the areal coverage of icebergs is the same or if the abundance of icebergs influences that parameter as well. I recommend that the authors briefly discuss the likely effects of changes in iceberg abundance on their simulations since it is one of their least constrained parameters and the thickness and abundance of icebergs will both be less for almost all other fjords around Greenland.
- I’d also like the authors to make it more clear that the iceberg geometries do not evolve in this simulation. You are essentially adding fixed iceberg blocks that do not change shape. I don’t take issue with that model configuration but it should be made clear in the method and that simplification should be discussed when you describe the potential influence of iceberg shape on the simulations.
Minor Comments
- Lines 56-61: I struggled a bit with this paragraph. The sentences summarizing results of the two Hughes references almost feel contradictory. This paragraph also felt a bit out of place because a lot of the preceding text focused on observations and then subsequent paragraphs focus on modeling. This paragraph describes modeling results but it is not all that obvious that you switched from observations to models. I suggest wrapping this text into one of the later paragraphs in the intro and rephrasing the two Hughes-focused sentences.
- Lines 68-78: Somewhere in this section or elsewhere in the intro, I recommend that you cite Hester et al. (2021; DOI: 10.1103/PhysRevFluids.6.023802) or FitzMaurice et al. (2017; doi:10.1002/2017GL073585), which both describe how melt rates can vary around icebergs due to differences in velocity across the varying faces.
- Line 107: Why is x_2 open water? Is that a requirement for stability? Figure 1 is fantastic for visualizing the set-up.
- Line 177: You describe the three-equation parameterization of melt as one that is “a number of constants whose values are plagued with uncertainty”. That set up the reader to expect to learn about multiple constants but you only focus on C_d. I recommend that you at least list the other constants and their values even if you focus on the one that you consider to be the most important.
- Line 137: I also recommend citing Astrom et al. (2021; https://doi.org/10.1017/jog.2021.14) when describing size distributions.
- Figure 2: This is terrific.
- Line 223: I don’t follow why you list all these subglacial discharge values when you later state that you only use three.
- Line 251: The comments about the icebergs being a heat sink for the plume make sense but without a simulation of the plume without icebergs, be careful with how you phrase your inference that the icebergs limit the extent of the plume.
- Line 257: Remove “of red” since that is really specific to the figure description and removing the color makes it more general.
- Lines 342-346: I had a difficult time following this description. Please try to revise.
- Line 393: Isn’t the parameterized melt rate for the standard thickness lower than the modeled melt rate for both the thicker and thinner simulations? I interpreted the text here as the opposite of what I saw in Figure 10.
- Lines 418-424: I think the difference in mélange areas is moreso because Enderlin et al. (2016) did not differentiate icebergs and sea ice, so all the areas in your simulations that are ice-free would be occupied by ice.
Citation: https://doi.org/10.5194/egusphere-2024-4081-RC2 - AC2: 'Reply on RC2', Lokesh Jain, 17 Mar 2025
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