the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Oct 2023
Ice-shelf freshwater triggers for the Filchner-Ronne Ice Shelf melt tipping point in a global ocean model
Carolyn Branecky Begeman
Xylar S. Asay-Davis
Darin Comeau
Alice Barthel
Stephen F. Price
Jonathan D. Wolfe
Abstract. Some ocean modeling studies have identified a potential tipping point from a low to high basal melt regime beneath the Filchner-Ronne Ice Shelf (FRIS), Antarctica, with significant implications for subsequent Antarctic Ice Sheet mass loss. To date, investigation of the climate drivers and impacts of this possible event have been limited, because ice-shelf cavities and ice-shelf melting are only now starting to be included in global climate models. Using a version of the Energy Exascale Earth System Model (E3SM) that represents both ocean circulations and melting within ice-shelf cavities, we explore freshwater triggers of a transition to a high melt regime at FRIS in a low resolution (30 km in the Southern Ocean) global ocean-sea ice model. We find that a realistic spatial distribution of iceberg melt fluxes is necessary to prevent the FRIS melt regime change from unrealistically occurring under historical reanalysis-based atmospheric forcing. Further, improvement of the default parameterization for mesoscale eddy mixing significantly reduces a large regional fresh bias and weak Antarctic Slope Front structure, both of which precondition the model to melt regime change. Using two different stable model configurations, we explore the sensitivity of FRIS melt regime change to regional ice-sheet freshwater fluxes. Through a series of sensitivity experiments prescribing incrementally increasing melt rates from the smaller, neighboring ice shelves in the eastern Weddell Sea, we demonstrate the potential for an ice-shelf melt ``domino effect'' should the upstream ice shelves experience increased melt rates. The experiments also reveal that modest ice-shelf melt biases in a model, especially at coarse ocean resolution where narrow continental shelf dynamics are not well resolved, can lead to unrealistic melt regime change downstream, and these ice-shelf melt teleconnections are sensitive to baseline model conditions. Our results highlight both the potential and the peril of simulating prognostic Antarctic ice-shelf melt rates in a low-resolution, global model.
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Matthew J. Hoffman et al.
Status: open (until 15 Dec 2023)
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RC1: 'Comment on egusphere-2023-2226', Anonymous Referee #1, 06 Dec 2023
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General comments:
This study investigates the tipping point of the Filchner-Ronne Ice Shelf (FRIS), based on a series of numerical experiments of a sea-ice and ocean model (including ice shelves) forced by present-day atmospheric boundary conditions. The sea ice-ocean model is a component of the Energy Exascale Earth System Model (E3SM) for wider climate sciences. In this study, the authors first show that by adjusting the iceberg meltwater distribution and the mesoscale eddy parameterization (GM coefficient), the unrealistic tipping event of the FRIS ice-shelf basal melt occurred in the standard configuration (CGM-UIB case) can be avoided in the CGM-DIB and VGM-DIB cases. Using the adjusted experimental setups, the authors further examine the increase in the FRIS melting in response to changes in upstream ice-shelf melting and show that a significant rise in EWIS ice-shelf melting can cause the FRIS to experience a tipping point. A rapid increase in ice-shelf basal melt substantially impacts inner ice-sheet changes, potentially contributing to the Antarctic ice-sheet mass loss and the subsequent global sea-level rise. In addition, while the substantial increase in ice-shelf basal melting due to the enhanced contribution of the warm Circumpolar Deep Water onto the southwestern Weddell continental shelf region has been reported in several modeling studies, it is still an interesting topic to be addressed in a coarse-resolution ocean model, which is a part of the climate model. For these reasons, I consider that the topic of this study falls within the scope of The Cryosphere. Although the manuscript in the present format may be clear for those familiar with E3SM and its ocean model biases, I found it very difficult for general readers (including me) to follow the logic smoothly. I suggest revising the manuscript to make it more understandable for a broader audience.Major comments:
1. While the study highlighted the use of E3SM in many places (e.g., the abstract (L4-6), the last two paragraphs of the introduction (L51-64), and the methods section), actually, this study used only its ocean, sea-ice, and ice-shelf components of it with specifying atmospheric boundary conditions (CORE-II forcing for global sea ice-ocean models). The description throughout the manuscript should be careful not to give the impression that the experiments were conducted with the full ESM. This study is based on a global sea ice-ocean model, and I think it is sufficient to briefly describe these components are part of the E3SM in the methods section.2. Throughout the manuscript, only a very limited area of the southern Weddell Sea (FRIS) is shown, and there is no information such as place names used in the manuscript. I think a map is required showing the broad area, including the Eastern Wedell Sea and major place names. In addition, it would be better to use the model representations of the coastline/ice-front line and grounding line (instead of the realistic coastal/grounding lines) to show the model’s horizontal resolution clearly.
3. In the experiments where the melt rate of the Eastern Weddell Sea is increased to 4-16m/yr (Section 2.3), I believe it is important to also express these rates in terms of volume (Gt/yr) per unit area. Considering observational data (Lauber et al. 2023), a melt rate increase to 4-16m/yr seems excessively high. If there are any justifications for this rate based on other modeling results or evidence, it should be included in the manuscript.
Lauber, J., Hattermann, T., de Steur, L., Darelius, E., Auger, M., Nøst, O. A., & Moholdt, G. (2023). Warming beneath an East Antarctic ice shelf due to increased subpolar westerlies and reduced sea ice. Nature Geoscience, 16(10), 877–885. https://doi.org/10.1038/s41561-023-01273-54. I understand that freshwater supply and distribution can determine the FRIS tipping, but can you estimate the tipping conditions in ***this model*** by analyzing the freshwater balance on the continental shelf in front of the FRIS? For example, how much Gt or more of freshwater on the southwestern Weddell continental shelf would be required to cause the tipping point?
5. There are two very relevant new papers. The first one is missing, and the second one requires an update of the citation.
Mathiot, P., & Jourdain, N. C. (2023). Southern Ocean warming and Antarctic ice shelf melting in conditions plausible by late 23rd century in a high-end scenario. Ocean Science, 19(6), 1595–1615. https://doi.org/10.5194/os-19-1595-2023
Haid, V., Timmermann, R., Gürses, Ö., & Hellmer, H. H. (2023). On the drivers of regime shifts in the Antarctic marginal seas, exemplified by the Weddell Sea. Ocean Science, 19(6), 1529–1544. https://doi.org/10.5194/os-19-1529-2023Specific comments:
6. L35-44: There are some useful literature (Thompson et al. 2018 for introducing Antarctic Slope Front/Current, Thoma et al. 2010 for remote linkage between Eastern Weddell Sea and Weddell Sea, and Kusahara and Hasumi 2013 for pathway of the EWIS meltwater). Please consider including the references.
Thompson, A. F., Stewart, A. L., Spence, P., & Heywood, K. J. (2018). The Antarctic Slope Current in a Changing Climate. Reviews of Geophysics, 56(4), 741–770. https://doi.org/10.1029/2018RG000624
Thoma, M., Grosfeld, K., Makinson, K., & Lange, M. A. (2010). Modelling the impact of ocean warming on melting and water masses of ice shelves in the Eastern Weddell Sea. Ocean Dynamics, 60, 480–489. https://doi.org/10.1007/s10236-010-0262-x
Kusahara, K., & Hasumi, H. (2013). Modeling Antarctic ice shelf responses to future climate changes and impacts on the ocean. Journal of Geophysical Research: Oceans, 118(5), 2454–2475. https://doi.org/10.1002/jgrc.201667. L73-74: This sentence is incorrect. Dinniman et al. (2016) summarized ocean models, which include ice-shelf cavities, and several ocean models with the same features have been developed since the publication.
Dinniman, M., Asay-Davis, X., Galton-Fenzi, B., Holland, P., Jenkins, A., & Timmermann, R. (2016). Modeling Ice Shelf/Ocean Interaction in Antarctica: A Review. Oceanography, 29(4), 144–153. https://doi.org/10.5670/oceanog.2016.1068. L104 Where did you restore the surface salinity, and with what intensity?
9. L109-110: I think that a map of the variable GM calculated in the model is helpful to see the difference from the constant-value case.
10. L115-136: Maps of iceberg melt flux are also helpful to understand the differences among the experiments.
11. Figure 1: Adding Gt/yr to the right axis also makes it easier to compare with other studies and between ice shelves.
12. Figure 2: Information of longitude and latitude is sparse, and it is very difficult to see the latitude information.
13. Figure 4: Is the small map for the T-S plot? If so, please enlarge the map and add depth contours.
14. Figure 7: Without vertical profiles of water properties, it is very difficult to understand the figure.
15. Figure 8: A map of the barotropic stream function is required, with pointing to the observation site M31W and transect C.
16. Figure 9: Could you add panels of vertical profiles of alongshore current to see the Antarctic Slope Current? Why did you use the potential density anomaly referenced to 1000 m? The other figures used the potential density referenced to the surface (Figs. 4, 7, and 12). Please remove TODO (I think this kind of error should be removed before submission, circulating the manuscript among the authors.).
17. Figure 10: It is very difficult to see the observed range. Please consider revising the color.
18. Figure 12: Please add the experiment names in the figure.
Technical corrections:
19. Title: It is not clear to me what the word “ice-shelf freshwater” stands for in the title. “Antarctic coastal freshening” may be a candidate to replace the word.
Citation: https://doi.org/10.5194/egusphere-2023-2226-RC1
Matthew J. Hoffman et al.
Matthew J. Hoffman et al.
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