the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Modes of Antarctic tidal grounding line migration revealed by ICESat-2 laser altimetry
Oliver J. Marsh
Anna E. Hogg
Helen Amanda Fricker
Laurie Padman
Abstract. Short-term tidal grounding line (GL) migration in Antarctica can impact ice dynamics at the ice sheet margins and obscures assessments of long-term GL advance or retreat. However, the magnitude of tidally-induced GL migration is poorly known, and the spatial pattern and modes of variability are not well characterised. Here we develop and apply a technique that uses ICESat-2 repeat-track laser altimetry to locate the inland limit of tidal ice shelf flexure for each sampled tide, enabling the magnitude and temporal variability of tidal GL migration to be resolved. We demonstrate its application at an ice plain north of Bungenstockrücken, in a region of the southern Ronne Ice Shelf subject to large ocean tides. We observe a 1,300 km2 area of ephemeral grounding over which the GL migrates by up to 15 km between low and high tide, and identify four distinct modes of migration: “linear”, “asymmetric”, “threshold” and “hysteresis”. The short-term movement of the GL dominates any long-term migration signal in this location, and the distribution of GL positions and modes contains information about spatial variability in the ice-bed interface. We discuss the impact of extreme tidal GL migration on ice shelf-ocean-subglacial systems in Antarctica and make recommendations for how GLs should be more precisely defined and documented in future by the community.
Bryony I. D. Freer et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-265', Pietro Milillo, 09 Mar 2023
This article discusses the development and application of a technique using ICESat-2 repeat-track laser altimetry to locate the inland limit of tidal ice shelf flexure and resolve the magnitude and temporal variability of tidal grounding line (GL) migration in Antarctica. The authors apply this technique to an ice plain north of Bungenstockrücken, in a region of the southern Ronne Ice Shelf subject to large ocean tides. They observe a 1,300 km2 area of ephemeral grounding over which the GL migrates by up to 15 km between low and high tide and identify four distinct modes of migration: “linear”, “asymmetric”, “threshold” and “hysteresis”. The short-term movement of the GL dominates any long-term migration signal in this location, and the distribution of GL positions and modes contains information about spatial variability in the ice-bed interface. The authors identify four distinct modes of GL migration: linear, asymmetric, threshold and hysteresis. I was surprised when reading about linear and threshold behaviors they did not mention recent well-known studies confirming these results (i.e. Milillo et al 2022 for linear behavior and Milillo et al 2019 for threshold behavior over the Thwaites Cavity). The authors recommend that these observations can be used to validate models of tidal ice shelf flexure, GL migration, and subglacial hydrology at the grounding zone (GZ). They find a 14 km grounding zone that could be explained with the Stubblefield et al 2021 Model. However, I haven’t found any reference to this paper in the manuscript. The study concludes with recommendations for future work, including the need for timestamped measurements of GL position accompanied by tide height and phase, continent-wide analysis of tidal GL migration, and improved representation of GL migration behavior in ice sheet models.
The paper is well written, well organized and provides significant results. I encourage the editor to accept this manuscript after few minor revisions.
I believe the authors could further improve the manuscript by referring the aforementioned literature studies as a further independent confirmation of the validity of their findings.
Milillo, P., Rignot, E., Rizzoli, P., Scheuchl, B., Mouginot, J., Bueso-Bello, J. L., ... & Dini, L. (2022). Rapid glacier retreat rates observed in West Antarctica. Nature Geoscience, 15(1), 48-53.
Milillo, P., Rignot, E., Rizzoli, P., Scheuchl, B., Mouginot, J., Bueso-Bello, J., & Prats-Iraola, P. (2019). Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica. Science advances, 5(1), eaau3433.
Stubblefield, A. G., Spiegelman, M., & Creyts, T. T. (2021). Variational formulation of marine ice-sheet and subglacial-lake grounding-line dynamics. Journal of Fluid Mechanics, 919, A23.
Citation: https://doi.org/10.5194/tc-2022-265-RC1 -
AC1: 'Reply on RC1', Bryony Freer, 18 May 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-265/tc-2022-265-AC1-supplement.pdf
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AC1: 'Reply on RC1', Bryony Freer, 18 May 2023
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RC2: 'Comment on tc-2022-265', Kasia Warburton, 13 Mar 2023
This article addresses the mapping of tidal grounding-line migration from ICESat-2 repeat-track laser altimetry, highlighting the large regions of transiently grounded ice in an ice plain of the Ronne Ice Shelf. The authors raise excellent points about the need for consistency in grounding line products given the scale of tidal grounding-line migration compared to long term retreat rates. They also identify several different patterns of migration, termed "linear", "asymmetric", "threshold", and "hysteresis" - although I agree fully with the authors that given the potential for hysteresis, this categorisation could be highly affected by the gaps in sampling and will be improved by increasing repeats by ICESat-2 (perhaps this point should be made a little louder).
This is a well-written and significant paper, and the summary and outlook section in particular provides concrete recommendations that will enhance our understanding of grounding-line processes. I have only two minor comments which the authors might like to consider in their final version:
The authors argue convincingly that the choice of lowest-sampled profile provides the best measurement of grounding-line migration. The mean profile is described as introducing observation bias, but the issue appears to be primarily the same issue as with the neutral tidal profile, that the point F cannot be accurately identified when the elevation anomaly is negative. This point could be made more clearly, perhaps with a note about why this limitation exists (e.g. a naive question might be what happens if the highest-sampled profile is taken as reference instead).
In l.263 the authors describe that they "then manually adjusted any choice of peak where it was still visibly incorrect". From both a reproducibility and automation point of view, this method should be made more explicit. Making the code available (currently tbc) should also be a priority.
Citation: https://doi.org/10.5194/tc-2022-265-RC2 -
AC2: 'Reply on RC2', Bryony Freer, 18 May 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-265/tc-2022-265-AC2-supplement.pdf
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AC2: 'Reply on RC2', Bryony Freer, 18 May 2023
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RC3: 'Comment on tc-2022-265', Tian Li, 19 Mar 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-265/tc-2022-265-RC3-supplement.pdf
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AC3: 'Reply on RC3', Bryony Freer, 18 May 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-265/tc-2022-265-AC3-supplement.pdf
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AC3: 'Reply on RC3', Bryony Freer, 18 May 2023
Bryony I. D. Freer et al.
Bryony I. D. Freer et al.
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