the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2025
Short and Long-term Grounding Zone Dynamics of Amery Ice Shelf, East Antarctica
Abstract. The detection of grounding line (GL) positions in Antarctica is crucial for investigating the stability and health of ice sheets and glaciers. In reality the GL position is not fixed and will migrate upstream or downstream in response to varying tidal states on an hourly to daily timescale, or in response to longer-term ice dynamic change. However, the magnitude of short and long-term GL migration is not well characterised in many parts of Antarctica. In this study, we employ the Differential Range Offset Tracking method to measure the tidal GL migration on the Amery Ice Shelf in East Antarctica. We delineate 32 GL positions for the year 2021, covering 1,172 km of coastline. The results show that GL migration in this region is not solely dictated by tide amplitude but is also significantly influenced by ice velocity and subglacial bed topography, providing new insights into the GL dynamics of the region. We also observe significant long-term GL retreat in the eastern part of the Amery Ice Shelf relative to the MEaSUREs Antarctic GL derived from 2000 SAR imagery, with the maximum retreat reaching up to 10 km. Our findings underscore the need for continuous, high-resolution GL monitoring around the whole Antarctic coastline, to improve predictive models of ice sheet responses to climate changes and their subsequent impact on global sea-level rise.
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RC1: 'Comment on egusphere-2025-849', Anonymous Referee #1, 18 Apr 2025
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**General Comments**
This manuscript presents a comprehensive assessment of grounding zone (GZ) dynamics across the Amery Ice Shelf (AmIS) using the Differential Range Offset Tracking (DROT) method. The authors produce a spatially extensive dataset of 32 grounding line (GL) positions over a 1,172 km stretch of coastline during 2021, allowing for the characterization of both short-term tidal GL migration and localized long-term retreat. A notable strength of the study lies in its systematic comparison of DROT-derived GLs with independent datasets, including contemporaneous DDInSAR measurements and long-standing GL products (e.g., MEaSUREs, Synthesized GL), establishing the method’s reliability and its applicability in coherence-limited regions. The use of DROT across such a large region represents a valuable technical advancement and offers significant potential for extending GL monitoring coverage elsewhere in Antarctica.
However, while the study is methodologically robust and offers a valuable GL dataset, its interpretation of GL migration modes and grounding zone width correlations shows limited novelty in light of recent work. In particular, the identification of linear, threshold, and asymmetric migration modes largely replicates the classification framework introduced by Freer et al. (2023), and the reported correlations between GZ width and glaciological parameters (e.g., bed slope, ice velocity) parallel earlier insights from Chen et al. (2023). Consequently, the manuscript’s broader scientific contribution to understanding the controls on GL behavior and GZ morphology is somewhat incremental rather than conceptually new. Nonetheless, this work provides a high-quality observational foundation and demonstrates the utility of DROT for grounding line science.
**Specific Comments**
- L35–40: Consider elaborating on the specific advantages of the DROT method in this context and providing a brief rationale for its selection over other grounding line detection techniques. This would help clarify the methodological motivation at an early stage in the manuscript.
- L220–222: It would be helpful to clarify the basis on which the GL migration along profile 4 is interpreted as permanent. Specifically, how does Figure 2d/Figure 1c support the conclusion that the observed inland migration exceeds short-term tidal variability?
- In the phase legend of Figure 3a, it is recommended to use 180° instead of 3.14.
- For Figures 3b–c, it is recommended to display the grounding lines derived from both methods simultaneously within the same panels, if feasible. This would enable a more direct and intuitive visual comparison between the results.
- Section 3.2 – Comparison with Other Grounding Line Measurements: It is recommended that the authors consider incorporating a comparison with the dataset available at https://nsidc.org/data/nsidc-0778/versions/1. This product, part of the MEaSUREs program, provides a continent-wide map of short-term grounding line migration zones derived from InSAR during 2018–2020. Given its closer temporal proximity to the 2021 DROT results presented in this study, it would serve as a valuable reference for contextual validation and potentially enhance the robustness of the comparative analysis.
- L304: The manuscript states a typical offset of 1.5 km between DROT- and DDInSAR-derived grounding lines. Could the authors clarify whether this value represents an average across specific regions? Additionally, the observed differences in this study appear smaller—what factors might account for this discrepancy, and how spatially consistent are these deviations across the Amery Ice Shelf?
- L355–359: The interpretation of a positive correlation between grounding line migration distance and the absolute double-differential tide range would benefit from further clarification. Does a lower absolute double-differential tide range necessarily imply smaller ice surface deformation? For instance, if the first and second SAR acquisitions occur at high tides and the third and fourth at low tides, substantial ice deformation may occur, yet the computed absolute double-differential tide range could remain small.
- L375–377: Figure 2e in Chen et al. (2023) similarly demonstrates that the grounding line position of the Lambert and Mellor Glaciers oscillates between two discrete states over timescales of several months. Also, given that the temporal sampling in the present study is sparser than that of Chen et al., it is possible that some short-term transitions or episodic changes may not have been fully captured.
- Figure 5b: The synchronized grounding line migration observed for the two glaciers suggests a common driving mechanism. Could this be attributed to tidal forcing, or do the authors propose that subglacial hydrological processes—such as simultaneous subglacial lake drainage or basal melting—could exert a temporally comparable influence on both glaciers? Further discussion on the plausibility of shared controls would enhance the interpretation.
- L455–456: Is this the reason behind the classification of this island as part of the grounding zone in Figure 1c?
- L570–571: It would be helpful to specify how the along-profile length over which the slopes are calculated was defined. What criteria were used to determine the extent of the landward and seaward segments used in the slope analysis?
Citation: https://doi.org/10.5194/egusphere-2025-849-RC1
Data sets
Additonal code and data for grounding line analysis presented in this article "Short and Long-term Grounding Zone Dynamics of Amery Ice Shelf, East Antarctica" Yikai Zhu et al. https://zenodo.org/doi/10.5281/zenodo.14912749
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Anna E. Hogg
Andrew Hooper
Benjamin J. Wallis
This study investigates the long- and short-term changes in the grounding line of the Amery Ice Shelf in East Antarctica, using satellite observations and a method called Differential Range Offset Tracking (DROT). Our findings show how the grounding line behaves in response to tides and other environmental factors, with implications for understanding ice shelf stability.
This study investigates the long- and short-term changes in the grounding line of the Amery Ice...