the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Dec 2023
Modelling GNSS-observed seasonal velocity changes of the Ross Ice Shelf, Antarctica, using the Ice-sheet and Sea-level System Model (ISSM)
Abstract. Recently, seasonal changes in sea ice cover have been found to elevate basal melt rates of the Ross Ice Shelf (RIS) calving front at sensitive regions. Melting at these sensitive regions has been found to impact ice sheet mass balance. However, the influence of these seasonally elevated basal melt rates on RIS flow variability is not yet fully understood. This paper aims to explore whether seasonal perturbations in basal melt rates of the RIS can explain intra-annual variations in ice flow measured by GNSS at four sites across the ice shelf. We use the automatic differentiation tool in the Ice-sheet and Sea-level System Model (ISSM) to identify regions of the RIS where changes in basal melt affect ice velocities at the GNSS sites. Next, we seasonally perturb Massachusetts Institute of Technology general circulation (MITgcm) basal melt rates in ISSM at these sensitive regions to try and replicate the GNSS ice flow observations. The GNSS observations display clear intra-annual velocity variability at the four sites, with two distinct peaks observed in austral summer and austral winter. We can replicate this intra-annual velocity variation for GNSS sites near the calving front by seasonally perturbing the basal melt rates at the identified sensitive regions of the ice shelf. We argue that the perturbed seasonal basal melt variability at sensitive regions along the calving front is a realistic scenario for the RIS. Thus, we suggest that the GNSS-recorded intra-annual velocity variations along the calving front could be partly driven by seasonal changes in basal melting today. We also try to replicate intra-annual velocity variability observed at the Siple Coast by seasonally perturbing basal melt rates at sensitive regions there. However, we are unable to replicate similar magnitudes of velocity variations to the GNSS measurements and suspect that the perturbed seasonal basal melt variability is unrealistic, with no observations of seasonally high basal melt rates at the Siple Coast grounding lines or pinning points. Thus, seasonal changes in basal melt cannot explain the observed intra-annual velocity variability at all the GNSS sites, and further work is needed. Our sensitivity maps highlight regions of the ice shelf where changes in basal melt most influence velocities, and are a valuable addition to fieldwork campaigns and modelling studies.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2793', Anonymous Referee #1, 06 Feb 2024
This manuscript presents simulations of seasonal velocity variability at 4 sites on the Ross Ice Shelf, in response to seasonal variations in basal melt rates. The results show that, for 2 of the sites, intraannual velocity variability can be driven by seasonal basal melt rate perturbations. For the other 2 sites, however, seasonal basal melt rate perturbations are not likely to be the primary drivers of velocity variability.
The experimental design in the paper is straightforward and presents clear and novel results. However, the writing and figures in the manuscript could be made clearer and I have some general and editorial comments below to address this.
General
- As stated in the submission guidelines for The Cryosphere, I strongly encourage the authors to deposit all scripts and configuration files for setting up and running the ISSM simulations in a FAIR-aligned repository, such as Zenodo.
- Add to the introduction paragraph on lines 80-88 a brief description of how this present study differs from Klein et al. (2020) and Mosbeux et al. (2023) which, as described in the preceding paragraph, provide an explanation for the intra-annual velocity variations for the RIS.
- Additionally, please add text in the Discussion that addresses why the conclusions of this present study differ from Klein et al. (2020), which claim that seasonal velocity variations are not driven by basal melt rate variability.
- Use "intraannual" throughout the text to refer to monthly to seasonal variability. For example, line 73 refers to monthly to seasonal variability as "interannual" but this should be changed to "intraannual". Please check the entire manuscript for other cases of this.
- Add a map of observed velocities of the ice shelf to Figure 1.
- Remove Figure 3 because Figure 5 shows the same data.
- It is not clear to me whether including both Figure 2 and Figure 4 is necessary and how the interpretation differs for the results shown in these two Figures. My understanding is that Figure 2 shows sensitivity of the final velocity for a 6-month simulation and Figure 4 shows sensitivity of the final velocity for a 2-year simulation. I also see that Figure 2 shows sensitivities that are above the selected threshold, whereas Figure 4 shows the full range of sensitivities. However, it seems like the text in Section 3.2, which describes the results in Figure 4, could also apply to the results in Figure 2. I may be wrong, in which case please feel free to disagree. If it is decided to keep both figures in the manuscript, please add text to Section 3.2 that explains why the results in the two figures are different and what additional information for interpretation is provided by Figure 4 that isn't already provided by Figure 2.
- Add a figure showing absolute modeled and observed velocities at each GNSS site to Supplementary Materials and reference this figure on line 266.
- The paragraph on lines 353-360 hypothesizes that perturbing the melt rate at the KIS grounding zone could modify driving stress at Site 4, through a modification of basal friction. Couldn't you use the ice sheet model to test this proposed process? ISSM simulates changes in driving stress and the corresponding change in basal friction due to the simulated melt rate perturbations. You could analyze the changes in the force balance at Site 4 to address this. Please either add this analysis to the paper or provide text explaining why this is not possible with your model configuration.
- Wherever possible, begin each paragraph in the Discussion section with a topic sentence that describes the main result that is being discussed in the paragraph. For example, on line 312, change the topic sentence to: "We model a seasonal signal in velocity variability that is similar in phasing and magnitude at GNSS Sites 1 and 2 but not Sites 3 and 4." Another example is on line 362, where the topic sentence could be changed to: "The melt rate perturbations used in our modelling experiments are realistic for Sites 1 and 2 but less realistic for Sites 3 and 4." Please go through the Discussion to find other opportunities to make changes to topic sentences to clarify the result being discussed in the paragraph.
Editorial[line 13] The word "today" seems out of place here. Can it be removed?
[line 126] I suggest adding a reference to https://doi.org/10.5623/geomat-2005-0004 to cite the CSRS-PPP specifically.
[line 175] Add a sentence to Section 2.4 stating that one set of simulations was run in which the basal melt rates were perturbed at locations where there was sensitivity in the velocities for any of the GNSS locations (as opposed to separate simulations where the melt rates were perturbed for each individual GNSS location).
[line 181] Replace "raw" with "unperturbed"
[Figures 2 and 4] Is the black line showing the grounding line? If so, state that in the caption and change the passive ice outline from black to a different color.
[Figure 4] Add labels and arrows showing the locations of Roosevelt Island, Crary Ice Rise, Steershead Ice Rise, the Shirase Coast Ice Rumplus, Byrd Glacier and any other locations that are refered to in the text when describing this figure.
[Figure 5] Color each dotted black line using the same colors as the solid lines to denote the melt rate perturbation magnitudes.
[line 264] The text states that "use of the lower sensitivity value did not significantly affect the final modelled velocity variations" and Figure 5 shows that is indeed correct for sites 1-3 but for site 4, it looks like the velocity peaks are about 30% larger for the highest perturbation magnitude. I suggest quantifying the differences between the two simulations that used different sensitivity thresholds.
[lines 340-341] This sentence needs to be reworded: "Figure 2 highlights that basal melt rates are perturbed at the Ross Island region for Sites 1, 2 and
3." to something like "Figure 2 shows that velocities at Sites 1, 2, and 3 are most sensitive to basal melt rate perturbations at the Ross Island region."[lines 350-352] This sentence is repetitive and states the same thing as the previous paragraph. Please delete this.
[line 361] Change this section heading to: "Comparison to observed basal melt rates beneath the Ross Ice Shelf"
[line 384] Please define what "short-term" basal melt rates are. This is the first mention of this term and it is not clear how this is defined.
[Figure 6] Similar to the previous comment, please define the timespans that "short-term" and "mean" are covering in the figure caption.
[lines 419-423] Are these "additional" experiments ones that are already described in the paper? If not, the configuration and results from these additional experiments need to be included in paper. They can be added to Supplementary Materials or an Appendix but please add figures of (1) the locations where melt rates were perturbed and (2) the resulting velocity variations.
Citation: https://doi.org/10.5194/egusphere-2023-2793-RC1 - AC1: 'Reply on RC1', Francesca Baldacchino, 15 Mar 2024
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RC2: 'Comment on egusphere-2023-2793', Anonymous Referee #2, 16 Feb 2024
The manuscript presents observations of velocity variations on the Ross Ice Shelf recorded at 4 GNSS sites. The authors identify areas of the ice shelf where changes in basal melt could affect the velocity at each of these sites. They apply perturbations to basal melt rates using the ISSM model and estimate how much melt is required (and in which areas) to reproduce the observed behavior.
Seasonal velocity fluctuations on the Ross Ice Shelf have been described and analyzed previously (e.g. Klein et al., 2020, Mosbeux et al., 2023), with a determination that basal melt was relatively unimportant and sea surface height, tides or other factors influencing sliding at the grounding line were likely to be the dominant driving factors. The work presented here returns to the basal melt hypothesis and appears to follow a similar analysis as Klein et al., but with different conclusions. In some places the approach is not clear, and I do not think these new conclusions are justified by the analysis as it is presented in the paper.
The findings of Klein et al., 2020 and Mosbeux et al., 2023 are referenced but then appear to be largely disregarded until the discussion. In particular, the potential influence of changes in sea surface height and ocean tides is completely overlooked. Tides are known to cause substantial variations in velocity over short periods (e.g. Anandakrishnan et al., 2003; Doake et al., 2002), and also potentially over long periods of up to a year (e.g. Murray et al., 2007). The GNSS processing smooths out short-term tidal effects, but I expect daily variability is large, being previously observed nearby at up to 100% of the mean (e.g. Brunt et al., 2010). It is not inconceivable that small, solar annual or semi-annual tides could drive the remaining <1% semi-annual variations in velocity shown in Figure 3 and it needs to be explained why they can be ignored.
Secondly, it is not clear to me why a more realistic melt forcing was not used? The forcing used here (and required to match behavior at sites 1 and 2) is symmetric with two peaks, when observations from Stewart et al., 2019 show only one dominant peak in the northwest region near Ross Island. Also the peak melting observed by Stewart et al., 2019 occurs consistently in February while here melt peaks are applied in April and October, and the magnitude of the melt perturbations appears to be substantially higher than observations, with 20 m/a additional melt required, presumably on top of a baseline rate?
My fundamental issue with the paper is that without an understanding or attempt to account for other more dominant factors that are driving variability, I don’t think it can be concluded with any confidence that seasonal melt is driving (or even influencing) seasonal velocity variability at any of the sites.
Specific comments:
Abstract: ‘sensitive regions’ are mentioned several times before it can be determined that this means regions where ice velocities are sensitive to basal melt
Line 12: “We suggest that… velocity variations… could be partly…. driven by melt”. This is a very tentative conclusion and implies that you don’t believe this to be the case either.
Line 55: sp. Siple Coast ice streams
Line 70: I would argue that GNSS doesn’t provide a unique opportunity to measure seasonal variations in velocity – satellite methods can also do this – although GNSS does have better resolution and accuracy.
Line 70: sp. MacAyeal
Line 80: While you discuss the work of Klein et al., and Mosbeux et al. in the previous paragraph, the aims and approach taken in this paper do not follow from this discussion.
Line 101-104: These two sentences contradict each other.
Line 129: What is meant by ‘data processing was iterated’? How are initial positions updated and until what criteria are met?
Line 132: What is the ‘reported processing uncertainty’?
Line 133: More detail is required here to understand the processing steps and error propagation.
Line 136: Is aliasing the correct word here?
Line 141: Where is this presented?
Line 159: Is melt varied across the whole domain or locally? Assuming these units are m/a (velocity) per m3/a (melt), it is not clear over what area of the ice shelf the melt is integrated.
Line 181: It would be useful to know what the baseline MITgcm basal melt rates are and how / if they vary seasonally before the perturbation is applied.
Fig 2 caption: The whole of the RIS and islands are also outlined in black so it is not clear which bit is ‘passive ice’.
Fig 3. What is meant by ‘errors’?
Line 247: I don’t see evidence that ‘local changes in basal melt influence the velocities at Site 4’? This also seems to contradict the conclusions (Line 486).
Line 265: What is the difference? It could be important to note here.
Figure 6: Is this the baseline dataset (i.e. perturbation of 0 m/a)? In which case why is there no velocity variability in the 0 m/a perturbation scenario (Fig 5). The mean and max imply seasonality in this baseline dataset.
Line 422: I don’t think it is justified to say that “the majority of Site 2’s intra-annual velocity variability is driven by seasonal changes in melting”.
Line 449: Duplicate reference of Liu and Miller, 1979.
Line 456: What is the mechanism for a short period of surface melting to lead to a velocity increase in the center of the ice shelf, far from any shear zone?
Line 460: This is a misrepresentation of what is stated in Mosbeux et al., (2023) who do tentatively attribute the 6-monthly signal to tides.Refs:
Anandakrishnan, S., Voigt, D. E., and Alley, R. B.: Ice stream D flow speed is strongly modulated by the tide beneath the Ross Ice Shelf, Geophys. Res. Lett., 30, 1361, https://doi.org/10.1029/2002GL016329, 2003
Brunt KM, King MA, Fricker HA, MacAyeal DR. Flow of the Ross Ice Shelf, Antarctica, is modulated by the ocean tide. Journal of Glaciology. 2010;56(195):157-161. doi:10.3189/002214310791190875
Doake, C., Corr, H. F. J., Nicholls, K. W., Gaffikin, A., Jenkins, A., Bertiger, W. I., and King, M. A.: Tide-induced lateral movement of Brunt Ice Shelf, Antarctica, Geophys. Res. Lett., 29, 67-1–67- 4, https://doi.org/10.1029/2001GL014606, 2002
Klein, E., Mosbeux, C., Bromirski, P. D., Padman, L., Bock, Y., Springer, S. R., and Fricker, H. A.: Annual cycle in flow of Ross Ice Shelf, Antarctica: Contribution of variable basal melting, J. Glaciol., 66, 861–875, https://doi.org/10.1017/jog.2020.61, 2020
Mosbeux, C., Padman, L., Klein, E., Bromirski, P. D., and Fricker, H. A.: Seasonal variability in Antarctic ice shelf velocities forced by sea surface height variations, The Cryosphere, 17, 2585–2606, https://doi.org/10.5194/tc-17-2585-2023, 2023
Murray, T., Smith, A., King, M. A., and Weedon, G. P.: Ice flow modulated by tides at up to annual periods at Rutford Ice Stream, West Antarctica, Geophys. Res. Lett., 34, L18503, https://doi.org/10.1029/2007GL031207, 2007
Stewart, C.L., Christoffersen, P., Nicholls, K.W. et al. Basal melting of Ross Ice Shelf from solar heat absorption in an ice-front polynya. Nat. Geosci. 12, 435–440 (2019). https://doi.org/10.1038/s41561-019-0356-0Citation: https://doi.org/10.5194/egusphere-2023-2793-RC2 - AC2: 'Reply on RC2', Francesca Baldacchino, 15 Mar 2024
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RC3: 'Comment on egusphere-2023-2793', Anonymous Referee #3, 14 Mar 2024
- AC3: 'Reply on RC3', Francesca Baldacchino, 15 Mar 2024
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