Interaction of marine ice-sheet instabilities in two drainage basins:  simple scaling of geometry and transition time

Feldmann, J.; Levermann, A.

doi:https://doi.org/10.5194/tc-9-631-2015

Articles | Volume 9, issue 2

https://doi.org/10.5194/tc-9-631-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-9-631-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 9, issue 2

Research article

|

02 Apr 2015

Research article |

| 02 Apr 2015

Interaction of marine ice-sheet instabilities in two drainage basins: simple scaling of geometry and transition time

J. Feldmann and A. Levermann

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Final revised paper (published on 02 Apr 2015)
Supplement to the final revised paper
Preprint (discussion started on 12 Sep 2014)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

AC C1810: 'Correction of a sentence', Johannes Feldmann, 16 Sep 2014
RC C2093: 'review of tc-2014-134', Stephen Cornford, 17 Oct 2014
RC C2330: 'review of tc-2014-134', Anonymous Referee #2, 12 Nov 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Johannes Feldmann on behalf of the Authors (18 Dec 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Dec 2014) by Frank Pattyn

RR by Stephen Cornford (05 Jan 2015)

RR by Anonymous Referee #2 (28 Jan 2015)

Suggestions for revision or reasons for rejection

The authors have clarified that the results they presented in their first submitted version of the paper was solely due to ocean-induced melt being applied upstream of the grounding line in their model. I thank the others for their clarification.

The authors argue that this `smearing’ (my term) is physically plausible. In the words of the authors, doing so avoids `loss of influence of the ocean onto a fully grounded ice sheet’.

I find this questionable. Why should there be a substantial ocean-induced melt over the grounded part? I can’t see any reasons for that. The subglacial drainage system drains water from the interior towards the ocean. The ocean water masses can hardly flow against the water flow and upstream over the grounding line. So how can the ocean gain access to the grounded ice and cause melting? The process is unclear to me, and I know of no observations showing zones of high melting upstream of the grounding line due to the ocean.

If this `smearing’ of ocean melt across the grounding only introduced some small errors in the simulation, then this would not be too much of an issue. But in the work presented the whole response of the grounded ice is entirely caused by this.
The authors argue that the behaviour is independent of forcing and they have now added an abstract (not in the original submitted version of the paper) where they reason that the response can be produced by applying a perturbation in surface accumulation as well. I have no difficulties in accepting this. Yet the paper still discussion the ocean-induced melt experiments and the authors appear to consider their treatment of the melt to be correct. I disagree. The treatment of the ocean-induced melt is quite simply incorrectly done in the model.
The paper could be rewritten by getting rid of the ocean melt experiments and showing only the upper surface mass-balance perturbations. Then the paper would then be methodologically sound. But as it stands with most of the discussion focusing on perturbations in ocean melt the paper the paper is fundamentally flawed.
I suggest rejecting the paper in its current form, but with the understanding that a rewritten paper were all the perturbations are applied over the surface, could be submitted. Giving the fact that the descriptions of the perturbation experiments will have to be rewritten, just about all the figures and the runs presented in the paper redone, and the discussion rewritten, I don’t see how one can only suggest a resubmission. It is also worrisome that in the first version of the paper there was no discussion about the fact that he whole response of the perturbations was due to this `smearing’ of ocean-induced melt over the grounding line.

The paper also raises the uncomfortable question if grounding line is treated accurately in the model. Is this `smearing’ of the melting rate over the grounding line somehow a consequence of how the grounding line is treated in the model? Can the authors, for example, apply significant ice-shelf melt WITHOUT the grounding line migrating? Or is it inherent in their model treatment of ocean melt that some melt will occur above the grounding line? And what about other fields? If this `smearing’ happens to the melt, then does it also happen to the basal slipperiness, i.e. does the basal slipperiness also gradually go to a finite value as the grounding is crossed over a the same distance as the basal melt rate goes from a finite value to zero?

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RR by Anonymous Referee #3 (30 Jan 2015)

Suggestions for revision or reasons for rejection

I have read the two initial reviews, the authors reply as well as the new version of the manuscript. I think the idea beyond this study is interesting and the results obtained deserve to be published. The new version has been improved after this first step review. Nevertheless, I have still some comments and suggestions to improve the manuscript, as well as some typos.

General Comments:
- despite the fact that it is shown that the way the initial perturbation is applied doesn't play a role in the obtained results, it would be nice to explain the basal melting perturbation more deeply. I am not able to understand what is really done by just reading the sentence page 4 (lines 25-27). The accumulation perturbation is even better explained in the appendix. You should make clearer what is really done to force the GL to retreat on RHS basin.

- The notation used (xc, xs, xd, bc) should be added in Figure 3. Also, from my understanding, only bc is varied. Are the elevation in xs and xd keep constant should be specified and values given somewhere.

- Because of the time involved (few ka) for the perturbation to propagate, I am wondering what would be the influence of accounting for isostasy? The bedrock topography will not stay symmetric during the perturbation phase. This missing process might change the conclusion of the paper. At least, this should be mentioned and discussed a bit in the conclusion section.

- I don't think the comparison with the Vialov profile is pertinent. By imposing from the model the dome elevation and length of the ice-sheet, this is no surprise that the Vialov shapes are in relatively good agreement. I guess that a parabola going through these two points with horizontal asymptote at the dome would do a reasonable job too? I would suggest to suppress this section.

Minor remarks:
- page 12, line 7: it should be mentioned that it is equivalent to a plane strain formulation for a 'true' flow line model.
- page 12, line 9: isn't it x=800km for the front position?
- page 12, line 11: the Delta_y spacial resolution is of no interest (should not have any influence on the solution).
- page 15, line 16: Isn't it Q_g instead of Q_q?
- page 16, line 3: cannot be compensated the decrease -> cannot be compensated by the decrease (?)
- page 17, line 7: why the Vialov profils are not shown in panel a also? (In fact i would suggest not to show the Vialov profile, see above)
- page 17, line 7: from our simulation -> from our steady simulation
- Equation 7: give value for Delta_t_min, DeltH,min. What is C? A constant? Give its value?
- page 20, line 15: it should be made reference to the Appendix where the accumulation perturbation is explained in detail.
- page 22, line 7: profiles differ at the end -> profiles differ only at the end
- Figure 1: add the other notation xc, ..., bc
- Figure 4, legend: The bottom panel is the c panel
- Last figure is 11 not 1.

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ED: Publish subject to minor revisions (Editor review) (09 Feb 2015) by Frank Pattyn

AR by Johannes Feldmann on behalf of the Authors (13 Mar 2015) Author's response Manuscript

ED: Publish subject to technical corrections (19 Mar 2015) by Frank Pattyn

AR by Johannes Feldmann on behalf of the Authors (19 Mar 2015) Manuscript