Diagnosing the sensitivity of grounding-line flux to changes in sub-ice-shelf melting

Zhang, Tong; Price, Stephen F.; Hoffman, Matthew J.; Perego, Mauro; Asay-Davis, Xylar

doi:https://doi.org/10.5194/tc-14-3407-2020

Articles | Volume 14, issue 10

https://doi.org/10.5194/tc-14-3407-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-14-3407-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 10

Research article

|

16 Oct 2020

Research article |

| 16 Oct 2020

Diagnosing the sensitivity of grounding-line flux to changes in sub-ice-shelf melting

Tong Zhang, Stephen F. Price, Matthew J. Hoffman, Mauro Perego, and Xylar Asay-Davis

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Final revised paper (published on 16 Oct 2020)
Supplement to the final revised paper
Preprint (discussion started on 11 Feb 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review of tc-2020-12', G. Hilmar Gudmundsson, 15 Feb 2020
- AC1: 'Response to Reviewer 1', Tong Zhang, 07 Jun 2020
RC2: 'Comment on 'Diagnosing the sensitivity of grounding line flux to changes in sub-ice shelf melting'', Anonymous Referee #2, 12 Mar 2020
- AC2: 'Response to Reviewer 2', Tong Zhang, 07 Jun 2020
RC3: 'Review of tc-2020-12', Anonymous Referee #3, 13 Mar 2020
- AC3: 'Response to Reviewer 3', Tong Zhang, 07 Jun 2020

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (08 Jun 2020) by Olivier Gagliardini

AR by Anna Wenzel on behalf of the Authors (17 Jun 2020) Author's response

ED: Referee Nomination & Report Request started (26 Jun 2020) by Olivier Gagliardini

RR by G. Hilmar Gudmundsson (30 Jun 2020)

Suggestions for revision or reasons for rejection

The manuscript has been improved greatly and is not in a much better shape.

Personally, I still find that the search for a correlation between locally derived buttressing numbers within an ice shelf and integrated grounding-line flux is a bit beside the point. There will be some correlation because for confined ice-shelves local changes in ice thickness impact both grounding-line flux and the local measure of buttressing. But maybe this is just a question of how to think about these processes. I like to think about the thickness affecting the stresses. The fact that we then many have a correlation between stresses at two different locations, or between stresses where the thickness perturbation was applied and GL flux, is somehow less fundamental to me. But everyone should be entitled to have their own favourite way of thinking about these things. The bottom line is that using locally defined buttressing numbers within an ice shelf as a diagnostic for impacts on GLF is not a particularly sound idea and, as the authors find, any such relationship is tenuous at the best.

The way the paper is written at the moment I feel there is a potential for some confusion. As far as I can see there are two main points in the paper. One relates to the usefulness of local buttressing numbers within an ice shelf as a diagnostic for impact on GL flux (bad idea), the other one is a purely methodological point: i.e. how to compute the impact of thickness perturbations on GL flux in a computationally efficient way. This second main point of the paper (use the adjoint method to speed up things) has nothing to do with the first point. But as the paper is written at the moment, it might appear to a reader that is not too careful, that the adjoint-based sensitivity calculation somehow links to the first point, or that it even offers a solution to the (lack) of correlation between locally calculated buttressing and GL flux.

I don’t see how the Rees approach can be considered ‘ad hoc’. It’s just computationally inefficient. In fact, one could argue that the Rees approach as the advantage that it does not have the inbuilt linearization of the adjoint method. The adjoint approach is much better at estimating the linear response than the finite-differences methods. It is both more accurate, and faster. But the (first-order) adjoin approach use by the authors cannot be used to estimate the range (amplitude) of the linear response as done by Rees. Also, in this particular example the adjoint method will not really make that much of a difference unless one only want to know the integrated impact on GL flux. As soon as one wants to calculate the perturbation at each and every point along the grounding line (as done by Rees) I suspect the computational efforts using the finite-differences and the adjoint methods will become similar.

Not sure about this as I’m not that familiar with the notion, but is (6) not generally true whenever atm. pressure is neglected?

I feel that the discussion about ice-shelf buttressing should reference the papers by:

Pegler, S. S.: Suppression of marine ice sheet instability, J. Fluid Mech., 857, 648–680, doi:10.1017/jfm.2018.742, 2018.

Pegler, S. S.: Marine ice sheet dynamics: the impacts of ice-shelf buttressing, J. Fluid Mech., 857, 605–647, doi:10.1017/jfm.2018.741, 2018.

Haseloff, M. and Sergienko, O. V.: The effect of buttressing on grounding line dynamics, J. Glaciol., 64(245), 417–431, doi:10.1017/jog.2018.30, 2018.

This could, for example, be done early in the paper (for example at the end of line 25) but there are number of other places were this could be added. These three papers are quite important addition to the literature.

Citation to Cornford 2020 should be undated to refer to the final published version.

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RR by Anonymous Referee #3 (09 Jul 2020)

ED: Publish subject to minor revisions (review by editor) (16 Jul 2020) by Olivier Gagliardini

AR by Tong Zhang on behalf of the Authors (25 Jul 2020) Author's response Manuscript

ED: Publish as is (21 Aug 2020) by Olivier Gagliardini