|Seroussi et al. explore the sensitivity of the Pine Island Glacier (PIG) to various climate forcings (change in surface mass balance, calving front position and sub ice shelf melting) using the Ice Sheet System Model (ISSM). They demonstrate the importance of sub ice shelf melting and that a better description of this boundary condition is required to improve the reliability of sea-level rise projections. This work is well described, with relevant results and authors fairly answered to previous reviewers’ comments. This clearly deserves publication. I however have one regret and one suggestion.|
ISSM, grounding line dynamics and MISMIP3d
Authors answer to one comment from the second reviewer that ISSM has been validated against MISMIP3d. Then, one could question why it is not mentioned in the paper? Actually, only the Shallow Shelf Approximation (SSA) implemented within ISSM has been evaluated so far against the MISMIP3d setup (and results were in conflict with other SSA models owing to parametrization of grounding line position, see Seroussi et al. 2014). The present study is made using the Blatter-Pattyn (BP) approximation. To that respect, the model used in this work has not been evaluated against MISMIP3d. This therefore justifies to not refer to the MISMIP3d benchmark in the present work.
Authors stressed in the past that solving higher-order approximations of the Stokes equations is important for proper modeling of the PIG flow (Morlighem et al., 2010). Consequently, they use PB in the present work. I regret to not see their BP model evaluated against MISMIP3d. I personally believe that any marine ice sheet model producing mass balance projections of actual glaciers should be first evaluated against MISMIP3d. This is not always the case (e.g. Joughin et al. 2014) and not here as well. It has to be noted that the authors made significant efforts to show that their results are not affected by mesh resolution (annex). I let at the appreciation of the editor whether further validations are required or not. To my opinion, validation against MISMIP3d would be a valuable annex to this paper or maybe a part of the Seroussi et al. 2014 (currently under revision in TCD). This would give more confidence in the result obtained in the present study and any further similar work the authors could produce. But I would understand that it is out of the scope of that paper.
Modeled grounding line retreat in disagreement with current observations?
Authors shows the evolution of their modeled grounding line in Figure 6. Modeled grounding line retreat seems in contradiction with current observations (Rignot et al., 2014): the model is not retreating in the main trunk (observations shows that it does) but exhibits large retreat in the eastern part of the ice shelf (not observed). I believe this should be clearly stated in the present work and some related caveats should be formulated when discussing the coming evolution of the PIG deduced from their simulations.
May this contraction with observations come from an inappropriate description of the grounding line dynamics (and evaluation against MISMIP3d may help to exclude this hypothesis)? I am more inclined to believe that it is mainly due to the melt rate distribution applied, here imposed from MIT GCM outputs. Then, concluding « coupling of the ice sheet with ocean circulation models is therefore desired for future studies to conduct more accurate simulations » seems a bit overstated to me. Authors rather show that imposed melt rate is a very sensitive parameter for a marine ice sheet model and that the melt distribution currently computed from the MIT GCM does not allow to properly reproduce some observed critical changes using this particular ice sheet model. I would suggest to reformulate related sentences.
I would suggest to use the same vertical axes in panel a, c, e and b, d, f of Figure 5. Comparison between panels would be easier.
Joughin, I.; Smith, B. E. & Medley, B. Marine Ice Sheet Collapse Potentially Underway for the Thwaites Glacier Basin, West Antarctica Science, 2014.
Morlighem, M.; Rignot, E.; Seroussi, H.; Larour, E.; Ben Dhia, H. & Aubry, D. Spatial patterns of basal drag inferred using control methods from a full-Stokes and simpler models for Pine Island Glacier, West Antarctica Geophysical Research Letters, 2010, 37.
Rignot, E.; Mouginot, J.; Morlighem, M.; Seroussi, H. & Scheuchl, B. Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011. Geophysical Research Letters, 2014.
Seroussi, H.; Morlighem, M.; Larour, E.; Rignot, E. & Khazendar, A. Hydrostatic grounding line parametrization in ice sheet models. The Cryosphere Discussions, 2014, 8(3), 3335-3365.