|Review of ‘Responses of sub-ice platelet layer thickening rate and frazil ice concentration to variations in Ice Shelf Water supercooling in McMurdo Sound, Antarctica’ following reviewer comments and revision|
The revisions and corrections made to this manuscript have significantly improved both its interest to the community, and clarity for the reader. However, some of my original comments have not been adequately considered, and in my opinion, would need to be properly addressed (both in response to reviewers and by modifying the article text) before the manuscript is ready for publication.
1. All references to ‘temperature’ should be changed to ‘potential temperature’ (assuming this is indeed the quantity used). For example, the text in P2-L15 was modified in response to Reviewer #1’s comments, to support the author’s assertion that temperature can be treated as vertically-uniform within the ISW plume. Unless the qualifier ‘potential’ is used, this is incorrect, and not supported by observations.
2. Distinction between applicability to crystal accretions beneath sea ice and ice shelves remains unclear. In their response to my review, the authors note that “Beneath the platelet ice layers the supercooling is produced by the pressure drop experience by ISW at it emerges from beneath an ice shelf and rise towards the sea surface, while regions of marine ice accretion beneath ice shelves have typically very low basal slopes.” I completely agree with this, and am therefore puzzled as to why the authors seem reluctant to identify this difference to the reader. This is exactly the mechanistic driver that would allow the sub-ice boundary layers to behave differently.
I am not suggesting that the work (tested for platelet layers beneath sea ice, and potentially applied to marine ice layers beneath ice shelves) is invalid. However, I do think that a statement clarifying the differences between the regimes is necessary for the reader (and may protect the authors of the present study in the event that their model is naively applied to a regime for which it has not been validated). Specifically, the potential sources of divergence are:
a. The difference in basal slope is likely to result in the ISW plume existing much closer to the in-situ freezing temperature than is observed in McMurdo Sound, with the result that in-situ supercooling is likely to be much smaller (and potentially similar to the resolution of present-day instruments. i.e. unobservable except by implication) beneath ice shelves;
b. The difference in basal slope also has implications for generating buoyancy-induced momentum, which is the implicit source of the background current in Hughes et al. (2014), and therefore would need to be excluded (or vastly reduced) for application to an ice shelf cavity.
c. The authors allude to another difference with their statement “The one indirect effect on the ISW plume might be an increased drag coefficient beneath the platelet layer, where more rapid freezing of the deposited crystals may create more irregularity in the form of the ice-ocean interface.” This is true, and will affect the sedimentation process.
d. In addition to the above, the length of time over which crystal accretion may occur is vastly different (i.e. about 1-3 years in McMurdo Sound vs tens-hundreds of years beneath ice shelves). Combined with the likely difference in degree of supercooling, this could potentially lead to very different internal structures of the crystal layers (e.g. marine ice layers more likely to collapse under accreted buoyancy), and hence present different physical boundaries to ocean flow, and an entirely different source of effective hydrodynamic drag (as suggested by Robinson et al., 2017, which the authors cite).
e. Finally, the observed supercooled plume in McMurdo Sound, having only recently experienced the step-change in pressure, is still adjusting to the change through active ice formation (onto both suspended frazil and to accreted platelet ice), and will therefore come to a point of equilibrium at some point beyond where the observations to date have been made. This represents a significantly different thermodynamic regime to the general situation of an ISW plume beneath an ice shelf.
This specifically relates to P11L8-10 of the revised manuscript, since in the general sub-ice shelf regime, the supercooled layer will almost certainly not approach the thickness observed in McMurdo Sound. This may have implications for the regime for which ‘the efficiency of converting ISW supercooling into frazil concentration … is determined by the suspension index’, since this is true only when ‘the thickness of a supercooled layer of ISW is large enough’ (P11L8-10) (i.e. greater than 65 m for the McMurdo Sound parameters).
I suggest that the addition of a well-crafted paragraph of text outlining the potential differences between the regimes would be sufficient to both demonstrate that the authors understand the implications of these differences (and I am convinced they do), and highlight to the reader where caution (and/or improved understanding) is required in applying this model to the sub-ice shelf regime.
3. Justification for values of parameters used, and acknowledgement of available observations. In their response, the authors point to the lack of ‘observational guidance’ as justification for the extensive tuning of specific parameters. I agree, there are very little data available to constrain the models. However, the values they have chosen do find support in the literature, and it would strengthen the paper to acknowledge these. In particular (P7L9-10):
a. ISW outflow properties: the chosen values for temperature and salinity coincide with those reported by Hughes et al., 2014;
b. Platelet layer basal drag coefficient: the value chosen fits appropriately within the range identified by Robinson et al., 2017;
c. Frazil ice crystal size distribution: unknown, but presumably these are chosen from somewhere – perhaps previous modelling studies? Similarly for the Shields criterion?
d. Ambient current speed: The chosen value is consistent with the lowest speeds reported by Robinson et al., 2014 (although lower than their reported residual flow).
e. In addition, the observations in both Hughes et al. (2014) and Robinson et al. (2014) papers show the homogeneous ISW layer (observed in the centre of the modelled plume flow) as being 150 - 200 m thick, and the supercooled portion extending to 60/70 m. I would have thought these would be useful reference points for this manuscript.
4. My concern about the apparent resolution in figures 5 & 6 still stands: the separation of the contour lines, especially around the core of the plume, implies greater resolution than the model contains. A potential solution may be to plot only every second contour line.