Summary

This paper presents novel simulations of an idealised representation of Ilulissat Icefjord, West Greenland, quantifying the impact of iceberg submarine melting on the circulation and water properties in the fjord and on the submarine melt rate of the Jakobshavn Isbrae icefront, from winter through to peak summer. The simulations show that the effect of icebergs is significant, causing substantial cooling and freshening over their draft as well as more modest but still significant changes in the deep basin, plus changes to the strength and pattern of fjord circulation. The authors go beyond this, and also show that these changes to the fjord water properties lead to a reduction in the neutral buoyancy depth of the subglacial discharge-driven plume, which consequent impacts on glacier submarine melt rates, fjord circulation and fjord water properties. These are novel findings, which go beyond previously published research on iceberg-ocean interactions in Greenland’s fjords, providing insights into the interactions between iceberg melt, plume dynamics and fjord circulation that were not previously available. However, I believe the paper is hampered by three main issues, which I outline below.

Firstly, the introduction as written does not provide sufficient context to motivate the work. The importance of icebergs in glacial fjords is a relatively new field of research, and many who have not been following that research closely will not necessarily appreciate the need to conduct the experiments presented here intuitively, and will likely not grasp the potential importance of the potential links between iceberg-induced changes to water column properties, the vertical distribution of glacier submarine melt rates and calving rates. I suggest that the introduction be lengthened (it is currently only three paragraphs) so that it can provide a more thorough narrative to introduce the work, ensuring that the potential importance of vertical variations in water properties (and how icebergs might affect those) for iceberg calving are explained. Introduction too short and does not provide sufficient context or background to motivate the work. This is not too onerous a task, but it is an important one if readers are to understand and appreciate the importance of the results.

Secondly, many of the key results in the manuscript focus on mixing and recirculation within the fjord. They will therefore depend somewhat on mixing and diffusion within the ocean model. I was therefore a little surprised that no analysis quantifying the sensitivity of the results to choices of model viscosity and diffusivity parameter values was presented. To remedy this, I think these sensitivity analyses need to be conducted; at the very least, a few simulations testing higher and lower values of vertical eddy viscosity, and horizontal and vertical diffusion coefficients of temperature and salinity in the IBP setup. It may be that they make little difference to the behaviour of IMAW and GMW, but this needs to be demonstrated with simulations in order to ensure the results are robust to these choices.

Thirdly, a considerable portion of the results and discussion is dedicated to (1) entrainment of GMW into the deep basin and (2) cooling of inflowing water by icebergs, which subsequently enters the deep basin, and the behaviour of these water masses form the basis for some of the key results in the manuscript. However, no definition or means of identifying GMW and its fate is provided, nor is the inflowing water tracked, which somewhat undermines the related results and discussion. To remedy this, you need to describe your method for identifying GMW, assuming one is used. In addition, I think it would greatly strengthen the manuscript if you were to perform a small number of additional experiments in which tracers are included at the ocean boundary (and perhaps also in the subglacial discharge) as these would unambiguously demonstrate the changes in GMW outflow dynamics and allow you to calculate precisely the relative contributions of IMAW and GMW to the deep basin.  

I believe this paper presents important and novel findings of iceberg-ocean interactions in Greenland’s glacial fjords, particularly with regard to glacier submarine melt and mélange dynamics; however, more detail, clearer methods and some additional context is required to make this paper robust enough to be published.

Below, I provide more specific comments going through the paper line by line:

Line 1: “marine terminating” should be “marine-terminating”

Line 3: consider changing “at depth” to “vertically” or “along the iceberg draft” or similar.

Line 3: “contributing to fjord stratification, thus impacting melt and dynamics at the front”. There’s a lot to unpack here. Firstly, “contributing to fjord stratification” is somewhat ambiguous. Secondly, regarding “melt”, this statement is written as if there is already substantial evidence showing that icebergs affect glacier submarine melt rates and that it is widely known, which I don’t think is the case (though this manuscript is of course a start). Thirdly, regarding “dynamics”, it’s not clear if this refers to the dynamics of the circulation at the ice-ocean interface or the dynamics of the ice front itself (i.e. calving). The former is a logical progression from a change in fjord stratification, but the latter is quite a jump. Consider rewording this sentence to focus on the known (albeit simulated) affects of iceberg melt on fjord water properties and circulation, but state that the impacts of those on glacier submarine melt rates are poorly known.

Line 3-4: “We model the high-silled…” -> suggest changing to “We model an idealised representation of the high-silled…”.

Line 4 and elsewhere: I suggest being more specific than just “the effect of icebergs”, and clarify (at least on the first use in the main text) that this means the effect of submarine iceberg melting.

Line 5: change “fjord properties” to “fjord water properties from winter through August” (or similar)

Line 6: “seasonality” doesn’t seem quite right here, given the simulations covered a winter period as an initialisation then the rising limb of the summer hydrograph. I suggest changing this to “primary driver of fjord circulation, glacier melt and iceberg melt during the melt season”

Line 6-7: Suggest changing “Icebergs are necessary to include to correctly…” to “Including submarine iceberg melting in the simulations is required to reproduce the observed water properties in…” (or similar).

Line 8: Consider providing representative values for the amount of freshening and amount of neutral buoyancy depth depression (though I appreciate these are sensitive to various model parameters)

Line 10: “increased entrainment of glacially modified water into the fjord” – should “entrainment” instead be “recirculation” or “reflux”? Also, consider linking this point to the depression of the depth of glacially modified outflow.

Line 10: Consider changing “ambient water” to “shelf water” or similar.

Line 11: Consider changing “limits melt to the deep section of the front…” to “limits the vertical extent of plume-enhanced glacier melting…” or similar, because melt will of course occur across the entire front.

Line 13: again “impact of icebergs” should really be “impact of submarine iceberg melting”

Line 14 and elsewhere: “melange" should be “mélange”  

Line 18: “controlled by the fjord geometry and fjord stratification” is somewhat ambiguous (what aspect of the glacier is controlled?) and excludes all the other important factors that affect glacier behaviour, particularly external forcing. If this point is necessary to introduce the manuscript, then I suggest a longer explanation is required.

Line 20: Clarify that this refers to “future sea-level contribution estimates”

Line 20: “Greenland ice sheet” should be capitalised as has been done on line 16. 

Line 22: I suggest that (1) submarine melt rate and/or our ability to model submarine melt rates, and; (2) our knowledge of water properties at the ice-ocean interface, should be included in this list. Consider also being more specific for each of the items currently listed. For example, with “calving”, does this mean overall calving rates, or some particular aspect of calving? Similarly, should it be “subglacial discharge volume”?

Line 22: “shape of the plume” – I assume this refers to the geometry of the source; however, plumes have not been introduced yet, so it’s not clear what is meant here (see first major comment).

Line 23: I suggest that “ocean driving retreat” is perhaps overly simplistic and not a fair representation of the current state of knowledge. There is for example quite considerable evidence that increasing runoff, driven by rising atmospheric temperatures, would increase submarine melt rates and potentially affect glacier terminus position. Consider changing this to something like “evidence that changes submerged glacier geometry due to changes in the magnitude and spatial distribution of glacier submarine melting are a key control on marine terminating glacier retreat rates”

Line 26: Again “melange” should be “mélange”

Line 27: Please provide at least one appropriate reference for this statement (the connection between dense mélange and glacier calving). It would also be clearer if the direction of this connection (that mélange presence suppresses calving) is stated in this sentence, rather than in the subsequent one.

Line 27-29: Consider also providing the counter example, that anomalously long periods of mélange absence are associated with anomalous glacier retreat, with appropriate references.

Line 31: Moon et al. (2018) didn’t present observations showing icebergs modify fjord water properties in fjords, so I don’t think it is an appropriate reference here.

Line 32: specify that the comparison between iceberg freshwater flux and subglacial discharge is on annual timescales (which I think has to be for this statement to be true)

Lines 34-35: whilst I agree with this statement, I do not think that it follows logically from the preceding sentences. I strongly recommend revising the preceding sentences to provide a more detailed summary of how icebergs could impact the glacier front – i.e. how do they affect fjord stratification and oceanic heat delivery to calving fronts? What is the vertical pattern of that? And, based on our understanding of undercutting-driven calving, how might that affect the glacier front? (see my first major comment)

Line 36: Change “the fjord” to “Ilulissat Icefjord”, and change “to the glacier front” to “for submarine melting and undercutting of Sermeq Kujalleq”. Or, if you wish to keep this sentence non-specific, then just say “a large west Greenland fjord” or similar.

Line 37: Change “construct a model of” to “construct an idealized model of” – I appreciate that it’s not that idealized, but the geometry is idealized.

Line 38: MITgcm has not been defined yet. Marshall et al. (1997) should also be referenced

Marshall, J., Adcroft, A., Hill, C., Perelman, L. & Heisey, C. A finite-volume, incompressible Navier Stokes model for studies of the ocean on parallel computers. J. Geophys. Res. 102, 5753 (1997).

Line 39: change “impact of icebergs” to “impact of submarine iceberg melting” or similar.

Line 40: “each phase of the discharge season” would imply that you also consider the descending limb of the seasonal runoff hydrograph, which you don’t, so I suggest changing this to “during winter, spring and peak summer” or just “throughout that time”.

Line 43: “stability” can mean lots of things, but instability usually implies continued retreat even once a forcing is removed, which I don’t think you mean here. Consider changing this to “calving rates” or similar.

Line 43: change “the glacier” to “Sermeq Kujalleq”

Line 50: “70 Gt/a” – I thought it was more like 55 Gt/a at most, so this seems a little high (see Mankoff et al. 2020). I also don’t think that Bondzio et al (2017) is an appropriate reference given it is a modelling study.

Mankoff, K.D., Solgaard, A., Colgan, W., Ahlstrøm, A.P., Khan, S.A. and Fausto, R.S., 2020. Greenland Ice Sheet solid ice discharge from 1986 through March 2020. Earth System Science Data, 12(2), pp.1367-1383.

Line 50: Suggest starting a new sentence and changing “leaving the fjord clogged with icebergs” to “The rapid supply of icebergs and the barrier presented by sill, often leave the fjord iceberg-congested”, and move this sentence to line 53 after the info about icebergs grounding on the sill.

Line 54: CTD has not been defined.

Line 58: Consider providing representative temperature and salinity values for the layers

Line 60: provide a definition for glacially-modified water i.e. a mixture of runoff, fjord water and meltwater

Line 63: Specify that you mean seasonality of water properties (and circulation?)

Line 73: It is not clear what “in front of the sill” means. Seaward of the sill? Please revise the wording so that your meaning is clear

Line 76: I’m surprised you used MITgcm in hydrostatic mode given the resolution of your domain and the inclusion of a steep sill. In high-resolution simulations with steep topography, in which vertical momentum may be important, it is common and recommended to use the non-hydrostatic mode. Perhaps it makes little difference here, but I suggest that it is at least worth running at least one simulation in non-hydrostatic mode to check.

Line 80: Specify the direction of each resolution value.

Line 81: Change “third dimension” to “vertical dimension”

Line 81: Can you clarify what is meant by “icebergs and plume width considerations”. I can appreciate how a relatively fine vertical resolution is required to faithfully represent the vertical distribution of iceberg freshwater flux, but it is not clear how it affected plume width in the horizontal direction.

Line 85: Why not just call it “source width” or “outlet width”. Calling it “plume width” is confusing, because this does not refer to a plume itself and because the width of a plume changes as it entrains ambient water.

Line 89: “much larger contributor” to what?

Line 91: Please note that the observations from Jackson et al (2017) were from a different fjord

Lines 94-101: please specify somewhere here what outlet height you use (or how you define it), what the velocity is, and which of those variables you modify in order to increase subglacial discharge throughout a simulation. Also, Cowton et al. (2015) use a fixed water velocity at the outlet and therefore accommodate all changes in runoff by changing the radius of the outlet. They demonstrated that their results were not sensitive to the choice of water velocity. However, I do not recall seeing a similar analysis for a sheet plume. Therefore, please also indicate with evidence (whether through simulations or otherwise) whether your results are sensitive to your choice of water velocity and outlet height.

Lines 94-101: please specify how you conserved mass in the domain so that the runoff did not just slowly fill up the basin (which would modify sea surface slopes and drive erroneous currents). It is typical to impose a small outward velocity at the boundary, equivalent to the subglacial discharge volume flux, but it is not clear whether that was done here.

Line 101: Please define what is meant by “bell-shaped”. Is it a Gaussian? I suggest it is also worth noting here that you do not simulate the falling limb of the runoff hydrograph as your simulations stop at the end of August.

Lines 116-119: I think this description needs some more detail and clarity with regard to where you are specifically describing boundary conditions imposed on the model boundary and where you are describing initial conditions throughout the domain. I think it would be useful to state where and when the observations in the fjord were acquired, which you use to set the conditions at and below sill depth. Assuming you initialise the model at and below sill depth with observations acquired in the fjord during summer, then that initialisation will have an imprint of glacially modified water and other summertime processes. Therefore, please provide evidence that the model reached a quasi-steady state in terms of water properties (not just circulation) during the winter portion of your simulations. Your Sup. Fig. 2 does a fairly good job at this and I recommend explicitly referring to that here.

Line 124: Change “Melt and negative salinity flux” to “Heat and salt fluxes” as there is no real freshwater flux in the IceBerg model of Davison et al. (2020)

Line 129: It is also worth mentioning that the only deterioration mechanism is submarine melt (i.e. no wave action, no mechanical breakup and no plumes – though they are crudely parameterized with the background velocity.

Line 146: again “seasonality” doesn’t seem like the right word for this, as it’s only the winter/spring and up to peak summer.

Line 152-154: My understanding of IcePlume is that the plume water is input to the model grid cell at the neutral buoyancy depth, which immediately becomes a horizontal outflow (and any subsequent dynamics are dictated by your choice of diffusion and mixing parameter values in MITgcm), and that there is no calculation of the plume vertical momentum above the neutral buoyancy depth, so I can’t work out how you calculated the continued vertical transport of the plume due to momentum. Did you modify the code? Perhaps I’m wrong and I just missed that option. Or is this sentence more a musing on what happens in reality but not in the model? If so, please clarify in the text.

Line 154: related to the above, is the 230 m outflow of GMW just the plume neutral buoyancy depth, or have you extended the plume model?

Line 154: Also related to the above, how is GMW defined here in such a way that you can specify a depth and even arrows on some figures? Did you use ptracers? I can’t see a definition in terms of water properties or currents anywhere in the manuscript, so it’s no clear how these arrows were generated if not from some tracer.

Line 160: As written, this implies that there was no inflow from Disko bay until August, which I don’t think is the case based on Figure 4.

Section 4.1: there is no mention of up-fjord currents above the GMW outflow, and indeed they don’t seem to exist on Figure 4, but shouldn’t they exist in this situation?

Line 161: the switch to melt rates in this paragraph is somewhat confusing, given that the first half the paragraph is about fjord circulation. I suggest you start a new paragraph for submarine melt rates.

Line 166: Define “early season” (May-July?) and clarify what is meant by “entrainment” – does this really mean reflux? Or eddy-driven mixing between deep basin water and GMW outflow? As written, it’s not clear what is being entrained nor what is doing the entraining.

Line 148: “properties remain constant since there is no circulation” but presumably there are some small changes in temperature and salinity because of diffusion (Sup. Fig. 2 suggests there are some changes in model avg properties), and Fig. 4 suggests there is circulation down to ~500 m.

Line 171: “Seasonality” in Section 4.2 subheading. See comments above re seasonality

Line 201: I’m not sure this reasoning holds. With greater subglacial discharge, the GMW will almost certainly be warmer because the more vigorous plume will entrain more deep basin water. This will drive more rapid iceberg melting, thereby cooling the GMW locally, such that the MITgcm diagnostic temperature remains constant. The fact that temperature remains constant in the diagnostics could therefore just be a reflection of the balance between iceberg-driven cooling and plume-driven warming.

Line 204: are these values just for the inflow? I assume they are, based on the context, but the preceding use of “volumetric flow rate” is somewhat confusing. I suggest you modify this to “increasing the up-fjord volume flux over the sill…”

Line 213: “entrainment of GMW into the inflow into the deep basin” is a bit confusing. Perhaps just “enhancing the reflux/recirculation of GMW into the deep basin” or similar would be clearer?

Line 213: “the melt rate” specify that this is now the glacier submarine melt rate. Actually, I suggest just changing this sentence to something like “These changes to the water column properties reduce average glacier submarine melt rates by X%/Y m/d and reduce the vertical extent of plume-enhanced melt rates by ~X m”.

Line 215: again, this is submarine melt, and specify that you are now referring to total submarine melt flux. Regarding “melt will be limited to the deep part of the glacier”, please specify that you are referring to plume-driven melting, because the rest of the submerged face will still be melting.

Line 219: It’s not clear how “GMW outflow” is defined in the observations, given that no current observations are presented.

Line 222: “the observed deep basin is dominated by melt” – do you mean “vertical changes in temperature and salinity and dominated by ice melt..” ?

Line 224: “we interpret this to indicate a large contribution of IMAW rather than GMW” – I agree with this interpretation but can you explain your reasoning? For example, based on the simulated circulation and/or simulated volume fluxes of GMW and IMAW.

Line 226: This is a neat suggestion that would be useful for those working in the field. This is something you can very easily test using your model output and range of experiments, by determining the isopycnal separating up and down-fjord currents. Given the potential utility of this information, I suggest you do this analysis and present it at least as text at this point in the manuscript, but possibly also as a supplementary figure.

Line 228: “we run three different plumes and three different iceberg distributions” should be “we run simulations using three plume/outlet widths and three iceberg distributions (Table 1)”. The reference to Table 1 would make more sense mid-sentence, otherwise readers may look to table one for the outcome of the sensitivity analysis.

Lines 232-234: I suggest removing the sentence beginning “Changes in the depth of the GMW outflow…” and instead changing the previous sentence to “Plume width is the primary controlling factor on the vertical extent of the plume, which impacts fjord circulation and water properties. I also suggest re-wording the next sentence to: “Decreasing plume width reduces the volume of deep basin water entrained into the plume. The plume thus rises higher in the water column and exports GMW closer to the fjord surface. The shallower and more concentrated outflow of GMW leads to reduced entrainment/reflux/recirculation of inflowing water at the sill…” As currently written, these connected and important processes appear bitty and almost unrelated.

Line 237: “entrainment into the plume is more efficient” I think you should be more specific here by stating that total entrainment of deep basin water per m³ s^-1 discharge is likely greater than with a narrow plume outlet, because of the greater surface area available for entrainment. Also “causing the plume to remain deep” should be “causing the plume to reach neutral buoyancy lower in the water column”

Line 237: “entrainment of GMW” see comments above re the use of entrainment in this context, which is particularly confusing in this instance because of the preceding discussion of an entirely different kind of entrainment.

Line 239: please quantify what is meant by “significantly higher” (double? An order of magnitude?) The reader shouldn’t have to go to the figure to check just how significant this difference is.

Line 241: This wording suggests that there are no changes in the amount of cooling and freshening in the upper layers that contain some icebergs in all experiments i.e. that there are only changes in the vertical extent over which cooling and freshening occurs. However, Fig. 9 suggests that are changes in water properties at these depths?

Lines 241-242: it’s not clear which of these contributions relate to the intermediate layer or the deep basin, or both? Please clarify your meaning

Line 300: “more realistic” – perhaps “more appropriate in Ilulissat Icefjord” would be more precise?

Line 304: no direct contact with the plume? Or the plume outflow/GMW?

Line 324: “seasonal surface warming in Disko Bay has little impact in the fjord when icebergs are included” – I think this correct, but it should be substantiated with reference to a figure and the key observation from that figure. For example, “when icebergs are included, as shown by the absence of a simulated summer surface warming in the fjord (Fig 3f-j)” or similar. Substantiating this statement is also necessary to substantiate the following statement regarding the driver of seasonal iceberg freshwater flux.

Line 328: “wintertime freshwater flux is estimates” – freshwater flux from what? Icebergs? Subglacial melt? Glacier submarine melt? Also consider changing m³ s^-1 to “cumecs” when using it in a sentence rather than as a unit.

Line 330: perhaps I’m wrong, but I thought that Moon et al (2018) kept their iceberg distributions static for each of their integrations, and that the seasonal differences in freshwater flux were largely due to changes in water temperature?

Line 336: “either the calving-multiplier effect or a destabilizing influence” There’s quite a lot to unpack here.. My understanding is that the calving-multiplier is defined as some ratio of calving flux and submarine melt flux (or rates if considering 2D), but that there could be a suite of processes that lead to the calving multiplier value at a given glacier and at a given time. In other words, the calving multiplier is just a useful term to describe the aggregated effect of lots of processes that we either can’t measure or don’t understand. So, to say melt and calving are connected through the calving multiplier is not very helpful, because they are by definition, even if there is no multiplier. Therefore, it also doesn’t make sense to distinguish a separate “destabilizing influence”, because that would be roped into any calving multiplier value. If you want to emphasise the point that changes in the rate and vertical distribution of melting can affect calving rates, potentially in a non-linear manner, I suggest you focus on describing mechanistically how you might expect that to occur (the following sentences to that to a degree), and then afterwards perhaps summarise those in terms of the calving multiplier.

Line 340-342: “melt is the main driver of calving”. As written, this implies that icebergs increase calving compared to periods where there are no icebergs. However, based on the next paragraph, I’m not sure if that’s what you mean because melt-driven undercutting would act as a stabilizing influence and reduce bottom-out rotation driven calving events, not increase them. This is never explicitly mentioned in the manuscript, but I think it would really help to clarify the argument that is being made in this and the following paragraph. However, it’s worth pointing out in the manuscript that bottom-out calving events do still occur even when the fjord hosts many icebergs. So perhaps the conclusion is something like “icebergs encourage a melt pattern that should hinder bottom-out calving, so maybe if there weren’t icebergs, we would see even more bottom-out calving?”.

Section 5.1. Somewhere here I think it would be worth mentioning the impact of icebergs on the total glacier submarine melt flux. Melt rates may not change that much at a given depth, but presumably the change in neutral buoyancy depth really affects the total melt flux?

Line 344: “strong control of the melt rate, both through discharge volume”. I suggest being more specific here and relating discharge volume to plume properties (i.e. vertical velocity)

Line 355: “we see the seasonal growth of rigid mélange as a consequence of decreased melting and calving” This is arguably one of the key outcomes of the paper, and yet it’s not clear exactly what is being suggested here? Decreased glacier melting or iceberg melting? Decreased calving allows time for the mélange to become rigid, even though less ice is being supplied to it? Also, how does this hypothesis explain years with very late or even no mélange formation? (assuming it can be applied to other fjords as well?) Given the importance of the proposition here, which is a completely different perspective on mélange-glacier interactions than has previously been supposed, I think this hypothesis needs to be explained much more clearly.

Line 355: Please also clarify whether you are suggesting that periods of mélange reduce calving in a binary manner, or whether you are suggesting that periods of more concentrated/thicker mélange will suppress calving more than times of less concentrated/thinner mélange. I would argue that the former is justified by your results, but that the manuscript presents much less evidence for the latter (if nothing else, the melt curves in Figure 10 are very similar for each different iceberg configuration, even though these iceberg configurations represent quite a wide spread of iceberg conditions).

Line 367: “increased entrainment and iceberg modification as the plume weakens” – entrainment and modification of what? (see similar comments throughout this response)

Section 5.3: given that some of the key results in the manuscript relate to mixing between water masses, I was surprised that there wasn’t a discussion here of the sensitivity of the results to choices of diffusivity and viscosity parameter values (see major comment above)

Line 402: I’m not sure that comparison to Fitzmaurice et al. (2018) is robust here. Fitzmaurice et al (2018) consider a melt parameterization for entire icebergs, so need to distinguish between periods in which the plume is attached to or detached from the iceberg. However, the IceBerg package of Davison et al. (2020) uses the 3 equation formulation for submarine melting of a portion of an ice wall, so melt rate is a function of the temperature and velocity at the ice-ocean interface, regardless of whether those currents are apparently caused by a plume or the ambient water motion. Of course, there are no plumes as such in the IceBerg package, and their parameterisation is crude, so that certainly is an area of improvement.

Line 410: “glacier’s response to external forcing” this statement is too general to be supported by the results. I suggest being much more specific and instead focus on the suggestion made earlier in the manuscript regarding mélange rigidity.

Line 416: “early in the season” – specify this means melt season.

Line 418” “undercutting and thus calving” – see my comment above regarding this and being specific about the direction of the relationship

Typos and very minor edits

Line 37: remove “rapidly calving” (as the fjord is the subject)

Line 38: change “ocean model” to “ocean circulation model”

Line 46: change “Western” to “West”

Line 47: “fastest”

Line 54: “year, however expendable” should be “year; however, expendable”

Line 78: switching between km’s and km. I don’t really mind which is used, “km” is probably more common, but be consistent.

Line 79: The sill is 5 km from the western boundary?

Line 79: change “in front” to “west”

Line 82: “monthly conditions” à “monthly condition”

Line 87: check the formatting for reference to Motyka et al. (2011)

Line 103: “Mid-July” à “mid-July”

Line 110: Change “accounted for” to “partially accounted for” or similar. Also change “boundary forcing” to “idealized boundary forcing”

Line 111: Change “of Disko Bay” to “provided at the open boundary, which in our simulations represent the seasonal changes observed in Disko Bay”

Line 113: Please specify the width in km of the sponge layer.

Line 115: please state what month this profile was obtained (it is stated in Fig 1, but it would be clearer to state it here too)

Line 131-133: Specify whether the iceberg concentration is uniform along the fjord

Line 164: smaller than what?

Line 172: Please provide a value for the freshwater flux

Line 170: remove “out”

Line 174: suggest changing “slight” to “weak”

Line 176: should be “in NoIBP”

Line 178: suggest “increases iceberg melt rates”

Line 179: “increases the inflow” – can you quantify this?

Line 190: as above, the use of “entrainment” here is not very clear. There are some other instances of entrainment in a similar manner later in the manuscript – please address them all.

Line 190: see comment above re ambient water vs shelf water

Line 196: clarify that this melt rate decrease is relative to the NoIBP simulation

Line 197: space in “watercolumn” needed

Line 253: “GWM” should be “GMW”

Line 258: semi-colon required before “however” (here and elsewhere)

Line 283: “Illustrated” should be all lower-case

Line 290: “iceberg induced” à “iceberg-induced”

Line 316: “high silled” à “high-silled”

Line 394: “impacta” à “impacts”

Line 415: “glacially modified” à “glacially-modified” (here and throughout)

Figures

The results figure locations all seemed a bit late to me, usually many pages later than their associated bit of text.

Figure 1:

• The black box in the inset is hard to see
• The colour bar blends in too much with the background – consider moving to one side of the figure, and make the text larger
• The green diamond is not very clear – consider yellow or magenta?
• BedMachine version 4 is now available – consider replotting the figure with this
• How are the sill and calving front defined? Is there a date for the calving front position? (present day will obviously not age well)

Figure 2:

• The text, especially tick labels and axis labels, are a little hard to read at 100%

Figure 3 and other along-fjord transects: please state in the figure caption whether these are centreline profiles or across-fjord averages (or something else?).

Figure 3: panel (k) contains observations from a fairly small number of casts (assuming Figure 1 shows all relevant casts). Please explain somewhere, in the figure caption or methods, how these observations are interpolated to produce the data plotted in this panel?

Figure 3, 4 & 9: a continuous colorbar is shown for each of these figure; however, the colours in the panels appear to be contoured, or at least represented with many fewer colours than shown in the bar. Please either provide a colorbar that is representative of the data as plotted, or plot the data using continuous colours as shown in the current colorbar of each figure.

Figure 8: I found the red star a bit confusing, as initially it looks like it could be a data point.

Figure 9: depth/y-axis not shown, despite being referred to in-text.

Review of manuscript Impact of icebergs on the seasonal submarine melt of Sermeq

Kujalleq, by Kajanto et al

This manuscript presents a study into the effects of icebergs on the circulation and water properties within Ilulissat Isfjord, a major ice-choked fjord in western Greenland. The study utilises the recent ‘IceBerg’ package developed for MITgcm (Davison et al 2020, 2021) to simulate the evolution of the fjord with and without icebergs, with the results then compared to (sparse) available observations. It builds upon the two earlier Davison papers in its application of the model to a new fjord system, and one of particular significance to this subject due to its exceptionally high ice concentration and proximity to Greenland’s largest outlet glacier.

The manuscript makes a useful contribution to the growing literature on Greenland’s fjords, clearly demonstrating the potential for icebergs to strongly modify fjord processes, and elucidating some of the mechanisms through which this can occur. I have one major comment which related to the experimental design which needs to be addressed to allow clear and confident interpretation of the presented results. Beyond this, I have some specific questions on aspects of the model set up and quite a long list of further comments, questions and points of clarity. Finally, I have attached a separate PDF with typological issues highlighted.

Major comments

1) Experimental design

The experiments are run from initiation in March through to August, at which point modelled fjord conditions are compared with observations. While this shows the transition from winter through to summer conditions, terminating the experiments in August seems premature and makes it difficult to assess the modelled evolution of the fjord. In particular, it is not clear how fjord water properties would continue to evolve beyond high summer, and if and how they return to something like the initial conditions in time to undertake this evolution again.

The fjord undergoes freshening and cooling over the duration of the model run, reaching something approximately resembling the observations by the end of the run in August. It’s not clear though whether the August temperatures are the end of the journey (with the fjord existing in a new quasi equilibrium state similar to the experiments by Davison et al 2022), or whether the modelled fjord would actually continue to cool and freshen into the autumn and winter if the model was allowed to run on. If it’s the latter, this implies the fjord is periodically returned to something resembling the original conditions by some other mechanism, only for the icebergs to resume the cooling process.

I think this is important for at least two reasons. Firstly, it is has implications for the validity of the comparison of model results with August observations. If it is necessary for the fjord to start off warmer / saltier at the start of the melt season in order to approximately match observations by August, then the difference between the applied initial conditions and the summer observations becomes critical in determining whether the model matches the observations. As the winter profile (initial conditions) comes from 2018, it’s not clear whether this does represent an appropriate starting point relative to the summer observations (which come from 2014). There is also no evidence provided to show that the assumption that winter conditions inside the fjord simply match shelf water properties at sill depth is appropriate. If water properties inside the fjord were already cooler/fresher than those at sill depth at the start of the melt season, would they end up even cooler/fresher by August (in which case the mismatch with observations will increase)?

Secondly, it raises interesting questions about the key processes and the seasonality of fjord water properties (which is a key aspect of the paper and the focus of the title). In the way the experiments are set up, you are implicitly assuming that there is an annual cycle of winter warming and summer cooling within the fjord. It’s not clear though from the evidence presented whether this is correct or whether given sufficient time a relatively consistent offset between shelf and fjord temperature/salinity would be established.

Without further investigation / discussion of these points, it’s difficult to assess the validity of the seasonal evolution of water properties, which is a key aspect of the study as it is currently presented. This could be partially addressed by running the experiments at least until the end of the melt season (which is I think fairly conventional for studies of seasonal processes) so that we can at least assess whether cooling and freshening continues beyond the August observations. I think though that at least one scenario should be run for > 1 year to allow for a proper spin up, reduce the dependency on the initial conditions and allow you to look into the questions of seasonal cyclicity raised above. If results diverge from those presented for the first melt season, then this raises an interesting discussion over whether other processes are serving to balance the impact of icebergs over this timescale. Finally, you could also run the model with a few steady discharge values to see what equilibrium conditions are eventually reached (similar to Davison et al, 2022). This would help to reduce the temporal dependency in the results, and allow you to assess what the impact of a given discharge and iceberg concentration would be on the fjord in the longer term. This would help to assess how far down this path the fjord has progressed on the timescale of the experiments presented here.

2) Model details

There are a few points in the paper where the description of model function differs from that in the original paper describing the IceBerg package by Davison et al. (2022). This raises questions over whether the model is being accurately described (or whether it has been modified), and if the results accurately interpreted.

L125-126. This sentence describes how ice adjacent currents are calculated, but it doesn't explain the representation of iceberg drift in the IceBerg package. According to Davison et al (2020, p10) 'Melt rates derived using the velocity-dependent three-equation formulation are sensitive to the current velocity at the ice-ocean interface. For icebergs, this is the difference between an iceberg’s drift velocity and the ambient water velocity at any given point on the iceberg. In ice mélange—a dense matrix of icebergs and sea ice often found adjacent to large tidewater glaciers—iceberg motion is typically slow relative to the surrounding currents; therefore, in this region of our domain, we assume the icebergs are fixed in place. Elsewhere in the domain, we calculate iceberg drift velocity as the average water velocity from the fjord surface to the iceberg keel depth (but we do not use this to update the location of each iceberg) ... We calculate the submarine melt rate of every face on each iceberg individually at each model vertical level using ice-parallel current speeds (relative to the calculated drift of the iceberg).'

This raises the question of whether or not drift is included in the present simulations? For a fjord like Ilulissat Isfjord, where dense melange is widespread, it may be more appropriate not to include drift?

L152-154. Unless it has been edited, I think the plume in IcePlume is programmed to terminate upon reaching neutral buoyancy rather than zero momentum (see Cowton et al, 2015).

L398-399. This line states that icebergs do not present an obstacle to flow in the model, but according to Davison et al (2020, p10), 'As well as drifting with ocean currents, icebergs also act as a barrier to water flow. We represent this effect using partial cells within MITgcm—essentially forcing a portion of some of the cells to be ‘dry’. The fraction of the cell that is dry is equivalent to the proportion of the cell volume occupied by icebergs. In this way, the blocking effect of all of the icebergs in a cell is represented using a single value, rather than representing individual icebergs as solid bodies within grid cells.' So unless the package has been modified, it seems the icebergs should be exerting a physical obstacle to flow in these simulations (albeit a simplified one based on a cell averaged approach)? This seems to be noted on L127-128.

Other specific comments:

L18. In what sense are the glaciers ‘controlled’ by the geometry and stratification?

L19-22. I feel this would benefit from a little elaboration. How does the ice-ocean interface create uncertainty in sea level contribution predictions?

L26-7. Be more specific - do you mean that calving is reduced when dense ice melange is present?

L80. Why not include Coriolis force? This is discussed later, but should be justified here.

L81-82. Why not vary the runoff smoothly by interpolating between monthly values? I think this is the default set up for MITgcm, and would prevent unrealistic step changes.

L92-93. I don't quite follow: if water is draining through the shear margins, wouldn't this suggest drainage close to the lateral margins rather than across the full width of the glacier?

L94. Why 1.2 km? I appreciate plume width is hard to constrain but why this value in particular? It seems very wide compared to the sort of values that are normally used (e.g. a recommendation of 200 m by Jackson et al 2017). Without further justification, it gives the impression it was chosen to give the best fit to observations – if this is the case, it should be stated.

L99. As above - a ‘narrow’ plume of 400 m wide is still wide by conventional standards (e.g. Jackson et al 2017, Slater et al 2022).

L102-103. It's unlikely that subglacial discharge at a glacier like Jakobshavn is ~0 in the winter months - it will be a lot smaller than summer but with such a large catchment and such high sliding velocities there will likely be a non-neglible winter discharge of subglacially-derived meltwater. This would affect the result that in the NoIBP scenario there is no circulation in the deep basin before May. It's very hard to quantify subglacial melt rates, but it might be worth trying using a discharge of a few m³/s in the winter to see if this has a noticeable effect on results.

L102-103.  What is the justification for such a large lag time? In a pressurised drainage system, there should be almost no lag between input to the system (i.e. surface runoff) and output from the system (i.e. subglacial discharge). There will be some subglacial storage which will serve to smooth the peaks, but this wouldn't cause the peak to be displaced by several weeks. For example, Mankoff et al. (2020) assume instantaneous routing between runoff and outlet discharge, and find good agreement in the timing of discharge peaks with observations (with a 7 day smoothing applied).

L103. What is the peak discharge of 1200 m³/s based on?

L114-116. Give the years and dates of these data here – presently this is only stated in the SI.

L117-118. I don't follow - why modify the forcing in this way (and at what point in the seasonal cycle were these values obtained?)?

L124. The description that ‘Melt and negative salinity flux are computed’ seems odd. What about heat? Should it state that melt rates, and thus salinity and heat fluxes, are calculated?

L148. See earlier comment regarding winter discharge.

L163-4. This is the case in the model (melt in areas of low current velocity is poorly constrained in iceplume), but there needs to be a bit more comment on whether or not this is deemed realistic – recent research suggests there should be much less discrepancy between melt rates within and outside of the plume area (Jackson et al. 2020; Sutherland et al. 2019).

L165-7. I don't follow. The deep basin starts off colder than the shelf (due to the initial conditions) but seems to be steadily warming through the summer due to inflow over the sill (shown in Figure 3/4)?

L167-8. Ambiguous - do you mean it is 2 C cooler than the equivalent Disko Bay temperature?

L177-8. Keep in mind this is likely underestimated due to poorly resolved boundary processes (see earlier comment)

L176-6. This doesn't sound very likely - even the plume melt rate in figure 5 only reaches 4.5 m/d in August (Figure 5). Is this a mistake, and if not where is it shown?

L201. See earlier question on 'drift'. And if drift is turned on, how do you distinguish between 'drift induced' melt and melt due to the flow of water past the berg? (Given that the net flow velocity past the berg is the difference between the drift velocity and the current speeds at any given depth).

L210. It's also notable that they result in a much larger export of freshwater and GMW from the fjord.

L218-9. How is the GMW outflow identified in the observations? Is this based purely on the T-S properties?

L220-2. Again, how is this determined? This section would benefit from a little elaboration.

L237. Does ‘entrainment of GMW’ here just mean entrainment of outflowing GMW into shelf water flowing inwards over the sill, or does it also include recirculation of GMW where the plume termination depth is deeper than sill depth? (The latter being perhaps a slightly different thing to 'entrainment').

L268-9. The fjord in the IBP scenarios gets steadily cooler and fresher below ~200m over the course of the summer (Figure 8). Does this trend continue if the simulation is allowed to run on for longer (into autumn and winter), such that model results and observations continue to diverge, or is a new equilibrium reached? See earlier major comment.

L271-2. How is this quantified?

L275-8. Muilwijk et al (2022) show that subglacial discharge represents a small fraction of GMW, but that upwelling of AW by the plume is very important in the formation of GMW. The role of plumes doesn’t seem to be properly captured by this sentence.

L278-9. Could this be tested based on whether the difference between the modelled and observed properties sits on a melt or runoff mixing line?

L294-5. It would be valuable to compare other aspects of the results to Davison et al (2022) as well, given the similarity in these studies. Davison et al used an idealised domain to investigate the impact of icebergs across a parameter space representing the diversity of Greenland's fjords. As Ilulissat Isfjord represents one end member of this range, it would be valuable to examine how closely it aligns with the predictions of Davison et al.

L299-300. Jackson et al. (2017) propose that a line plume of ~200 m width gives best agreement with their observations – this is much narrower than the tested range of 400-4000 m, so it doesn’t really justify the choice of plume widths used in this study (see earlier comment).

L331-2. Should qualify that this is true over the parameter space considered

L340-2. I don't follow - wouldn't the change in geometry due to undercutting make icebergs more likely to rotate top-first into the fjord?

L353-5. I feel this needs some substantiation. The currents in question are a summer phenomenon, whereas rigid sea ice format occurs towards the end of winter. Would a weaker plume during one summer really affect sea ice formation the following winter / spring? I'm not saying it's impossible, but it seems highly speculative and would benefit from stronger justification.

L357. Need to be more specific with terminology, to make clear you are talking about recirculation of deep waters rather than some other form of entrainment. Same for 'iceberg modification'.

L361-3. This is broad statement which doesn’t really do justice to the rich literature on calving, including on the impact of undercutting on calving (e.g. O'Leary and Christoffersen 2013; Ma and Bassis 2019; Benn et al. 2017; Slater et al. 2021).

L377-382. The comparison of model results and observations hinges on implications of the experimental design, and may need to be reconsidered (see earlier major comment).

L381. It is hard to compare these two plots. Could additional curves (or even additional plots) be added to Figure 8 to allow comparison? (Also, there is no Figure 3m).

L385. Why not vary this smoothly (see earlier comment)?

L392. As earlier, is this 'drift induced', or is it water flowing past stationary grounded icebergs?

L392-4. I'm not sure I follow this. I would assume that freshening causes upwelling and outflow of GMW, and that it is entrainment into this flow that drives subsurface inflow to the fjord (as well as entrainment into the main plume)? In which case it's not obvious to me why the rate of entrainment/inflow should be greater due to the negative salinity approach. I can see that if you used a real freshwater flux from the melting icebergs this would increase the outflow, such that there was a net outflow from the fjord (equal to the meltwater flux), but it's less obvious to me why this would reduce the inflow to the fjord in absolute terms. If you used a real freshwater flux, there would also be a question over whether it is appropriate to add a physical volume of meltwater whilst not simultaneously decreasing the volume of the fjord occupied by icebergs, as the two should approximately balance each other out.

L406-7. Given that the results presented already over-estimate cooling and freshening, this raises further questions over why the impact of icebergs in the model seems to overestimate the impacts of icebergs, and should probably be referenced in this context.

L410-1. Have 'misleading interpretations' been presented in the paper? If not, perhaps better to simply state that it's important to take icebergs into account when studying and simulating these systems.

L416. ‘Entrainment’ is unspecific – clarify the process in question.

L416-8. This sentence is hard to follow. Also the influence of discharge on the plume and frontal melt rates has been demonstrated in many other places (more so that here, where it isn't really the focus of the paper).

L419. Need to be specific about the mechanism here - is it purely due to changes in the stratification?

L421. This seems to be overstating things given the speculative nature of this connection. A 'potential link' would seem more appropriate.

See annotated PDF for further minor corrections.

References                                                                            

Benn, Douglas I, Jan Åström, Thomas Zwinger, Joe Todd, Faezeh M Nick, Susan Cook, Nicholas RJ Hulton, and Adrian Luckman. 2017. 'Melt-under-cutting and buoyancy-driven calving from tidewater glaciers: new insights from discrete element and continuum model simulations', Journal of Glaciology, 63: 691-702.

Davison, Benjamin Joseph, Tom Cowton, Andrew Sole, Finlo Cottier, and Pete Nienow. 2022. 'Modelling the effect of submarine iceberg melting on glacier-adjacent water properties', The Cryosphere, 16: 1181-96.

Jackson, R. H., E. L. Shroyer, J. D. Nash, D. A. Sutherland, D. Carroll, M. J. Fried, G. A. Catania, T. C. Bartholomaus, and L. A. Stearns. 2017. 'Near-glacier surveying of a subglacial discharge plume: implications for plume parameterizations', Geophysical Research Letters, 43: 6886-94.

Jackson, RH, JD Nash, C Kienholz, DA Sutherland, JM Amundson, RJ Motyka, D Winters, E Skyllingstad, and EC Pettit. 2020. 'Meltwater intrusions reveal mechanisms for rapid submarine melt at a tidewater glacier', Geophysical Research Letters, 47: e2019GL085335.

Ma, Yue, and Jeremy N Bassis. 2019. 'The Effect of Submarine Melting on Calving From Marine Terminating Glaciers', Journal of Geophysical Research: Earth Surface, 124: 334-46.

Mankoff, K. D., B. Noël, X. Fettweis, A. P. Ahlstrøm, W. Colgan, K. Kondo, K. Langley, S. Sugiyama, D. van As, and R. S. Fausto. 2020. 'Greenland liquid water discharge from 1958 through 2019', Earth Syst. Sci. Data, 12: 2811-41.

Muilwijk, Morven, Fiamma Straneo, Donald A Slater, Lars H Smedsrud, James Holte, Michael Wood, Camilla S Andresen, and Ben Harden. 2022. 'Export of ice sheet meltwater from Upernavik Fjord, West Greenland', Journal of Physical Oceanography, 52: 363-82.

O'Leary, M., and P. Christoffersen. 2013. 'Calving on tidewater glaciers amplified by submarine frontal melting', The Cryosphere, 7: 119-28.

Slater, DA, DI Benn, TR Cowton, JN Bassis, and JA Todd. 2021. 'Calving multiplier effect controlled by melt undercut geometry', Journal of Geophysical Research: Earth Surface, 126: e2021JF006191.

Sutherland, DA, Rebecca H Jackson, Christian Kienholz, Jason M Amundson, WP Dryer, Dan Duncan, EF Eidam, RJ Motyka, and JD Nash. 2019. 'Direct observations of submarine melt and subsurface geometry at a tidewater glacier', Science, 365: 369-74.