the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Mar 2024
Ice mélange melt drives changes in observed water column stratification at a tidewater glacier in Greenland
Abstract. Glacial fjords often contain ice mélange, a frozen conglomeration of icebergs, brash ice, and sea ice, that have been postulated to influence both glacier dynamics and fjord circulation through coupled mechanical and thermodynamic processes. Ice mélange meltwater can alter stratification of the water column by releasing cool, fresh water across a range of depths in the upper layer of the fjord. This meltwater input can subsequently modify the depth at which the subglacial discharge plume reaches neutral buoyancy and therefore the underlying buoyancy-driven fjord circulation and heat exchange with warm ocean shelf waters. Despite a spate of recent modelling studies exploring these proposed feedbacks, we lack in situ observations quantifying changes to the water column induced by ice mélange meltwater. Here we use a novel dataset collected before and after the melt, breakup, and down-fjord transport of an ephemeral ice mélange in front of Kangilliup Sermia (Rink Isbræ) to directly investigate the extent to which ice mélange meltwater can modify glacier-adjacent water properties. We find that even a short-lived ice mélange (4 days) can cause substantial cooling (0.18 °C) and freshening (0.25 g kg-1) of the water column that leads to stratification change down to the depth of the outflowing discharge plume. We compare our observations to an adjacent fjord, Kangerlussuup Sermia, where ice mélange seldom forms in the summertime, and show that the presence or absence of ice mélange melt creates fundamental differences in their upper layer hydrography. These observations provide critical constraints for recent modelling studies that have suggested ice mélange meltwater needs to be included in ocean circulation models for glaciers with deep grounding lines and high ice fluxes, which are precisely the glaciers exhibiting the largest magnitude terminus retreats at present.
-
Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
-
Preprint
(2923 KB)
-
Supplement
(60673 KB)
-
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2923 KB) - Metadata XML
-
Supplement
(60673 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-504', Benjamin Davison, 11 Apr 2024
Summary: the authors present new hydrographic observations in two of Greenland’s fjords that quantify the substantial impact of a 4-day period of ice mélange melt on water column properties. The observations are compared to output from standalone plume and iceberg-melt models to demonstrate that the observed changes in water column properties can be explained by ice mélange melt during the period of mélange presence, in addition to contributions from a subglacial discharge-driven plume.
Overall, the paper is very well-written, clear and convincing. It provides much needed observational evidence that support and add more detail to several modelling studies of iceberg-ocean interaction, which I think will be a great interest to the community.
I have just a few minor suggestions, questions and comments listed below, but otherwise I think the study is ready for publication:
Minor comments
Line 25: can you go any further with this statement? Do your observations support the notion that ice mélange meltwater needs to be included (or its effects parameterised) in ocean circulation models or in ocean boundary conditions used to force glacier models? As written, this is implied but it’s not clear the absence of a direct statement is deliberate.
Line 29: I’m not sure that “ice tongues” is the correct terminology here? I think ice tongues strictly refers to small, ice shelf-like features, which there are a few of in Greenland, but I don’t think they existed at the majority of its now marine-terminating glaciers as the second part of the sentence implies.
Line 34: consider “days to years”, since this study focuses on a 4-day period of mélange.
Line 36: “the influence of ice mélange meltwater and its temporal variability” seems a bit ambiguous to me and somewhat contradictory to the first part of the sentence that cites several modelling studies that examined the influence of ice mélange meltwater on fjord water properties and circulation. Can you rephrase to make your meaning clearer, which I think is that the cited studies focused on long-lived mélange rather than ephemeral mélange?
Lines 46/47: Can you be clearer about the directionality of the impact of changes in freshwater flux on fjord-shelf exchange? i.e. does an increase in freshwater flux enhance or reduce exchange? As written, it states that any change in freshwater flux corresponds to an increase in exchange, which doesn’t seem right to me.
Equation 6: Shouldn’t Aicebe the total iceberg-ocean contact area, not the area of the fjord surface covered by ice mélange?
Lines 191-194: can you clarify what the errors on the T and S measurements represent? As presented, they seem large relative to the observed changes in T and S, which might or might not be important for the interpretation depending on how the errors are defined.
Line 203: erroneous “then”?
Line 229-232: are these melt rates from icebergs with a particular keel depth? Or averaged across icebergs with a range of keel depths? I think this should be specified in the text.
Line 242: “these time spans for complete melting of the ice mélange” – I found this a bit confusing. Wasn’t the preceding paragraph providing time-spans for melting of 14% of the ice mélange? So it is expected that 86% of the mélange must have been exported as solid ice? (plus or minus the uncertainties relating to the initial ice volume, but the main thing here is that there was more than enough ice available). I recommend providing a figure to illustrate the time-spans required to produce Vmelt under the range of estimated melt rates.
Line 252: erroneous “surface”?
Line 300 and surrounding text: I’m not sure that evidence has been presented to support this statement. Hydrographic observations are only available before and after the icebergs were present in the fjord, so there is no information about how the water properties changed during that period of 4 days – the modification may have happened steadily over the 4-days or it may have mostly happened in the first day. We do know that the icebergs were only held in place for ~12 hours before becoming mobile, which either suggests that rigid mélange caused all of the modification in the first 12 hours, or that modification continued despite the mélange become mobile.
Line 358/9: “progressively cooling the upper layer of the fjord over the course of the summer” – I think it would be clearer if this statement came after the explanation given the following paragraph. I also think it would be relevant here to state whether any ephemeral mélange events occurred before the one described here, and if so give the dates of that occurrence and describe how that might or might not have affected the pre-event water properties.
Paragraph starting line 361: I think this analysis assumes that the mélange events have no impact on ‘offshore’ water properties. The offshore properties used here are from a co-located CTD-cast and mooring outside of the sill where the two fjords branch, some 60 km from the glacier terminus. If the authors agree, I think this assumption should be acknowledged and the direction of impact on this analysis should be given.
I think it would be worth adding a paragraph to the discussion examining why ephemeral mélange events occur at some fjords but not others (some combination of mid-depth plume outflow and large calving events, but not so much calving that you get near-permanent mélange?), or at least to give some indication as to how widespread they are or might be around Greenland. As written, some readers could conclude that most fjords are either like Kangerlussuup Sermia without ephemeral mélange events, or they are like Sermilik Fjord where there is near-permanent mélange, and only a few have these ephemeral events.
Figure 3a: I think it would help if labels I, II and III were briefly summarised in the figure caption, as well as in the main text, just to save the reader scrolling around the manuscript.
Figure 3a: I found the square markers a bit confusing. The caption states they are at intervals of 50 m, but I count 9 black square markers before (above?) the panel (b) inset, which includes data from 50-100 m. Does that just mean the water column properties don’t change smoothly in T-S space with depth? If so, then I’m not sure that the square markers aid the interpretation of panel (a). Or should I just be comparing the position of a black square with a position of a blue square in T-S space? If so, then is there some way you can make it clear which squares correspond to the same depth on the black and blue curves?
Figure 4: what do the grey dots represent? I can’t see a description of them in the caption. Please can you also specify the dates of the presented CTD casts? It’s not clear if the RNK data is from before or after the mélange event.
Benjamin Davison
Citation: https://doi.org/10.5194/egusphere-2024-504-RC1 -
AC1: 'Reply on RC1', Nicole Abib, 17 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-504/egusphere-2024-504-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Nicole Abib, 17 May 2024
-
RC2: 'Comment on egusphere-2024-504', Twila Moon, 25 Apr 2024
-
AC2: 'Reply on RC2', Nicole Abib, 17 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-504/egusphere-2024-504-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Nicole Abib, 17 May 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-504', Benjamin Davison, 11 Apr 2024
Summary: the authors present new hydrographic observations in two of Greenland’s fjords that quantify the substantial impact of a 4-day period of ice mélange melt on water column properties. The observations are compared to output from standalone plume and iceberg-melt models to demonstrate that the observed changes in water column properties can be explained by ice mélange melt during the period of mélange presence, in addition to contributions from a subglacial discharge-driven plume.
Overall, the paper is very well-written, clear and convincing. It provides much needed observational evidence that support and add more detail to several modelling studies of iceberg-ocean interaction, which I think will be a great interest to the community.
I have just a few minor suggestions, questions and comments listed below, but otherwise I think the study is ready for publication:
Minor comments
Line 25: can you go any further with this statement? Do your observations support the notion that ice mélange meltwater needs to be included (or its effects parameterised) in ocean circulation models or in ocean boundary conditions used to force glacier models? As written, this is implied but it’s not clear the absence of a direct statement is deliberate.
Line 29: I’m not sure that “ice tongues” is the correct terminology here? I think ice tongues strictly refers to small, ice shelf-like features, which there are a few of in Greenland, but I don’t think they existed at the majority of its now marine-terminating glaciers as the second part of the sentence implies.
Line 34: consider “days to years”, since this study focuses on a 4-day period of mélange.
Line 36: “the influence of ice mélange meltwater and its temporal variability” seems a bit ambiguous to me and somewhat contradictory to the first part of the sentence that cites several modelling studies that examined the influence of ice mélange meltwater on fjord water properties and circulation. Can you rephrase to make your meaning clearer, which I think is that the cited studies focused on long-lived mélange rather than ephemeral mélange?
Lines 46/47: Can you be clearer about the directionality of the impact of changes in freshwater flux on fjord-shelf exchange? i.e. does an increase in freshwater flux enhance or reduce exchange? As written, it states that any change in freshwater flux corresponds to an increase in exchange, which doesn’t seem right to me.
Equation 6: Shouldn’t Aicebe the total iceberg-ocean contact area, not the area of the fjord surface covered by ice mélange?
Lines 191-194: can you clarify what the errors on the T and S measurements represent? As presented, they seem large relative to the observed changes in T and S, which might or might not be important for the interpretation depending on how the errors are defined.
Line 203: erroneous “then”?
Line 229-232: are these melt rates from icebergs with a particular keel depth? Or averaged across icebergs with a range of keel depths? I think this should be specified in the text.
Line 242: “these time spans for complete melting of the ice mélange” – I found this a bit confusing. Wasn’t the preceding paragraph providing time-spans for melting of 14% of the ice mélange? So it is expected that 86% of the mélange must have been exported as solid ice? (plus or minus the uncertainties relating to the initial ice volume, but the main thing here is that there was more than enough ice available). I recommend providing a figure to illustrate the time-spans required to produce Vmelt under the range of estimated melt rates.
Line 252: erroneous “surface”?
Line 300 and surrounding text: I’m not sure that evidence has been presented to support this statement. Hydrographic observations are only available before and after the icebergs were present in the fjord, so there is no information about how the water properties changed during that period of 4 days – the modification may have happened steadily over the 4-days or it may have mostly happened in the first day. We do know that the icebergs were only held in place for ~12 hours before becoming mobile, which either suggests that rigid mélange caused all of the modification in the first 12 hours, or that modification continued despite the mélange become mobile.
Line 358/9: “progressively cooling the upper layer of the fjord over the course of the summer” – I think it would be clearer if this statement came after the explanation given the following paragraph. I also think it would be relevant here to state whether any ephemeral mélange events occurred before the one described here, and if so give the dates of that occurrence and describe how that might or might not have affected the pre-event water properties.
Paragraph starting line 361: I think this analysis assumes that the mélange events have no impact on ‘offshore’ water properties. The offshore properties used here are from a co-located CTD-cast and mooring outside of the sill where the two fjords branch, some 60 km from the glacier terminus. If the authors agree, I think this assumption should be acknowledged and the direction of impact on this analysis should be given.
I think it would be worth adding a paragraph to the discussion examining why ephemeral mélange events occur at some fjords but not others (some combination of mid-depth plume outflow and large calving events, but not so much calving that you get near-permanent mélange?), or at least to give some indication as to how widespread they are or might be around Greenland. As written, some readers could conclude that most fjords are either like Kangerlussuup Sermia without ephemeral mélange events, or they are like Sermilik Fjord where there is near-permanent mélange, and only a few have these ephemeral events.
Figure 3a: I think it would help if labels I, II and III were briefly summarised in the figure caption, as well as in the main text, just to save the reader scrolling around the manuscript.
Figure 3a: I found the square markers a bit confusing. The caption states they are at intervals of 50 m, but I count 9 black square markers before (above?) the panel (b) inset, which includes data from 50-100 m. Does that just mean the water column properties don’t change smoothly in T-S space with depth? If so, then I’m not sure that the square markers aid the interpretation of panel (a). Or should I just be comparing the position of a black square with a position of a blue square in T-S space? If so, then is there some way you can make it clear which squares correspond to the same depth on the black and blue curves?
Figure 4: what do the grey dots represent? I can’t see a description of them in the caption. Please can you also specify the dates of the presented CTD casts? It’s not clear if the RNK data is from before or after the mélange event.
Benjamin Davison
Citation: https://doi.org/10.5194/egusphere-2024-504-RC1 -
AC1: 'Reply on RC1', Nicole Abib, 17 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-504/egusphere-2024-504-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Nicole Abib, 17 May 2024
-
RC2: 'Comment on egusphere-2024-504', Twila Moon, 25 Apr 2024
-
AC2: 'Reply on RC2', Nicole Abib, 17 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-504/egusphere-2024-504-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Nicole Abib, 17 May 2024
Peer review completion
Journal article(s) based on this preprint
Data sets
G. Catania et al. Water temperature, salinity and hydrostatic pressure collected from subsurface moorings deployed by the research vessel Sanna in the Uummannaq Bay, west Greenland, from 2013-09-17 to 2015-07-12 https://www.ncei.noaa.gov/archive/accession/0173969
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 383 | 86 | 29 | 498 | 41 | 20 | 25 |
- HTML: 383
- PDF: 86
- XML: 29
- Total: 498
- Supplement: 41
- BibTeX: 20
- EndNote: 25
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
David A. Sutherland
Rachel Peterson
Ginny Catania
Jonathan D. Nash
Emily L. Shroyer
Leigh A. Stearns
Timothy C. Bartholomaus
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2923 KB) - Metadata XML
-
Supplement
(60673 KB) - BibTeX
- EndNote
- Final revised paper