the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 May 2021
Winter growth and tidal variability of the sub-ice platelet layer observed with electromagnetic induction soundings
Abstract. Here, we present the first electromagnetic induction time-series measurements of ice shelf-influenced fast ice and sub-ice platelet layer thickness over winter and in late spring in McMurdo Sound. Significant increases in sub-ice platelet layer thickness (~0.5–1 m) co-occurred with strong southerly wind events and satellite-observed polynya activity suggesting wind-driven surface circulation of supercooled Ice Shelf Water outflow from the McMurdo-Ross ice shelf cavity. Temporal variability observed in sub-ice platelet layer thickness on diurnal timescales correlated with tidally-induced current patterns previously observed in McMurdo Sound. The thickness of the sub-ice platelet layer increased on spring and neap ebb tides corresponding with northward currents circulating out from the ice shelf cavity. The late spring spatial distributions of first-year and second-year fast ice and sub-ice platelet layer thickness in McMurdo Sound were assessed with drill hole and electromagnetic induction surveys and were comparable to a previous four-year dataset. We resolved second-year fast ice thicknesses of 4 m with a substantial sub-ice platelet layer beneath of up to 11 m using electromagnetic induction techniques suggesting that the longer temporal persistence of the two-year-old fast ice allowed a substantially thicker sub-ice platelet layer to form. The variability observed in the sub-ice platelet layer indicates that a combination of the tides, wind-driven polynya activity and the presence of multi-year ice influences the circulation of Ice Shelf Water in the upper surface ocean and consequently sub-ice platelet layer formation over a range of timescales.
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RC1: 'Comment on tc-2021-61', Anonymous Referee #1, 15 Jul 2021
Several findings are reported here using applications of EMI soundings to investigate Subice platelet layers in McMurdo Sound. 1. Ground-based surveys with concurrent drill hole and snow depth measurements on large spatial scales. 2. Small spatial scale surveys conducted over several days with high resolution. and 3. Time series measurements using a fixed point EMI to map variations of SIPL at various time scales. The authors then relate each of these sets on SIPL properties to driving forces. Previous studies have measured the spatial distributions (only) in late spring while this study made winter survey measurements and time series measurements at a fixed site.
The authors’ conclude that the variability observed in the sub-ice platelet layer indicates that a combination of the tides, wind-driven polynya activity and the presence of multi-year ice influences the circulation of Ice Shelf Water in the upper surface ocean and consequently sub-ice platelet layer formation over a range of timescales.
General Comment.
I found that the structure of the paper, jumping from the three different types of measurements was laborious to follow. This was especially true when three separate sets of results are presented with quite different aims and then are not discussed until after all the results are presented so there is a cognitive break in this reader’s ability to follow a discussion and interpretation from a set of results that occurred several pages before. My recommendation is to revise the paper into a shorter version covering the ground-based spatial surveys (large and possibly small spatial scales) and leave out the time series measurements at a fixed point and the detailed tides dependence calculation, which are inconclusive after a lengthy analysis. These single point time series measurements (possibly coupled with the small-scale surveys?) may form a second paper. The findings from the ground surveys on large spatial scales are worthy of publication.
Some detailed comments follow.
Detailed Comments
Figure 1. The color bar scales with only end points indicated, gave no indication of the intermediate scales color values, which appear to be also nonlinear? The middle one is also reversed from the other two, with blue being the thickest ice, while blues are near the thinnest snow depths and SIPL thicknesses. Recommend providing some intermediate values on the color bar scales rather than just zero and maximum values and redoing the snow depth so that thin and thick colors are the same as the other two rather than reversed.
Figure 2. Lacks labeling of the blue contours relating to SIPL thickness. The caption reads that these are in 0.10m steps from 0 to 10m which would correspond to a hundred blue lines a number much greater than the number of lines displayed unless there are ~50 lines packed in near the bottom? I assume the scales goes from 0 at the bottom to 10m at the top but the spacing at 0.10m is not clear. Possibly place a few numbers on the blue lines to indicate the scale.
Line s 165-170 While the experimental data gives SIPL conductivity values of 800mSm-1 for First year Ice, and 1000mSm-1 for Second year ice, the authors choose neither of the above for the forward modeling (900 mSm-1)? Are the forward modeling results significantly different for 800 or 1000 than they are for 900? Would it be better to show two modeling runs for the two actual measured conductivities than for an intermediate not observed value?
Pg.9, Figure 3c. The color of the dots are hard to compare with the EMI trace they correspond to for year as they appear (to me) as dark blue which has no parallel EMI trace color in 3c. Which year are those drill hole dots for SIPL thickness taken? Especially these seemingly dark blue (?)dots, they don’t seem to correspond very well to any of the measured EMI Traces, especially in the left hand side? Can you explain?
Figure 6. The lack of horizontal time scales on b and c lead to some confusion with the spatial scale on Fig 6a. I don’t see the merit in putting the spatial scale figure a in with the time scale figures b, c, d. Suggest breaking this into two figures, one with 6a only to show the location of the time series within the broader spatial scales and another figure with b, c, d with better labeling on each for the time scales.
Fig 6d While on different days, the drill hole SIPL thickness(3Nov) and EMI thicknesses measure from 4 Nov on seem quite different? Can you comment?
Line 291 Measured conductivity of 900mSm-1 was derived from drill hole measurements but differed from the 800 (FYI) or 1000(SYI) discussed above?
Line 295 In the figure looks like EMI is underestimated for sea ice relative to drill holes and SIPL is overestimated for EMI compared to drill holes? Text indicates EMI overestimated for both.
Lines 33=-335-337 reads “The magnitude of change in I, Q and SIPL thickness at the WSN over the tidal range of each flood (positive from trough to peak) and ebb (negative from peak to trough) spring and neap tide was quantified and is shown in Fig. 8 with linear fits applied. Opposing trends were observed in Q and I with the tides (Fig. 8a). I decreased on ebb tides and increased on flood tides. In contrast, Q 335 increased on ebb tides and decreased on flood tides.” This was investigated on pgs 15 and 16 with forward modeling parameters and attributed to variations in seawater conductivity as opposed to SIPL conductivity. It’s supposedly discussed in Section 4.5 relating to seawater salinity and temperature. But the beginning of the Discussion section (4.1) jumps back to the Spatial surveys data. I find the interleaving of two or three quite different studies, the large scale spatial surveys, the small-scale surveys with a time components, and the single site high temporal scale measurements extremely confusing to follow coherently.
Section 4.5 Discussion following the results referred to above. Here after rejection of the two effects modeled on pgs 15 and 16 of changes in seawater conductivity and SIPL conductivity, we now find that both of these are probably not the cause. Instead a new discussion is injected where the variations of the thickness of the SIPL is examined and related to other observations of the SIPL using underwater video. Again this discussion is inconclusive as to whether this third cause is responsible. Makes for a lot of difficult reading to arrive at a somewhat null result?
Citation: https://doi.org/10.5194/tc-2021-61-RC1 -
RC2: 'Comment on tc-2021-61', Anonymous Referee #2, 20 Jul 2021
This paper reports observations of the evolution of the sub-ice platelet layer beneath fast ice in McMurdo Sound through late winter and spring of 2018. Overall, I found it a difficult paper to review. I found no issues in the analyses or the results, but, even after several readings, I was still a little unclear about what the authors key findings are. I think the reason for this is that the paper is presented more in the form of a data report than a study addressing specific questions. Unfortunately, that means that the manuscript appears to be of limited interest to anyone who is not working on the sub-ice platelet layer in McMurdo Sound. The authors presumably had the aim of answering some broader science questions when they went to the not inconsiderable effort of collecting the data. However, those broader questions and how the results help to answer them are never clearly shared with the reader.
Once I had rationalised my general misgivings about the paper, there were a couple of places where, with hindsight, the problems are clearly apparent. The first is the title “Winter growth and tidal variability …”, two arguably unrelated aspects of the sub-ice platelet layer are treated in the same paper. Now, both are related to the processes of formation of the layer and the temporal variability of those processes. Only the timescales differ. However, the processes are barely touched on in the paper, and are never used to link the two sets of observations. For this reason, it took me some time to figure out what actually had been measured at the various sites, and even now I’m not sure that I have it all clear in my mind. The only motivation for putting all these observations together in the same paper would appear to be that the data were collected as part of the same campaign. But that link has little relevance to those not involved in the fieldwork and whose main interest in these measurements is the insight they can give into sub-ice platelet layer development.
The lack of a focus on processes becomes apparent in a statement towards the end of the paper. Following a quite long, but inconclusive, discussion of what might have caused the tidal variability, there is the statement (lines 550-551) that coincident oceanographic data were collected that will contribute to understanding the tidal variability in the sub-ice platelet layer. If the motivation were to advance our understanding of the processes driving tidal variability, wouldn’t it make more sense to publish all the data that can contribute to that understanding together in a single paper? It would definitely be of more use to the wider community. Once again, that points to the primary motivation for this submission being the publication of a dataset rather than the solution of a scientific problem.
For this reason, I would not recommend that the paper by published in the Cryosphere in its current form. I should be clear that my recommendation does not in any way reflect on the quality of the data themselves, or the data collection and processing. The authors have clearly completed a challenging field campaign to high standards and have collected a great dataset that could potentially be valuable to others. However, the paper as currently written does not really do the data justice and does not communicate their value to the reader, nor how they can be used to advance understanding of sub-ice platelet layer formation.
I would actually recommend that the authors split the data over two papers, one focussing on “winter growth” and the other on “tidal variability”. Each one needs clear statements of the problem being addressed and how the data contribute towards new understanding of that problem. The latter paper should ideally wait until the oceanographic data are available, so that a more complete and rigorous discussion of the processes driving variability can be given.
Citation: https://doi.org/10.5194/tc-2021-61-RC2
Status: closed
-
RC1: 'Comment on tc-2021-61', Anonymous Referee #1, 15 Jul 2021
Several findings are reported here using applications of EMI soundings to investigate Subice platelet layers in McMurdo Sound. 1. Ground-based surveys with concurrent drill hole and snow depth measurements on large spatial scales. 2. Small spatial scale surveys conducted over several days with high resolution. and 3. Time series measurements using a fixed point EMI to map variations of SIPL at various time scales. The authors then relate each of these sets on SIPL properties to driving forces. Previous studies have measured the spatial distributions (only) in late spring while this study made winter survey measurements and time series measurements at a fixed site.
The authors’ conclude that the variability observed in the sub-ice platelet layer indicates that a combination of the tides, wind-driven polynya activity and the presence of multi-year ice influences the circulation of Ice Shelf Water in the upper surface ocean and consequently sub-ice platelet layer formation over a range of timescales.
General Comment.
I found that the structure of the paper, jumping from the three different types of measurements was laborious to follow. This was especially true when three separate sets of results are presented with quite different aims and then are not discussed until after all the results are presented so there is a cognitive break in this reader’s ability to follow a discussion and interpretation from a set of results that occurred several pages before. My recommendation is to revise the paper into a shorter version covering the ground-based spatial surveys (large and possibly small spatial scales) and leave out the time series measurements at a fixed point and the detailed tides dependence calculation, which are inconclusive after a lengthy analysis. These single point time series measurements (possibly coupled with the small-scale surveys?) may form a second paper. The findings from the ground surveys on large spatial scales are worthy of publication.
Some detailed comments follow.
Detailed Comments
Figure 1. The color bar scales with only end points indicated, gave no indication of the intermediate scales color values, which appear to be also nonlinear? The middle one is also reversed from the other two, with blue being the thickest ice, while blues are near the thinnest snow depths and SIPL thicknesses. Recommend providing some intermediate values on the color bar scales rather than just zero and maximum values and redoing the snow depth so that thin and thick colors are the same as the other two rather than reversed.
Figure 2. Lacks labeling of the blue contours relating to SIPL thickness. The caption reads that these are in 0.10m steps from 0 to 10m which would correspond to a hundred blue lines a number much greater than the number of lines displayed unless there are ~50 lines packed in near the bottom? I assume the scales goes from 0 at the bottom to 10m at the top but the spacing at 0.10m is not clear. Possibly place a few numbers on the blue lines to indicate the scale.
Line s 165-170 While the experimental data gives SIPL conductivity values of 800mSm-1 for First year Ice, and 1000mSm-1 for Second year ice, the authors choose neither of the above for the forward modeling (900 mSm-1)? Are the forward modeling results significantly different for 800 or 1000 than they are for 900? Would it be better to show two modeling runs for the two actual measured conductivities than for an intermediate not observed value?
Pg.9, Figure 3c. The color of the dots are hard to compare with the EMI trace they correspond to for year as they appear (to me) as dark blue which has no parallel EMI trace color in 3c. Which year are those drill hole dots for SIPL thickness taken? Especially these seemingly dark blue (?)dots, they don’t seem to correspond very well to any of the measured EMI Traces, especially in the left hand side? Can you explain?
Figure 6. The lack of horizontal time scales on b and c lead to some confusion with the spatial scale on Fig 6a. I don’t see the merit in putting the spatial scale figure a in with the time scale figures b, c, d. Suggest breaking this into two figures, one with 6a only to show the location of the time series within the broader spatial scales and another figure with b, c, d with better labeling on each for the time scales.
Fig 6d While on different days, the drill hole SIPL thickness(3Nov) and EMI thicknesses measure from 4 Nov on seem quite different? Can you comment?
Line 291 Measured conductivity of 900mSm-1 was derived from drill hole measurements but differed from the 800 (FYI) or 1000(SYI) discussed above?
Line 295 In the figure looks like EMI is underestimated for sea ice relative to drill holes and SIPL is overestimated for EMI compared to drill holes? Text indicates EMI overestimated for both.
Lines 33=-335-337 reads “The magnitude of change in I, Q and SIPL thickness at the WSN over the tidal range of each flood (positive from trough to peak) and ebb (negative from peak to trough) spring and neap tide was quantified and is shown in Fig. 8 with linear fits applied. Opposing trends were observed in Q and I with the tides (Fig. 8a). I decreased on ebb tides and increased on flood tides. In contrast, Q 335 increased on ebb tides and decreased on flood tides.” This was investigated on pgs 15 and 16 with forward modeling parameters and attributed to variations in seawater conductivity as opposed to SIPL conductivity. It’s supposedly discussed in Section 4.5 relating to seawater salinity and temperature. But the beginning of the Discussion section (4.1) jumps back to the Spatial surveys data. I find the interleaving of two or three quite different studies, the large scale spatial surveys, the small-scale surveys with a time components, and the single site high temporal scale measurements extremely confusing to follow coherently.
Section 4.5 Discussion following the results referred to above. Here after rejection of the two effects modeled on pgs 15 and 16 of changes in seawater conductivity and SIPL conductivity, we now find that both of these are probably not the cause. Instead a new discussion is injected where the variations of the thickness of the SIPL is examined and related to other observations of the SIPL using underwater video. Again this discussion is inconclusive as to whether this third cause is responsible. Makes for a lot of difficult reading to arrive at a somewhat null result?
Citation: https://doi.org/10.5194/tc-2021-61-RC1 -
RC2: 'Comment on tc-2021-61', Anonymous Referee #2, 20 Jul 2021
This paper reports observations of the evolution of the sub-ice platelet layer beneath fast ice in McMurdo Sound through late winter and spring of 2018. Overall, I found it a difficult paper to review. I found no issues in the analyses or the results, but, even after several readings, I was still a little unclear about what the authors key findings are. I think the reason for this is that the paper is presented more in the form of a data report than a study addressing specific questions. Unfortunately, that means that the manuscript appears to be of limited interest to anyone who is not working on the sub-ice platelet layer in McMurdo Sound. The authors presumably had the aim of answering some broader science questions when they went to the not inconsiderable effort of collecting the data. However, those broader questions and how the results help to answer them are never clearly shared with the reader.
Once I had rationalised my general misgivings about the paper, there were a couple of places where, with hindsight, the problems are clearly apparent. The first is the title “Winter growth and tidal variability …”, two arguably unrelated aspects of the sub-ice platelet layer are treated in the same paper. Now, both are related to the processes of formation of the layer and the temporal variability of those processes. Only the timescales differ. However, the processes are barely touched on in the paper, and are never used to link the two sets of observations. For this reason, it took me some time to figure out what actually had been measured at the various sites, and even now I’m not sure that I have it all clear in my mind. The only motivation for putting all these observations together in the same paper would appear to be that the data were collected as part of the same campaign. But that link has little relevance to those not involved in the fieldwork and whose main interest in these measurements is the insight they can give into sub-ice platelet layer development.
The lack of a focus on processes becomes apparent in a statement towards the end of the paper. Following a quite long, but inconclusive, discussion of what might have caused the tidal variability, there is the statement (lines 550-551) that coincident oceanographic data were collected that will contribute to understanding the tidal variability in the sub-ice platelet layer. If the motivation were to advance our understanding of the processes driving tidal variability, wouldn’t it make more sense to publish all the data that can contribute to that understanding together in a single paper? It would definitely be of more use to the wider community. Once again, that points to the primary motivation for this submission being the publication of a dataset rather than the solution of a scientific problem.
For this reason, I would not recommend that the paper by published in the Cryosphere in its current form. I should be clear that my recommendation does not in any way reflect on the quality of the data themselves, or the data collection and processing. The authors have clearly completed a challenging field campaign to high standards and have collected a great dataset that could potentially be valuable to others. However, the paper as currently written does not really do the data justice and does not communicate their value to the reader, nor how they can be used to advance understanding of sub-ice platelet layer formation.
I would actually recommend that the authors split the data over two papers, one focussing on “winter growth” and the other on “tidal variability”. Each one needs clear statements of the problem being addressed and how the data contribute towards new understanding of that problem. The latter paper should ideally wait until the oceanographic data are available, so that a more complete and rigorous discussion of the processes driving variability can be given.
Citation: https://doi.org/10.5194/tc-2021-61-RC2
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