Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability

Metzner, Enrico P.; Salzmann, Marc

doi:https://doi.org/10.5194/egusphere-2023-106

Preprints

https://doi.org/10.5194/egusphere-2023-106

Preprints

20 Feb 2023

| 20 Feb 2023

Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability

Enrico P. Metzner and Marc Salzmann

Abstract. The Arctic Ocean cold halocline layer (CHL) separates the cold surface mixed layer (SML) from the underlying warm Atlantic water, and thus provides a precondition for sea ice formation. Here, we introduce a new method in which the CHL base depth is diagnosed from vertical stability and compare it to two existing methods. Vertical stability directly affects vertical mixing and heat exchange. When applied to measurements from ice-tethered profilers, ships, and moorings, the new method for estimating the CHL base depth provides robust results with few artifacts. Comparatively large differences between our new method and two existing methods for detecting the CHL base depth were found in regions which are most prone to a CHL retreat in a warming climate. CHL base depth exhibits a seasonal cycle with a maximum depth in winter and also spring, when the SML depth is also at its maximum, but the amplitude of the CHL base depth's seasonal cycle is lower than for the SML for all three methods as expected. We also propose a novel method for detecting the cold halostad layer and study the seasonal cycle employing conservative assumptions to avoid a misclassification (including a lower bound of 50 m for the thickness). Detection of a cold halostad layer was largely confined to the Canada Basin and to the regions off the eastern coast of Greenland and also Svalbard.

Received: 26 Jan 2023 – Discussion started: 20 Feb 2023

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18 Oct 2023

Technical note: Determining Arctic Ocean halocline and cold halostad depths based on vertical stability

Enrico P. Metzner and Marc Salzmann

Ocean Sci., 19, 1453–1464, https://doi.org/10.5194/os-19-1453-2023,https://doi.org/10.5194/os-19-1453-2023, 2023

Short summary

Enrico P. Metzner and Marc Salzmann

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-106', Igor Polyakov, 06 Mar 2023
R E V I E W

of the manuscript (egusphere-2023-106) entitled“Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability” by E. P. Metzner and M. Salzmann

The manuscript proposes a new method for estimating the depth of the Cold Halocline Layer (CHL). The topic is important and warrants a lot of attention in the published paper. Thus, I am very positive that the authors have the potential to produce a nice publication.
However, at the current stage, the manuscript suffers from several major shortcomings:
I did not find a convincing and satisfactory comparison of the three methods defining the halocline. The majority of the materials presented in the manuscript are about the illustration of the application of the methods and not their comparison. This comparison should include an evaluation of each method's performance and an explanation of the benefits of using each method. Right now, the way it is done is not satisfactory. I would like to see, for example, individual temperature (T) and salinity (S) profiles where the authors show what each method provides and explain the physical reasons for that. I found Fig. 3, which is devoted to method comparison, to be hard to read and not informative.

It looks like the authors misinterpret the water structure of the Arctic Ocean, which has direct implications for their comparative analysis of the three methods used for the definition of the halocline base. In the Eurasian Basin, CHL is the layer where T is close to the freezing point (that is why it is called cold) and S rapidly increases. However, below the CHL, there is the second portion of halocline—the lower halocline water—in which T and S increase with depth. The authors should define what they investigate. In the Canadian Basin, this structure is further complicated by the presence of two other halocline water varieties: Pacific summer and winter waters. Showing the Nordic Seas and Siberian shelves should be excluded from their analysis, for example.

By the way, it looks to me like the authors miss two golden opportunities to give a beautiful explanation to their methods. a) Fig. 2 from a single ITP record shows a mismatch of results between the authors’ stability method (ST) and the density ratio (DR) method after winter convection. I may be wrong, but it is worth checking whether the ST method captures the beginning of the new halocline formation, which is not captured by the two other methods. The halocline formation is a very important topic of Arctic oceanography. b) When the authors analyzed halostad, a possible interpretation may be that they analyzed the boundaries of a variety (winter or summer) of Pacific water. It is worth checking, and if it is true, this may be a nice touch to "sell" the method.

The way materials presented is often not good enough – see my detailed comments provided to the authors.

Despite these deficiencies, I believe that the authors can improve the manuscript to the level suitable for publications. That is why I give a “major revision” to this manuscript.

Below are my specific comments.

Comments:
Line 16: As defined, it is not CHL, but CHL+Lower Halocline Water in the Eurasian Basin.

Line 17: 300m may be correct for the Canadian Basin but not for the Eurasian Basin.

Line 43: The halocline has a complex structure indeed, but this was first described long time ago – see papers by e.g. Rudels, Aagaard, Carmack, Steele.

Line 57: Same as for line 43, plus add here Pacific waters.

Data description.

Please provide vertical resolution of original data, time covered by observations, show distribution of data coverage in time and space for annual and seasonal coverage, show separately spatiotemporal coverage provided by ITP and other sources. Provide accuracy of observations.

I do not understand the need of this complex vertical data interpolation (lines 89-90). I suspect that the original (raw) data have 1 or 2m resolution which should be sufficient for the purposes of the study.

Exclude all points from the shelves where there is no typically halocline found. The same is true for the Nordic Seas.

Regions are shown in Fig 1 but not used. What is the purpose for that?

Line 101: Why T is given in K, not in C?

Line 113: Shallower, not smaller.

Section 2.2.4 and further discussion: All these methods should be illustrated using individual profiles where everything is clearly marked and visible. Same section: You may use definition of the upper Atlantic Water layer by using 0^oC isotherm.

Section 2.2.5: The authors may want to clearly define halostad by giving some physical interpretation for this layer.

Line 144: Taking 0.001m threshold seems misleading since the original data have a 1m vertical resolution (or even coarser).

Section 2.3. I found the use of kriging more misleading than helpful in this study.Actually there is no need in that at all since the authors used another much simpler method which serves for the purposes.

Line 168: Filtration of outliers using such a severe threshold of 25/75% (which is less than 1.5 standard deviation) needs an explanation. Sensitivity study where the authors show how sensitive their estimates to different thresholds may be helpful.

Line 198: An example where the authors misinterpret water masses: I think they found that the CHL disappeared but the lower halocline water (not AW) is below the SML.

Line 202: Please provide specific for the cases when the criteria were not met: what criteria, why, etc.

Line 204: I did not understand what is written there.

Fig 2: I think this, plus Fig 7, is a good plot to be used for the method interpretation. I suggest to move line definition from the panel - the authors have space below the map. This case would be nice to illustrate further by using individual profiles of T & S for different regimes.

Similar plot for the Canadian Basin, plus individual profiles from there, would be a good illustration for regional halocline differences.
3. Not a good figure. I would completely eliminate it since I found no information in the current version.

Section 3.2. What does “occurrence” mean in this context?

Fig 4: Please eliminate point in the Nordic Seas and over the shelves. I suggest to eliminate panels with kriging. The point here is not to show the differences between spatial interpolation methods, but between halocline definition. So, I suggest to show additional panels for differences between methods: e.g. ST-DR and ST-TD. Again, please explain “occurrence”. I did not understand the ”nearest-neighbor” method: If the nearest data point to the grid point is used, why there was averaging then?

5: The same as for Fig 4: Please eliminate estimates from kriging and compare methods of halocline definition and not the methods of spatial interpolation. I.e. show CHL-DR minus CHL-ST and CHL-TD minus CHL_ST.

Lines 245-247: Often, some analysis of SML is given (like in this paragraph). This is not the topic of the study, first. SML parameters may be given, if they serve the purpose of illustrating the methods. Otherwise, please skip these places. As they are now, they raise questions about newness of these results (e.g. asking for comparison with the previous papers on the subject).

Fig 6: The caption is not clear – the difference is not defined. “(e) to (f)” is not clear. Cos fit is not illustrated.

Fig 7. Is good, but a) add T &S (like in Fig 2) b) move line definition from the panel.

Fig 8: What is the point of giving fraction of grid points? What is halostad depth? Give a physical interpretation (Pacific water?).

Line 314: What is “semi-saline”?

Line 321: How can we see the point made for the Barents Sea?

Line 322: This discussion and comparison with Steele work needs much more thourough analysis – not just a single line.

Line 325 – What does this “lack: mean?
Citation: https://doi.org/10.5194/egusphere-2023-106-CC1
RC1:
'Comment on egusphere-2023-106', Igor Polyakov, 06 Mar 2023
R E V I E W

of the manuscript (egusphere-2023-106) entitled
“Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability” by E. P. Metzner and M. Salzmann

The manuscript proposes a new method for estimating the depth of the Cold Halocline Layer (CHL). The topic is important and warrants a lot of attention in the published paper. Thus, I am very positive that the authors have the potential to produce a nice publication.
However, at the current stage, the manuscript suffers from several major shortcomings:
I did not find a convincing and satisfactory comparison of the three methods defining the halocline. The majority of the materials presented in the manuscript are about the illustration of the application of the methods and not their comparison. This comparison should include an evaluation of each method's performance and an explanation of the benefits of using each method. Right now, the way it is done is not satisfactory. I would like to see, for example, individual temperature (T) and salinity (S) profiles where the authors show what each method provides and explain the physical reasons for that. I found Fig. 3, which is devoted to method comparison, to be hard to read and not informative.

It looks like the authors misinterpret the water structure of the Arctic Ocean, which has direct implications for their comparative analysis of the three methods used for the definition of the halocline base. In the Eurasian Basin, CHL is the layer where T is close to the freezing point (that is why it is called cold) and S rapidly increases. However, below the CHL, there is the second portion of halocline—the lower halocline water—in which T and S increase with depth. The authors should define what they investigate. In the Canadian Basin, this structure is further complicated by the presence of two other halocline water varieties: Pacific summer and winter waters. Showing the Nordic Seas and Siberian shelves should be excluded from their analysis, for example.

By the way, it looks to me like the authors miss two golden opportunities to give a beautiful explanation to their methods. a) Fig. 2 from a single ITP record shows a mismatch of results between the authors’ stability method (ST) and the density ratio (DR) method after winter convection. I may be wrong, but it is worth checking whether the ST method captures the beginning of the new halocline formation, which is not captured by the two other methods. The halocline formation is a very important topic of Arctic oceanography. b) When the authors analyzed halostad, a possible interpretation may be that they analyzed the boundaries of a variety (winter or summer) of Pacific water. It is worth checking, and if it is true, this may be a nice touch to "sell" the method.

The way materials presented is often not good enough – see my detailed comments provided to the authors.

Despite these deficiencies, I believe that the authors can improve the manuscript to the level suitable for publications. That is why I give a “major revision” to this manuscript.

Below are my specific comments.

Comments:
Line 16: As defined, it is not CHL, but CHL+Lower Halocline Water in the Eurasian Basin.

Line 17: 300m may be correct for the Canadian Basin but not for the Eurasian Basin.

Line 43: The halocline has a complex structure indeed, but this was first described long time ago – see papers by e.g. Rudels, Aagaard, Carmack, Steele.

Line 57: Same as for line 43, plus add here Pacific waters.

Data description.

Please provide vertical resolution of original data, time covered by observations, show distribution of data coverage in time and space for annual and seasonal coverage, show separately spatiotemporal coverage provided by ITP and other sources. Provide accuracy of observations.

I do not understand the need of this complex vertical data interpolation (lines 89-90). I suspect that the original (raw) data have 1 or 2m resolution which should be sufficient for the purposes of the study.

Exclude all points from the shelves where there is no typically halocline found. The same is true for the Nordic Seas.

Regions are shown in Fig 1 but not used. What is the purpose for that?

Line 101: Why T is given in K, not in C?

Line 113: Shallower, not smaller.

Section 2.2.4 and further discussion: All these methods should be illustrated using individual profiles where everything is clearly marked and visible. Same section: You may use definition of the upper Atlantic Water layer by using 0^oC isotherm.

Section 2.2.5: The authors may want to clearly define halostad by giving some physical interpretation for this layer.

Line 144: Taking 0.001m threshold seems misleading since the original data have a 1m vertical resolution (or even coarser).

Section 2.3. I found the use of kriging more misleading than helpful in this study.Actually there is no need in that at all since the authors used another much simpler method which serves for the purposes.

Line 168: Filtration of outliers using such a severe threshold of 25/75% (which is less than 1.5 standard deviation) needs an explanation. Sensitivity study where the authors show how sensitive their estimates to different thresholds may be helpful.

Line 198: An example where the authors misinterpret water masses: I think they found that the CHL disappeared but the lower halocline water (not AW) is below the SML.

Line 202: Please provide specific for the cases when the criteria were not met: what criteria, why, etc.

Line 204: I did not understand what is written there.

Fig 2: I think this, plus Fig 7, is a good plot to be used for the method interpretation. I suggest to move line definition from the panel - the authors have space below the map. This case would be nice to illustrate further by using individual profiles of T & S for different regimes.

Similar plot for the Canadian Basin, plus individual profiles from there, would be a good illustration for regional halocline differences.
3. Not a good figure. I would completely eliminate it since I found no information in the current version.

Section 3.2. What does “occurrence” mean in this context?

Fig 4: Please eliminate point in the Nordic Seas and over the shelves. I suggest to eliminate panels with kriging. The point here is not to show the differences between spatial interpolation methods, but between halocline definition. So, I suggest to show additional panels for differences between methods: e.g. ST-DR and ST-TD. Again, please explain “occurrence”. I did not understand the ”nearest-neighbor” method: If the nearest data point to the grid point is used, why there was averaging then?

5: The same as for Fig 4: Please eliminate estimates from kriging and compare methods of halocline definition and not the methods of spatial interpolation. I.e. show CHL-DR minus CHL-ST and CHL-TD minus CHL_ST.

Lines 245-247: Often, some analysis of SML is given (like in this paragraph). This is not the topic of the study, first. SML parameters may be given, if they serve the purpose of illustrating the methods. Otherwise, please skip these places. As they are now, they raise questions about newness of these results (e.g. asking for comparison with the previous papers on the subject).

Fig 6: The caption is not clear – the difference is not defined. “(e) to (f)” is not clear. Cos fit is not illustrated.

Fig 7. Is good, but a) add T &S (like in Fig 2) b) move line definition from the panel.

Fig 8: What is the point of giving fraction of grid points? What is halostad depth? Give a physical interpretation (Pacific water?).

Line 314: What is “semi-saline”?

Line 321: How can we see the point made for the Barents Sea?

Line 322: This discussion and comparison with Steele work needs much more thourough analysis – not just a single line.

Line 325 – What does this “lack: mean?
Citation: https://doi.org/10.5194/egusphere-2023-106-RC1
- AC1: 'Reply to reviewer comment by I. Polyakov', Marc Salzmann, 16 May 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-106/egusphere-2023-106-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-106-AC1
RC2:
'Comment on egusphere-2023-106', Marylou Athanase, 22 Mar 2023
Review on the manuscript EGUSPHERE-2023-106

Summary:
The present study proposes a new method for the detection of the cold halocline layer base depth in the Arctic Ocean. The authors define a new criterion based on vertical stability and compare the results to those obtained with two previously used methods: one based on the density ratio, and one based on temperature differences. Using the ITP and UDASH databases, they derive pan-Arctic maps of the cold halocline layer base depth using all three methods.

General assessment:
The topic is well within the scope of Ocean Science and is of particular importance given the complex and varying structure of the Arctic halocline throughout basins. However, several crucial points would need to be addressed, regarding the manuscript organization as well as the clarity of the method and completeness of the results. Below are listed my major and minor comments. For these reasons, I recommend that the manuscript undergo major revisions before being potentially suitable for publication.

Major comments:
The manuscript lacks a clear, physical explanation of what the CHL and cold halostad are, and how the proposed method detects their boundaries. I would also suggest the authors emphasize the goal aimed to be achieved by defining a new criterion, e.g., enabling the robust detection of the CHL base depth across basins and seasons using only one criterion.

It is my understanding that here, the authors equate the CHL base to the point of maximum stability. Wouldn’t the point of maximum stability rather be within the “cline” you are considering, and not necessarily at its base? If I misunderstood, I would suggest the authors clarify their method and the physical reasoning behind it, as stated above.

The manuscript still lacks an actual evaluation of the performance of each of the detection methods presented here. I like the large scales comparisons, but it lacks some quantitative estimates (which ideally would take into account the varying seasons and basins) and some idea of which method performs best. Because of the diversity of situations, different tests may lead to different rankings of the presented methods. And that is fine, as long as these various results are explicitly presented and discussed.

The organization of the manuscript lacks fluid connections, both in the introduction and in the presentation of the results. Subsections seem to be organized thematically but without a clear logical order. I would suggest the authors consider reorganizing the overall manuscript (introduction and results) and the abstract as follows:

1) Clear introduction of what SML, CHL, and cold halostad are, and why they matter (as already done in part for the CHL).
2) Presenting the previous methods that have been used to define these layers, and underlining eventual (knowledge) gaps in these methods.
3) Introducing the new method, the physical reasoning behind its development, and the goal it aims to achieve.
4) Demonstrating how the results of the new method compare to results from previous methods (qualitatively and quantitatively, by adding some basin-wise statistics for example), and what are the gains of the new method.

Minor comments:

In the introduction in general:
It would be good to improve logical connections between paragraphs. For example, L26-27: what is the connection between Atlantification discussions and CHL characteristics? You could finish the previous paragraph by commenting on possible changes in the strength of stratification within the CHL that could either boost or hinder further Atlantification.
In general, I would suggest reshaping your introduction as follows (as stated above):
- Defining the broad concepts of cold halocline layer and cold halostad
- Underlining their importance for our understanding of the present and future Arctic ocean characteristics
- Listing criteria for their definition used so far, and eventually pros and cons
- Explaining clearly the motivation behind creating a new set of criteria, and the problem you aim to address
- Finally, introduce the organization of the manuscript (as you already do)

L22-26: Remind the readers in 1-2 sentences what Atlantification is. In fact, you partly do so in the following sentence, but this should come earlier. And isn’t ice loss a symptom/characteristic of Atlantification, too?

L38-39: Here, and generally throughout the manuscript, please define concepts as early as they appear. This sentence should come as you mention the density ratio, 2 lines above. In short, put the sentence line 37-38 at the end of this paragraph.

L34-52: It would be best to reshape this series of 3 paragraphs into one, listing all existing used criteria to define the base of the CHL and eventually their pro and cons.

L57: You already introduced this concept above when citing Bertosio (2020) and (2022). Would be best to merge the descriptions of upper and lower CHL into one paragraph here, keeping the relevant citations.

L75: “Level III data”: This processing level naming convention is rather opaque for unfamiliar readers. State more clearly what this entails (visual inspection, vertical interpolation, salinity spikes or bias corrections… etc)

L91: “Gaussian filter”: as above, please introduce each processing method explicitly.

L89: Is this reasonable for all profiles? Could some profiles, especially the oldest ones, have a vertical sampling of 5 to 10 m? If that is the case, please state so and briefly discuss why you think such a high-resolution interpolation is appropriate and reasonable. I would also suggest the authors consider a vertical resolution that is less fine and closer to the native profiles’ vertical resolutions.

L139: it seems this sentence has grammar issues.

L141: …“only the lowest of these layers is identified as a cold halostad.“ Why is that, physically?

L169: 25^th/75^th percentile: This is an extremely stringent test. Can you explain why you took such a high threshold? As the other reviewer stated, it would be good to know how sensitive your results are to the “outlier” threshold you chose.

L254: In general, I would try to limit references to previous specific figures from previous papers. It is easier if you directly remind the readers what were the findings shown that figure through text, citing the source paper, in discussing your results in light of it.

L314: If you mean “comparatively low-salinity” then use “low salinity”.

Fig. 2 and 7: I like these figures, and it would be great to have one of such plots for -when possible- each subregion and season. Even on one ITP, you could for example indicate when the buoys are in Canada vs. Makarov vs. Amundsen / Nansen basins.

Fig. 3: This is an interesting visualization, but it is rather under-used in the manuscript. I would suggest the author consider replacing it with plots presenting the vertical profiles, either in the introduction to present your various concepts (SML, upper and lower CHL, cold halostad), or in your results by grouping profiles in similar regions or seasons.

Fig. 4 and 5: I still do not quite get the goal of using 2 interpolation methods (NN and kriging). If there is no other objective that showing the pan-Arctic results in 2 different ways, I would suggest the authors pick one of these methods and eliminate the other, in order to make the manuscript more fluid.
Citation: https://doi.org/10.5194/egusphere-2023-106-RC2
- AC2: 'Reply to reviewer comment by M. Athanase', Marc Salzmann, 16 May 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-106/egusphere-2023-106-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-106-AC2

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-106', Igor Polyakov, 06 Mar 2023
R E V I E W

of the manuscript (egusphere-2023-106) entitled“Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability” by E. P. Metzner and M. Salzmann

The manuscript proposes a new method for estimating the depth of the Cold Halocline Layer (CHL). The topic is important and warrants a lot of attention in the published paper. Thus, I am very positive that the authors have the potential to produce a nice publication.
However, at the current stage, the manuscript suffers from several major shortcomings:
I did not find a convincing and satisfactory comparison of the three methods defining the halocline. The majority of the materials presented in the manuscript are about the illustration of the application of the methods and not their comparison. This comparison should include an evaluation of each method's performance and an explanation of the benefits of using each method. Right now, the way it is done is not satisfactory. I would like to see, for example, individual temperature (T) and salinity (S) profiles where the authors show what each method provides and explain the physical reasons for that. I found Fig. 3, which is devoted to method comparison, to be hard to read and not informative.

It looks like the authors misinterpret the water structure of the Arctic Ocean, which has direct implications for their comparative analysis of the three methods used for the definition of the halocline base. In the Eurasian Basin, CHL is the layer where T is close to the freezing point (that is why it is called cold) and S rapidly increases. However, below the CHL, there is the second portion of halocline—the lower halocline water—in which T and S increase with depth. The authors should define what they investigate. In the Canadian Basin, this structure is further complicated by the presence of two other halocline water varieties: Pacific summer and winter waters. Showing the Nordic Seas and Siberian shelves should be excluded from their analysis, for example.

By the way, it looks to me like the authors miss two golden opportunities to give a beautiful explanation to their methods. a) Fig. 2 from a single ITP record shows a mismatch of results between the authors’ stability method (ST) and the density ratio (DR) method after winter convection. I may be wrong, but it is worth checking whether the ST method captures the beginning of the new halocline formation, which is not captured by the two other methods. The halocline formation is a very important topic of Arctic oceanography. b) When the authors analyzed halostad, a possible interpretation may be that they analyzed the boundaries of a variety (winter or summer) of Pacific water. It is worth checking, and if it is true, this may be a nice touch to "sell" the method.

The way materials presented is often not good enough – see my detailed comments provided to the authors.

Despite these deficiencies, I believe that the authors can improve the manuscript to the level suitable for publications. That is why I give a “major revision” to this manuscript.

Below are my specific comments.

Comments:
Line 16: As defined, it is not CHL, but CHL+Lower Halocline Water in the Eurasian Basin.

Line 17: 300m may be correct for the Canadian Basin but not for the Eurasian Basin.

Line 43: The halocline has a complex structure indeed, but this was first described long time ago – see papers by e.g. Rudels, Aagaard, Carmack, Steele.

Line 57: Same as for line 43, plus add here Pacific waters.

Data description.

Please provide vertical resolution of original data, time covered by observations, show distribution of data coverage in time and space for annual and seasonal coverage, show separately spatiotemporal coverage provided by ITP and other sources. Provide accuracy of observations.

I do not understand the need of this complex vertical data interpolation (lines 89-90). I suspect that the original (raw) data have 1 or 2m resolution which should be sufficient for the purposes of the study.

Exclude all points from the shelves where there is no typically halocline found. The same is true for the Nordic Seas.

Regions are shown in Fig 1 but not used. What is the purpose for that?

Line 101: Why T is given in K, not in C?

Line 113: Shallower, not smaller.

Section 2.2.4 and further discussion: All these methods should be illustrated using individual profiles where everything is clearly marked and visible. Same section: You may use definition of the upper Atlantic Water layer by using 0^oC isotherm.

Section 2.2.5: The authors may want to clearly define halostad by giving some physical interpretation for this layer.

Line 144: Taking 0.001m threshold seems misleading since the original data have a 1m vertical resolution (or even coarser).

Section 2.3. I found the use of kriging more misleading than helpful in this study.Actually there is no need in that at all since the authors used another much simpler method which serves for the purposes.

Line 168: Filtration of outliers using such a severe threshold of 25/75% (which is less than 1.5 standard deviation) needs an explanation. Sensitivity study where the authors show how sensitive their estimates to different thresholds may be helpful.

Line 198: An example where the authors misinterpret water masses: I think they found that the CHL disappeared but the lower halocline water (not AW) is below the SML.

Line 202: Please provide specific for the cases when the criteria were not met: what criteria, why, etc.

Line 204: I did not understand what is written there.

Fig 2: I think this, plus Fig 7, is a good plot to be used for the method interpretation. I suggest to move line definition from the panel - the authors have space below the map. This case would be nice to illustrate further by using individual profiles of T & S for different regimes.

Similar plot for the Canadian Basin, plus individual profiles from there, would be a good illustration for regional halocline differences.
3. Not a good figure. I would completely eliminate it since I found no information in the current version.

Section 3.2. What does “occurrence” mean in this context?

Fig 4: Please eliminate point in the Nordic Seas and over the shelves. I suggest to eliminate panels with kriging. The point here is not to show the differences between spatial interpolation methods, but between halocline definition. So, I suggest to show additional panels for differences between methods: e.g. ST-DR and ST-TD. Again, please explain “occurrence”. I did not understand the ”nearest-neighbor” method: If the nearest data point to the grid point is used, why there was averaging then?

5: The same as for Fig 4: Please eliminate estimates from kriging and compare methods of halocline definition and not the methods of spatial interpolation. I.e. show CHL-DR minus CHL-ST and CHL-TD minus CHL_ST.

Lines 245-247: Often, some analysis of SML is given (like in this paragraph). This is not the topic of the study, first. SML parameters may be given, if they serve the purpose of illustrating the methods. Otherwise, please skip these places. As they are now, they raise questions about newness of these results (e.g. asking for comparison with the previous papers on the subject).

Fig 6: The caption is not clear – the difference is not defined. “(e) to (f)” is not clear. Cos fit is not illustrated.

Fig 7. Is good, but a) add T &S (like in Fig 2) b) move line definition from the panel.

Fig 8: What is the point of giving fraction of grid points? What is halostad depth? Give a physical interpretation (Pacific water?).

Line 314: What is “semi-saline”?

Line 321: How can we see the point made for the Barents Sea?

Line 322: This discussion and comparison with Steele work needs much more thourough analysis – not just a single line.

Line 325 – What does this “lack: mean?
Citation: https://doi.org/10.5194/egusphere-2023-106-CC1
RC1:
'Comment on egusphere-2023-106', Igor Polyakov, 06 Mar 2023
R E V I E W

of the manuscript (egusphere-2023-106) entitled
“Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability” by E. P. Metzner and M. Salzmann

The manuscript proposes a new method for estimating the depth of the Cold Halocline Layer (CHL). The topic is important and warrants a lot of attention in the published paper. Thus, I am very positive that the authors have the potential to produce a nice publication.
However, at the current stage, the manuscript suffers from several major shortcomings:
I did not find a convincing and satisfactory comparison of the three methods defining the halocline. The majority of the materials presented in the manuscript are about the illustration of the application of the methods and not their comparison. This comparison should include an evaluation of each method's performance and an explanation of the benefits of using each method. Right now, the way it is done is not satisfactory. I would like to see, for example, individual temperature (T) and salinity (S) profiles where the authors show what each method provides and explain the physical reasons for that. I found Fig. 3, which is devoted to method comparison, to be hard to read and not informative.

It looks like the authors misinterpret the water structure of the Arctic Ocean, which has direct implications for their comparative analysis of the three methods used for the definition of the halocline base. In the Eurasian Basin, CHL is the layer where T is close to the freezing point (that is why it is called cold) and S rapidly increases. However, below the CHL, there is the second portion of halocline—the lower halocline water—in which T and S increase with depth. The authors should define what they investigate. In the Canadian Basin, this structure is further complicated by the presence of two other halocline water varieties: Pacific summer and winter waters. Showing the Nordic Seas and Siberian shelves should be excluded from their analysis, for example.

By the way, it looks to me like the authors miss two golden opportunities to give a beautiful explanation to their methods. a) Fig. 2 from a single ITP record shows a mismatch of results between the authors’ stability method (ST) and the density ratio (DR) method after winter convection. I may be wrong, but it is worth checking whether the ST method captures the beginning of the new halocline formation, which is not captured by the two other methods. The halocline formation is a very important topic of Arctic oceanography. b) When the authors analyzed halostad, a possible interpretation may be that they analyzed the boundaries of a variety (winter or summer) of Pacific water. It is worth checking, and if it is true, this may be a nice touch to "sell" the method.

The way materials presented is often not good enough – see my detailed comments provided to the authors.

Despite these deficiencies, I believe that the authors can improve the manuscript to the level suitable for publications. That is why I give a “major revision” to this manuscript.

Below are my specific comments.

Comments:
Line 16: As defined, it is not CHL, but CHL+Lower Halocline Water in the Eurasian Basin.

Line 17: 300m may be correct for the Canadian Basin but not for the Eurasian Basin.

Line 43: The halocline has a complex structure indeed, but this was first described long time ago – see papers by e.g. Rudels, Aagaard, Carmack, Steele.

Line 57: Same as for line 43, plus add here Pacific waters.

Data description.

Please provide vertical resolution of original data, time covered by observations, show distribution of data coverage in time and space for annual and seasonal coverage, show separately spatiotemporal coverage provided by ITP and other sources. Provide accuracy of observations.

I do not understand the need of this complex vertical data interpolation (lines 89-90). I suspect that the original (raw) data have 1 or 2m resolution which should be sufficient for the purposes of the study.

Exclude all points from the shelves where there is no typically halocline found. The same is true for the Nordic Seas.

Regions are shown in Fig 1 but not used. What is the purpose for that?

Line 101: Why T is given in K, not in C?

Line 113: Shallower, not smaller.

Section 2.2.4 and further discussion: All these methods should be illustrated using individual profiles where everything is clearly marked and visible. Same section: You may use definition of the upper Atlantic Water layer by using 0^oC isotherm.

Section 2.2.5: The authors may want to clearly define halostad by giving some physical interpretation for this layer.

Line 144: Taking 0.001m threshold seems misleading since the original data have a 1m vertical resolution (or even coarser).

Section 2.3. I found the use of kriging more misleading than helpful in this study.Actually there is no need in that at all since the authors used another much simpler method which serves for the purposes.

Line 168: Filtration of outliers using such a severe threshold of 25/75% (which is less than 1.5 standard deviation) needs an explanation. Sensitivity study where the authors show how sensitive their estimates to different thresholds may be helpful.

Line 198: An example where the authors misinterpret water masses: I think they found that the CHL disappeared but the lower halocline water (not AW) is below the SML.

Line 202: Please provide specific for the cases when the criteria were not met: what criteria, why, etc.

Line 204: I did not understand what is written there.

Fig 2: I think this, plus Fig 7, is a good plot to be used for the method interpretation. I suggest to move line definition from the panel - the authors have space below the map. This case would be nice to illustrate further by using individual profiles of T & S for different regimes.

Similar plot for the Canadian Basin, plus individual profiles from there, would be a good illustration for regional halocline differences.
3. Not a good figure. I would completely eliminate it since I found no information in the current version.

Section 3.2. What does “occurrence” mean in this context?

Fig 4: Please eliminate point in the Nordic Seas and over the shelves. I suggest to eliminate panels with kriging. The point here is not to show the differences between spatial interpolation methods, but between halocline definition. So, I suggest to show additional panels for differences between methods: e.g. ST-DR and ST-TD. Again, please explain “occurrence”. I did not understand the ”nearest-neighbor” method: If the nearest data point to the grid point is used, why there was averaging then?

5: The same as for Fig 4: Please eliminate estimates from kriging and compare methods of halocline definition and not the methods of spatial interpolation. I.e. show CHL-DR minus CHL-ST and CHL-TD minus CHL_ST.

Lines 245-247: Often, some analysis of SML is given (like in this paragraph). This is not the topic of the study, first. SML parameters may be given, if they serve the purpose of illustrating the methods. Otherwise, please skip these places. As they are now, they raise questions about newness of these results (e.g. asking for comparison with the previous papers on the subject).

Fig 6: The caption is not clear – the difference is not defined. “(e) to (f)” is not clear. Cos fit is not illustrated.

Fig 7. Is good, but a) add T &S (like in Fig 2) b) move line definition from the panel.

Fig 8: What is the point of giving fraction of grid points? What is halostad depth? Give a physical interpretation (Pacific water?).

Line 314: What is “semi-saline”?

Line 321: How can we see the point made for the Barents Sea?

Line 322: This discussion and comparison with Steele work needs much more thourough analysis – not just a single line.

Line 325 – What does this “lack: mean?
Citation: https://doi.org/10.5194/egusphere-2023-106-RC1
- AC1: 'Reply to reviewer comment by I. Polyakov', Marc Salzmann, 16 May 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-106/egusphere-2023-106-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-106-AC1
RC2:
'Comment on egusphere-2023-106', Marylou Athanase, 22 Mar 2023
Review on the manuscript EGUSPHERE-2023-106

Summary:
The present study proposes a new method for the detection of the cold halocline layer base depth in the Arctic Ocean. The authors define a new criterion based on vertical stability and compare the results to those obtained with two previously used methods: one based on the density ratio, and one based on temperature differences. Using the ITP and UDASH databases, they derive pan-Arctic maps of the cold halocline layer base depth using all three methods.

General assessment:
The topic is well within the scope of Ocean Science and is of particular importance given the complex and varying structure of the Arctic halocline throughout basins. However, several crucial points would need to be addressed, regarding the manuscript organization as well as the clarity of the method and completeness of the results. Below are listed my major and minor comments. For these reasons, I recommend that the manuscript undergo major revisions before being potentially suitable for publication.

Major comments:
The manuscript lacks a clear, physical explanation of what the CHL and cold halostad are, and how the proposed method detects their boundaries. I would also suggest the authors emphasize the goal aimed to be achieved by defining a new criterion, e.g., enabling the robust detection of the CHL base depth across basins and seasons using only one criterion.

It is my understanding that here, the authors equate the CHL base to the point of maximum stability. Wouldn’t the point of maximum stability rather be within the “cline” you are considering, and not necessarily at its base? If I misunderstood, I would suggest the authors clarify their method and the physical reasoning behind it, as stated above.

The manuscript still lacks an actual evaluation of the performance of each of the detection methods presented here. I like the large scales comparisons, but it lacks some quantitative estimates (which ideally would take into account the varying seasons and basins) and some idea of which method performs best. Because of the diversity of situations, different tests may lead to different rankings of the presented methods. And that is fine, as long as these various results are explicitly presented and discussed.

The organization of the manuscript lacks fluid connections, both in the introduction and in the presentation of the results. Subsections seem to be organized thematically but without a clear logical order. I would suggest the authors consider reorganizing the overall manuscript (introduction and results) and the abstract as follows:

1) Clear introduction of what SML, CHL, and cold halostad are, and why they matter (as already done in part for the CHL).
2) Presenting the previous methods that have been used to define these layers, and underlining eventual (knowledge) gaps in these methods.
3) Introducing the new method, the physical reasoning behind its development, and the goal it aims to achieve.
4) Demonstrating how the results of the new method compare to results from previous methods (qualitatively and quantitatively, by adding some basin-wise statistics for example), and what are the gains of the new method.

Minor comments:

In the introduction in general:
It would be good to improve logical connections between paragraphs. For example, L26-27: what is the connection between Atlantification discussions and CHL characteristics? You could finish the previous paragraph by commenting on possible changes in the strength of stratification within the CHL that could either boost or hinder further Atlantification.
In general, I would suggest reshaping your introduction as follows (as stated above):
- Defining the broad concepts of cold halocline layer and cold halostad
- Underlining their importance for our understanding of the present and future Arctic ocean characteristics
- Listing criteria for their definition used so far, and eventually pros and cons
- Explaining clearly the motivation behind creating a new set of criteria, and the problem you aim to address
- Finally, introduce the organization of the manuscript (as you already do)

L22-26: Remind the readers in 1-2 sentences what Atlantification is. In fact, you partly do so in the following sentence, but this should come earlier. And isn’t ice loss a symptom/characteristic of Atlantification, too?

L38-39: Here, and generally throughout the manuscript, please define concepts as early as they appear. This sentence should come as you mention the density ratio, 2 lines above. In short, put the sentence line 37-38 at the end of this paragraph.

L34-52: It would be best to reshape this series of 3 paragraphs into one, listing all existing used criteria to define the base of the CHL and eventually their pro and cons.

L57: You already introduced this concept above when citing Bertosio (2020) and (2022). Would be best to merge the descriptions of upper and lower CHL into one paragraph here, keeping the relevant citations.

L75: “Level III data”: This processing level naming convention is rather opaque for unfamiliar readers. State more clearly what this entails (visual inspection, vertical interpolation, salinity spikes or bias corrections… etc)

L91: “Gaussian filter”: as above, please introduce each processing method explicitly.

L89: Is this reasonable for all profiles? Could some profiles, especially the oldest ones, have a vertical sampling of 5 to 10 m? If that is the case, please state so and briefly discuss why you think such a high-resolution interpolation is appropriate and reasonable. I would also suggest the authors consider a vertical resolution that is less fine and closer to the native profiles’ vertical resolutions.

L139: it seems this sentence has grammar issues.

L141: …“only the lowest of these layers is identified as a cold halostad.“ Why is that, physically?

L169: 25^th/75^th percentile: This is an extremely stringent test. Can you explain why you took such a high threshold? As the other reviewer stated, it would be good to know how sensitive your results are to the “outlier” threshold you chose.

L254: In general, I would try to limit references to previous specific figures from previous papers. It is easier if you directly remind the readers what were the findings shown that figure through text, citing the source paper, in discussing your results in light of it.

L314: If you mean “comparatively low-salinity” then use “low salinity”.

Fig. 2 and 7: I like these figures, and it would be great to have one of such plots for -when possible- each subregion and season. Even on one ITP, you could for example indicate when the buoys are in Canada vs. Makarov vs. Amundsen / Nansen basins.

Fig. 3: This is an interesting visualization, but it is rather under-used in the manuscript. I would suggest the author consider replacing it with plots presenting the vertical profiles, either in the introduction to present your various concepts (SML, upper and lower CHL, cold halostad), or in your results by grouping profiles in similar regions or seasons.

Fig. 4 and 5: I still do not quite get the goal of using 2 interpolation methods (NN and kriging). If there is no other objective that showing the pan-Arctic results in 2 different ways, I would suggest the authors pick one of these methods and eliminate the other, in order to make the manuscript more fluid.
Citation: https://doi.org/10.5194/egusphere-2023-106-RC2
- AC2: 'Reply to reviewer comment by M. Athanase', Marc Salzmann, 16 May 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-106/egusphere-2023-106-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-106-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Marc Salzmann on behalf of the Authors (08 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Jun 2023) by Ilker Fer

RR by Marylou Athanase (15 Jun 2023)

Suggestions for revision or reasons for rejection

This is my second round of reviews for this manuscript. In my first review of the original manuscript, my main concerns regarded (i) the reasoning being the choice of the ST criteria, (ii) the lack of direct evaluation of the methods, and (iii) improving the manuscript's readability.
The authors have answered all my initial comments, and I find that the revised manuscript addresses well these three main concerns.
The introduction and method description were greatly improved. The result section is now clearer and presented in a more linear way. I appreciate the addition of Figures 3, 5, and 7.

I still have a few remarks regarding editing and text clarifications, and one main comment which I trust the authors will be able to address. Overall, I believe the manuscript is close to being fit for publication, and would recommend acceptance after some minor revisions.

Main comment:

Since the choice of the DR criterion used for computing the ST criterion is, to some extent, somewhat arbitrary, it would be nice to add 1 or 2 sentences discussing this choice & how it should be optimized in the last section of the manuscript. You already touched on this topic by indicating that the ST criterion could be slightly changed to provide a better match with the DR criterion. This raises two questions:
- Should one aim for a match between both methods?
- If yes, are there particular reasons or limitations to why the adjustment of the ST criterion, following your own recommendations, could not be included in this manuscript as well? Adding an attempted “optimal ST criterion” for future users could strengthen your manuscript even more.

Editing remarks:

L. 6 and 338: Could you clarify what is meant by “robust” here? Most likely is done in the core text, but if mentioned in the abstract it should be clear what the criteria are for readers.

L. 28-30: This is confusing: you first refer to the advection of shelf water onto more saline waters as a halocline formation mechanism. Yet in the following sentences, you state that this mechanism is supported by 2 other studies, but then cite the formation of halocline via ice melting on top of warm AW. Although there are similarities in these mechanisms, they are not exactly the same. In the first, the input of freshwater comes from the fresher shelf waters, themselves fed by continental runoff and/or ice melt. In the latter, only direct ice melt on top of saltier waters is at play. Please clarify this statement.

L. 40: This is again confusing. I understand what you mean, but calling the PWW both a part of the halocline and a halostad layer can appear contradictory. You could consider saying that the halocline in the Canada Basin is constituted of PHW, in which we distinguish a halocline layer formed by the PSW, which overlays a cold halostad formed by PWW, and below the PHW is found the strongly stratified LHW of Atlantic-origin. Adding a schematic figure of what these layers look like in a T-S profile could also help. You could point to the local stability maxima as well.

L. 50: typo “ist”

L. 59: This sentence should rather be merged with line 52, here it is a little off-topic.

L. 67: “among others by others” typo

L. 86-87: This sentence should be moved to the end of the previous paragraph (l. 59), as it results from emphasizing the importance of the halocline in the Arctic system.

L.88: The manuscript outline is long and too detailed. Consider shortening this part, to no more than 1 sentence per section (1-2 lines) without detailing each sub-section. You might also not need to acknowledge your reviewers’ suggestions.

L. 148: “as demonstrated below” What does this refer to? This section? A figure?

Fig. 3: Nice figure.
- tick labels are occasionally overlaying each other, consider fixing that
- panel label and plot lines are occasionally overlaying each other, consider fixing that
- if possible, would be great to have a profile of density as well.

L. 263-310. This section is interesting but quite dense. Consider breaking it down into several paragraphs, e.g., by adding a line break l. 271 (before “In addition”) and l. 283 (when transitioning to the Canada Basin).

L. 275-277: Somewhat convoluted phrasing. In general, it is easier when sentences are straight to the point. In this case, you could just say that “the ST method also detects more frequent cases of halocline BD below 120 m compared to the DR method.”

L. 282: Any comments on this? You don’t make any use of Fig. 7c otherwise.

L. 312-316: This is merely a suggestion, and you may choose or not to follow it: since you have already explained your concepts in section 2, you can consider removing the reminder of these concepts at the beginning of your subsections in section 3. Here, for example, lines 312-318 mostly repeat information provided in section 2.
Since the paper is not overly long, you can however decide to maintain these reminders if you prefer to do so.

L. 335: Redefine all acronyms in the summary section

L. 374: This is a bit colloquial (and rather pessimistic wording). Consider saying rather that e.g., “mismatch rates seem to be notably varying across ITPs tested. This suggests that further testing and refining are needed in order to account for regional and temporal variability of CHS characteristics”

L. 381: I would suggest rephrasing your very last sentence so that it highlights the potential of AI methods without discarding too harshly traditional methods (including your newly proposed one).

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RR by Igor Polyakov (22 Jun 2023)

Suggestions for revision or reasons for rejection

R E V I E W

of the manuscript (egusphere-2023-106) entitled
“Technical note: Determining Arctic Ocean cold halocline and cold halostad layer depths based on vertical stability” by E. P. Metzner and M. Salzmann

The authors made substantial revision of the original manuscript following the reviewers’ comments. However, there are still several issues that need the authors’ attention. They are related to the application of the methods and the quality of data processing and analysis. I think this study still requires major revision.
Comments:
1. Lines 3-4: The sentence is hard to read. Needs editing.
2. Abstract: You can drop all abbreviations from the abstract if they are used only once. They will need to be defined in the text anyway.
3. Lines 6-7: These two sentences can be merged.
4. Lines 10-13: The last two sentences do not belong to the end of Abstract. The authors may use GCM argument (ll. 9-10) to close this short discussion.
5. Line 15. “Inflow regions” instead of “inflow”?
6. Discussion of halocline formation (e.g. ll. 26-30 and throughout the text). Now the text reads as the freshwater capping is involved in and helps the halocline formation. It is partially true since the freshwater is the source for the density gradient in the halocline. But the formation of the halocline is driven by a) horizontal advection (injection) of denser water from shelves in winter and b) vertical mixing (overcoming stability of the surface fresher layer) due to winter sea ice formation and brine rejection. Thus, the physical drivers overcome stability of the water column to form the halocline.
7. Lines 36-37: Logic is broken in the LHW part, the sentence needs editing.
8. Lines 44-46: Editing is needed. It would be helpful if the authors would illustrate statements like that with figures.
9. Line 50: “ist”.
10. Line 59, The authors may want to add reference to Shimadat etal 2006:
K. Shimada, T. Kamoshida, M. Itoh, S. Nishino, E. Carmack, F. McLaughlin, S. Zimmermann, A. Proshutinsky, Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean. Geophys. Res. Lett. 33, L08605 (2006).
11. Line 98: This referencing to reviewers is not standard. I suggest to cut them.
12. Line 104: I suggest to change “In a halocline” to “in the Arctic halocline” and “must be” to “is”.
13. Section 2.2. I suggest the authors illustrate these features using vertical profiles of temperature, salinity, and density. Means would probably work best.
14. Line 152: “Halocline”, not “halcoline” (but sounds beautiful).
15. Line 155: Please show this method using a profile.
16. Lines 156-163: Please show examples (profiles).
17. Line 167: Define “smoothed”.
18. Line 176: The authors need to cite the source, not our paper.
19. Line 2003: Please define “slopes … change”. They change everywhere, so the sentence is not clear.
20. Lines 203-206: This is a way too long sentence which can be easily split for 2-3.
21. Line 212: July 15 is not “After July”.
22. Figure 3: This new Figure and its analysis are my major concerns.
a. Since density is used for the analysis, I suggest it is also shown.
b. I see a problem with the salinity profiles which show local minima just above the high gradient areas. It is a well-known problem and comes from data processing. It creates artificial instabilities (higher over lower salinity layers). I think that is partially the reason for the high level of noise in the density ratio profiles. This problem may affect all authors’ results so it should be addressed properly. There are several ways to reduce the problem. One way is to use raw temperature and conductivity and use local correlations to better adjust the profiles. Probably, it will be prohibitively expensive time-/effort-wise. Another one is to apply smoothing, but it requires a lot of considerations. I suggest the authors consider that, fix the problem in the salinity profiles and describe it carefully in the text.
c. I am also wondering why the density ratio profiles change the sign from negative to positive within 60-170m whereas both temperature and salinity (as well as alpha and beta) are monotonic. I think within this depth range the density ratio should be positive. Please check. After fixing the problems described in b-c, the profiles and definitions (and further results!) may look differently.
d. The authors may consider using log axis for the density ratio to show the 0.05 value.
e. (f-h) Why SML depth is at 130m? Density profile would help.
f. I suggest to use different colors for SML and CHL segments. If they occupy the same depth, one can barely separate them so both are clearly seen.
g. Overall figure structure: Instead of “ITP74,” I suggest to use general title “Methods” and subtitles for the three right columns using methods’ abbreviations.
h. Caption: please define stability (and its unit). “threshold values,” “temperature difference” should be briefly explained.
23. Lines 219-222: The sentence needs editing.
24. Line 222: This sentence needs an explanation helping to see that “the convection affected halocline water” (e.g. …”as expressed by …”). There are many sentences throughout the text like that (e.g., lines 228, 229-230, etc.). The text will benefit from clarifying them.
25. Lines 236-237. This sentence (“Reasons..”) can be deleted.
26. Lines 240-250 etc. The authors often use reference to figures inside the sentences (e.g., ll. 242 twice, 246). Traditionally, however, these references are placed at the end of the sentences.
27. Lines: 249-254: I disagree that the differences between these methods are in different direction of search. This is a very misleading explanation. I think the major reason is, as the authors rightly stressed prior to this paragraph, a better physical justification, reasoning for the ST method. I suggest to remove this discussion or edit providing a better explanation.
28. Line 259: “The underlying grid…” – this sentence can be removed.
29. Figure 6: I suggest to remove panels f-j and corresponding text referencing it. In the Arctic Ocean, there are no differences. For d-e: How the differences were computed if the points in (a-c) were not occupying the same positions?
30. Lines 265-280: Please do not use negative definitions in describing the ST method. E.g. Line 266 states that the ST “overestimates” halocline depth compared with another method. That really reads as negative towards the ST.
31. Same text: I suggest you drop off the definition of regions using ovals. You may refer to the regions using their geographical names.
32. Bottom of page 13: I strongly suggest that the authors drop off any mentioning of the Norwegian Sea since this is a very different region with very different water masses and driving forces. They can mask this area in Figs.
33. Figure 7: Instead of “relative frequency” the authors may use “histogram”.
34. Section 3.3: Lines 312-319 is a repetition of Intro and should be removed. Lines 319-323 belong to “Methods”.
35. Line 332: Please direct the readers where exactly they can see the cold halostad near the coasts of Greenland in Fig. 8b.
36. Lines 329-332: Fig 8c may benefit from showing Standard Errors – they may help address the problem stated in the text.

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ED: Reconsider after major revisions (22 Jun 2023) by Ilker Fer

AR by Marc Salzmann on behalf of the Authors (29 Jul 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (01 Aug 2023) by Ilker Fer

RR by Igor Polyakov (06 Aug 2023)

Suggestions for revision or reasons for rejection

Editor, OS

RE: Review of the manuscript egusphere-2023-106

Dear Editor,
In this review, I limited my task by checking only the authors’ response to my major point of concern, namely excitation of spurious salinity extremes in high-gradient segments of the profiles. This is a well-known problem based on different sensitivity and time constants of temperature and conductivity sensors.
I am not happy with the authors’ response to my comment. They argued that they noticed the same issue in modeling data. This argument is left to the conscience of those who processed the modeling data. The authors also suggested that PWW could be one of the reasons for this spiritual salinity extremes. These extremes, however, are visible in ITP74 salinity profiles. I am not here to argue that salinity profiles can exhibit false extremes in areas with significant T&S gradients. There is an abundance of literature on this subject. This is a serious problem, and because of the nature of the task in front of the authors, it should be addressed. Right now, the issue is not handled properly. In its turn, this issue may affect the authors’ results (e.g., the DR method may select density values affected by the spurious salinity extremes).
The optimum solution would be to reprocess the data and to use lagged correlations inside a running window to locally adjust the T and S profiles. However, due to the relatively coarse ITP resolution, it may be prohibitively time-consuming and may not totally fix the problem. Another (easier) technique would be to apply extra smoothing (the authors did so anyway), but attention should be given to explain the overall impact of this smoothing. The problem must be resolved in some way. Otherwise, this potentially extremely important work would leave holes in the authors' conclusions.
I leave the decision to the editor, but I would advise the authors to take this issue carefully.
All the best -

Igor Polyakov

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RR by Marylou Athanase (18 Aug 2023)

ED: Publish subject to minor revisions (review by editor) (23 Aug 2023) by Ilker Fer

AR by Marc Salzmann on behalf of the Authors (01 Sep 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Sep 2023) by Ilker Fer

AR by Marc Salzmann on behalf of the Authors (04 Sep 2023)

Journal article(s) based on this preprint

18 Oct 2023

Technical note: Determining Arctic Ocean halocline and cold halostad depths based on vertical stability

Enrico P. Metzner and Marc Salzmann

Ocean Sci., 19, 1453–1464, https://doi.org/10.5194/os-19-1453-2023,https://doi.org/10.5194/os-19-1453-2023, 2023

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Enrico P. Metzner and Marc Salzmann

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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The Arctic Ocean cold halocline layer (CHL) separates the cold surface mixed layer from the underlying warm Atlantic water, and thus provides a precondition for sea ice formation. Here, we introduce a new method for detecting the CHL base and compare it to two existing methods. We show that the largest differences between the methods are found in the regions which are most prone to a CHL retreat in a warming climate, and we discuss advantages and disadvantages of the three methods.


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