Surface Water Mass Transformation in North Atlantic Based on NCEP CSFR Reanalysis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of the paper " Surface water mass transformation in North Atlantic based on 2 NCEP CSFR reanalysis " by Kukushkin and Gulev. 

 

In this paper, the authors investigate the space-time variability of the transformation of water masses in the North Atlantic, on multi-year time scales, with specific emphasis on its seasonal variability. The transformation is attributed to surface density flux, under the assumption that this mechanism is dominant over lateral mixing which is justified. The analysis is based on data from the updated, high-resolution version of the NCEP CFSv2 reanalysis dataset.

 

The study revealed large seasonality in water mass transformation, which is expected given the seasonal variations in oceanic heat and water budgets. It also identified geographic regions where surface fluxes are most intense. The novelty of the study, in my view, lies in the demonstration of a significant trend in the transformation of the major water masses in the N. Atlantic since the end of the last century. This trend appears to be correlated with anomalies in mixed layer volume and potentially with other variables that reflect large-scale climatological processes. Overall, the results presented in the study are consistent with findings from other research. In particular, they highlight modifications in oceanic processes in the context of global climate change.

 

The paper is well written, and the quality of the figures is good, effectively supporting the presentation and interpretation of the results. I therefore recommend the publication of the paper.

Nevertheless, I have a number of questions and comments regarding the form and content of this work. I hope they will help to improving the quality of the presentation of the results.

 

General comment: It’s standard practice to provide the full name of a quantity or variable before introducing its acronym (e.g., MLD, SST, STMW … ). However, this convention is not always followed in the manuscript.

Ln 28-29: which atmospheric and oceanic characteristics are you referring to?  

Ln 30-36: references should be provided for the two major hypotheses concerning the evolution of the surface boundary layer.

Ln 40-43: I did not observe the application of the Monin–Obukhov length scale in the analysis or interpretation of the results. It is unclear why the authors include its definition in the Introduction if it is not used in the study.

Ln 71-73: please revise the text and clarify your idea.

Ln 74: the word model is not appropriate

Ln 77-80: The text should be revised to ensure that references to results are not included in the Introduction, as the results appear later in the manuscript.

Ln 123-124: The authors should provide a justification for the choice of a 5-meter vertical extent for the surface layer. Is this a conventional thickness commonly accepted in the community? If not, why is a 5-meter thickness preferred over other possible values such as 10 meters or 200 meters?

Ln 98: degree is missed after 1/15.

Ln 118-119: The method used to analyse water mass formation is not novel. It would be helpful if the authors could explain how this approach differs from, improves, or is preferable to existing methods in the literature.

I wonder if the name “transformation” and the process of transformation is related to renewal of a water mass? My thoughts are mainly based on units representing volume transfer per unit time. Perhaps could be useful at this stage (Ln 119-120) to expand the explanation of the process of transformation.

Ln 154: The formula should follow standard notation conventions for expressions involving double integrals. Additionally, the variables delta and rho-prime are not defined in the text. Please provide their definitions and also the time period of integration.

Ln 158: Correct the value of Sv: 10^6 and also many other cases when the power is used.

Ln: 202-224: The authors use data from the year 1997 to illustrate the seasonal variability of the density flux.

Why the year 1997 was chosen instead of using seasonal averages computed over the full set of analysed years? Justifying this choice is important to ensure the representativeness of the seasonal variability analysis.

Ln 253: Same question regarding the results show in Fig. 3.

 

I find it necessary to include a discussion of the results in the context of previous studies. Section 4, entitled Discussion and Conclusions, would be an appropriate place for this comparison.

 

Additionally, I recommend sharpening the conclusion by clearly highlighting the novelty and originality of the results.

 

Comments for author File: Comments.pdf

Author Response

For research article

 

Surface water mass transformation in North Atlantic based on NCEP CSFR reanalysis

 

Response to Reviewer 1 Comments
1. Summary
The author thanks the reviewer for his time to re-review the article and for his valuable comments, which helped to improve the quality of the article. Below is a step-by-step response to the comments suggested by the reviewers. In the attached revised version of the manuscript, all corrections related to this review are highlighted in yellow.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Yes
Are all the cited references relevant to the research?	Yes
Is the research design appropriate?	Yes
Are the methods adequately described?	Yes
Are the results clearly presented?	Yes
Are the conclusions supported by the results?	Yes
3. Point-by-point response to Comments and Suggestions for Authors
Comments 1:
General comment: It’s standard practice to provide the full name of a quantity or variable before introducing its acronym (e.g., MLD, SST, STMW … ). However, this convention is not always followed in the manuscript. Response 1: Thanks for the comment, the abbreviated transcripts have been added to the text. Comments 2: Ln 28-29: which atmospheric and oceanic characteristics are you referring to?  Response 2: Thanks for the comment, the characteristics are described in more detail in the text Comments 3: Ln 30-36: references should be provided for the two major hypotheses concerning the evolution of the surface boundary layer. Response 3: Thanks for the comment, relevant links have been added to the article Comments 4: Ln 40-43: I did not observe the application of the Monin–Obukhov length scale in the analysis or interpretation of the results. It is unclear why the authors include its definition in the Introduction if it is not used in the study. Response 4: Thanks for the comment, the mention of the Monin-Obukhov length is used as part of a literature review of the theoretical component of this issue. Although the Monin-Obukhov length is not used in this particular study, it is important for understanding the theoretical part of the interaction of turbulence and buoyancy flows. Comments 5: Ln 71-73: please revise the text and clarify your idea. Response 5: Thanks for the comment, rephrased in a more understandable form. Comments 6: Ln 74: the word model is not appropriate Response 6: Thanks for the comment, corrected in text. Comments 7: Ln 77-80: The text should be revised to ensure that references to results are not included in the Introduction, as the results appear later in the manuscript. Response 7: Thanks for the comment, corrected in text. Comments 8: Ln 123-124: The authors should provide a justification for the choice of a 5-meter vertical extent for the surface layer. Is this a conventional thickness commonly accepted in the community? If not, why is a 5-meter thickness preferred over other possible values such as 10 meters or 200 meters? Response 8: Thanks for the comment, more detailed justification has been added to the text. Comments 9: Ln 98: degree is missed after 1/15. Response 9: Thanks for the comment, сcorrected. Comments 10: Ln 118-119: The method used to analyse water mass formation is not novel. It would be helpful if the authors could explain how this approach differs from, improves, or is preferable to existing methods in the literature. I wonder if the name “transformation” and the process of transformation is related to renewal of a water mass? My thoughts are mainly based on units representing volume transfer per unit time. Perhaps could be useful at this stage (Ln 119-120) to expand the explanation of the process of transformation. Response 10: Thanks for the comment, justification for using the density flux values has been added to the text. Clarification of the term transformation and transformation rate has been added to the text. Comments 11: Ln 154: The formula should follow standard notation conventions for expressions involving double integrals. Additionally, the variables delta and rho-prime are not defined in the text. Please provide their definitions and also the time period of integration. Response 11: Thanks for the comment, formula has been rewritten, variables and integration period was defined. Comments 12: Ln 158: Correct the value of Sv: 10^6 and also many other cases when the power is used. Response 12: Thank you, the degree is written in superscript, and so on in the text Comments 13: Ln: 202-224: The authors use data from the year 1997 to illustrate the seasonal variability of the density flux.   Why the year 1997 was chosen instead of using seasonal averages computed over the full set of analysed years? Justifying this choice is important to ensure the representativeness of the seasonal variability analysis. Response 13: The authors use 1979, the first year of the CFSR series, to illustrate the spatial features of the density flux distribution. Using the averaged full series of years would have given a smoother picture of the general appearance. Using a specific year gives a more detailed picture. In 1979, the NAO index was near zero, which preceded the positive phase of the NAO in the 1980s, which allows us to consider an average year in atmospheric activity. This motivation added to text. Comments 14: Ln 253: Same question regarding the results show in Fig. 3. Response 14: Motivation added to text. Comments 15: I find it necessary to include a discussion of the results in the context of previous studies. Section 4, entitled Discussion and Conclusions, would be an appropriate place for this comparison. Additionally, I recommend sharpening the conclusion by clearly highlighting the novelty and originality of the results.
Response 15: In conclusion, comparisons with already known studies on this topic have been added, and a justification for the novelty of the study has been added.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Review of “Surface water mass transformation in North Atlantic based on NCEP CSFR reanalysis.”

 

The authors discuss ocean water transport (transformations , m³ s-¹)  in the Noth Atlantic.  To me it is not quite clear what their main contribution is (The study reveals key regional features and seasonal variations…”) The study focus on two hypotheses, first, H1 ,  that atmospheric forcing causes vertical mixing in the North Atlantic waters, and second, H2, that the mechanical energy budget explains mixed layer dynamics.

 

Major issues

To me it was rather difficult to read the text and understand their arguments. It may be my fault.  The study presents two hypotheses, and I would have anticipated that there were clear responses to these hypotheses in the conclusion section, but I could not see that.  For example: “Water masses of different densities respond differently to seasonal and annual..” line 346 , the second section in the conclusion.  I don’t see that as a response to the hypotheses and I do not see other response either.

Minor  issues

Line 40 maybe spell out as “Monin–Obukhov similarity theory”

Line 49 MLD should be spelled out the first time, this also apply to line 95, except that here the terms are spelled out, but no acronyms – that  are used later on.

Line 72 twas →was

Line 105 AMOC spell out

Line 154 the sigma letter usually means “sum”

Line 164 concentration → cover?

Line 191 transformation (units m3 sec-1) → transport

Line 201 The highest values are

Line 202 western → easterly ?

Line 239 allocated → identified

Line 254 TS spell out Temperature Salinity

Figure 4 Explain vertical lines in Figure text

Line 318 fluctuations → variability

Line 349 Here you introduce the NAO acronym

 

Author Response

For research article

Surface water mass transformation in North Atlantic based on NCEP CSFR reanalysis

 

 

Response to Reviewer 2 Comments
1. Summary
The author thanks the reviewer for his time to re-review the article and for his valuable comments, which helped to improve the quality of the article. Below is a step-by-step response to the comments suggested by the reviewers. In the attached revised version of the manuscript, all corrections related to this review are highlighted in yellow.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Can be improved
Are all the cited references relevant to the research?	Yes
Is the research design appropriate?	Can be improved
Are the methods adequately described?	Must be improved
Are the results clearly presented?	Must be improved
Are the conclusions supported by the results?	Must be improved
3. Point-by-point response to Comments and Suggestions for Authors
Comments 1: Major issues To me it was rather difficult to read the text and understand their arguments. It may be my fault.  The study presents two hypotheses, and I would have anticipated that there were clear responses to these hypotheses in the conclusion section, but I could not see that.  For example: “Water masses of different densities respond differently to seasonal and annual..” line 346 , the second section in the conclusion.  I don’t see that as a response to the hypotheses and I do not see other response either. Response 1: Thank you very much for your comment, perhaps due to the ambiguity of the text, the reviewer decided that the hypotheses concern our study. The hypotheses are given as an example of early studies on the interaction of the ocean and the atmosphere and refer to the literature review on the topic of these studies in the introduction. In order to avoid a false understanding of the main purpose of the study, the corresponding text has been added to the introduction. In the conclusion, the main novelty of the work has been added. Comments 2: Line 40 maybe spell out as “Monin–Obukhov similarity theory” Response 2: Thank you very much for your comment, corrected. Comments 3: Line 49 MLD should be spelled out the first time, this also apply to line 95, except that here the terms are spelled out, but no acronyms – that  are used later on. Response 3: Thank you very much for your comment, corrected, all acronyms are spelled out. Comments 4: Line 72 twas →was Response 4: Thank you very much for your comment, corrected. Comments 5: Line 105 AMOC spell out Response 5: Thank you very much for your comment, corrected. Comments 6: Line 154 the sigma letter usually means “sum” Response 6: Thank you very much for your comment, the formula has been revised in accordance with the comments of previous reviewers, sigma has been replaced by xy, which means the area of the water area. Comments 7: Line 164 concentration → cover? Response 7: Thank you very much for your comment, corrected. Comments 8: Line 191 transformation (units m3 sec-1) → transport Response 8: Thank you very much for your comment, but in the community it is accepted to call this value precisely the transformation rate. In this study, if we talk about the calculation and the value of transformation, we mean the transformation rate, as in Speer Tziperman 1992. Explanation about this added to ln 145-148 Comments 9: Line 201 The highest values are Response 9: Thank you very much for your comment, corrected. Comments 10: Line 202 western → easterly ? Response 10: Thank you very much for your comment, the text states correctly. Western boundary currents are usually called warm currents in the western part of the ocean, such as the Gulf Stream, Kuroshio, Brazilian, Mozambique, Madagascar, East Australian Comments 11: Line 239 allocated → identified Response 12: Thank you very much for your comment, corrected. Comments 12: Line 254 TS spell out Temperature Salinity Response 12: Thank you very much for your comment, corrected. Comments 13: Figure 4 Explain vertical lines in Figure text Response 13: Thank you very much for your comment, explanation added to Figure text. Comments 14: Line 318 fluctuations → variability Response 14: Thank you very much for your comment, corrected. Comments 15: Line 349 Here you introduce the NAO acronym Response 14: Thank you very much for your comment, corrected, NAO spells out in introduction

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study explores the variability of density flux and water mass transformation in the North Atlantic, looking at both seasonal and climatic timescales.  The paper is well-written, thorough, and clearly demonstrates a solid grasp of the complex ocean-atmosphere interactions at play. However, further discussion on the model's limitations is needed.

Minor Comments:
Figure 8: The black solid line is not mentioned in the caption or legend. Besides, only (b) exists, no (a). Check this.

 

This paper explores the variability of density flux and water mass transformation in the North Atlantic, looking at both seasonal and climatic timescales. The authors investigate how atmospheric processes influence the formation of subpolar, subtropical, and Labrador Sea waters, using NCEP CFSv2 reanalysis data. Their findings offer insights into how the atmosphere impacts vertical circulation, which could be valuable for future modeling efforts. This work stands out for its effective use of high-resolution NCEP CFSv2 reanalysis data, which bolsters the reliability of its analysis. It provides a clear and thorough examination of both seasonal and interannual water mass transformations, particularly for critical North Atlantic water masses, and rigorously links these changes to atmospheric variability like the NAO. The methodology is robust, with transparent calculations of density flux and transformation rates, including both thermal and salinity components, making the study's analytical approach highly credible.

The methodology is generally thorough, but here are some specific improvements and further controls to consider:

1. For the density flux calculations and transformation rate estimations, a sensitivity analysis of key parameters is lacked.
2. Further discussion on the model's limitations is needed. A more explicit discussion of the limitations of the NCEP CFSv2 reanalysis model can be discussed. Acknowledging potential uncertainties, such as data assimilation errors or model bias, would add critical depth to the analysis.
3. Figure descriptions need to be imporved. Provide more detailed and explanatory captions for all figures, especially those depicting complex transformations of subtropical and subpolar modal waters. Besides, in the Figure 8, The black solid line is not mentioned in the caption or legend. Only (b) exists, no (a). Overall, this paper makes a notable contribution to oceanography, especially in improving our understanding of surface water mass transformations in the North Atlantic. By effectively using NCEP CFSv2 reanalysis data, the authors provide valuable insights into the seasonal and interannual dynamics of water mass formation, driven by atmospheric processes. The study is well-written, thorough, and clearly demonstrates a solid grasp of the complex ocean-atmosphere interactions at play. However, further discussion on the model’s limitations and the inclusion of additional influencing factors would make the analysis even more comprehensive and impactful.

Author Response

For research article

 

Surface water mass transformation in North Atlantic based on NCEP CSFR reanalysis

 

Response to Reviewer 3 Comments
1. Summary
The author thanks the reviewer for his time to re-review the article and for his valuable comments, which helped to improve the quality of the article. Below is a step-by-step response to the comments suggested by the reviewers. In the attached revised version of the manuscript, all corrections related to this review are highlighted in yellow.
2. Questions for General Evaluation	Reviewer’s Evaluation	Response and Revisions
Does the introduction provide sufficient background and include all relevant references?	Yes
Are all the cited references relevant to the research?	Yes
Is the research design appropriate?	Yes
Are the methods adequately described?	Can be improved
Are the results clearly presented?	Yes
Are the conclusions supported by the results?	Yes
3. Point-by-point response to Comments and Suggestions for Authors
Comments 1:

Minor Comments:
Figure 8: The black solid line is not mentioned in the caption or legend. Besides, only (b) exists, no (a). Check this.

Response 1:

Thank you very much for your comment, this picture was replaced to avoid copyright issues.

Comments 2:

For the density flux calculations and transformation rate estimations, a sensitivity analysis of key parameters is lacked.

Response 2:

Thank you very much for your comment, discussion on sensitivity tests of density flux magnitude to variability of its components added to methods chapter. Authors performed such analysis before, reference added.

Comments 3:

Further discussion on the model's limitations is needed. A more explicit discussion of the limitations of the NCEP CFSv2 reanalysis model can be discussed. Acknowledging potential uncertainties, such as data assimilation errors or model bias, would add critical depth to the analysis.

Response 3:

Thanks for the note, indeed using one data source requires adding text about its limitations. The paragraph about the limitations of NCEP CFSR was added to the conclusion.

Comments 4:

Figure descriptions need to be imporved. Provide more detailed and explanatory captions for all figures, especially those depicting complex transformations of subtropical and subpolar modal waters. Besides, in the Figure 8, The black solid line is not mentioned in the caption or legend. Only (b) exists, no (a).

Response 4:

A discussion of the reasons for the dynamics described in the figures has been added to the figures descriptions.

Author Response File: Author Response.pdf