Stratification and Mixed Layer Depth around Iceland, characterization and inter-annual variability

Ruiz-Angulo, Angel; Portela, Esther; de Marez, Charly; Macrander, Andreas; Ólafsdóttir, Sólveig Rósa; Meunier, Thomas; Jónsson, Steingrímur; Pérez-Hernández, M. Dolores

doi:10.5194/egusphere-2025-2102

Preprints

https://doi.org/10.5194/egusphere-2025-2102

Preprints

21 May 2025

| 21 May 2025

Stratification and Mixed Layer Depth around Iceland, characterization and inter-annual variability

Angel Ruiz-Angulo, Esther Portela, Charly de Marez, Andreas Macrander, Sólveig Rósa Ólafsdóttir, Thomas Meunier, Steingrímur Jónsson, and M. Dolores Pérez-Hernández

Abstract. The ocean around Iceland witnesses some of the most important transformations of water masses that drive the global ocean circulation. Here, we analyze 29 years (1990–2019) of quarterly hydrographic sections data collected around Iceland. The hydrographic properties around Iceland show important spatial variability. Based on temperature, salinity, and stratification structure, we classified the Icelandic waters in three distinct regions: the South, the North and Northeast regions. The warm and salty Atlantic Waters that dominate the south show the deepest winter mixed layers (~500 m) while the North and Northeast show shallower depths (~100 m). Based on the decomposition of total stratification into temperature and salinity contributions, we find that, in the South, the subsurface stratification is mainly dominated by temperature, in the Northwest salinity dominates, while in the North, the seasonality of the North Icelandic Irminger Current and East Icelandic Current alternate the temperature and salinity contribution to stratification. The interannual variability of the mixed layer and of its thermohaline properties is also large around Iceland. Mixed layer waters were generally colder in the 90’s, then warmed until approximately 2015, and became colder again from 2015 to 2018. In the Northeast, a clear multidecadal mixed layer warming trend clearly emerges from the interannual variability as the Atlantic Water progresses northeastward, which is responsible for transforming locally, the upper stratification from salinity dominated into temperature dominated, allowing for the formation of deeper mixed layers. This is associated with the “Atlantification” of the Arctic. Elsewhere, we observe density-compensated changes in mixed layer temperature and salinity, without clear trends. This study provides an unprecedented and detailed description of the seasonal to multi-decadal variability of the mixed layer depth and stratification around Iceland, and their link with the changing North Atlantic under global warming.

Received: 05 May 2025 – Discussion started: 21 May 2025

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Journal article(s) based on this preprint

29 May 2026

Stratification and mixed layer depth around Iceland: Characterization and inter-annual variability

Angel Ruiz-Angulo, Esther Portela, Charly de Marez, Andreas Macrander, Sólveig Rósa Ólafsdóttir, Thomas Meunier, Steingrímur Jónsson, and M. Dolores Pérez-Hernández

Ocean Sci., 22, 1727–1743, https://doi.org/10.5194/os-22-1727-2026,https://doi.org/10.5194/os-22-1727-2026, 2026

Short summary

Angel Ruiz-Angulo, Esther Portela, Charly de Marez, Andreas Macrander, Sólveig Rósa Ólafsdóttir, Thomas Meunier, Steingrímur Jónsson, and M. Dolores Pérez-Hernández

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2102', Anonymous Referee #1, 20 Jul 2025
This manuscript takes advantage of a long-term hydrographic data set from a series of stations encircling Iceland to assess major characteristics of upper-ocean variability and to evaluate long-term trends in the region. The study shows that salinity governs stratification at stations northwest of Iceland, while temperature governs stratification to the south. To the north of Iceland, alternating impacts from the North Icelandic Irminger Current and the East Icelandic Current lead to a mixed response.

The data set used for the study offers a rich supply of information, and the authors have chosen an interesting question to pursue. Parts of the analysis would benefit from more detail. I feel that the manuscript will likely be suitable for publication after careful revision.
The overall analysis of the manuscript addresses several distinct issues that are not always tightly linked together. One focal point is temperature vs salinity controls on mixed-layer depth or stratification, including consideration of the seasonal cycle. A second thread considers long-term trends in mixed-layer temperature and salinity along with the quantities with which they correlate. A third aspect assesses the multi-year linear trends in mixed-layer temperature in summer and winter. Analyses explore the mixed-layer evolution on the seasonal scale using a one-dimensional mixed-layer model and look at historic temperature vs salinity domination on a regional and seasonal scale. These are interesting and related analyses, but they are not fully linked together to provide clear and targeted interpretation of the results. For example, the long-term temperature trends in Figure 6 are interesting but not well connected with the rest of the analysis. In a rewriting, the manuscript should be more tightly focused to identify clear and linked results, well grounded in robust statistics.

One gap in the manuscript is a lack of statistical detail. This gap is particularly noticeable in Figure 5, in which the authors show time series of mixed-layer temperature and salinity anomalies, mixed-layer depth, and the North Atlantic Oscillation. The authors discuss correlations between these records but do not report correlation coefficients or statistical significance. To show that the patterns that the authors observe in their plots are robust, they should report quantifiable statistical metrics.

Similarly in Figure 6, the authors fit linear trends to plotted mixed-layer temperature records, but they do not specify the slope of the linear trends. In addition, the caption to Figure 6 does not indicate what the p values represent. These pieces of information should be added.

Given the discussion in Figure 5 and given the character of the records in Figure 5, I was surprised by the decision to fit trends in Figure 6. The discussion in Figure 5 emphasizes the specific relations between plotted quantities rather than long-term trends, so I was expecting Figure 6 to report correlations. It would be interesting to see the correlations between NAO, mixed-layer depth, and MLT mapped out for the full set of stations.

Since the analysis of Figure 6 focuses on trends, and the overall goals of the manuscript are directed toward alpha and beta oceanic regimes, the authors could/should expand the manuscript discussion to indicate how the trends (and regression coefficients, perhaps) inform their understanding of alpha vs beta ocean regions.

Line 133. “great score”. In this usage, “great” sounds like a word that expresses an opinion. This point needs to be quantified, and more neutral wording should be used to express the skill of the density threshold method.

Lines 206-207. “The southern stations ST5 and SB5, have a minimal contribution from salinity, which may be associated with the numerous river discharges and the proximity to the continental shelf.” This is an interesting point. Does the fresh water budget support this hypothesis? It would be useful to quantify the volume of freshwater discharge and its expected impact on salinity. Precipitation or oceanic circulation would be other factors that could influence salinity.

Figure 3. The station labels are much smaller than the other figure labels and are too small to read clearly. The figure should be redrafted with larger labels.

Line 214. “hydrographic onset”. The meaning of this is unclear. Does it refer to the top of the hydrographic profile or the seasonal onset of a change in the hydrographic profile?

Figure 4. The figure shows open circles for deep mixed layers. The justification for this is not clear, since deep mixed layers can be as dynamically relevant as shallow mixed layers. Further explanation is needed.

Line 270. Correlations with the NAO should be quantified. As noted above, the manuscript should report correlation coefficients and evaluate statistical significance.

Figure 5. The repetition of panels g, h, and i seems unnecessary. Could the NAO time series be superimposed on the panels above (along with the addition of concrete correlation statistics)?

Line 286. “aliasing”. The term “aliasing has a specific meaning in time series analysis, and the usage here seems inconsistent with that usage. This could be described as “superimposed on”.

Lines 295 and following. Choice of one-dimensional model. The Price-Weller-Pinkel model has been used extensively over the last four decades for upper ocean analyses. It is not the only possible model, and other recent studies have made use of GOTM or a stripped-down form of KPP. Thus, it’s important to justify the choice of the PWP model.

Lines 295 and following. The focus of the one-dimensional mixed layer analysis on winter only also needs clarification and should be more carefully described to explain that the analysis is really looking a the winter-to-summer transition. The PWP model has previously been used over a broad range of latitudes and for all seasons. Thus, a priori, there's not an obvious reason to exclude summer.

Figure 7 calculations. How is the mixed-layer model initialized? Does it start with stratification typical of February? It's surprising that the mixed layer in the model appears to deepen at the outset. I would have expected it to be initialized with a profile that matches the climatological observations. This should be explained.

Figure 7 color scale. Please check colors. Green/red contrasts can be challenging for readers with limited color vision.

Line 389. “not correlated with the NAO”. The lack of correlation should be quantified in the main body of the text, particularly if it is referenced in the conclusions. Better language would specify that the correlation with the NAO is not statistically different from zero.

Line 390. “at times reaching 2 degrees C”. This number should be reported as a rate, in units of change in temperature per unit time. Please specify the time interval over which this estimate is computed.

Minor grammatical points
Line 17: “sections” -> “section”

Lines 19 and 20: It would be good to make a decision about consistent capitalization of regions (“South” or “south”, etc.)

Line 24. “alternate the temperature and salinity contribution to stratification”. Wording is unclear. Maybe the authors could write, “while in the North, the North Icelandic Irminger Current and East Icelandic Current alternate seasonally, shifting the region between temperature-dominated and salinity-dominated stratification.”

Line 28. Remove comma after “locally”.

Line 32. “their link” Wording is confusing. The word their implies a plural reference point, but the grammatical structure of the sentence does not clearly identify what this reference should be. Maybe “This study provides an unprecedented and detailed description of the seasonal to multi-decadal variability of mixed-layer depth and stratification around Iceland, showing links between this regional variability and the changing North Atlantic….”

Lines 42-46. Capitalization and punctuation are inconsistent for numbered points. All three items could be capitalized as separate sentences, or all three could be started with lower case letters, with semi-colons to separate the items. But mixed punctuation and capitalization is confusing.

Line 45. Remove “with”

Line 55. “drives” -> “drive”

Line 57. “heat fluxes are the main drivers” or “heat flux is the main driver”?

Line 57. “on the center” -> “in the center”

Line 58. “Nordic Seas have been previously described as a ‘melting pot’”. Inconsistent plurals. The Nordic Seas region is a melting pot? Or Nordic Seas are melting pots?

Line 59. “Nordic Seas are also a large repository”. Same thing. “The Nordic Seas region is a large repository”?

Line 65. “of the Arctic Amplification” -> “of Arctic Amplification”

Line 65. “the decrease” -> “a decrease”

Line 74. “forcings” -> “forcing”

Line 74. “to control” -> “for controlling”

Line 74. Add comma after “mixing”

Line 76. “of the strong” -> “of strong”

Line 99. “hinders” -> “hinder”

Line 101. Add comma after “MLD”

Line 114. “IB” -> “IH”

Lines 128-137. It’s standard practice to subscript theta in sigma_theta.

Line 137. The line following equation (10) continues the sentence containing equation 1 and should not be capitalized or start a new paragraph. Equation 1 should be punctuated with a comma rather than a period

Line 137. “decomposed on” -> “decomposed to show”

Line 138. “contribution of the salinity” -> “contribution of salinity”

Line 146. “Where” is a continuation of the sentence containing equations (3) and (4). No capitalization and no indenting.

Line 148. Add comma after “salinity” since this is a compound sentence.

Line 151. “have” -> “has”. (The sentence structure implies that only one component needs to have an impact, so the verb should assume a singular subject.)

Line 164. No indent. Please check all equations for this issue.

Line 187. “strike out” has a couple of distinct usages, but this reads as if it is using the baseball metaphor, which means to fail completely. Maybe use “are strikingly saltier”.

Line 205. Missing words. Maybe "despite the fact that stratification ...."

Line 251. “correlates” -> “correlate”

Line 258. “in the order” -> “on the order” OR “are the same order of magnitude as”

Line 261. “neither …. nor” is not used correctly here. Change to “do not seem correlated with the MLT/MLS or with the ….:

Line 264. “it is”. The text is not clear about the meaning of “it”. Clarify whether “it” is station LB8 or the winter MLD.

Line 266. Start a new sentence: “front, fresher” -> “front. Fresher”. Add a verb: “MLs associated” -> “MLs are associated”

Line 270. “of NAO” -> “of the NAO”

Line 271. “MLS” -> “MLS,”

Lines 325-326. “distribution (Fig. 8) … southern (northern) … alpha- (beta-) …” Avoid using opposites in parentheses since opposites are also used for clarifications (e.g. Fig. 8 is likely not a match to “northern”). In general, this opposite-in-parentheses structure is difficult for readers to parse. If the point is worth making, then it can be spread into two sentences.

Line 325 and discussion of Fig. 8. The method underlying the results in Fig. 8 is shown in the Introduction. Here the text could reference Equation (5) to point readers to the relevant aspect of the computational approach.

Line 338. “200” -> “2000”?

Line 364. “northwestward, hence”. This is a comma splice. Start a new sentence instead.

Line 366. Don’t put opposites in parentheses. Write a clear two-part sentence instead.

Line 370. “variability, they”. Comma splice. Start a new sentence with “they”

Line 374. Add a comma after “ML’s”
Citation: https://doi.org/10.5194/egusphere-2025-2102-RC1
- AC1: 'Reply on RC1', Angel Ruiz-Angulo, 30 Oct 2025
  
  We sincerely thank the reviewer for all the detailed and insightful comments. The original version of the manuscript was missing some key liaisons to complete the story, and the reviewer’s suggestions have greatly improved the clarity and coherence of the work.
  We have attached the main responses in the attached file. All the minor grammatical changes have been addressed in the new version of the manuscript. Repl
  
  Citation: https://doi.org/10.5194/egusphere-2025-2102-AC1
RC2:
'Comment on egusphere-2025-2102', Anonymous Referee #2, 22 Aug 2025

Please see the attached document CommentsToAuthors.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2102-RC2
- AC2: 'Reply on RC2', Angel Ruiz-Angulo, 31 Oct 2025
  
  We thank the reviewer for the time and effort dedicated to evaluating our manuscript. We have carefully addressed all the comments and made the requested corrections. Our detailed responses to each point are provided in the attached document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2102-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-2102', Anonymous Referee #1, 20 Jul 2025
This manuscript takes advantage of a long-term hydrographic data set from a series of stations encircling Iceland to assess major characteristics of upper-ocean variability and to evaluate long-term trends in the region. The study shows that salinity governs stratification at stations northwest of Iceland, while temperature governs stratification to the south. To the north of Iceland, alternating impacts from the North Icelandic Irminger Current and the East Icelandic Current lead to a mixed response.

The data set used for the study offers a rich supply of information, and the authors have chosen an interesting question to pursue. Parts of the analysis would benefit from more detail. I feel that the manuscript will likely be suitable for publication after careful revision.
The overall analysis of the manuscript addresses several distinct issues that are not always tightly linked together. One focal point is temperature vs salinity controls on mixed-layer depth or stratification, including consideration of the seasonal cycle. A second thread considers long-term trends in mixed-layer temperature and salinity along with the quantities with which they correlate. A third aspect assesses the multi-year linear trends in mixed-layer temperature in summer and winter. Analyses explore the mixed-layer evolution on the seasonal scale using a one-dimensional mixed-layer model and look at historic temperature vs salinity domination on a regional and seasonal scale. These are interesting and related analyses, but they are not fully linked together to provide clear and targeted interpretation of the results. For example, the long-term temperature trends in Figure 6 are interesting but not well connected with the rest of the analysis. In a rewriting, the manuscript should be more tightly focused to identify clear and linked results, well grounded in robust statistics.

One gap in the manuscript is a lack of statistical detail. This gap is particularly noticeable in Figure 5, in which the authors show time series of mixed-layer temperature and salinity anomalies, mixed-layer depth, and the North Atlantic Oscillation. The authors discuss correlations between these records but do not report correlation coefficients or statistical significance. To show that the patterns that the authors observe in their plots are robust, they should report quantifiable statistical metrics.

Similarly in Figure 6, the authors fit linear trends to plotted mixed-layer temperature records, but they do not specify the slope of the linear trends. In addition, the caption to Figure 6 does not indicate what the p values represent. These pieces of information should be added.

Given the discussion in Figure 5 and given the character of the records in Figure 5, I was surprised by the decision to fit trends in Figure 6. The discussion in Figure 5 emphasizes the specific relations between plotted quantities rather than long-term trends, so I was expecting Figure 6 to report correlations. It would be interesting to see the correlations between NAO, mixed-layer depth, and MLT mapped out for the full set of stations.

Since the analysis of Figure 6 focuses on trends, and the overall goals of the manuscript are directed toward alpha and beta oceanic regimes, the authors could/should expand the manuscript discussion to indicate how the trends (and regression coefficients, perhaps) inform their understanding of alpha vs beta ocean regions.

Line 133. “great score”. In this usage, “great” sounds like a word that expresses an opinion. This point needs to be quantified, and more neutral wording should be used to express the skill of the density threshold method.

Lines 206-207. “The southern stations ST5 and SB5, have a minimal contribution from salinity, which may be associated with the numerous river discharges and the proximity to the continental shelf.” This is an interesting point. Does the fresh water budget support this hypothesis? It would be useful to quantify the volume of freshwater discharge and its expected impact on salinity. Precipitation or oceanic circulation would be other factors that could influence salinity.

Figure 3. The station labels are much smaller than the other figure labels and are too small to read clearly. The figure should be redrafted with larger labels.

Line 214. “hydrographic onset”. The meaning of this is unclear. Does it refer to the top of the hydrographic profile or the seasonal onset of a change in the hydrographic profile?

Figure 4. The figure shows open circles for deep mixed layers. The justification for this is not clear, since deep mixed layers can be as dynamically relevant as shallow mixed layers. Further explanation is needed.

Line 270. Correlations with the NAO should be quantified. As noted above, the manuscript should report correlation coefficients and evaluate statistical significance.

Figure 5. The repetition of panels g, h, and i seems unnecessary. Could the NAO time series be superimposed on the panels above (along with the addition of concrete correlation statistics)?

Line 286. “aliasing”. The term “aliasing has a specific meaning in time series analysis, and the usage here seems inconsistent with that usage. This could be described as “superimposed on”.

Lines 295 and following. Choice of one-dimensional model. The Price-Weller-Pinkel model has been used extensively over the last four decades for upper ocean analyses. It is not the only possible model, and other recent studies have made use of GOTM or a stripped-down form of KPP. Thus, it’s important to justify the choice of the PWP model.

Lines 295 and following. The focus of the one-dimensional mixed layer analysis on winter only also needs clarification and should be more carefully described to explain that the analysis is really looking a the winter-to-summer transition. The PWP model has previously been used over a broad range of latitudes and for all seasons. Thus, a priori, there's not an obvious reason to exclude summer.

Figure 7 calculations. How is the mixed-layer model initialized? Does it start with stratification typical of February? It's surprising that the mixed layer in the model appears to deepen at the outset. I would have expected it to be initialized with a profile that matches the climatological observations. This should be explained.

Figure 7 color scale. Please check colors. Green/red contrasts can be challenging for readers with limited color vision.

Line 389. “not correlated with the NAO”. The lack of correlation should be quantified in the main body of the text, particularly if it is referenced in the conclusions. Better language would specify that the correlation with the NAO is not statistically different from zero.

Line 390. “at times reaching 2 degrees C”. This number should be reported as a rate, in units of change in temperature per unit time. Please specify the time interval over which this estimate is computed.

Minor grammatical points
Line 17: “sections” -> “section”

Lines 19 and 20: It would be good to make a decision about consistent capitalization of regions (“South” or “south”, etc.)

Line 24. “alternate the temperature and salinity contribution to stratification”. Wording is unclear. Maybe the authors could write, “while in the North, the North Icelandic Irminger Current and East Icelandic Current alternate seasonally, shifting the region between temperature-dominated and salinity-dominated stratification.”

Line 28. Remove comma after “locally”.

Line 32. “their link” Wording is confusing. The word their implies a plural reference point, but the grammatical structure of the sentence does not clearly identify what this reference should be. Maybe “This study provides an unprecedented and detailed description of the seasonal to multi-decadal variability of mixed-layer depth and stratification around Iceland, showing links between this regional variability and the changing North Atlantic….”

Lines 42-46. Capitalization and punctuation are inconsistent for numbered points. All three items could be capitalized as separate sentences, or all three could be started with lower case letters, with semi-colons to separate the items. But mixed punctuation and capitalization is confusing.

Line 45. Remove “with”

Line 55. “drives” -> “drive”

Line 57. “heat fluxes are the main drivers” or “heat flux is the main driver”?

Line 57. “on the center” -> “in the center”

Line 58. “Nordic Seas have been previously described as a ‘melting pot’”. Inconsistent plurals. The Nordic Seas region is a melting pot? Or Nordic Seas are melting pots?

Line 59. “Nordic Seas are also a large repository”. Same thing. “The Nordic Seas region is a large repository”?

Line 65. “of the Arctic Amplification” -> “of Arctic Amplification”

Line 65. “the decrease” -> “a decrease”

Line 74. “forcings” -> “forcing”

Line 74. “to control” -> “for controlling”

Line 74. Add comma after “mixing”

Line 76. “of the strong” -> “of strong”

Line 99. “hinders” -> “hinder”

Line 101. Add comma after “MLD”

Line 114. “IB” -> “IH”

Lines 128-137. It’s standard practice to subscript theta in sigma_theta.

Line 137. The line following equation (10) continues the sentence containing equation 1 and should not be capitalized or start a new paragraph. Equation 1 should be punctuated with a comma rather than a period

Line 137. “decomposed on” -> “decomposed to show”

Line 138. “contribution of the salinity” -> “contribution of salinity”

Line 146. “Where” is a continuation of the sentence containing equations (3) and (4). No capitalization and no indenting.

Line 148. Add comma after “salinity” since this is a compound sentence.

Line 151. “have” -> “has”. (The sentence structure implies that only one component needs to have an impact, so the verb should assume a singular subject.)

Line 164. No indent. Please check all equations for this issue.

Line 187. “strike out” has a couple of distinct usages, but this reads as if it is using the baseball metaphor, which means to fail completely. Maybe use “are strikingly saltier”.

Line 205. Missing words. Maybe "despite the fact that stratification ...."

Line 251. “correlates” -> “correlate”

Line 258. “in the order” -> “on the order” OR “are the same order of magnitude as”

Line 261. “neither …. nor” is not used correctly here. Change to “do not seem correlated with the MLT/MLS or with the ….:

Line 264. “it is”. The text is not clear about the meaning of “it”. Clarify whether “it” is station LB8 or the winter MLD.

Line 266. Start a new sentence: “front, fresher” -> “front. Fresher”. Add a verb: “MLs associated” -> “MLs are associated”

Line 270. “of NAO” -> “of the NAO”

Line 271. “MLS” -> “MLS,”

Lines 325-326. “distribution (Fig. 8) … southern (northern) … alpha- (beta-) …” Avoid using opposites in parentheses since opposites are also used for clarifications (e.g. Fig. 8 is likely not a match to “northern”). In general, this opposite-in-parentheses structure is difficult for readers to parse. If the point is worth making, then it can be spread into two sentences.

Line 325 and discussion of Fig. 8. The method underlying the results in Fig. 8 is shown in the Introduction. Here the text could reference Equation (5) to point readers to the relevant aspect of the computational approach.

Line 338. “200” -> “2000”?

Line 364. “northwestward, hence”. This is a comma splice. Start a new sentence instead.

Line 366. Don’t put opposites in parentheses. Write a clear two-part sentence instead.

Line 370. “variability, they”. Comma splice. Start a new sentence with “they”

Line 374. Add a comma after “ML’s”
Citation: https://doi.org/10.5194/egusphere-2025-2102-RC1
- AC1: 'Reply on RC1', Angel Ruiz-Angulo, 30 Oct 2025
  
  We sincerely thank the reviewer for all the detailed and insightful comments. The original version of the manuscript was missing some key liaisons to complete the story, and the reviewer’s suggestions have greatly improved the clarity and coherence of the work.
  We have attached the main responses in the attached file. All the minor grammatical changes have been addressed in the new version of the manuscript. Repl
  
  Citation: https://doi.org/10.5194/egusphere-2025-2102-AC1
RC2:
'Comment on egusphere-2025-2102', Anonymous Referee #2, 22 Aug 2025

Please see the attached document CommentsToAuthors.pdf

Citation: https://doi.org/10.5194/egusphere-2025-2102-RC2
- AC2: 'Reply on RC2', Angel Ruiz-Angulo, 31 Oct 2025
  
  We thank the reviewer for the time and effort dedicated to evaluating our manuscript. We have carefully addressed all the comments and made the requested corrections. Our detailed responses to each point are provided in the attached document.
  
  Citation: https://doi.org/10.5194/egusphere-2025-2102-AC2

Peer review completion

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

AR by Angel Ruiz-Angulo on behalf of the Authors (21 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Dec 2025) by Agnieszka Beszczynska-Möller

RR by Anonymous Referee #1 (30 Dec 2025)

Suggestions for revision or reasons for rejection

In this revision of the manuscript submitted earlier in 2025, the authors have made a number of revisions, notably improving the statistics to demonstrate the robustness of their trend analysis. These revisions are helpful and clarify the impact of the results. I continue to think that the manuscript contains a valuable set of analyses that will be of interest to the oceanographic research community. In my original review, I also specified, “In a rewriting, the manuscript should be more tightly focused to identify clear and linked results, ….” In the revision, the authors have not (yet) made significant revisions to help readers connect the results. Thus, I have further suggestions aimed at making this manuscript more impactful for readers.

1. Introduction. The manuscript would benefit from a rewriting of the introduction to clearly specify the objectives of this study. In the present version and in the original submission, the major articulation of the manuscript objectives seems to appear in the final sentence of Section 1: “In this study, by using a long time series of hydrographic observations around Iceland spanning 29 years, we aim at unraveling the different scales of variability of the water-mass properties, MLD, and stratification around Iceland.” The manuscript in fact has a broader scope than this, addressing trends, correlation with the NAO, and vertical and horizontal patterns of variability. The introduction should be reframed so that it explains to readers what questions will be addressed and why these questions make sense as a package contained within a single manuscript. As a general rule of thumb, the questions answered in the conclusions section of the manuscript should be posed in the introduction.

2. Cardinal directions. Oddly, one of the challenges for me as a reader comes in capitalization of regions. In general, in English, directions should be capitalized if they refer to specific places (e.g. “The North” as a specific named region) but not as a general direction (e.g. “hydrographic stations north of Iceland”). In the manuscript, regions are sometimes capitalized and sometimes not. The abstract specifies three important regions: the South, the North, and the Northeast. This makes sense, so in reading the manuscript, I sought a clear definition of the three regions. But the manuscript discussion turns out to be more complicated. At line 23, the abstract mentions the Northwest. I wasn’t sure if this was a typo or a new specific location. The authors have options for this. They could remove capitalization of regions to allow a little flexibility, or they could retain capitalization and clearly define the geographic bounds of each region and identify the stations contained within each region. At line 202, capitalization of “Northwestern” stations implies a specific region, but it isn’t defined earlier. The figures discuss different sets of stations in confusing ways that do not naturally help readers organize information to create a clear mental picture of the patterns of variability. I would recommend creating a table defining regions and matching stations to named regions.

3. Part of the clarity gap involves changes in notation. In Figure 5 and Table 1, stations are identified without numbers (e.g. FX), while elsewhere they appear with identifying numbers (e.g. FX9). This requires readers to do a lot of cross-checking to see if the comments refer to the same stations.

4. The first reference to the NAO is in line 258. The acronym needs to be defined, and more importantly, if the NAO is an important metric for variability, then the hypothesis that the NAO matters should be laid out early in the manuscript. Typically this should appear in the introduction.

5. Minor comments:
a. Title: Comma really should be a semi-colon: Stratification and mixed-layer depth around Iceland: Characterization and interannual variability
b. Line 80: “high latitude” -> “high-latitude”
c. Lines 81 and 85: Correct accent in Sallée.
d. Line 83: “underdgoing” -> “undergoing”
e. Line 86: “on Climate Change, IPCC”. It’s not clear if this references a specific IPCC publication, in which case it’s missing a year, or if it is a general reference to climate change studies, in which case more detail would help.
f. Line 93: “and associated” -> “and the associated”; “delays” -> “delay”; “South of Iceland” -> “south of Iceland”
g. Line 99: “of the oceanic” -> “oceanic”
h. Line 114: “these stations” -> “stations”
i. Lines 130 and 132: “Montegut” -> “Montégut”
j. Line 133: “shows to” -> “proves to” OR “appears to”; OR “shows itself to”
k. Line 138: no indent after equation
l. Lines 128 and 138: I don’t understand the notation used for potential density. After studying this for a while, I’m guessing that the intent was to use sigma_theta, but the notation didn’t work properly in the main text, and subscripting didn’t work in equation 1.
m. Lines 138, 151: N^2 should be in math font to match its representation in equations (1) and (2).
n. Line 156: “furthermore” -> “further” (The term “furthermore” is too argumentative.)
o. Lines 163, 166: K^2 should be in math font
p. Line 175: “dispersions” -> “variance”?
q. Lines 184-185: “due to seasonal warming of the upper layers”: Phrase seems misplaced. I would move it to the start of the sentence: “Overall, due to seasonal warming of the upper layers, the summer profiles …..”
r. Line 209: “exhibits” -> “exhibit”
s. Line 252: “West” -> “west”
t. Line 255: “moderate” -> “moderately”
u. Line 258: “certain correlation (R)”. The word “certain” is unclear. Can it be removed? Is the intent to say that the correlation is statistically significant?
v. Line 261: “explanation,” -> “explanation;”
w. Line 268: “North” -> north”
x. Line 297: “South” -> “south”
y. Line 298: “28-years” -> “28-year”
z. Lines 344-345: It’s best to avoid the “southern (northern) …. temperature (salinity)” construction because readers have a difficult time parsing information. Instead, break the sentence into two parts. For example, “The spiciness distributions shown in Figure 8 reveal that temperature dominates on the southern side of Iceland, marked by an alpha-ocean regime, while salinity dominates on the northern side, marked by a beta-ocean.”
aa. Line 370: “South” -> “south”

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RR by Anonymous Referee #2 (05 Jan 2026)

ED: Reconsider after major revisions (11 Jan 2026) by Agnieszka Beszczynska-Möller

AR by Angel Ruiz-Angulo on behalf of the Authors (11 Mar 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (16 Mar 2026) by Agnieszka Beszczynska-Möller

RR by Anonymous Referee #2 (18 Mar 2026)

RR by Anonymous Referee #1 (29 Mar 2026)

Suggestions for revision or reasons for rejection

In this revision, the authors have addressed most of the issues raised by reviewers in an appropriate way. Results of this study provide an interesting delineation between temperature-dominated and salinity-dominated parts of the North Atlantic. There remain a number of minor issues. New material incorporating the PWP model raises some questions that should be addressed carefully in the text.

1. line 273. "thus, we prefer to not to consider this". The fact that the two-year lag is statistically significant but not explained does not really justify a preference not to discuss the two-year lag. It's possible that the number of degrees of freedom is reduced for the 2-year lag, and its not actually significant. Even in the event that it is statistically significant, an alternative approach would be to explain, "However, we have not identified a physical explanation for a 2-year lag and will not explore this correlation further in this paper." That leaves open the opportunity for a future study to delve further into the matter.

2. Table 1. Do the numbers in Table 1 represent zero lag? I'm confused by the text at line 271-272, which talks about lagged correlations, while the table doesn't specify lags. More detail is needed in the table caption and probably also in the main text.

3. lines 337-338. "It is worth mentioning that the 1D model estimates the MLD from temperature-based profiles while ..." If the PWP model estimates MLD from temperature, then how does it allow you to evaluate the impact of alpha vs beta oceans. I found online information that implies that PWP was originally written for temperature only but that newer versions incorporate freshwater fluxes, which should allow assessment of temperature and salinity effects. https://uop.whoi.edu/techdocs/technote/9701tn.html

4. lines 341-342. "The choice of PWP model was made to support the idea that β- and transition
oceans do not develop deep mixed layers, which ..." The PWP model should not be selected to support an idea---that sounds like circular logic. Rather it should be selected if it is the right tool to test a hypothesis. I'd suggest revising this to say, "The PWP model was chosen to test the idea that β- and transition oceans do not develop deep mixed layers, which ..." I also think this sentence looks like it should appear in line 336, just after the statement saying that the Price et al (2986) one-dimensional model is used.

5. lines 409-411. "Even if the α-ocean area is warming, it is not expanding northwestward. Hence, the EGC is acting as a barrier bringing PSW in the area and maintaining the β-ocean state on the northwesternmost side of the Strait." Some precision in language is warranted. The fact that the alpha ocean is not expanding does not prove that the EGC is a barrier, but rather is consistent with the hypothesis that the EGC acts as a barrier. Or it implies that the EGC might be acting as a barrier.

6. Minor style/grammar issues.
a. line 18. "the north and northeast regions" -> "the north, and the northeast."

b. line 22. "that, the" -> "that the"

c. line 22. "south, by" -> "south and by"

d. line 39. "play" -> "playing". (Or alternatively put a comma after "circulation".)

e. lines 54-55. Change hyphens to em dashes.

f. line 56. "retreat and" -> "retreat, and"

g. line 56. "drives" -> "drive"

h. line 57. Sentence too long. I'd suggest starting a new sentence with "Sea-ice retreat"

i. line 81. "IPCC" acronym is not defined, and the relevant citation needs to be clear about the specific IPCC report that is being cited.

j. lines 85-86. "on Climate Change" looks to be an erroneous citation for the IPCC report. Correct citation.

k. line 96. "supply and" -> "supply, and"

l. line 137. Missing parentheses around (2018).

m. line 138. "was" -> "were"

n. line 142. "where" should not be indented after equation (1).

o. line 151 and following. Paragraph appears to be erroneously indented relative to equation.

p. line 155, line 158. N should be italicized to match notation in equation (2).

q. line 156. "any" -> "either"

r. line 158. "stratified" -> "stratified,"

s. line 160. "MLD we" -> "MLD, we"

t. line 160. Remove "one-dimensional" since the same information is repeated in the next sentence.

u. lines 167, 168. Mathematical terms should be in the same font used for mathematical terms in equations.

v. line 169. "0.25 local" -> "0.25, local"

w. line 169. "with ERA" -> "with the ERA"

x. line 171. "allows to" -> "allows us to"

y. line 179. "positive (negative)". This structure is hard for readers to parse and can easily be avoided by slightly stretching out the sentence: "is positive in alpha oceans and negative in beta oceans."

z. line 183. "seasonal" -> "seasonal,"

aa. line 184. "are shown in Figure 2" -> "(Figure 2)"

bb. line 204. "summer but" -> "summer, but"

cc. line 261, line 264. "West" -> "west"

dd. line 268. "where": Should this be "when"? The phrasing appears to refer to timing rather than geographic location.

ee. line 270. "is average" -> "is near its long-term average".

ff. line 273. "we consider that a 2-year" -> "in our assessment, a 2-year

gg. line 283. "2016) but" -> "2016), but"

hh. line 293. "except the last decade where" -> "except during the last decade, when"

ii. line 334. "summer stratification" -> "stratification"

jj. line 357. "soobtained" -> "so obtained"

kk. line 358. "roughly with" -> "is roughly"

ll. line 370. "Icelan" -> "Iceland"

mm. line 379. "Then in " -> "In "

nn. line 381. "Then in" could remain as is, but this also might be smoother if changed to "In"

oo. line 428. "north and" -> "north, and"

pp. lines 429-431. Change to "of both the increase in surface temperatures, i.e., northward shift of warmer isotherms over the Iceland Faroe Ridge (de Marez et al., 2025) and the"

qq. line 434. "subjected" -> "subject"

rr. line 435. "variability (ii)" -> "variability. (ii)"

ss. line 438. "and accounting for" -> "and experience" (I'm not sure what you mean, so this is my guess.)

tt. line 433 and line 443. "Four main" seems to be undermined by point (v) at the end of the paragraph.

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ED: Publish subject to minor revisions (review by editor) (09 Apr 2026) by Agnieszka Beszczynska-Möller

AR by Angel Ruiz-Angulo on behalf of the Authors (04 May 2026) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (08 May 2026) by Agnieszka Beszczynska-Möller

AR by Angel Ruiz-Angulo on behalf of the Authors (13 May 2026) Manuscript

Journal article(s) based on this preprint

29 May 2026

Stratification and mixed layer depth around Iceland: Characterization and inter-annual variability

Angel Ruiz-Angulo, Esther Portela, Charly de Marez, Andreas Macrander, Sólveig Rósa Ólafsdóttir, Thomas Meunier, Steingrímur Jónsson, and M. Dolores Pérez-Hernández

Ocean Sci., 22, 1727–1743, https://doi.org/10.5194/os-22-1727-2026,https://doi.org/10.5194/os-22-1727-2026, 2026

Short summary

Angel Ruiz-Angulo, Esther Portela, Charly de Marez, Andreas Macrander, Sólveig Rósa Ólafsdóttir, Thomas Meunier, Steingrímur Jónsson, and M. Dolores Pérez-Hernández

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Jan 2026	208	200	20	428
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The ocean around Iceland is a key region for water mass transformation that drives global ocean circulation. We use 29 years of hydrographic data to examine the spatial and temporal variability of mixed layer depth and stratification, identifying three distinct regions: South, North, and Northeast. We present a comprehensive view of seasonal to multi-decadal variability in upper ocean structure and its link to a changing North Atlantic under global warming.


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