Variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS (Nansen and Amundsen Basins Observing System) hydrography data

Zhurbas, Nataliya; Kuzmina, Natalia

doi:https://doi.org/10.5194/os-16-405-2020

Articles | Volume 16, issue 2

https://doi.org/10.5194/os-16-405-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-16-405-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 2

Research article

|

02 Apr 2020

Research article |

| 02 Apr 2020

Variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS (Nansen and Amundsen Basins Observing System) hydrography data

Nataliya Zhurbas and Natalia Kuzmina

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Final revised paper (published on 02 Apr 2020)
Preprint (discussion started on 03 Jun 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'review of the ms', Anonymous Referee #1, 28 Jun 2019
- AC1: 'Discussion: Reply to RC1', Nataliya Zhurbas, 08 Jul 2019
  - AC4: 'Discussion: a short addition to reply to RC1', Nataliya Zhurbas, 24 Jul 2019
RC2: 'os-2019-54', Anonymous Referee #2, 01 Jul 2019
- AC3: 'Discussion: Reply to RC2', Nataliya Zhurbas, 24 Jul 2019
EC1: 'General comments', Ilker Fer, 08 Jul 2019
- AC2: 'Discussion: Reply to EC1', Nataliya Zhurbas, 12 Jul 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Nataliya Zhurbas on behalf of the Authors (13 Aug 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Aug 2019) by Ilker Fer

RR by Anonymous Referee #1 (16 Aug 2019)

Suggestions for revision or reasons for rejection

R E V I E W

of the manuscript (os-2019-54) entitled
“Assessment of variability of the thermohaline structure and transport of Atlantic
water in the Arctic Ocean based on NABOS CTD data”
by Nataliya Zhurbas and Natalia Kuzmina

This is my second reading of the manuscript after recommended major revision. This is this rear case when the manuscript is essentially not improved after revision. Unfortunately, my major critical points:
1. All findings of the paper should be placed in the context of the existing literature, and
2. Care should be taken to separate spatial and temporal variability,
have not been properly addressed.
Based on that I recommend rejection with potential resubmission. I provide my major critical points to the authors again below (this time skipping numerous minor comments).

Major critical points:
1) My previous comment #1 required the authors to place their research in the context of what has been done in this research area in the past. This is essential requirement because it would identify new and exciting results coming from the authors’ research. E.g. after reading the entire Section 3.1 I left wondering what new is in this section. The same is about the part of Section 3.2 devoted to temperature, salinity, and density changes. This is also true for Section 3.3 which, despite the title, does not discuss water transport changes. All these three sections include multiple repetitions.
2) I do not think that the part of “Results” devoted to temperature, salinity and density changes carries any new information. One of the major emphases is the attempt to separate the Barents and Fram Strait branches of the Atlantic Water. First, there is an extensive literature on that subject which is not presented in the manuscript. Secondly, I doubt that using just temperature and salinity data would allow successful separation of the water masses because of their strong mixing along the Eurasian slope which makes the boundary between these water masses extremely eluded. Chemical data analysis is the key for the successful separation.
3) Analysis of water transports based on geostrophic estimates is the golden nugget of the manuscript. However, now it is buried under unnecessary (to my view) discussions of temperature and salinity.
4) Still a lot should be done to demonstrate that the estimates of water transports presented in the paper are not affected by spatial variability.

My recommendations would be:
- Focusing the manuscript on analysis of water transports based on geostrophic estimates. Remove from the text discussion of temperature, salinity and (mostly) density changes.
- Provide solid evidence that the estimates carry signal and not just synoptic noise.
- Compare these estimates with the existing ones (e.g. recent Pnyushkov et al. studies). I would like to see cross-sections of these inferred currents.
- Do analysis in depth of spatial changes versus temporal changes.

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RR by Anonymous Referee #2 (27 Aug 2019)

Suggestions for revision or reasons for rejection

Comments on Zhurbas and Kuzmina 2nd version

This is a resubmission and I give my comments from top to bottom regardless of importance.

Line 10: Correct this sentence.

Line 23: Can the geostrophic current be computed in another way?

Lines 39 ff: Perhaps too many references.

Lines: 49-50: How do we know that the shallow shelf water is cold and why?

Line 55: There are a few geostrophic transport estimates for the exchanges through Fram Strait: e.g. Rudels (1987), Schlitzholz and Houssais (2002), Rudels et al. (2008), Marnela et al., (2013). Some of these discuss water mass transformations and also the effects of the sloping bottom (based on Jakobsen and Jensen, 1926).

Line: 60: Internal waves and eddies might disturb the density field and therefore also make geostrophic transport estimates unreliable.

Line 65: Is array the correct word to use here?

Lines 65-75: This part can be shortened.

Lines 116-118: What does this sentence mean?

Lines 148-151: Although the statement that S-max should always be below the T-max is correct, these lines do not make it clearer. A value at a point does not say anything about the stratification. Knowledge of the equation of state is required. I suggest that this is dropped.

Lines 170 ff: According to figure 1 all stations in the section are in St. Anna Trough and there are no stations in the Voronin Trough. I suggest that the discussion about the importance of the Voronin Trough is removed or modified. The outflow farther to the east hangs on the slope and contains fairly warm (>0C) Atlantic water as well as colder low salinity Barents Sea branch water. This branch mixes with the Fram Strait branch north of Severnaya Zemlya.

Lines around 240, Figure 5: I am surprised that the cold, less saline water at the slope is not commented upon here. This is water of the less dense Barents Sea branch that has entered the Nansen Basin when the slope narrows north of Severnaya Zemlya.

Lines 277-287: There is not only a decrease in the Fram Strait branch Atlantic water temperature but also in its salinity. These subsurface changes suggest mixing with a less dense part of the Barents Sea branch.

Line 310, Figures 7a and 8: Which stations are from the Voronin Trough?

Figure 8e and 8f: Looking at the TS diagram I would expect the cold Barents Sea outflow to enter the Nansen Basin water column at the level where the deep small-scale interleaving is found. Looking at figure 8f, it is not dense enough to reach that deep. However, colder water is more compressible, and if its density is referred to the depth where the interleaving is found, its density might correspond to that of the interleaving.

Line 341: Since ther are many bends and “knees” in these TS curve and just for clarity, I would drop the “knee” and only refer to the salinity and temperature minima, (region II).

Lines 368-380: If the Barents Sea branch water mixes isopycnally with the Fram Strait branch water we would not see any perturbations in the density field, but a cooling and freshening of the Fram Strait branch, which is observed. With a broad definition of the Fram Strait branch and a narrow definition of the Barents Sea branch, it is to be expected that the Barents Sea branch becomes absorbed in the Fram Strait branch. The uPDW was originally introduce to explain the stable-stable stratification found below the temperature maximum in the Amerasian Basin and was explained by a vertical redistribution of heat from the Atlantic water to deeper layers by saline and dense entraining plumes.

Lines 390-405: I am not convinced by this discussion without also looking at salinity and density profiles. There are two major outflow of Barents Sea branch water from the Kara Sea. The denser part sinks down the St. Anna Trough and enters below 1000m. The less dense outflow occurs west of Severnaya Zemlya and enters into and below the Fram Strait branch Atlantic core. To separate these two inflow by looking at observation in St. Anna Trough and northeast of Severnaya Zemlya is difficult and perhaps not worth the effort.

Lines 413-414: How do we know from the section at 82N that there is an eastward (geostrophic?) flow parallel to the section. Are data from the entire section array in the St. Anna Trough considered?

Lines 424-425: There is also the possibility that the uppermost layer, containing the temperature maximum, is transformed by sea ice melting and cooling into a low salinity upper layer and that the temperature maximum observed farther to the east derives from deeper layers.

Line 428: What surrounding waters?

Lines 440-450: Cannot the increased distance of the Atlantic water core from the slope at 126E be the beginning of the recirculation of the Fram Strait branch toward Fram Strait?

Line 486: Change “two time smaller than” to “half”

Lines 569-580: The 3 Sv are entering the Nansen Basin. The three branches refer to the splitting of the Norwegian Atlantic Current further south. A total of 2Sv of Atlantic water entering the Nansen Basin is low compared to existing estimates (e.g. Tsubouchi et al., 2012).

Line 605: I cannot see why there cannot be a barotropic velocity component in the presence of sea ice.

Lines 660ff: If the flow rate decreases along the path of the flow and at the same time the characteristics becomes weaker due to mixing with surrounding waters, how does this fit with the continuity equation. Where does the water go? Is the boundary current losing mass to a return flow on the basin side of the boundary current?

Discussion and summary sections: I find these parts too long and too loose. They add questions but provide only more or less speculative answers. These sections should be shortened and more focused.

Summary: This work examines the circulation and transformations of the inflowing Atlantic water mainly in the Eurasian Basin of the Arctic Ocean. The transport estimates are obtained from geostrophic calculations and are generally significant lower than other published estimates. The presentation is overly pedagogical and repeating earlier statements. (I found that some misgivings and questions I had when reading were taken up several pages later. Since I wrote the comments while reading through the text, I found that some of my questions were addressed later in the manuscript. I think that it would shorten the text considerably if such questions are addressed directly. In any case, this manuscript can be considered a part in the ongoing discussion of the circulation of the Atlantic water in the Arctic Ocean and could be published with minor revisions and shortening.

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ED: Reconsider after major revisions (28 Aug 2019) by Ilker Fer

AR by Nataliya Zhurbas on behalf of the Authors (09 Oct 2019) Manuscript

ED: Referee Nomination & Report Request started (10 Oct 2019) by Ilker Fer

RR by Anonymous Referee #1 (22 Oct 2019)

RR by Anonymous Referee #2 (28 Oct 2019)

Suggestions for revision or reasons for rejection

This is the third time I read through this manuscript and I still have difficulties. I think it is mainly due to the way the results are presented. My impression is that the authors underestimate their readers, and that they devote much time and space on explaining processes that every oceanographer knows (or should know) and on describing features in the Arctic Ocean known by anyone working in the field.

One example is the geostrophic boundary current. It is well known that there is an eastward moving geostrophic boundary current along the Eurasian continental slope, with or without a barotropic reference velocity. The baroclinic part is distinguished by the spreading of the isopycnals towards the slope. This can be mentioned once, but it is not necessary to repeat this statement at every discussed section. Especially not since one of the main results of the study is the geostrophic transport in the boundary current. Should that transport at one section be westward, the observed distribution of the isopycnals could be discussed. No such instances occurred according to table 1.

In any case, I do not think it is fair of a reviewer to bring up something new this late in the game, but the above remarks might be of some use for the authors' next manuscript.

Below I have listed some minor points.

Abstract: I would have preferred present rather than past tense here, but that is a matter of taste.

Line 43: I think that Basin should be written with b if it is not in a name (e.g. Eurasian Basin)

Line 51: Why does a baroclinic flow follow the sea bed topography?

Line 61 Which Arctic Basin?

Line 75 Why does mixing with other waters reduce the flow rate? It should rather increase the flow rate, unless only the Atlantic water is meant and that the mixing removes Atlantic water out of its TS definition.

Line 85: Change “whole” to “entire”

Line 118: What does “if we limit ourselves to geostrophic estimates of volume flow” means?

Line 163: “shallow shelf waters” Is here brine enriched cold shelf bottom water meant?

Line 223: “Barents sea Branch water”

Line 232: What latitudes are meant here?

Line 295: No stations from the Voronin Trough.

Line 323: remove “more”

Line 336: The BSBW cannot be a flow.

Line 337: Salinity at mid-depth (800-1000m) below 34.9 indicates input from the shelf and slope, most likely Brents Sea branch water. Salinities below 34.9 are not found at this depth range west of St. Anna Trough.

Line 350: Where in the western Nansen Basin is the UPDW observed? Give reference or station.

The UPDW in the Eurasian Basin is mainly formed by the intrusive mixing of the Barents Sea branch with the Fram Strait branch. In the Amerasian Basin the most characteristic UPDW is found (increasing salinity and decreasing temperature with depth). It is created by interactions between the Atlantic water and entraining dense and saline boundary plumes at the continental slope.

Line 364: How can a flow rate be determined by a water mass? I assume what is meant is that the geostrophic transport of Atlantic water east of 126oE is mainly provided by the Fram Strait branch.

Lines 365-370: What does this paragraph mean?

Lines 380-385: I do not understand the argument in this paragraph, especially not the meaning of the sentence “The probable presence of the eastward constituent of indefinite vale makes questionable the results of…” I would remove this paragraph.

If a discussion of a possible eastward flow at 82oN is really necessary, I would compute the transports through the 80oN section and the 82oN section, take the difference, and if the transport at 80oN is larger than that at 82oN, the difference could be an estimate of the transport moving eastward and not crossing the 82oN section.

Lines 405-410: The salinity maximum is removed, mainly by mixing with the Barents Sea branch inflow. The deep salinity maximum has no relation to the Atlantic water salinity maximum.

Line 583: I cannot follow this argument about the ice cover. In the Beaufort Gyre the wind drives the ice towards the center of the gyre increasing the sea level and leading to a dynamic topography that drives the circulation of the gyre. This topographic effect is seen both in the hydrography and by altimeter observations by satellites. I cannot see why a similar situation can arise in the Nansen Basin.

Summary. I think that is ready for publication and I hope that the comments above can be of some use.

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ED: Reconsider after major revisions (07 Nov 2019) by Ilker Fer

AR by Nataliya Zhurbas on behalf of the Authors (16 Dec 2019)

ED: Publish subject to minor revisions (review by editor) (16 Jan 2020) by Ilker Fer

AR by Nataliya Zhurbas on behalf of the Authors (26 Jan 2020) Author's response Manuscript

ED: Publish as is (03 Feb 2020) by Ilker Fer

AR by Nataliya Zhurbas on behalf of the Authors (13 Feb 2020)

Short summary

Based on hydrography data (2002–2015) it was found that the average geostrophic volume transport of Atlantic water (AW) decreases along the Eurasian Basin slope from 103º E to 142º E and has a small value in the Makarov Basin. Maximum values of the AW volume transport, temperature, and salinity were observed in 2006. A trend of increase in AW salinity over time (AW salinization in early 2000s) was discovered. These results can be useful to anyone interested in recent change in the Arctic Ocean.