the Creative Commons Attribution 4.0 License.
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| 07 Dec 2023
The Polar Front in the northwestern Barents Sea: structure, variability and mixing
Abstract. In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front (the Barents Sea Polar Front). Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project, in the region between Hopen Trench and Olga Basin. Ocean stratification, currents, and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (>2 °C) and salty (>34.8) Atlantic Water intruding below the colder (<0 °C) and fresher (<34.4) Polar Water, setting up a baroclinic front and geostrophic currents reaching 25 cm s-1, with estimated eastward transport of 0.3±0.2 Sv (1 Sv = 1×106 m3 s-1). We observe anomalous warm and cold-water patches on the cold and warm side of the front, respectively, collocated with enhanced turbulence, where dissipation rates of turbulent kinetic energy range between 10-8 and 10-7 W kg-1. Short-term variability below the surface mixed layer arises from tidal currents and mesoscale eddies. While the effects of tidal currents are mainly confined to the bottom boundary layer, eddies induce significant shifts in the position of the front, and alter the isopycnal slopes and the available potential energy of the front. Substantial water mass transformation is observed across the front, likely a result of eddy-driven isopycnal mixing. Despite the seasonal changes in the upper layers of the front (0–100 m) influenced by atmospheric forcing, sea ice formation, and brine rejection, the position of the front beneath 100 m depth remained relatively unperturbed.
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CC1: 'Comment on egusphere-2023-2864', Maria Dolores Pérez-Hernández, 28 Dec 2023
This study focuses on understanding the Polar Front over the sill between the Hopen Trench and Olga Basin, one of the four areas where AW meets Polar Water in the Barents Sea. The Polar Front is important for biological activity and mixing in the area. The results arise from two detailed fieldworks where hydrographic data from ship and glider sections are analyzed with altimetry, wind, and sea ice concentrations. This study is very interesting and highlights the high variability that the Polar Front has in terms of existing in location, shape, forcings, and time. The dataset used is available, and the study is relevant to the field. I suggest publishing it after some changes.
My main concern is Section 4.2, ‘Polar Front structure and seasonal variability.’ Here are some comments to help improve it:
- The way it is written reads more like a general variability than a seasonal cycle, and it finishes with an average view. Results will be better understood if the section starts with the average view and, from there, moves towards seasonality.
- The seasonal cycle cannot be fully resolved with the available dataset. Nevertheless, a section in August could be used as representative of summer, 11 sections can be combined into a fall average section, and 4 sections can be averaged as a winter section.
- This section also has some minor issues, like using the term ‘Atlantic-origin water’ instead of ‘modified Atlantic Water’ as stated in Table 3.
- It is not said whether negative distances are located north or south of the sill in the caption of Figure 3.
- At some point, it is stated that the velocities from altimetry match the DAC, and while that is true for October 19, the agreement is not as evident on the other two dates.
- Transport estimations are only given for the average section (Figure 5). You could also estimate seasonal transports or, if not, a table with the transport for each time frame.
- Overall, the text between lines 306 and 317 should be carefully revisited as they had some misleading errors.
- Line 306-307. AW is separated from the surface by a warm and fresh layer; it is not cooler but fresher in the upper 60m.
- Line 312. I don’t see a cooling from December to February, as the years don’t match. I see cooling on the glider dataset for November and December 2019 and the ship dataset for February 2021. These colder sections are relative to the August 2019 and October 2020 sections.
- Line 314-315. In February, AW is not present (being AW defined with temperatures higher than 2ºC and salinities higher than 35.06). From what is visible in Figure 3 a, lower 2 subplots, the northern side of the Front fits better with the description of modified AW given in Table 3.
- Line 317. Assuming that negative distances are south of the sill, the average position lies 10km north of the sill, where the core of the positive velocities is found (Figure 5). Yet, it can reach as far as 10 km south of the sill in the 50 m depth and narrows from there to the bottom.
- Lines 415-416 and 420. The seasonality of the isopycnals is arguable. It says that the isopycnal tilt is flat in winter, while in Figure 3, the February sections have quite a tilt. Although the Glider sections of December have flat isopycnals, some of the October sections also present nearly flat isopycnals. So, this goes again with Section 4.2; perhaps a seasonal composite could be a better approach to assess seasonality or just blend it all under ‘variability’.
Some other minor issues:
- Line 86 to 87. This sentence is confusing; I suggest rephrasing or avoiding mentioning Figure 1b. Here, the text refers to the data used in the study, while Figure 1b introduces a larger area.
- Line 94. Please explain how salinity was calibrated (AUTOSAL, Portasal,other?)
- Section 2.2. Two paragraphs above, it said that the cruises will be referred to as fall and winter cruises, but in this section, the names of the vessels are used. You could recall the season after the cruise name at the beginning or go with the seasonal names.
- Line 121. ‘of the PF location (Figure 2)’.
- Line 201. EUMETSAT OSI-SAF (2017).
- Line 291. AW depth exceeds 200m depth? Do you mean that the entire water column is AW? or that it spreads to waters shallower than 200m?
- Lines 297-298. Between November and December 2019, in Figure 1e, the sea ice rose to 10%. So perhaps you should extend the time frame to the end of January 2020.
- Line 330 is the ‘maximum’ southward ‘extension’ of the ‘PF.’
- Line 407. This increase in salinity during winter is not mentioned in Section 4.2.
- Lines 410-411, in Figure 3, a northward progression of the AW/mAW is observed near the bottom.
- Figures
- Figure 1 The caption should state which SST and sea ice product is used, as the references to Figure 1 start in the introduction.
- Figure 2. The caption should state what the blue, orange, and yellow triangles are.
- Figure 5 could benefit from having a lower row where the standard deviation section is shown to understand in which depths the front varies more.
- DACs are integrated in the figure with altimetry? Which depth range?
Citation: https://doi.org/10.5194/egusphere-2023-2864-CC1 -
AC3: 'Reply on CC1', Eivind Hugaas Kolås, 01 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2864/egusphere-2023-2864-AC3-supplement.pdf
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RC1: 'Comment on egusphere-2023-2864', Anonymous Referee #1, 05 Jan 2024
Kolås and colleagues provide a detailed analysis of the oceanographic characteristics of the polar front in the northwestern Barents Sea (BS). The paper uses a comprehensive observational dataset (a subset of the companion paper submitted to JRL, as I understand it) obtained as part of the Nansen Legacy project with traditional and autonomous platforms (gliders), which is undeniably more detailed in terms of spatio-temporal variability, but also more complete in terms of physical variables than any previous dataset. To this end, the authors make a very valuable and serious contribution to the community. In addition, the writing and overall form are excellent and well streamlined, making life really easy for the reader.
This study is rather technical (which is fine!) and I find that the authors could discuss implications slightly more. The BS has long been thought a "hot spot" for marine productivity.... Due to strong vertical and/or cross-frontal mixing. However, this study confirms that the front is a place of "moderate turbulent mixing" (mainly at the surface and at the bottom) and that subsurface mixing occurs more along the isopycnals. And in fact, to the best of my knowledge, the front was never demonstrated as a significantly more productive area than the rest of the BS. The area studied is the only northern gateway to the western Barents Sea. As the front extends to the east, we might expect the same effect to occur here in the West. Interestingly, this is not the case seasonally, and from one year to another. The implications, both in terms of biogeochemical tracer and heat, are that the PF front acts more as a barrier to the AW domain (rather than a mixing machine) and to the ongoing "Atlantification". To go beyond, the AW have to subduct, transporting heat, salt and… carbon along isopycnals... which could eventually be sequestered further in the Nansen Basin.
Another interesting aspect of this study is that it provides the first very interesting evidence of the baroclinic structure of FP. This structure could favor baroclinic instabilities. So I can see why the authors took the time to study eddies (although they were created elsewhere). These vortices could temporarily tilt the isopycnals even further and provide additional mixing. It's fascinating that this physical feature has been studied for decades and still holds so many mysteries.
I found no major problems with the manuscript, which I recommend for direct publication. I recommend only very minor edits/additions for which (I think) it is not necessary to send the manuscript to the reviewers again:
- Please specify somewhere that altimetry provides only surface geostrophic velocities. No need extra work but you could provide existing (very few) studies that evaluated those products for quality control in the region.
- Figure 2: please provide brief information in the caption about transects A, D, F so that the reader so remains in the blue until Figure 9.
- Figures 3 and 4: curious arrangement… panel labels at the bottom, colorbars squeezed in the middle. It would help to pop out the colorbars. Panels are un-scaled, you can notice through the y-axis which should be the same everywhere. This is okay and probably complex to fix, but I recommend to improve the visual. I would just use the same “frame” and leave blank where there is no data (no obligation, you probably tried already, just my suggestion).
- Line 327, just say add “following” section, it helps to grasp the nice flow of the article. Nice transitions.
- Figure 6: I recommend to draw some box, or arrow to describe the eddy position, structure, etc… because from isopycnals it looks like 2 eddies which are merging and it creates a bit of confusion when reading.
- In the discussion I would have liked a word on the intensity of the density gradient.
- I would also like a comparison of magnitude of the dissipation rate with previous estimates and/or close-by regions, besides Fer and Drinkwater (2014).
- If the structure of the PF is baroclinic, then the use of altimetry must be at least questioned.
Citation: https://doi.org/10.5194/egusphere-2023-2864-RC1 -
AC1: 'Reply on RC1', Eivind Hugaas Kolås, 01 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2864/egusphere-2023-2864-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2023-2864', Anonymous Referee #2, 25 Jan 2024
Review of “The Polar Front in the northwestern Barents Sea: structure, variability and mixing” by Kolås et al.
The authors provide a nice description of the structure of the Polar Front in the Barents Sea based on extensive ship board data (a fall and a winter cruise) as well as data from several gliders. Even though this region has been extensively studied, the study provides some novel aspects of the dynamics of the front, in particular that it is baroclinic at the sill, unlike its barotropic nature encountered further south. The paper is well-written, easy to read and does not suffer from any major issues in the methodology or interpretations. On several occasions during the read I however asked myself “So what?”. Where possible it might be nice for the authors to clarify the motivation for what they are doing or what the implications of their findings could be. That being said, the baroclinic nature as well as the level of mixing at the front (weaker than at the surface and at the bottom, but quite large for the mid-water column!) are important additions to the literature that are well placed in “Ocean Science”. Consequently, I can recommend this manuscript for publication after minor revisions; the minor points below do not warrant a second round at the reviewers.
Minor points:
Fig. 1a This figure does not provide the important information. Most of the topographic features are not labelled and the isobaths are very hard to understand/interpret. The sill is not label or marked even though it is the key location of the paper. Fig. 11 of the authors’ JGR preprint is much clearer and I’m wondering why the authors don’t use a modified version of that figure here, in particular the labeling and the water depth as a color scale and not just as contour lines.
L143 Do you mean “southward wind” or “northerly winds”?
L199 It might be nice to explicitly give the equation for C_D as a function of sea ice concentration rather than only referring to the 2005 paper.
L214 You give a reason, but that still leaves the question as to why you decided to do that.
L221 How are L_x and L_z estimated?
L237 Is this recalculation of salinity from the sorted density profiles common oceanographic practice? Then please cite examples from the literature. Otherwise, it is a worthwhile methodological advancement that should be motivated, justified, and also advertised a bit more prominently than only with this single sentence.
L280 Does this bias your estimation of EKE at a grid point when sea ice is present (only) at certain periods of time (which might e.g. be high, or low, EKE time periods)?
L318 Why do you not calculate the gradients from each transect directly and then average the gradients to substitute the numbers currently given in L319. Note that averaged T/S will smooth (among others due to differences in the horizontal location of the strongest gradients) the gradients substantially compared to what is presumably present in each of the individual sections.
L323-330 These lines are repetitive. Consider “This reversal is discussed in the next section.” And then “Simultaneously, …”
L336-339 I can’t quite follow this Eulerian vs. Lagrangian view.
Fig. 6 caption “specified in the lower left corner” I can’t see it.
Fig. 7 caption Consider rephrasing “at B5 in fall (left) and at B7 in winter (right)”.
Fig. 8 right part of figure: delta time = 1 day is a different amount of centimeters on the printed page for the upper panel (October 2020) than for the lower panel (February 2021). Consider making it equal.
Fig. 9 caption “in Figure 2” (a space is missing in front of the “2”).
L432 “We expect the contribution”
L440 “in mid-October”
L442 “between the averaging box and the position”
L456 There are other possible explanations (L 454 “may be related to”). There might be a sea ice related bias (see comment L280). There might be an uneven distribution of events driven by external (non-climate change related) interannual variability in the 2 decades. E.g. (I’m just making up numbers/causal relations for point of illustration) EKE could be high during high NAO phases. In the first decade there were 3 years with high NAO and in the second decade there were 6 years with high NAO even though there is no long-term trend in the frequency of high NAO events.
L501 “scientists” “Haakon cruise”
Citation: https://doi.org/10.5194/egusphere-2023-2864-RC2 -
AC2: 'Reply on RC2', Eivind Hugaas Kolås, 01 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2864/egusphere-2023-2864-AC2-supplement.pdf
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AC2: 'Reply on RC2', Eivind Hugaas Kolås, 01 Mar 2024
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EC1: 'Comment on egusphere-2023-2864', Katsuro Katsumata, 26 Jan 2024
Both the reviewers #1 & #2 recommend only minor revisions before publication. Please proceed with revising the manuscript. Although this remains optional, I would encourage the authors to consider the Community Comment as well, which I found constructive. I look forward to receiving the revision.
Citation: https://doi.org/10.5194/egusphere-2023-2864-EC1
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