Diapycnal mixing across the photic zone of the NE Atlantic

van Haren, Hans; Brussaard, Corina P. D.; Gerringa, Loes J. A.; van Manen, Mathijs H.; Middag, Rob; Groenewegen, Ruud

doi:https://doi.org/10.5194/os-17-301-2021

Articles | Volume 17, issue 1

https://doi.org/10.5194/os-17-301-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-17-301-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 1

Research article

|

17 Feb 2021

Research article |

| 17 Feb 2021

Diapycnal mixing across the photic zone of the NE Atlantic

Hans van Haren, Corina P. D. Brussaard, Loes J. A. Gerringa, Mathijs H. van Manen, Rob Middag, and Ruud Groenewegen

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Final revised paper (published on 17 Feb 2021)
Preprint (discussion started on 03 Aug 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review for manuscript # os-2020-73', Anonymous Referee #1, 04 Sep 2020
- AC1: 'reply to reviewer 1', H. van Haren, 27 Oct 2020
RC2: 'Review of "Diapycnal mixing across the photic zone of the NE-Atlantic"', Anonymous Referee #2, 06 Sep 2020
- AC2: 'reply to reviewer 2', H. van Haren, 27 Oct 2020
RC3: 'Comments on OS-2020-73 by Hans van Haren et al.', Anonymous Referee #3, 20 Sep 2020
- AC3: 'reply to reviewer 3', H. van Haren, 27 Oct 2020
- AC4: 'modified text with changes highlighted', H. van Haren, 27 Oct 2020

Peer-review completion

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

AR by H. van Haren on behalf of the Authors (27 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (28 Oct 2020) by Ilker Fer

RR by Anonymous Referee #2 (12 Nov 2020)

RR by Anonymous Referee #3 (21 Nov 2020)

RR by Anonymous Referee #1 (24 Nov 2020)

Suggestions for revision or reasons for rejection

Review for the reviesed manuscript # os-2020-73" Diapycnal mixing across the photic zone of the NE-Atlantic" by van Haren et al.

As indicated in the previous review of this manuscript, this paper discusses dissipation rates of turbulent kinetic energy, eddy diffusivities and vertical turbulent nutrient fluxes inferred from upper-ocean hydrographic and nutrient data taken during a cruise on a transect from 60°N to 30°N along about 17°W in the North Atlantic.
While the new version of the manuscript is somewhat improved, in particular by adding statistical and measurement uncertainty, I still find that results presented in the manuscript are not sufficient to support the authors’ interpretations and conclusions.

Remaining major concerns

My major remaining concern continues to be the authors’ claim “the lack of correspondence between turbulent mixing and stratification along the transect suggests that nutrient availability for phytoplankton in the euphotic surface waters may not be affected by global warming”, which is not supported by their results. As stated in my first review, comparing the strength of thermocline mixing in different oceanic regions (at different latitudes, here) cannot lead to any conclusions on local changes of the strength of turbulent mixing due to locally increasing stratification. As I am detailing in the response below, the added wording does not help to justify their claim.

Furthermore, my second concern related to the presented vertical nutrient fluxes due to turbulent mixing has not fully been addressed. Although adding accuracy and precision of the nutrient measurements has improved the manuscript, it remains unclear if the presented nutrient fluxes are relevant for near-surface primary production and thus for the biological carbon pump as a whole. In my earlier review, I suggested to the authors to compare the results to previous studies and to provide a comparison to nutrient uptake in the upper ocean. Unfortunately, it seems that my suggestion was misinterpreted by the authors.

I provide further details to these concerns in my reply to the responses below. For clarity, I am presenting my former remarks, then the reply by the reviewers followed by my reply.

Reviewer’s previous major remarks: (3) I cannot approve the approach chosen here as a whole. Comparing the strength of upper thermocline mixing at different latitudes cannot lead to any conclusions on local changes of the strength of turbulent mixing e.g. due to locally increasing stratification.

Reply by the authors: We do not agree with this statement, because all sampling is done in the upper 500 m where the local water depth was at least 1100 m, and, except for 3 stations, most stations were over (much) deeper waters >2000 m. So, sampling was well away from bottom topography, in the NE-Atlantic where semidiurnal tides, and inertial motions, dominate the internal wave field, in summertime under overall moderate-good weather conditions across the entire survey. As a result, the dominant convection (in the upper 20-30 m) and internal wave induced mixing (in the stratified layers below) are much less variable across the transect due to different forcing than due to the highly intermittent occurrence of turbulent bursts as the reviewer correctly indicates above. Those bursts are inherent to turbulence, and less so dependent on the generation process. We added text to better explain this, lines 419-421:’ If shear-induced turbulence in the upper ocean is dominant it may thus be latitudinally independent (Jurado et al., 2012; deeper observations present study). There are no indications that the overall open ocean internal wave field and (sub)mesoscale activities are energetically much different across the mid-latitudes.

Reply by the reviewer: I do not want to get into a (perhaps endless) discussion on internal wave forcing and energy fluxes. However, even if the energy residing in the internal wave field were comparable at the different latitudes (which the authors do not demonstrate in this study nor was this shown in the study by Jurado et al. 2012) it can not be concluded that the energy fluxes into internal waves and from internal waves into turbulence would be the same. In fact, this would be rather unlikely, because internal wave-wave interaction processes are dependent on latitude (see e.g. Henyey et al. 1986 or Gregg et al. 2003). Wave-wave interaction at higher latitudes is more efficient in fluxing energy to turbulence than at lower latitudes and we would thus expect an elevated flux of internal wave energy to turbulence at 60°N compared to 30°N. Furthermore, parametric subharmonic instability of the M2 tide leads to elevated energy flux into turbulence and enhanced mixing in the region between about 20°-30° away from the equator (see e.g. Hibiya et al. 2007). This process may be responsible for the somewhat elevated eddy diffusivities in the southern part (30°N-32°N) of the transect. I am mentioning these two processes because the added text, in particular “no indications that the open-ocean internal waves field … are energetically not much different across the mid-latitudes” does not help to understand the claim that eddy diffusivities across the transect are of comparable magnitude.
Apart from issues with the added sentence, I strongly disagree with the statement in the authors response above “… internal wave induced mixing (in the stratified layers below) are [is] much less variable across the transect due to different forcing than due to the highly intermittent occurrence of turbulent bursts …” In the thermocline, internal wave energy is dissipated by turbulence. Certainly, the bursts of turbulence are highly variable, but if resolved adequately in time, they merely reflect the dissipation of internal wave energy. There are numerous factors that impact local internal wave energetics and their energy flux to turbulence (see e.g. McKinnon et al. 2017 for a recent review). Additionally, wind stress curl may efficiently excite near-inertial waves if rotating anticyclonically at frequency close to the Coriolis frequency even at low wind speeds.
I retain my position that the authors should remove the discussion on mixing and nutrient fluxes in a changing climate from the manuscript. The data analysis presented in the study does not allow to draw conclusions on this matter.

Reviewer’s previous comment: As a revision strategy, … Instead, the focus could be shifted to a detailed discussion of an upper-ocean nutrient budget including statistical uncertainties and a comparison to the net community production.

Reply by the authors: The outcome of our paper is the suggestion that climate change might not affect fluxes as strongly as current paradigm suggests. The intention is to inspire discussion/further research. The nutrient budget and comparison to the net community production have been described by Mojica et al. (2016), which we will not repeat in our paper which is more oriented to physics processes than biology. We explained this better now. Our manuscript is an extension of that work.

Reply by the reviewer: I have two issues with the revised version and the response above. First of all, my last suggestion to refer to the publication by Cyr et al. (2015) was misunderstood, because I did not want to point to the results of that particular study but rather their table 1 listing about 20 published studies on vertical nutrient fluxes by turbulent mixing on page 2326. I should have been more detailed with my comment, here. This list also includes several studies in the subtropical and subpolar North Atlantic. In particular, the results by Martin et al. (2010) are of relevance here. Their measurement program was conducted in boreal summer in an area (PAP, 49°N 16°30’W) very close to the transect which data are analyzed here. The study by Martin et al. revealed vertical nutrient fluxes due to turbulent mixing that are very similar to the numbers reported by the authors here. However, additional analysis by Martin et al. showed that these nutrient fluxes account for only about 2% of the nutrient uptake within the euphotic zone. They conclude that other processes must be responsible to supply nitrate to the euphotic zone that are much more relevant than vertical fluxes due to mixing.
In the study by Mojica et al. (2016), no numbers for the calculated vertical turbulent nutrient fluxes are given. This is different from the study here. When comparing these numbers with previous studies, it appears that they are too small to sustain significant production in the euphotic zone. Is it possible that rare intense mixing events relevant for the vertical supply of nutrients were not captured by the measurement program (see e.g. Hummels et al., 2020)? A discussion of this issue needs to be added to the manuscript.

References
Cyr, F., D. Bourgault, P. S. Galbraith, and M. Gosselin (2015), Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada, J. Geophys. Res. Oceans, 120, 2308–2330.
Gregg, M. C., T. B. Sanford, and D. P. Winkel (2003), Reduced mixing from the breaking of internal waves in equatorial waters, Nature, 422, 513–515.
Henyey, F. S., J. Wright, and S. M. Flatte (1986), Energy and action flow through the internal wave field - an eikonal approach, J. Geophys. Res., 91(C7), 8487-8495.
Hibiya T., M. Nagasawa and Y. Niwa (2007) Latitudinal dependence of diapycnal diffusivity in the thermocline observed using a microstructure profiler, Geophys. Res. Lett, 34, L24602.
Hummels, R., M. Dengler, W. Rath, G. Foltz, F. Schütte, T. Fischer and P. Brandt (2020), Surface cooling caused by rare but intense near-inertial wave induced mixing in the tropical Atlantic, Nature Comm., 11, 3829.
Martin, A. P., M. I. Lucas, S. C. Painter, R. Pidcock, H. Prandke, H. Prandke, and M. C. Stinchcombe (2010), The supply of nutrients due to vertical turbulent mixing: A study at the Porcupine Abyssal Plain study site in the northeast Atlantic, Deep Sea Res., Part II, 57(15), 1293–1302.
MacKinnon, J. A., et al. (2017) Climate Process Team on Internal Wave–Driven Ocean Mixing. Bulletin of the American Meteorological Society 98, 2429-2454.

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ED: Reconsider after major revisions (30 Nov 2020) by Ilker Fer

AR by H. van Haren on behalf of the Authors (21 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (01 Jan 2021) by Ilker Fer

Dear Hans,

Happy 2021! Thanks for the revised submission of your manuscript.

I think you have addressed the comments of the reviewers satisfactorily and I do not see the need to send the manuscript out for further comments. I doubt that you and reviewer 3 will converge; however, together with the open discussion and concerns raised, your findings and analysis can be documented in the literature.

I have some other concerns, which must be addressed before the manuscript could be published. I list them below together with some minor issues.

Thank you,

Ilker

1- Description of the Thorpe scale analysis (Section 2.2):

Using Eq(1) in the form of Thorpe 1977 but with the 0.64 factor from Dillon is misleading. In this equation, “d” must be replaced with the Thorpe scale, LT which is the r.m.s. of d over the overturn (or over constant vertical scale of 7 m as you calculated). Although not ideal, it is acceptable to use a fixed vertical averaging scale. Please revise throughout by introducing LT = rms(d), and in line 223, LO/LT = 0.8.

Also because potential density is introduced for analysis (line 211) (and also C_T and S_A earlier), the exact definition of N using in situ density is confusing. Please start the description using sigma_theta from line 204 (and N approximated as sqrt(g/rho0 dsigma_theta/dz) ). Also, because rho changes in space, use partial derivative with z.

2. Vertical scale for N (100 m) versus LT (7 m):

This leads to at least two issues. Firstly, the Thorpe scale analysis is typically based on a background stratification over the overturn scale. There is a mismatch (and inconsistency) between 7 m and 100 m. Secondly, when mixed layer depth is about 30 m (Fig 7a), calculating N over 100 m will always give you unrealistically large stratification in the mixed layer. Your analysis (of epsilon, Krho and turbulent fluxes) in the upper layer (0-15 m averages) will be biased. All green data points in Fig 5 are likely in error. Furthermore, given that MLD is 30 m or so (i.e., > 15 m), by definition N2 = 0 (or at least cannot be resolved by density profiles), and the application of the Osborn approach (for Krho) in the upper 15 m is not allowed since N2 leads to singularity. One approach is to exclude the 0-15 m results from the paper.

In any case I would recommend repeating the analysis using 7 m vertical scale for both N2 and LT. I would also recommend screening the data (excluding epsilon and Krho) over segments when there are very few displacements in a 7-m window, and when N is less than a noise level (hence Krho and turbulent fluxes undefined).

3. Turbulent fluxes (and Fig 8-9): I find the presentation of figures 8-9 confusing. Some data points are excluded without mentioning. Take the vertical gradient value, first red circle data point in panel c. It has no corresponding data in panel a. The next data points near 33N in panel a do not have a corresponding gradient in panel c but have a flux value in (d). Please carefully go through what you plot and ensure the dataset is correctly presented. At latitudes 60-63N blue dots and circles are almost collocated in panel b, hence zero vertical gradient, yet there are substantial gradient values (blue crosses) in panel c. This is all very confusing.

In Fig 9, panel a, the second and third set of data points have the opposite sign gradient (dot and circles are reversed). But their turbulent fluxes are plotted positive. This is erroneous. The authors appear to have ignored the sign of turbulent fluxes (also in other occasions. e.g. DFe at 53N and PO4 at 30N, these must have a gradient sign opposite to other data points.

Minor points:

Li52: delete “near the surface”

Li 54: [average] temperature decreased

Li 111: end the paragraph with a concluding sentence that the sampled dataset is adequate for a discussion on the variability of turbulence, stratification and turbulent nutrient fluxes with latitude.

Li 175: [A]bsolute [S]alinity, S_A (that is capital first letters, and subscript A)… [potential] density anomalies…

Li 231: should be Gregg et al (2018)

Li 238: downgradient turbulent fluxes, with z traditionally defined upward has a minus sign, i.e. -Kz d()/dz.

Li 246: Where does 7x10-5 come from? If it is half of peak-to-peak raw variations of 1.4x10-3, it should be 7x10-4? Note this value (7x10-5) is mentioned again in Fig 3’s caption.

Li 304: dissipation rate[s]

Fig 3: In practice a proper application of Thorpe scale analysis with detection of overturns (even with very high-quality density measurements) cannot resolve epsilon values of 10-11 or less, yet panel c shows such small values. It must be an artifact of using dz=7 m and that 7 m window having a few non-zero d values, resulting in a low epsilon. A data screening excluding segments with few data points can improve this.

Fig 3. Li 748: relaced average over with r.m.s calculated over

Fig 3, 4 and throughout as needed: x-label for N should be log(N) as with other parameters. Also in the caption of Fig 3 (and other captions where log is mentioned) note that it is logarithm base 10.

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AR by H. van Haren on behalf of the Authors (08 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (12 Jan 2021) by Ilker Fer

AR by H. van Haren on behalf of the Authors (14 Jan 2021) Manuscript

Short summary

Changes in ocean temperature may affect vertical density stratification, which may hamper turbulent exchange and thus nutrient availability for phytoplankton growth. To quantify varying physical conditions, we sampled the upper 500 m along 17 ± 5° W between [30, 63]° N in summer. South to north, temperature decreased with stratification while turbulence and nutrient fluxes remained constant, likely due to internal waves breaking and little affected by the physical process of global warming.