Cape Verde Frontal Zone in summer 2017: lateral transports of mass, dissolved oxygen and inorganic nutrients

Burgoa, Nadia; Machín, Francisco; Rodríguez-Santana, Ángel; Marrero-Díaz, Ángeles; Álvarez-Salgado, Xosé Antón; Fernández-Castro, Bieito; Gelado-Caballero, María Dolores; Arístegui, Javier

doi:https://doi.org/10.5194/os-17-769-2021

Articles | Volume 17, issue 3

https://doi.org/10.5194/os-17-769-2021

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

https://doi.org/10.5194/os-17-769-2021

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

Articles | Volume 17, issue 3

Research article

|

11 Jun 2021

Research article |

| 11 Jun 2021

Cape Verde Frontal Zone in summer 2017: lateral transports of mass, dissolved oxygen and inorganic nutrients

Nadia Burgoa, Francisco Machín, Ángel Rodríguez-Santana, Ángeles Marrero-Díaz, Xosé Antón Álvarez-Salgado, Bieito Fernández-Castro, María Dolores Gelado-Caballero, and Javier Arístegui

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Final revised paper (published on 11 Jun 2021)
Preprint (discussion started on 05 Nov 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review of "Lateral transports of mass, inorganic nutrients and dissolved oxygen in the Cape Verde Frontal Zone in summer 2017" by Nadia Burgoa et al.', Anonymous Referee #1, 11 Dec 2020
- AC1: 'Reply on RC1', Nadia Burgoa, 31 Dec 2020
  - AC2: 'Reply on AC1', Nadia Burgoa, 31 Dec 2020
RC2: 'Review of "Lateral transports of mass, inorganic nutrients and dissolved oxygen in the Cape Verde Frontal Zone in summer 2017" by Nadia Burgoa et al.', Anonymous Referee #2, 05 Jan 2021
- AC3: 'Reply on RC2', Nadia Burgoa, 08 Feb 2021
RC3: 'Review comment of os-2020-98', Anonymous Referee #3, 12 Jan 2021
- AC5: 'Reply on RC3', Nadia Burgoa, 08 Feb 2021
AC4: 'Reply on RC3', Nadia Burgoa, 08 Feb 2021

Peer-review completion

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

AR by Nadia Burgoa on behalf of the Authors (20 Mar 2021) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Mar 2021) by Piers Chapman

RR by Anonymous Referee #2 (15 Apr 2021)

RR by Josep L. Pelegrí (18 Apr 2021)

Suggestions for revision or reasons for rejection

This is a very much improved version of the original manuscript. I would like to congratulate the authors for the efforts carried out, which have led to this much clearer version. I value very much their data representation and analysis, particularly the inverse model analysis.
Reading this improved version, two main ideas come to my mind, which I point next. I also enclose the pdf file with a number of editing comments and suggestions.
(1) The authors state that their inverse model does not include diapycnal mixing. The authors argue that this is not necessary as the overall mass balance is good and the imbalances for individual layers are not very large. The largest imbalances occur in the surface waters (SW; -0.66 Sv) and central waters (CW; 0.60 Sv), although these values are less than the calculated uncertainties (0.9 and 2.0 Sv, respectively) (Table A2). They ascribe these imbalances to the role of mesoscale eddies that are not properly sampled.
However, in my view, the data suggest something different. The 0.66 Sv gain in the SW fits fairly well with the loss of 0.6 Sv in the CW. In contrast, the intermediate waters (IW) only experience a loss of 0.03 Sv. The implication to me is that there are about 0.6 Sv of water upwelling in the coastal zone.
Considering a coastal upwelling region 600 km and 100 km wide, the imbalance of 0.6 Sv would represent an upwelling (actual transformation of CW into SW) at a mean rate of 10 m/day over the entire region, which is very reasonable.
In table A3, the authors provide additional information on the CW imbalance split into the subtropical and tropical domains. They find that most of the imbalance takes place in the subtropics (0.48 Sv as compared with 0.12 Sv in the tropics). This is again consistent with our knowledge that upwelling off NW Africa is higher in the subtropical than in the tropical waters.
One possible way to corroborate these ideas is by repeating the inverse model analysis but closing the box with the eastern section rather than with the coastline. If the above argument is correct, you will find that the imbalances in the SW and CW layers decrease substantially.
(2) The mass and property imbalances in the CW and SW are used to draw conclusions about net primary production in SW and net remineralization in CW. This is a very important result, probably among the most important ones in the manuscript. However, the high water mass imbalances in the SW and CW leaves the reader thinking how much of the oxygen and inorganic nutrients imbalances are related to physics (water-mass imbalance) and how much is related to biogeochemical processes (mostly primary production and remineralization). This is particularly troublesome as some of the imbalances have uncertainties that are larger than the estimated values.
I believe that the inverse model analysis should be carried out again considering the point (1) above. You can do something simple such as setting the imbalance in the SW as and additional water-mass transfer from the CW to the SW (upwelling transfer). You will likely find that the physics of the model does much better, even with lower a priori uncertainties so that your error bars are smaller. This is essentially equivalent to having 0.6 Sv of CW (and associated biogeochemical properties) being transferred from the central to the surface layers. In my opinion, only after you do this is that your results will be meaningful.
One additional step forward would be to split this upwelling transfer among the subtropical (0.48 Sv) and tropical (0.12 Sv) domains. In this way you will be able to discuss your results in terms of what happens in the subtropics and what happens in the tropics. For example, I would expect that primary production and remineralization is much greater in the tropics than in the subtropical but right now you are finding precisely the opposite.
Some additional minor comments and suggestions:
(3) In the enclosed pdf I suggest some modifications for the abstract, plus a number of editing suggestions.
(4) p. 5, l. 5: “to validate the temperature interpolated to the XBT positions and set the signal to noise ratio...” I agree with the signal-to-noise ratio but you do not use interpolations to validate a measurement but the other way round.
(5) p. 5, l. 8: 10%? Is this correct, 10% and even 5% is far too high: 5% of 20C is 1C, which is very large.
(6) p. 5, l. 13-14, please clarify.
(7) p. 6, l. 20: I’m not sure about how the journal handles citations to manuscripts in preparation but I would say this is highly irregular. I suggest simply to state “(not shown)”.
(8) p. 7, l. 2-3: “to avoid any issues related with the temporal evolution of structures, the volume is closed with land instead of with the eastern transect”. Please clarify.
(9) p. 9, l. 27: explain this beforehand, in section 2.2.
(10) p. 10, l. 14-15 and elsewhere (including tables): I suggest not mentioning the results for DW. You are only sampling a tiny fraction of these DWs, what is the sense of providing these values?
(11) P. 12, l. 12: Luyten et al. (1983) did not document this shadow zone, they provided a theory that explains its existence. You may rather for example cite Kawase and Sarmiento (JGR, 1985).
(12) Fig. A1: I suggest you show the grid every 1 degree.
(13) Fig. A11a: I suggest to separate SW from CW and remove the DW results.
(14) Fig. A15: perhaps you are missing an axis for phosphate transports?
(15) I suggest adding an additional figure like A15 but for the net values, both within the original domain and within the domain that excludes the upwelling region.

Referee Report: PDF

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RR by Anonymous Referee #3 (19 Apr 2021)

ED: Publish subject to minor revisions (review by editor) (27 Apr 2021) by Piers Chapman

AR by Nadia Burgoa on behalf of the Authors (10 May 2021) Author's response Manuscript

ED: Publish as is (14 May 2021) by Piers Chapman

AR by Nadia Burgoa on behalf of the Authors (14 May 2021)

Short summary

The circulation patterns in the confluence of the North Atlantic subtropical and tropical gyres delimited by the Cape Verde Front were examined during a field cruise in summer 2017. The collected hydrographic data, O₂ and inorganic nutrients along the perimeter of a closed box embracing the Cape Verde Frontal Zone allowed for the independent estimation of the transport of these properties.