Why did deep convection persist over four consecutive winters (2015–2018) southeast of Cape Farewell?

Zunino, Patricia; Mercier, Herlé; Thierry, Virginie

doi:https://doi.org/10.5194/os-16-99-2020

Articles | Volume 16, issue 1

https://doi.org/10.5194/os-16-99-2020

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

https://doi.org/10.5194/os-16-99-2020

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

Articles | Volume 16, issue 1

Research article

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20 Jan 2020

Research article | Highlight paper |

| 20 Jan 2020

Why did deep convection persist over four consecutive winters (2015–2018) southeast of Cape Farewell?

Patricia Zunino, Herlé Mercier, and Virginie Thierry

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Final revised paper (published on 20 Jan 2020)
Supplement to the final revised paper
Preprint (discussion started on 17 May 2019)
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Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review of Two superimposed cold and fresh anomalies enhanced Irminger Sea deep convection in 2016-2018', Femke de Jong, 21 Jun 2019
- AC1: 'Authors Reponse to Referee 1', Patricia ZUNINO, 07 Oct 2019
RC2: 'Review of: “Two superimposed cold and fresh anomalies enhanced Irminger Sea deep convection in 2016-2018” by Zunino, Mercier and Thierry', Anonymous Referee #2, 25 Jun 2019
- AC2: 'Authors Reponse to Referee 2', Patricia ZUNINO, 07 Oct 2019
RC3: 'Please see the attached document CommentsToAuthors.pdf', Anonymous Referee #3, 26 Jun 2019
- AC3: 'Authors Reponse to Referee 3', Patricia ZUNINO, 07 Oct 2019

Peer-review completion

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

AR by Patricia ZUNINO on behalf of the Authors (14 Oct 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Oct 2019) by Neil Wells

RR by Femke de Jong (07 Nov 2019)

Suggestions for revision or reasons for rejection

The authors have tried to address the main comments on their method, previously using a density criterion only to determine a mixed layer and now considering temperature and salinity as well. Their current criterion is still rather wide. The authors state that “The temperature threshold of 0.1°C and the salinity threshold of 0.012 were selected because they correspond to a threshold of 0.01 kg m-3 in density that was previously shown to perform well in the subpolar gyre (Piron et al., 2016)”. This is only true if either salinity or temperature is at this threshold, if both are the density difference can be as big as 0.2 kg/m^3.

The authors attempt to compare their method with earlier publications, however the comparison is made incorrectly stating “Following de Jong et al. method’s, three MLD were defined as the depths were the standard deviations were smaller than 0.05 kg m-3 , 0.05°C and 0.005 for density, temperature and salinity, respectively”, the error being that the standard deviations were not used as a threshold to determine the bottom of the MLD but as a final check leading to discarding misidentified MLDs (as described in the paper). Since this comparison now features in the supplementary materials it will need to be corrected.

The present manuscript mostly compares the identified MLDs to those we identified in the winter of 2014-2015 (de Jong et al., 2018). For those MLD we used a criteria of “0.015°C for temperature, 0.005 for salinity, and 0.0025 kg m–3 for potential density” “The resulting MLDs of all three variables had to be within 50 dbar to be accepted, after which the temperature-derived MLDs were chosen as final MLDs.” Running this criteria on the specific float profiles shown in Figure 2 of the manuscript results in the deepest MLD (of 570 dbar) observed in the profile of float 5904772, cycle 33. For the other profile shown in the Figure 2, and the supplementary material, our methods gives a MLD of only 150 dbar. We made the criteria this strict to sort out actively mixed profiles at the moorings for recently mixed profiles which may have been formed elsewhere and advected to the moorings. Since the authors are looking more of a region than at one specific spot they could make an argument that they can use a wider criterion. In that case I ask that that at least the SI is corrected to show our methods accurately and the difference with earlier estimates be discussed more clearly in the discussion.

In the second paragraph of the results section there is a discussion on the percentage of floats that had deep MLD. This is not very informative, as 1 float with 1 deep ML in the winter of 2017 would give the same percentage as 1 float recording 20 deep MLs in 2015. The percentage of profiles with deep MLs out of all recorded profiles would give a much better indication of the intensity of convection. Overall, this difference in intensity between the winters is mostly neglected in the text ,focusing only on depth (for example lines 364-366 and in the conclusions line 461-467). This is a missed opportunity since the authors have already investigated the likely cause for this difference in intensity, being weaker surface fluxes in the winters after 2014-2015.

Line by line comments
Line 50: the “exceptional” here is disputable because we don’t have a long enough record to say whether it is really that unusual and because there is such a distinct difference in intensity over the four winters.

Line 156: The use of Q and Q3 as acronyms for parameters that are not associated is confusing, especially if they occur in subsequent sentences. Consider using a different acronym for Q3. How useful is the third quartile if there are only three profiles with deep MLDs in a winter?

Line 199: The Monte Carlo method does not address the bias in a reanalysis product as a whole, as observed by Josey et al.

Line 259: “Despite Bsurf* is mainly explained by Q, the accumulated FWF* amounts to ~10 % of the accumulated Q with opposite sign. The air-sea buoyancy flux is 10% lower on average than the air-sea heat flux.” This seems to be saying the same thing twice. Consider rephrasing.

Lines 330-340: The discussion of anomalies here would be more interesting/easier to follow if it was immediately linked to local convection and advection pattern as described in lines 393-397.

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ED: Publish subject to minor revisions (review by editor) (11 Nov 2019) by Neil Wells

AR by Patricia ZUNINO on behalf of the Authors (20 Nov 2019) Author's response Manuscript

ED: Publish as is (02 Dec 2019) by Neil Wells

Short summary

The region south of Cape Farewell (SCF) is recognized as a deep convection site. Convection deeper than 1300 m occurred SCF in 2015 and persisted during three additional winters. Extreme air–sea buoyancy fluxes caused the 2015 event. For the following winters, air–sea fluxes were close to the climatological average, but local cooling above 800 m and the advection below 1200 m of a fresh anomaly from the Labrador Sea decreased stratification and allowed for the persistence of deep convection.