Internal tides in the Solomon Sea in contrasted ENSO conditions

Tchilibou, Michel; Gourdeau, Lionel; Lyard, Florent; Morrow, Rosemary; Koch Larrouy, Ariane; Allain, Damien; Djath, Bughsin

doi:https://doi.org/10.5194/os-16-615-2020

Articles | Volume 16, issue 3

https://doi.org/10.5194/os-16-615-2020

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

https://doi.org/10.5194/os-16-615-2020

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

Articles | Volume 16, issue 3

Research article

|

15 May 2020

Research article |

| 15 May 2020

Internal tides in the Solomon Sea in contrasted ENSO conditions

Michel Tchilibou, Lionel Gourdeau, Florent Lyard, Rosemary Morrow, Ariane Koch Larrouy, Damien Allain, and Bughsin Djath

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Final revised paper (published on 15 May 2020)
Preprint (discussion started on 26 Sep 2019)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'os-2019-94', Anonymous Referee #1, 19 Oct 2019
- AC1: 'In attached file is the reply to reviewer 1', lionel gourdeau, 24 Dec 2019
RC2: 'Review of "Internal tides in the Solomon Sea in contrasted ENSO conditions" (Tchilibou et al)', William Kessler, 12 Nov 2019
- AC2: 'In attached file is the reply to reviewer 2', lionel gourdeau, 24 Dec 2019

Peer-review completion

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

AR by lionel gourdeau on behalf of the Authors (24 Dec 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Jan 2020) by Erik van Sebille

RR by Anonymous Referee #1 (24 Jan 2020)

Suggestions for revision or reasons for rejection

os-2019-94

Revision

The paper documents M2 internal tides amidst the Solomon Sea circulation from models with and without tides in different ENSO states and is worthy of eventual publication. The internal tides appear sensitive to stratification and mesoscale activity, which vary over one El Niño and one La Niña. Water mass transformation is greatest at the straits in the Solomon Sea, where the internal tides are generated. In my earlier review, I indicated the manuscript had not addressed 2 major points: (1) mechanisms for internal wave variability and (2) model-data comparison. The same is true of the current version.

(1) The changes are described well between ENSO states, but the reason behind them is asserted to be due to mesoscale eddies or vertical stratification. The authors are likely correct, but nothing is proven. The relevant terms could be calculated from the model results, but the authors contend this is beyond the scope of this work. The path by which baroclinic energy changes is not identified despite having a model available and all terms available to diagnose these changes. Is stratification affecting p’ at the generation sites, mesoscale currents refracting internal waves along their propagation paths, or something else? If the authors are unwilling to determine what factors are affecting the internal waves, then they should clearly identify in the text what is proven and what is speculation or coincidence: ie, Stratification and mesoscale eddy activity change with ENSO, internal tides change with ENSO, and due to this coincidence we speculate these ENSO-related changes affect the internal waves.

(2) The realism of the model’s internal tides are also far from proven in the manuscript. Some relatively simple comparisons would help. Internal tide values produced in the model are not compared to readily available observations from Zhao e.g., https://doi.org/10.1029/2018JC014475). Maybe just comparison of mean internal tide energy levels or averaged energy density at the relatively coarse resolution of the altimetric internal tides in the Solomon Sea. In their response, the authors provided some comparison between a mooring, the Honiara tide gauge, and their model. This comparison would also be worthwhile to include to improve confidence in the model. Amplitude and phase of the modelled and observed M2 signal for the mooring and tide gauge could be compared in a table or pretty quickly in the text. Also the manuscript asserts, “Our modeling results show that the diapycnal mixing induced by the internal tides is particularly useful to erode the salinity maximum of the upper thermocline water,” but I do not see any estimate of an eddy diffusivity (K) or comparison of model K to observed fine-scale K parameterisations (Alberty, 2017) or previously modelled K (Melet 2011).

My minor comments from the earlier review were mostly addressed.

Some further small comments by line number:

- σθ has units

94 - Thorpe scale is capitalized and there are several other capitalization mistakes throughout the manuscript

97 - 4.1-23 10-8 [W kg-1] - no brackets needed for units

252 - ∇hH

265 - ∇h is the horizontal gradient (∂x, ∂y) operator. (∇h ·) is the divergence operator

Fig 5 - label x axis with units

Many figures- The repetitive references to the topography are overkill and can be placed instead in the methods, acknowledgements and/or cited in the references or at least shortened considerably- Bathymetry as in Fig. 1. SI convention should be used, e.g., W[space]m-2 and other units too. Also it’s good practice to label the colour bar with EKE or whatever is plotted and not just label with units. I realize the editor would like the bathymetry reference but I have never encountered this with any other journal. Sometimes the bathymetry part is longer than the actual caption. By similar logic, anything involving the CARS database must also be similarly referenced.

441 - “higher dissipative modes” - no evidence to support them being dissipative. How about just higher modes?

Fig 9 and 10 - some spurious vectors in the baroclinic flux and illegible arrows in the barotropic flux. Consider scaling or averaging over some number of adjacent cells.

Table 1 has inconsistent notation in the headings.

588 - 0.06 psu

628 - What is the value of Qnet? What fraction of the incoming solar radiation is it?

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RR by William Kessler (05 Feb 2020)

Suggestions for revision or reasons for rejection

2nd review of Tchilibou et al ("Internal tide in the Solomon Sea ...")

Overall comment:
The revision is much improved. The authors' responses are comprehensive and convincing. I am happy to see that the review process prompted the authors to go over their calculations and identify an error (re Fig.14). Their rewrite of section 5 is excellent and clear. The figures are much better. In my opinion, the paper is now very close to acceptance. I have only minor suggestions for the authors to consider as they prepare their final version.

Specific comments/suggestions:

Re the authors' response:

Fig.1 of the response to R1 shows Honiara tides, but is labeled 2S,147E. Honiara is actually at 10S,160E. Just a mislabeling?

Figs.3-4 of the response to R2 are very interesting. Is a figure like 3 published anywhere? (no reference is given where the result is stated on L392)

Minor comments:

L29: 'where' is unclear. As written it appears to refer to the equator, but I think you mean the Solomon Sea. How about:
'Intense equatorward LLWBCs transit the Solomon Sea, where active ...'

L30: 'constraints' is not the right word here. Also, is this 'dynamical'? How about '... the mixing induced by these features can play a role...'

L36 and elsewhere: Should say 'Vertical (modes)'. Especially in the abstract, the reader may not know what kind of mode is referred to.

L44: 'far from the strong currents' => 'in quieter regions'

L50-51: The last sentence is unclear. I think you are trying to say that the local circulation is a stronger influence on the tides than the stratification, right? How about:
'dominant role of local circulation changes, which have a larger effect than the stratification on the tides'

L73: I'm a bit embarrassed to pitch my own work, but in a sentence listing work describing the circulation of the Solomon Sea, probably Kessler, Hristova and Davis (2019, Prog.Oc.) should be listed. Perhaps especially on L714-715 where the issue of the merging of the NVJ/SEC into the NGCU is a major topic of that paper.

L290: 'We verified' => 'Inspection of the model velocity and temperature showed that...'.

L298-302: This sentence is unclear. It raises several questions:
- Why does the bathymetric control at Vitiaz have a stronger effect during El Nino? I thought the effect was that it limited the amount of water that could be pushed through Vitiaz, thus the El Nino flow increase through Vitiaz is not as large as it would be if the strait was wider. That does not seem to be 'a stronger effect'.
- Although the Melet result seems correct, it was one experiment with one model. I keep hoping someone will try this with another model, or at finer resolution. In any case it would be worth noting this: 'Some model experiments suggest ...'. We have no observational confirmation of this.

L607-8: The difference between CARS and either model run east of the Solomon Sea is more than a shift in latitude (Fig.12). It is described as a bias on L602, which seems more accurate.
- Is it timing? CARS is an average over decades, right? Thus the difference might be temporal, as suggested by the CORA05 comparison.
This is not crucial to the paper, but it is unfortunate.
I especially wonder about the S-max just south of New Britain (6.5S,151E) in CARS, that does not appear in either model solution (Fig.12).

L639: How was the transit time determined? That would be an interesting calculation to do.

L643: 'within' => 'averaged over' (isn't the point here that averaging over the different impacts east and west gives a small value because those have opposite signals?)

Fig.10: Perhaps use the same scale for the two modes so the difference would be obvious. There is no reason that the details of mode 2 need to be very visible. At least note this in the caption.

Fig.12 needs a vector scale. It looks like the vectors are the same for both panels a and b But Fig.14 suggests they are substantially different. Please check.

Fig.14: This is a very informative figure. It looks like the temperatures are differences, but the velocity vectors are totals. The caption should say this.
Is there a way to say or show how large the total temperature changes are between El Nino and La Nina? That would give context to the values shown here. If you don't want to add two more panels, perhaps just say something like 'While the SW temperature differences between Tides and NoTides are about 0.5C, the changes between El Nino and La Nina are about X', and are concentrated in the region Y'.

Minor notes of English usage and etc (the English is generally very good):

L30: cumulatED => cumulatIVE

L39 and many other places: Don't use season names (fall, summer) which are ambiguous (especially since this is the southern hemisphere!). Does OS allow, e.g., JFM, etc? That would be clearer.

L40: 'complexity OF predicting'

L106: 'some' => 'a few' (goes better with 'specific locations')

L110: 'A lot' is too colloquial (familier). => 'Several studies have focused ...'

L227: 'we will use the vertical mode decomposition (3) to define...'

L243: The verb 'resume' in English means 'reprendre'. => 'The equations can then be written'

L288-289: 'summer', 'fall' again. (Also L665).

L371: Is it necessary to use the abbreviation 'SSI'? I had to go back to find out what this meant. It seems to be used infrequently enough that it could just be written out.

L445: 'drastic' is a very strong word. How about 'substantially'?

L592: 'closeR' (need the 'r' since this is a comparison)

L594: 'discrepancy' is not the right word; it implies that something is wrong. => 'difference'

L611: '..., but inside the Sea only long-term comparisons are possible'

L612: 'on' => 'in'

L627: 'that in turn affects' (not 'return')

L630: omit 'of'

L664: 'is under the influence of ENSO' => 'is influenced by ENSO'

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ED: Publish subject to minor revisions (review by editor) (05 Feb 2020) by Erik van Sebille

AR by lionel gourdeau on behalf of the Authors (31 Mar 2020) Author's response Manuscript

ED: Publish as is (02 Apr 2020) by Erik van Sebille

AR by lionel gourdeau on behalf of the Authors (06 Apr 2020) Manuscript

Short summary

This paper focuses on internal tides in the marginal Solomon Sea where LLWBCs transit. The objective is to characterize such internal tides and to give some insights into their impacts on water mass transformation in this area of interest for the global circulation. Results are discussed for two contrasted ENSO conditions with different mesoscale activity and stratification. Such study is motivated by the next altimetric SWOT mission that will be able to observe such phenomena.