A modelling study of eddy-splitting by an island/seamount

Yang, Shengmu; Xing, Jiuxing; Chen, Daoyi; Chen, Shengli

doi:https://doi.org/10.5194/os-13-837-2017

Articles | Volume 13, issue 5

https://doi.org/10.5194/os-13-837-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/os-13-837-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 5

Research article

|

25 Oct 2017

Research article |

| 25 Oct 2017

A modelling study of eddy-splitting by an island/seamount

Shengmu Yang, Jiuxing Xing, Daoyi Chen, and Shengli Chen

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Final revised paper (published on 25 Oct 2017)
Preprint (discussion started on 05 Dec 2016)

Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Reviewer report', Jarle Berntsen, 12 Dec 2016
- AC1: 'Reply to SC1', yang shengmu, 14 Dec 2016
  - RC2: 'Reviewer report', Anonymous Referee #2, 27 Mar 2017
SC2: 'Review of "A modelling study of eddy-splitting by an Island/Seamount"', Liang Sun, 15 Dec 2016
- AC2: 'Reply to SC2', yang shengmu, 19 Dec 2016
RC1: 'Interactive comment on “A modelling study of eddy-splitting by an Island/Seamount” by Shengmu Yang et al.', Youyu Lu, 30 Jan 2017
- AC3: 'Reply to RC1', yang shengmu, 13 Feb 2017

Peer-review completion

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

AR by Yang Shengmu on behalf of the Authors (19 Apr 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 May 2017) by Eric J.M. Delhez

RR by Anonymous Referee #3 (16 May 2017)

Suggestions for revision or reasons for rejection

The manuscript describes a set of numerical simulations to quantify effects of cylindrical islands/seamounts on the beta-drift of surface-intensified eddies over a flat bottom. Apparently, the most striking result is the development of secondary eddies which the authors call “eddy-splitting”. The parameter range of eddy-splitting is characterized by two nondimentional parameters related to the eddy size, the obstacle size and submerged depth. This interesting study can be substantially improved if the authors attempt to clarify physical mechanisms behind the eddy-splitting in relation to previous studies.

Generally, the evolution of rotating stratified fluid near vertical boundaries can be decomposed into three major components: balanced part related to the PV redistribution, internal boundary Kelvin wave, and unbalanced gravity waves (the later is likely unimportant here, e.g., Reznik and Sutyrin, J. Fluid Mech, v.527, p.235, 2005). For better understanding of the vortex interaction with boundaries, It’d be illuminating to show baroclinic PV structure on the beta-plane before the interaction (upper anticyclonic anomaly and lower cyclonic anomaly with maximum at some depth, like in fig. 7, cf. Herbette et al. 2005). The interaction of such dipolar (hetonic) structure with a cylindrical obstacle may result in splitting both cyclonic and anticyclonic PV anomalies and formation of self-propagation structures like illustrated in fig. 8 after the interaction. This process was described using more simple QG model without Kelvin waves (Wang and Dewar, J. Phys. Oceanogr., v.33, p.2446, 2003). Some evidence of Kelvin waves propagating clockwise around the island can be seen in fig. 4 c, d. It’d be useful to show the evolution of both upper and lower temperature fields in more details, e.g., between t = 10 and t = 40 with smaller interval to see how the Kelvin waves trap and transport water. Can the site of the secondary dipole separation from the island be related to the speed of Kelvin wave (which does not depend of the island size) ?

In my opinion, the physical mechanisms of the secondary eddies formation in the case of an island are complicated enough and deserve to be described in a separate manuscript, part I. Effects of additional PV anomaly on the top of submerged seamount could be described in more details in Part 2 comparing with existing dynamical framework (Sutyrin et al., Geophys. Astrophys. Fluid Dyn, v.105, p. 478, 2011).

In summary, the manuscript is not free of errors in logic; alternative explanations are not explored as appropriate; biases, limitations, and assumptions are not clearly stated; previous work and current understanding is not cited and represented correctly; information is not conveyed clearly enough to be understood by the typical reader. Therefore, it needs substantial revision to make it suitable for publishing in the OS.

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RR by Jarle Berntsen (07 Jun 2017)

Suggestions for revision or reasons for rejection

Report on "A modelling study of eddy-splitting by an Island/Seamount"

by

Tang, Xing, Chen, and Chen

General comments

This is the second version of the manuscript that I review.
In my first review, I stated that "The manuscript is very interesting,
and adds to the literature on interactions between eddies and
topography. By introducing two non-dimensional parameters for island size and submergence depth,
they identify no splitting, weak splitting, and splitting regimes. To my knowledge, this is new.
Therefore, the paper should be published."

The authors have improved the quality of the manuscript based on the inputs from the reviewers.
The improvements/changes of the paper are, however, minor. There is for instance one additional Figure and the others are
as before. I still feel that there are issues that they could have addressed 'deeper', and there are still
language issues.

With the model system they have, they could have addressed in more detail the dynamical situation when the
eddy approaches and interacts with island/seamount. In the discussion on page 8, near the end of section 4.2, they refer to
conservation of integrated angular momentum and show eddies after 30 and 50 days in Figure 7.
The scales used are different. Are the strength of the eddies the same? Is angular momentum conserved?
We see two snap-shots, but not enough to see the evolution.
In the Figure text of Figure 9 they say that we see 'Eddy evolution'.
What we see is the temperature after 50 days for island with different
diameter, and not the evolution over time for the specific cases.
They could at least studied in more detail one case, for instance the cases shown in Figs. 6 or 7.

In section 4.2 in the first paragraph they for instance say that 'the eddy looses mass' and that
'separated water is dammed at the downstream of the jet'. These statements could have been followed up
with analysis of the numerical results and figures to support them.

Specific Comments

Page 3 lines 6 and 7: However large > However, large

Page 3 line 13: splitting. But > splitting, but

Page 4 line3: Much previous > Previous

On page 5 I find units given in math mode rather than in usual text mode. Check throughout.

Page 5 line 20: affect model results > affect the model results

Page 10 lines 4 and 5: No need to explain R again.

Page 10 line 7: part of topography > part of the topography

Page 11 line 7: result of numerical > results of the numerical

Page 11 line 23: settings of experiments > settings of the experiments

Page 12 line 19: and combine with > in combination with

Page 12 line 22: eddy interacts > eddy interacting

Page 12 line 24: eddy. Results > eddy. The results

Page 13 line 1 and 2: Please rewrite the first sentence starting 'In short'

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RR by Youyu Lu (15 Jun 2017)

Suggestions for revision or reasons for rejection

Interactive Comment on Revised Submission of os-2016-88 “A modelling study of eddy-splitting by an Island/Seamount” by Shengmu Yang et al.
This revised manuscript has addressed most of my comments on the originally submitted version. I am fine with the scientific content now, but still see the need of improving the English. It should not be the duty of reviewers to correct the wording, so below I will just give a few examples for the authors to consider in revising the manuscript. On the other hand, I will have no issue if the Editor of the Journal sees the English presentation to be acceptable.
1. Abstract: L 8 “in the South China Sea” should be “under the condition of South China Sea”; “MITgcm” should be spelled out; L9 “The speed of” should be “The propagation speed of”. L10 “some eddies may split” should be “under certain conditions the eddy may split”; L12 “sensitivity experiments of …. indicate that” should be “sensitivity experiments with varying island or seamount geometry indicate that”; L16 “seamount depth” should be “seamount submergence depth”;
2. Introduction: There seems to be a lot jumping back and forth in this section between describing general characteristics of eddies and the particular condition in SCS. Can the authors consider re-organizing the sequence of presentation, such that the general characteristics are described first, followed by describing the SCS: which aspects are consistent with the general and which aspects are special? Alternatively, you can start with the first sentence “Eddies are common…”, then in the second sentence “Eddies are frequently observed in the South China Sea”, followed by describing previous studies on eddies in SCS, and one the way pointing out which aspects of SCS eddies are consistent with general characteristics (citing relevant papers), and which aspects are special.
3. P4L7 “abstracted to”?
4. P5L21 “geostrophic balance” was already mentioned at P4L11. Please try to avoid repeating.
5. L7P3-4, seems repeating P6L17.
6. P11L6 “Above all”?
7. P12L12 “by defining two dimensional parameters” is in fact “… the dependence on … can be summarized using two dimensionless parameters”.
8. L13P7, “Y. Liu” either deleting it or changing to “Y. Lu”.
9. Fig.2 caption: insufficient information. Trajectory of 50 days but also the snapshot at day 50? Temperature at what depth? I suggest checking all the figure captions!

Referee Report: PDF

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ED: Reconsider after major revisions (16 Jul 2017) by Eric J.M. Delhez

AR by Yang Shengmu on behalf of the Authors (23 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Aug 2017) by Eric J.M. Delhez

RR by Anonymous Referee #3 (05 Sep 2017)

RR by Jarle Berntsen (06 Sep 2017)

Suggestions for revision or reasons for rejection

General comments

This is the third version of the manuscript that I review.
In my first review, I stated that "The manuscript is very interesting,
and adds to the literature on interactions between eddies and
topography. By introducing two non-dimensional parameters for island size and submergence depth,
they identify no splitting, weak splitting, and splitting regimes. To my knowledge, this is new.
Therefore, the paper should be published."

In the revision process, the quality of the manuscript has improved. Most of the issues raised are
adequately addressed, even if there are aspects that could have been investigated further. This is
always the case. In the last revision, the addition of Figs. 7 and 8
gave further insight in the evolution of the eddy interaction with the island.

I do not think that more iterations on the manuscript will improve the quality significantly.
We are close to 'accept', but a minor revision to check the manuscript carefully may be necessary.

In that process, the following remarks should be considered:

On Figs. 4 and 7: They both show the evolution of
eddy-splitting induced by interaction with an island of 20 km.
Fig 7 give more details from days 20 to 36.
I expected the results after 20 and 30 days, given in both Figs. to be equal.
They are not.
The reason is probably that the initial position/starting point of the
eddies in the two Figs. are different.
This should be clarified if the Figs are kept as is.
Why is a different initial state used for the results in Fig. 7?
I may have missed something here?

In Fig. 9 there are 4 plots. Only a) and b) are mentioned in the caption.
They give PV anomaly at 1000 m depth. In the two lower plots 1000 m depth
is indicated, but I see no anomalies in these plots.
Fig. 9 with caption need to be re-done before publication.

Page 2 line 21: In contrast to with a > In contrast to the case with a

Page 3 line 18: affects coastal ocean > affects the coastal ocean

Page 9 line 11: ones is inner > one is the inner

Page 9 line 11: is outer > is the outer (same also on line 12)

Page 9 line 20: Fig. 11 is referenced here, before Fig. 9 is discussed. get this
in correct order.

Page 17 line 20: Chinese signs are used: Please translate to English.

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ED: Publish subject to minor revisions (Editor review) (13 Sep 2017) by Eric J.M. Delhez

AR by Yang Shengmu on behalf of the Authors (17 Sep 2017) Author's response Manuscript

ED: Publish as is (18 Sep 2017) by Eric J.M. Delhez

AR by Yang Shengmu on behalf of the Authors (22 Sep 2017)

Short summary

Motivated by eddy-splitting near Dongsha island, the eddy's trajectory and the effect of topography on eddy evolution were explored using the MITgcm. Warm eddies propagate to the southwest while cold eddies propagate to the northwest in open oceans. The results of the model indicate that the eddy would split in a qualitative range, and the location of the eddy split-off is related to the island size. In addition, eddy-splitting is an important way to transform energy between different scales.