The connection of the Indonesian Throughflow, South Indian Ocean  Countercurrent and the Leeuwin Current

Lambert, Erwin; Le Bars, Dewi; de Ruijter, Wilhelmus P. M.

doi:https://doi.org/10.5194/os-12-771-2016

Articles | Volume 12, issue 3

https://doi.org/10.5194/os-12-771-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/os-12-771-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 3

Research article

|

02 Jun 2016

Research article |

| 02 Jun 2016

The connection of the Indonesian Throughflow, South Indian Ocean Countercurrent and the Leeuwin Current

Erwin Lambert, Dewi Le Bars, and Wilhelmus P. M. de Ruijter

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Final revised paper (published on 02 Jun 2016)
Preprint (discussion started on 25 Sep 2015)

Interactive discussion

Status: closed

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

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RC C956: 'Review of a manuscript entiteled ``The Dynamic Connection of the Indonesian Throughflow, South Indian Ocean Countercurrent and the Leeuwin Current'' by Lambert et al.', Anonymous Referee #1, 21 Oct 2015
- AC C1017: 'Response to first review', E. Lambert, 26 Oct 2015
  - RC C1072: 'Re. the SICC is not a Parsons jet:', Anonymous Referee #1, 30 Oct 2015
RC C1070: 'The Dynamic Connection of the Indonesian Throughflow, South Indian Ocean Countercurrent and the Leeuwin Current', Anonymous Referee #2, 30 Oct 2015

Peer-review completion

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

AR by E. Lambert on behalf of the Authors (29 Jan 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Feb 2016) by Matthew Hecht

RR by Anonymous Referee #1 (25 Feb 2016)

Suggestions for revision or reasons for rejection

Summary and Recommendation:

The authors have improved on the text and description of the solutions, however, my main concerns regarding the 2-layer experiments and their interpretation remain. On the other hand, this is an interesting and important topic, and I believe that the authors’ work can be published after major revisions.

General Comments:

1) The authors interpret the different positions of the front in the western basins in their 2-layer model solutions as being the result of local processes, i.e., inertial terms vs. viscosity. In my view, this interpretation may be correct, however, the experiments are not sufficiently well designed (or at least the analysis is not detailed enough) to proof that. As discussed in the manuscript and the authors’ response, there are several competing mechanisms which can determine the position of the front. These mechanisms depend on several parameters including the eastern boundary layer thickness at the island, the magnitude of the jump across the front, and the mechanism allowing for the frontal current (viscosity vs inertial). Ideally, only the latter would change to proof the authors point, but this is not the case. Since the authors want keep their current experiments, i suggest that they either add an in-depth analysis of the experiments (e.g., they could compare their numerical solutions to inviscid, analytical solutions, calculate position of fronts as in Dewar, 1991), or they could reformulate their text to indicate that their described dynamical picture is a valid hypothesis, but acknowledge that their solutions may be interpreted in different ways.

Detailed Comments:

1) Lines 148-156: I agree that there is still much to be learned about the LC system. One common conclusions of the studies mentioned in this paragraph appears to be, however, that the continental slope is an essential model component in order to simulate a realistic LC. Specifically, when a model (OGCM or 2-layer model) is forced with a meridional surface density gradient, it does not generate a barotropic eastern boundary current without a continental slope. This is a bit problematic given that the 2-layer model configuration used here has a flat bottom and vertical walls. Please discuss in more detail why you think you do not need to consider the effects of a continental slope.

2) Line 193-194: ``And why are poleward eastern boundary currents like the LC not found in other basins?’’ If this is really a central question that you want to address in your paper, you should discuss this issue in more detail, and perhaps attempt an answer in the discussion section. There are, in fact, other poleward eastern boundary currents, e.g., the Slope Current in the North Atlantic and the Iberian Poleward Current. It has been suggested that whether the eastern boundary current is directed pole- or equatorwards depends on the relative strength of the offshore meridional surface density gradient and the coastal winds (e.g., Godfrey and Weaver, 1991: Is the Leeuwin Current driven by Pacific heating and winds? Progress in Oceanography). If you have an alternative explanation, it would be useful to contrast the two ideas.

3) Lines 267-281: Perhaps you could give the range of w that results from this buoyancy forcing and also the range of Ekman pumping velocities, to give the reader an idea of the relative strength of these two forcings? Since you have eliminated the equations for the buoyancy forcing and definition of the constants involved, it might also be helpful to convert the colorbar in Figure 3a into velocity units.

4) Lines 288-296: For the model to equilibrate, it is necessary that the mass transfer w between the layers integrated over the domain is zero. I understand that in your 2-layer model, the model reduces to a 1-layer system in the outcrop region (there are different ways to deal with such an outcrop; please indicate clearly how that occurs in your model version). Thus, it appears (and you also stated that in the original version of the manuscript) that, in case of an outcrop, the w no longer balances over the domain, and the model cannot reach a steady state. Given the structure of the buoyancy forcing and the size and position of the outcrop, this imbalance may be substantial. So, I think you need to back up the claim that the model is close to equilibrium with actual numbers. How does the total upper layer volume change between model years 45-55?
It also appears as if the layer-1 volume in the solution shown in Fig. 6b was much less than in Fig. 7. When all solutions were started from the same state of rest, that suggests that the volume change is substantial. Please discuss the effect of different layer-1 volume on the key properties of your solutions.

5) Lines 296-298: A statistical steady-state is obtained when the model equilibrates, and not by averaging. I think you want to say that you analyze the model variables averaged over years 45 - 55.

6) Line 475: Change ``OCean’’ to ``Ocean’’

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RR by Anonymous Referee #3 (22 Mar 2016)

ED: Publish subject to minor revisions (Editor review) (28 Mar 2016) by Matthew Hecht

AR by E. Lambert on behalf of the Authors (15 Apr 2016) Author's response Manuscript

ED: Publish subject to technical corrections (05 May 2016) by Matthew Hecht

AR by E. Lambert on behalf of the Authors (10 May 2016) Manuscript

Short summary

In the upper layers of the South Indian Ocean, three anomalous currents are observed: The South Indian Ocean Countercurrent, the Leeuwin Current and the Indonesian Throughflow. Using two models of different complexity, we show how these three currents can be dynamically connected, unravelling the large-scale connectivity. Further, we produce a conceptual theory of how this current system follows an idealized pattern of wind stress and buoyancy forcing.