Multidecadal preconditioning of the Maud Rise polynya region

van Westen, René M.; Dijkstra, Henk A.

doi:https://doi.org/10.5194/os-16-1443-2020

Articles | Volume 16, issue 6

https://doi.org/10.5194/os-16-1443-2020

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

https://doi.org/10.5194/os-16-1443-2020

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

Articles | Volume 16, issue 6

Research article

|

23 Nov 2020

Research article |

| 23 Nov 2020

Multidecadal preconditioning of the Maud Rise polynya region

René M. van Westen and Henk A. Dijkstra

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Final revised paper (published on 23 Nov 2020)
Preprint (discussion started on 16 Apr 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review #1', Anonymous Referee #1, 22 May 2020
- AC1: 'Point-by-point reply to reviewer 1', René van Westen, 06 Jul 2020
RC2: 'Reviewer decision: Accepted with Major Revisions.', Anonymous Referee #2, 04 Jun 2020
- AC2: 'Point-by-point reply to reviewer 2', René van Westen, 06 Jul 2020

Peer-review completion

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

AR by René van Westen on behalf of the Authors (12 Aug 2020) Manuscript

ED: Referee Nomination & Report Request started (20 Aug 2020) by Mario Hoppema

RR by Anonymous Referee #1 (10 Sep 2020)

Suggestions for revision or reasons for rejection

I am overall satisfied with the response to my comments. I have few additional comments which are listed below. Most are minor, but the first one is rather important.

- CESM first EOF capture 4.1% of variance. This is a very small percentage. First it raises questions about whether it is significant or not. North (1992) (if I recall correctly) provides a method to evaluate the significance of EOFs.
Are these others EOFs well separated from the first one?

Secondly, I'm surprised that the authors gloss over this aspect. There is one remark that the CESM 1st EOF explains less variance than that of POP (which is already quite small).
Are we looking at noise?

This deserves more discussion. I expect that most readers will pause there. To reinforce the paper and its interest, this aspect would need to be addressed.

- line 23, page 1: "a negative freshwater flux" it doesn't mean much without being clear about the sign convention for the fluxes. Maybe use a more explicit term such "net evaporation"

- line 20-23, page 2: "Analysis ,,, (Juling et al, 2018)". Strange sentence. Responsible for what?

- line 19-20, page 5 : "The SOM is a large-scale intrinsic ocean mode in the Southern Ocean (Le Bars et al., 2016) and was first found in a stand-alone ocean simulation of the POP (the ocean component of the CESM)."

The first part of the sentence make it sound like the SOM is an established observed mode.
Please rephrase to make it clear that has only been seen in models at this point (well one model really, POP).

- line 6-7, page 12: "The individual particles are hard to distinguish in Figure 7b, therefore we present the final distributions of the particles which are initially released at 200 m, 500 m and 1000 m depths in the Polynya region, are shown in Figure 8a, 8c and 8e, respectively."
Something strange, please rephrase.

- Line 11, page 17: "and eventually they reach" remove they

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RR by Anonymous Referee #2 (29 Sep 2020)

Suggestions for revision or reasons for rejection

Review of the manuscript:
Multidecadal Preconditioning of the Maud Rise Polynya Region
By Authors
René M. van Westen and Henk A. Dijkstra

Reviewer Decision: Acceptable after minor revision.

I am happy to see that the authors have addressed most of my mentioned concerns. However, the revisions bring into light some new concerns and I hope the authors can address them.

1) Has either SOM or SOM* ever been observed in nature? or any model other than CESM/POP? If not then explicitly state it and appropriately reduce the relevance throughout the manuscript. The authors on multiple occasions throughout the manuscript over sell this idea of SOM and SOM* but that is misleading since as far as we know this is a CESM/POP simulation artifact only captured in 4 CESM simulations so far. My main concern here is that the authors need to be upfront about the hypothetical nature of this study.
2) Lines 37-40 in Section 3: I’m really not convinced about your reasons for choosing different area for calculating SOM* index. The new area is (a) significantly smaller than the originally proposed area, and (b) is away from the region where EOF pattern has maxima/minima. Just because the correlation between SOM* index and Weddell Gyre strength is greater, that alone doesn't justify your choice.
3) The authors also need to make it clear that the fact that subsurface heat accumulation is useful in Weddell Sea Polynya formation in model studies, it has not been validated in any observational studies as a pre-requisite for Maud Rise Polynya formation.
4) Line 31 in Section 3: Why is there the phase difference between PC1 and SOM/SOM* index ? Is it significant ? Would it affect the proposed causal relationship with the Weddell Gyre strength?
5) Line 34 in Section 3: what is the correlation coefficient between PC1 and WG strength? for both models?
6) Lines 48-50 in Section 3: Despite the 60-month smoothing the percentage of variance explained is small implying a large part of regions variability is dominated by spatial noise. Correct?
7) Line 68 in Section 3: how long would a high-resolution CESM simulation need to be to have SOM? more than 100 years? Recent high-resolution model simulations that best reproduce the Maud Rise Polynya (Kurtakoti et al. 2018; Kaufman et al. 2020) show no evidence of SOM or SOM* in their ~130 years long CESM simulation. In fact, there is no periodicity in the subsurface heat accumulation in the Weddell Sea.
8) There is a line count reset in Section 3 after Fig. 3. Please fix that.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (30 Sep 2020) by Mario Hoppema

AR by René van Westen on behalf of the Authors (06 Oct 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (15 Oct 2020) by Mario Hoppema

Dear Drs. Van Westen and Dijkstra,

Thank you for the revised version of your manuscript. I am satisfied with your response to the referees, and your manuscript is almost ready for publication. Please consider the list of my final comments below and prepare the final version of your manuscript taking into account these comments.

P1, L19 „convection below the region of the MRP” I think it is not clear what is meant with this. Please modify the text.

P3, L9-10 “start equilibrating after about 150 years.” What is the criterion for starting to equilibrate? And what is the criterion for accepting the increase in the radiative imbalance?

P3, L9-10 “The radiative imbalance is only slightly positive over the last 100 years of the simulation.” I think you mean here that it is slightly increasing, right? It is positive all the time.

P3, L11-12 “relatively small trends (compared to the first part of the simulation) occur over the last 100 years which can easily be removed through a quadratic detrending.” The figure shows that the trends between years 80 and 150 are exactly as large as those in the last 100 years.

P3, L17 “reasonably agrees with observations” How is reasonably defined here? Please be more quantitative.

P5, L16 e.g. does not fit here, probably better: i.e.
P8, L4 … a SOM period of about 40 – 50 years (add: SOM)

P8, L4 Please be more quantitative about “similar”. What is similar here?
P8, L19 delete: below (superfluous)

P8, L24 Change the second “sea-ice fractions” to values: The September sea-ice fraction (blue curve in Figure 4a) shows relatively low values with respect to …

P10, L10 As demonstrated in section 4.1, both … (use section number)
P10, L16 … by 80 months. (why give the imprecise number)

P11, L8 OceanParcels
P13, L12-14 This sentence is not clear to me. Please rephrase and make clear.

P16, L7 I suggest to emphasize the real years here as the transition from model years to real years comes unexpected to the reader: Note that observations show that prior to the MRP of the year 2016-2017, no …

P16, L14 “where 1 Sv Psu is about 109 g of salt per second” The salinity can be expressed on the practical salinity scale, where salinity is dimensionless (psu is sometimes used but is not an existing unit). How is g salt calculated from this?

P16, L22 … over the Polynya region which are also … (which instead of and)

P16, L32-34 “The potential density at 900 m depth for each September is shown in Figure 10b and also shows Taylor columns over Maud Rise during other years.” This sentence is grammatically not correct.

P16, L34 “stratification at 900 m depth” Stratification at a certain depth does not exist, right? It is always along a depth range. Please use correct terminology.

P17, L9 delete: In this paper, (superfluous)
P17, L9 … from a 250 years control simulation … (2 ½ centuries is not multi-century; multi = many)

P17, L12 “broadly in agreement” What does this mean. Please explain where the agreement lies and where not.

P20, L12 Please add country
P20, L11-13 Please indicate what this is: a technical report, etc. and access date.
P20, L23 Add pages
P20, L25 Reference is incomplete

Fig 9 panel (a): Near the color scale is the term heat advection. This is misleading, as with advection one would expect a different unit than TW. If this concerns heat indeed (and thus TW), the term should be changed: advective heat
Similar to panel 9 (b) and panels 9 (c) and 9 (d)

With best wishes
Mario Hoppema

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AR by René van Westen on behalf of the Authors (16 Oct 2020) Author's response Manuscript

ED: Publish as is (16 Oct 2020) by Mario Hoppema

AR by René van Westen on behalf of the Authors (16 Oct 2020)

Short summary

During the mid-1970s and quite recently in 2017, a large open-water area appeared in the Antarctic sea-ice pack, the so-called Maud Rise polynya. From several model studies, the reoccurrence time of this polynya seems arbitrary. In this study, we address the reoccurrence time of the polynya using a high-resolution climate model. We find a preferred multidecadal return time in polynya formation. The return time of the polynya is associated with a large-scale ocean mode in the Southern Ocean.