Arctic sea-ice-free season projected to extend into autumn

Lebrun, Marion; Vancoppenolle, Martin; Madec, Gurvan; Massonnet, François

doi:https://doi.org/10.5194/tc-13-79-2019

Articles | Volume 13, issue 1

https://doi.org/10.5194/tc-13-79-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-13-79-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 13, issue 1

Research article

|

10 Jan 2019

Research article |

| 10 Jan 2019

Arctic sea-ice-free season projected to extend into autumn

Marion Lebrun, Martin Vancoppenolle, Gurvan Madec, and François Massonnet

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Final revised paper (published on 10 Jan 2019)
Supplement to the final revised paper
Preprint (discussion started on 02 May 2018)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Paper comments', Anonymous Referee #1, 01 Jun 2018
- AC1: 'Reply to referee #1', Marion Lebrun, 08 Aug 2018
RC2: 'Comments on "Arctic sea ice-free season projected to extend into fall" by Lebrun et al.', Anonymous Referee #2, 30 Jun 2018
- AC2: 'Reply to referee #2', Marion Lebrun, 08 Aug 2018

Peer-review completion

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

AR by Marion Lebrun on behalf of the Authors (05 Sep 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Sep 2018) by Dirk Notz

RR by Anonymous Referee #1 (25 Sep 2018)

Suggestions for revision or reasons for rejection

General comments
The authors have clarified some of their reasoning in the paper and provided more arguments for the changes in ice retreat and advance timing. However, there is no new analysis in the study. This is disappointing. I think that there are many interesting elements to the work presented here but feel that the authors have not dug in deeply enough to understand these elements. Because of this, I believe that the important new material in the study is modest - basically that solar heating occurring over the summer is slow to be released during the fall freeze-up and delays the ice advance. This is based on simulations with a simple single column model and only limited results on this mechanism (just the SST annual cycle) are shown from the coupled simulations. There are a number of questions that remain regarding the changes in ice retreat and advance timing as outlined in the major comments below. I would encourage the authors to address some aspects of these questions and thereby provide more insights on why the trend in ice advance timing exceeds the ice retreat trends in the 21st century (as raised in my original review). I recommend another round of major revisions and would encourage some new analysis to address these comments.

Major comments
1. The discrepancies between the long term and short term variations in ice retreat and advance timing are interesting but not at all explored in the study. The authors offer a number of possibilities for why there might be this difference, with references to previous work, but nothing is conclusive or backed up by analysis. More analysis on what drives the ice retreat timing, ice advance timing and how this varies across timescales would be useful.

2. The authors provide results from the 0-layer thermodynamic model but with very little comparison to the coupled climate simulations. The 0-layer model excludes many factors (not just dynamics). For example, in the coupled model solar energy can penetrate through leads and ice so will not have such an abrupt transition as in the 0-layer model and the surface fluxes will affect the atmospheric state and feed back onto the surface fluxes, among others. I’d suggest that more analysis is done with the coupled model to assess whether the 0-layer model results are indeed the dominate process occurring in the coupled runs, how things change with time scale, etc.. For example, on Figure 6b, why are the solar and net heat fluxes for IPSL not shown in addition to the SST? Do these look similar to the 0-layer model in Fig 6a? Do they change on interannual versus longer timescales (see 1 above)? Making more comparisons between the 0-layer results and the coupled model would help to strengthen the paper. It could also help to answer why the simple mechanism at work in the 0-layer model does not act on short timescales in the coupled modeling systems (again, see 1 above).

3. The 0-layer model results provide a reason why the ice advance timing is delayed but do not clearly indicate why (at least to me) it would be delayed more relative to retreat timing in the 21st century. Some physical arguments for why this occurs are suggested on lines 309-321. However, there is no analysis to back them up. It would be useful to assess some of these factors in the coupled model simulations to determine their influence, relative magnitude, etc. A clearer reason with corroborating material as to why Rlong increases to greater than one (which is one of the main conclusions of the manuscript) would be useful. I think that this could be done without drastically inflating the manuscript, especially if it is just to show support for the mechanisms that are already speculated in the manuscript.

Minor comments.

Line 54: Wording/typo – need to include “be” in the sentence

Line 72-73: Wording needs to be revised (“in accord the observed in situ increase …”)

Line 109-112: It might also be good to say what the caveats are from a forced model. For example, the surface state doesn’t feedback to the atmosphere which can affect the processes that drive changes in ice advance and retreat timing.

Section 3.1. You should mention here that there is an inherent discrepancy in comparing observations, which are influenced by internal variability, to a multi-model mean.

Section 3.1. Last sentence: It would be useful to mention that the Barents Sea is subject to internally-generated decadal scale variations driven by ocean heat transport anomalies (for example see Yeager et al., 2015) which could explain some of the discrepancies between observations and models in that region.

Lines 244-245. It should be mentioned that internal variability may also play some role here in the discrepancy between the climate model and satellite data.

Lines 361-362. “This points to dynamical processes as most likely drivers …” I believe that you are making this argument because of the results from the 0-layer model. However, as mentioned above, the 0-layer model is missing many things in addition to dynamics. For example, the coupling to the atmosphere, which will change atmospheric conditions and fluxes could play an importan

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RR by Anonymous Referee #2 (25 Sep 2018)

Suggestions for revision or reasons for rejection

Main comments
1. CMIP5 model trends and R-values vary quite a bit spatially (Fig. 4,5 and S4). While this is not the main focus of the study, the manuscript would be improved by discussing the CMIP5 model differences instead of relating them generally to mean state errors. While the selected CMIP5 models may all produce mostly positive R-values, the large spatial differences between individual models indicate that the multi-model mean obscures a substantial amount of variability.

2. I suggest changing the header of the “Conclusions” section to “Summary and Discussion” for this manuscript. I understand and appreciate the authors’ desire to connect the key points of the manuscript to broader implications. However, I maintain from my original comments that “Conclusions” should focus on the findings of the study, which have been shown in the results section. For example, the connections drawn in lines 363-367 should either be done as “discussion” or introduced in Section 3.5. They should not be introduced for the first time as conclusions. Furthermore, since the authors hope to keep lines 368-380, I suggest limiting this part of the discussion to two distinct topics: (1) photosynthetic organisms and (2) shipping. These two examples are the most clearly impacted by the authors’ conclusions (a greater change in ice advance versus retreat) out of those listed.

Other comments
Lines 54-55: The distinction between melt season and open water season has improved in this submission.

Line 78: The phrase “for sure” is too informal. I recommend saying “... and this is true in the Alaskan Arctic where they have been analyzed.”

Line 132: Figure S2 is more useful than the original Table S1 and should continue to be included in the supplementary material. To bolster this point, I recommend that the authors add additional text near line 132 describing why the interpolation error analysis using satellite observations is expected to be similar in the CMIP5 models.

Line 139: “Tangible” not the correct word here. Perhaps the word “visible” instead?

Lines 175-177: It would be clearer to describe when exactly each timescales is used. It is too easy for the reader to mistakenly connect Rshort with 36 years and Rlong with 200 years.

Lines 203-206: At this location, the manuscript would benefit from a more substantial explanation of model differences.

Line 222: As far as I understand, the last paragraph was about satellite observations, but the topic sentence of this paragraph seems to be making a conclusion about the R-values of the CMIP5 models, which haven’t been discussed yet. I recommend taking out the word “thus” or rephrasing this sentence.

Line 261: It would be clearer if there was a sentence here signaling that in the next section you will explore this mechanism (in addition to the header of Section 3.4).

Lines 283-284: Please clarify if the 100-200% statistic refers to 100-200% of the winter contribution.

Lines 309-310: Points (i)-(iii) seem to provide arguments for progressive emergence found in the 1-D model that are applicable to the CMIP5 models. If this is true, the sentence, “There are physical arguments…in the course of this century,” should explain this instead of mentioning only the 1-D model.

Line 357: The phrase “turns up” is too informal. Perhaps the word “appears” instead?

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ED: Reconsider after major revisions (26 Sep 2018) by Dirk Notz

AR by Marion Lebrun on behalf of the Authors (05 Nov 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Nov 2018) by Dirk Notz

RR by Anonymous Referee #1 (25 Nov 2018)

ED: Publish subject to minor revisions (review by editor) (04 Dec 2018) by Dirk Notz

AR by Marion Lebrun on behalf of the Authors (12 Dec 2018) Author's response Manuscript

ED: Publish as is (14 Dec 2018) by Dirk Notz

AR by Marion Lebrun on behalf of the Authors (14 Dec 2018)

Short summary

The present analysis shows that the increase in the Arctic ice-free season duration will be asymmetrical, with later autumn freeze-up contributing about twice as much as earlier spring retreat. This feature is robustly found in a hierarchy of climate models and is consistent with a simple mechanism: solar energy is absorbed more efficiently than it can be released in non-solar form and should emerge out of variability within the next few decades.