The role of Barents–Kara sea ice loss  in projected polar vortex changes

Kretschmer, Marlene; Zappa, Giuseppe; Shepherd, Theodore G.

doi:https://doi.org/10.5194/wcd-1-715-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-715-2020

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

https://doi.org/10.5194/wcd-1-715-2020

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

Articles | Volume 1, issue 2

Research article

|

20 Nov 2020

Research article |

| 20 Nov 2020

The role of Barents–Kara sea ice loss in projected polar vortex changes

Marlene Kretschmer, Giuseppe Zappa, and Theodore G. Shepherd

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Final revised paper (published on 20 Nov 2020)
Supplement to the final revised paper
Preprint (discussion started on 27 Jul 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review of the paper entitled "The role of Arctic sea ice loss in projected polar vortex changes" by Kretschmer at al.', Anonymous Referee #1, 02 Aug 2020
RC2: 'Review of "The role of Arctic sea ice loss in projected polar vortex changes" by Kretschmer et al.', Anonymous Referee #2, 26 Aug 2020
AC1: 'Answers to reviewer´s comments', Marlene Kretschmer, 18 Sep 2020

Peer-review completion

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

AR by Marlene Kretschmer on behalf of the Authors (18 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Sep 2020) by Tim Woollings

RR by Anonymous Referee #2 (08 Oct 2020)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

The authors have improved the manuscript, but the authors did not address my first major comment about the nonlinear response of the SPV and I am very confused by their response. It is not clear to me how framing it as a deduction rather than induction helps to clarify anything. I agree with their point 1) in their response and this is an important implication of the analysis. However, another deduction/implication of the assumed causal link from Fig 5 is that BK-SIC loss cannot explain the nonlinearity of the SPV, which contradicts one of the main conclusions of the paper. The authors state that Fig 5 shows that a weak causal influence of BK-SIC can explain the nonlinearity of the SPV (L337), but I don’t think Fig 5 shows this at all. In the last few sentences of their response to this point, the authors state that they think the trend in the residual is linear. I disagree with this. To me, it looks like most of the nonlinearity in the SPV response (Fig 5a,d) actually comes from the residual (Fig 5c,f) not the BK SIC (Fig 5 b,e). Maybe my eyes are deceiving me, which is why I suggested the authors do something more quantitative, so we do not have to debate about what it looks like.

In their response, the authors also state that the evidence for nonlinear response to BKSIC is presented in Figure 1. However, the nonlinearity in the BK-SIC is primarily seen in the models with >5K warming and extremely weak when looking at all models(Fig 1c), but the nonlinearity of the SPV is apparent in all models, not only the >5K models(Fig 1a). This again appears to contradict the conclusion that the nonlinearity is driven by BK-SIC.

The authors then use this nonlinearity of the SPV as evidence for the causal link between BK-SIC and SPV and claim it is a ‘problem’ or a ‘puzzle’ to explain the nonlinear response if it is not sea ice(L226-227, L332-334). However, based on the evidence presented, a small causal link can’t explain it either, so this is a problem or puzzle either way. In their response, the authors say that there is no published study that makes the case for a nonlinear response from other processes. I am not aware of any other study that has shown the nonlinearity in the SPV response, so of course there is no evidence that other processes are to blame. Also, the amount of nonlinearity in the SPV response is quite small, which makes it more difficult to rule out other processes.

Overall, there a still some nice work in this study, so I still think it should be published. The causal analysis (Fig 3) and the implications that even a very small causal effect could still have an impact on the response and model spread of the SPV is an important result. However, the evidence presented by the authors does not support a link between nonlinearity of the SPV response and BK-SIC. The authors should have less focus on this result/conclusion, or provide additional evidence for it.

Minor comment:
L47-49:If the authors are only referring to BK sea ice here (as stated in their response), then Blackport and Kushner 2017 , Hoshi et al. 2017, Nakamura et al. 2016 and Screen 2017b, should be removed as references here. These studies look at the response to either observed or projected Pan-Arctic sea ice loss (not just BK).

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ED: Publish subject to minor revisions (review by editor) (12 Oct 2020) by Tim Woollings

AR by Marlene Kretschmer on behalf of the Authors (02 Nov 2020) Author's response Manuscript

ED: Publish as is (05 Nov 2020) by Tim Woollings

AR by Marlene Kretschmer on behalf of the Authors (11 Nov 2020)

Short summary

The winds in the polar stratosphere affect the weather in the mid-latitudes, making it important to understand potential changes in response to global warming. However, climate model projections disagree on how this so-called polar vortex will change in the future. Here we show that sea ice loss in the Barents and Kara (BK) seas plays a central role in this. The time when the BK seas become ice-free differs between models, which explains some of the disagreement regarding vortex projections.