Stratospheric influence on North Atlantic marine cold air  outbreaks following sudden stratospheric warming events

Afargan-Gerstman, Hilla; Polkova, Iuliia; Papritz, Lukas; Ruggieri, Paolo; King, Martin P.; Athanasiadis, Panos J.; Baehr, Johanna; Domeisen, Daniela I. V.

doi:https://doi.org/10.5194/wcd-1-541-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-541-2020

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

https://doi.org/10.5194/wcd-1-541-2020

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

Articles | Volume 1, issue 2

Research article

|

17 Oct 2020

Research article |

| 17 Oct 2020

Stratospheric influence on North Atlantic marine cold air outbreaks following sudden stratospheric warming events

Hilla Afargan-Gerstman, Iuliia Polkova, Lukas Papritz, Paolo Ruggieri, Martin P. King, Panos J. Athanasiadis, Johanna Baehr, and Daniela I. V. Domeisen

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Final revised paper (published on 17 Oct 2020)
Preprint (discussion started on 05 May 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 manuscript by Afargan-Gerstman et al.', Anonymous Referee #2, 26 May 2020
RC2: 'Review of "Stratospheric influence on marine cold air outbreaks in the Barents Sea"', Anonymous Referee #1, 29 May 2020
RC3: 'More context is needed to interpret the results', Anonymous Referee #3, 30 Jun 2020
AC1: 'Response to reviewers', Hilla Gerstman, 05 Aug 2020
AC2: 'Revised manuscript (highlighted changes)', Hilla Gerstman, 05 Aug 2020

Peer-review completion

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

AR by Hilla Afargan Gerstman on behalf of the Authors (05 Aug 2020) Author's response

ED: Referee Nomination & Report Request started (11 Aug 2020) by Thomas Birner

RR by Anonymous Referee #1 (22 Aug 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

General comments:
In this manuscript, the authors are evaluating whether there is a relationship between marine cold air outbreaks (MCAOs) and Sudden Stratospheric Warmings (SSWs) in the Barents, Norwegian, and Labrador Seas. The authors conclude that changes in the large-scale tropospheric circulation account for 42% of the MCAO variance in the Barents Sea and 31% of the variance in the Norwegian Sea. They also make a convincing case that there is a significant increase in the correlation between the large-scale tropospheric flow pattern and MCAOs after SSWs in the Barents and Norwegian Seas. The connection to the large-scale tropospheric flow to MCAOs is further found to be correlated with the Scandinavian Trough pattern in the Barents and Norwegian Seas and Greenland Blocking pattern in the Labrador Sea. This manuscript fits within the scope of WCD in that it addresses stratosphere-troposphere coupling and prediction on sub-seasonal to seasonal time scales. In this revised manuscript, the authors have diligently addressed the earlier concerns and I now think that this could be published after some minor revisions outlined below.

Specific comments:
1 The connection to physical processes linking the stratosphere to the surface has been improved in this version, although after reading again, I am still left wondering what physical feature(s) is(are) transporting the cold air into the regions of focus in this study. It makes sense that the lower 500 hPa heights would be associated with the colder air and the related storminess nearby. Papritz et al. (2019) address these processes that are likely relevant in the Norwegian Sea, particularly the tropopause polar vortex (e.g. Cavallo and Hakim 2010).
2 Line 50: The Labrador Sea is also mentioned in the abstract and on Line 55, but just the Norwegian and Barents Seas here.
3 Line 129: Please provide a justification for choosing M >= 4K to define moderate-to-strong MCAOs in DJFM climatology.
4 Lines 135-160: The interpretation provided that the storminess increases in the Barents, Norwegian and Labrador Seas at the time of MCAOs in the respective regions is not quite accurate. However, the conclusion reached by the authors on lines 142-144 is the correct conclusion. Cold air outbreaks occur when there are northerly winds over those regions, which means that the storms must move slightly east of those regions so that the cold sector (west side) is over the seas themselves. For example, in Figure 2j, there is an increase in storminess in the lower right side of the Barents Sea box but decrease in upper left side (not stated correctly on lines 137-138) and similarly for the Norwegian Sea (Fig. 2k) and Labrador Sea (Fig. 2l). This also applies to the conclusions section on line 265.
5 Line 140: On point (iii), it looks like in Figure 2g that the strongest northerly wind anomalies are over the Barents Sea with weaker northerly wind anomalies over the Norwegian Sea.
6 Lines 180-190: It is nice that the authors have now considered statistical significance in this revision. However, it is not clear on line 187 what "All correlations...are found to be statistically significant" refers to. It is good that the correlations themselves have p < 0:05, but what about the differences in correlations between the SSW and climatology or SSW and no SSW? This seems to be addressed in the Labrador Sea correlations only. It should be stated which correlations are and are not significant; perhaps a table with the corresponding
p-values would be most helpful.
7 Line 209: Where are the authors getting that there is an increase in 15% in the explained variance for the Barents Sea for the ZDI index vs. MCAO? It looks like it should be 18% from Figure 3a.
8 Line 245: It is easier to see from Figure 6a,b that the circulation over the Barents Sea is "anomalously cyclonic" rather than "cyclonic."

Technical corrections:
1 Figure 2: The caption should state what the black and green boxes are in each of the panels (as it is in Figure 1 caption).
2 It is not necessary to say `historical' on line 90
3 Line 145: enhanced --> moderate-to-strong

References
Cavallo, S. M. and G. J. Hakim, 2010: The composite structure of tropopause polar cyclones from a mesoscale model. Mon. Wea. Rev., 138 (10), 3840-3857, doi:10.1175/2010MWR3371.1.
Papritz, L., E. Rouges, F. Aemisegger, and H. Wernli, 2019: On the thermodynamic pre-conditioning of arctic air masses and the role of tropopause polar vortices for cold air outbreaks from Fram Strait. J. Geophys. Res.: Atmos.

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RR by Anonymous Referee #2 (24 Aug 2020)

RR by Anonymous Referee #3 (31 Aug 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 done a good job of addressing the comments from the first round of review. Their effort has greatly improved the manuscript. I think by both broadening the analysis to include all SSW events rather than just a limited subset from the reanalysis period, as well as, including the additional North Atlantic regions into the analysis, the paper provides a compelling synoptic climatological analysis on the occurrence of anomalous MCAO activity after SSW. I have a few minor/technical comments about the framing of the analysis and have included these below.

Minor comments:
Title change: The title needs to be more specific to the analysis presented in the paper. The authors only consider sudden stratospheric warming events and not other ‘stratospheric influences’ like those from final warming events, strong events, etc. Perhaps something like, “Modulation of North Atlantic marine cold air outbreaks following sudden stratospheric warming events” would be more appropriate to the analysis presented.

The authors state that they “aim to shed light on the predictability of MCAOs” on line 58 and re-emphasize this point elsewhere in the discussion of the results. However, this aim should be reworded/scaled back. The analysis does not get into a discussion about our ability to predict MCAO at various timescales (nor should it, as that is not the focus of the analysis), rather shows that MCAOs occur more frequently in some locations and less so in others after SSWs. Perhaps this fact could be used by a simple statistical model to increase predictability, but that is not explicitly discussed. This aim should be reframed, perhaps in terms of mitigating societal/economic impacts (i.e., those discussed in the intro) due to changes in frequency and locations.

Fig. 2: A minor suggestion to consider: perhaps you can flip your color bar for the first row of the figure (for MACO) so that cold air is represented by cold colors and warm air is warm colors.

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ED: Publish subject to minor revisions (review by editor) (13 Sep 2020) by Thomas Birner

AR by Hilla Afargan Gerstman on behalf of the Authors (24 Sep 2020)

ED: Publish as is (29 Sep 2020) by Thomas Birner

AR by Hilla Afargan Gerstman on behalf of the Authors (02 Oct 2020) Author's response Manuscript

Short summary

We investigate the stratospheric influence on marine cold air outbreaks (MCAOs) in the North Atlantic using ERA-Interim reanalysis data. MCAOs are associated with severe Arctic weather, such as polar lows and strong surface winds. Sudden stratospheric events are found to be associated with more frequent MCAOs in the Barents and the Norwegian seas, affected by the anomalous circulation over Greenland and Scandinavia. Identification of MCAO precursors is crucial for improved long-range prediction.