Effects of Arctic stratospheric ozone changes on spring precipitation in the northwestern United States

Ma, Xuan; Xie, Fei; Li, Jianping; Zheng, Xinlong; Tian, Wenshou; Ding, Ruiqiang; Sun, Cheng; Zhang, Jiankai

doi:https://doi.org/10.5194/acp-19-861-2019

Articles | Volume 19, issue 2

https://doi.org/10.5194/acp-19-861-2019

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

Special issue:

The Polar Stratosphere in a Changing Climate (POLSTRACC) (ACP/AMT...

https://doi.org/10.5194/acp-19-861-2019

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

Articles | Volume 19, issue 2

Research article

|

23 Jan 2019

Research article |

| 23 Jan 2019

Effects of Arctic stratospheric ozone changes on spring precipitation in the northwestern United States

Xuan Ma, Fei Xie, Jianping Li, Xinlong Zheng, Wenshou Tian, Ruiqiang Ding, Cheng Sun, and Jiankai Zhang

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Final revised paper (published on 23 Jan 2019)
Preprint (discussion started on 07 Jun 2018)

Interactive discussion

Status: closed

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

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- Supplement

SC1: 'Role of zonal asymmetry in the ozone field?', Peer Johannes Nowack, 08 Jun 2018
- SC2: 'Responses to Short Comments', F. Xie, 28 Jun 2018
- AC3: 'Revised paper', F. Xie, 13 Sep 2018
RC1: 'Review of Ma et al.', Anonymous Referee #1, 30 Jun 2018
- AC1: 'Responses', F. Xie, 13 Sep 2018
- AC6: 'Revised paper', F. Xie, 13 Sep 2018
RC2: 'review of Ma et al., Effects of Arctic stratospheric ozone changes on spring precipitation in the northwestern United States', Anonymous Referee #2, 05 Jul 2018
- AC2: 'Responses', F. Xie, 13 Sep 2018
- AC5: 'Revised paper', F. Xie, 13 Sep 2018
AC4: 'Revised paper', F. Xie, 13 Sep 2018

Peer-review completion

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

AR by Anna Mirena Feist-Polner on behalf of the Authors (17 Sep 2018) Author's response

ED: Referee Nomination & Report Request started (21 Sep 2018) by Bjoern-Martin Sinnhuber

RR by Anonymous Referee #1 (27 Sep 2018)

Suggestions for revision or reasons for rejection

I thank the authors for their consideration of the reviewers’ comments. They have largely improved many of the cosmetic issues with the original manuscript, but many of the major scientific flaws of the original manuscript remain. Furthermore, the statistical significance contours added to the model results show that the key model results of this study that are foundation for the authors’ conclusions in the abstract are not statistically significant. So, unfortunately, I cannot recommend publication of this manuscript for the following reasons:

1. Figure 10, which presents the authors’ central conclusion (both Arctic ozone and SST forcings are necessary to reproduce the observed signals), does not actually show statistically significant precipitation and circulation signatures over the Pacific Northwest. In my view, this casts some serious doubt on the robustness of the observed relationship documented in this study, if models cannot faithfully reproduce the observed relationship. The most statistically significant features in the model results are in Fig. 7, which are driven by ozone anomalies only and are of the opposite sign to those in the observations.

2. The authors are incorrect in stating that midwinter stratospheric anomalies couple down to the troposphere and impact the North Atlantic sector, but that springtime stratospheric anomalies instead impact the North Pacific sector (lines 67–71). Smith et al. (2014) and Calvo et al. (2015) have shown that Arctic stratospheric ozone anomalies primarily affect the Atlantic sector during spring (see Fig. 4 of Smith et al. 2014; see Figs. 3–4 of Calvo et al. 2015). As I stated in my original review, the authors need to provide a convincing explanation for why their patterns are focused in the North Pacific sector, instead of the North Atlantic sector (as would be anticipated from many previous studies).

More minor concerns:

3. The authors’ argument that Arctic stratospheric ozone anomalies are more important than stratospheric polar vortex variability for Pacific Northwest precipitation is very misleading in my view. They have added Fig. 12 to address this issue, but they compare February vortex variability with April precipitation, but March ozone with April precipitation. For this to be a true comparison, they need to compare March vortex variability with April precipitation (or February ozone with April precipitation).

4. The SST anomalies associated with Pacific Northwest precipitation variations and ASO variations closely resemble those associated with ENSO and the PDO (see Fig. R5 in authors’ responses). While the authors have presented some correlation analyses in their responses, I’m not entirely convinced that the observed signals in this manuscript aren’t being aliased by concurrent ENSO and PDO variability. It would be nice to show that the observed signals still exist if only neutral ENSO years are sampled, as very large El Niño and La Niña events occur within their composites (Table 2).

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RR by Anonymous Referee #2 (17 Oct 2018)

Suggestions for revision or reasons for rejection

Having reviewed the initial submitted version of the manuscript, I will not provide a summary of the manuscript here. My only significant concern with the revised version is that the authors have not completely shown that the circulation and precipitation anomalies are caused by ASO anomalies alone. The new Figure 12 comparing correlations with SPV and ASO, if anything, strengthens the idea that anomalies in ozone and anomalies in the stratospheric circulation reenforce each other. I am not sure this point really comes through in the tone of the discussion. But at some point it becomes an argument about semantics and I think the reader can decide for themselves.

I think the authors have sufficiently addressed the concerns raised by the reviewers on the first version here. I might add that the work presented here is much more easily accepted and put into context when combined with the coupled model simulations presented in Xie et al. (2017). The authors may wish to develop a more direct link with the Xie et al. work and stress that the coupled model simulations presented in Xie et al. (2017) were able to produce SST anomalies that were somewhat like the Victoria mode anomalies from the observed SSTs used here. This point of what causes the SST anomalies, which are imposed in the experiments here, is an integral part of the argument about the role of ASO anomalies and should be strengthened here.

Minor comments:
Line 74-75: I think ‘Comparing with the effect of the winter stratospheric dynamical processes...’ should perhaps be ‘Similar to the effects of winter stratospheric dynamical processes...’

Lines 154-156 are difficult to understand, particularly because the WACCM4-GHG scheme has not been defined.

Lines 353 – 355 ‘Second, the ASO changes generate SST anomalies over the North Pacific...’ should really have a reference to Xie et al. (2017).

Lines 402 – 405 ‘Such circulation anomalies force an anomalous cyclone in the western United States in the middle and upper troposphere, which likely enhances cold and dry airflow from the North American continent to the North Pacific...’ At least in the climatological monthly average the winds over the northwestern United States continue to be from the west, bringing moisture in from the Pacific. The anomalous cyclone is not a reversal of the winds, but a reduction in the winds. I do not believe you have shown any westerly transport of dry air from the North American continent to the North Pacific – this would require winds from the east. Not just a reduction in westerly winds. I believe a much more justifiable interpretation of the changes is a reduction in the transport of moisture inland from the Pacific.

Figure 8 – Can the authors check that the revised Figure 8 actually shows significance at the 95% level. To my eye Figure 8 in the revised version looks identical to Figure 10 in the original submission where significance was plotted at the 90% level. The significance is denoted with dots and I am not able to find a single dot that has changed between the original Figure 10 and the revised Figure 8 – even in places where a single, isolated dot appears by itself.

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ED: Reconsider after major revisions (08 Nov 2018) by Bjoern-Martin Sinnhuber

Based on the detailed reviewer comments, the authors have expanded and very much improved their revised manuscript. I am very grateful to the reviewers for their advice. However, while referee #2 concludes that all previous comments have been sufficiently addressed and recommends publication of the manuscript with only a few minor modifications, referee #1 is not convinced and suggests rejection of the manuscript. The main concern of referee #1 is that the model responses over the northwestern United States due to stratospheric ozone forcing are not statistically significant and the response due to ozone changes alone is of a different sign as the observed correlation. I agree with referee #1 on this point. However, I do feel that this study contains valuable results that justify publication in Atmos. Chem. Phys.

In order to be acceptable for publication, I ask the authors to take the additional comments of the referees into account. In particular also to arrive at a more balanced and appropriate discussion of the role of ASO vs. SST variations in forcing of northwestern US precipitation anomalies. The revised discussion should in particular reflect the concerns raised by referee #1. As suggested by referee #2, a more explicit link to the coupled model study of Xie et al. (2017) would help to improve the discussion of ASO vs. SST forcing.

The statement that dynamical coupling affects primarily the Atlantic sector while ozone changes affect primarily the Pacific sector does not seem to be well supported by other studies. I suggest to de-emphasize this point unless there is specific and robust evidence.

Please take into account also the additional minor comments of both referees.

In addition, a reference to the HadISST data set should be included. Why is in one case (Fig. 8) HadISST used and in another case (Fig. 9) CESM SST forcing data?

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AR by Fei Xie on behalf of the Authors (04 Dec 2018) Author's response Manuscript

ED: Publish subject to technical corrections (20 Dec 2018) by Bjoern-Martin Sinnhuber