Projection of North Atlantic Oscillation and its effect on tracer transport

Bacer, Sara; Christoudias, Theodoros; Pozzer, Andrea

doi:https://doi.org/10.5194/acp-16-15581-2016

Articles | Volume 16, issue 24

https://doi.org/10.5194/acp-16-15581-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The Modular Earth Submodel System (MESSy) (ACP/GMD inter-journal...

https://doi.org/10.5194/acp-16-15581-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 24

Research article

|

16 Dec 2016

Research article |

| 16 Dec 2016

Projection of North Atlantic Oscillation and its effect on tracer transport

Sara Bacer, Theodoros Christoudias, and Andrea Pozzer

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Final revised paper (published on 16 Dec 2016)
Supplement to the final revised paper
Preprint (discussion started on 20 May 2016)
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: 'Review comments', Anonymous Referee #1, 23 Jun 2016
- AC1: 'Reply to Referee #1', Sara Bacer, 19 Sep 2016
RC2: 'Review of "Projection of North Atlantic Oscillation and its effect on tracer transport"', Anonymous Referee #2, 05 Jul 2016
- AC2: 'Reply to Referee #2', Sara Bacer, 19 Sep 2016

Peer-review completion

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

AR by Sara Bacer on behalf of the Authors (21 Sep 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (29 Sep 2016) by Peter Haynes

RR by Anonymous Referee #2 (14 Oct 2016)

Suggestions for revision or reasons for rejection

This revised paper shows rather convincingly that in the EMAC model coupled runs the NAO shifts eastward with a positive trend into the future. They show that this impacts the resultant pollution patterns with increases in pollution up to +15% to the higher amplitude NAO events. This I believe can be published.

That said, there are a number of technical problems with the paper that should be addressed prior to publication. However, I don’t really see a real impediment to publication at this point.

Minor Comments:
1. P1, L7: “interchange”: meaning not clear.

2. P1, L10: “increased pollutant depletion”: awkward phrasing

3. P1, L13: what do the authors mean by “teleconnection activity”

4. P1, L13: the last sentence in the abstract seems a bit strong, that “local air quality conditions over Europe … will become more extreme” particularly since the authors suggest in the sentence above that portions of Europe will see less pollution.

3. P3, L17-21: repetitive with P2, L25-28

4. Section 2: I assume sea-ice is also modeled? Please mention explicitly.

5. P4, L13: “the development of the climate”: delete “the development of”

6. P4, L27: “climatology of aerosols” Do the aerosols change with time? This has been implicated as important in simulating the NAO.

7. P5, L4: “all associated feedbacks”: is the carbon cycle modeled?

8. Equation 1 and Figure 8. What are the units? With pressure in pascals and the mixing ratio in mol/mol the units are a bit strange…

9. Page 6, line 4-5: The first figure in the supplement indeed suggests that the NAO pattern shifts between the present and future climate. However, the clause, “but they do not between any of the decadal timescale within each period” needs amending. This sentence is not clear and the individual panels are shown on a basis of 30 years not in decades. Please clarify.

10. Page 6, L16: Perhaps make clear the linear regression coefficient is with respect to time.

11. Page 6, L24: Please give units.

12. Page 6, L31: The short-term trend in the NAO at the end of the century does not look significant in the sense that if extended the simulation by 10 years one would get a different result. Thus, it is not clear of the importance of this line.

13. It is perhaps worth stressing that the results in Figure 4 are not surprising, but in fact are consistent with the positive trend: an increase in high level events in the future and a decrease in low level events.

14. The units on Figure 6 are not given. One should regress CO against the principal component and then multiply by the standard deviation of the principal component to get typical changes in CO (in ppb) due to NAO variability. The correlation and regression should look exactly the same (except for units) so it is not necessary to show figure 5, only figure 6. The significant regions, however, should be marked in Figure 6. The difference map between the future regression and the present-day regression should be shown. These changes would act to limit the rather lengthy explanations in the text. The text describing figures 5 and 6 should be shortened.

15. P7, L19-20; P8, L13-15: It seems unnecessary to explain exactly what a positive or negative correlation (regression) means.

16. Differences between high and low NAO (Figure 7). This is probably a nice way of showing future changes due to the NAO. However, it is likely that in the future the events categorized as high have, on average, a higher NAO index than those characterized as high in the present; similarly and the future events categorized as low in the future are likely less negative than those characterized as low in the present. This alone would explain Figure 7; that is, it is probably not true that the response to the NAO becomes more extreme in the future, just that there are more positive events in the future. This distinction should be clarified. The averages of the future and present high and low NAO indexes that are used in Figure 7 should be given. Discuss whether the results given in Figure 7 are consistent with the regressions shown in Figure 6 (taken into account the differences in the high and low events). That is, are the results the same one would get assuming linearity or are the high end events getting more extreme?

17. Figure 8. Please include the units. Also, showing a plot of future minus present would be helpful in visualizing the differences.

18. P10, L7-8. Please check the wording here. I thought the future showed an increase in the amplitudes of the positive NAO and a reduction in the amplitudes of the negative NAO.

19. Conclusions. Which is more important, the shift in the NAO or the change in its amplitude? (Or maybe the authors could not determine this). However, it seems it should be at least mentioned.

20. P10, L16. The “Therefore” here needs some more explanation I think.

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ED: Reconsider after minor revisions (Editor review) (25 Oct 2016) by Peter Haynes

AR by Sara Bacer on behalf of the Authors (03 Nov 2016) Author's response Manuscript

ED: Reconsider after minor revisions (Editor review) (20 Nov 2016) by Peter Haynes

The paper is very close to being suitable for publication. My only remaining concern is about absolute clarity -- see the first comment below. There are also some straightforward technical changes that are needed.

Please address the first comment below (or respond appropriately) and the other more straightforward comments. Then I will be pleased to accept the paper for publication.

p1 l9-10: My understanding is that this paper addresses 2 independent points — (i) will there be a future change in the NAO and (ii) will there be a future change in the structure and hence the chemical implications of the NAO as defined as the leading mode of variability? I don’t believe that any of your results directly address — ‘will there on average be a chemical change in future’, but what is addressed is ‘will positive NAO vs negative NAO have different chemical implications in future compared to the present.

This needs to be made transparently clear to the reader and a sentence such as ‘We find that at end of the century during positive NAO phases south-west Mediterranean and north Africa will see higher
pollutant concentrations with respect to the past, while a wider part of north Europe will benefit from lower pollution.’ in the abstract is not very clear in my view — a casual reader may interpret it as saying ‘We find at the end of the century … a wider part of north Europe will benefit from lower pollution’. A possible clarification would be: ‘We find that at the end of the century south-west Mediterranean and north Africa will during positive NAO phases see higher
pollutant concentrations with respect to the past, while a wide part of north Europe will, during NAO phases, see lower pollutant concentrations.’

(The suggested sentence might seem unnecessarily repetitive, but at least it is transparently clear.)

Please consider carefully whether the distinction between (i) and (ii) above is made completely clear throughout the paper as a whole.

p4 l31: ‘Saravan’ > ‘Saravanan’

p6 l1: ’shown in the electronic supplement’ — please make explicit reference to the Figures in the supplement — e.g. in this case ‘Fig 1S (where S indicates supplement)’. Please do the same throughout the paper.

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AR by Sara Bacer on behalf of the Authors (23 Nov 2016) Author's response Manuscript

ED: Publish as is (25 Nov 2016) by Peter Haynes

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Short summary

We investigate the influence of the North Atlantic Oscillation on atmospheric pollutant transport in the 21st century under a global climate-change scenario, using a coupled atmosphere–chemistry–ocean general circulation model. We find that, at the end of the century, the south-western Mediterranean and northern Africa will see higher pollutant concentrations during positive NAO phases with respect to the past, while a wider part of north Europe will see lower pollutant concentrations.