|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.
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.