|Review of manuscript acp-2017-647: “The role of the winter residual circulation in the summer mesopause regions in WACCM”, by Maartje S. Kuilman and Bodil Karlsson.|
The authors have substantially reduced the length of the manuscript, and modified part of the text, in response to my comments. I think the paper goes more to the point, and thus has improved. However, I am not fully convinced about the novelty of the results; the response of the authors to this concern has been a bit vague.
I am somewhat torn on what to recommend for this paper. I think there is some interest in these results, but I believe the paper needs some new figures and a clarification of certain aspects of the presentation of the results, before it is ready for publication.
- What is the motivation for trying to reproduce KB16 results with WACCM? Are there processes included in WACCM and not in KMCM that justify the study?
I am not entirely satisfied with the response given to this question. WACCM includes a chemistry module in the middle atmosphere, and has different GW parameterizations and dynamical cores than those in KMCM. But this is generally true for any pair of general circulation models. What have we learned from this paper that we did not know from the previous papers, particularly KB16?
With this comment I would like to encourage the authors to find an attractive way to present the results and highlight their relevance. The way the paper is motivated in the Introduction section (and the way the results are summarized in the Conclusions section), gives me the impression that the paper is an exercise of reproducibility of previous results – which is always good news, but perhaps not enough for an article to be published in ACP.
- Figures 1 and 2. These figures show two things: 1) the IHC mechanism controls the mean T of the summer polar mesopause (Fug. 1); and 2) that T in the summer polar mesopause does not covary statistically significantly with T in the winter stratosphere (Fig. 2). The authors seem to use interchangeably “summer mesopause” and “summer polar mesopause” throughout the paper; but Fig. 2 shows that the summer mesopause away form the pole has statistically significant anomalies of T, while this is not the case for the polar region.
So what controls the summer polar mesopause T in a climatological (average) sense? IHC is clearly shown to play a role. It would also be nice to mention that the presence of GW in the summer hemisphere is a crucial factor, as shown in the response figures. Without them, the summer polar mesopause will be very high (or even absent).
As for the interannual variability, the IHC only seem to reach 50N, but not further north (line 376-383).
- I really think the paper would benefit from including plots of the EP flux divergence and the Tranformed Eulerian Mean velocities (either v* or w*). Those are more direct measures of the wave forcing and the BDC than stratospheric T, and they should not be difficult to compute from monthly mean output (WACCM does provide output of zonal mean flux diagnostics v’T’, u’v’ etc that speeds up the process). And this way I believe it would add value to the results as compared to KB16.
- Line 133. Analogically → Analogously?
- Line 176. Simply WACCM since it has already been defined.
- Line 272: degrees → K (Kelvin).
- Line 280. They attributed that the warming → They attributed the warming.
- Line 288: turning of → turning off
- Lines 289-290. I may have misunderstood it, but if T in the equatorial mesosphere has a different sign in both models, there is no qualitative agreement.
- Line 343: less effect → weaker effect.
- Lines 358. It seems not quite conventional to use T in the extratropics as a proxy for the strength of the BDC, when the model provides with all the variables needed to calculate it. Please comment on this choice.
- Line 386. I do not understand this sentences. Does it mean that the BDC modifies the summer stratospheric meridional gradient of T?
- Lines 395-398. But in Fig. 2 we see that the anomalies are not statistically significant.
- Lines 468-471. “… it is the equatorial mesosphere that is governing the temperature in the summer mesopause regions...”. I agree, but the equatorial T is ultimately driven by the BDC changes forced by the winter planetary wave forcing, according with your results.
- The authors may consider to include the first part of section 3 in a new section 3.1, and rename the old section 3.1 as section 3.2 (for a matter of symmetry in the presentation).