Dear authors, please see the attached PDF for my review of your manuscript.

Summary of the review

The present paper presents a thorough statistical investigation of the relationship of transient eddy activity (or storm-track activity) with westerly jet intensity and meridional width for the North Atlantic and Pacific. By applying a novel classification method of jet intensity and meridional width to reanalysis data, the authors have revealed that the relationship between the eddy activity and time-mean flow tend to differ fundamentally between inter-monthly and sub-monthly time scales. They have also revealed that for a given jet intensity between weak and modest levels, a narrower jet tends to accompany weaker eddy activity than a broader jet. The findings in this work can be a clue for fully understanding the counterintuitive “midwinter minimum of the North Pacific storm-track activity”. I consider this work to be worthy of being published, since the finding of those robust relationships makes an important contribution to the storm-track dynamics, although they still require deeper dynamical understanding. However, I find some aspects of the manuscript that require clarification and improvement as listed below. I therefore consider a minor revision is necessary to properly address those comments before the acceptance of this paper. 

Major scientific comments:

[A] L100~103: In a line similar to Schemm et al. (2021), Okajima et al. (2022, J. Clim., 35 (4), 1137–1156) investigated the midwinter minimum in the North Pacific storm-track activity. They found that the net eddy conversion/generation rate normalized by the eddy total energy is indeed reduced in midwinter, to which a reduced conversion rate of baroclinic energy gain and an increase rate of barotropic energy loss in midwinter both contribute. It is a relevant work, I believe, to be cited at the end of this paragraph.

[B] L151~166: In these two paragraphs, statistical analyses by Nakamura (1992, N92) are reviewed. For deeper understanding through fairer comparison with this study, the review should include the following aspects. Specifically, N92 used eddy statistics and mean-flow properties based on sampling not only for individual calendar months but also for 31-day moving windows, which should be mentioned properly, the latter of which is equivalent to the 30-day moving average used in this work. In fact, N92 used the moving average for constructing scatter plots between eddy statistics (activity) and a given mean flow property, including zonal wind speed or meridional temperature gradient. So, any description that may lead to such misunderstanding that N92 only used statistics for individual calendar months should be avoided. Note that Okajima et al. (2022) also used statistics for 31-day moving windows for their analyses. Another important aspect for the scatter plots in N92 is that the mean flow properties were sampled along the instantaneous local storm-track axis defined as latitudinal maxima of eddy amplitude, which differs from the present study. Therefore, the westerly wind speed and associated baroclinicity plotted in the scatter plots in N92 tend to be weaker than the corresponding values in this study especially over the midwinter North Pacific, where the storm-track tends to be located poleward of the intense narrow jet stream.

[C] L156: “a monthly mean, with monthly means computed around a central date every 10 d” makes no sense, and even “monthly means computed around a central date every 10 d” is quite misleading. This is because “monthly mean” typically signifies averaging over a given calendar month. Going through the manuscript, it will be finally realized that “monthly mean” or “monthly average” in this paper actually denotes “30-day running mean”. If this is indeed the case, “a monthly mean, with monthly means computed around a central date every 10 d” should be replaced with “statistics based on 30-d running means evaluated at 10d intervals”. In the rest of the manuscript, “monthly mean” should be replaced for clarification with “30-day moving average” or “monthly-scale averaging”, wherever necessary. Individual occasions for such replacement are given as “minor comments” below.

[D] L236•237: “periods of enhanced baroclinic conversion are followed by a weakened U and increased EKE” sounds quite interesting, but I wonder if such a lead-lag relationship has been robustly extracted in this work through statistical analysis. The same comment applies also to a similar statement in L479~481.

[E] L315~318: It is indeed the case that “the NP jet is more tightly constrained by the descending branch of the Hadley circulation” in midwinter. In addition, the NP jet is tightly constrained also by the prominent planetary-wave trough over the Far East, as pointed out by Nakamura et al. (2002, J. Clim.) as well as Nakamura and Sampe (2002, GRL). This aspect should be noted in the text.

[F] L339: As stated here, the jet with a comparable strength tends to be broader in winter than in summer (Fig. 6), which is an important finding. From a different viewpoint, the jet with a comparable width tends to be stronger in winter than in summer, reflecting the greater equator-pole temperature difference. This is especially the case over the midwinter North Pacific, where an eddy-driven subpolar jet is merged with the subtropical jet due to the prominent planetary-wave trough over the Far East, as argued by Nakamura et al. (2004, AGU Monogr. 147, 329–345).

[G] L408: As mentioned here, Fig. 9 clearly shows the tendency for transient eddies over the midwinter North Pacific to be more confined meridionally under the stronger jet than under the weaker jet. This finding is in good agreement with what is indicated in Fig. 3 of Nakamura and Sampe (2002 GRL), which should be cited here. Another important aspect indicated by Fig. 9 is that the strong jet is more zonally extended and cyclone development tends to occur closer to the jet core region if compared to the weak jet condition.

[H] L485: As commented in [D]. the midwinter North Pacific jet is constrained not only by the Hadley circulation but also by the planetary-wave trough over the Far East. The description here should be modified accordingly.

[I] L524•525: The eddy energetics modulated under the intensified narrow jet in the midwinter North Pacific has been investigated also by Okajima et al. (2022), which may be cited here.

Minor comments:

1. L22: “narrower with smaller EKE” sounds more appropriate in context than “narrower, and therefore EKE smaller”.
2. L121•122: “10-day temporal high-and low-pass filters” rather than “a 10-day temporal high-and low-pass filter”.
3. L167: Better to describe explicitly here if this jet speed classification is based on 10-day lowpass-filtered fields or 30-day running mean fields.
4. p.8, L4 Fig. 2 caption: “30-d moving averages” is better than “30-d averages”.
5. L177: “monthly-scale averaging” is more appropriate than “monthly averaging”.
6. L181: “simple monthly-scale averaging” sounds more appropriate than “monthly averaging”.
7. L201: It is certainly the case that “a distinct negative relationship emerging for averaged jet velocities exceeding 55 m/s” for the NA in Fig. 2d. This feature is, however, hinted even in Fig. 2b, which should be stated here.
8. L222: It should be stated here how the degrees of freedom are estimated for assessing this significance.
9. L225: Is this tendency “EKE generally decreases with increasing jet strength” apparent on sub-monthly timescales? If so, it should be stated explicitly here.
10. L233: “all calendar months” is clearer than “all months”.
11. p.11, L3 Fig. 4 caption: “individual calendar months” is better than “individual months”.
12. L253: “monthly-scale averaging” is more appropriate than “monthly averaging”.
13. L267: “the EKE decrease” is better than “the decrease”.
14. L276~280: Here, the variance/covariance decomposition is described in referring Appendix A. Please clarify that “the variance of monthly means” is based on averages for individual (calendar) months.
15. L351: “narrower summer jet” rather than “more narrow summer jet
16. L358: Based on Fig. 7a, the threshold jet speed over NP for EKE sensitivity to jet width seems to be “70 m/s”, rather than “80 m/s” as stated here.
17. L369: “the jet cannot be regarded simply as a fixed background state” sounds more appropriate than “the jet cannot be regarded as a fixed background state”.
18. p.20, Fig. 9: The labels along the abscissa (10E, 10W) and ordinate (10N, 10S) may be misleading. Better to replace them with (+10, –10).
19. P.20, L4 Fig. 9 caption: “in thick blue contours” is better than “in blue contours”.
20. L441: “a reduction of the SLP anomaly” is ambiguous. “a weakening of the SLP anomaly” is better.
21. L442: Isn’t it “U maximum“ rather than “KE maximum”? “KE” has not been defined yet.
22. What is meant by “left jet exit region”? Isn’t it simply “jet exit region?
23. Isn’t it “U is greater by definition“ rather than “KE is higher by definition”?
24. “wavy U pattern“ rather than “wavy KE pattern”?
25. “when the jet is stronger and more zonal” is more specific than “when the jet is more zonal”.
26. “monthly-scale averages” is more appropriate than “monthly averages”.
27. L483: “monthly-scale statistics” sounds more appropriate than “monthly averaged values”.