Please see the attached file.

Review of manuscript titled “An ERA5 Climatology of Synoptic-Scale Negative Potential Vorticity-Jet Interactions over the Western North Atlantic” by Lojko, Winters, Oertel, Jablonowski, and Payn.

Sorry for the delay in this review. This is partly because it occurred over the Easter period, and partly because I needed to do some background reading to help with the review. Please note that this is an independent review in that I have not accessed any comments already uploaded from other reviewer(s).

Summary

This is a interesting manuscript, which highlights the relevance of synoptic-scale negative potential vorticity (NPV) features in close proximity to the (North Atlantic) jet stream for jet stream strength, the propagation of waves on the jet stream and, ultimately, downstream predictability. The study provides a nice climatology of the locations and frequencies of NPV-jet ‘interactions’. A clustering technique (nicely illustrated) is used to produce three flow configurations where the interaction point occurs at different locations along a jet ridge. The relationship between moisture transport (a prerequisite for the convective tilting of vertical wind shear) in the production of NPV is examined. The role of NPV in setting the conditions for inertial instability, and its relationship to ageostrophic winds, makes it interesting for the authors to quantify the associated wave activity flux (since the constituent momentum fluxes and the flux of geopotential by the ageostrophic wind could both be strong in the vicinity of NPV-jet interactions). While I do not dispute the findings of this study, I have a few “major” and several “minor” comments and questions which mean that I suggest that this manuscript would be suitable for publication following major revision.

Major comments

1. [line 64] Rowe and Hitchman, 2016 are cited., but I wonder if enough explicit discussion is made of the potential role for inertial instability in linking the various stands of the current study? In particular, it might help clarify the meaning of “NPV-jet interactions”, which doesn’t seem to be pinned-down in the current text (unless I missed it). When discussing inertial instability, Holton comments that “near neutrality often occurs on the anticyclonic shear side of upper-level jet streaks”. So, the argument here (if I understand it) is that NPVs develop where this near neutrality is tipped into instability when convection tilts vertical shear into the vertical. The “interaction” is then the adjustment (via ageostrophic winds) to remove the instability, which (slowly?) dissipates the NPV and shifts the jet northwards and strengthens it(?)
2. [equation 1] There seems to be a few mistakes (typos?) in the printed equation. I think the second meridional-component should be ψ_y²  - ψψ_yy . Also, I would have expected to see the flux associated with the phase propagation “C_u * M” (as in Takaya and Nakamura, 2018). Later on, the various terms within Eq(1) are plotted. In Figure 9, to me it looks like “Part 1” and “Part 4” are the zonal components and “Part 2” and “Part 3” are the meridional components. Please could you clarify all these aspects?
3. [line 312] “support the results presented here that NPV-jet interactions have approximately increased by a relative amount of 11% from 2000-2021…” I also wonder what the impact of increasingly better observations over the period is on the diagnosed NPV trends in ERA5. Either directly, or through better constraint of vertical wind sheer, subsquently tilted within the data assimilation model, etc? Even if the 22-year trends are “real”, I suspect that more evidence is needed to distinguish these from natural low-frequency variability.

Minor comments

There are quite a lot of typos in this manuscript, and some missing details in figure captions. I have suggested corrections for some of these, but others may not have been highlighted.

1. [line 95] I think hourly data are available from the ERA5 archive (even if the authors choose to use 6-hourly).
2. [line 102] …are filtered TO ONLY INCLUDE THE WESTERN… (?)
3. [Figure 1] I found this figure very helpful.
4. [line 146-147] “Over 80% … shorter than 200km”. This seems a slightly confusing remark, since it is the NPV’s with length scale larger than 1650km which are the focus of this study.
5. [line 148] in THIS study.
6. [line 149] “will smooth the PV field” (not the NPV field)?
7. [line 155] “Figure 2c …” This sentence seems poorly written.
8. [line 156] “decreases QUASI-exponentially with distance away from the jet stream, AS INDICATED BY DX/DY IN FIG.2C”(?)
9. [line 171] “centred on the location of NPV-jet ‘INTERACTION’ leads to …”
10. [line 186] “enhanced led…” Typo?
11. [line 209] “PV”, not “PVU”?
12. [line 215] The second terms IN THE SQUARE BRACKETS refer…”
13. [line 232] “frequency of NPV FEATURES are…”?
14. [line 235-236] “This gradient largely follows the general pattern of mesoscale convection...” Presumably also associated with the gradient in planetary vorticity?
15. [lines 245-246] Are only the “interaction points” used in the plot, or the entire synoptic scale NPV features? I think it is the latter, but it may be useful to note that this is not the frequency of interactions, per se. 
16. [line 253] “Wind speed DIFFERENCES are …”?
17. [line 255] Change at the “98th percentile” to “at the 2% level”?
18. [line 272-273] “for particular years (NOT SHOWN)”?
19. [lines 324-327] “The maximum frequency of NPV-jet events lie adjacent to the interaction point (ORANGE DOT) along the equator ward side of the jet…”. I don’t understand why the maximum frequency is not located at exactly the centre, since fields are all centred on the interaction point. Did I miss something or can this be better described here or in the methods section?
20. [line 329] Change “positive PV gradient anomalies” to “large amplitudes in PV gradient”?
21. [line 330] Change “where the PV gradient rapidly sharpens towards much higher PVU values” to “where stratification is particularly large”?
22. [lines 340-347] I suspect that the last sentence here is the main explanation. Could this paragraph be rewritten to incorporate this sentence in the discussion about lack of local moist processes? 
23. [lines 350-351] “no matter if the synoptic-scale NPV is located on the poleward (fig. 6d-e) or equator ward (fig. 6f) flank’. I don’t see this! Are we looking at the orange dot and thick grey contour?
24. [Fig. 6] This is a very nice clear figure. Would it be useful to also show the composite-means of NPV somewhere in the figure?
25. [Fig. 6 (j)-(l)] Does this advection have the wrong sign? I see non-divergent wind in (k) and (l) crossing 2PVU in a sense that would leave the opposite sign for the advection(?)
26. [Fig. 6 caption] Change “as a result of performing a centred composite” to “as a result of performing the clustering on fields centred on their respective interaction point (indicated by the orange dot)”? Change “and the PV gradient anomaly” to “and the magnitude of the PV gradient anomaly”? For panels (g-i), need to include “The thin black contour shows where the magnitude of ageostrophic wind exceeds 15m/s”(?) Change “in m” to “in panel m” as the m could otherwise refer to metres.
27. [lines 352- 353] Is this sentence a re-telling of the previous one? I find it confusing - maybe delete(?) 
28. [line 359” Change “that diagonally extend” to “tend to cross”?
29. [line 362] Change “positive irrotational” to “positively divergent irrotational”, or simply “divergent”?
30. [line 367] Change “(Fig. 6j-l)” to “(Fig. 6l)”?
31. [line 373] “Despite THE NPV-JET INTERACTION POINT being far from…”
32. [line 384-385] “This maintenance of the amplified WAF packet coincides with the maintenance of the more amplified ridge that is of comparable magnitude to the day of NPV-jet interaction”. Is the link between the propagating ridge and WAF mainly associated with the PHASE propagation part of the WAF (which seemed to be omitted in Eq.1)?
33. [line 403] “synoptic-scale latent heating”. Why is this necessarily synoptic-scale?
34. [line 408] ‘RELATIVE vorticity”?
35. [line 419] “As a final test (NOT SHOWN)…” ?
36. [Fig. 7] Would it be useful to show the vertical wind shear?
37. [Fig. 7 caption] “The vectors show the non-divergent winds obtained from the vorticity inversion”. Does the inversion assume that relative vorticity is zero outside the grey box? If so, does this imply that the vectors outside the grey box may not be non-divergent? 
38. [line 436] “momentum FLUX term”?
39. [Fig. 8 top panel colour bar]. Should this be labelled to indicate that it is the magnitude of the non-divergent wind anomaly? Also the word “magnitude” seems to be needed in several places in the manuscript where a shaded/contoured field represents a vector field.
40. [Fig. 9 bottom panels] Please clarify which “Parts” relate to zonal and meridional WAF components
41. [Fig. 9 caption] Change to “Section 2.2.3”? Also, I don’t understand the rationale for excluding the normalisation by the base-state windspeed.
42. [line 479] I think there is confusion between the terms with a “U” and a “V” in them, and which relate to x- and y-components.
43. [line 505] Change “unique” to “based purely on”?