An ERA5 Climatology of Synoptic-Scale Negative Potential Vorticity-Jet Interactions over the Western North Atlantic

Lojko, Alexander; Winters, Andrew Charles; Oertel, Annika; Jablonowski, Christiane; Payne, Ashley Elizabeth

doi:https://doi.org/10.5194/egusphere-2024-382

Preprints

https://doi.org/10.5194/egusphere-2024-382

Preprints

19 Feb 2024

| 19 Feb 2024

An ERA5 Climatology of Synoptic-Scale Negative Potential Vorticity-Jet Interactions over the Western North Atlantic

Alexander Lojko, Andrew Charles Winters, Annika Oertel, Christiane Jablonowski, and Ashley Elizabeth Payne

Abstract. Recent numerical modelling and theoretical work deduce that potential vorticity (PV) can turn negative in the Northern Hemisphere as a result of localized, convective heating embedded in vertical wind shear. It is further postulated that negative potential vorticity (NPV) may be relevant for the large-scale circulation as it has been observed to grow upscale into mesoscale bands when in close proximity to the jet stream, accelerating jet stream winds and degrading numerical weather prediction skill. However, these observations are largely confined to case studies. A composite investigation is proposed to evaluate whether strengthening of the jet stream is a robust response to NPV. This research focuses on synoptic-scale bands (>1650 km) of NPV that are in close proximity (<100 km) to the jet stream (NPV-jet) using ERA5 data from January 2000 – December 2021. Climatological characteristics show that NPV-jet interactions occur most frequently over the Western Atlantic (>1.2 % occurrence per year at particular grid-points) during Boreal Winter along 40° N. This latitude band has also seen a 11 % increase (relative-change) in NPV-jet interactions over the 22 year time-period. Separating NPV-jet interactions into distinct, large-scale flow patterns using K-means clustering illustrates the presence of a trough-ridge couplet adjacent to positive integrated vapor transport (IVT) anomalies, conducive to warm-conveyor belts and mesoscale convective systems. Even when NPV is positioned in a more adiabatic environment (i.e., far away from regions of strong IVT anomalies), robust, positive PV gradient and wind speed anomalies exist along the jet stream. Inspecting three detailed case-studies that serve as archetypes to the clusters, it is shown that the presence of NPV near the jet stream enhances wave activity flux due to NPV mutually strengthening momentum transport and the ageostrophic flux of geopotential. The results show that NPV-jet interactions can in-situ strengthen the mid-latitude jet stream and could be dynamically relevant in enhancing downstream development, despite NPV's theorized origin from sub-mesoscales.

Received: 09 Feb 2024 – Discussion started: 19 Feb 2024

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Journal article(s) based on this preprint

11 Apr 2025

An ERA5 climatology of synoptic-scale negative potential vorticity–jet interactions over the western North Atlantic

Alexander Lojko, Andrew C. Winters, Annika Oertel, Christiane Jablonowski, and Ashley E. Payne

Weather Clim. Dynam., 6, 387–411, https://doi.org/10.5194/wcd-6-387-2025,https://doi.org/10.5194/wcd-6-387-2025, 2025

Short summary

Alexander Lojko, Andrew Charles Winters, Annika Oertel, Christiane Jablonowski, and Ashley Elizabeth Payne

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2024-382', Anonymous Referee #1, 13 Mar 2024

Please see the attached file.

Citation: https://doi.org/10.5194/egusphere-2024-382-RC1
EC1: 'Two additional minor items', Michael Riemer, 20 Mar 2024

Dear authors,
in addition to reviewer comments, I have two minor points that I have noted when going through your manuscript:
- There is recent work by Prince and Evans (2022) that seems highly relevant and should be discussed: https://doi.org/10.1175/JAS-D-22-0094.1
- Fig. 6 shows a centered composite but depicts outlines of continents, which gives an incorrect sense of geographical definiteness. It is preferable to remove the continent outlines and show the composite with pseudo-lat and pseudo-lon (0,0) denoting the center of the composite.
Thank you and kind regards, Michael

Citation: https://doi.org/10.5194/egusphere-2024-382-EC1
RC2:
'Comment on egusphere-2024-382', Anonymous Referee #2, 29 Apr 2024
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
[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(?)

[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?

[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.
[line 95] I think hourly data are available from the ERA5 archive (even if the authors choose to use 6-hourly).

[line 102] …are filtered TO ONLY INCLUDE THE WESTERN… (?)

[Figure 1] I found this figure very helpful.

[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.

[line 148] in THIS study.

[line 149] “will smooth the PV field” (not the NPV field)?

[line 155] “Figure 2c …” This sentence seems poorly written.

[line 156] “decreases QUASI-exponentially with distance away from the jet stream, AS INDICATED BY DX/DY IN FIG.2C”(?)

[line 171] “centred on the location of NPV-jet ‘INTERACTION’ leads to …”

[line 186] “enhanced led…” Typo?

[line 209] “PV”, not “PVU”?

[line 215] The second terms IN THE SQUARE BRACKETS refer…”

[line 232] “frequency of NPV FEATURES are…”?

[line 235-236] “This gradient largely follows the general pattern of mesoscale convection...” Presumably also associated with the gradient in planetary vorticity?

[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.

[line 253] “Wind speed DIFFERENCES are …”?

[line 255] Change at the “98th percentile” to “at the 2% level”?

[line 272-273] “for particular years (NOT SHOWN)”?

[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?

[line 329] Change “positive PV gradient anomalies” to “large amplitudes in PV gradient”?

[line 330] Change “where the PV gradient rapidly sharpens towards much higher PVU values” to “where stratification is particularly large”?

[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?

[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?

[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?

[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(?)

[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.

[lines 352- 353] Is this sentence a re-telling of the previous one? I find it confusing - maybe delete(?)

[line 359” Change “that diagonally extend” to “tend to cross”?

[line 362] Change “positive irrotational” to “positively divergent irrotational”, or simply “divergent”?

[line 367] Change “(Fig. 6j-l)” to “(Fig. 6l)”?

[line 373] “Despite THE NPV-JET INTERACTION POINT being far from…”

[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)?

[line 403] “synoptic-scale latent heating”. Why is this necessarily synoptic-scale?

[line 408] ‘RELATIVE vorticity”?

[line 419] “As a final test (NOT SHOWN)…” ?

[Fig. 7] Would it be useful to show the vertical wind shear?

[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?

[line 436] “momentum FLUX term”?

[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.

[Fig. 9 bottom panels] Please clarify which “Parts” relate to zonal and meridional WAF components

[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.

[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.

[line 505] Change “unique” to “based purely on”?
Citation: https://doi.org/10.5194/egusphere-2024-382-RC2
AC1: 'Comment on egusphere-2024-382', Alexander Lojko, 31 Jul 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-382/egusphere-2024-382-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-382-AC1
AC2: 'Response to Reviewers', Alexander Lojko, 31 Jul 2024

Publisher’s note: this comment is a copy of AC1 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2024-382-AC2

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2024-382', Anonymous Referee #1, 13 Mar 2024

Please see the attached file.

Citation: https://doi.org/10.5194/egusphere-2024-382-RC1
EC1: 'Two additional minor items', Michael Riemer, 20 Mar 2024

Dear authors,
in addition to reviewer comments, I have two minor points that I have noted when going through your manuscript:
- There is recent work by Prince and Evans (2022) that seems highly relevant and should be discussed: https://doi.org/10.1175/JAS-D-22-0094.1
- Fig. 6 shows a centered composite but depicts outlines of continents, which gives an incorrect sense of geographical definiteness. It is preferable to remove the continent outlines and show the composite with pseudo-lat and pseudo-lon (0,0) denoting the center of the composite.
Thank you and kind regards, Michael

Citation: https://doi.org/10.5194/egusphere-2024-382-EC1
RC2:
'Comment on egusphere-2024-382', Anonymous Referee #2, 29 Apr 2024
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
[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(?)

[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?

[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.
[line 95] I think hourly data are available from the ERA5 archive (even if the authors choose to use 6-hourly).

[line 102] …are filtered TO ONLY INCLUDE THE WESTERN… (?)

[Figure 1] I found this figure very helpful.

[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.

[line 148] in THIS study.

[line 149] “will smooth the PV field” (not the NPV field)?

[line 155] “Figure 2c …” This sentence seems poorly written.

[line 156] “decreases QUASI-exponentially with distance away from the jet stream, AS INDICATED BY DX/DY IN FIG.2C”(?)

[line 171] “centred on the location of NPV-jet ‘INTERACTION’ leads to …”

[line 186] “enhanced led…” Typo?

[line 209] “PV”, not “PVU”?

[line 215] The second terms IN THE SQUARE BRACKETS refer…”

[line 232] “frequency of NPV FEATURES are…”?

[line 235-236] “This gradient largely follows the general pattern of mesoscale convection...” Presumably also associated with the gradient in planetary vorticity?

[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.

[line 253] “Wind speed DIFFERENCES are …”?

[line 255] Change at the “98th percentile” to “at the 2% level”?

[line 272-273] “for particular years (NOT SHOWN)”?

[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?

[line 329] Change “positive PV gradient anomalies” to “large amplitudes in PV gradient”?

[line 330] Change “where the PV gradient rapidly sharpens towards much higher PVU values” to “where stratification is particularly large”?

[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?

[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?

[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?

[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(?)

[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.

[lines 352- 353] Is this sentence a re-telling of the previous one? I find it confusing - maybe delete(?)

[line 359” Change “that diagonally extend” to “tend to cross”?

[line 362] Change “positive irrotational” to “positively divergent irrotational”, or simply “divergent”?

[line 367] Change “(Fig. 6j-l)” to “(Fig. 6l)”?

[line 373] “Despite THE NPV-JET INTERACTION POINT being far from…”

[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)?

[line 403] “synoptic-scale latent heating”. Why is this necessarily synoptic-scale?

[line 408] ‘RELATIVE vorticity”?

[line 419] “As a final test (NOT SHOWN)…” ?

[Fig. 7] Would it be useful to show the vertical wind shear?

[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?

[line 436] “momentum FLUX term”?

[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.

[Fig. 9 bottom panels] Please clarify which “Parts” relate to zonal and meridional WAF components

[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.

[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.

[line 505] Change “unique” to “based purely on”?
Citation: https://doi.org/10.5194/egusphere-2024-382-RC2
AC1: 'Comment on egusphere-2024-382', Alexander Lojko, 31 Jul 2024

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-382/egusphere-2024-382-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2024-382-AC1
AC2: 'Response to Reviewers', Alexander Lojko, 31 Jul 2024

Publisher’s note: this comment is a copy of AC1 and its content was therefore removed.

Citation: https://doi.org/10.5194/egusphere-2024-382-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Alexander Lojko on behalf of the Authors (01 Aug 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (26 Aug 2024) by Michael Riemer

RR by Anonymous Referee #2 (29 Aug 2024)

RR by Anonymous Referee #3 (23 Sep 2024)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

rejected

Were a revised manuscript to be sent for another round of reviews:

I would be willing to review the revised manuscript.

I would not be willing to review the revised manuscript.

Suggestions for revision or reasons for rejection

Review for ‘egusphere-2024-382-manuscript-version3’.

Summary

The manuscript explores the characteristics of negative PV features in the upper-troposphere which interact with the North Atlantic jet stream. Such features are of interest because they are diabatically generated, can influence the evolution of the jet stream and its predictability, and have been linked to clear air turbulence processes. Yet the extent to which their lifecycle (generation, evolution, dissipation) is simulated by numerical models is not well understood. The contributions of the present study are to (1) compute a climatology of negative PV features interacting with the jet stream in ERA5, (2) categorize the features based on the large-scale flow in which they are embedded, and (3) analyse three contrasting case studies to illustrate their impact on the jet stream.
I read this manuscript with interest and found it to be well-written and contain numerous interesting results. Overall, I would be very happy to see this work published, subject to one concern I feel needs addressing. I have made numerous other minor suggestions, listed below, which the authors may wish to also consider. Note that I did not review the earlier version of this manuscript. As such, I have read the revised manuscript afresh, and subsequently modified my notes after reading the previous reviews, to avoid duplication.

Major Comment

My main concern follows similar lines to RC2 major comment 2, namely that some of the signals attributed to NPVs in this work would be present even if the PV value did not go below zero (i.e. it was small, but not a NPV feature). RC2 focussed on Fig 6 (and their concern has been at least partially addressed by including a discussion of the 100-300km cases in Fig 7), but in my view the same concern holds for the relative vorticity inversion of Fig 8 (new numbering). Specifically, you invert (if i've understood correctly) the part of the relative vorticity field that lies within the NPV contour and then claim (L517) that ‘the anticyclonic vorticity associated with NPV appeared to contribute to approximately 50% of the total non-divergent wind field’. I fear this may mislead readers. There is always negative relative vorticity on the southern side of the jet (by definition of vorticity) so claiming it is all ‘associated with the NPV’ is not a fair assessment. Indeed, one could argue that the NPV feature should only be associated to that part of the (absolute) vorticity field that is negative, and I suspect inverting this (much smaller) anomaly would produce a much smaller estimate of the impact on the wind field. I note that you do include a caveat along these lines towards the end of the discussion section (L675) but feel the conclusions drawn in section 3.3.1 need adjusting to reflect this important caveat, or even better, the alternative inversion calculation presented as well for comparison. I suspect similar is also true for the inferences drawn from the WAF calcuations in sections 3.3.2 and 3.3.3, but am less familiar with that diagnostic to know for sure. Please reread those sections with this concern in mind and adjust the language used as appropriate.

Minor Comments

L24: As written, the third sentence of the Introduction overplays the role of diabatic processes relative to dry dynamics. Suggest a minor change to something like ‘These eddies, which can include a strong contribution from cloud diabatic heating processes, act to perturb the jet stream’s large-scale circulation away from its base state.’
L36: Does the irrotational wind cause the outflow air to move just polewards, or in all directions?
L72: How can the anticyclonic relative vorticity associated with NPV be an order of magnitude larger than the PV anomaly, when they have different units? Please clarify.
L140: Could you explain what you mean by a 10 point Gaussian smoother? What is the half width of the smoothing kernel?
L241: What is the streamfunction anomaly computed relative to? The seasonal mean streamfunction?
L265: Please describe the relative vorticity inversion calculation in more detail. What domain is the calculation performed over? If it’s not a global inversion, what boundary conditions are applied?

Hide

ED: Publish subject to revisions (further review by editor and referees) (31 Oct 2024) by Michael Riemer

AR by Alexander Lojko on behalf of the Authors (09 Dec 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Dec 2024) by Michael Riemer

RR by Anonymous Referee #3 (13 Jan 2025)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

rejected

Were a revised manuscript to be sent for another round of reviews:

I would be willing to review the revised manuscript.

I would not be willing to review the revised manuscript.

Suggestions for revision or reasons for rejection

Review for ‘egusphere-2024-382-manuscript-version4’ (second review).

Summary

I’d like to thank the authors for addressing my comments, and in particular for refining their methodology in response to my major comment. The new calculation provides a fairer quantification of the influence of diabatic heating on the jet stream winds, though – as noted in the response – is still very much an approximation. However, I think it is a useful exercise and (subject to the minor comment) I am happy to see the paper accepted.

Minor Comment

Paragraph starting L539: If I’ve understood correctly, the new methodology attributes to the NPV feature the change in relative vorticity between the time of NPV identification and that of a background flow of a region slightly upstream at an earlier time. To my mind this is much better than inverting the full relative vorticity anomaly as was done before, because it represents (at least some approximation to) the influence of diabatic heating over the preceding period. However:
- I do not think it is fair to attribute this wind to ‘the NPV feature’ which to me suggests it is fully associated with that part of the absolute vorticity field that is below zero. As you now point out, this is very small. I do agree that this quantity is some approximation to the influence of the diabatic heating which caused the negative PV. Can you rephrase?
- There are clearly several approximations going on here which I think need spelling out more clearly, to aid the reader. First, relative vorticity is not conserved by adiabatic flow. Second, the shape of the ridges evolve over the preceding period. Thirdly, you omit the influence of diabatic heating in any regions of positive PV (which I could imagine may counteract the acceleration associated with the negative PV region).
- Why do you claim this estimate is ‘conservative’? Please clarify.

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ED: Publish subject to revisions (further review by editor and referees) (16 Jan 2025) by Michael Riemer

AR by Alexander Lojko on behalf of the Authors (19 Jan 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (25 Jan 2025) by Michael Riemer

RR by Anonymous Referee #3 (03 Feb 2025)

ED: Publish as is (05 Feb 2025) by Michael Riemer

AR by Alexander Lojko on behalf of the Authors (06 Feb 2025)

Journal article(s) based on this preprint

11 Apr 2025

An ERA5 climatology of synoptic-scale negative potential vorticity–jet interactions over the western North Atlantic

Alexander Lojko, Andrew C. Winters, Annika Oertel, Christiane Jablonowski, and Ashley E. Payne

Weather Clim. Dynam., 6, 387–411, https://doi.org/10.5194/wcd-6-387-2025,https://doi.org/10.5194/wcd-6-387-2025, 2025

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Alexander Lojko, Andrew Charles Winters, Annika Oertel, Christiane Jablonowski, and Ashley Elizabeth Payne

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Jul 2024	19	11	0	30
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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Recent studies show that convective storms can produce anticyclonically rotating vortices (~10 km) referred to as negative potential vorticity (NPV), which can elongate to larger scales (~1000 km). Our composite analysis shows that elongated NPV frequently occurs along the Western North Atlantic tropopause where they are observed to accelerate jet stream winds and influence its evolution. This may impinge on aviation turbulence and weather forecasting despite its small-scale origin.


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