The role of tropopause polar vortices in the intensification of summer Arctic cyclones

Gray, Suzanne L.; Hodges, Kevin I.; Vautrey, Jonathan L.; Methven, John

doi:10.5194/wcd-2-1303-2021

Articles | Volume 2, issue 4

https://doi.org/10.5194/wcd-2-1303-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wcd-2-1303-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 2, issue 4

Research article

|

23 Dec 2021

Research article |

| 23 Dec 2021

The role of tropopause polar vortices in the intensification of summer Arctic cyclones

Suzanne L. Gray, Kevin I. Hodges, Jonathan L. Vautrey, and John Methven

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Final revised paper (published on 23 Dec 2021)
Preprint (discussion started on 07 Jun 2021)

Interactive discussion

Status: closed

RC1: 'Comment on wcd-2021-30', Anonymous Referee #1, 13 Jun 2021

This submission undertakes a comprehensive analysis (based on 40 years of ERA5 reanalysis) of summer Arctic TPV influences on the intensification of near-surface cyclones. As the authors point out, there has been relatively little attention devoted this topic.

The ideas, methods etc. used in the paper are described well and clearly, and the rationale and aims are succinctly expressed. In part, the investigation includes a careful stratification of the upper and low-level systems which serves to reveal much about the connections between these, and the accompanying analysis reveals some very interesting (and perhaps unexpected) results.

The submission is poised to make a significant contribution to the literature, but not in its present form. Before I would be able to recommend acceptance, there are a number of issues which need to be addressed.

Lines 29-31: In this connection with the AFZ very valuable to reference the works of

Crawford, A., and M. Serreze, 2015: A new look at the summer Arctic Frontal Zone. Journal of Climate, 28, 737-754, doi: 10.1175/jcli-d-14-00447.1.

Crawford, A. D., and M. C. Serreze, 2016: Does the summer Arctic frontal zone influence Arctic Ocean cyclone activity? Journal of Climate, 29, 4977-4993, doi: 10.1175/JCLI-D-15-0755.1.

Crawford, A. D., and M. C. Serreze, 2017: Projected changes in the Arctic Frontal Zone and summer Arctic cyclone activity in the CESM Large Ensemble. Journal of Climate, 30, 9847-9869, doi: 10.1175/JCLI-D-17-0296.1.

Line 37: ‘Cavallo et al., 2009’ should be Cavallo and Hakim, 2009’. (Similar error at lines 66, 104, 244, 261, 495, … and maybe elsewhere.)

Lines 41-65: A informative survey is presented here in connection with Arctic cyclone numbers and trends, and how results may depend on a range of techniques used. Beneficial here to cite analysis of Screen and coauthors, 2018: Polar climate change as manifest in atmospheric circulation. Current Clim. Change Reports, 4, 383-395, and Rudeva et al., 2015: Variability and trends of global atmospheric frontal activity and links with large-scale modes of variability. Jnl. Clim., 28, 3311-3330.

Lines 73-78: This text presents a nice clarification of many terms which, in this broad topic, have often been confused. Incidentally, in connection with ‘radiative cooling re-building the PV reservoir’ it would be helpful to reference the study of

Cavallo, S. M., and G. J. Hakim, 2012: Radiative impact on tropopause polar vortices over the Arctic. Monthly Weather Review, 140, 1683-1702, doi: 10.1175/MWR-D-11-00182.1.

Line 93: Change ‘Cavallo et al. (2010)’ to ‘Cavallo and Hakim (2010)’ – similar change required at ll. 119, 177, 453, 552, …

Line 127: See also the detailed vorticity budget analysis of this storm by

Ryota Ishiyama, and Hiroshi L. Tanaka, 2021: Analysis of vorticity budget for a developing extraordinary Arctic cyclone in August 2016. Sola, doi: 10.2151/sola.2021-020.

Line 146: I suggest starting a new paragraph with the ‘In this study …’. This makes it clear that the introductory survey has finished, and the authors are now going on to detail what they plan to achieve in the paper.

Line 352-354: It is not clear whether we are looking at statistically significant differences between the months. This should be checked. If, e.g., an F test does not indicate significance perhaps this comment should be deleted, as we would just be looking at noise. If the differences are above the noise level, some (thermo)dynamic explanation or hypotheses should be offered. One thought that comes to mind is that by removing the ‘>= 2 days’ constraint might mean radiative influences become more apparent, and result in result in a signal in midsummer.

Lines 445-447: Role of stratospheric influence in the storm of September 2010 was analysed by Tao W, Zhang J, Zhang X (2017) The role of stratosphere vortex downward intrusion in a long-lasting late-summer Arctic storm. Quart. J. Roy. Meteor. Soc. 143:1953-1966

Lines 572-575: It would be worthy of note in the paper to mention the importance of this stratospheric influence on rapid development outside the Arctic, e.g., Kouroutzoglou et al., 2015: On the dynamics of a case study of explosive cyclogenesis in the Mediterranean. , 127, 49-73.

Lines 597-600: A gremlim seems to have gotten into the author list for these two papers by Steve Cavallo and Greg Hakim.

Citation: https://doi.org/10.5194/wcd-2021-30-RC1
RC2: 'Comment on wcd-2021-30', Anonymous Referee #2, 06 Jul 2021

General comments:

This research investigated the relationship between Arctic cyclone (surface cyclone) and tropopause polar vortices (TPVs) using the same tracking algorithm. The authors analyzed the features of the track density and composited structure for Arctic cyclones by separating matched and unmatched cyclones. The results showed that most of the Arctic cyclones are far from the TPVs at their initiation, and about one-third of the Arctic cyclones developed associated with the TPVs. They also showed that while the genesis of the unmatched cyclones is along the Eurasia coastline, that of the matched cyclone generated over the Arctic Ocean, North America, and the Canadian Arctic Archipelago. The rearward title of relative vorticity for matched cyclones is less than that for unmatched cyclones. The topic is very interesting, and this study shows the relationship between Arctic cyclones and TPVs clearly for the first time. The findings in this study would promote the understanding of the cyclone development over the Arctic in summer. Therefore, the reviewer recommends for publication of this article in WCD after minor revisions.

Comments:

Lines 37: Tao et al (2017, QJRMS) also showed the importance of TPVs for the intensification of an Arctic cyclone.

Tao, W., Zhang, J., & Zhang, X. (2017). The role of stratosphere vortex downward intrusion in a long-lasting late-summer Arctic storm. Quarterly Journal of the Royal Meteorological Society, 143(705), 1953–1966. https://doi.org/10.1002/qj.3055

Line 215: Is the percentage of the cyclone associated with the TPVs within 2º is intermediate between 1º and 3º in Fig. 5? While the author showed the percentage of the cyclone associated with the TPV within 1º, 3º, and 5º at each stage (Fig. 5), the percentage of the matched cyclone (i.e., cyclones satisfied both criteria) is not shown in the results.

Line 279-281: Do the track density and cyclone features show similar results to the Figs. 1-3 in each month? Or these features have monthly variability?

Lines 285: Does the timing of the maximum relative vorticity match with that of the minimum SLP? Or they have some lag?

Fig. 4: Including the NAO index would be better for understanding Line 304-317.

Lines 334-336: Why the number of unmatched cyclones is similar to matched cyclones? Isn't a cyclone classified as an unmatched cyclone when it does not satisfy the criteria of a matched cyclone?

Lines 340-341: Is there any relationship between the location of the local maximum of genesis density for the matched cyclone and surface condition (e.g., cyclone generated over the marginal ice zone in Inoue and Hori (2011, SOLA))?

Inoue, J., & Hori, M. E. (2011). Arctic cyclogenesis at the marginal ice zone: A contributory mechanism for the temperature amplification? Geophysical Research Letters, 38(May), 1–6. https://doi.org/10.1029/2011GL047696

Lines 407-409 and 450-452: Do these sentences indicate that the PV at the middle-troposphere in the matched cyclones is due to the frictional processes? If so, is the middle-tropospheric PV for each cyclone influenced by the surface condition (over the land, ocean, and sea ice)?

Citation: https://doi.org/10.5194/wcd-2021-30-RC2
RC3:
'Comment on wcd-2021-30', Anonymous Referee #3, 07 Jul 2021
General comments:
This study evaluates how often pre-existing tropopause-level features are present for Arctic cyclone development. Specifically, the authors seek to quantify the frequency that the tropopause-level disturbance called tropopause polar vortices (TPVs) are linked to Arctic cyclones during several stages of the cyclone’s lifecycle. The analysis is performed by the authors computing tracks of TPVs and Arctic cyclones from ERA5 based on certain flow metrics and distance thresholds. Using these methods, the authors find a rather surprising result that at most, 10 percent of Arctic cyclones have a nearby TPV at genesis, while 35 and 38 percent of Arctic cyclones are in close proximity to a TPV at the maximum growth rate and intensity, respectively. This is surprising because the alternative explanation that is offered is that Pettersen type A cyclogenesis must be more common, where the tropopause-level anomaly is generated as a result of baroclinic interactions with low levels.

The study is novel in that the relation of TPVs and Arctic cyclones has never before been systematically quantified as has similarly been established in midlatitudes between tropopause-level features and surface cyclone development. Although I point out some serious flaws below, I do think that this preliminary work warrants further analysis and ultimately could successfully answer the scientific question to make a positive and notable contribution after major revisions. In particular, my most primary concerns at this stage are:

1)   The main argument presented for type A cyclogenesis as I read it is that TPVs are not evident near the unmatched Arctic Cyclones since relative vorticity does not increase with height. By thermal wind balance, this implies that there should not be a minimum potential temperature anomaly at all levels (up to the tropopause). However, the cross-sections for each case in the composites are all taken in the same (along-track) direction. Unlike baroclinic waves, where there is usually a westward tilt with height, in the case of TPVs, TPVs could lie in any direction from the center of the surface cyclone, and may not be large enough to span even half a quadrant. Dynamically, this would still create favorable baroclinic conditions needed for an Arctic Cyclone to form or intensify due to a TPV. One additional complication for using the along-track direction is that there may be multiple TPVs influencing the surface cyclone over the course of its lifecycle, and hence a surface cyclone’s trajectory may not be straightforward as it is in midlatitudes where there is typically one relatively large upshear PV anomaly. In addition, it is further used as evidence that in the unmatched cases, there must be a diabatic and/or frictional process to create upper-level PV in conservative flow (lines 406- 409), but the reasoning does not account for additional features that may be (conservatively) advected from elsewhere to influence the nearby flow. For example, there can be multiple TPVs rotating around a larger-scale feature, or TPVs moving toward the cyclone around the flow of lower-latitude Rossby waves or wave breaks. Thus, cross-section direction is likely unique for each individual case, and important characteristics may be inadvertently missed by fixing cross-sections to along-track direction. While it would be helpful for the authors to state the details of their calculation instead of referring to Bengtsson et al. (2009), I am somewhat familiar with their method. Nonetheless, the tilt method is similar to that of the cross-sections by projecting vorticity in a certain direction and thus would suffer from the same shortcomings as the cross-sections.

2)   The choice of matching cyclones within up to 5 degrees (or 555 km) is too restrictive. A reason is given on lines 210-214, but even when just considering the Coriolis parameter, alone, the Rossby radius of deformation varies quite substantially between the pole and 65N. Cavallo and Hakim (2010) find that TPVs radii can range up to around 1000 km, and Aizawa and Tanaka (2016) find that Arctic Cyclones can have radii up to ~2500 km. This puts a maximum spatial scale for which one could expect from observed cases in the peer-reviewed literature that the possible influence between an upper-level and lower-level PV anomaly to be somewhere in the 1000-2500 km range. I appreciate the sensitivity test to the chosen threshold at the beginning of Section 3.2, but there should also be tests done at larger threshold distances. Perhaps a sensitivity test could be done by varying the chosen threshold and performing Monte-Carlo tests between Arctic Cyclones and a random drawing of TPVs. See the details in Lillo et al. (2021) (Figure 17) for a similar test between cold air outbreak locations and TPVs.

3)   I do also have some concerns that this analysis could very likely be filtering out a large portion of TPVs. The spectral filtering of T5-T63 would remove features equal to or less than about 400 km. This is near the mean radius of TPVs (Cavallo and Hakim 2010) and therefore, given the ERA5 resolution of 30 km, would be eliminating perhaps as many as half of all TPVs. See in particular Figure 2a in Cavallo and Hakim (2010). Also compare with Figure 6a in Hakim and Canavan (2005). Note the differences in radii, where Hakim and Canavan (2005) may also suffer from the effects of spectral filtering while Cavallo and Hakim (2010) use a regularly-spaced grid and conversely does not suffer the same effect. The spectral filtering here may also be removing more features toward the pole, and this is evident in Figure 2a,b, where there is a very dampened summer Arctic cyclone maximum near the pole in contrast to Serreze and Barrett (2008). I suggest using the track data that is described in Szapiro and Cavallo (2018), which uses a watershed segmentation approach to identify minima. They also compare directly with Hakim and Canavan (2005), and hence this would keep results most consistent with previous results. While those results do not use ERA5, I would think it is quite likely these data with ERA5 are freely available if requested. This would be far more advantageous than just using the same tracking algorithm for both surface cyclones and TPVs.
Specific comments:
1)   Lines 168,192: What is the rationale for requiring systems to be mobile?

2)   Line 184: Are theta anomalies identified with respect to a (climatological) time mean? If so, tracking minima in theta anomalies will also potentially remove some TPVs. For example, in certain regions of the Arctic where TPVs are very common/frequent, a relatively weak (but dynamically significant) TPV for that location may not necessarily be accompanied by a negative anomaly.

3)   This approach takes the view that there can only be one TPV associated with an Arctic Cyclone. For example, reducing the matching radius from 5 to 2 degrees between maximum growth and maximum intensity. But why can there not be more than one TPV associated with an Arctic Cyclone? Perhaps one TPV could be responsible for an initial growth cycle of an Arctic Cyclone until it becomes vertically tilted, but then another nearby TPV could then “take over” to contribute to either further growth or sustained longevity. It also seems that as an Arctic Cyclone matures and obtains a larger radius, that the scale of potential influence from other nearby TPVs could also increase considerably due to the cyclone’s larger scale.

4)   In comparing the Figure 2 results to Cavallo and Hakim (2009) around line 261, it seems notable to point out that Figure 2d shows fewer TPVs over the Northern Siberian coast /Arctic Frontal Zone area; In particular, compare with Figure 1 of Cavallo and Hakim (2010). This may be important to note given the results found later in this study that there are a relatively high number of unmatched TPVs and Arctic cyclones near the Arctic Frontal Zone (around line 340).

Lines 276-282: The long-lived TPV described in Szapiro and Cavallo (2018) started out relatively small in July 2006 and grew upscale until September. This is contrary to the speculation here. Perhaps the explanation is that the TPVs continue to grow radiatively until an external factor can influence it, such as the jet stream, which is weak in the summer as described in Cavallo and Hakim (2012). This is also consistent with the finding here that TPVs of Arctic genesis have longer

Technical corrections:

It is confusing that it states that minimum lifetimes are required to be 1 day for TPVs on line 192. Is it not 2 days that is often referred to later for both TPVs and Arctic Cyclones?

Lines 264-265: There should be a reference to Figure 2(f) at the end of this sentence as it refers to TPVs that form outside the

Line 372: Insert “(ξ)” after Relative vorticity since this notation is not defined

References:

Aizawa, T. and H. L. Tanaka, 2016: Axisymmetric structure of the long-lasting summer Arctic cyclones. Polar Science, 10 (3), 192-198.

Bengtsson, L., K. I. Hodges, and N. Keenlyside, 2009: Will extratropical storms intensify in a warmer climate? J. Climate, 22, 2276-2301.

Cavallo, S. M. and G. J. Hakim, 2009: Potential vorticity diagnosis of a tropopause polar cyclone. Mon. Wea. Rev., 137 (4), 1358-1371.

Cavallo, S. M. and G. J. Hakim, 2010: The composite structure of tropopause polar cyclones from a mesoscale model. Mon. Wea. Rev., 138 (10), 3840-3857, doi:10.1175/2010MWR3371.1.

Cavallo, S. M. and G. J. Hakim, 2012: Radiative impact on tropopause polar vortices over the arctic. Mon. Wea. Rev., 140 (5), 1683-1702.

Hakim, G. J. and A. K. Canavan, 2005: Observed cyclone-anticyclone tropopause asymmetries. J. Atmos. Sci., 62 (1), 231-240.

Lillo, S. P., S. M. Cavallo, D. B. Parsons, and C. Riedel, 2021: The role of a tropopause polar vortex in the generation of the January 2019 extreme arctic outbreak. J. Atmos. Sci., Accepted, doi:10.1175/JAS-D-20-0285.1.

Serreze, M. C. and A. P. Barrett, 2008: The summer cyclone maximum over the central Arctic ocean. J. Climate, 21 (5), 1048-1065, doi:10.1175/2007JCLI1810.1.

Szapiro, N. and S. M. Cavallo, 2018: Tpvtrack (v1.0): A watershed segmentation and overlap correspondence method for tracking tropopause polar vortices. Geoscientific Model Development, 11 (12), 5173-5187.
Citation: https://doi.org/10.5194/wcd-2021-30-RC3
AC1: 'Response to reviewers', Suzanne L. Gray, 12 Aug 2021

We thank the three reviewers for taking the time to read our paper and provide these detailed and thoughtful reviews. Please see the attached file for our point-by-point response.

Citation: https://doi.org/10.5194/wcd-2021-30-AC1

Peer review completion

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

AR by Suzanne L. Gray on behalf of the Authors (12 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Aug 2021) by Stephan Pfahl

RR by Anonymous Referee #3 (02 Sep 2021)

ED: Publish subject to revisions (further review by editor and referees) (06 Sep 2021) by Stephan Pfahl

AR by Suzanne L. Gray on behalf of the Authors (01 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (05 Nov 2021) by Stephan Pfahl

Dear Sue and co-authors

Thanks for again revising your manuscript based on the reviewer's suggestions, and in particular for including the example case, which helps illustrating your matching procedure. Before being able to accept your manuscript for final publication, I have two minor questions that I'd like you to respond to:

- In your response to the reviewer's first comment, you state that "We do not assume that Arctic cyclones evolve from a tilted structure to a less tilted structure.". This is a bit in contrast to your statement in line 240 of the revised manuscript: "These feature separation thresholds were chosen assuming that the Arctic cyclones evolved from a tilted vertical structure to a less tilted vertically-aligned structure at maturity (maximum intensity)", which I think the reviewer was referring to. I understand that the assumption of the tilt is only needed to argue for the smaller required distance at the time of maximum intensity, but maybe you can still adapt the formulation a bit to avoid this contrast between the manuscript and your response.
- In your response to another comment, you state that: " The composites are generated using cross-sections are centred on the cyclones and so will include all upper-tropospheric features (and any TPVs) in the vicinity of those cyclones at the reference time used for compositing.". I'm not sure that I understand this point: Of course, TPVs in any direction are considered in your matching approach, but in the vertical cross section, I only see anomalies along the direction of cyclone motion, right? So theoretically there may be vorticity anomalies in another direction (e.g., perpendicular to the track) that are not considered in these plots (corresponding to the "satellite TPVs" mentioned by the reviewer). Can you comment on this?

Best regards,
Stephan

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AR by Suzanne L. Gray on behalf of the Authors (15 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (18 Nov 2021) by Stephan Pfahl

AR by Suzanne L. Gray on behalf of the Authors (19 Nov 2021) Manuscript

Short summary

This research demonstrates, using feature identification and tracking, that anticlockwise rotating vortices at about 7 km altitude called tropopause polar vortices frequently interact with storms developing in the Arctic region, affecting their structure and where they occur. This interaction has implications for the predictability of Arctic weather, given the long lifetime but a relatively small spatial scale of these vortices compared with the density of the polar observation network.