Evaluating Long-Term Variability of the Arctic Stratospheric Polar Vortex Simulated by CMIP6 Models

Round 1

Reviewer 1 Report (Previous Reviewer 1)

The manuscript “Evaluating Long-term Variability of the Arctic Stratosphere 2 Polar Vortex Simulated by CMIP6 Models”, improved substantially, the authors made an effort to include the comments of the reviewers in this corrected version.

I appreciate the detailed response document to my initial comments.

 

Author Response

Thanks for your comments! According to the comments of 2nd reviewer and editor, we have made further improvement for this manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report (Previous Reviewer 2)

The work contains valuable results on the stratospheric polar vortex dynamics in the Arctic. In comparison with the previous version, MERRA2 is selected as the primary data source for quality assessment of 16 CMIP6 models. The text has been modified, particularly, by changing the approach to the use of reanalysis data, but some necessary changes have been also included in the Introduction, and the design of the paper is improved. In my opinion, the work has a high scientific level and it can be published in Remote Sensing after minor revision. A series of remarks are contained in the attached file.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer #2’s Comments:

Siyi Zhao et al. (Author)

We are very grateful for the Reviewer#2's detailed comments and suggestions, which help us improve this paper significantly. Based on the comments and suggestions from the reviewer, we add some details to some sections in order to make the manuscript more informative.

 

Please see our point-by-point reply to comments.

Specific Responds:

(1) L. 37. It is worth adding more key words because the available ones repeat the title of the article.

Response: Thank you for your comment. We added ‘Planetary wave’ and ‘Wave-mean flow interaction’ as two key words (see L37-38).

 

(2) L. 62. ‘sea-ice’ – the hyphen seems unnecessary (as in ‘sea-surface’ in L. 63).

Response: Thank you for your comment and corrected (see L63, 64).

 

(3) L. 104. ‘  (ENSO )’ – a space after the parenthesis is not needed.

Response: Thank you for your comment and we have deleted the space (see L106).

 

(4) L. 112. ‘more realistic in the simulations of high-top models than that in the simulations of low-top models’ – a comment is desirable on how low-top and high-top models are differentiated.

Response: Thank you for your comment. The models were classified into a high-top and low-top ensemble based primarily upon their lid height, with a threshold between high-top and low-top at 1 hPa (see L113-115). The models have the model top above the stratopause and relatively fine stratospheric vertical resolution (high-top), and those that have a model top below the stratopause (low-top) (Charlton-Perez, et al., 2013; Hurwitz, et al., 2014).

 

(5) L. 171-173. ‘In this study, daily meteorological data obtained from the NASA Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) product, which has a horizontal resolution of 1.25°×1.25° (latitude × longitude) and there are 42 pressure levels…’ – a proper predicate is absent in the first part of the sentence.

Response: Thank you for your comment and corrected (see L171).

 

(6) L. 177. ‘PV’ – the abbreviation for planetary vorticity appears for the first time in the main text and should be explained.

Response: Thank you for your comment! In this study ‘PV’ denotes potential vorticity and we have added the full name where it appears for the first time (L20).

 

(7) L. 219. Table 1. ‘top level sigma = 0.0002’ – this description of INM-CM5-0 is significantly distinct from the others and should be commented on.

Response: Thank you for your comment. According to the official website of INM-CM5-0, available online: https://doi.org/10.22033/ESGF/CMIP6.5070, the top level of INM-CM5-0 is sigma = 0.0002, which is about 0.20hPa (see 2-nd column in Table 1). In the same time, Volodin et al. (2017) pointed that the upper boundary of the calculating area lies at the altitude of about 60 km.

 

(8) L. 233-234. ‘The fractional area of the polar vortex is defined as the percentage area of the polar vortex that covers a hemisphere divided by the total area of the polar vortex’ – this definition is unclear in comparison with Fig. 9 where fractional area is calculated for Eurasia and North America, not for a hemisphere.

Response: Thank you for your comment. We have corrected the definition of fractional area of the polar vortex as the area covering Eurasia/ North America divided by the total area of the polar vortex, respectively (see L240-241).

 

(9) L. 247. ‘latitude and longitude of the ith grid’ – probably, ‘of the ith grid point’ (see also L. 273).

Response: Thank you for your comment and corrected (see L256, 282).

 

(10) L. 261. ‘u and v represents zonal and meridional wind, respectively’ – the plural form should be, ‘u and v represent’ .

Response: Thank you for your comment and corrected (see L270).

 

(11) L. 274. ‘The model mean and MERRA2 mean are the weighted area-averaged mean’ – probably the plural form should be, ‘are the weighted area-averaged means’ .

Response: Thank you for your comment and corrected (see L283).

 

(12) L. 315 (Table 2). Are the deviations of Reanalysis from MERRA-2 (–0.37%) and ERA-I (2.30%) consistent with the deviations of MERRA-2 and ERA-I from the Reanalysis (±1.32%)?

Response: Thank you for your comment. The polar vortex area of Reanalysis in the previous manuscript is not correct and we have corrected it as 2113.67 (10⁴km²). Thus, the deviations of Reanalysis from MERRA-2 and ERA-I should be –1.30% and 1.34%. The denominator changes according to the formula for the deviation by. To avoid confusion and misleading, we removed the rows and columns associated with Reanalysis mean in the revised paper.

 

(13) L. 307-308. ‘averaged over the isentropic levels between 430– 600K’ – a scheme of altitude averaging is not very obvious and should be briefly described.

Response: Thank you for your comment. In this study, we averaged the potential vorticity at the isentropic surface between 430-600K directly.

 

(14) L. 315 (Table 2). The right side of the last column on P. 10 in Table 2 is cropped.

Response: Thank you for your comment and corrected (see Table 2).

 

(15) L. 352. ‘The simulated polar vortex area is larger than the MERRA2 dataset for all CMIP6 models except CESM2- WACCM,  NorESM2-LM and  NorESM2-MM’ – the  new approach  uses a comparison with  MERRA2, where  IPSL- CM5A2-INCA also exhibits a negative bias –0.83% (Table 3). Therefore, the subsequent calculation of the correlation coefficient excluding only three models becomes weakly motivated.

Response: Thank you for your comment and corrected. When results of the four models (CESM2-WACCM, NorESM2-LM, NorESM2-MM and IPSL-CM5A2-INCA) are excluded, the correlation coefficient between area bias and strength bias is –0.48, statistically significant at 95% confidence level (see L363-365).

 

(16) L. 365. What is ‘average mean’?

Response: Thank you for your comment and corrected (L376). The average mean indicates that the potential vorticity is averaged in the lower stratosphere (averaged over the isentropic levels between 430–600 K).

 

(17) L. 369. This text should not be crossed out.

Response: Thank you for your comment. We added this text in the revised manuscript (see L382-383).

 

(18) L. 463. ‘A significant positive correlation’ – what indicates the significance of this correlation?

Response: Thank you for your comment. ‘A significant positive correlation’ in the previous manuscript is not correct and we have corrected it as ‘A positive correlation’ (L477).

 

(19) L. 476-478. ‘with the trend of zonal mean zonal wind at 100hPa averaged over 45–75°N and the trend of zonal mean zonal winds between 1 and 100hPa averaged over 60–90°N’ – from this caption, it is unclear, which trend is shown in Fig. 7.

Response: Thank you for your comment. We have revised the figure caption for clarity. As shown below: Figure 7. Correlation between the trend in EP-flux divergence of zonal wavenumber-1 accumulated in early winter from 10 December to 10 January at 1-10 hPa averaged over 45–­­­75°N and the trend in zonal mean zonal winds averaged at 1-100hPa over 60–90°N in the late winter from 10 January to 28 February for the period 1980/81–­­­2013/14 (see L489-492).

 

(20) L. 571. ‘in Feb.’ – it is worth indicating the full name of month. Then the words ‘in February’ at the end of the sentence are not needed.

Response: Thank you for your comment and corrected (see L589).

 

(21) L. 622. ‘soft-ware’ – the hyphen is unnecessary.

Response: Thank you for your comment and corrected (see L663).

 

(22) L. 624. ‘S.Z. , C.Z.’ – a space before the comma is not required.

Response: Thank you for your comment and corrected (see L665).

 

(23) L. 698. ‘Polvani, L.M.; & Waugh, D.W.’ – spaces between initials are absent here, unlike most other references.

Response: Thank you for your comment and corrected (see L740, 742).

 

(24) L. 702. This reference should be [25].

Response: Thank you for your comment and corrected (see L751).

 

References

Charlton-Perez, A. J.; Baldwin, M. P.; Birner, T.; Black, R. X.; Butler, A. H.; Calvo, N.; Davis, N. A.; Gerber, E. P.; Gillett, N.; Hardiman, S.; Kim, J.; Krüger, K.; Lee, Y. –Y.; Manzini, E.; McDaniel, B. A.; Polvani, L.; Reichler, T.; Shaw, T. A.; Sigmond, M.; Son, S. –W.; Toohey, M.; Wilcox, L.; Yoden, S.; Christiansen, B.; Lott, F.; Shindell, D.; Yukimoto, S.; & Watanabe, S. On the lack of stratospheric dynamical variability in low-top versions of the CMIP5 models, Journal of Geophysical Resarch: Atmospheres, 2013, 118, 2494–2505.

Hurwitz, M. M.; Calvo, N.; Garfinkel, C. I.; Butler, A. H.; Ineson, S.; Cagnazzo, C.; Manzini, E.; & Peña-Ortiz, C. Extra-tropical atmospheric response to ENSO in the CMIP5 models. Climate Dynamics, 2014, 43, 3367–3376.

Volodin, E. M.; Mortikov, E. V.; Kostrykin, S. V.; Galin, V. Y.; Lykosov, V. N.; Gritsun, A. S.; Diansky, N.A.; Gusev, A.V.; & Yakovlev, N. G. Simulation of modern climate with the new version of the INM RAS climate model. Izvestiya, Atmospheric and Oceanic Physics, 2017, 53(2), 142–155.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comment to authors

The manuscript “Evaluating Long-term Variability of the Arctic Stratospheric Polar Vortex Simulated by CMIP6 Models” presents an evaluation of the performance of 16 CMIP6 models in the analysis of the polar vortex, in addition to two databases, a reanalysis and an observational MERRA2. The analysis is interesting for a period of time appropriate for trends and anomaly values. The research is interesting; however, there are some issues that the authors must clarify. For example, in the methodology, a greater description is required on how the CMIP6 data were analyzed concerning the reanalysis and MERRA2, that is, about the spatial and temporal resolution of all the databases, that is, was a remap of grids done?

The authors use the ERA-Interim reanalysis, why not use the ERA5 version for the calculations?

It is not clear to me why including a “reanalysis mean” seems redundant in the analysis.

Several concepts are mentioned at the beginning that I would expect to be discussed based on the results, for example, the role of climatic indices such as ENSO, the same on the behavior of the jet stream, and the SST influence. The wording of the text becomes confusing as much of the discussion is included in the results section

I suggest making adjustments to the manuscript to be evaluated and eventually accepted.

 

Specifics  comments

 

L122 Define the acronym AO

 

L1550 Please correct the typo MEERA

 

L271 Table 2 The acronyms PV and PVS generate confusion. I suppose it is the center where PV is maximum, which indicates PVS?

 

L305-309 The relationship of the bias inside and outside the vortex is not very clear, because the amplitude of the variations is very large.

Reviewer 2 Report

The article studies the representation of the Arctic stratospheric polar vortex by 16 CMIP6 models in comparison with the data of ERA-Interim and MERRA-2 reanalysis, covering the time range from 1980 to 2014. The obtained results show small discrepancies for the vortex area and shape, but with an underestimation of the polar vortex strength by CMIP6 models on average. The manuscript contains a lot of interesting material that will be useful in other stratosphere studies. In particular, it is worth to mention Fig. 5, which displays an underestimation of zonal wind velocity by most of the analyzed models in the first half of winter and the following overestimation by many of them in the second half.

One of the positive aspects of the paper is its systematical structure with the inclusion of many models, two reanalyses and calculations for a long time range exceeding three decades. Without any doubt, a similar work for atmospheric conditions after 2014 would also be very interesting, and I hope that such a study may appear in the future. Among other things, a trend in the vortex location needs further research.

Notes and comments are collected in the separate file. Most of them are technical, a few more important remarks concern the clear presentation of the results for readers. Particularly, I consider useful a brief explanation of how optimal the calendar winter is to characterize the polar vortex. Some captions to figures should be expanded (see comments to L. 388, 409). There are almost no typos in the main text, but they appear in the list of references and, of course, should be corrected.

In general, the article proposes interesting results with a good design and, in my opinion, can be published in Remote Sensing after minor revision.

Comments for author File: Comments.pdf