Comments on "Observational perspective on SSWs and blocking from EP fluxes" by Kamilya Yessimbet, Andrea K. Steiner, Florian Ladstädter, Albert C. Ossó

In this paper, the authors examined eight major boreal Sudden Stratospheric Warming (SSW) events between 2007 and 2019 to understand the vertical coupling between the troposphere and stratosphere, as well as the relationship between SSWs and blocking events using Global Navigation Satellite System (GNSS) radio occultation (RO) observations. They classified the eight SSW events into two types of groups, i.e., "reflecting" events and "absorbing" events; two events fell into the former group, while the other six events fell into the latter one. The reflecting events were found to be displacement-type SSWs with a downward propagation of wave activity from the stratosphere to the troposphere during vortex recovery, accompanying the formation of blocking in the North Pacific region. On the other hand, the absorbing events were found to be split-type or mixed-type ones, showing the subsequent formation of blocking in the Euro-Atlantic region. The authors also showed an enhancement of the polar tropopause inversion layer as the result of SSWs, which was stronger for the absorbing events. These results are consistent with former studies and can actually reinforce the former results. For this point, the authors describe that "these results could help clarify the open question of whether split or displacement events trigger consistently different reactions in the tropospheric circulation, on which there is not yet consensus in the scientific community" (L366-367). If so, the authors should try to make dynamical discussions to answer this question as far as possible. Hence, I recommend the paper be revised with attention to the following details.

Specific Comments:

(1) L.154-156: In this analysis, eddy meridional heat fluxes are estimated at 100 hPa, while zonal mean zonal winds are calculated at 10 hPa. What does the two-day lag mean in this case? Please add comments on this point.

(2) L.159, 164: "Fp" should be "F_p"

(3) L.189-191: Negative eddy meridional heat fluxes mean the occurrence of downward propagation of wave activity. As discussed in Kodera et al. (2016), downward propagation of wave activity could enhance meandering of zonal flows, which gives a favorable condition for blocking occurrence. Therefore, the development of the North Pacific blocking might be seen during the negative eddy meridional heat flux peak. In order to more clearly see the relationship between the downward wave propagation and the blocking occurrence, I recommend the authors to make 3-d analyses by the use of 3-d wave activity fluxes, such as Plumb's (1985) one. As in Kodera et al. (2013), it might be shown that downward propagation of wave packets over North America induces a ridge over the North Pacific as well as a trough over eastern Canada in the upper troposphere.

(4) L.195-204: For the reflecting events, easterlies remain only in the upper stratosphere and westerlies still dominates the lower to middle stratosphere with a maximum in the lower stratosphere, as seen Fig. 3. In such a situation, refractive index squared might be negative in the upper flank of the westerly maximum. If so, it could be a favorable condition to wave reflection for upward propagating wave packets from the lower atmosphere, as discussed by Perlwitz and Harnik (2003). Could the authors add any discussions on this point?

(5) L.244-263: As for the absorbing events, downward descending easterlies attain to the lower stratosphere, as shown in Fig. 5, which brings about a negative signal of the Northern Annular Mode in the troposphere and generally gives a favorable condition for blocking occurrence. In this case, upward wave packets, which could be occasionally originated from blocking highs, would supply easterly momentum to maintain warming events. Such a positive feedback mechanism seems to occur in these events. I also recommend the authors to make 3-d analyses here in order to show the details, as in Comment (3). In this case, it is interesting whether to see features that continuous upward wave packets originated from blockings might contribute to the continuation SSWs.

(6) L.325-329: Since influences induced by the absorbing events can reach the UTLS region and make large temperature anomalies there, the polar tropopause inversion layer would be more evidently enhanced. Please add further discussions on this point.

References

Kodera, K., Mukougawa, H., and Fujii, A.: Influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings, J. Geophys. Res. Atmos., 118, 8333–8345, https://doi.org/10.1002/jgrd.50650, 2013.

Kodera, K., Mukougawa, H., Maury, P., Ueda, M., and Claud, C.: Absorbing and reflecting sudden stratospheric warming events and their relationship with tropospheric circulation, J. Geophys. Res. Atmos., 121, 80–94, https://doi.org/10.1002/2015JD023359, 2016.

Perlwitz, J., and Harnik, N.: Observational evidence of a stratospheric influence on the troposphere by planetary wave reflection. J. Climate, 16, 3011–3026, https://doi.org/10.1175/1520-0442(2003)016<3011:OEOASI>2.0.CO;2, 2003.

Plumb, R. A.: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 42, 217−229, https://doi.org/10.1175/1520-0469(1985)042<0217:OTTDPO>2.0.CO;2, 1985.

Summary:

This work examines the structure, origin, and tropospheric influence of eight major SSW events between 2007 and 2019. All analysis is based on GNSS RO derived temperature soundings. The data are horizontally gridded to provide the geostrophic winds needed to derive factors that characterized SSW events such as EP flux and EP flux divergence. Displacement (type 1), splitting, and mixed SSW events (type 2) are compared and contrasted. The study focuses on the relation between the lower stratosphere (100 - 10 hPa), the troposphere, and tropospheric blocking during the evolution of SSW events. Results showed that GNSS RO observations are capable of capturing the main features of the SSW events. Examination of wave reflecting and absorbing SSW events highlighted the relatively short duration of the wind reversal as well as the formation of the North Pacific blocking pattern during wave reflecting events. Results also showed that the behavior of the enhanced upward EP flux prior to the SSW events differed between the two types of warming events. Furthermore, the TIL was found to depend on the magnitude of the SSW near the tropopause.

Strengths:

The paper relates a two-dimensional, zonally averaged, dynamical analysis of SSW events to the evolution of tropospheric blocking events, providing an analysis of connections between changes in both the stratosphere and the troposphere. The detailed examination of the two main patterns identified provided a useful and engaging approach to characterizing all the warming events in the study. The dependence of the TIL on the structure of the warming event was an intriguing result.

Weakness:

1) The use of geostrophically derived winds in the stratosphere can lead to errors in the EP flux calculation. As shown in Boville (1987) the stratospheric heat flux and momentum flux errors can be as large as 40%. For example, Fig. 1 arrows at 60N and 10 hPa are nearly vertical, differing from the NH winter climatology shown in Butchart (2022, Fig. 4a). While the citations in the text claim reasonable errors when using the geostrophic approximation, some comparison with corresponding reanalysis results should be shown to justify reliance on the geostrophic approximation, especially for the stratospheric fields.

Butchart, N.: The stratosphere: a review of the dynamics and variability, Weather Clim. Dynam., 3, 1237–1272, https://doi.org/10.5194/wcd-3-1237-2022, 2022.

Boville, B. A.: The validity of the geostrophic approximation in the winter stratosphere and troposphere, J. Atmos. Sci.,44,pp 443-457, 1987.

2) It is not clear how the high vertical resolution of the GNSS RO observations contributed to the results as the vertical grid used seemed similar to current model and reanalyses. Is this work mainly a feasibility study for future work based on GNSS RO observations? More explanation can be done here.

Minor Comment:

Line 151: Kordera et al., 2016 define recovery at 50 hPa, however, the vertical line denoting recover appears to based on 10 hPa temperatures in Fig. 3a. Some explanation is needed.

Recommendation: Publish after address the two weaknesses noted above.

This paper examines the use of GNSS radio occultation (RO) data for investigating key features of sudden stratospheric warmings (SSWs). The authors grid RO data following established techniques and then calculate geostrophic winds from this gridded product. Wave diagnostics – meridional heat flux and Eliassen-Palm (EP) flux – are then extracted from these data. SSWs are identified and classified based off these data, and the evolution of wave forcing and vertical coupling are shown. The authors provide thorough discussion about this evolution for sets of known SSW types: displacement and split events, and reflecting and absorbing events. They acknowledge their sample size is small, but find evidence in support of previous work that displacement events are followed by North Pacific blocking and split events are followed by North Atlantic blocking.

The work is presented well, though the authors could consider some rearrangement of two figures. This work strictly uses observational data – along with dynamical theory – which is still somewhat novel. Other observational studies have looked at more limited samples or other features of SSWs such as only the stratospheric fields. And the thorough, deep analysis of a small, but representative set of events provides useful information for the community. The authors adeptly touched on several outstanding issues in our understanding of the wave-mean flow interaction and stratosphere-troposphere coupling that occurs around SSWs.

However, the manuscript would benefit greatly from additional work on a few topics. These are mostly related to how the GNSS data compare with other, well-studied data sets and what the GNSS data provide that is new.

1) Details of the GNSS RO data are missing. As the authors surely know, the RO method observes bending of signals through the atmosphere, which is foremost related to changes in density. This knowledge can then be used to derive geopotential height and pressure to a high degree of accuracy through the hydrostatic equation. Following this, temperature may be calculated, but only by assuming no moisture – i.e., “dry temperature.” This is a reasonable assumption in the stratosphere but will lead to large inaccuracies in the tropospheric fields. Given how much of the discussion and results rely on temperature below 300 hPa, the authors should address the limitations of its use. This is covered in the references they cite but the reader of this work would benefit from additional, relevant information here.

Alternatively, accurate temperature and humidity fields may be derived from RO using one-dimensional variational analysis. But it’s not clear if the authors used such a product, in which case the limitations of that needs to be addressed.

2) RO profile density may be an important topic to document for this study, but no details are given. RO missions and profile counts vary with time, but some measure of the sampling density should be given. Given the nature of RO sampling, this can likely be well-represented by zonal mean statistics. It would also be useful for the authors to document the occurrence, or likely rarity, of grid points that are missing RO profiles.

3) Limited comparisons with reanalyses may be a real benefit to the manuscript. As it stands, the manuscript doesn’t give the reader a sense of what the relatively high vertical resolution of the RO observations adds to our understanding of SSWs. This is most evident in the tropopause inversion layer (TIL) results. Details of the Brunt Vaisala frequency N^2 would seem to be most sensitive to vertical resolution, and RO may be able to provide additional insight, but it’s not clear what that is.

Some comparison of N^2 with, say, ERA5 for all or a limited sample of SSWs may support the authors’ claims on the benefits of the high vertical resolution of RO observations.

Additionally, the authors might consider adding one or two sentences about how their diagnosed dates of the SSWs compare with other, reanalysis-based studies.

4) Line 96: The citation to Scherllin-Pirscher et al. (2014) is not included in the references section.

5) Figures 6 and 7 could benefit from vertical stacking into two rows of 4 panels. As they’re presented, some of the details are squished into a narrow space.

6) Line 114: Suggest starting a new paragraph at “A standard algorithm…”

7) Line 150: This final sentence of this paragraph feels more appropriate in the previous section with other definitions.

8) Line 155: Recommend “concurrent with” rather than “due to” as the heat flux is a proxy for the wave activity flux that drives the zonal wind reversal.

You may consider a similar slight wording change on line 199: “led to a deceleration.”