Review of “Modulation of the Northern polar vortex by the Hunga Tonga-Hunga Ha’apai eruption and associated surface response”, by Kuchar et al.

This paper examines the impact of the January 2022 Hunga Tonga-Hunga Ha’apai (HT) volcanic eruption on the Northern hemisphere (NH) stratosphere, and an indirect surface climate response via a stratosphere-troposphere coupling pathway. Specifically, the study uses the Earth System model SOCOLv4 with and without the HT forcing (10 ensemble members each) to show how the water vapor injection from the eruption modifies the radiative balance and dynamics of the upper stratosphere, weakening the NH polar vortex during winter-spring 2023. The resulting signal propagates downward through the stratosphere and drives anomalies in NH surface pressure and temperature. Assessment of the 2-sigma statistical significance of the simulated responses is done via Student’s t-test. The paper also speculates on the possibility of similar occurrences during the next few years if stratospheric water vapor levels from the eruption remain elevated. Some general evaluation/comparison of the model simulated aerosol and water vapor anomalies with observations is provided in Appendix A.

General comments:

The paper presents some interesting results, as a possible stratosphere-troposphere coupling mechanism and subsequent surface climate response indirectly caused by the HT stratospheric water vapor injection is of considerable interest. This is primarily a model study, as comparisons with observations are limited to the HT forcing agents, H2O and aerosols. The paper is generally well written and structured but should have additional significant clarification and/or explanation in certain places (although not quite at the "major revisions" level). These and a few other specific concerns are detailed below and should be addressed prior to publication.

The authors compare the model with observations of the HT H2O and aerosols in the Appendix. But what about comparison with observations of the model responses, e.g., temperature and ozone? For example, do the features seen in Figure 1 (for temperature and ozone) and Figure A4 (HNO3) show up in de-seasonalized MLS data? Comparing the model with vs. without the anomaly against de-seasonalized data can be difficult given the background variability, but do the model features show up at least qualitatively in the observations? The authors should discuss this. This is especially important for the downward propagation of the signal (Figure 2B) and the surface responses. The downward propagation seems to be robust in the model stratosphere, but does it show up in observations or reanalysis, at least qualitatively? And are the surface responses detectable in reanalysis data? There is only a brief mention of comparisons with stratospheric/mesospheric observations documented in other studies (L65-66). If these features are difficult to ascertain from observations due to atmospheric variability, this should be made clear. The authors seem to allude to this difficulty somewhat (L112-113), but this should be clarified.

I also found the statement on L40, “validate these simulations with observational data” to be somewhat misleading, as the model is evaluated/compared with observations in the Appendix only for H2O and aerosol (forcers), but not the model responses in temperature, ozone, etc. This statement should be qualified, or observational comparisons of the model responses should be included.

While there is a statistically significant signal in SLP and (a small area) in surface temperature (Figure 3), it’s not clear if the signal is really propagating down from the stratosphere through the troposphere. There is a vague hint of this in Figure 2, but this appears to be mostly not statistically significant in the troposphere. Do maps of, e.g., geopotential height anomalies, show significant signals in the mid and upper troposphere similar to SLP? If so, it would be important and compelling to show maps like Figure 3A, for example, at 500, 300, and/or 200 hPa. If the responses in the mid-upper troposphere are not similar and/or not significant, the authors should provide a more detailed and clear explanation as to why the surface response is still expected to be part of the “dynamical stratosphere-troposphere-surface coupling in the NH following the eruption” as stated on L97-98, and what the mechanisms are that drive the surface response. For example, is this an indication of a surface amplification of the stratospheric signal as discussed in previous studies (e.g., Baldwin et al., Rev of Geophys., 2021; Domeisen et al., JGR, 2020).

Specific comments:

L16 -17: “Massive” is appropriate for the amount of H2O injected. But for SO2, suggest changing to "modest amount of SO2", or something to that effect. 0.4 Tg is is not “massive” compared to Pinatubo, and “massive SO2” also contradicts "lower emissions of SO2" stated on L27.

L19: "have been marked"? It’s not clear what "marked" means here. Have been "studied" or "documented"? Please clarify.

L25: Note that the statistical significance of “surface warming over Eurasia” following major volcanic eruptions has been challenged recently (eg, DallaSanta and Polvani, ACP, 2022). Suggest tempering this statement.

L57-59: Does radiative cooling by the H2O anomaly drive any of the temperature response around the stratopause, or is it all due to the reduction in ozone heating? If H2O cooling is (or is not) a factor here, should briefly state this to clarify.

L 66-67: “no significant mesospheric temperature anomaly is found….”  Suggest being more specific here, e.g., change to: "….no significant persistent mesospheric temperature anomaly…."  since there's a brief significant cold anomaly in March in Fig, 2B.

L75: “we observe” should be changed to “we calculate” or something to that effect. Fig. 1S-T shows model calculations, not observations.

L77: Should define "NAM" here since this is the first time it's used in the main text, and point to section A3. Also, it would be very helpful here to give a brief description of what the positive/negative NAM is in geophysical terms, e.g., stronger/weaker zonal jet, SLP and temperature changes, etc.

L85-86: The negative anomalies close to the surface in spring in Figure 2C are not statistically significant. This at least should be mentioned. Also, the anomalies appear to extend only through April 2023 in Figure 2C; May is not included. Either the text should be modified accordingly, or the plot should be extended to include May.

L89-90: "...negative modulation of the stratospheric PV..." I assume this means a weakened polar vortex as is stated in the next sentence. If so, suggest clarifying to read something to the effect of: "This pattern is characteristic of a weaker stratospheric PV associated ..."

Also, the cited Kidston et al., 2015 reference (their Box 1) states that there is “a net poleward shift of the tropospheric jet… when the stratospheric winds are strong and westerly.”  However, the statement on L89-90: “…. negative modulation of the stratospheric PV associated with a poleward shift of the tropospheric jet stream.” seems to contradict this. This should be checked and/or clarified.

L154 - should state "return to the troposphere" somewhere here when discussing “… transport to higher latitudes …. via the Brewer-Dobson circulation (BDC).”

The excess H2O returning to the troposphere and subsequent rainout is the actual removal process (along with PSC sedimentation as is stated).

As a suggestion following from L43-44, “an outlook of how these dynamically-induced events could be further explored” (and L123-125): It would be of interest to mention that examining the response in the Southern hemisphere could be investigated in the future, especially since the upper stratospheric cooling is also significant in the SH lower latitudes in Figure 1P-T. It would also be interesting to examine if/how a possible future stratospheric response (Figure 2) could be impacted by the phase of the QBO.

Technical corrections:

Captions for Figures 1,2, and 3: When mentioning “FDR correction”, suggest pointing to section A2, since “FDR” is not discussed/defined in the main text.

Figure 2: It would be very helpful to indicate the latitude ranges at the top of each panel.

Figure 2C. Suggest reversing (or changing somehow) the colors to be consistent with panels A and B. Red colors are positive anomalies in 2A-B but are negative anomalies in Fig. 2C. This was somewhat confusing, and I had to frequently look at the color legend to be reminded of the differing color schemes.

L75: Should this be "(see Fig. 1I-J"?

L89: Change “characteristic for” to ”characteristic of”

L108: To clarify, suggest inserting “negative” before “temperature response”

Figure A1: should state that this is a global average.

Figure A3 caption, first word: Should be “Seasonal”.

Review of “Modulation of the Northern polar vortex by the Hunga Tonga-Hunga Ha’apai eruption and associated surface response,” Kuchar et al. (2024) 

General comments:

            The study authored by Kuchar et al. offers an analysis of the impacts of the water vapor and sulfur dioxide injections from the Hunga Tonga-Hunga Ha’apai eruption into the stratosphere and mesosphere using simulations with the Earth System Model SOCOLv4. They compare an ensemble of 10 simulations without the volcanic forcing to 10 simulations with the volcanic forcing to identify anomalies. They conclude that the enhanced water vapor resulted in significant dynamical responses including a weakening of the Northern Hemisphere polar vortex which is a consequence of the decreased temperature gradient. They also assert that is signal propagates downwards to drive surface-level temperature and pressure anomalies. Providing a mechanism by which volcanic eruptions influence stratospheric dynamics and ultimately the surface is a scientifically relevant complement to the radiative analyses.

Given that the study is primarily an analysis of model output, however, a more rigorous validation of the simulations’ recreation of the atmospheric response to the HT eruption is critical. Specifically, while the appendix compares the water vapor and sulfate aerosol anomalies from the model to observation, and I agree that there is large agreement in the general shape and evolution of the anomalies, how might the differences in, for example, spatial extent of the anomalies impact the conclusions? Similarly, to fully explore the performance of the model, the predictions should also be compared to observations. For example, how well do the temperature anomalies match?

Overall, the paper is exceptionally well-written, and no improvements to the presentation are necessary. Additional context on the model performance, and a brief discussion of any limitations associated with that performance, however, are needed.

Specific comments:

• Line 19: “marked” is ambiguous, in what direction?
• Lines 30-31 and 52-53: It’s likely worthwhile to also reference the work investigating the contribution of heterogeneous reactions to the HT impacts (e.g. Santee et al., JGR, 2023; and Evan et al., Science 2023)
• NAM acronym is only defined in the Figure 2 caption
• Lines 57-59: do the water vapor anomalies contribute radiatively here?

Technical corrections:

Figure 1: the top and bottom rows are slightly offset

References:

Santee, M. L., Lambert, A., Froidevaux, L., Manney, G. L., Schwartz, M. J., Millán, L. F., et al. (2023). Strong evidence of heterogeneous processing on stratospheric sulfate aerosol in the extrapolar Southern Hemisphere following the 2022 Hunga Tonga-Hunga Ha'apai eruption. Journal of Geophysical Research: Atmospheres, 128, e2023JD039169. https://doi.org/10.1029/2023JD039169

Evan, S., Brioude, J., Rosenlof, K.H., Gao, R., Portmann, R. W., Zhu, Y., et al. (2023). Rapid ozone depletion after humidification of the stratosphere by the Hunga Tonga Eruption. Science, 382, https://doi.org/10.1126/science.adg2551