the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2024
Modulation of the Northern polar vortex by the Hunga Tonga-Hunga Ha'apai eruption and associated surface response
Abstract. The January 2022 Hunga Tonga-Hunga Ha'apai (HT) eruption injected sulfur dioxide and unprecedented amounts of water vapor (WV) into the stratosphere. Given the manifold impacts of previous volcanic eruptions, the full implications of these emissions are a topic of active research. This study explores the dynamical implications of the perturbed upper atmospheric composition using an ensemble simulation with the Earth System Model SOCOLv4. The simulations replicate the observed anomalies in the stratosphere and lower mesosphere's chemical composition and reveal a novel pathway linking water-rich volcanic eruptions to surface climate anomalies. We show that in early 2023 the excess WV caused significant negative anomalies in tropical upper-stratospheric/mesospheric ozone and temperature, forcing an atmospheric circulation response that particularly affects the Northern Hemisphere polar vortex (PV). The decreased temperature gradient leads to a weakening of the PV, which propagates downward similarly to sudden stratospheric warmings (SSWs) and drives surface anomalies via stratosphere-troposphere coupling. These results underscore the potential for HT to create favorable conditions for SSWs in subsequent winters as long as the near-stratopause cooling effect of excess WV persists. Our findings highlight the complex interactions between volcanic activity and climate dynamics and offer crucial insights for future climate modeling and attribution.
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Status: final response (author comments only)
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CC1: 'Comment on egusphere-2024-1909', Simon Lee, 29 Aug 2024
Beginning on L116, the authors state "For the first time since records began in the mid 20th century, three SSW-like events have been detected during the extended winter 2023/2024. Ineson et al. (2024), using a large model ensemble, showed that such event series has a return period of about 250 years". There are several major problems with such a statement. First, what constitutes an "SSW-like" event, versus the extensively-defined and rigorously-tested definitions of major SSWs? If the authors do wish to claim that three "SSW-like" events happened for the first time, then they would need to provide evidence (and indeed, go to quite some effort to justify their definition of 'SSW-like events' versus the existing definitions of actual major SSWs). So, I can only assume that the authors are conflating whatever their "SSW-like" events are, with records of major SSWs... according to all published criteria used to define major SSWs , winter 2023/2024 saw only TWO major events. It is indeed correct to say that no observed winter has seen three major SSWs, but one cannot then use that in the context of discussing whatever an "SSW-like" event is! Furthermore, the analysis undertaken by the cited Ineson et al. (2024) paper use the well-defined classification criteria for major SSWs. Thus, one cannot then use their results – which are specifically about major SSWs – to then talk about "SSW-like" events. This is immensely confusing and risks generating hype about the SSWs in 2023/2024 that is entirely unfounded.
Citation: https://doi.org/10.5194/egusphere-2024-1909-CC1 -
CC2: 'Reply on CC1', Amy Butler, 29 Aug 2024
I wanted to echo Dr. Lee's sentiments about the statement on lines 116-117: “For the first time since records began in the mid 20th century, three SSW-like events have been detected during the extended winter 2023/2024.” It’s likely this statement originates from (non-peer reviewed) media reports: https://blog.metoffice.gov.uk/2024/03/06/one-in-250-year-event-underway-high-in-the-atmosphere/
There were only two major SSWs in 2023-2024. Across multiple reanalysis products, the daily-mean zonal-mean winds only reversed twice: once on 16 Jan 2024, and again on 4 Mar 2024. While there was a near-major SSW in February, the winds did not fall below 0 m/s. Even if the winds had fallen below 0 m/s, they would not have been separated from the 4 Mar 2024 event by 20 consecutive days of westerlies, so again, only 2 major SSWs would have been counted.
The authors have tempered their statement by calling the events “SSW-like”. It’s not clear what an “SSW-like” event is, but in any case, the return periods in Ineson et al. 2024 are based on major SSWs only (with the same “separation criteria” also mentioned above), so that the cited 1-in-250 year return period would not apply to the 2023-24 winter. Rather, the relevant numbers should be those winters with two major SSWs. In this case, the return date is once in every 8-9 years.
Winters with minor SSW events or a mix of minor and major events are common. For example, adjusting the threshold from 0 m/s to 5 m/s increases the observed SSW frequency by 40% (Butler and Gerber 2018).
In the context of this paper, there is also no need for an overstatement of what happened- the 2023-2024 winter was dynamically active in the stratosphere by many other measures. I suggest this statement be removed or re-written.
Citation: https://doi.org/10.5194/egusphere-2024-1909-CC2
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CC2: 'Reply on CC1', Amy Butler, 29 Aug 2024
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AC1: 'Comment on egusphere-2024-1909', Ales Kuchar, 05 Sep 2024
We agree with the comments of Dr. Lee and Dr. Butler that there is no need for an overstatement of what happened in the 2023-2024 winter in the context of this paper. As shown below, we document two major SSWs in the 2023-2024 winter. Therefore, we will limit the sentence to the following statement: "Two major SSWs have been detected during the extended winter 2023/2024 (see Fig. A7). Our model-projected forcing ..."
Citation: https://doi.org/10.5194/egusphere-2024-1909-AC1 -
RC1: 'Comment on egusphere-2024-1909', Anonymous Referee #1, 13 Sep 2024
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”.
Citation: https://doi.org/10.5194/egusphere-2024-1909-RC1 -
RC2: 'Comment on egusphere-2024-1909', Anonymous Referee #2, 15 Sep 2024
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
Citation: https://doi.org/10.5194/egusphere-2024-1909-RC2
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