the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Long-lasting high-latitude volcanic eruptions as a trigger for sudden stratospheric warmings: An idealized model experiment
Abstract. The temporary enhancement of the stratospheric aerosol layer after major explosive volcanic eruptions can trigger climate anomalies beyond the duration of the radiative forcing. Whereas the mechanisms responsible for long-lasting response to volcanic forcing have been extensively investigated for tropical eruptions, less is known about the dynamical response to high-latitude eruptions. Here we use global climate model simulations of an idealized long-lasting (6 months) northern hemisphere high-latitude eruption to investigate the climate response during the first three post-eruption winters, focusing on the dynamics governing the stratospheric polar vortex. Our results reveal that two competing mechanisms contribute to determining the post-eruption evolution of the polar vortex: 1) A local stratospheric mechanism whereby increased absorption of thermal radiation by the enhanced aerosol layer yields a polar vortex strengthening via a thermal wind response. 2) A bottom-up mechanism whereby surface cooling yields an increase in atmospheric wave activity that propagates into the winter stratosphere, leading to a weakening of the polar vortex, also seen as an increased occurrence of sudden stratospheric warming events (SSWs). The local stratospheric mechanism dominates in the first post-eruption winter, while the bottom-up mechanism dominates in the follow-up winters. The identification of a deterministic response such as increased SSWs following high-latitude volcanic eruptions calls for increased attention to these events as an important source of interannual variability and a possible source of increased seasonal predictability of northern hemisphere regional climates.
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Status: open (until 04 Jul 2024)
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RC1: 'Comment on egusphere-2024-1302', Anonymous Referee #1, 23 Jun 2024
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General comments:
This study uses CESM1-WACCM4 for an idealized experiment with six-months volcanic aerosol injection in the Northern Hemisphere high latitude stratosphere. The authors emphasizes that potential increased sudden stratosphere warming events are found in response to the eruption. If the results are solid, then it can add valuable new insight to the research field. However, current experiment design, results and discussions in the paper are not convincing enough to make solid conclusions as stated in the paper.
Below are some major questions that need to be clarified/addressed:
- The study uses CESM1-WACCM4 with specified chemistry, which is already an old version of the model, how this old version is suitable for this study is not convincingly stated in the manuscript.
- The experiment design, why the injection mass maintains over 6 months without any change and over wide vertical range (10-27 km), is it possible for a volcanic eruption? This looks more like a stratospheric aerosol modification case instead of a volcanic eruption case.
- How is the aerosol distribution? What changes do aerosol zonal mean have along the time? How are the shortwave and longwave radiation and stratospheric temperature changes related to the aerosol distributions? These are all unclear questions, which make it hard to understand the mechanism explanation in the following parts.
- The manuscript emphasizes two competing mechanisms for explaining the results, one is the local stratospheric aerosol induced warming mechanism, the other one is the local surface cooling induced wave activity change mechanism, however, no results or supplement figures prove if the warming is at the same location as the aerosol loading area, or the increased wave propagation originates from the cooling areas.
Besides, why there is a cooling in the first winter in both the cpl and atm-only runs are not well explained. In the winter, there is no shortwave radiation, then the aerosol induced stratospheric warming due to longwave radiation absorption should be a dominating effect compared to the first summer, but why it’s the opposite? Connection to strengthened polar vortex in the winter needs to be explained better.
Moreover, the aerosol induced stratospheric warming should be weakened in the second and third winter due to decreased aerosol, thus the strengthened polar vortex in the first winter can also be weakened due to less aerosol in the second and third winter, how can you rule this out from your second mechanism?
- There are 27 sudden stratospheric warming events counted in the third winter, but there are only 20 ensemble members, so there is more than one SSW event in one member. What is the definition of the SSW event used here? Is it only based on the U50? Is it reasonable to count like this? More clarifications are needed. A figure showing the wind changes and marked SSW events in each member would make it clearer.
- Discussions needs to be largely improved. Comparison with other related studies and limitations of this study are limited.
- The structure and English writing of the manuscript can be improved. The discussion and summary can be separated with improved discussions and clear conclusions in the summary.
Specific comments
L37-46: Too long for an introduction on this well-known statement.
L51: “the aerosols tend to stay longer in the polar stratosphere (Graf et al., 1994, 2007)” Correct? This study (“Initial atmospheric conditions control transport of volcanic volatiles, forcing and impacts” https://acp.copernicus.org/articles/24/6233/2024/) shows longer lifetime of volcanic aerosol in the NHET after tropical eruptions compared to extratropical eruptions.
L57-61: Are Tropical or NH extratropical eruptions described here? Any difference will it have after tropical and NH extratropical eruptions?
L111-112: “that is comparable to the interactive chemistry model version”
Not convincing with this simple statement.
What is the aerosol module used in this model? How is the model’s ability on simulating volcanic aerosol evolutions and NH high latitude dynamics like polar vortex, SSW etc.? These are important aspects that needs to be evaluated for this study.
L126-127: Better moving this sentence to the previous paragraph where describing the coupled ocean and related runs.
L132: EVA is not height dependent in Toohey 2016.
L137-138: a midlatitude location at which latitude?
Figure 1 and L136-142: Not clear how this scaled is performed based on extinction and aerosol mass from the figure and the text description. What is the original EVA forcing?
Figure 1 and L147-148: The months in Figure 1 and decline on Oct 1 is confusing, if the decreasing starts on Oct. 1 as written in the text, then Nov., Dec., Jan next year is used for the first winter calculation?
L154: It can be quite different for a 45° N injection compared to a 65° N injection, how this assumption affect your results needs to be discussed.
L162-167 Better to start description of cpl and then atm-only to keep it consistent across the whole paper.
Figure 2 and L194-206: Better to adjust the order of fig 2(a) and 2(b) and related descriptions, same consistency reason. The time axes Jan, May, Sep. is confusing, better to use Jan. Jul. instead? Any significance test results?
L196-199: Needs to be rephrased. The seasonal variation of the solar radiation is not the reason of different anomalies in the first and second winter.
Figure 2c: why is there a break in the tropics (around 0 degree) in the aerosol mass distribution?
L210-211: “at 65°N” is confusing.
L212-213: Better to convert unit to avoid this exponential value expression.
L218-223: Unclear explanation. More analysis are needed to differentiate the shortwave and longwave radiation effect and the direct radiation effect and dynamical effect to understand the different stratospheric temperature responses in the summer and winter.
Figure 3 and L233-234: Is this the 2 STD of experiment or control ensemble runs? How different are they compared to the control runs? Figure 3a and 3b show different length, better to use Jan and July?
L237-240: “high latitude into midlatitudes” and “subtropics into midlatitudes”, one is equatorward, oppositely, the other one is poleward, confusing.
Figure 4: better to make it larger, it’s not easy to see the details.
L246-248: where shows the local heating? Figure 3a and 4a only shows the temperature difference to the control run, but what is the temperature gradients of experiment and control runs?
L255-259: “locate sources of wave activity”? But the cooling and the upward wave activity locates at different areas, then how can this explain the bottom-up mechanism?
L275-277: what is the direct thermal forcing?
L277-279: Do you mean the inconsistent results are due to the U50 definition, then why use this index? what if other indices are used, can they show consistent results?
L280-282: North Pacific … North Atlantic and Siberia, they are all ocean, how to understand the reasoning “pointing to a possible influence of the change in land-sea thermal contrast”?
Figure 5: as written in the general comments, clarification on the definition/counting/presenting of SSW events are needed.
L329-330: why is it a stratospheric cooling? If thermal response to aerosol injection, then it’s stratospheric warming.
Section 3.3: Does it contribute a lot to the main purpose of the paper by just describing the detailed spatial patterns. What connections do they have with previous results? How aerosol distribution lead to the temperature responses? How do they relate to the different cpl and atm-only configurations? Addressing these questions are helpful to improve the quality of the study.
Summarizing discussions: The first three paragraphs just repeat most of descriptions in the results section. Discussions are needed to compare with other related studies. Like how’s SSW response to volcanic eruption in the observations and studies using other models? What different reasons do they have if showing different results? How the specified chemistry model configuration affects the result? What kind of impact will it have on the results if including aerosol microphysics and stratospheric chemistry in the model? This study (“Volcanic forcing of high-latitude Northern Hemisphere eruptions” https://www.nature.com/articles/s41612-023-00539-4#:~:text=High%2Dlatitude%20explosive%20volcanic%20eruptions,Pinatubo%20eruption. ) shows initial polar vortex stability affects the aerosol distribution, but the forcing is produced with EVA, how will these affect the results. These needs to be discussed.
L416-422: How this relates to equatorial eruptions? “WACCM4 is insensitive to the injection latitude”, is the volcanic forcing produced by EVA? How can this conclusion be made here?
L427: what is “dynamic surface response”?
L430-432: Is this too arbitrary? No model-observation comparisons were made, and the aerosol forcing is much stronger than any volcanic forcing used in previous CMIP5/CMIP6 simulations.
L434-435: Don’t understand this conclusion. Figure 3 shows stronger stratospheric warming in cpl than atm-only in the second and third winter.
L439: what is “an intrinsic reason originating in the model”?
Technical corrections
L43-44: surface and stratospheric meridional temperature gradients?
L62: effect -> affect; as the positive phase of … -> Leading to a positive phase of…?
L81: Icelandic volcanism?
L90-91: history and current activity makes these types of eruption?
L96: the response within of NH stratospheric polar vortex?
L194: short-wave -> shortwave
L199: substantially
L272: N America -> North America
L462: smaller size -> smaller magnitude?
Citation: https://doi.org/10.5194/egusphere-2024-1302-RC1
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