the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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: open (until 23 Sep 2024)
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CC1: 'Comment on egusphere-2024-1909', Simon Lee, 29 Aug 2024
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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
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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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