Review of “Was the strengthening of the Northern polar vortex in 2024/2025 associated with the Hunga Tonga eruption?”, by Kuchar et al.

This study examines the impact of the water vapor anomaly from the January 2022 Hunga Tonga (HT) volcanic eruption on the Northern Hemisphere polar vortex, specifically focusing on the winter of 2024/2025. The paper compares the observed responses with simulations from the SOCOLv4 model with and without the HT forcing, using 10 ensemble members each. The authors show that the Northern polar stratosphere in winter 2024/2025 was characterized by the descent of a significant HT-induced water vapor anomaly (up to 2 ppmv) in MLS observations. The SOCOLv4 simulations, along with an offline radiative transfer model, are used to calculate the direct H2O radiative cooling and the indirect cooling caused by reduced shortwave ozone heating due to chemically induced ozone loss. The authors argue that this cooling then strengthened the meridional temperature gradient and the stratospheric polar vortex, which then inhibited wave forcing of the vortex, leading to a stronger than usual polar vortex throughout most of the 2024/2025 winter. The authors also suggest that the winter of 2025/2026 may have an increased probability for sudden stratospheric warmings as the HT water vapor anomaly diminishes.

General comments:

The paper presents some interesting results which are relevant within the scope of ACP. The possible link between the HT water vapor anomaly and the stronger than normal Northern polar vortex during the 2024/2025 winter (~3 years after the eruption) is of considerable interest. The SOCOLv4 model results qualitatively illustrate the possible mechanism(s) involved. However, I have some concerns about the interpretation and presentation of results where corrections and/or clarifications are necessary, as discussed below. I would characterize some of these as major revisions which should be addressed for the paper to be published.

Specific comments:

Abstract, L11-12: “...shift toward sudden stratospheric warmings in the winter 2025/2026 as the WV forcing declines.” Do the authors mean more SSWs here? As WV declines back towards pre-Hunga levels, one would expect stratospheric dynamics would also tend toward more “normal” pre-Hunga behavior, i.e., a tendency towards more frequent SSWs and a weaker SPV compared to 2024/2025. Please clarify.

L20: “WV inputs of HT were distributed…” – should specify what locations and time periods are being referred to here, e.g., Southern hemisphere subtropics in the days/weeks following the eruption (see the Hunga APARC report, section 2.3). Similar comment for the following sentence (L20-21), i.e., when were the positive residuals of WV over high latitudes.

L24: Return of stratospheric air to the troposphere at mid-high latitudes by the BDC is also likely an important sink of the H2O anomaly.

L36: Suggest changing to “linked to a stratospheric WV increase.” not “linked to the stratospheric WV increase.”  since you’re not specifically talking about HT here.

 L59: “…illustrate the enormous strength of the SPV.” Please be more specific here, i.e., I assume the authors are referring to the fact that although there is large variability, the SPV is generally stronger than average during ~December-February (2024/2025).

L59-74: The authors show MLS observations of water vapor, ozone, and temperature anomalies for the winters of 2024/2025 (Figure 2) and 2025/2026 (Figure A3). However, there is no discussion of the statistical significance of these results, for example, how these anomalies compare to the range of interannual variability (as done in Figure 1) seen over the MLS time period (2004-2025). The H2O anomaly from HT is quite large in 2024/2025, and is likely outside the range of variability and is therefore more easily detectable. But what about temperature and ozone? MERRA-2 temperature anomalies in the Northern polar region during winter 2015/2016 and 2021/2022 are not impacted by HT but show patterns similar to Figure 2. So are the patterns in 2024/2025 uniquely due to HT? Perhaps the HT forcing contributed to the observed responses in 2024/2025, with some partial attribution to natural variability. But this should be discussed and clarified.

Also, observations for the 2023/2024 winter are not shown. If the HT H2O had not yet reached the Northern polar region (or was too weak to drive a temperature/ozone response) in the 2023/2024 winter, it would be helpful to at least briefly mention this point.

L65: “negative ozone (< 0.5 ppmv) anomaly” is confusing, since this implies that ozone anomalies of 0 to + 0.5 ppmv are also negative.

L66-67: “descend downward” is redundant.

L68-70: “In March, we observe a significant increase in both temperature and ozone…”. However, the stratospheric ozone anomalies have both alternating positive and negative bands. Also, the temperature anomaly is positive only in the stratosphere, with a strong negative anomaly in the mesosphere. These points should be corrected/clarified.

L71: “The by then ….” Wording should be corrected.

L72-74: “The close correspondence between the WV and ozone anomalies and the diagnosed temperature response suggests that they are the primary drivers of the initial cooling observed in OW3.”

This point is not clear as written. The large WV anomaly in the stratosphere (> +1 ppmv) corresponds to both a positive ozone anomaly in early winter (as discussed on L67-68), and a negative ozone anomaly in Dec-Feb. And what is the “initial cooling observed in OW3”? Is this the cooling at the beginning of the winter? This cooling is rather weak in the plots shown. There is a correlation between the negative ozone and temperature anomalies in the middle and lower stratosphere during Dec-Feb. This appears to be due to weaker than normal wave driving as shown in Figure 1B. If so, this should be clearly discussed as a dynamical process, not radiative as is currently implied (L71-74).

 L76-116: While the model H2O anomaly in Figure 3 is statistically significant throughout the winter, the model ozone, temperature, and zonal wind responses are significant only in small areas (Fig.3). This would imply that the HT-induced responses are not necessarily detectable above the background variability. This is not mentioned (as far as I could tell), but should be discussed.

L82: “… we understand MW3 as the upcoming OW4.” Wording is not clear. Perhaps: “we assume MW3 matches with the upcoming OW4.”? Or something to that effect?

L86-88: should clarify that the positive ozone anomaly is only for Sept.-Nov. and over a very narrow vertical extent. And this feature is shown by a very faint green color in Figure 3B and is hard to see. It would be good to make it a darker or more visible green if possible.

L88-89: should clarify that the weaker poleward transport is reflected by the EHF in Fig. 3D. But the negative ozone and temperature anomalies are mostly not statistically significant.

L94: “more muted” - suggest using “weaker” as this is more precise wording.

L128: need to define “EPP”

L190-191: “We report the Holton-Tan mechanism…” What is meant by this? Do you mean the model simulates qualitatively the Holtan-Tan mechanism? Please clarify.

Figure A1: SWOOSH appears to show a H2O anomaly of ~0.5 ppmv at 10-20 hPa in early 2022, whereas MLS shows a maximum of up to 2 ppmv in the global average at this time (APARC, 2025, Fig. S3.14). This is a big difference and should be noted/discussed and/or the SWOOSH data should be checked for accuracy here. Also, the contours overlaid on the shading are difficult to interpret. Suggest putting the SWOOSH data in a second panel with shading/contours consistent with the model.

L209: “in consequence of” should be “as a consequence of”

Figure A4: This needs more explanation. It took me some time to understand that the bottom, middle, and top rows are o3 only, h2o only, and o3+h2o, respectively. This needs to be explained clearly in the caption. Panels F and I are probably not necessary. I think it’s better just to clearly label what’s being input. And it needs to be stated that the x-axis scales are all different. This was also confusing. I assume the input o3 and h2o for 2024-10-10 are held constant throughout the 200-day integration? And is integrating for 200 days needed for convergence? And why was this specific date chosen, as opposed to sometime in mid-winter?  This all needs to be clearly stated/discussed.

Figure A6: With the current colors, it is very hard to distinguish between the blue and green for easterly/westerly winds, especially since the dots are small. Suggest using red colors for the westerlies, or something with greater contrast for easterly/westerly. Also, the dots should be made larger. Should also mention/clarify in the caption that the dots correspond to each of the 10 ensemble members. And what time period does the 10hPa equatorial ZW correspond to? Is it the average over the winter? This should be clearly stated in the caption.

L246-249: The sentence: “A similar configuration occurred in OW3, when the QBO was also in its westerly phase” should be checked/corrected or clarified, since OW3 as I understand it (winter 2024/2025 according to Fig. 2) shows QBO easterlies in Figure A6.

And L250-251: It’s also unclear that “… the forced SPV response in MW2 arises from the HT-induced WV”. MW2 (2023/2024) has equatorial winds in the westerly QBO phase (Fig. A6) which create conditions that favor a stronger SPV as stated at the beginning of the paragraph. Perhaps both mechanisms contributed to the stronger SPV, but this needs to be clearly discussed.

L252-255: It would be interesting to know if reanalysis SLP in February 2026 show a pattern similar to the model in Figure A8.

Figure A8: To be clear, suggest adding “SOCOLv4” at the beginning of the caption right before “Monthly anomaly”.

The paper poses the question of whether the Hunga Tonga eruption led to a strengthening of the Northern hemisphere polar vortex in winter 2024/2025. They argue that water vapor injected from the Hunga Tunga eruption makes its way into the polar vortex and starts a chain of processes that lead to a stronger polar vortex. They make use of satellite, reanalysis and model (SOCOLv4) datasets to support their mechanism. Whilst this paper is certainly addressing an important and timely problem and the topic could make a good contribution to the literature, there are some major methodological flaws that we would like to see addressed.

Major comments:

- Line 65 and Figure 2: There needs to be more evidence presented to link the water vapor anomaly to the ozone and temperature anomalies. We are not convinced that the ozone and temperature anomalies are actually collocated with the water vapor. Are these ozone and T anomalies large compared to the usual climate anomalies during strong vortex years?

- Radiative transfer calculations: Radiative–convective equilibrium (which is valid in the tropics) does not hold in the polar regions and is the correct calculation to use to diagnose the radiative heating rates. The relevant balance is radiative–advective equilibrium (Caballero and Merlis, 2025; Cronin and Jansen, 2015). 

- Mechanistic explanation: the paper presents a variety of coincident observed & simulated variables to demonstrate influence on the SPV from WV anomalies. The mechanistic explanation is plausible but at present the paper does not clearly show a causal link. The response in simulation runs with a strong WV anomaly should be compared with runs with a weak WV anomaly to diagnose a different response. Other strong SPV years should be considered; is the response distinct from these years as a result of the increased WV?

- Line 129: “The key elements are the slow Brewer-Dobson descent of the HT WV anomaly, enhanced HOx and ClOx chemistry leading to … ozone loss” It is not clear to us where these statements are drawn from. It has not been clearly demonstrated that 1) the descent of the WV anomaly is slow and 2) HOx and ClOx chemistry are responsible for the ozone loss. Which months and at what height levels are these chemical cycles causing ozone loss? Is the heterogeneous chemistry adequately capturing these processes in the SOCOL model? Can you quantify how “slow” the WV anomaly descent is? A slower Brewer-Dobson circulation relative to climatology would be contradicted by the more rapid dissipation of the WV anomaly in the simulations.

Minor comments:

- Line 93: “These temperature anomalies are comparable with those … simulated” is contradicted by the following sentence and not reflected in figure 3C. The colourbar in 3C has too many levels to be easily interpreted, but it looks like a minimum temperature anomaly around -5K. 

- Line 190: What exactly do you mean by “We report the Holton-Tan mechanism (Holton and Tan, 1980)”? Do you mean that you can demonstrate a strong link between the QBO phase and the strength of the polar vortex in your model?

References: 

Caballero, R., and T. M. Merlis, 2025: Polar Feedbacks in Clear-Sky Radiative–Advective Equilibrium from an Airmass Transformation Perspective. J. Climate, 38, 3399–3416, https://doi.org/10.1175/JCLI-D-24-0031.1.

Cronin, T. W., and M. F.Jansen (2016), Analytic radiative-advective equilibrium as a model for high-latitude climate, Geophys. Res. Lett., 43, 449–457, doi:10.1002/2015GL067172.