Review of “On the magnitude and sensitivity of the QBO response to a tropical volcanic eruption” by Brown et al.

This study uses the UM-UKCA model to investigate the response of the QBO to tropical eruptions. They then try to give a mechanistic explanation through the use of a common 2D dynamical model, which is nicely described in Section 2.4.

The paper does a good job at framing the issue overall (clarity over their definition of the QBO phase and state is greatly appreciated!), although they could include in their initial discussion some more recent studies (some done with their same model) of Pinatubo, for instance. The paper is well written and the results are explained clearly. I have just a few suggestions for improvement here and there, mainly about contextualizing this work a bit better, but other than that, the paper is in a really good shape and will make a good contribution to the literature.

L 68: “Sulfate geoengineering is, in effect, equivalent to a sustained volcanic eruption, although somewhat different to a short-lived, explosive eruption” A bit of a convoluted phrase. “somewhat different” – how? 

L 108: Also se Pitari et al. (2016) which discusses this exact issue in the context of the four main volcanic eruptions.

L 130: “Inacted”?

S2.2 Interested here as to why the authors don’t reference or compare their results with those from Dhomse et al. (2020)? It seems like the model is the same, maybe a slightly different version, however the amount of SO2 is different, and Dhomse found a better agreement for lower injections of SO2. Their simulations are also AMIP-style. This needs to be discussed (and perhaps, if the models’ versions are compatible, one could look at those already available simulations which are part of ISA-MIP and compare the QBO response…). In terms of the location of the injection (in altitude), the authors can also refer to the range explored in the ISA-MIP experiments (Quaglia et al., 2022) – at least acknowledging that different altitudes of injection produce different clouds with, potentially, different QBO effects.

L. 433 Stop is inside the parenthesis but should be outside.

Figure 9 and 10 are really, really hard to interpret due to the different contours. I would suggest duplicating the figure for T and for SO4 mass rather than try to cram everything in one panel.

Overall, a panel showing AOD (both global over time, or 30N-30S, and latitudinal) could be very useful to understand better how the QBO is affecting the aerosols.

L 578 (paragraph) I agree with the authors’ assessment that it is hard with a continuous perturbation to make similar attributions. However, I would stress here that “the preference for sulfate geoengineering to halt the QBO following e-QBO conditions” only refers to SG applied at the equator (see the cited Richter et al., 2017 and Kravitz et al., 2019), and not even in all climate models (see Niemier et al., 2020, but also Jones et al., 2022, Fig. 8)

L. 622 feedbacks “among” maybe “between” is better

L. 644 “aerosols are”

L. 672 (paragraph) In parts, this paragraph feels rather weak and just thrown out there to give a broader scope to the paper. I don’t have much doubt that this research is useful, mind you, I just feel like this could be better justified. “species such as ozone and stratospheric water vapour have different signals depending on the zonal wind shear and their distribution is important for climate change projections” sounds rather weak – if there was a volcanic eruption capable of modifying the QBO, the main effect on climate change projections wouldn’t be the distribution of ozone. We should know it, but the aerosols’ distribution (which is affected by the QBO more than WV) would affect the climate more. Similarly, ozone is influenced by many factors in case of a volcanic eruption (see Aquila et al., 2013). “despite an increased probability of a QBO disruption in a warmer future climate” I can see why you would say this and can think of a few references for it (like Aubrey et al., 2021) but you should 1) cite them and 2) contextualize this a bit better. Seems like a shame to conclude this paper this way, the authors should work on this a bit more.

Data availability – This won’t do, you need to make it public by ACP guidelines. 

References

Aquila, V., Oman, L. D., Stolarski, R., Douglass, A. R., & Newman, P. A. (2013). The Response of Ozone and Nitrogen Dioxide to the Eruption of Mt. Pinatubo at Southern and Northern Midlatitudes, Journal of the Atmospheric Sciences, 70(3), 894-900. Retrieved Dec 19, 2022, from https://journals.ametsoc.org/view/journals/atsc/70/3/jas-d-12-0143.1.xml

Aubry, T.J., Staunton-Sykes, J., Marshall, L.R. et al. Climate change modulates the stratospheric volcanic sulfate aerosol lifecycle and radiative forcing from tropical eruptions. Nat Commun 12, 4708 (2021). https://doi.org/10.1038/s41467-021-24943-7

Dhomse, S. S., Mann, G. W., Antuña Marrero, J. C., Shallcross, S. E., Chipperfield, M. P., Carslaw, K. S., Marshall, L., Abraham, N. L., and Johnson, C. E.: Evaluating the simulated radiative forcings, aerosol properties, and stratospheric warmings from the 1963 Mt Agung, 1982 El Chichón, and 1991 Mt Pinatubo volcanic aerosol clouds, Atmos. Chem. Phys., 20, 13627–13654, https://doi.org/10.5194/acp-20-13627-2020, 2020.

Jones, A., Haywood, J. M., Scaife, A. A., Boucher, O., Henry, M., Kravitz, B., Lurton, T., Nabat, P., Niemeier, U., Séférian, R., Tilmes, S., and Visioni, D.: The impact of stratospheric aerosol intervention on the North Atlantic and Quasi-Biennial Oscillations in the Geoengineering Model Intercomparison Project (GeoMIP) G6sulfur experiment, Atmos. Chem. Phys., 22, 2999–3016, https://doi.org/10.5194/acp-22-2999-2022, 2022

Kravitz, B., MacMartin, D. G., Tilmes, S., Richter, J. H., Mills, M. J., Cheng, W., et al. (2019). Comparing surface and stratospheric impacts of geoengineering with different SO2 injection strategies. Journal of Geophysical Research: Atmospheres, 124, 7900– 7918. https://doi.org/10.1029/2019JD030329

Pitari, G.; Di Genova, G.; Mancini, E.; Visioni, D.; Gandolfi, I.; Cionni, I. Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time. Atmosphere 2016, 7, 75. https://doi.org/10.3390/atmos7060075

Quaglia, I., Timmreck, C., Niemeier, U., Visioni, D., Pitari, G., Brühl, C., Dhomse, S., Franke, H., Laakso, A., Mann, G., Rozanov, E., and Sukhodolov, T.: Interactive Stratospheric Aerosol models response to different amount and altitude of SO2 injections during the 1991 Pinatubo eruption, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2022-514, in review, 2022.

Review of “On the magnitude and sensitivity of the QBO response to a tropical volcanic eruption” by Brown et al. 

This study examines the QBO response to two magnitudes of volcanic eruption (one Pinatubo-like, and one 4x stronger) using an idealized 2D model and a comprehensive (3D) chemistry-climate model. The underlying mechanisms of the response are clearly explained and this study helps clarify previous results in the literature. The paper is very clearly written and the figures well presented. My recommendation is to accept the paper, pending only the minor corrections suggested below (by line number).

130: “with explosive eruptions inacted”: Do you mean “enacted”, or “activated”? Not sure what this is trying to say.

324: “between 40 hPa and 5 hPa” → “between 40 hPa and 15 hPa”  ?

462-463: Fig. 3e also shows a slight upward movement of the westerly QBO phase for the 60 Tg eruption (roughly months 5-8), though it’s modest compared to the upward shift for the e-QBO cases. The corresponding January case (Fig. S1e) is more pronounced.

499: I’m not sure from Fig. 10c that the QBO westerly phase has become broader. (The broadening is also mentioned a couple times in the Conclusions.) It looks to me like it’s roughly the same width but has shifted off the equator, into the NH. But I find it hard to tell just by comparing the filled and line contours in this plot. Maybe a latitudinal profile of the wind (at 20 hPa, say) would make this clearer? Perhaps also with a metric of the width, e.g. full width at half maximum. However it seems to me that this result is not essential to the paper, and if the statement about broadening is not supported it could simply be removed.

514: “confining it more to the easterly phase”: not sure what “it” refers to here.

551-552: Because the wave forcing is weaker? (Consistent with a longer QBO period.) Perhaps worth saying this explicitly, if that’s what is meant.

643: “sustaining the meridional aerosol” → “sustaining the meridional aerosol distribution” ?

658: “secondary circulation causes easterly shear zones to descend faster than westerly shear zones”: switch “easterly” and “westerly”