the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The influence of springtime Arctic ozone recovery on stratospheric and surface climate
Gabriel Chiodo
Marina Friedel
Svenja Seeber
Andrea Stenke
Timofei Sukhodolov
Franziska Zilker
Abstract. Stratospheric ozone is expected to recover by mid-century due to the success of the Montreal Protocol in regulating the emission of ozone-depleting substances (ODSs). In the Arctic, ozone abundances are projected to surpass historical levels due to the combined effect of decreasing ODSs and elevated greenhouse gases (GHGs). While ozone recovery has been shown to be a major driver of future surface climate in the Southern Hemisphere during summertime, the dynamical and climatic impacts of elevated ozone levels in the Arctic have not been investigated. In this study, we use two chemistry climate models (SOCOL-MPIOM and CESM-WACCM) to assess the climatic impacts of Arctic ozone recovery on stratospheric dynamics and surface climate in the Northern Hemisphere (NH) during the 21st century. Under the high-emission scenario (RCP8.5) examined in this work, Arctic ozone returns to pre-industrial levels by the middle of the century. Thereby, it warms the lower Arctic stratosphere, reduces the strength of the polar vortex, advancing its breakdown, and weakening the Brewer-Dobson circulation. In the troposphere, Arctic ozone recovery induces a negative phase of the Arctic Oscillation, pushing the jet equatorward over the Atlantic. These impacts of ozone recovery in the NH are smaller than the effects of GHGs, but they are remarkably robust among the two models employed in this study, cancelling out some of the GHG effects. Taken together, our results indicate that Arctic ozone recovery actively shapes the projected changes in the stratospheric circulation and their coupling to the troposphere, thereby playing an important and previously unrecognized role as driver of the large-scale atmospheric circulation response to climate change
Gabriel Chiodo et al.
Status: open (until 26 Jun 2023)
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RC1: 'Comment on egusphere-2023-672', Anonymous Referee #2, 17 May 2023
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Reasons for rejectionThis paper has three major issues that concern me; I'm disappointed that none of my pre-review remarks were considered:1) The reference to ozone recovery could well be misplaced. Recovery from what? Recovery should be reserved ONLY for recovery from the increases in CFCs. The paper doesn't show that this is all that is going on in their simulations, and it's very likely not. The choice of the high emission RCP8.5 scenario means other things are changing that are known to affect 21st century ozone (e.g. CH4 and N2O). These should be evaluated and the writing fully revamped to avoid sending a misleading message.2) Experiment design, item 1: The paper uses a fixed 3-D pre-ozone hole ozone climatology. This means that the ozone distribution will be inconsistent with the wave dynamics, a known problem in such simulations. Substantially more work needs to be done to demonstrate (or not) that the results are not an artifact of this choice. A useful reference to think about is https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018MS001478 but there are others; the authors need to cover this literature and deal with the experiment design carefully.3) Experiment design, item 2: The paper uses 5 member ensembles and argues that 20 years of evolution represent 100 independent points. This is unlikely to be true. I think much more needs to be done to justify such a remarkable claim. The problem may be related to item 2) above (are they still independent when ozone is allowed to vary with the dynamics?). The paper also needs to fully explain why in this case one can have so few realizations and avoid the issues identified in a strong body of other work on NAO changes showing the need for many realizations. For example, in Deser et al referenced in the paper, 40 realizations were needed; it doesn't seem plausible that adding in the ozone change (if done self-consistently per comment 1 above) would have such a big impact, unless it is an artifact as noted under item 1). https://link.springer.com/article/10.1007/s00382-016-3502-zCitation: https://doi.org/
10.5194/egusphere-2023-672-RC1 -
AC1: 'Author comment to referee report', Gabriel Chiodo, 25 May 2023
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General comment: we note that we had included a point-by-point response to referee#2 in the “access review” and all their comments were addressed. The paper had been corrected before going into public discussion. We believe that the referee has overlooked them, and therefore use them below in our replies. Our responses below refer to the version of the manuscript initially submitted and which went through the “access review”. The referee’s comments (RC) are quoted in italic, while the authors’ comments (AC) are shown in bold-face.
[RC] Reasons for rejection: This paper has three major issues that concern me; I'm disappointed that none of my pre-review remarks were considered.
[AC] As indicated in our general comment above, the referee’s comments had been fully taken into account in the access review, and we made some amendments in the manuscript. We believe that the referee has overseen these responses.
[RC] The reference to ozone recovery could well be misplaced. Recovery from what? Recovery should be reserved ONLY for recovery from the increases in CFCs. The paper doesn't show that this is all that is going on in their simulations, and it's very likely not. The choice of the high emission RCP8.5 scenario means other things are changing that are known to affect 21st century ozone (e.g. CH4 and N2O). These should be evaluated and the writing fully revamped to avoid sending a misleading message.
[AC] We define the ozone “recovery” in terms of future long-term changes in Arctic ozone with respect to present-day (2005-2020 baseline). As stated in the original manuscript near L123, our recovery definition differs from that of the WMO due to different baseline being used (quoting it: “We note that our definition of ozone recovery slightly differs from the WMO, in that our period of reference is present-day (2005-2020) instead of 1980”).
Our definition of “recovery” also includes the long-term changes that are attributable to forcings other than ODS, such as GHGs. The effects of the individual GHG forcings in RCP scenarios, including the high-emission scenario considered here (RCP8.5) have been already extensively evaluated, even for the two models used in this paper (see Revell et al., 2012 for SOCOL – see Butler et al., 2016 for WACCM). In the Arctic, Methane (CH4) and Nitrous Oxide (N2O) nearly offset each other in terms of the stratospheric ozone changes they induce (see e.g., Fig. 2A in Butler et al., 2016), while rising CO2 levels drive an ozone increase. As a result, the projected Arctic ozone abundances in this scenario surpass historical levels by 20-30 DU (Fig. 1 in Butler et al., 2016), which is termed by the WMO as “super-recovery” (WMO 2010; 2014; 2018; 2022). These effects are robust across models and are not the focus of our paper, which instead deals with the dynamical impacts.
To make it clear that our definition of “recovery” also differs from that of the WMO in terms of the attributable ozone changes, we have added the following sentence near L132 of the paper that is now in discussion: “Lastly, our definition of recovery also includes the long-term trends induced by GHGs, aside from the phase-out of ODS. These induce a "super-recovery" in ozone with respect to 1980 levels (WMO, 2022)”.
[RC] Experiment design, item 1: The paper uses a fixed 3-D pre-ozone hole ozone climatology. This means that the ozone will be inconsistent with the dynamics, a known problem in such simulations. More work needs to be done to be certain that the results are not an artifact of this choice. A useful reference to think about may be https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018MS001478
[AC] As stated in the original version of the manuscript (i.e. the version which was provided in the first “access review”) near L116, prescribing an ozone forcing does not affect the climatology and variability of the two models considered in this study, as long as the climatology is derived from the same underlying chemistry-climate model and same boundary conditions (“In both models, the climatology and variability of stratospheric, tropospheric, and surface climate is nearly identical in both configurations (interactive vs prescribed, but consistent with boundary conditions, ozone - (Smith et al., 2014), even under present-day conditions (Friedel et al., 2022a)”). We further verified this, by comparing the temperature and wind climatology of the two ensembles (“Recovery” vs “No Recovery”) over the “baseline” period used to derive the ozone climatology prescribed throughout the 21st century in “No Recovery” (2005-2019). We only found small temperature differences (< 1 K) in the upper stratosphere, likely due to an under-estimation of the heating arising from the diurnal ozone cycle, as reported in Smith et al., (2014). We found no significant changes anywhere below 10 hPa and in particular, the polar vortex strength and variability is nearly identical in both ensembles; for example, prescribing an ozone climatology does not alter the SSW frequency in both models (Friedel et al., 2022a; Oehrlein et al., 2020). Hence, we believe that our results are not an artifact of the models’ configuration in the “No recovery” ensemble.
To make this point clearer, we had added the following sentence near L118 of the paper that is now in discussion: “We verified this by comparing the two ensembles (the "Recovery" experiments with interactive ozone vs "No Recovery" experiments with prescribed ozone) over the reference period used to obtain the ozone climatology imposed in the "No Recovery" ensemble (2005-2019). This comparison reveals only marginal differences of less than 1 K in the upper stratosphere (above 10 hPa, not shown), which are likely due to the underestimation of the heating arising from the diurnal ozone cycle (which is not captured by the monthly-mean 3-D ozone climatology, as shown in Smith et al. (2014)). However, these differences are much smaller than the dynamical impacts of long-term ozone trends in the Arctic and global stratosphere, as shown below”.
[RC] - Experiment design, item 2: The paper uses 5 member ensembles and argues that 20 years of evolution represent 100 independent points. I think more needs to be done to justify this in light of 2) above (are they still independent when ozone is allowed to vary with the dynamics?) and to better explain why in this case one can have so few realizations and avoid the issues identified in other work on NAO changes. For example, in Deser et al 40 realizations were needed; it doesn't seem plausible that adding in the ozone change (if done self-consistently per comment 1 above) would have such a big impact, unless it is an artifact as noted under item 1). https://link.springer.com/article/10.1007/s00382-016-3502-z
[AC] As stated in our previous reply, the ozone configuration does not significantly affect the variability, nor the background state of the two CCMs. Hence, we discard the possibility raised by the referee, according to which our results may be an artifact of the configuration.
As the reviewer indicates, projections in the NAO are very uncertain in CMIP models, and our two models are no exception to this. Take Figure 6 (panels a and d) to appreciate how different the springtime SLP projections are in our two models and in particular, the little resemblance to a “canonical” NAO pattern in any of our two models; hence, it is not correct to frame our results in the context of NAO projections, which are, indeed, very uncertain.
Lastly, we disagree with the reviewer on the little plausibility of any ozone effects on SLP (Fig. 6 – panels c and f) due to uncertainty in SLP caused by internal variability. In this respect, we wish to emphasize three aspects that give us confidence.
1. There is strong coherence between stratospheric and troposphere/surface circulation changes (negative stratospheric NAM – high SLP anomaly over the Arctic). Hence, the effects of ozone reported in our paper are consistent with our understanding of stratosphere-troposphere coupling on intraseasonal time-scales. Decades of research have consistently shown that a weak stratospheric polar vortex (as a result of Arctic ozone recovery in our study) can induce surface circulation changes (see e.g., Domeisen & Butler, 2020; Baldwin et al., 2021).
2. There is a striking similarity across the two models (compare panels c and f in Fig. 6), indicating robustness in the ozone effects, despite the two models being very different in their sensitivity (Fig. A1) and in their projected changes in SLP (Fig. 6).
3. There is a body of evidence showing that inter-annual changes in Arctic ozone (which are on the order of 15-20%) can induce surface climate anomalies (see Calvo et al., 2015; Ivy et al., 2017; Friedel et al., 2022): note that the long-term Arctic ozone changes reported in this paper are roughly of the same magnitude (Fig. 2).
We also note, however, that ozone-induced SLP changes are small compared to those induced by GHGs and are generally only at the fringe of significance; this pattern might become more significant with a larger number of ensemble members. The marginal significance of the tropospheric signals is noted in several places of the manuscript, such as L375 (“While the effects in the stratosphere are very detectable, those in the troposphere are only on the fringe of significance, although they are very robust across the two models used in this study”).
In light of the above, we reaffirm the validity of our results and believe that they are portrayed and discussed in a balanced way in our manuscript.
References
Baldwin, M. P., Ayarzagüena, B., Birner, T., Butchart, N., Butler, A. H., Charlton-Perez, A. J., et al.: Sudden stratospheric warmings. Reviews of Geophysics, 59, e2020RG000708. https://doi.org/10.1029/2020RG000708, 2021
Butler, A. et al.: Diverse policy implications for future ozone and surface UV in a changing climate, Environ. Res. Lett. 11 064017, DOI 10.1088/1748-9326/11/6/064017, 2016.
Calvo, N, L M Polvani, and S Solomon: On the Surface Impact of Arctic Stratospheric Ozone Extremes. Env. Res. Lett. 10, no. 9 Environ. Res. Lett. 10, doi:10.1088/1748-9326/10/9/094003, 2015.
Domeisen & Butler: Stratospheric drivers of extreme events at the Earth’s surface, Commun Earth Environ 1, 59, https://doi.org/10.1038/s43247-020-00060-z, 2020.
Friedel, M., G. Chiodo, A. Stenke, D. Domeisen, S. Fueglistaler, J. Anet, and T. Peter: Springtime Arctic ozone depletion forces Northern Hemisphere climate anomalies, Nature Geoscience, DOI:10.1038/s41561-022-00974-7, 2022.
Ivy et al: Observed connections of Arctic stratospheric ozone extremes to Northern Hemisphere surface climate, Environ. Res. Lett., doi: 10.1088/1748-9326/aa57a4, 2017.
Oehrlein, J., G. Chiodo, and L.M. Polvani: The effect of interactive ozone chemistry on weak and strong stratospheric polar vortex events, Atmospheric Chemistry and Physics, DOI:10.5194/acp-20-10531-2020, 2020.
Revell, L. E., Bodeker, G. E., Huck, P. E., Williamson, B. E., and Rozanov, E.: The sensitivity of stratospheric ozone changes through the 21st century to N2O and CH4, Atmos. Chem. Phys., 12, 11309–11317, https://doi.org/10.5194/acp-12-11309-2012, 2012.
Citation: https://doi.org/10.5194/egusphere-2023-672-AC1 -
RC2: 'Reply on AC1', Anonymous Referee #2, 26 May 2023
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I will comment quickly only on my point 1. It is simply inappropriate to define "recovery" any way you like, without regard to well established conventions in the field by now, as employed in the international WMO assessment process and many previous papers. This should be changed throughout if this paper is to be publishable, and the paper should also conduct runs that make clear what drives the conclusions: CFCs? I doubt it in 2100. CH4? N2O? CO2? These each have vastly different implications.
Citation: https://doi.org/10.5194/egusphere-2023-672-RC2
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RC2: 'Reply on AC1', Anonymous Referee #2, 26 May 2023
reply
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AC1: 'Author comment to referee report', Gabriel Chiodo, 25 May 2023
reply
Gabriel Chiodo et al.
Gabriel Chiodo et al.
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