the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Nov 2024
Stratospheric Aerosol Intervention Experiment for the Chemistry-Climate Model Intercomparison Project
Abstract. A new Stratospheric Aerosol Intervention (SAI) experiment has been designed for the Chemistry- Climate Modeling Initiative (CCMI-2022) to assess the impacts of SAI on stratospheric chemistry and dynamical responses and inter-model differences using a constrained setup with a prescribed stratospheric aerosol distribution and fixed sea-surface temperatures (SSTs) and sea-ice. This paper describes the details of the experimental setup and the prescribed aerosol distribution. Furthermore, we discuss differences in the Whole Atmosphere Community Climate Model (WACCM6) results between the interactive stratospheric aerosol configuration with coupling to land, ocean, and sea ice that was used to produce the stratospheric aerosol distribution and the results of the constrained SAI experiment. With this, we identify and isolate the stratosphere-controlled SAI-induced impacts from those influenced by the coupling with the ocean. We confirm earlier suggestions that the SAI-induced positive phase of the Northern Atlantic Oscillation in winter, with the corresponding winter warming over Eurasia, is directly driven by the effect of SAI on the stratosphere-troposphere coupling. We further show that the resulting stratospheric responses are largely similar between the fully coupled and constrained experiments, demonstrating the suitability of the simplified setup to study impacts in the stratosphere in a multi-model framework. Only small differences arise in the stratospheric ozone and dynamical SAI responses between the two experiments due to minor differences in the aerosol distributions and their coupling with local changes in temperatures, upwelling, and chemistry, alongside interactive coupling with the ocean and sea ice.
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Status: open (until 07 Jan 2025)
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RC1: 'Comment on egusphere-2024-3586', Anonymous Referee #1, 11 Dec 2024
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Everything in the paper seems to be correct, but it is not an exciting paper. The paper does not have interesting new scientific discoveries. Rather, it reads like a long technical report. The descriptions of how each variable responds are overly comprehensive. This would make a nice introductory paper for a special issue where the results of the experiment they describe are carried out by multiple models. The results are based on only one, albeit excellent, model, but I wonder how model-dependent the results are. And I wonder how forcing created with WACCM will interact with other models with different physics, chemistry, and resolution. By the way, the paper does not describe the resolution of the model simulations.
There are small edits for the authors to consider in the attached annotated manuscript.
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RC2: 'Comment on egusphere-2024-3586', Anonymous Referee #2, 27 Dec 2024
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The main goal of the paper is to present the CCMI-2022 simulation setup senD2-sai as an alternative to running a fully coupled model with Stratospheric Aerosol Injection (SAI), and to justify its validity by showing WACCM6 senD2-sai output and comparing it to a fully coupled simulation (SSP2-4.5-SAI), which is also used to generate the aerosol input. The main conclusion is that the setup is valid to isolate the direct effects of SAI on stratospheric and tropospheric climate that are not mediated by changes in the ocean. However, the differences with the fully coupled simulation highlight that the setup is not valid to study surface climate, as the response in surface temperature and precipitation is very different. The stratospheric response is isolated in senD2-sai as a strengthened polar vortex in both hemispheres, which impact surface climate by inducing a positive NAM and SAM. The ocean-mediated response is El Niño-type including its known teleconnections affecting wave activity in the troposphere and stratosphere. Therefore, the conclusion is that the setup is useful to study the effects of SAI on the stratospheric dynamics and chemistry in a homogenized framework, but that doing this has non-negligible limitations due to the missing ocean feedbacks.
The manuscript includes noteworthy and novel insights on the separation between the stratospheric only effects and the effects including the ocean feedbacks. The analyses are robust, the figures clear. However, the large number of simulations and having to remember their technical description and slight differences somewhat difficults following the paper and obscures the main points. I have some questions and suggestions to improve the readabilty and facilitate interpretation of the results. I also include some specific comments on the text below.
General comments:
For instance, could the number of simulations used be reduced? For the reader it is diffucult to remember the differences between all the simulations, so I suggest keeping a smaller number of simulations in the main results description, and discuss the complementary runs in a separate discussion section. For example, I understand the interest of comparing with refD2 from CCMI-2022, but can be in a separate section after the main results are presented.
If I understand correctly, the only difference between SSP2-4.5 and refD2 is the implementation of eruptive sulful injections and the evolution of ODS. In this sense, the cleanest comparison would have been to create a simulation exactly equal to refD2 but including the SAI. However, this is not available and that is why SSP2-4.5 and SSP2-4.5-SAI have been used. Is this correct? Please explain the reasoning for having both simulations in the manuscript in Section 2.2.
Also, Figures 3, 5 and 6 show the differences between SSP2-4.5 SAI in 2080 – 2099 and the control experiment (SSP2-4.5) in 2020 – 2030. Is it really necessary to use control simulations, SSP2-4.5 and senD2-fix, for the present period? Could you compare the SAI simulations in the later period to the earlier period of the same simulation, when SAI should not have an impact yet? This is typically done in climate change studies and would simplify the interpretation.
The senD2-sai setup is not coupled to the ocean and thus the SST and sea ice need to be prescribed. The proposed setup is to prescribe climatological SST and sea ice from the present. However, approach largely dampens the climate variability in these simulations (as seen in Fig. 1), and this is important because the type of responses that are analyzed are modes of variability (NAO, SAM, ENSO). Is there an alternative way to construct these simulations? For example, prescribe the SST from a coupled model SSP-4.5-SAI simulation, similar to what is done for refD2? This would likely improve the total model response to SAI. Is this not proposed to avoid imposing an ocean response which is suspect of being model dependent? Has this been discussed in previous literature? This discussion should be included in the paper.
Since an important motivation for the study is the need to increase the number of models that perform SAI experiements, it would be good to add the information of how many CCMI-2022 models do not have a coupled ocean currently.
Specific comments:
L47: Keeble et al. 2021 looks at CMIP6 models.
L53: ‘stratospheric aerosol distribution or optical properties’. What does this ‘or’ mean? Is it an option or are both things equivalent?
L80: ‘SSP2-4.5 and refD2’ . The name SSP2-4.5 is a bit confusing, since refD2 follows that same IPCC scenario. Could this be called perhaps Ref_coupled, and the other one SAI_coupled or something similar?
L119: Is it correct that you changed the interhemispheric temperature gradient condition by an interhemispheric symmetry aerosol distribution condition? It could be stated more clearly.
L150-151: the globally averaged precipitation change is half as large in the senD2-sai compared the coupled simulation, and this is not clear from the text, which only states ‘the feedback from the atmosphere and land changes in the globally averaged precipitation in senD2-sai is similar to that in SSP2-4.5 SAI’.
L187-189: might this explain the strange change in values in Figure 3 top row central and right panels, at 50N/S near the tropopause?
L243: correct typos ‘stratosphre’ and ‘coupld’
L253: ‘Compared to the present day and independent of SAI’ does this mean in scenarios that do not implement SAI? It could be stated more clearly.
L275: Here and at other places the difference in wave driving between senD2-sai and SSP2-4.5SAI is mentioned, but it is not shown. Could you include a figure of the changes in Eliassen-Palm flux divergence, at least in the supplementary material?
L285: Here an El Niño like response is mentioned but this has not been shown yet in the paper, so please include a mention that it will be shown later.
L293-294: ‘the discussed differences between the prescribed and interactive experiment are similar’ The phase ‘differences are similar’ sounds a bit strange, I suggest rephrasing to ‘changes are similar’.
L320-321: ‘The symmetric injetion in both hemispheres using SAI has been shown to counter the weakening of the AMOC due to climate change’. This seems at odds with the fact that a strong cooling is observed in the North Atlantic in Figure 9 central panels (SSP2-4.5).
L352: ‘dominated the response in the region’. Suggested: ‘in the Pacific region’
L363-367: ‘This austral summer negative SAM and weakening of eddy-driven jet are therefore likely connected to the equatorial Pacific and the El-Nino-like response resulting in changes in tropospheric winds and Kelvin wave fluxes into the stratosphere to contribute to the winter and spring stratospheric easterly response.’ I do not understand this sentence. First, it is unclear how Kelvin waves propagating in the stratosphere influence the SAM. L’Heureux and Thompson (2006) do not mention Kelvin waves. Second, the beginning of the sentence refers to the austral summer SAM response, but the end of the sentence refers to the ‘winter and spring response’.
L455: modeling groups (add ‘s’)
Citation: https://doi.org/10.5194/egusphere-2024-3586-RC2
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