the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering
Amy H. Butler
Daniele Visioni
Yan Zhang
Ben Kravitz
Douglas G. MacMartin
Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that Stratospheric Aerosol Injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations.
Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections: an equatorial injection, an annual injection of equal amounts of SO2 at 15° N and 15° S, an annual injection of equal amounts of SO2 at 30° N and 30° S, and a polar strategy injecting SO2 at 60° N and 60° S only in spring in each hemisphere.
We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres and changes in the speed of the residual circulation. In conjunction with the differences in direct radiative impacts at the surface, these drive different impacts on the extratropical modes of variability (Northern and Southern Annular Mode), including important consequences on the northern winter surface climate, as well as on the intensity of tropical tropospheric Walker and Hadley Circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical and chemical processes driving the atmospheric response to SAI, as well as lay the ground for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.
- Preprint
(5219 KB) -
Supplement
(8230 KB) - BibTeX
- EndNote
Ewa M. Bednarz et al.
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2023-495', Ulrike Niemeier, 20 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-495/egusphere-2023-495-RC1-supplement.pdf
-
RC2: 'Comment on egusphere-2023-495', Anonymous Referee #2, 07 Jun 2023
The article explores the effects of different injection strategies for stratospheric aerosol geoengineering on various aspects of the stratospheric and tropospheric circulation. The results are important to highlight the different possible impacts of different strategies which had not been explored before. The paper is well presented and interesting, but I have some concerns as listed below that should be considered before publication.
- Methods: I understand that only one simulation is analyzed for each type of injection strategy. This could be problematic when extracting the forced signal at high latitudes, where the large internal variability can dominate. I would recommend using several members and extracting the ensemble mean response in order to obtain more robust results. If this is not possible I would ask the authors to at least acknowledge the possible uncertainties due to this limitation, in particular for the NH polar vortex response and the surface response.
- Figure 1 and 2, L179-180: ‘increases equator-to-pole meridional temperature gradients near the tropopause and lower stratosphere and thus forces strengthening of stratospheric jets in both hemispheres’
This argument should be rephrased more carefully. In the lower stratosphere temperature is lowest in the tropics except in SH summer. As SAI warms this region, the eq-to-pole gradient becomes smaller in absolute value. This implies a reduction of the (negative) wind shear, thus an acceleration of the wind above the temperature perturbation and a deceleration below. This is seen clearly in Fig. 2 (top left) for EQ. However, the high latitude strengthening of the zonal wind is also linked to the structure of the cooling over the polar cap, which is present in both hemispheres in the lowermost stratosphere in all strategies except POLAR, and also in middle and high latitude in the SH. Could you explain why you get this cooling regions? In general the cooling of the middle and upper stratosphere is quite an outstanding feature in all strategies and it should be discussed as it can influence not only the winds but also ozone.
- Another question is why the warming in the lower stratosphere is weaker in the case of polar injection. Is it because the injected material is transported into the troposphere and removed from the atmosphere?
- It would be useful to add letters to the figure panels.
- L120: the confinement to the tropical pipe will only work above ~20 km, while below that level there is strong horizontal mixing. It could be interesting (beyond this work) to investigate that case.
- L110: ‘throughout a year’ → throughout each year
- L214-215: ‘Warming in the lower stratosphere also reduces the stability of the stratosphere itself, thereby accelerating the deep branch of the BDC.’The BDC is forced by wave driving, so I would expect that thermal changes induce wind changes which modify wave propagation conditions and this drive the BDC. In order to explore this mechanism, it would be good to include the Eliassen-Palm flux divergence in order to examine the associated changes. Also the heat flux plots could be extended in altitude to show the stratosphere.
- L381-382: These conclusions cannot be extracted from the analysis because you are not comparing to the past period as you do for the previous figures, so the reader does not know if the BDC is stronger for SAI than for SSP2-4.5. It would be necessary to include those maps too or at least mention how is the BDC in the reference simulation.
- L292: typo ‘an qualitatively’
- L329: typo ‘near-air surface’ should be near-surface air
- L375: the different sign above and below the climatological ozone maximum should be noted (due to opposite-sign vertical gradients)
Citation: https://doi.org/10.5194/egusphere-2023-495-RC2
Ewa M. Bednarz et al.
Ewa M. Bednarz et al.
Viewed
HTML | XML | Total | Supplement | BibTeX | EndNote | |
---|---|---|---|---|---|---|
289 | 159 | 12 | 460 | 26 | 10 | 7 |
- HTML: 289
- PDF: 159
- XML: 12
- Total: 460
- Supplement: 26
- BibTeX: 10
- EndNote: 7
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1