Preprints
https://doi.org/10.5194/acp-2022-372
https://doi.org/10.5194/acp-2022-372
 
13 Jun 2022
13 Jun 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Climate response to off-equatorial stratospheric sulfur injections in three Earth System Models – Part 2: stratospheric and free-tropospheric response

Ewa M. Bednarz1, Daniele Visioni1, Ben Kravitz2,3, Andy Jones4, James M. Haywood4,5, Jadwiga Richter6, Douglas G. MacMartin1, and Peter Braesicke7 Ewa M. Bednarz et al.
  • 1Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA
  • 2Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN, USA
  • 3Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
  • 4Met Office Hadley Centre, Exeter, UK
  • 5College of Engineering, Maths and Physical Science, University of Exeter, Exeter, UK
  • 6National Centre for Atmospheric Research, Boulder, CO, USA
  • 7IMK-ASF, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany

Abstract. The paper constitutes part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric sulfate aerosol injections (SAI) at various latitudes as simulated by three state-of-the-art Earth System Models – CESM2(WACCM6), UKESM1.0, and GISS-E2.1-G. We use a set of five sensitivity experiments with constant annual injections of SO2 in the lower stratosphere at either 30° S, 15° S, 0°, 15° N or 30° N. We identify the similarities and differences in the simulated responses amongst the models as well as demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics and chemistry in driving the inter-model differences.

Building on part 1 (Visioni et al., 2022), we explain the simulated differences in the aerosol spatial distribution between the models: CESM2 shows significantly faster shallow branches of the Brewer Dobson circulation facilitating transport of the relatively larger-sized aerosol to higher latitudes; UKESM shows a relatively isolated tropical pipe and older tropical age-of-air confining the relatively smaller-sized aerosols to the tropics; and the two GISS versions with either bulk or modal aerosol microphysics show elevated sulfate levels at higher latitudes as the result of smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age-of-air).

We then elucidate the role of these factors in driving the stratospheric responses to SAI. We find a large spread in the magnitudes of the tropical lower stratospheric warming amongst the models, which can be partially attributed to the differences in aerosol distribution and sizes. Regarding the stratospheric ozone responses, we find a good agreement in the tropics between the models with modal microphysics, with lower stratospheric ozone changes consistent with the SAI-induced modulation of the large-scale circulation and the resulting changes in transport. In contrast to the relative agreement at low latitudes, we find a large inter-model spread in the Antarctic ozone responses that can largely be explained by the differences in the simulated latitudinal distributions of aerosols as well as the degree of implementation of heterogeneous halogen chemistry on sulfate. Finally, we also find large differences in stratospheric water vapour responses amongst the models, with CESM2 and GISS with modal microphysics both showing significant increases in stratospheric water vapour under SAI consistent with the increase in cold point temperatures that were largely not reproduced in UKESM.

For the GISS runs with bulk microphysics, the SAI simulations show contrastingly different stratospheric responses to the models using the modal aerosol treatment, including the absence of lower stratospheric warming as well as significant reductions in stratospheric water vapour and ozone. The results point towards the importance of detailed treatment of aerosol processes, although some problems in halogen chemistry in GISS are identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as the sources of uncertainty in model projections of climate impacts from SAI.

Ewa M. Bednarz et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-372', Alan Robock, 14 Jul 2022
  • RC2: 'Comment on acp-2022-372', Anonymous Referee #2, 22 Jul 2022
  • AC1: 'Authors' response to reviewers' comments', Ewa Bednarz, 14 Sep 2022

Ewa M. Bednarz et al.

Ewa M. Bednarz et al.

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Short summary
The paper constitutes part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric sulfate aerosol injections (SAI) at various latitudes as simulated by three state-of-the-art Earth System Models. We identify similarities and differences in the model responses as well as demonstrate the role of biases in the climatological circulation and aspects of the model microphysics and chemistry in driving the inter-model differences.
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