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.