Quantifying CanESM5 and EAMv1 sensitivities to volcanic forcingfor the CMIP6 historical experiment

Abstract. Large volcanic eruptions reaching the stratosphere have caused marked perturbations to the global climate including cooling at the Earth's surface, changes in large-scale circulation and precipitation patterns and marked temporary reductions in global ocean heat content. Many studies have investigated these effects using climate models, however uncertainties remain in the modelled response to these eruptions. This is due in part to the diversity of forcing datasets that are used to prescribe the distribution of stratospheric aerosols resulting from these volcanic eruptions, as well as uncertainties in optical property derivations from these datasets. To reduce these uncertainties in the Coupled Model Intercomparison Project 6 (CMIP6) a combined stratospheric aerosol dataset, the Global Space-based Stratospheric Aerosol Climatology, GloSSAC, was constructed. Along with information from ice-cores and sun photometers, GloSSAC was used to generate aerosol distributions, characteristics and optical properties and construct a consistent stratospheric aerosol forcing dataset for models participating in CMIP6. This version 3 of the stratospheric aerosol forcing has been endorsed for use in all contributing CMIP6 simulations. Recent updates to the underlying GloSSAC from version 1 to version 1.1 affected the 1991 to 1994 period and necessitated an update to the stratospheric aerosol forcing from version 3 to version 4. As version 3 remains the official CMIP6 input, quantification of the impact on radiative forcing and climate is both relevant and timely for interpreting results from experiments such as the CMIP6 historical simulations. This study uses the Canadian Earth System Model version 5 (CanESM5) to estimate the difference in instantaneous radiative forcing in simulated post-Pinatubo climate response when using version 4 instead of version 3. Differences in temperature, precipitation, and radiative forcings are generally found to be small compared to internal variability. To further elucidate sensitivities that are representative of the CMIP6 model suite, additional simulations are performed using the Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1), which also indicates that the impact of the update to GloSSAC version 4 on climate are relatively minor. An exception to this is differences in temperature anomalies in the stratosphere, which can be as large as 3 °C following the eruption of Mt. Pinatubo.