Model-Dependent Stratospheric Aerosol Effective Radiative Forcing in the VolMIP volc-pinatubo-full Ensemble

One of the central aims of VolMIP was to perform a coordinated model intercomparison with stratospheric aerosol optical properties implemented consistently across participating models. However, it remains unclear to what degree the same aerosol optical properties implemented in different models produce the same radiative forcing. Diagnosing effective radiative forcing (ERF) is usually achieved through dedicated model simulations where sea surface temperatures (SSTs) are held fixed—the ERF represents the instantaneous forcing from the aerosol plus rapid adjustments, but not changes in top-of-atmosphere radiative flux associated with surface temperature changes. Fixed SST experiments were not part of the VolMIP protocol. Here, we show first results using a novel technique to diagnose ERF from the coupled VolMIP volc-pinatubo-full simulations, which assumes that ERF is a linear function of the prescribed stratospheric aerosol optical depth (SAOD). We find a multi-model mean ERF-to-SAOD ratio of -22 W/m2, a value that is broadly consistent with prior estimates, but we also find a wide inter-model spread in the 6 models participating in VolMIP, with values ranging from -17 to -26 W/m2. We furthermore explore the role of cloud processes in the inter-model spread of the ERF-to-SAOD ratio.