Volcanic Eruptions in Global Climate Models: Lessons Learned from the VolMIP-Tambora Interactive Stratospheric Aerosol Ensemble

In 2016, as part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating an eruption resembling the 1815 Mt Tambora eruption (VolMIP-Tambora ISA ensemble). This VolMIP-Tambora ISA ensemble uses a single prescribed set of injection parameters, which prevents individual models from choosing their injection parameters to make their results match a desired set of observations, thus allowing for an idealized experiment to compare models to each other. The pre-study provided the ancillary ability to assess radiative forcing uncertainties in aerosol climate models for a large stratospheric-injecting equatorial eruption. Preliminary results showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD) in the visible wavelength range. Now, we pinpoint the primary sources of intermodel inconsistencies in volcanic aerosol formation, evolution and duration in the stratosphere that largely contribute to the inconsistencies in modeled global stratospheric AOD. We identify specific physical and chemical processes that are missing in some of the models and/or parameterized differently between models, which cause these differences. Lastly, we explain the additional improvements which need to be made to these state-of-the-art models that would allow for them to reasonably be compared to observations, which would ultimately be the best gauge of model performance.