Evaluating the Uncertainties of the Global Atmospheric Sulphur Budget in a Multi-Model Framework

Sulphate aerosol in the stratosphere recently became an interactive part of many global climate models and its uncertainties are not yet well constrained. Stratospheric sulphate aerosol is modulated by natural emissions of several sulphur-containing species, including volcanic eruptions, as well as anthropogenic emissions. If not directly injected into the stratosphere by large volcanic eruptions, sulphate aerosols and their precursors are transported into the stratosphere via the tropical tropopause. While there have been some model intercomparison activity for the large volcanic eruptions, the background conditions of sulphur species and in particular the stratospheric aerosol layer have thus far not been addressed at all. Evaluating the background conditions in global models allow to identify modelling issues that are usually masked by larger perturbations such as volcanic eruptions, yet may still be of importance after such a perturbation. Some key factors causing differences between models include different microphysical schemes, chemical schemes, as well as transport and cross-tropopause fluxes. In this work, we use 8 models and available observational data to quantify the full background atmospheric sulphur cycle (burdens, fluxes, and their variability) and investigate its uncertainties within the framework of the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP). We focus on stratospheric aerosol and its transport with the Brewer-Dobson-Circulation, as well as the influence of the polar vortices. We find significant inter-model variations in the background burden of the major sulphur species. Seasonal cycles agree well in the southern hemisphere, whereas the northern hemisphere shows more inter-model differences due to the individual representations of the northern polar vortex.