Volcanic drivers of stratospheric sulfur in GFDL ESM4

Stratospheric injections of sulfur dioxide from major volcanic eruptions perturb the Earth's global radiative balance and dominate variability in stratospheric sulfur loading. The atmospheric component of the GFDL Earth System Model (ESM4.1) uses a bulk aerosol scheme and previously prescribed the distribution of aerosol optical properties in the stratosphere. To quantify volcanic contributions to the stratospheric sulfur cycle and the resulting climate impact, we modified ESM4.1 to simulate stratospheric sulfate aerosols prognostically. Driven by explicit volcanic emissions of aerosol precursors and non-volcanic sources, we conduct ESM4.1 simulations from 1989 to 2014, with a focus on the Mt. Pinatubo eruption. We evaluate our interactive representation of the stratospheric sulfur cycle against data from Moderate Resolution Imaging Spectroradiometer, Multi-angle Imaging SpectroRadiometer, Advanced Very High Resolution Radiometer, High Resolution Infrared Radiation Sounder, and Stratospheric Aerosol and Gas Experiment II. To assess the key processes associated with volcanic aerosols, we performed a sensitivity analysis of sulfate burden from the Mt. Pinatubo eruption by varying injection heights, emission amount, and stratospheric sulfate's dry effective radius. We find that the simulated stratospheric sulfate mass burden and aerosol optical depth in the model are sensitive to these parameters, especially volcanic SO2 injection height, and the optimal combination of parameters depends on the metric we evaluate. Plain Language Summary Major volcanic eruptions emit sulfur dioxide into the stratosphere and affect the Earth's global radiative balance as well as the stratospheric sulfur abundance. The GFDL Earth System Model (ESM4.1) previously uses prescribed aerosol optical properties, and in this paper, we replace it with explicit volcanic emissions to study the volcanic contribution to the stratospheric sulfur cycle and its impact. We simulate years from 1989 to 2014, with a focus on the Mt. Pinatubo eruption as a benchmark. We also evaluate the new improvements against observations and performed sensitivity analyses of the sulfate burden from the Mt. Pinatubo eruptions. We find that the simulated stratospheric sulfate amount and aerosol optical depth are sensitive to injection height, emission amount, and aerosol size, and while the injection height is the most sensitive, the best combination of parameters depends on the chosen observational metric.