Impact of volcanic eruptions on decadal variability of stratospheric age of air

A major uncertainty in current climate model simulations concerns the evolution of the stratospheric Brewer-Dobson circulation. Models show decreasing mean age, indicating an accelerating circulation, while observations show opposite decadal age variations in the Northern hemisphere. Here, we quantify the effect of the stratospheric volcanic aerosol on mean age variability, using mean age from reanalysis-driven TRACZILLA and CLaMS simulations for the 1989-2010 period, showing decadal variations consistent with observations. Our method is based on a multilinear regression technique using satellite observations of aerosol optical depth including Pinatubo and smaller volcanoes after 2002. We find that the decadal age variations can be primarily attributed to the volcanic signals. In particular the smaller volcanic eruptions after 2002, which are not included in most climate model simulations, cause most of the positive mean age decadal change in the Northern hemisphere after 2002. Consequently, the discrepancy between climate models and observations regarding decadal mean age changes (2002-2010) is likely related to the representation of volcanic stratospheric aerosol in the models