The Arctic Polar Vortex Response to Volcanic Forcing of Different Strengths

Tropical volcanic eruptions injecting sulfur into the stratosphere are assumed to not only scatter radiation and cool Earth's surface but also alter atmospheric circulation and in particular to strengthen the stratospheric polar vortex in boreal winter. The exact impact is difficult to estimate because of the small number of well observed eruptions and the high internal variability of the vortex. We use large (100-member) ensembles of simulations with an Earth system model for idealized volcanic aerosol distributions resulting from sulfur injections between 2.5 and 20 Tg. We suggest the existence of a threshold somewhere between 2.5 and 5 Tg(S) below which the vortex does not show a detectable response to the injection. This nonlinearity is introduced partly through the infrared aerosol optical density which increases much stronger than linear with increasing particle size occurring for increasing injection amount. Additionally, the dynamical mechanism causing the vortex strengthening seems not to set in for small aerosol loading. Furthermore, we add to the recent discussion concerning a possible downward propagation of the circulation response leading to a winter warming in Northern Eurasia. At latitudes northward of about 50 degrees N, our simulations do show such an average warming pattern that is statistically significant for injections of 10 Tg(S) or more.