The Arctic Polar Vortex Response to Volcanic Forcing of Different Strengths
Creators: name: Azoulay, Alon
name: Schmidt, Hauke
name: Timmreck, Claudia
Publisher: name: FID GEO
Year: 2021

description: 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°N, our simulations do show such an average warming pattern that is statistically significant for injections of 10 Tg(S) or more.
description: Plain Language Summary: Large volcanic eruptions can inject sulfur containing gases into the stratosphere where they build sulfate aerosols. These particles (a) scatter incoming sunlight away from the Earth, resulting in a temporary global mean cooling at the surface, and (b) absorb infrared radiation and thereby warm the lower stratosphere. This heating is thought to strengthen the Arctic polar vortex, circumpolar westerly winds in the winter stratosphere. The exact effect of volcanic aerosol on the polar vortex is, however, unknown. Here, we aim to understand the dependence of the vortex strengthening on the amount of injected sulfur. For five different eruption strengths, we simulate the atmospheric response 100 times. We show that the simulated vortex response depends nonlinearly on eruption strength and is indistinguishable from zero for the smallest injection, suggesting a threshold below which the dynamical mechanism leading to the vortex strengthening does not work. The stratospheric Arctic vortex is of interest because there are strong indications that it influences wintertime climate in the troposphere. Starting at an eruption strength similar to the Pinatubo eruption in 1991, our model simulates an increased likelihood of a warmer than normal winter in Northern Eurasia despite the global cooling.
description: Key Points: Model simulations suggest a threshold below which stratospheric sulfur injections from volcanoes do not affect the Arctic polar vortex. Statistically significant winter warming in Northern Eurasia is simulated for large eruptions.
description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
description: Deutsche Forschungsgemeinschaft Research Unit VolImpact