Long-lasting high-latitude volcanic eruptions as a trigger for sudden stratospheric warmings: An idealized model experiment

Abstract. The temporary enhancement of the stratospheric aerosol layer after major explosive volcanic eruptions can trigger climate anomalies beyond the duration of the radiative forcing. Whereas the mechanisms responsible for long-lasting response to volcanic forcing have been extensively investigated for tropical eruptions, less is known about the dynamical response to high-latitude eruptions. Here we use global climate model simulations of an idealized long-lasting (6 months) northern hemisphere high-latitude eruption to investigate the climate response during the first three post-eruption winters, focusing on the dynamics governing the stratospheric polar vortex. Our results reveal that two competing mechanisms contribute to determining the post-eruption evolution of the polar vortex: 1) A local stratospheric mechanism whereby increased absorption of thermal radiation by the enhanced aerosol layer yields a polar vortex strengthening via a thermal wind response. 2) A bottom-up mechanism whereby surface cooling yields an increase in atmospheric wave activity that propagates into the winter stratosphere, leading to a weakening of the polar vortex, also seen as an increased occurrence of sudden stratospheric warming events (SSWs). The local stratospheric mechanism dominates in the first post-eruption winter, while the bottom-up mechanism dominates in the follow-up winters. The identification of a deterministic response such as increased SSWs following high-latitude volcanic eruptions calls for increased attention to these events as an important source of interannual variability and a possible source of increased seasonal predictability of northern hemisphere regional climates.