Volcanic aerosol modification of the stratospheric circulation in E3SMv2 – Part 1: Wave–mean flow interaction

Abstract. Following tropical volcanic eruptions, westerly zonal-wind accelerations have been observed in the winter hemisphere polar vortex region. This wind response has been reproduced in some (but not all) simulated eruption studies. As the primary effect of volcanic aerosols during the initial post-eruption period is to heat the tropical stratosphere, the midlatitude zonal-wind response is often explained as a thermal wind effect. Several studies have shown that this explanation is insufficient in understanding the relative significance of the aerosol direct effect and indirect dynamical feedbacks. In this work, we use a Transformed Eulerian Mean (TEM) framework to identify the dynamical origins of stratospheric wind anomalies following the simulated 1991 eruption of Mt. Pinatubo. A paired set of volcanic and non-volcanic 15-member ensembles is used to isolate the volcanic impact. A TEM decomposition of the net zonal-wind forcing is then performed to close the differenced momentum budget between the two ensembles. Zonal-wind accelerations near 30–40° N and 3–30 hPa are identified with significance in the Northern Hemisphere (NH) during both the summer and winter. We find each of these seasonal acceleration episodes to have distinct dynamical drivers. In the summertime, the response is primarily governed by an accelerated meridional residual circulation. In the wintertime, the response is eddy-driven, where an equatorward deflection of planetary waves was robustly identified near 30N and 30 hPa. We additionally identified that a deficit of wave forcing in the tropical stratosphere dampens the amplitude of the quasi-biennial oscillation (QBO) for at least 2 years following the eruption.