Abstract
Abstract. Observations and simple theoretical arguments suggest that the Northern Hemisphere (NH) stratospheric polar vortex is stronger in winters following major volcanic eruptions. However, recent studies show that climate models forced by prescribed volcanic aerosol fields fail to reproduce this effect. We investigate the impact of volcanic aerosol forcing on stratospheric dynamics, including the strength of the NH polar vortex, in ensemble simulations with the Max Planck Institute Earth System Model. The model is forced by four different prescribed forcing sets representing the radiative properties of stratospheric aerosol following the 1991 eruption of Mt. Pinatubo: two forcing sets are based on observations, and are commonly used in climate model simulations, and two forcing sets are constructed based on coupled aerosol–climate model simulations. For all forcings, we find that simulated temperature and zonal wind anomalies in the NH high latitudes are not directly impacted by anomalous volcanic aerosol heating. Instead, high-latitude effects result from enhancements in stratospheric residual circulation, which in turn result, at least in part, from enhanced stratospheric wave activity. High-latitude effects are therefore much less robust than would be expected if they were the direct result of aerosol heating. Both observation-based forcing sets result in insignificant changes in vortex strength. For the model-based forcing sets, the vortex response is found to be sensitive to the structure of the forcing, with one forcing set leading to significant strengthening of the polar vortex in rough agreement with observation-based expectations. Differences in the dynamical response to the forcing sets imply that reproducing the polar vortex responses to past eruptions, or predicting the response to future eruptions, depends on accurate representation of the space–time structure of the volcanic aerosol forcing.
Highlights
The Northern Hemisphere (NH) stratospheric winter polar vortex, which shows considerable interannual and intraseasonal variability, has been observed to be stronger than normal in winters after major volcanic eruptions (Kodera, 1995; Labitzke and van Loon, 1989)
In simulations of the post-Pinatubo eruption period with the MPI-ESM with four different volcanic aerosol forcings, an enhanced polar vortex – which is expected based on limited observations and simple theoretical arguments – was not a robust response
The responses that were significant and robust across all four forcings in the NH winter stratospheric include: (1) positive temperature anomalies in the lower tropical stratosphere, (2) enhanced Fz in the NH midlatitudes (40– 60◦ N) and wave drag in the midlatitude middle stratosphere, (3) enhanced meridional residual circulation, (4) dynamical cooling of the tropical lower stratosphere and heating of the midlatitude lower stratosphere
Summary
The Northern Hemisphere (NH) stratospheric winter polar vortex, which shows considerable interannual and intraseasonal variability, has been observed to be stronger than normal in winters after major volcanic eruptions (Kodera, 1995; Labitzke and van Loon, 1989). While this observation is based on a relatively small sample (limited to the winters after the 1963 Agung, 1982 El Chichón and 1991 Pinatubo eruptions), the theoretical argument to explain such a strengthening appears clear: namely, that heating of the lower stratosphere through the absorption of radiation by volcanic sulfate aerosols enhances the equator-to-pole temperature gradient in the lower stratosphere, which, through the thermal wind equation, leads to stronger westerly winds (Robock, 2000 and references therein). The degree to which secondary feedback mechanisms – such as changes in ozone or upward propagating planetary waves (e.g. Graf et al, 2007; Stenchikov et al, 2006) – affect the vortex strength is at present unclear
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