The Anomalous State of the Stratosphere Following the 1991 Eruption of Mt Pinatubo

Abstract:

Large volcanic eruptions at low latitudes increase the stratospheric aerosol loading for prolonged periods of time, and consequently lead to an increase in the planetary albedo and fraction of diffuse light, perturb the radiative budget of the lower stratosphere, and affect lower stratospheric ozone. In light of proposed geo-engineering with artificial enhancement of the stratospheric aerosol layer, accurate understanding of the impact of the natural analogue provided by volcanoes is imperative. Here, I provide an overview over a mounting body of evidence from observations that suggest very anomalous stratospheric dynamics following the Pinatubo eruption, much more so than after the eruption of El-Chichon. Analysis of general circulation models forced with observed sea surface temperatures and prescribed stratospheric aerosol shows that the models generally fail to reproduce the anomalous dynamic state, and it is unclear how, or even whether, the anomalous lower stratospheric dynamics are linked to the Pinatubo aerosol. Comparison of 3 different aerosol datasets based on observations shows large differences in the stratospheric aerosol radiative heating following the Pinatubo eruption. A suite of numerical model calculations using a comprehensive chemistry-climate model, and a primitive equation model, are used to study the impact of the change in the lower stratospheric radiative budget on temperature and dynamics, and their sensitivity to magnitude and distribution of the forcing from aerosol, and sea surface temperature distribution.