Zander, Stefanie (2010) Die Auswirkung von Vulkaneruptionen auf die atmosphärische Zirkulation der Südhemisphere unter besonderer Berücksichtigung der Ausbruchstärke. (Bachelor thesis), Christian-Albrechts-Universität, Kiel, Germany, 55 pp.

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Abstract

Although there has been much research into the changes that occur in the atmospheric circulation in the Northern Hemisphere after strong stratospheric volcanic eruptions like Mt. Pinatubo in 1991, there are few comparable findings on their impact on the circulation in the Southern Hemisphere. Due to the asymmetry in the distribution of the Earth’s landmasses, the circulation is much stronger and more zonal in the Southern Hemisphere than in the Northern Hemisphere, and so the southern hemispheric circulation is more resistant to external influences. For this paper, simulations with the atmospheric general circulation and aerosol model ECHAM5-HAM were used to investigate and compare the anomalies in the southern hemispheric circulation occurring after simulated volcanic eruptions of varying magnitude. The anomalies are presented using the Southern Annular Mode (SAM) index, and the strength of the volcanic eruptions is determined by the injected amount of sulfur dioxide, which varies from 17 Mt SO2, corresponding to the observed stratospheric injection of the Mt. Pinatubo eruption in 1991, up to 700 Mt SO2, the estimated emission associated with the eruption of the Los Chocoyos volcano 84 thousand years ago. Analysis of the model simulations shows that, if the volcanic forcing is strong enough, significantly increased SAM index values will occur, and that these SAM index anomalies will increase up to a certain limit with respect to the eruption strength. Significant anomalies develop for the weaker eruptions mainly at stratospheric heights (at pressure levels around 30 hPa), but they spread to the lower levels of the stratosphere and troposphere only after larger eruptions. The occurrence of the anomalies for weaker volcanic eruptions is very dependent on the fully developed winter stratospheric polar vortex. However, for stronger eruptions, the maximal anomalies, especially in the stratosphere, become increasingly independent of the seasonality of the polar vortex. Particularly large SAM index anomalies in the stratosphere can be found in the troposphere after some delay in time and this is seen as an evidence for ”downward coupling” mechanisms taking place.

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