12th Conference on Middle Atmosphere

Wednesday, 6 November 2002: 2:50 PM
Changes in stratosphere-troposphere circulation induced by volcanic aerosol
Mihaela Caian, Imperial College of Science, Technology and Medicine, London, United Kingdom; and J. D. Haigh
A coupled atmosphere-ocean GCM has been used to simulate the response of the troposphere and stratosphere to the injection of stratospheric aerosol by volcanic eruptions. Aerosol optical depths, based on the reconstruction of Sato at al (JGR, 1993), have been inserted above the tropopause to represent the period from December 1990 to February 1992 and thus the eruption of Mt. Pina Tubo. For most of the experiments the atmosphere had 19 levels up to 4.6 hPa but a ?time-slice? experiment was also carried out with an increase to 64 levels, up to 0.01 hPa, for the period December 1991 to February 1992, in order to study at higher accuracy winter time stratosphere-troposphere interactions in response to the aerosol forcing. The ocean comprised 20 levels.

The aerosol produces a forcing below the tropopause, controled by vertical stability variations to the second order. This leads to an increase of the tropospheric pumping, more emphasised in the ?fountain" area of maximum mass transport, an to a locally intensified meridional circulation in the stratosphere. The vertical dipole induced in the heating rates at the tropopause both generates an accelerated exchange of energy and mass and confines the induced circulation. The mechanism of the increased circulation is investigated with a simple primitive equations linear model. The changes to the stratosphere-troposphere system dynamics and interactions are also analysed through energy conversion diagnostics.

The sensitivity of the induced changes to some of the parameters of the perturbation (e.g. duration , strength, location) is performed. A possible negative feedback mechanism leading to damping of the induced circulation, for a hypothetical constant forcing, is also analysed.

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