Extended Abstract (100K)14.9
Modulation of the Southern polar vortex and tropospheric variability by forcing in the tropical stratosphere and implications for ozone
David Noone, California Institute of Technology, Pasadena, CA; and Y. Yung and R. L. Shia
To explore the role that variations in the stratospheric circulation have on the Southern Hemisphere climate, an atmospheric general circulation model is employed. A forcing term is added to the thermodynamic equation to represent the additional heating associated with radiative effects of stratospheric aerosols following the eruption of Mount Pinatubo in 1991. The imposed warming of approximately 2 K/day is realized in the region of the climatological thermal minimum of the tropics. For an atmosphere at quasi-equilibrium, the model indicates that the heat may be disposed from the tropical stratosphere once the local temperature has increased by 5K. Following geostrophy, these thermal changes induce a reduction in the strength of the summer easterlies and an increase in the wintertime westerly jet at mid-latitudes. This response equivalently describes an intensification of the nocturnal polar vortex, although the meridional location of the wind maximum remains fixed. The magnitude of the response in the mid-stratosphere is typical of that associated with variations in the Southern Annular Mode. Although additional heat has been added, the polar stratosphere becomes cooler. The tropospheric response is, however, less robust. The direct downward control of the zonally symmetric flow is small even though the stratospheric overturning has increased. Changes in the tropics arise due to modification of the column heat budget and allow a reduction in the diabatic heating at the equator. This acts to reduce the intensity of the time mean circulation and consequently there is less activity in the baroclinic zone. This suggests a relatively small perturbation to the climatological distribution of planetary wave generation in the troposphere. As such, the magnitude of the feedback to stratospheric flow via wave-pumping is not substantial. It is suggested that a more important influence in the troposphere follows from changes to the ozone concentrations. Based on the GCM results, an off-line two-dimensional chemical transport model is used to compute the changes in ozone associated with the imposed dynamic forcing. Changes in the abundance and seasonality of ozone modify the radiative flux of both incoming solar and outgoing longwave radiation and provides impetus for additional thermal forcing in the troposphere beyond that expressed through the dynamic terms modeled by the GCM. This study highlights the importance of the coupling between the ozone photochemistry and dynamics in modulating the climate of the high Southern latitudes.
Session 14, Atmospheric Chemistry in the Southern Hemisphere
Thursday, 27 March 2003, 9:00 AM-12:00 PM
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