Thursday, 27 March 2003: 11:45 AM
Modulation of the Southern polar vortex and tropospheric variability by forcing in the tropical stratosphere and implications for ozone
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.
Supplementary URL: