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RESEARCH
Science
Atmospheric
Sciences
WEATHER
FORECASTING
Stratosheric polar vortex influences winter cold, researchers
say
James
E. Kloeppel, Physical Sciences Editor
(217) 244-1073;
kloeppel@uiuc.edu
12/1/2001
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Photo
by Bill Wiegand
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UI professor
of atmospheric sciences Walter Robinson has proposed a mechanism
to explain how the behavior of the stratosphere may afftect
the tropospheric weather patterns.
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CHAMPAIGN, Ill. A
mechanism to explain how the behavior of the stratosphere may affect
tropospheric weather patterns has been proposed by scientists at the
University of Illinois. If correct, the idea could be included in models
to better understand the climate system and predict the weather.
"Recent observations have suggested that the strength of the stratospheric
polar vortex influences circulation in the troposphere," said Walter
Robinson, a UI professor of atmospheric sciences. "We believe there
is a weak forcing in the stratosphere, directed downward, that is pinging
the lower atmosphere, stimulating modes of variability that are already
there."
The polar vortex is a wintertime feature of the stratosphere. Consisting
of winds spinning counterclockwise above the pole, the vortex varies
in strength on long time scales because of interactions with planetary
waves global-scale disturbances that rise from the troposphere.
"The polar vortex acts like a big flywheel," Robinson said.
"When it weakens, it tends to stay weakened for a while."
Other researchers have noted a statistical correlation between periods
when the polar vortex is weak and outbreaks of severe cold in many Northern
Hemisphere cities.
"When the vortex is strong, the westerlies descend all the way
to Earths surface," Robinson said. "This carries more
air warmed by the ocean onto the land. When the vortex is weak, thats
when the really deep cold occurs. The correlation could prove useful
for weather forecasting.
To explain this behavior of the atmosphere, Robinson and postdoctoral
research associate Yucheng Song proposed a dynamical mechanism in which
stratospheric forcing, through the mechanism of "downward control,"
weakly forces the Arctic oscillation a mode of variability in
sea-level pressure. This forcing is then reinforced in the troposphere
by interactions with transient eddies in the lower atmosphere, creating
a substantial amplification of the signal.
"The polar vortex does not create new modes of variability in the
troposphere," Robinson said. "It stimulates pre-existing modes
that are fundamental to the dynamics of the lower atmosphere."
Robinson and Song demonstrated the mechanism in a simple global climate
model of the atmosphere. By applying a torque to the stratosphere, the
researchers could study the effects upon internal modes of variability
in the troposphere.
Atmospheric scientists use big numerical models both to forecast the
weather and to better understand the climate system. Because of computational
cost and complexity, current models include only a sketchy representation
of stratospheric dynamics. To improve simulations of the lower atmosphere,
Robinson said, it is possible that the models must also capture the
variability of the polar vortex.
Robinson will discuss the latest model results at the American Geophysical
Union meeting in San Francisco, Dec. 10-14. The National Science Foundation
funded the work.
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