"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 Earth's surface," Robinson said. "This carries more air warmed by the ocean onto the land. When the vortex is weak, that's 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.