Stratosphere's role in weather suggested
Posted: August 1, 2003

What happens in the stratosphere, the atmospheric layer just above where commercial airplanes fly, may have a larger influence on our climate and weather than previously thought, according to research funded by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the National Science Foundation.

"The stratosphere is an active player in providing memory to the climate system," said Dr. Mark P. Baldwin, Senior Research Scientist at NorthWest Research Associates, Bellevue, Wash. He is lead author of a paper in the August 1 issue of Science.

Baldwin and his co-authors suggest, although the stratosphere is mostly clear and weather free, it appears changes to the stratospheric circulation can affect weather patterns for a month or more. Wind patterns in the lower stratosphere tend to change much more slowly than those near the surface.

Once the winds in the lower stratosphere become unusually strong or weak, they tend to stay that way for at least a month. "This is the key," Baldwin said, "to understanding how the stratosphere can affect our weather." Large-scale waves that originate in the troposphere, the level of the atmosphere closest to the Earth's surface, appear to be sensitive to the slowly shifting winds in the stratosphere. The waves allow stratospheric changes to feed back, affecting weather and climate on the Earth's surface.

Knowing the stratosphere plays this role could be helpful in predicting weather patterns well beyond the seven-to-10-day limit of current weather prediction models. The stratospheric effect could be compared to the effects of El Nino in that they both provide predictability of average weather patterns. However, the stratospheric effects last only two months at most, and the effects only occur from late fall to early spring.

A better understanding of the stratosphere's effect on the troposphere could also be useful in gaining additional insight into the climatic effects of stratospheric ozone depletion, solar changes and variations in aerosol amounts associated with major volcanic eruptions.

The stratospheric wind shifts can be thought of as changes to the strength of the belt of westerly winds that circulate around the globe at high latitudes. Scientists call these winds the "stratospheric polar vortex." The waves from the troposphere first create fluctuations in the strength of the polar vortex, and then the changes in the vortex strength feed back to affect a hemispheric-scale weather pattern known as the Arctic Oscillation.

When the Arctic Oscillation, also known as the North Atlantic Oscillation, is in its positive phase, there are stronger westerly winds at mid-latitudes, especially across the Atlantic. Northern Europe and much of the United States are warmer and wetter than average, while Southern Europe is drier than average, according to Baldwin. "In effect, the stratosphere can act as a predictor of the state of the Arctic Oscillation," he said.

NASA funds this research through its Earth Science Enterprise, a program dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of weather and natural hazards using the unique vantage point of space.

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