Whither Comes Weather? Scientists Suggest Stratosphere's Role
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
"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 Niño 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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