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 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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