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NSF PR 98-69 - October 22, 1998
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El Niño and Climate More Predictable Than Previously
Thought
Fluctuations in the earth's climate from year to year,
such as those that are associated with El Niño,
are considerably more predictable than had been previously
believed, according to a paper appearing in this week's
issue of Science. The research was jointly
funded by the National Science Foundation (NSF), NOAA,
and NASA.
"For more than 30 years, the so-called 'butterfly effect'
has been the dominant paradigm for weather forecasting,"
says scientist J. Shukla of the George Mason University
Center for Ocean-Land-Atmospheric Studies (COLA),
lead author of the Science paper. "It
has now been demonstrated that there are important
exceptions to the 'butterfly effect' and that certain
aspects of climate are far more predictable than previously
thought."
The "butterfly effect" is a reference to the chaotic
nature of day-to-day atmospheric fluctuations, explains
Jay Fein, director of NSF's climate dynamics program,
which funds COLA research. Such weather events cannot
be predicted precisely beyond one to two weeks in
the future. For several decades, the prevailing view
in scientific circles was that it was not possible
to predict weather and climate variations beyond this
intrinsic limit. Research by Shukla and his colleagues
at COLA has shown that, although weather cannot be
predicted beyond a few days away, atmospheric circulation
and precipitation, averaged for an entire season,
are potentially predictable. "Indeed, there is predictability
in the midst of chaos," Shukla says. "We now have
a scientific basis for climate prediction, and that
suggests that the large scale effects of all future
major El Niño events should be predictable
several months in advance."
Seasonal averages especially in the tropics are most
predictable because the tropical atmosphere responds
directly to slowly varying conditions at the earth's
surface. Shukla and his colleagues at COLA have run
models of the global climate to show that seasonal
mean weather conditions are determined by sea surface
temperature, soil wetness, vegetation and snow cover.
In particular, variations in sea surface temperature
such as those that are associated with El Niño
can significantly alter weather in the tropics for
an entire season, or longer.
High predictability of the tropical atmosphere can
also enhance the predictability of the North American
region. Shukla says that if changes in the tropical
Pacific sea surface temperature are large, the seasonal
average atmospheric circulation over the north Pacific
and North America is also highly predictable.
"It is no accident that seasonal predictions made by
several research groups around the world for last
winter (1997 - 1998) were quite accurate," Shukla
points out. "Those unprecedented forecasts were just
the first examples of the accurate predictions of
major El Niño events. We can expect more such
in the future."
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