Fluctuations in the earth's climate from year to year, such as those that are associated with El Nino, 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 Nino 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 Nino 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 Nino events. We can expect more such in the future."
Jay Fein, NSF