Scientists Hit Pay Dirt In Predicting El Nino
BOSTON — It's been a futile kind of fun to learn from time to time that the warming of the tropical eastern Pacific waters - called El Nino - had caused nasty weather around the world. There was little we could do with that intriguing hindsight. No longer! For the first time in the history of their science, meteorologists now can predict a major climatic effect - namely, El Nino - up to a year in advance.
This unprecedented ability already is paying off for Australia, Brazil, and Peru. Devastating droughts or floods in those countries are linked directly to the state of the tropical eastern Pacific. Being forewarned, farmers and their governments can take evasive action.
The drought that followed the 1982-83 El Nino caught Peruvians off guard. The overall value of their harvest dropped 14 percent below normal. Not so in 1987. Heeding an experimental El Nino forecast, they planted a drought-resistant array of crops that showed a 3 percent gain in value.
Semi-arid northeast Brazil didn't get the word that year and lost most of its crops. But it did benefit in 1992. Timely action in that drought year cut crop loss. Preemptive water-use restrictions and investment in a new dam also kept the regional capital, Fortaleza, from running out of water as rainfall dropped 45 percent below normal. Both countries now factor El Nino forecasts into their planning.
This forecasting breakthrough is a fruit of the 10-year international Tropical Oceans and Global Atmosphere (TOGA) program that ended in 1995. TOGA scientists will publish their summary reports this year. But in a review released last December, the US National Academy of Sciences already has certified that what began as a research program has produced an unexpected payoff. It has produced computer-based models of the tropical Pacific "which now demonstrate skill in the prediction of tropical sea-surface temperatures months to a year or so in advance."
It addition, TOGA's observing system of satellites, ground-based stations, and string of 70 instrumented ocean buoys lets scientists keep tabs on those temperatures. Meteorologists were flying blind before they had that system. Now, as TOGA scientist Edward Sarachik of the University of Washington in Seattle explains, "You can go to a Web site and ... see what's happening in the tropical Pacific [right] now." (For more information, see: http://www.pmel.noaa.gov/toga-tao/elnino.html/.)
That's valuable knowledge. Scientists link El Nino warming and its opposite subnormal cooling together with what they call the Southern Oscillation. This is a fairly regular shifting of atmospheric pressure across the central Pacific that changes winds and ocean currents. It is the shifting patterns of this joint phenomenon, which scientists call ENSO, that change tropical ocean surface temperatures and influence global weather. A clear correlation between those ocean temperatures and rainfall lets scientists translate their skill at ENSO prediction into useful regional climate forecasts in the tropics and subtropics. Doing the same for higher latitudes in the Northern Hemisphere will be trickier.
While scientists know ENSO affects weather in those latitudes, the linkage is subtle. The large-scale, air-circulation pattern there seems linked to the underlying pattern of sea-surface temperatures. Together, these appear to determine the course of the jet-stream winds and the tracks of storms that produce regional weather. Writing in the January issue of the Bulletin of the American Meteorological Society, Ngar-Cheung Lau of Princeton University in New Jersey notes that computer simulations suggest that much of the year-to-year variability in this weather-guiding system is internally generated. It would act the way it does even without a signal from the tropics. Yet, Dr. Lau notes that it seems clear that ENSO does affect mid-latitude weather.
Until scientists learn more about how the tropics affect this self-willed system, they won't be able to do for Europe, North America, and northern Asia what they have done for Australia, Brazil, and Peru. But given their success in translating ENSO predictions into useful warnings for some countries, they like to think they're on their way.