Watching El Niño

McPhaden's office monitors the ocean conditions that can lead to either El Niño or La Niña. The information comes from the TAO project, satellites or ships in the Pacific. TAO is a collection of more than 70 buoys that float in the Pacific Ocean along the equator. Several thousand feet of cable tether them to the ocean floor in a grid pattern that spans about one-third of the circumference of the globe. NOAA's Pacific Marine Environmental Laboratory and Japan's Agency for Marine-Earth Science and Technology jointly maintain the buoys.

image: NOAA

American and Japanese scientists worked for ten years to get the buoys in place after one of the largest known El Niños was a surprise to scientists. "The 1982-83 El Niño caught the research community completely by surprise," McPhaden says. "It was the largest El Niño in recorded history up to that time. And it was neither predicted nor was it even detected until nearly at its peak."

The system has been in place since 1994 and has been operating almost continuously ever since. "What we're looking for on a day-by-day basis," says McPhaden, "is the large scale…patterns of sea surface temperature and upper ocean temperatures and winds and how they're evolving. The state of the ocean and its tendencies tell us whether we may be moving into an El Niño or a La Niña or whether it may be simply sort of a neutral situation for the next few seasons."

All this equipment and effort is starting to yield some new information. One is a better understanding of what must happen to create an El Niño. McPhaden says several ingredients are involved. "One," he says, "is a prior build-up of heat content in the tropical Pacific pretty much across the entire basin; second is a weakening of the trade winds which develops first in the western and then the central Pacific; and third, of course, is the sea surface temperatures."

Another may be a clue in predicting El Niños. Writing in the Bulletin of the American Meteorological Society, McPhaden reports that, "a buildup in heat content along the equator has preceded all El Niños since 1980 by two to three seasons." However, he points out that sometimes the build-up is a false start, noting, "not all the time do those conditions actually play out in terms of an El Niño."

However, there's still plenty to learn about El Niño and La Niña. McPhaden calls it a "chicken and egg problem"— they know that the warmer water can weaken the trade winds and the weaker trade winds can warm water, but "what changes first?" asks McPhaden. "That's a good question."

This research appeared in the May, 2004 issue of the Bulletin of the American Meteorological Society. It was funded by the National Oceanic and Atmospheric Administration and Japan's Agency for Marine-Earth Science and Technology.