The warm, moist air and currents push on toward South America, and veer north into subtropical and temperate regions, often with dramatic weather consequences. Alaska typically sees a mild winter, but the collision of tropical air with colder, drier air produces waves of powerful storms that begin to batter Southern California, triggering floods and mudslides.
Farther inland, the tropical pulses create stronger weather fronts, deeper lows and sharp changes in temperature across the continent. The Gulf Coast also gets more tropical moisture.
The strongest El Nino on record occurred in 1982-1983. But by some measures -- chiefly, water temperature -- the current event has equaled that, and may get stronger, Lukas says.
This summer, buoys signaled that surface water temperatures in the "cold tongue" have risen as much as 9 degrees Fahrenheit above normal. More recently, Glantz said, there have been readings 11 to 12 degrees above normal in spots off Peru.
But Lukas says extreme readings may not lead to correspondingly extreme weather elsewhere: "Some of the strongest [weather] anomalies in North and South America have occurred during weak [El Nino] events."
Just why El Ninos occur isn't clear. Eileen Shea, executive director of the Center for the Application of Research on the Environment, says they may simply be the planet's way of dissipating heat from the tropics -- like steam blowing from a tea kettle.
Many questions remain
But scientists have many questions. How often did El Ninos occur in the distant past? Is global warming increasing their frequency and severity? If so, are we all facing more extreme weather? Could we get "stuck" in an El Nino?
Today, anyone with access to the Internet can read the forecasts and follow on colorful maps as an El Nino develops. "This information is now in the hands of the public and policy-makers," Lukas said. It is now up to them to find ways to prepare for or profit from the events as they play out.
"The opportunities are there," he says.
For more on El Nino, try the Internet at www.dir.ucar.edu/esig/enso
Pub Date: 8/29/97