CNN El Nino Ocean Weather article 8/18/97

Forecasters look at oceans to predict next year's weather

August 18, 1997

Ask any meteorologist what the weather's going to be like next month, and all you're likely to get is hand-wringing.

But next year? Why, that's easy!

The forecast for early 1998: Wetter than usual in the southern and southwestern United States, with higher than normal temperatures along the West Coast and drier weather in the Pacific Northwest. Southern Australia and Africa's Sahel region are expected to be warmer than usual. Wetter than normal conditions are forecast for southern Africa and the Amazon basin, but northern Brazil should be relatively dry.

So how do they know all that? Simple: Just by looking at the ocean.

In recent years, meteorologists have discovered that much of the year-to-year variation in the Earth's climate is controlled by a single oceanographic phenomenon -- the famed El Nino of the tropical Pacific.

El Nino is a warming of the waters off equatorial South America that develops every few years, reaching a peak in November or December. Its effects propagate around the world, influencing the weather in Australia, Africa, South Asia and the tropical parts of the Americas.

This year, for example, meteorologists can already see a whopper of an El Nino developing. They expect it to be the biggest in 15 years.

"The physics behind El Nino are fairly well understood," says Lisa Goddard, a project scientist at the International Research Institute for Climate Prediction in La Jolla, California.

Unfortunately, the physics of the rest of the world's oceans are more mysterious. But if they can understand the Atlantic, Indian and Southern oceans as well as they do the tropical Pacific, perhaps scientists can forecast things like droughts, heat waves and cold snaps a year ahead of time.

"You can't say it's going to be rainy on August 10, but you can say over next season -- August, September, October -- there's an increased likelihood for rain in a particular area," Goddard says.

Scientists discovered in the mid-1980s what makes El Nino tick, and have spent the last decade learning to predict months ahead of time whether one will develop. The big problem, however, is that El Nino doesn't significantly affect the weather in many parts of the globe.

So meteorologists have turned their attention to those other parts, especially the Atlantic. By determining how the Atlantic evolves over months and years, they can estimate how many hurricanes will develop in an upcoming season or tell Europeans if they're facing an unusually cold winter.

Who cares? Well, farmers for one. Peruvian cotton growers switch to rice in El Nino years, because rice likes the rain the weather pattern brings. In northeastern Brazil, they plant drought-resistant varieties of corn and beans when El Nino rears its ugly head. And in Africa and Colombia, an impending El Nino year prompts stronger malaria prevention efforts.

In more temperate parts of the world, cities could choose to invest in new snowplows if the coming winter looks cold and snowy.

Coastal residents might buy more insurance if a stormy hurricane season is forecast. And maybe their insurers would raise their rates.

El Nino predictions have succeeded because researchers went beyond describing how winds and waves move around in the tropical Pacific. They noticed a pattern to the interactions of the ocean and atmosphere in that part of the world, and were able to simulate that behavior with a computer model.

In 1986, the Lamont-Doherty Earth Observatory in Palisades, New York, devised a computer program that mathematically described how the currents and winds of the tropical ocean interact to create either an El Nino or an opposite state known as La Nina -- or a condition partway between the two.

That program and others have been refined in the last decade to the point where meteorologists can say fairly confidently a year ahead of time whether an El Nino is on the way. They simply plug in current temperatures, pressures and other physical factors, then roll the computer's clock ahead a year and see what happens.

But in the Atlantic, researchers haven't gotten that far.

The problem is that things get much more difficult to understand as you move away from the equator, and the Atlantic is much less dominated by its tropical waters than the Pacific. So it changes more slowly and less dramatically han the Pacific, making patterns harder to distinguish.

But several recent studies have found intriguing patterns in the North Atlantic.

In a recent issue of the British journal Nature, Rowan Sutton and M.R. Allen of Oxford University describe a sort of slow-motion Atlantic version of El Nino.

The oceanographers analyzed thousands of water temperature measurements by ships at sea since World War II. Those measurements revealed a cyclical pattern that repeats at 12- to 14-year intervals, in which warm water develops off the coast of the Carolinas, then slowly moves northeast, taking about six years to reach the vicinity of Iceland.

By then, relatively cool water has moved into the Carolinas, reversing the situation that existed six years before. The cool water takes another six or seven years to move north, bringing things back to the cycle's beginning.

Because the temperature of the water off the Carolinas strongly influences the development of hurricanes in the Atlantic, meteorologists are extremely interested in the pattern Sutton and Allen describe, since it's the kind of natural phenomenon that makes year-to-year weather prediction possible.

"It is very intriguing. Though we don't have all the answers yet," Sutton says.

The pattern could be a stable phenomenon that has a lot to do with weather in the North Atlantic. Or it could be an insignificant fluke that disappears in the next decade, never to be seen again. With only 50 years of temperature measurements, Sutton notes, it's hard to tell.

And even if the patterns that have been noticed in the North Atlantic are consistent, says Mike McCartney of the Woods Hole Oceanographic Institution, meteorologists will be reluctant to use them for year-to-year weather prediction until they understand them as well as they do El Nino.

"At this point, we're still dealing with fundamental physics questions," McCartney says. "But some clever modeler may make a breakthrough ... like the El Nino people did in the 1980s.

Copyright 1997 Associated Press

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