Meteorology: Warming and cooling of the tropical Pacific causes ripples that are felt across the globe in disrupted rainfall and juiced-up tropical storms. As the biggest El Nino of the century makes the transition into a La Nina event, scientists learn more about the relationship between the two.

A wide band of the Pacific Ocean that so warmed last year to produce the biggest El Nino of the century has been shifting gears since the spring. That vast reservoir has become so cool, so fast that the blockbuster El Nino is giving way to the birth of its climatological cousin, La Nina.

For the Northwest, after experiencing generally warmer and drier weather under El Nino, change is in the winds.

-- Skiers may be smiling, because Las Ninas tend to deliver arctic blasts into Western Washington. The chilly air is a key ingredient for snow, expected to dust the lowlands as well as accumulate into a heavy snowpack in the mountains.

-- Seattle-Tacoma International Airport managers are already stocking up on de-icing fluid. A shortage of the ice-cutting chemical here could mean delayed departures, diminished profits and disrupted connections at other airports.

-- Major flooding doesn't happen every La Nina winter. But flipping back through National Weather Service river records finds debilitating floods here in November 1995 and February 1996, a La Nina winter.

-- Chillier-than-normal ocean waters could affect the survival

of vulnerable salmon smolt, depending upon the timing of the baby salmon's journey to sea.

"I would consider it two extremes of a swinging pendulum," Lee-Lueng Fu, a chief scientist at NASA's Jet Propulsion Laboratory, said of the El Nino/La Nina dichotomy.

The oscillation, however, is irregular. Every swing the tropical Pacific makes toward superwarm waters isn't immediately countered by a swing toward supercool water. "That makes the whole subject interesting," Fu said.

A series of buoys that bob along the equator recording water temperature, wind speed and other essential information sensed the seeds for the demise of El Nino as far back as January. But even though a slug of cold water lurks below - superchilled liquid that crept from higher latitudes, taking about 10 years to arrive at the equator - its presence below the surface wasn't enough.

Trade winds back in the mix

When El Nino calls the shots, trade winds reverse their usual direction and get lazy. La Nina coaxes the trades to retrace their regular tracks and sometimes increase in strength.

"There was a slow elevation of this cold water toward the surface. What was missing in the formula was the trade winds," said Mike McPhaden, a research oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory, which tracks El Nino-based ocean warming and La Nina-based ocean cooling.

When the trades picked up speed, the thin veneer of warm water at the surface was chilled by water from below. The result? The tropical Pacific's transition from warm to cold was speedier than ever seen before - a 15-degree drop in water temperature from early May through early June in water that stretches 3,000 miles from east to west along the equator.

After that spectacular drop, the tropical Pacific's now holding steady. Water temperature has plateaued at about 7 degrees cooler than normal in the center of the cool pool, McPhaden said.

"I do think conditions are favorable for this cold pool of water to continue to grow, mainly because it's being fed from below by a fairly extensive pool of cold water," he said.

That observational view is confirmed by computer models and satellite images taken by TOPEX/Poseidon, a U.S.-French $700 million satellite launched in 1992.

The buoys cover a narrow band across the equator, providing a close-up view of water conditions. The satellite has a more global view that's essential in learning more about the transition from an El Nino to a La Nina, Fu said.

It can track changes in sea level, which normally fluctuates inches at a time. How high or low that level is helps determine the dynamic interplay of water temperatures.

The TOPEX/Poseidon's view, accurate to about an inch, picked up a 7-inch sea-level drop in the central, equatorial Pacific. Nature doesn't like that degree of imbalance. The drop in sea level triggered an adjustment and redistribution of water.

"A depression of sea level by 7 inches means the cold-water layer below the surface will rise by 120 feet," Fu said.

La Nina packs a wallop

Knowing how the cold water is redistributed allows scientists to estimate heat-storage capacity as well as heat deficit. Water that reaches a certain warmth evaporates, releasing heat into the atmosphere that feeds storms. When the finger of cold water stretches beyond its normal range, it shifts the breeding grounds for tropical storms.

In La Nina years, for example, more hurricanes are expected to hammer the Atlantic coastline. The devastating events could make the price tag for the climate phenomenon more costly in money and in loss of life than El Nino-induced storms, said Michael Glantz, a senior scientist with the National Center for Atmospheric Research.

Commodity trackers, oil companies and international scientists are signed up for the world's first La Nina summit later this month, organized by Glantz. What was once envisioned as 20 people sitting around a table sharing papers has grown into a full-fledged conference sorting through what's known about the phenomenon, he said.

"For example, there are a lot of phrases thrown around. `It's the opposite of El Nino.' `It's the counterpart.' `It's the alter ego,' " he said. "But is it? . . . Is there symmetry? If you get 10 degrees warmer (water) can you get 10 degrees colder? I don't think so. It can only get as cold as the deep water."

Could be a colder, snowier winter

In projecting how that chilly puddle of water may make its influence felt here next winter, scientists look to climate statistics gathered during past La Nina winters.

La Nina's deepest fingerprints here, said University of Washington meteorology Professor Cliff Mass, should be on snowfall and temperature.

The winter is expected to be colder than normal with more snow. Sea-Tac airport in a typical winter gets 8 to 9 inches of snow and a mere 5 inches during El Nino winters. That snowfall total jumps to 16.9 inches during La Nina winters.

There's more of a tendency for low-pressure systems to linger over the western part of the United States with high-pressure ridges off shore. The jet stream, which steers the path of storms, moves up north toward Alaska during La Nina winters, then swings down the coast, Mass said.

The chilly air mass the jet stream brings is a building block for snow.

"We have a chance of doing skiing in the lowlands. You can get on the Burke-Gilman Trail," Mass said.

But as Seattle skis, Bellingham should prepare to bundle up and batten down.

"Bellingham should get it. The classic path for the cold air is through the Fraser River Valley," he said. "They can expect to get some of those major, arctic blasts. They can get winds of 60 to 70 mph as the cold air invades into Western Washington."

Diedtra Henderson's phone message number is 206-464-8259. Her e-mail address is: dhenderson@seattletimes.com