LA NINA - WINTER'S NEW WEATHER GIRL.

Byline: Keith Stone Daily News Staff Writer

The official explanation for Southern California's wacky weather swings for many years has been: El Nino, El Nino, El Nino.

Now some scientists are blaming El Nino's rogue sister, La Nina, for driving some of this past winter's most dramatic climate changes.

El Nino typically brings heavy rains to Southern California; La Nina has left it drier. Total rainfall since July 1 at the Los Angeles Civic Center was 1.69 inches lower than normal.

Some meteorologists say La Nina has left her fingerprints across the country this winter: heavy rains in the Pacific Northwest, icebox cold from the Great Lakes to the Atlantic states, drier-than-normal weather from Southern California to Florida, and heavy snow in Appalachian states.

``The precipitation pattern of more or less rain or snow is exactly what we would expect, based on our past studies of the impact of La Nina on the United States' winter,'' said Jim O'Brien, a Florida State University meteorologist and oceanographer.

La Nina's name is a play on El Nino, the Spanish term for the Christ child. Peruvian fisherman gave the better-known warm-water pattern that name after they began noticing warm-water fish in their nets around Christmas.

O'Brien asserts that La Nina, known officially as a cold event, has gotten the cold shoulder for too long in favor of her brother, a warm event.

``The scientific community has been concentrating on warm events and ignoring the fact that the cold events have just as important impacts on human beings,'' O'Brien said.

Other meteorologists are less quick to blame La Nina for winter's woes.

They say La Nina accounts for only a quarter to a third of the climate engine that fuels weather extremes in the top half of the United States.

``This is a piece of the puzzle that explains maybe a significant portion of the variability in some areas,'' said Vern Kousky, a research meteorologist for the National Weather Service. ``It might be one piece of the puzzle that we understand.''

Other pieces that don't seem to fit include what meteorologists call the North Atlantic Oscillation, another weather pattern that only now is being studied. And then there's the atmosphere's ``chaotic internal variability,'' and a mysterious mix of cold and warm waters in the northern Pacific that signal more of an El Nino than a La Nina.

``A lot of the cause and effects still need to be sorted out,'' said Tom Murphree, climate researcher at the Naval Postgraduate School in Monterey.

``But I definitely agree,'' he said, ``that it looks suspicious, that La Nina is playing a fairly large role in weather changes in North America.''

Scientists say they know that La Nina has settled in because its signature in the Pacific Ocean is unmistakable: colder-than-normal waters in the central and east-central equatorial Pacific, compared with abnormally warm waters that spawn El Nino in that same area.

When the last El Nino episode ended in 1994-95, water temperatures swung like a giant pendulum to the cooler, La Nina condition.

``The Pacific Ocean is like a big sloshing bathtub - and sometimes it sloshes warm and sometimes it sloshes cold,'' said O'Brien.

In recent months, sea surface temperatures south of of Hawaii on the equator have been hovering around 77 degrees Fahrenheit, or 2 degrees below the normal temperature.

Driving these phenomena is a never-ending tug of war between the ocean and the atmosphere. Weakening trade winds spark an El Nino by allowing a buildup of warm water on the equator to move eastward, pulling the jet stream south where hitchhiking storms pick up more moisture than usual.

During La Nina, the water becomes abnormally cool, acting on the jet stream in an opposite fashion.

The strength of El Nino and La Nina dictates where the jet stream and its storms hit California, in the same way a garden hose is aimed. Exactly where the jet stream enters can vary for each pattern slightly during the winter.

El Ninos tend to bring wet weather to Southern California, but weaker ones can cause drought.

La Nina typically sends the jet stream into the Pacific Northwest, causing heavy rain, and then across Canada and down into the United States, where it sends arctic air onto the Northern Plains.

At the same time, the jet stream entirely misses Southern California, Southwestern states and Gulf states, leaving them drier than normal.

From Texas to Florida, wildfires reigned this summer, driven by record heat.

The dry weather along the bottom half of the nation is almost definitely a result of La Nina, meteorologists say.

``With more certainty, we can point to California, Florida and Texas being related. There was only one player in that game,'' said Ants Leetma, a senior scientist at the National Centers for Environmental Prediction.

``What is causing the record snowstorms and cold in the Midwestern and Eastern United States is something that is not related to La Nina,'' Leetma said.

Researchers say their suspicions are reinforced by oceanic and atmospheric conditions typically not tied to La Nina, such as warmer-than-usual waters in the northeastern Pacific.

``It keeps us scratching our heads all the time,'' said Steven Zebiak, senior research scientist at the Lamont Doherty Earth Observatory of Columbia University. ``We are getting some pieces of the puzzle - that is enough to keep us going.''

CAPTION(S):

2 maps