LA NINA MOOD SWINGS; DRY WEATHER PATTERN BRINGS HIGHS, LOWS.

Byline: Douglas Haberman Daily News Staff Writer

El Nino's rogue sister, La Nina, has left Los Angeles with half as much rain as normal so far this winter and produced wild temperature swings, including a roller coaster of record low and high temperatures, weather experts said.

La Nina, a global weather pattern caused by unusually cold equatorial waters, has meant drier weather for Southern California, in sharp contrast to the cold, wet weather last winter that was served up by the century's most potent El Nino.

``This is what I would call a moderate La Nina,'' said Bill Patzert, a research oceanographer at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena. ``We're definitely below normal for our rainy season.''

At the Los Angeles Civic Center, only 1.87 inches of rain fell between July 1 and Dec. 24, compared to the average of 4.49 inches during the same period, according to the National Weather Service.

The scant precipitation is notable, said NWS meteorologist Bill Hoffer, because ``we're right in our rainy season right now. We should be getting it.''

By the same time last year, 5.03 inches of rain had fallen at the Civic Center, the result of El Nino, which disrupts weather around the world and is caused by abnormally warm equatorial waters. In a normal year, an annual average of 15 inches of rain falls at the Civic Center, but in a La Nina year, rainfall generally will plunge to between nine and 12 inches. El Nino brought about 31 inches of rain.

Both El Nino and La Nina are marked by volatile weather swings, as evidenced by the cold snap that swooped down from the north to make Christmas week chilly in Los Angeles, if not white for some in the mountains, Patzert said.

But the warm, dry days the week before last - with temperatures running in the 80s and setting records in some places - are more typical of a La Nina, he said.

``There's a slight tilt for a warmer, drier winter in Southern California'' in La Nina, agreed Stephen Zabiak, a meteorologist and senior research scientist at Columbia University in New York.

Unexpected the norm

El Nino and La Nina ``rearrange the pieces on the weather board,'' Patzert said. ``You have to keep looking over your shoulder.''

Other experts agree: Although warmer weather is typically part of a La Nina, ``cold outbreaks are more common in La Nina years'' too, said Nicholas Graham, director of the climate forecast division of the International Institute for Climate Prediction at the Scripps Institution of Oceanography.

At the same time, episodic warm winds called Santa Anas are just as likely, he said.

Scientists are only now beginning to fully understand these weather engines. 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.

When the previous El Nino episode ended in 1994-95, water temperatures swung like a giant pendulum to the cooler, La Nina condition. That year, Los Angeles received less rain than normal, and some meteorologists said La Nina left her fingerprints elsewhere: 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.

Climatic tug of war 

Both El Nino and La Nina stem from abnormal surface water temperatures in the tropical Pacific Ocean. In an El Nino, the tropical Pacific's temperatures can rise as much as 8 degrees to 9 degrees above normal.

In a La Nina year, they are 5 degrees or 6 degrees colder than normal. ``They don't go as far from average as in an El Nino,'' Graham said.

This never-ending tug of war between the ocean and the atmosphere cause El Nino and La Nina. Weakening trade winds drive an El Nino by allowing a buildup of warm water on the equator to move eastward, pulling the jet stream off course and allowing hitchhiking storms to 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. This La Nina probably will end next spring, Graham said, and it's unlikely to come right back.

``It's pretty hard for them to last two years,'' he said.

Cloud seeding continues

Largely due to El Nino's bounty of rainfall, Southern California is not facing drought. Still, Los Angeles County and city officials are trying to increase localized rainfalls.

The county Department of Public Works and the city's Department of Water and Power have cloud-seeding programs to boost the amount of rainfall that produces water for use by residents of this semiarid region.

The DWP program, at an annual cost of about $75,000, uses airplanes to seed clouds over the eastern Sierra Nevada range and near Owens Valley, where a huge portion of the city's water supply originates.

The program helps the agency save at least three times the cost of the program by generating water it no longer has to buy from the Metropolitan Water District, said Vee Miller, a DWP hydrologic engineer.

The county program, costing slightly less than $200,000 a year, uses ground-based machines in the San Gabriel Mountains that send silver iodide into storm clouds; the chemical acts as nuclei around which raindrops form, increasing the amount of rainfall.

The storm water is collected in 15 county dams and later put into groundwater aquifers, including in the Sylmar and San Fernando areas.

Water pumped from local groundwater basins costs only 30 percent of the water transported to Los Angeles from Northern California or the Colorado River, according to a report on the county program.

CAPTION(S):

Chart, Map

Chart: L.A. RAINFALL

Map: TRACKING THE WEATHER

El nino and its sister, La Nina are global climate patterns that scientists say have dramatic, albeit opposite, effects on weather. Spawned in the tropical Pacific Ocean, they help aim the jet stream at different ends of the West Coast.

An El Nino jet stream tends to enter toward the south, and La Nina's to the north, but the exact track may vary.

SOURCE: Tom Murphree, Naval Postgraduate School