A LOOK AHEAD: With hurricanes battering the Caribbean and drought feared in some areas of the United States, the Southland is . . . Casting a Wary Eye on La Nina

La Nina--the widely anticipated counterpoint to last winter’s devastating El Nino--has arrived on schedule, and although Southern California may escape, there’s another siege of unusual and sometimes destructive weather in store for much of Central and North America.

As expected, there already have been more--and bigger--hurricanes than usual in the Caribbean and along the Atlantic Coast. Forecasters say that it probably will be stormier and colder than normal this winter in the Pacific Northwest.

And despite the recent drenchings that produced floods in south Texas, meteorologists agree that the Deep South and much of the Southwest should be noticeably warmer and drier than usual over the next six months, with moderate to severe drought conditions in some areas.

But the weather in Southern California will probably be rather unremarkable during the coming winter. If anything, forecasters say, the current La Nina oceanic and meteorological phenomenon could provide a relatively balmy contrast to the stormy, typical El Nino weather here last winter.

There could be a few more hot--and occasionally damaging--Santa Ana winds than usual. And despite the rain that fell on Sunday, there could be a little less precipitation than usual.

Nicholas Graham, a meteorologist with the International Research Institute for Climate Predictions at the Scripps Institution of Oceanography in La Jolla, said computer models show roughly an 80% probability of normal or drier than normal weather this winter, with the odds slightly favoring drier.

Bolstering this prediction, the National Oceanic and Atmospheric Administration notes that during seven out of the eight La Nina rainfall seasons since 1949, the Southland has experienced subaverage precipitation.

The average annual rainfall in downtown Los Angeles during those La Nina years was 11.61 inches. The overall average since record-keeping began in 1877 is 15.14 inches.

Southland Can Weather Drought

But even if the rainfall is a lot lower than average this winter, there is little threat of drought in California. Subterranean water tables are high and reservoirs throughout the state are still overflowing with runoff from last year’s El Nino-enhanced precipitation.

Meteorologists say that the first signs of La Nina began emerging early last summer as El Nino finally ebbed into oblivion. That’s because the two phenomena are interrelated.

Graham explained that the Pacific Ocean is essentially one vast basin, “with the water sort of sloshing back and forth.”

During normal conditions, low-level equatorial trade winds blow from east to west, shoving water ahead of them that tends to pile up near Indonesia, raising the sea level a foot higher there than it is off the coast of Peru. During El Nino, the trade winds slacken or even reverse, and the water sloshes back toward South America.

The sloshing surface water presses down on cooler water beneath it that normally wells to the surface off Peru. As a result, the surface water there stays warmer than usual, and starts ebbing back west across the Pacific.

This vast pool of warmer than usual surface water--often depicted on television last winter as a reddish smear spreading west from Peru--interacts with the atmosphere to disrupt normal weather patterns.

During most El Ninos, high-altitude Pacific jet stream winds are intensified and extended all the way across the ocean to California and Baja California. A large low-pressure center over the central Pacific spins off an unusual number of storms, and the amplified eastbound storm track tends to funnel more precipitation than normal into Southern California.

That’s what happened last fall, winter and spring, when 31.03 inches of rain fell on downtown Los Angeles--the third-highest total on record. The effect spread east across the southern third of the United States, with wetter than normal weather throughout the region and cooler than normal temperatures in Texas, Louisiana, Mississippi, Alabama, Georgia and Florida.

At the same time, the polar jet stream stayed farther north than usual as it circled the globe from west to east. The result last winter was much warmer temperatures than usual in southern Alaska, western Canada, the Pacific Northwest and the north-central Plains states, with drier than normal weather in Indiana and Ohio.

Fortunately, El Nino also sets up ocean and wind currents that ultimately reverse the phenomenon. Sometimes, the reversal goes beyond normal, and the water off the west coast of Peru becomes unusually cool. That condition is known as La Nina, and it started happening early last summer.

By the end of June, sea surface temperatures along the Pacific equator were dropping rapidly. By the end of September, the equatorial surface water was as much as 4 degrees cooler than normal in some areas, and the temperatures were continuing to drop.

“Moderate to strong La Nina conditions will likely remain in place through the remainder of 1998 and into at least the first one to two months of 1999,” the National Weather Service said. “The degree of confidence that the current La Nina will continue without interruption through March is near 90%.”

Change in Sea Temperatures

Once again, sea surface temperatures in the equatorial waters of the Pacific are interacting with the atmosphere to disrupt normal weather patterns, and forecasters said it’s shaping up as a typical La Nina winter.

During most La Ninas, high-altitude Pacific jet stream winds swing a little farther north than usual, and the polar jet stream swings a bit farther south. A large high-pressure center over the northern Pacific tends to block the storm flow, and the two jet streams, converging over the Northwest, tend to carry the storms that do get through well north of Southern California.

The usual result is normal to drier than normal weather in Southern California, with much drier than normal weather in Arizona, New Mexico, northern Texas and Florida and warmer than usual temperatures in the South and the central Atlantic states.

During most La Nina winters, more rain than usual falls in the Pacific Northwest and on Indiana and Ohio. Temperatures in eastern Alaska, northwestern Canada, the Pacific Northwest and the north-central Plains states usually are cooler than normal.

“But what we’re talking about is probabilities, not certainties,” Graham said.

Scientists have learned that although weather is largely governed by major cycles like El Nino and La Nina, it is also affected by random factors that are often unpredictable.

Mike Smith, a meteorologist who heads WeatherData Inc., cited recent Caribbean hurricanes as an example.

He said that while the more northern than usual swings of the current La Nina jet streams have permitted the incursion of hurricane precipitation into southern Texas, the precise paths of those hurricanes is almost random. The result, he said, is some wetter than normal areas of the Southwest that are right next to areas that, as expected, are drier than normal.

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Tracking La Nina

El Nino and La Nina are extreme phases of a naturally occurring oceanic and meteorological cycle driven by large-scale changes in sea-surface temperatures in the eastern equatorial Pacific. El Ninos occur when sea-surface temperatures are warmer than usual; La Ninas occur when they are cooler. El Ninos occur, on average, about once every three to five years, with La Ninas occurring in some, but not all, of the intervals. Both phenomena deflect high-altitude jet streamwinds that encircle the globe, influencing weather in the United States and elsewhere.

LAST WINTER

* El Nino: A strong El Nino condition intensified Pacific jet stream winds that extended across the ocean to California and Baja California. As low pressure over the central Pacific spun off storms, the amplified Pacific storm track sent unusually heavy rain into Southern California and across the Southwest and Deep South. Meanwhile, polar jet stream winds stayed farther north than usual, leading to higher temperatures in southern Alaska, western Canada, thePacific Northwest and the north-central Plains states.

THIS WINTER

* La Nina: The current La Nina condition is expected to intensify. The polar jet stream probably will swing farther south than usual, and the Pacific jet stream probably will swing farther north. The two wind streams, converging over the Northwest, are likely to carry most storms north of Southern California. Southern California probably will have normal to low levels of rain. Despite recent storms, the weather in the Southwest and Deep South probably will be much dryer than normal, with higher temperatures in the South and central Atlantic states.

SOURCE: CLIMATE PREDICTION CENTER