San Jose Mercury News

Posted at 10:18 p.m. PST Thursday, Jan. 10, 2002

Evidence of El Nino spotted

Oceanic warming could bring harsh weather

BY FRANK SWEENEY

Mercury News

El Niño, the unwelcome tropical ocean phenomenon that shreds climate patterns around the globe, may be coming back after a five-year absence to threaten California with a siege of wild weather next winter.

The classic signs -- warmer water, more rain and weakening of the trade winds -- are beginning to show up far out in the Pacific Ocean, prompting National Oceanic and Atmospheric Administration (NOAA) scientists to issue a long-range prediction this week.

El Niño is the abnormal warming of sea surface waters in the equatorial eastern Pacific that causes weather patterns to shift worldwide. Climatologists have linked it to droughts, floods, typhoons and other costly weather extremes around the globe.

In California, past occurrences of El Niño have sent powerful Pacific storms packing high winds and torrential rains our way, with towering surf pounding coastal communities and mudslides rushing down rain-soaked coastal mountain slopes. The 1997-98 El Niño was the strongest of the century, costing 22,000 lives and $36 billion in damage worldwide.

``Since El Niño occurs every two to seven years, we're not surprised'' at its likely reappearance, said Jim Laver, acting director of NOAA's National Center for Climate Prediction in Camp Springs,Md., which made the announcement.

``We see something happening,'' Laver said. ``If you need a year to plan for it, start now. We don't want any surprises.''

Two key signals

Laver said two key signals have emerged to indicate El Niño may be developing. The atmospheric convection -- an updraft that creates chronically cloudy weather and heavy rains -- over Indonesia has moved east to near the international date line, where sea surface temperature has risen to 2 degrees above normal.

``That may seem very subtle, but it makes a difference in the ocean,'' Laver said. ``We've seen more convection in the central equatorial Pacific than we have seen in the last four years. When the convection moves away from Indonesia, something is probably happening.''

Over the next several months, pulses of warmer water could spread east as El Niño intensifies, he said. The phenomenon requires many months to show any effects and is not responsible for the heavy rains seen in Northern California this winter.

It would take a moderate-to-strong El Niño for California to get heavy precipitation next winter, ``and even a strong event doesn't guarantee stronger rain,'' Laver said. ``The process takes a long time, but when we see these signs we're fairly confident of some sort of event.''

NOAA scientists base their prediction on satellite and buoy measurements across the Pacific, as well as climate models in supercomputers.

But, said Michael McPhaden, director of NOAA's Pacific Marine Environmental Laboratory in Seattle, ``the models do not all agree with one another. There's quite a spread in the forecasts. It might be a bit of a crapshoot.''

Still, McPhaden concurs that there is strong evidence of sea surface warming and weakening trade winds over the central Pacific. ``In some ways, that's what happened in the beginning of the 1997-1998 El Niño,'' he said.

McPhaden said the temperature increase is ``just the beginning. Right now most of the warming is concentrated near the date line. We expect it to grow and spread eastward.''

An early warning system

McPhaden's laboratory operates a sort of El Niño early warning system -- a network of 70 buoys moored along the equator from the west coast of South America to New Guinea.

Called the Tropical Atmosphere-Ocean Array, the buoys measure air temperature and pressure, relative humidity, wind direction and speed, and water temperatures on the surface and as deep as 1,600 feet below. They radio data to a NOAA polar-orbiting weather satellite, which relays the information to the Tropical Atmosphere-Ocean Array project office at the laboratory in Seattle.

From there, the information is made available to the international scientific community. Combined with data from geostationary weather satellites, these measurements allowed meteorologists and oceanographers to warn of El Niño five years ago, and again now.

El Niño is Spanish for ``the Christ child,'' a term first used by Peruvian fishermen to describe a warm ocean current that showed up around Christmas every few years to disrupt fishing. Essentially, El Niño is a dramatic reversal of the normal sea temperature, wind and atmospheric pressure conditions in the tropical Pacific.

What triggers it is a scientific unknown. Does the atmospheric pressure flip-flop cause sea water to heat up, or does the warmer water cause the atmosphere to respond?

Major storms are guided around the globe by the jet streams -- upper atmosphere high-speed rivers of air. El Niño makes the polar jet move south, nearly merging with the subtropical jet stream. The result: heavy precipitation in the Southern regions of the United States, less rain farther north in Canada.

``We're about due, since typically the recurrent rate for El Niño is three to four years,'' McPhaden said.

Contact Frank Sweeney at fsweeney@sjmercury.com or (408) 920-5675.

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