San Francisco — When the United States National Weather Service recently issued its winter forecast for North America, it based its predictions partly on conditions brewing almost half a world away in the tropical Pacific Ocean. For most of this year, scientists have been watching the tropics for the development of another ``El Nino,'' a momentous shift in wind direction and water temperatures that can alter weather patterns around the world. In 1982-83, the most recent El Nino, the most dramatic of the century, deluged some regions of the globe with fierce storms and floods, and parched others with drought. It also had far-flung economic effects, from the destruction of Peru's fishing industry to agricultural devastation in Australia by brushfires.
Two major indicators are pointing to another, but more moderate, El Nino. Scientists who study the phenomenon have predicted such an event in 1986.
First, surface temperatures in much of the Pacific have risen more than 1 degree Celsius above normal. This is the most extensive warming since the last El Nino, according to an advisory issued last month by the federal Climate Analysis Center in Maryland.
In addition, during November the easterly trade winds across the equatorial Pacific appear to have slackened, says Chet Ropelewski, a research meteorologist at the center. ``The easterly trades are associated with the upswelling of colder water from the lower depths, which keeps the surface temperature relatively cool,'' he explains. It is the warmer-than-normal surface temperatures in this region and the relaxation of the trade winds that appear during an El Nino, he says.
Even so, Mr. Ropelewski adds, ``we have a long way to go to approach anything like that ['82-'83] magnitude.'' During that eight-month episode, surface temperatures in the Pacific Basin were at least 2 degrees Celsius above normal, with some areas warming by 5 to 6 degrees.
The last El Nino left in its wake a cadre of scientists who had become captivated by what had happened. ``It had captured the attention of the scientific world, and afterward we were all asking whether we could predict the next one,'' says James O'Brien, professor of meteorology and oceanography at Florida State University.
Four scientific teams in the US, including Dr. O'Brien's, subsequently developed computerized models designed to forecast an El Nino. ``The important thing is that the four separate models all went positive by [this] summer,'' he says. The findings prompted the National Weather Service to issue an October advisory of a warming event in the central Pacific. The advisory was updated in mid-November, and a December update is likely soon.
What does such a warming event in the tropical Pacific portend for winter weather in the US, if anything?
In general, an El Nino means the Northwest - Washington, Idaho, and Montana, as well as British Columbia and Alberta - is more likely to experience milder temperatures in January and February. The outlook for the Gulf states, on the other hand, is for a wetter, cooler winter than normal.
But the correlation between El Nino and winter weather patterns in the US is ``a weak relationship, of limited value in long-term forecasting,'' emphasizes John Wallace, chairman of the atmospheric sciences department at the University of Washington. ``I'd say it can shade the odds [of correctly predicting winter weather in the Northwest and the Gulf], but that's about it.''
``The simple existence of a warming event [in the tropical Pacific] doesn't make forecasting any easier,'' concurs Donald Gilman, chief of long-range forecasting for the National Weather Service. ``In fact, it complicates it.''
Still, he says the warming event in the tropical Pacific influenced the national winter forecast issued Nov. 28, leading forecasters to make a relatively ``bold prediction for precipitation.'' The weather service predicted a 60 percent chance of above-average rain or snow along the Gulf coast, northern Florida, the edge of the East coast, all of New England, and Michigan, he says.
Scientists are still trying to determine if the changes in water temperature, wind, and atmospheric pressure are significant enough to constitute an El Nino. An advisory was issued last spring when similar changes were observed, but it turned out to be a false alarm. By late August, however, data began to point to this latest event.
Because the shifts can take several months to develop, ``It's not easy in real time to see when we're in an El Nino event,'' Dr. Wallace says. The most telling months are just beginning, he says. ``Six months from now, with hindsight, we'll be able to tell much more.''