WASHINGTON -- For the third time in 10 years, a huge expanse of unusually warm water has developed in the equatorial Pacific, setting in motion a so-called El Nino system that could disrupt weather patterns worldwide, National Weather Service scientists said yesterday.
Evidence of another of the periodic upsets had been accumulating for several months. After analyzing satellite data and other information collected in December, the Weather Service's Climate Analysis Center concluded yesterday that the system has formed and probably will reach its peak in the next six months.
Typically, the phenomenon produces abnormally warm winters in southeastern Alaska, the westernmost and easternmost regions of Canada and the northern tier of the contiguous United States. At the same time, areas from Arizona to Florida and along the southern Atlantic coast experience cooler-than-normal temperatures and greater-than-normal rainfall.
The likely effect on the western United States is uncertain. El Ninos have produced both very wet and very dry weather.
For example, a severe El Nino in the winter of 1982-'83 brought storms to California, prompting floods and mudslides and causing 1,500 deaths worldwide, but another in 1986-87 went nearly unnoticed.
The effects usually fade with the coming of spring in March, but David Rodenhuis, director of the Climate Analysis Center, said yesterday, "We should say this is still January, so hold on to your hat."
The warming of an expanse of the tropical Pacific that sometimes reaches a fourth of the way around the world leads to massive changes in cloud patterns and shifts in the jet stream, the high-altitude current of air that creates weather patterns.
The warm Pacific pool being watched is about twice the size of the one in 1986 and 1987, and the water temperature, as high as 86 degrees, averages about one degree higher than in January 1987.
"We could continue on with a weather pattern like what we have been experiencing for the last six weeks," said Edward O'Lenic, head of forecasting operations at the Climate Analysis Center.
If the jet stream remains split off the West Coast as it is now, he said, a mild weather pattern will continue to dominate most of the continent's interior.
If it forms a single stream, then moves north into British Columbia and dives to the midsection of the United States, it would bring colder temperatures to the Great Lakes and the eastern United States.
On a worldwide scale, El Ninos have been associated with extreme droughts in southern Africa, Australia and the tropics of Brazil.