Changes in the Pacific Ocean are making it more likely that winter weather in much of the United States will exhibit unusual warmth alternating with sharp cold, scientists at the Jet Propulsion Laboratory in Pasadena, Calif., reported yesterday.
The researchers said the pattern, prevalent this winter and last, might predominate for 20 or 30 years. The finding was based on calculations of the movement and temperature of ocean surface waters, and the varying amounts of heat they bear, based on measurements made by instruments aboard the Topex/Poseidon earth satellite.
The data reflect a natural oscillation in ocean conditions, not a sign of global climate change. Nor is the development tied directly to El Nino-La Nina weather patterns, year-to-year variations in ocean temperatures that set off atmospheric chain reactions in many parts of the world, including North America.
But if the satellite images do indeed signal the beginning of a new climatic regime in the Pacific, there will be "fewer and weaker El Ninos and more La Ninas," said Bill Patzert, a research oceanographer at the Pasadena laboratory.
In the natural weather phenomenon known as La Nina, sea-surface temperatures in the eastern tropical Pacific are cooler than normal. This sets off a train of atmospheric events.
El Nino is marked by abnormally warm sea-surface temperatures in the tropical Pacific, which touches off a different set of winter weather consequences, often including heavy rains across the southern tier of the United States.
But there is also a larger natural oscillation going on in the Pacific, this one involving a flip-flop in sea-temperature patterns on a scale of decades. When the ocean flips from one of these states to another, Patzert said, "it resets the stage for the climate system; it provides a new background on which smaller events like El Nino and La Nina can occur."
In one of these alternating states of what is called the Pacific Decadal Oscillation, sea-surface temperatures are warmer in the eastern equatorial Pacific but cooler throughout much of the rest of the Pacific basin. That pattern predominated from the mid-1970s through most of the 1990s.
Now, for the last two years, the opposite pattern has appeared: cooler water in the eastern tropical Pacific but warmer elsewhere. That pattern last predominated from the mid-1940s to the mid-'70s.
While Patzert and other scientists said they believed that a flip from one phase of the oscillation to another has occurred, they also said it was too soon to tell whether it represented a true shift from one multidecadal regime to the other.
"There simply has not been enough time" since the shift took place, said Wayne Higgins, a senior meteorologist at the government's Climate Prediction Center at Camp Springs, Md. Another five to 10 years of data may be required, he said.
If a longer-term shift has occurred, and La Nina materializes more frequently, this winter's highly variable pattern of weather in the United States probably would become more familiar.