Pacific Ocean Showing Signs of Major Shifts in the Climate
By William K. Stevens, The New York Times, Jan. 20, 2000
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 naturally occurring oscillation in ocean conditions, not a sign of global climate change.
If the satellite images do indeed signal the beginning of a new climatic regime in the Pacific, there will be "fewer and weaker El Niños and more La Niñas," said Dr. Bill Patzert, a research oceanographer at the Pasadena laboratory.
In the natural weather phenomenon known as La Niña, sea-surface temperatures in the eastern tropical Pacific are lower than normal.
This sets off a train of atmospheric events that affect weather patterns around the globe, especially in North America in the winter.
Sea surface temperatures in general have a major effect on atmospheric circulation patterns, and in large measure govern where storms develop and cold and warm air masses go.
El Niño is marked by abnormally high 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.
La Niña and El Niño typically last a year or two, but there is also a longer-term 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, Dr. Patzert said, "it resets the stage for the climate system; it provides a new background on which smaller events like El Niño and La Niña can occur."
In one of these alternating states of what is called the Pacific Decadal Oscillation, sea-surface temperatures are higher in the eastern equatorial Pacific but lower throughout much of the rest of the Pacific basin. That pattern predominated from the mid-1970's through most of the 1990's.
It was also a period of more frequent and stronger editions of El Niño.
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-1940's to the mid-1970's.
While Dr. Patzert and other scientists said they believed that a flip from one phase of the oscillation to another had 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. Five to 10 additional years of data may be required, Mr. Higgins said.
The shift is only two years old and whether it will last for a full 20 or 30 years remains to be seen.
If a longer-term shift has occurred, and La Niña materializes more frequently as a result, this winter's highly variable pattern of weather in the United States would probably become more familiar.
In it, the positions of warm and cold air masses and storm tracks shift so that the eastern part of the country is often exposed to warm weather.
Then the masses drift and sudden cold hits the East. Meanwhile, the Northwest becomes stormier. Then the pattern repeats.
The Climate Prediction Center has forecast that La Niña will persist into the spring, then fade. What will happen after that is unclear.
There is an additional complication just now: Atmospheric conditions in the North Atlantic have shifted in recent days to a new pattern in which the Northeast and Middle Atlantic are exposed to the likelihood of cold, stormy weather.
How that will play out over the next few weeks is not certain; but government forecasters predict that February will be warmer than normal in the southern tier of the country but colder than normal in the northern tier, with above-normal precipitation in the Northeast, Ohio Valley, Midwest and Pacific Northwest.
If the Pacific Decadal Oscillation persists in its new state, experts say, it might also portend drier weather and more frequent droughts in the southern tier.
Moreover, increased hurricane activity is associated with this phase of the oscillation, and there has been such an increase in the last five years.
Scientists believe that large-scale climatic fluctuations like the Pacific oscillation affect the global temperature.
The last time the oscillation was in its present state, from about 1945 to about 1976, a global warming trend that had begun early in the century leveled off.
Then it resumed when the oscillation flipped to its opposite state, rising in the 1990's to the highest level ever recorded.
Since the mid-1970's, federal scientists say, the average global surface temperature has risen at a rate equal to 3.5 degrees per century. (The world is 5 to 9 degrees warmer now than in the depths of the last ice age 18,000 to 20,000 years ago.)
The dominant view among experts is that emissions of heat-trapping industrial waste gases like carbon dioxide are responsible for at least part of the last century's global warming of about one degree.