Changing weather

Changes in climatic trends may be part of a decade-long pattern known as the Pacific Decadal Oscillation

Published: Tuesday 29 February 2000

scientists believe the world is on the verge of a change in climate patterns that could last 20 to 30 years. "If the trend is confirmed the change could have a major effect on Asia," says Michel Deque, a climate modelling expert with the French weather service, Meteo France.

Scientists at the National Aeronautical and Space Administration's ( nasa) Jet Propulsion Laboratory ( jpl ) at Pasadena, California, studying new imagery from the us- French topex/ Poisedon satellite, believe the abnormally warm ocean temperatures in the entire western Pacific and Asiatic oceans, may be a part of a larger, longer-lasting climate pattern.

"In contrast with the more spectacular but shorter duration El Nio and La Nina events, this multiple-year trend may be part of a decade-long pattern known as the Pacific Decadal Oscillation," says William Patzert, an oceanographer at jpl.

According to this theory, the Pacific Ocean switches between warm and cool every 20, 30 or 40 years. In the warm phase, an abnormally warm wedge of water appears near the Americas, while the sea surface cools near the Asian Rim from Japan to Australia.

The alternate cool phase occurs when the sea surface around the Asian Rim gets abnormally warm; at the same time, the ocean off the western United States is colder than usual. This leads to weather patterns similar to those produced by La Nina: the periodic arrival of abnormally cold pacific waters off Central America. The eastern Pacific is presently in the grip of a particularly long-running La Nina.

One result of warmer waters around the Asian Rim, he said, could be rainier summers and milder winters in Japan, China and Korea while there would be an increase risk of typhoons in Southeast Asia and other tropical areas.

The cool-phase conditions was first notice in mid-1998 and it has grown clearer in the past 18 months. Ants Leetmaa, who heads the National Oceanic and Atmospheric Administration's Climate Prediction Centre, suspects that the shift began in 1995, if not earlier, based on worldwide changes in rainfall patterns, hurricane numbers and other variables. But no one can say for certain.

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