These mood swings include a climate phenomenon known as Pacific Decadal Oscillation, or PDO. It's an El Nino- like shift in the ocean's temperature that scientists once thought cycled every 15 to 20 years. However, there's new NASA research that now shows there may be a second, much longer, PDO cycle that lasts about 70 years.

Dr. Yi Chao, an oceanographer at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and colleagues Drs. Michael Ghil and James McWilliams of the University of California, Los Angeles, have found evidence of the PDO's two-part structure in a study of the past 92-year record of sea-surface temperatures in the North and South Pacific.

The results of their study appears in the August 1, 2000, issue of Geophysical Research Letters.

If El Nino is a brief sonata, then the Pacific Decadal Oscillation is a much larger symphony.

"The El Nino is well defined," said Chao.

"We know when it is born, can see its rise and fall and measure its strength. We can forecast its consequences. But the PDO is larger, longer and more difficult to visualize. An explanation might be that it isn't just one thing - - it's potentially two big events going on."

In their study, Chao and his colleagues found large-scale temperature oscillations taking place in the Pacific basin approximately every 15 to 20 years.

"While we are only talking about a one-to-two degree centigrade difference in sea-surface temperature, we are talking about a huge area. This temperature difference has a big impact on the climate of North America."

The change in location of cold and warm water in the Pacific alters the path of the jet stream, the conveyor belt for storms across the continent.

Chao's study supports and expands the previous studies by University of Washington researchers that gave a name to the phenomenon only five years ago.

However, in addition to this regular and relatively short fluctuation in the Pacific basin's temperature, Chao also has found evidence of another temperature shift that appears to take place on a much longer time scale, about 70 years.

At the beginning of this century, sea-surface temperatures seem to gently drop to a low in the 1930s, gradually rise again until the 1970s, and then begin a similarly paced decline to the present.

"While we were only able to see one cycle in our data, tree-ring records, which go back 200 to 300 years, and fishery data, show a similar time-scale shift," added Chao. In Chao's analysis of the past century's sea-surface temperatures, the PDO also reveals striking symmetry between the northern and southern Pacific.

In its "cool" phase, the PDO is a giant horseshoe-shaped arc of warmer-than-normal water off the coast of Japan, enclosing a wedge of cooler-than-normal water near the equator. In Chao's study, this same approximately 20- year cycle appears around 1976, 1957, 1941 and 1924.

"What's striking is that the PDO pattern is similar in both the North and South Pacific and covers a huge area from the Aleutian Islands to the South Pacific," said Chao.

"No computer models developed so far have been able to reproduce this symmetric pattern across the equator. This symmetry is a key to understanding what creates the PDO."

"Looking into the future," Chao continued, "we are now analyzing the temperature below the sea surface. The goal is to get a three-dimensional picture of the PDO that might help us reach the ultimate goal, a realistic computer model linking the ocean and the atmosphere that will help us predict Earth's climate."