Artist's concept of the TOPEX/Poseidon spacecraft in orbit. Photo: NASA

A giant horseshoe pattern of higher than normal sea-surface heights developing over the last year is beginning to dominate the entire western Pacific and Asiatic oceans, new imagery from the U.S.-French TOPEX/Poseidon satellite shows.

Scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., studying the new data believe these abnormally warm ocean temperatures, which contrast with a cool La Nina, may be part of a larger, longer-lasting climate pattern.

The latest data, taken December 30, 1999 through January 8, 2000, show that this slower-developing condition covers most of the Pacific Ocean and has significant implications for global climate change, especially over North America, said Dr. William Patzert, an oceanographer at JPL.

"In contrast with the more spectacular but shorter duration El Nino and La Nina events, this multiple-year trend may be part of a decade-long pattern known as the 'Pacific decadal oscillation,'" Patzert said. "The persistence of these abnormally high and low Pacific sea-surface patterns, along with warmer and colder than average ocean temperatures, tells us there is much more than an isolated La Nina occurring in the Pacific Ocean."

Satellite data from the National Oceanic and Atmospheric Administration clearly illustrate the pattern. Sea-surface temperatures, which directly affect the atmosphere on a daily basis, show the same warm and cool water patterns.

"These warmer and cooler than normal sea-surface temperatures influence our atmosphere every day, while sea-surface heights are a measure of how much heat is stored in the ocean below," Patzert said. "When you put these two pieces of the climate puzzle together, they will tell us both about what is influencing today's weather and how much heat is being stored in the ocean to fuel future planetary climate events."

The Pacific decadal oscillation waxes and wanes approximately every 20 to 30 years, alternating between its present phase, with a warm horseshoe pattern of higher than normal sea-surface heights connecting the north, west and southern Pacific, in contrast to a cool wedge of lower than normal sea-surface heights in the eastern equatorial Pacific. After that the Pacific switches to the opposite phase, showing a reversal of the warm and cool regions; the horseshoe becomes cool and the wedge warms.

Pacific Ocean view with data from TOPEX/Poseidon. Photo: NASA

The strength of this climate trend is seen in the current TOPEX/Poseidon satellite image. Sea-surface height is shown relative to normal (green) height and reveals cooler water (blue and purple) measuring between 8 and 24 centimeters (3 and 9 inches) lower than normal along the coast of Central and South America, and stretching out into the equatorial Pacific. The giant horseshoe of warmer water (red and white) dominating the western and mid-latitude Pacific has higher than normal sea-surface heights of between 8 and 24 centimeters (3 and 9 inches). For the past year, warmer waters have been expanding slowly and are now beginning to dominate the western and north Pacific.

Although it is too early to definitively label these basin-wide conditions as a strong, multiple-year Pacific decadal oscillation, the current image suggests that simple labels or explanations such as a continuing La Nina/El Nino climate condition could be misleading, Patzert said. In the coming year, scientists using TOPEX/Poseidon data will continue to monitor the development of these conditions and their implications for climate in the next several years.

The U.S.-French TOPEX/Poseidon mission is managed JPL for the NASA's Earth Science Enterprise, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.