Thursday, June 14, 2001

Isles’ ‘wake’ may
affect climate

Research shows wind and ocean
patterns are changed greatly in a
stretch thousands of miles westward

By Helen Altonn

Though barely visible on a world map, the tiny Hawaiian islands "can affect a long stretch of Earth's largest ocean," University of Hawaii researcher Shang-Ping Xie says.

University of Hawaii

A scientist in the International Pacific Research Center and UH Meteorology Department, Xie was part of an international team that discovered Hawaii's "astounding effect on wind and water."

He was chief author of a paper by the researchers, "Far-reaching Effects of the Hawaiian Islands on the Pacific Ocean-Atmosphere System," that is published in the journal Science, which comes out tomorrow.

The Hawaiian Islands stimulate extraordinary interaction between wind and ocean that extends thousands of miles downwind -- an island effect much larger than has ever been observed, the team reported.

Using data from NASA and other Earth-observing satellites, researchers from the United States, Japan and China found an unusually long island "wake" and a narrow eastward flowing ocean current extending nearly 5,000 miles from Asia to Hawaii.

"The Pacific could only be sketchily observed with ship-based instruments," Xie said. "Advanced satellite technology, however, is changing all this and giving us fascinating new images of this ocean."

Working with Xie were Timothy Liu of NASA's Jet Propulsion Laboratory in Pasadena; Qinyu Liu of the University of Qingdao, Qingdao, China, and Masami Nonaka, Japanese scientist with the International Pacific Research Center -- Frontier Research System for Global Change.

Japanese funding is largely routed through the latter organization for the research center, founded by the United States and Japan in 1997 at the UH to increase knowledge of climate variation in the Asia-Pacific Region.

Liu said wind wakes caused by islands should dissipate within 180 miles downstream, according to conventional theories and observations, and they shouldn't be felt in the western Pacific.

"But we were able to observe a pattern that stretches more than 2,000 kilometers (1,800 miles) in the atmosphere and the ocean, from the western side of the Hawaiian Islands to beyond Wake Island in the western Pacific," he said.

Recognized for the first time, this pattern is a narrow but long break in the steady Pacific trade winds and the North Equatorial Current, Liu said.

"The system is triggered by the high Hawaiian Islands and sustained by positive ocean-atmosphere feedback," he said.

The series of events begins when the steady westward trade winds and North Equatorial Current encounter the Hawaiian Islands "standing tall in the middle of the Pacific Ocean," the researchers said.

Winds are forced by the islands to split, creating areas of weak winds behind the islands and strong winds on the islands' flanks. Individual wakes form behind the islands but merge about 150 miles to the west, the scientists said.

Winds associated with the broad wake spawn a narrow eastward countercurrent that draws warm water from west to east and triggers a positive feedback loop between the ocean and atmosphere.

The effects of the islands roll on for thousands of miles to the west because of that loop.

Climate sensitivity is much greater than believed, the scientists learned, with surface winds reacting to sea surface temperature variations as small as a few tenths of one degree.

This knowledge is important for scientists struggling to determine how the ocean and atmosphere affect each other.

The interaction is a key element in climate events ranging from El Nino to global warming and ice ages and the study is expected to help improve climate models used to predict climate changes.

University of Hawaii

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