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The Honolulu Advertiser
Posted on: Friday, June 15, 2001

Huge 'wake' created by Islands cuts trade winds

 •  Graphic: 'Wake' affects wind, ocean

By Jan TenBruggencate
Advertiser Science Writer

When Mark Twain referred to Hawai'i as an anchored "fleet of islands," the analogy was closer than he could have guessed.

University of Hawai'i researchers and others, in an article published in today's issue of Science, say the Islands create a "wake" that has effects on the winds and ocean for thousands of miles to the west.

While such wakes are not unusual, the one to the west of Hawai'i is the largest and longest ever identified.

The height of the Hawaiian Islands causes a disturbance in the flow of the trade winds, helping create an extended region of calmer air downwind, and also helping generate a counter-current that feeds warm water from the western Pacific all the way to the leeward sides of the Islands.

"Wakes behind ocean islands are commonly observed but they usually do not affect large-scale ocean circulation. ... Owing to the steady trades and the broad width of the island chain, the Hawaiian wake exerts a steady and substantial forcing on the ocean," the authors said.

Lead author Shang-Ping Xie is an associate professor of meteorology at the University of Hawai'i and a researcher with the University's International Pacific Research Center, a program of the School of Ocean and Earth Science and Technology.

Co-author Masami Nonaka is associated with the IPRC and the Frontier Research System for Global Change in Tokyo. The other authors are Timothy Liu of the Jet Propulsion Laboratory in Pasadena, California, and Qinyu Liu of the University of Qingdao in China.

The authors were out of the state and not available for comment yesterday.

Using satellite data and mathematical models, they identified an immense, elongated region of disturbed air and ocean leading west from the islands. It starts at the lee areas of each of the islands, but coalesces into single feature as it moves away.

"As the broad, steady northeasterly trades impinge on Hawai'i, a number of mechanical wakes form behind the individual islands. These individual wakes dissipate rather quickly, and a broad wake takes their place" about 200 miles downwind, they wrote.

Sea surface temperature records show that in addition to the changes in the wind, there is a change in water temperature. The authors identified a warm "tongue" of water that extends westward from the leeward sides of the Islands.

The warm water region in turn is associated with a narrow current that flows counter to the prevailing westerly-flowing North Equatorial Current. The counter-current extends as far as 5,000 miles to the west, into warm Asian waters. It is called the Hawaiian Lee Counter Current.

The authors say they conclude that the warm tongue is caused by warm water carried on the counter current from the Western Pacific.

The warm water may also influence cloud formation, causing higher moisture in the atmosphere above it.

"Our study shows how tiny islands, barely visible on a world map, can affect a long stretch of Earth's largest ocean. The Pacific could onlyÊbe sketchily observed with ship-based instruments; advanced satellite technology, however, is changing all this and giving us fascinating new images of this ocean," said author Xie, in a press release accompanying the publication.