Hawaii Casts Looooong Shadow

Stirring up the ocean. Various measures of wind speed and cloud cover show Hawaii's Pacific trail.

Mountain peaks on the Hawaiian Islands cause northeasterly trade winds to fork, driving a 3000-kilometer wake in the Pacific Ocean, researchers report in the 15 June issue of Science. The wake is 10 times longer than any ever seen and suggests that the atmosphere and ocean are more delicately coupled than previously thought.

When winds hit a mountain peak, two streams of air spread out on either side. When the mountain sits atop an isolated oceanic island like the Big Island of Hawai'i, the wind currents blow a V-shaped pattern in the water, giving the illusion that the islands are moving. Theories for the well-known phenomenon set a strict upper limit on the length of the wakes: 500 kilometers. "We thought there was no way you could go beyond that," says oceanographer Shang-Ping Xie of the University of Hawaii.

Xie and his collaborators, however, were shocked by several wide-area, cloud-penetrating microwave images from the recently launched QuikSCAT satellite. They pictures revealed a 3000-kilometer oceanic wake trailing westward from the Hawaiian Islands. Additional measurements from a second satellite, the Tropical Rain Measurement Mission (TRMM) confirmed that the wake is real and that it also drives an eastward current that brings warm water 8000 kilometers from the Asian coast to Hawaii. Oceanic conditions such as temperature measured by the TRMM also suggested an explanation for the exceptionally long wake. According to calculations by Xie's team, once the wind starts the wake, the ocean can't stop it. "The dissipation in the ocean is so small that the waves can propagate almost infinitely far to the west," he says.

"Who would have guessed that the Hawaiian Islands could have such far-reaching effects?" says oceanographer Eric Lindstrom of NASA headquarters in Washington, D.C. "But their pictures are very convincing." Xie hopes to convince several upcoming research voyages from Hawaii to take a closer look at the wake.

Related sites

Hawai'i From Space
The Ocean Atlas of Hawai'i
NASA Oceanography

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