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Mauna Kea’s orographic effect

Orographics is the effect of the terrain on weather. It often refers to the influence of mountains or mountain ranges on airflow. A major effect is orographic lifting, or the ascending air flow caused by mountains.

The high volcanoes of Hawaii have the capability of generating mammoth waves in the upper atmosphere. A sailplane pilot in 1969 surfed the skies above Mauna Kea, catching the mountain wave and riding it to 22,000 feet.

A mountain wave is caused by a flow of air passing over a mountain or mountain range. To visualize, think of a rock in a stream and water passing over the rock and forming standing waves slightly downstream from the rock. The water is flowing rapidly through the wave but the wave's position remains stationary in relationship to the rock.

But the big wave is an illusive and dangerous phenomenon. In the lee of a mountain, beneath its smooth laminar flow, lurks a twisting, writhing, swirling mass of air known as the "rotor." Violent turbulence can be encountered and sailplanes have been torn apart in its grip.

Sky Surfing!
The Thrill of Catching a 'Wave' 12,000 Feet Above the Snows of Mauna Kea

by Woodson K. Woods
Honolulu magazine
May 1, 1969

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Largest wind wake in the world

Little Islands – Big Wake

On a map of the world, the Hawaiian Islands are barely a speck in the 64 million square miles of the Pacific Ocean. But oceanographers recently discovered that these tiny dots on the map have a surprising effect on ocean currents and circulation patterns over much of the Pacific.

In the Northern Hemisphere, a system of persistent winds blows from northeast to southwest, from North and South America toward Asia, between the equator and 30 degrees north latitude. These northeasterly winds are called trade winds. Typically, the trade winds continue on an uninterrupted course across the Pacific — unless something gets in their way, like an island.

Mauna Kea and Mauna Loa on the island of Hawaii both tower nearly 14,000 feet (about 4,300 meters) above sea level. In addition, Haleakala on the island of Maui stands at over 10,000 feet (3,055 meters) high.

Hawaii’s high mountain landscape presents a substantial obstacle in the path of the trade winds. The elevated topography blocks the airflow, effectively splitting the trade winds in two. This split causes a zone of weak winds, called a “wind wake,” to form on the leeward side (away from the wind) of the islands.

The finding that a small island chain can have such a great impact on the Pacific Ocean is significant to the future of ocean observation, said Shang-Ping Xie, professor and researcher at the University of Hawaii’s International Pacific Research Center and Department of Meteorology. “If you look at a large region of the Pacific Ocean, you can barely see the Hawaiian Islands. But if you look at the winds or currents in that region, you can clearly see the influence of the Hawaiian Islands. It’s very pronounced.”

Xie and his colleague Timothy Liu, senior research scientist at NASA’s Jet Propulsion Laboratory, discovered a very unusual wake west of the Hawaiian Islands — one that extends 3,000 kilometers, which is roughly 10 times longer than any wake observed elsewhere.

They also discovered that the wind wake drives an eastward “counter current” that brings warm water 8,000 kilometers from the Asian coast to Hawaii. This warm water drives further changes in wind, allowing the island effect to extend far into the western Pacific.

“It’s logical to think that water would flow in the same direction that the winds blow,” said Liu. “But this current is actually going in the opposite direction — against the trade winds and back towards Hawaii.”

“The current may actually extend all the way to the Asian coast, which is about a quarter of the Earth’s circumference,” added Xie.

According to Liu, the counter current had been observed by oceanographers near the Hawaiian Islands years before the long wake was discovered. “Drifting buoys off the coast of Hawaii routinely measure currents,” he said. “So researchers were able to document the existence of a current that goes against the trade winds, but they didn’t know exactly what the mechanism was.”

One theory, according to Xie, was that the force of the ocean current hitting the islands generated eddies. “It’s like the story about the blind man feeling the elephant,” said Liu. “When he felt the leg, he thought it was a tree. So Japanese oceanographers would see a little part of the current here, the Hawaiians would see another part there, but they did not have a complete picture of what was happening. Satellite data enabled us for the first time to see the whole process across the Pacific, linking what people know in Asia with what people know in Hawaii.”

“It is this active interaction between wind, ocean current, and temperature that creates this uniquely long wake west of Hawaii,” Xie said.

The researchers’ discovery of the long wake testifies to the strong interaction between the atmosphere and ocean, which has strong implications for global climate research.

“A lot of people can’t even locate Hawaii on a map,” Xie concluded. “So, in that sense — it’s very surprising that an almost invisible geographic feature can have such a profound effect on the ocean-atmosphere system.”

This article contributed from
Distributed Active Archive Center (DAAC) Alliance: Supporting Earth Observing Science 2003
Earth Observatory
NASA Earth Science Enterprise
Data and Services
Little Islands – Big Wake
http://earthobservatory.nasa.gov/Study/Wake/
By Laurie J. Schmidt
Physical Oceanography DAAC
GSFC Earth Sciences DAAC
Download pdf: “Little Islands – Big Wake”

See also
Xie, S.-P., W.T. Liu, Q. Liu, and M. Nonaka. 2001
Far-reaching effects of the Hawaiian Islands on the Pacific Ocean-Atmosphere System
Science
292(5524):2057-2060.

 
 

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