Satellites Reveal Hawaiian Isles' Long Tail of Wind and Water
June 14, 2001

Still image of Hawaiian Wake. Click for animation.

The Hawaiian Islands trigger an extraordinary interaction between wind and ocean that extends thousands of kilometers. This island effect is much larger than has ever been observed by scientists before.

       Using data from Earth-observing satellites, researchers discovered this unusually long island "wake," which includes a narrow eastward-flowing ocean current extending 8,000 kilometers (4,900 miles) from Asia to Hawaii. The scientists' report appears in the June 15 issue of the journal Science.

       While scientists have known of an eastward current off of Asia for some years, this new research shows that it extends from Asia to the Hawaiian Isles. Some researchers say such a current could possibly have aided the islands' early settlers.

       "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 only be 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 Dr. Shang-Ping Xie, from the University of Hawaii's International Pacific Research Center and Meteorology Department.

       "According to conventional theories and observations, the wind wakes caused by islands should dissipate within approximately 300 kilometers (180 miles) downstream and should not be felt in the western Pacific," said co-author Dr. Timothy Liu of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "But we were able to observe a pattern that stretches more than 3,000 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. This pattern, never recognized before, is a narrow but long break in the steady Pacific trade winds and the North Equatorial Current. It is triggered by the high Hawaiian Islands and sustained by positive ocean-atmosphere feedback."

       In their paper, "Far-Reaching Effects of the Hawaiian Islands on the Pacific Ocean-Atmosphere," the authors describe a chain of events that begins when the steady westward trade winds and north equatorial current encounter the volcanically formed Hawaiian Islands standing tall in the middle of the Pacific Ocean. The islands force the winds to split, creating areas of weak winds behind the islands and strong winds on the islands' flanks. Individual wakes form behind the islands, but these merge into a broader wake about 240 kilometers (150 miles) to the west. The winds associated with this broader wake spawn a narrow eastward countercurrent that draws warm water from west to east. When the winds encounter these warm surface waters, they rise with convection. Cooler winds move in to feed the rising air, creating a rotating effect and reinforcing the current. The current, in turn, helps drive the winds, setting up a positive feedback between ocean and atmosphere that continues for thousands of kilometers to the west.

       An animation describing the wake is available online at

       http://www.jpl.nasa.gov/videos/earth/hawaii.

       The study shows that the surface winds react to sea surface temperature variations as small as a few tenths of one degree, indicating a climate sensitivity much higher than has been previously thought. This new knowledge of ocean- atmosphere interplay will help improve climate models used to predict phenomena like El Niņo and global warming.

       For their paper, Xie and his colleagues used data from NASA's QuikScat satellite, the European Remote Sensing satellites and the U.S./Japan Tropical Rainfall Measuring Mission. The SeaWinds on QuikScat project is managed for NASA's Earth Science Enterprise by JPL. TRMM is a joint U.S./Japanese mission managed by NASA's Goddard Space Flight Center, Greenbelt, MD.

       JPL is a division of the California Institute of Technology in Pasadena.