Satellites Reveal Hawaiian Isles' Astounding Effect on Wind and Water
An "island wake" is an area of weak winds created behind an island that act as a wind-barrier. The wind wake west of the Hawaiian Islands, and has recently been shown to impact on ocean currents south of Japan. Dr. Masami Nonaka, a full-time researcher at the International Pacific Research Center (IPRC) who has been involved in this study provides an overview.





Masami Nonaka*, Shang-Ping Xie (*FRSGC,IPRC, University of Hawaii)



T he Hawaiian Islands trigger an extraordinary interaction between wind and ocean that extends thousands of kilometers downwind, an island effect much larger than has ever been observed before, shown by an analysis of data from Earth-observing satellites. This unusually long island "wake" and a narrow eastward flowing ocean current extending 8,000 kilometers from Asia to Hawaii. While scientists have known of an eastward current off of Asia for some years, according to this new research this current may extend thousands of kilometers from Asia to Hawaii.

According to conventional theories and observations, the wind wakes caused by islands should dissipate within 300 km downstream and should not be felt in the western Pacific. But we were able to observe a pattern that stretches more than 3,000 km in the atmosphere and the ocean, from the western side of the Hawaiian Islands to the western Pacific (Figure). This pattern, never recognized before, is a narrow but long break in the steady Pacific trade winds and the North Equatorial Current. The system is triggered by the high Hawaiian Islands and sustained by positive ocean-atmosphere feedback.

Sea surface temperature (color scales in C) and wind vectors (m/sec).
Color Scales

(Reprinted with permission from "Far-Reaching Effects of the Hawaiian Islands on the Pacific Ocean-Atmosphere System" Shang-Ping Xie, W.Timothy Liu, Qinyu Liu, Masami Nonaka ,Vol.292, No.5524, 15 June 2001, 2057-2060, Figure 3 (left) , 2059. Copyright American Association for the Advancement of Science.)

Figure caption: Sea surface temperature (color scales in degree) and wind vectors (m/sec). Large-scale background fields have been subtracted to isolate island effects that have a smaller latitudinal scale. Anomalous winds blow onto the warm temperature band. Eastward vectors, which oppose the background trade winds and hence correspond to the wind wake, can be traced far into the west.

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A chain 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 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 around 240 km to the west. The winds associated with this broader wake spawn a narrow eastward countercurrent that draws warm water from west to east and initiates a positive feedback loop between the ocean and the atmosphere. This feedback loop lets the effects of the islands continue for thousands of miles to the west.

The study has implications for climate research. Scientists have been struggling to determine how much the ocean and atmosphere affect each other. The strength of this interaction is a key parameter in climate events ranging from El Nino to ice ages and global warming. 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 should help improve climate models.

For this study, we used data from the following satellites for their study: NASA's QuikScat, which provides high-resolution measurements of near-surface ocean winds; the European Remote Sensing (ERS), which offers a longer record of ocean winds, but at coarser resolutions; the U.S.-Japan Tropical Rainfall Measuring Mission (TRMM), which looks at wind speed, sea surface temperature, and clouds.

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. (English translation is based on a news release authored by G. Speidel.)


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