Climate of Hawaii - Effect On Trade Winds

Effect On Trade Winds

The top image above shows the winds around the Hawaiian Islands measured by the Seawinds instrument aboard QuikSCAT during August 1999. Trade winds blow from right to left in the image. The bottom image shows the ocean current formed by the islands’ wake. Arrows indicate current direction and speed, while white contours show ocean temperatures. The warm water of the current generates winds which sustain the current for thousands of miles.

Despite being a tiny speck within the vast Pacific Ocean, the Hawaiian Islands have a surprising effect on ocean currents and circulation patterns over much of the Pacific. In the Northern Hemisphere, trade winds blow from northeast to southwest, from North and South America toward Asia, between the equator and 30 degrees north latitude. Typically, the trade winds continue across the Pacific — unless something gets in their way, like an island.

Hawaii's high mountains present a substantial obstacle to 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", on the leeward side of the islands.

Aerodynamic theory indicates that an island wind wake effect should dissipate within a few hundred kilometers and not be felt in the western Pacific. However, the wind wake caused by the Hawaiian Islands extends 1,860 miles (3,000 km), roughly 10 times longer than any other wake. The long wake testifies to the strong interaction between the atmosphere and ocean, which has strong implications for global climate research. It is also important for understanding natural climate variations, like El Niño.

There are number of reasons why this has been observed only in Hawaii. First, the ocean reacts slowly to fast-changing winds; winds must be steady to exert force on the ocean, such as the trade winds. Second, the high mountain topography provides a significant disturbance to the winds. Third, the Hawaiian Islands are large in horizontal (east-west) scale, extending over four degrees in longitude. It is this active interaction between wind, ocean current, and temperature that creates this uniquely long wake west of Hawaii.

The wind wake drives an eastward "counter current" that brings warm water 5,000 miles (8,000 km) from the Asian coast. This warm water drives further changes in wind, allowing the island effect to extend far into the western Pacific. The counter current had been observed by oceanographers near the Hawaiian Islands years before the long wake was discovered, but they did not know what caused it.