Mesoscale eddies are common in the lee of the Hawaiian Islands. Persistent cyclones (anticyclones) have been observed downstream of the northern (southern) corner of the leeward side of the island of Hawai‘i. In addition, smaller submesoscale eddies have been observed in the lee of other islands, such as Oahu and Lanai. The region has been subject to numerous model studies of the lee eddies, but the last comprehensive observational study was completed over twenty years ago. Now, with the increase in available surface drifting buoys, the coverage of some lee areas by High Frequency Radar (HFR), as well as twenty years of satellite data an observational study is well timed. Surface drifting buoys were used to determine the frequency, timing, and characteristics of anticyclones in the lee of both Hawai‘i and Oahu. The anticyclones in the lee of Hawai‘i are formed most commonly in summer months, when both the wind and ocean forcing is the steadiest. The outline of the wake created by the anticyclonic drifting buoys extends westward from the island over 1500 km and is directed to the southwest. An analysis of wind and ocean forcing using Empirical Orthogonal Function (EOF) methodology revealed an overlap in spatial signature with the wind-forced annual sea level anomaly patterns, as well as partial overlap with a non wind-induced 100-day signature. Due to the overlap, exact forcing cannot be determined without better quality observations of ocean currents to the south of the island. The eddies in the lee of Oahu were smaller, had a much shortened leeward extent, and were observed more often due to the presence of HFR currents. The anticyclones and most of the cyclones in the lee of Oahu all occurred during steady Northeast Trade winds and can be explained by wind stress curl induced forcing. Three cyclones formed during trade wind reversals where wind stress curl forcing was zero, leaving a vorticity residual that can only be explained by ocean stress induced vorticity.
|Commitee:||Ascani, Francois, Carter, Glenn, Powell, Brian, Francis, Oceana|
|School:||University of Hawai'i at Manoa|
|School Location:||United States -- Hawaii|
|Source:||DAI-B 81/8(E), Dissertation Abstracts International|
|Keywords:||Anticyclones , Hawai‘i , Surface drifters|
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