Trade Wind Reversal Impacts on the Lee Dynamics of Oahu

Ocean island lees have been well studied in many regions, including the Hawaiian Islands and Canary Islands. However, in the Hawaiian Islands, most of the research has been on the lee created by the largest island, the island of Hawaii. The smaller islands have lee regions that can be hard to detect with satellite data due to low spatial resolutions. These lee dynamics are dominated by the presence of the NE Trade winds. The role of trade wind reversals on lee dynamics of the smaller islands has not been examined. Here, using drifter data and high frequency radar, a year long observational study was conducted in order to investigate the island lee created by Oahu, a smaller, but regionally important island. A distinct cyclonic zone was observed off the northwestern corner of Oahu, with a distinct anticyclonic zone observed off the southwestern corner of Oahu. The zones could be observed in both the HFR currents and the mean currents derived from the drifting buoys. Vorticity balance analysis indicates that vortices, both cyclonic and anticyclonic, that formed during normal northeast trade wind conditions are primarily wind-induced. However, cyclonic vortices that formed in Oahu's western waters during trade wind reversals were not wind-induced. During prolonged trade wind reversals, the near shore flow pattern was reversed, impacting the transport of nutrients and local heat fluxes.