Generation of mesoscale eddies in the lee of the Hawaiian Islands

The ocean west of the main Hawaiian Islands is characterized by enhanced eddy kinetic energy arising from the abundance of locally generated mesoscale eddies, most frequently in the area west of the island of Hawaii. Two mechanisms of eddy generation in the wake of an island are examined with numerical model experiments. The first, eddy generation and shedding by an oceanic flow around an oceanic barrier, requires the existence of strong westward flows to the north and south of the island of Hawaii. Model solutions show such westward flows and generation of eddies by these flows although the intensity of the eddies and the generation frequency are much lower than that derived from altimetry. As a result, these eddies contribute an insignificant amount of eddy kinetic energy in the region. The second, eddy generation and shedding by an atmospheric flow around an atmospheric barrier, is based on oceanic upwelling and downwelling induced by surface wind shear, effectively introducing sinks and sources to the ocean interior. Previous idealized modeling studies have shown that oceanic eddies can be generated by sufficiently strong forcing (source or sink), providing an explanation why eddy occurrences in the lee of the island of Hawaii coincide with periods of strong trade winds. Eddy generation characteristics in the model experiments are consistent with this mechanism in terms of time of occurrence, strength and the resulting eddy kinetic energy. Major discrepancies are in eddy propagation and therefore eddy distribution in the regional domain due to the complex nature of eddy-eddy interactions.