The Vorticity Balance of the Ocean Surface in Hawaii from a Regional Reanalysis

Abstract

he ocean surface vorticity budget around the Hawaiian Islands is examined using an 18-month model reanalysis generated using four-dimensional variational state estimation with all available observations (satellite, in situ, and high-frequency radio). To better resolve the ocean surface currents and reduce the representation error of the radio-measured surface currents, this study developed a new vertical scheme for the Regional Ocean Modeling System. A new, detailed description of the ocean surface vorticity is created, revealing a region dominated by cyclonic (anticyclonic) vorticity to the north (south) of the mean position of the Hawaii Lee Countercurrent. Advection of vorticity is the primary process that transports the vorticity generated in the lee of the islands by the wind wake. In this island lee, the zonal wavenumber spectra show a cascade of vorticity/energy from the submesoscale toward the larger scales. Latitudinal differences in the advection of vorticity spectra indicate the propagation of a first baroclinic mode Rossby wave in the region dominated by anticyclonic vorticity to the west of the island of Hawaii, while the region dominated by cyclonic vorticity is disrupted by the wake of the smaller islands.