Linear wind-forced beta plumes with application to the hawaiian lee countercurrent*

Abstract

Two numerical ocean models are used to study the baroclinic response to forcing by localized wind stress curl (i.e., awind-forced β plume, which is a circulation cell developing to thewest of the source region and composed of a set of zonal jets) with implications for the Hawaiian Lee Countercurrent (HLCC): an idealized primitive equation model [Regional Ocean Modeling System (ROMS)], and a global, eddy-resolving, general circulation model [Ocean GeneralCirculationModel for the Earth Simulator (OFES)]. In addition, theoretical ideas inferred from a linear continuously stratified model are used to interpret results. In ROMS, vertical mixing preferentially damps higher-order verticalmodes. The damping thickens the plume to the west of the forcing region, weakening the near-surface zonal jets and generating deeper zonal currents. The zonal damping scale increasesmonotonically with the meridional forcing scale, indicating a dominant role of vertical viscosity over diffusion, a consequence of the small forcing scale. In the OFES run forced by NCEP reanalysis winds, the HLCC has a vertical structure consistent with that of idealized β plumes simulated by ROMS, once the contribution of the North Equatorial Current (NEC) has been removed.Without this filtering, a deep HLCC branch appears artificially separated from the surface branch by the large-scale intermediate-depth NEC. The surface HLCC in two different OFES runs exhibits sensitivity to the meridional wind curl scale that agrees with the dynamics of a β plume in the presence of vertical viscosity. The existence of a deep HLCC extension is also suggested by velocities of Argo floats. © 2013 American Meteorological Society.