Semidiurnal Baroclinic Wave Momentum Fluxes at Kaena Ridge, Hawaii

Abstract

Abstract Kaena Ridge, Hawaii, is a site of energetic conversion of the semidiurnal barotropic tide. Diffuse baroclinic wave beams emanate from the critical-slope regions near the ridge crest, directed upward and southward from the north flank of the ridge and upward and northward from the south flank. Here, the momentum fluxes associated with generation at the ridge are estimated. Continuous vertical profiles of density and velocity from 80 to 800 m were obtained from the Research Platform Floating Instrument Platform (FLIP) over the southern edge of the ridge, as an aspect of the Hawaii Ocean Mixing Experiment. Data are used to estimate the Reynolds stress, Eulerian buoyancy flux, and the combined Eliassen–Palm flux in the semidiurnal band. An upward–southward stress maximum of ~0.5 × 10−4 m2 s−2 appears at depths of 300–500 m, generally consistent with beam-like behavior. A strong off-ridge buoyancy flux (~0.3 × 10−4 m2 s−3) combines with large along-ridge Reynolds stresses to form an Eliassen–Palm flux whose along-ridge and across-ridge magnitudes are comparable. The stress azimuth rotates clockwise with increasing altitude above the ridge crest. The principal upward–southward beam is found to be at depths 100–300 m shallower than are predicted by an analytic two-dimensional (2D) model and a 3D numerical simulation. This discrepancy is consistent with previous observations of the baroclinic energy flux. If these observed tidal momentum fluxes were to diverge in a 100-m-thick near-surface layer, the forcing would be comparable to a moderate wind stress. Pronounced lateral gradients of baroclinic tidal stresses can be expected offshore of Hawaiian topography.