Tidally Forced Internal Waves and Overturns Observed on a Slope: Results from HOME

Abstract

Abstract Tidal mixing over a slope was explored using moored time series observations on Kaena Ridge extending northwest from Oahu, Hawaii, during the Survey component of the Hawaii Ocean Mixing Experiment (HOME). A mooring was instrumented to sample the velocity and density field of the lower 500 m of the water column to look for indirect evidence of tidally induced mixing and was deployed on a slope in 1453-m water depth for 2 months beginning in November 2000. The semidiurnal barotropic tidal currents at this site have a significant cross-ridge component, favorable for exciting an internal tidal response. A large-amplitude response is expected, given that the slope of the topography (4.5°) is nearly the same as the slope of the internal wave group velocity at semidiurnal frequency. Density overturns were inferred from temperature profiles measured every 2 min. The number and strength of the overturns are greater in the 200 m nearest the bottom, with overturns exceeding 24 m present at any depth nearly 10% of the time. Estimates of turbulent dissipation rate ɛ were made for each overturn by associating the measured Thorpe scale with the Ozmidov scale. The average ɛ between 1300 and 1450 m for the entire experiment is about 10−8 m2 s−3, corresponding to an average Kρ of 10−3 m2 s−1. Both ɛ and Kρ decrease by about an order of magnitude by 1200 m. The occurrence of overturns and the magnitude of ɛ are both highly correlated with the tide: both with the spring–neap cycle as well as the phase of the semidiurnal tide itself. Dissipation rate varies by at least an order of magnitude over the spring–neap cycle. It appears that tidal frequency vertical shear within 200 m of the boundary leads to significant strain (vertical divergence). Most of the overturns occur during the few hours when the vertical strain is greatest. The buoyancy frequency N calculated from reordering these overturns is a factor of 3 lower than the background N. This is consistent with the following kinematic description: the internal tide first strains the mean density field, leading to regions of low N that subsequently overturn. Less regularly, overturns also occur when the internal tide strain has created relatively high stratification within 200 m of the bottom.