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Internal Tide Radiation from Mendocino Escarpment

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  • 1 Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, Washington
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Abstract

Strong semidiurnal internal tides are observed near Mendocino Escarpment in full-depth profile time series of velocity, temperature, and salinity. Velocity and density profiles are combined to estimate the internal tide energy flux. Divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment. A baroclinic energy flux of 7 kW m−1 radiates from the escarpment, corresponding to 3% of the 220 kW m−1 fluxing poleward in the surface tide. Energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site ∼90 km north of the escarpment. Coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection. Beyond the first surface reflection north of the escarpment, the energy flux drops by 2 kW m−1 and beams are no longer discernible. Turbulence, as inferred from finescale parameterizations, is elevated by over two orders of magnitude relative to the open-ocean interior in localized 500-m-thick layers at the bottom over the ridge crest, near the surface at the station closest to the first surface reflection to the north, slightly north of the first bottom reflection to the north, and on the south flank of the escarpment. Despite its intensity, turbulent dissipation integrated over the ridge crest is only 1% of the energy flux in the internal tides. Thus, the bulk of surface tidal losses at the escarpment is radiating away as internal waves. High turbulent dissipation rates near the surface reflection suggest that loss of energy flux there may be turbulent. This turbulence may arise from (i) Wentzel–Kramers–Brillouin amplification of semidiurnal shear as the internal tide propagates into high near-surface stratification or (ii) superposition of incident and reflected waves enhancing nonlinear transfers to small scales and turbulence production. Localized mixing due to internal tide beams impinging on the base of the mixed layer may be an important unconsidered cause of nutrient and water-mass fluxes between the surface layer and the upper pycnocline.

Corresponding author address: Eric Kunze, APL and School of Oceanography, University of Washington, 1013 NE 40th Street, Seattle, WA 98105-6698. Email: kunze@ocean.washington.edu

Abstract

Strong semidiurnal internal tides are observed near Mendocino Escarpment in full-depth profile time series of velocity, temperature, and salinity. Velocity and density profiles are combined to estimate the internal tide energy flux. Divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment. A baroclinic energy flux of 7 kW m−1 radiates from the escarpment, corresponding to 3% of the 220 kW m−1 fluxing poleward in the surface tide. Energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site ∼90 km north of the escarpment. Coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection. Beyond the first surface reflection north of the escarpment, the energy flux drops by 2 kW m−1 and beams are no longer discernible. Turbulence, as inferred from finescale parameterizations, is elevated by over two orders of magnitude relative to the open-ocean interior in localized 500-m-thick layers at the bottom over the ridge crest, near the surface at the station closest to the first surface reflection to the north, slightly north of the first bottom reflection to the north, and on the south flank of the escarpment. Despite its intensity, turbulent dissipation integrated over the ridge crest is only 1% of the energy flux in the internal tides. Thus, the bulk of surface tidal losses at the escarpment is radiating away as internal waves. High turbulent dissipation rates near the surface reflection suggest that loss of energy flux there may be turbulent. This turbulence may arise from (i) Wentzel–Kramers–Brillouin amplification of semidiurnal shear as the internal tide propagates into high near-surface stratification or (ii) superposition of incident and reflected waves enhancing nonlinear transfers to small scales and turbulence production. Localized mixing due to internal tide beams impinging on the base of the mixed layer may be an important unconsidered cause of nutrient and water-mass fluxes between the surface layer and the upper pycnocline.

Corresponding author address: Eric Kunze, APL and School of Oceanography, University of Washington, 1013 NE 40th Street, Seattle, WA 98105-6698. Email: kunze@ocean.washington.edu

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