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Shear and Baroclinic Energy Flux on the Summer New England Shelf

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

Observations are presented of internal wave properties and energy fluxes through a site near the 70-m isobath on the New England shelf in late summer. Data collected from a shipboard ADCP and microstructure profiler over a three-week period and projected onto dynamic vertical modes reveals large variations in the magnitude and vertical structure of internal waves. Baroclinic energy and shear were primarily associated with low-mode near-inertial and semidiurnal waves and, at times, high-frequency solibores. The energies in each mode varied by factors from 2 to 10 over several days and were not significantly correlated with one another. The associated shear variance was concentrated in the thermocline. However, the strength and vertical range of shear varied significantly throughout the research period and depended sensitively on both the magnitude and evolving vertical mode content of the wave field. Shear during the quasi-two-layer solibores was strong enough to temporarily lower the 4-m Richardson number below the threshold for shear instability. Energy flux through the site came primarily from the mode-1 internal tide, in both linear and nonlinear (solibore) forms. The average energy flux from the first five baroclinic modes was 130 W m−1. A comparison of energy fluxes from each mode and locally measured average dissipation rates suggests that near-inertial and high-mode waves were generated near the experimental site.

Current affiliation: IOD/SIO, La Jolla, California

Corresponding author address: Dr. J. A. MacKinnon, IOD/SIO, 9500 Gilman Drive, Mail Code 0209, La Jolla, CA 92093-0209. Email: jen@coast.ucsd.edu

Abstract

Observations are presented of internal wave properties and energy fluxes through a site near the 70-m isobath on the New England shelf in late summer. Data collected from a shipboard ADCP and microstructure profiler over a three-week period and projected onto dynamic vertical modes reveals large variations in the magnitude and vertical structure of internal waves. Baroclinic energy and shear were primarily associated with low-mode near-inertial and semidiurnal waves and, at times, high-frequency solibores. The energies in each mode varied by factors from 2 to 10 over several days and were not significantly correlated with one another. The associated shear variance was concentrated in the thermocline. However, the strength and vertical range of shear varied significantly throughout the research period and depended sensitively on both the magnitude and evolving vertical mode content of the wave field. Shear during the quasi-two-layer solibores was strong enough to temporarily lower the 4-m Richardson number below the threshold for shear instability. Energy flux through the site came primarily from the mode-1 internal tide, in both linear and nonlinear (solibore) forms. The average energy flux from the first five baroclinic modes was 130 W m−1. A comparison of energy fluxes from each mode and locally measured average dissipation rates suggests that near-inertial and high-mode waves were generated near the experimental site.

Current affiliation: IOD/SIO, La Jolla, California

Corresponding author address: Dr. J. A. MacKinnon, IOD/SIO, 9500 Gilman Drive, Mail Code 0209, La Jolla, CA 92093-0209. Email: jen@coast.ucsd.edu

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