Estimates of Dissipation in the Ocean Mixed Layer Using a Quasi-Horizontal Microstructure Profiler

Blair J. W. Greenan Ocean Sciences Division, Bedford Institute of Oceanography, Maritimes Region, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

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Neil S. Oakey Ocean Sciences Division, Bedford Institute of Oceanography, Maritimes Region, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

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Fred W. Dobson Ocean Sciences Division, Bedford Institute of Oceanography, Maritimes Region, Fisheries and Oceans Canada, Dartmouth, Nova Scotia, Canada

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Abstract

Some recent measurements of the mixed layer in oceans and lakes have indicated that the rate of the dissipation of turbulent kinetic energy, ε, is much higher than expected from a purely shear-driven wall layer. This enhancement has usually been attributed to wave breaking. In this study, measurements of dissipation in the open-ocean mixed layer on the continental shelf off Nova Scotia are integrated with air–sea flux estimates and directional wave spectra to further study this issue. A microstructure profiler gliding quasi-horizontally provides estimates of ε starting within 2 m of the ocean surface as it slowly descends through the mixed layer. Dissipation rates were found to be enhanced relative to the wind stress production and indicated that ∼6% of the wind energy at 10 m is dissipated in the ocean mixed layer. In addition, results from this experiment demonstrate that the WAVES scaling for ε, based on wind and wave parameters, is valid for the case of a simple windsea in which the swell can be easily separated. In more complex wave conditions ε remains enhanced relative to the classical wall layer; however, the WAVES scaling does not hold.

Corresponding author address: Dr. Blair J. W. Greenan, Ocean Sciences Division, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, NS B2Y 4A2, Canada.

Email: greenanb@mar.dfo-mpo.gc.ca

Abstract

Some recent measurements of the mixed layer in oceans and lakes have indicated that the rate of the dissipation of turbulent kinetic energy, ε, is much higher than expected from a purely shear-driven wall layer. This enhancement has usually been attributed to wave breaking. In this study, measurements of dissipation in the open-ocean mixed layer on the continental shelf off Nova Scotia are integrated with air–sea flux estimates and directional wave spectra to further study this issue. A microstructure profiler gliding quasi-horizontally provides estimates of ε starting within 2 m of the ocean surface as it slowly descends through the mixed layer. Dissipation rates were found to be enhanced relative to the wind stress production and indicated that ∼6% of the wind energy at 10 m is dissipated in the ocean mixed layer. In addition, results from this experiment demonstrate that the WAVES scaling for ε, based on wind and wave parameters, is valid for the case of a simple windsea in which the swell can be easily separated. In more complex wave conditions ε remains enhanced relative to the classical wall layer; however, the WAVES scaling does not hold.

Corresponding author address: Dr. Blair J. W. Greenan, Ocean Sciences Division, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, NS B2Y 4A2, Canada.

Email: greenanb@mar.dfo-mpo.gc.ca

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