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Blair J. W. Greenan
,
Neil S. Oakey
, and
Fred W. Dobson

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.

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Mary-Elena Carr
,
Neil S. Oakey
, and
Marlon R. Lewis

Abstract

A meridional transect along 150°W in March 1988 revealed a statistically significant maximum in turbulent kinetic energy dissipation, ε at 1°S–0° between 40-m and 60-m depth. The wind stress and buoyancy flux were poorly correlated with the observed mixed layer dissipation measured below 10 m. Dissipation modeled using similarity scaling was larger than the observed mixed layer dissipation (below 10 m) away from the equator and smaller than observed at 0°. The ratio of observed and modeled dissipation at the equator was highly correlated to the vertical velocity shear at the base of the mixed layer. The turbulent stress divergence computed as the residual of annual mean terms between 0 and 60 m for the 41/2-day time series was close to the sum of the annual mean terms of the zonal momentum balance of Bryden and Brady. For 21/2 days at 0° the change in heat content of the top 15 m was consistent with the observed one-dimensional fluxes within the uncertainty of the measurements. The 4-day average of penetrative irradiance out of the layer was twice as large as that of the turbulent heat flux.

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Richard K. Dewey
,
William R. Crawford
,
Ann E. Gargett
, and
Neil S. Oakey

Abstract

A free-falling instrument has been built to measure temperature, salinity and turbulent shear from near surface to within 15 cm of the ocean bottom. A probe guard mounted at the lower end of the instrument protects sensitive shear and temperature sensors from bottom sediments. The noise level in the shear signal corresponds to a dissipation rate of 3.0 × 10−7 W m−3, with vibrations of the probe guard providing the largest component of the noise. The signals are transmitted through a neutrally buoyant line to the ship where they are displayed in real time and recorded for later analysis. The profiling technique is capable of 20 profiles per hour through 50 m of water. Simultaneous profiles of turbulent microstructure and density can now be consistently obtained through the entire water column.

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Miles A. Sundermeyer
,
James R. Ledwell
,
Neil S. Oakey
, and
Blair J. W. Greenan

Abstract

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m2 s−1. Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m2 s−1. The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

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