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Variations in Apparent Mixing Efficiency in the North Atlantic Central Water

Variations in Apparent Mixing Efficiency in the North Atlantic Central Water Microstructure data from the North Atlantic Tracer Release Experiment (NATRE) are presented, providing detailed profiles of the thermal variance χχ in the upper 360 m of the Canary Basin for the fall and spring seasons. The Osborn––Cox model is used to compute the diffusivity K T . The diffusivity for the depth range 240––340 m is found to be 1.0(±±0.04) ×× 10 −−5 m 2 s −−1 in the fall and 2.2(±±0.1) ×× 10 −−5 m 2 s −−1 in the spring, in good agreement with dye-inferred diffusivities at similar depths. Measured turbulent kinetic energy (TKE) dissipation rates were found to be contaminated by hydrodynamic noise, so the Osborn dissipation method was not used to compute K ρρ . However, data segments for which the TKE dissipation rate ( εε ) was large enough to be unaffected by noise were used to compute the ““apparent mixing efficiency”” ΓΓ d . The computed ΓΓ d values are used to investigate variations in apparent mixing efficiency with respect to density ratio ( R ρρ ) and turbulence Reynolds number εε /( ννN 2 ), in an attempt to elucidate the underlying mechanisms of mixing in the NATRE region. Observed variations of ΓΓ d are compared with existing theoretical models of mixing due to: salt fingers, a combination of salt fingers and turbulence, ““conventional”” high Reynolds number turbulence, and low Reynolds number buoyancy-modified turbulence. Significant variations of ΓΓ d with respect to both R ρρ and εε /( ννN 2 ) are found. Although Monte Carlo tests show that some of the observed variations could be noise-induced, a substantial portion of the systematic variations the authors observed were not reproduced by Monte Carlo simulations. These trends are found to be statistically significant, and the authors conclude that they represent real variations in the apparent mixing efficiency. The authors find that ΓΓ d is an increasing function of εε /( ννN 2 ) and a decreasing function of R ρρ ; these variations are not fully consistent with any of the available mixing models. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Physical Oceanography American Meteorological Society

Variations in Apparent Mixing Efficiency in the North Atlantic Central Water

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Publisher
American Meteorological Society
Copyright
Copyright © 1997 American Meteorological Society
ISSN
1520-0485
DOI
10.1175/1520-0485(1997)027<2589:VIAMEI>2.0.CO;2
Publisher site
See Article on Publisher Site

Abstract

Microstructure data from the North Atlantic Tracer Release Experiment (NATRE) are presented, providing detailed profiles of the thermal variance χχ in the upper 360 m of the Canary Basin for the fall and spring seasons. The Osborn––Cox model is used to compute the diffusivity K T . The diffusivity for the depth range 240––340 m is found to be 1.0(±±0.04) ×× 10 −−5 m 2 s −−1 in the fall and 2.2(±±0.1) ×× 10 −−5 m 2 s −−1 in the spring, in good agreement with dye-inferred diffusivities at similar depths. Measured turbulent kinetic energy (TKE) dissipation rates were found to be contaminated by hydrodynamic noise, so the Osborn dissipation method was not used to compute K ρρ . However, data segments for which the TKE dissipation rate ( εε ) was large enough to be unaffected by noise were used to compute the ““apparent mixing efficiency”” ΓΓ d . The computed ΓΓ d values are used to investigate variations in apparent mixing efficiency with respect to density ratio ( R ρρ ) and turbulence Reynolds number εε /( ννN 2 ), in an attempt to elucidate the underlying mechanisms of mixing in the NATRE region. Observed variations of ΓΓ d are compared with existing theoretical models of mixing due to: salt fingers, a combination of salt fingers and turbulence, ““conventional”” high Reynolds number turbulence, and low Reynolds number buoyancy-modified turbulence. Significant variations of ΓΓ d with respect to both R ρρ and εε /( ννN 2 ) are found. Although Monte Carlo tests show that some of the observed variations could be noise-induced, a substantial portion of the systematic variations the authors observed were not reproduced by Monte Carlo simulations. These trends are found to be statistically significant, and the authors conclude that they represent real variations in the apparent mixing efficiency. The authors find that ΓΓ d is an increasing function of εε /( ννN 2 ) and a decreasing function of R ρρ ; these variations are not fully consistent with any of the available mixing models.

Journal

Journal of Physical Oceanography – American Meteorological Society

Published: Apr 7, 1997

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