Surface Mixing Layers in the Sargasso Sea

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  • 1 Institute of Ocean Sciences, Patricia Bay, Sidney, B.C., Canada V8L 4B2
  • 2 Woods Hole Oceanographic Institution, Woods Hole MA 02543
  • 3 Institute of Oceanography, University of British Columbia, Vancouver, B.C. V6T IW5
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Abstract

Observations of turbulent energy dissipation rate ε in the deep surface mixed layer at a mid-Sargasso site are presented: two occupations of this site include a large range of local meteorological forcing. Two frontal passages and a large time interval between profiles during the first series of measurements preclude examination of the turbulent kinetic energy balance: qualitatively, a profile taken during the strongest wind-wave forcing of the observation set suggests that layer deepening was not being driven directly from the surface, but by a shear instability at the mixed layer base. A quantitative assessment of terms in the steady-state locally balanced model of the turbulent kinetic energy budget proposed by Niiler (1975) has been possible for two profiles having dissipation characteristics and surface meteorological conditions which allow us to argue for the absence of all but a few of the possible source/sink terms in the turbulent kinetic energy balance. In one case, a steady-state local balance is possible. In the other case, a local balance can be maintained by giving up the steady-state assumption. i.e., by including the time rate of decay of the turbulent kinetic energy. Other possible balances exist. The analysis of the surface mixed-layer turbulent kinetic energy balance highlights two major uncertainties-parameterization of the wind-wave forcing term and lack of reliable dissipation measurements in the upper 10–20 m of the water column.

Abstract

Observations of turbulent energy dissipation rate ε in the deep surface mixed layer at a mid-Sargasso site are presented: two occupations of this site include a large range of local meteorological forcing. Two frontal passages and a large time interval between profiles during the first series of measurements preclude examination of the turbulent kinetic energy balance: qualitatively, a profile taken during the strongest wind-wave forcing of the observation set suggests that layer deepening was not being driven directly from the surface, but by a shear instability at the mixed layer base. A quantitative assessment of terms in the steady-state locally balanced model of the turbulent kinetic energy budget proposed by Niiler (1975) has been possible for two profiles having dissipation characteristics and surface meteorological conditions which allow us to argue for the absence of all but a few of the possible source/sink terms in the turbulent kinetic energy balance. In one case, a steady-state local balance is possible. In the other case, a local balance can be maintained by giving up the steady-state assumption. i.e., by including the time rate of decay of the turbulent kinetic energy. Other possible balances exist. The analysis of the surface mixed-layer turbulent kinetic energy balance highlights two major uncertainties-parameterization of the wind-wave forcing term and lack of reliable dissipation measurements in the upper 10–20 m of the water column.

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