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- Author or Editor: A. E. Gargett x
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Abstract
Data obtained from the cycling mode of a towed system operated in the North Pacific are used to investigate the relationship of small-scale mixing to the finestructure which seems to be such a common characteristic of vertical profiles of oceanic water properties. The signal from a high-frequency response platinum-film thermometer is used, with the local mean vertical temperature gradient, to produce variables proportional to heat flux and an eddy diffusivity for heat. Along with measured values for the local vertical salinity and density gradients, this information is used to examine some questions of interest in the general understanding of turbulence and finestructure. First, it is shown that in regions where the vertical density structure is distinctly layered, there is no noticeable tendency for mixing to occur preferentially on “sheets,” the high-density-gradient regions which separate “layers" of lower density gradient. Instead, mixing events seem to occur at random with respect to the density field, suggesting that such events do not arise predominantly as small-scale Kelvin-Helmholtz instabilities on pre-existing finestructure. Instead, the evidence points to a much closer connection between turbulent mixing and the processes which act to produce and destroy finestructure. Between 60 and 70% of the turbulence encountered in our tows is associated with “active” regions, areas where the vertical profiles of temperature and/or salinity show finestructure inversions. The density profile in such regions is often “steppy” but invariably statically stable on vertical scales greater than about a meter, so that some basically horizontal process, such as inertial waves or density-driven interleaving, is required to produce the inversions in T and S. Whatever process is involved in forming the finestructure inversions in the “active” regions also produces an incidence of turbulence which is higher by a factor of 2 than that which is typical of “inactive” regions. This increased incidence of turbulence could arise either directly through increased vertical shears or else, partly indirectly, through double-diffusive processes which become possible in local regions produced by the inversions. The observations show fairly strong statistical evidence for increased microstructure activity in regions where the local vertical gradients of T and S are suitable for double-diffusive processes.
Abstract
Data obtained from the cycling mode of a towed system operated in the North Pacific are used to investigate the relationship of small-scale mixing to the finestructure which seems to be such a common characteristic of vertical profiles of oceanic water properties. The signal from a high-frequency response platinum-film thermometer is used, with the local mean vertical temperature gradient, to produce variables proportional to heat flux and an eddy diffusivity for heat. Along with measured values for the local vertical salinity and density gradients, this information is used to examine some questions of interest in the general understanding of turbulence and finestructure. First, it is shown that in regions where the vertical density structure is distinctly layered, there is no noticeable tendency for mixing to occur preferentially on “sheets,” the high-density-gradient regions which separate “layers" of lower density gradient. Instead, mixing events seem to occur at random with respect to the density field, suggesting that such events do not arise predominantly as small-scale Kelvin-Helmholtz instabilities on pre-existing finestructure. Instead, the evidence points to a much closer connection between turbulent mixing and the processes which act to produce and destroy finestructure. Between 60 and 70% of the turbulence encountered in our tows is associated with “active” regions, areas where the vertical profiles of temperature and/or salinity show finestructure inversions. The density profile in such regions is often “steppy” but invariably statically stable on vertical scales greater than about a meter, so that some basically horizontal process, such as inertial waves or density-driven interleaving, is required to produce the inversions in T and S. Whatever process is involved in forming the finestructure inversions in the “active” regions also produces an incidence of turbulence which is higher by a factor of 2 than that which is typical of “inactive” regions. This increased incidence of turbulence could arise either directly through increased vertical shears or else, partly indirectly, through double-diffusive processes which become possible in local regions produced by the inversions. The observations show fairly strong statistical evidence for increased microstructure activity in regions where the local vertical gradients of T and S are suitable for double-diffusive processes.
Abstract
Existing large-eddy simulations (LES) of Langmuir supercells (LS) do not include rotational terms. Despite the fact that the actual coastal ocean is certainly affected by rotation, such simulations are found to provide excellent agreement with a wide range of features of LS observed in the shallow coastal ocean. This note explains why it is indeed acceptable to compare results of a nonrotational LES of LS in a laterally unbounded domain with observations of LS made in a rotating fluid with a lateral boundary.
Abstract
Existing large-eddy simulations (LES) of Langmuir supercells (LS) do not include rotational terms. Despite the fact that the actual coastal ocean is certainly affected by rotation, such simulations are found to provide excellent agreement with a wide range of features of LS observed in the shallow coastal ocean. This note explains why it is indeed acceptable to compare results of a nonrotational LES of LS in a laterally unbounded domain with observations of LS made in a rotating fluid with a lateral boundary.
Abstract
Turbulence in the ocean surface layer is generated by time-varying combinations of destabilizing surface buoyancy flux, wind stress forcing, and wave forcing through a vortex force associated with the surface wave field. Observations of time- and depth-averaged vertical velocity variance of full-depth turbulence in shallow unstratified water columns under destabilizing buoyancy forcing are used to determine when process domination can be assigned over a wide range of mixed forcings. The properties of two turbulence archetypes, one representing full-depth Langmuir circulations and the other representing full-depth convection, are described in detail. It is demonstrated that these archetypes lie in distinct regions of the plane of 
Abstract
Turbulence in the ocean surface layer is generated by time-varying combinations of destabilizing surface buoyancy flux, wind stress forcing, and wave forcing through a vortex force associated with the surface wave field. Observations of time- and depth-averaged vertical velocity variance of full-depth turbulence in shallow unstratified water columns under destabilizing buoyancy forcing are used to determine when process domination can be assigned over a wide range of mixed forcings. The properties of two turbulence archetypes, one representing full-depth Langmuir circulations and the other representing full-depth convection, are described in detail. It is demonstrated that these archetypes lie in distinct regions of the plane of
Abstract
We present time series of tritium (3H) concentrations at varying depths in the water column at Ocean Station P(50°N, 145°W) in the northeast Pacific. Measurements started in the fall of 1974, at the time of the GEOSECS mapping of the North Pacific, and extended through 1982. When combined with the GEOSECS spatial maps, various features of the observed tritium time series suggest that the vertical distribution of tritium (and presumably other conservative tracers) at Station P is determined mainly by advection along isopycnals.
Abstract
We present time series of tritium (3H) concentrations at varying depths in the water column at Ocean Station P(50°N, 145°W) in the northeast Pacific. Measurements started in the fall of 1974, at the time of the GEOSECS mapping of the North Pacific, and extended through 1982. When combined with the GEOSECS spatial maps, various features of the observed tritium time series suggest that the vertical distribution of tritium (and presumably other conservative tracers) at Station P is determined mainly by advection along isopycnals.
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.
Abstract
Results from three separate velocity profilers operated nearly simultaneously in the northwest Atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters. This exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a WKB-scaled level from k = 0.01 cpm through k 0 ≈ 0.1 cpm, then falls as k −1 to a buoyancy wavenumber k 0 = (N 3/ε)1/2 determined by the local average Väisälä frequency N and the volume-averaged dissipation rate ε. Various consequences of the observed shear spectral shape are explored.
Abstract
Results from three separate velocity profilers operated nearly simultaneously in the northwest Atlantic in 1975 are used to form a composite shear spectrum over vertical wavelengths from 100 m down to a few centimeters. This exercise constitutes an intercomparison of the three different measurement techniques and reveals a shear spectrum which is approximately fiat at a WKB-scaled level from k = 0.01 cpm through k 0 ≈ 0.1 cpm, then falls as k −1 to a buoyancy wavenumber k 0 = (N 3/ε)1/2 determined by the local average Väisälä frequency N and the volume-averaged dissipation rate ε. Various consequences of the observed shear spectral shape are explored.
