Turbulence Production

Turbulence Generation by Swimming Marine Organisms     |    Shear Instability     |    Gregg-Henyey

Oceanographers are interested in turbulent mixing because it redistributes temperature, salinity and dissolved material in seawater. It is also thought to control the meridional thermohaline overturning circulation (global conveyor belt). Away from the surface boundary layer where direct atmospheric forcing by wind as well as diurnal and seasonal cooling can produce turbulence, and bottom boundary layers, where friction produces turbulence, turbulence production and mixing in the stratified ocean interior, are largely caused by the breaking of internal gravity waves (Fig. 1). Near-inertial waves generated by atmospheric storms and radiating downward from the surface plus tidal-frequency internal waves radiating from rough topography provide the main supply of energy for turbulence. Long-wavelength (vertical wavelength λ_z ~ 1 km, horizontal wavelength λ_h ~ 100 km) waves propagate rapidly and carry most of the energy but are unlikely to break. The short-wavelength (vertical wavelength λ_z ~ 10 m) internal waves that become unstable travel very slowly and must be created locally. They arise from nonlinear interactions among longer-wavelength waves cascading energy from long to short wavelengths.

Fig.1: Typical history of a turbulent patch from a growing instability (Kelvin-Helmholtz billow), breaking to form turbulence which then decays. Also shown are time-series of shear production u'w'U_z, buoyancy-flux w'b', dissipation rate ε, turbulent available potential energy APE’ and background potential energy PE (a). Lower panels show the vertical path of a float through heaving density field as it becomes turbulent (b) and in a density coordinate system (c) (from Sun et al. 1996 JPO).

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