8.6: Important Concepts
- Page ID
- 30102
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- Friction in the ocean is important only over distances of a few millimeters. For most flows, friction can be ignored.
- The ocean is turbulent for all flows whose typical dimension exceeds a few centimeters, yet the theory for turbulent flow in the ocean is poorly understood.
- The influence of turbulence is a function of the Reynolds number of the flow. Flows with the same geometry and Reynolds number have the same streamlines.
- Oceanographers assume that turbulence influences flows over distances greater than a few centimeters in the same way that molecular viscosity influences flow over much smaller distances.
- The influence of turbulence leads to Reynolds stress terms in the momentum equation.
- The influence of static stability in the ocean is expressed as a frequency, the stability frequency. The larger the frequency, the more stable the water column.
- The influence of shear stability is expressed through the Richardson number. The greater the velocity shear, and the weaker the static stability, the more likely the flow will become turbulent.
- Molecular diffusion of heat is much faster than the diffusion of salt. This leads to a double-diffusion instability which modifies the density distribution in the water column in many regions of the ocean.
- Instability in the ocean leads to mixing. Mixing across surfaces of constant density is much smaller than mixing along such surfaces.
- Horizontal eddy diffusivity in the ocean is much greater than vertical eddy diffusivity.
- Measurements of eddy diffusivity indicate water is mixed vertically near oceanic boundaries such as above seamounts and mid-ocean ridges.