Chapter 8 - Equations of Motion With
8.6 Important Concepts
- 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
- 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
- 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
- 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
- Measurements of eddy diffusivity indicate water is mixed vertically near
oceanic boundaries such as above seamounts and mid-ocean ridges.