Frictional Coupling With The Ocean

Figure 3.4 The difference between (a) laminar flow and (b) turbulent flow. The arrows represent the paths taken by individual parcels of water.

Figure 3.4 The difference between (a) laminar flow and (b) turbulent flow. The arrows represent the paths taken by individual parcels of water.

The effect of wind stress on the sea-surface is transmitted downwards as a result of internal friction within the upper ocean. This internal friction results from turbulence and is not simply the viscosity of a fluid moving in a laminar fashion (see Figure 3.4).

Friction in a moving fluid results from the transfer of momentum (mass x velocity) between different parts of the fluid. In a fluid moving in a laminar manner, momentum transfer occurs as a result of the transfer of molecules (and their associated masses and velocities) between adjacent layers, as shown schematically in Figure 3.5(a), and should therefore strictly be called molecular viscosity. At the sea-surface, as in the rest of the ocean, motion is rarely laminar, but instead turbulent, so that parcels of water, rather than individual molecules, are exchanged between one part of the moving fluid and another (Figure 3.5(b)). The internal friction that results is much greater than that caused by exchange of individual molecules, and is known as eddy viscosity.

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