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The oceans
The oceans

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4.2 The effect of the wind on the oceans

The wind provides one of the main forces that move the surface of the oceans. When the wind moves across water, surface friction transfers energy from the wind to the water through wind stress. Wind stress has the Greek symbol τ (tau) and is proportional to the square of the wind speed, W:

τ = cW2
Equation label: (Eqn 3)

where c is the constant of proportionality. So if the wind speed increases from 1 m s−1 to 4 m s−1 then wind stress will increase by a factor of 16. This large jump in the wind stress happens because when the winds are weak, the surface of the ocean is relatively flat (Figure 16a) and there are few wave tops for the wind to push against. As energy is transferred from the wind to the ocean, the surface becomes rougher and 'stretched', so more of the surface is in contact with the wind (Figure 16b). The increased surface area leads to more energy being transferred to the ocean and larger surface waves.

Described image
Figure 16 A typical sea surface when the wind speed is (a) low and (b) higher.

Once the surface of the water is moving, some of the wind's energy is transferred downwards into the water column through internal friction called eddy viscosity. The result is that momentum is transferred downwards and the mixed layers you saw in Figure 11 develop.

  • What effect will a rise in wind speed have on the thickness of the layer of well-mixed water?

  • It will increase because the surface waters will be mixed to a greater depth as more energy is supplied.

When the wind blows across the oceans, the surface waters are mixed and they start to move. But the direction of movement is not simply the same as the direction of the winds. The Earth is rotating and moving currents are affected by the Coriolis force, which arises from the rotation of the planet. Moving objects in the Northern Hemisphere are deflected to the right and those in the Southern Hemisphere are deflected to the left. The understanding of how the rotation of the planet affects moving ocean currents was developed through an experiment in the polar seas as described next.