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The oceans
The oceans

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4.4 Divergence and convergence

Focusing on a closed surface wind circulation such as that in the North Atlantic Ocean (Figure 18), you can see a clockwise pattern repeated in the sea-surface current map in Figure 15. A large closed surface water circulation is called a gyre, and this particular one is the North Atlantic Gyre.

  • What will be the effect of Ekman drift in the centre of the North Atlantic Gyre?

  • There will be a slow movement of water into the centre of the gyre.

    The drift of water into the centre of the gyre causes a surface convergence that pools water and actually raises the surface of the ocean by approximately a metre.

  • What will happen to the level of water in the centre of a gyre in the Northern Hemisphere where the circulation of the winds and surface currents is anticlockwise?

  • There will be a divergence. This will depress the surface of the ocean.

A clockwise circulation is called anticyclonic, and an anticlockwise one is cyclonic. Because of Ekman transport, an anticyclonic circulation causes a convergence of surface waters and a cyclonic circulation a divergence. Figure 19 shows that surface convergence and divergence have an effect beneath the surface.

Described image
Figure 19 Effect of (a) a cyclonic and (b) an anticyclonic wind on the surface of the ocean in the Northern Hemisphere. In (a) there is a divergence at the surface which depresses the surface of the ocean and raises water from beneath the thermocline towards the surface (upwelling). In (b) the surface waters converge which pushes the sea surface upwards and depresses the thermocline (downwelling).