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.