ocean currents

ocean currents Ocean currents are analogous to atmospheric winds. The general circulation of the world ocean is driven partially by atmospheric winds and partly by the thermohaline effects across the world ocean (see sea ice and climate). The ocean currents are the movement of water resulting from these two driving forces. The world ocean has a complex network of shallow ocean currents which are dominated by the prevailing winds; the larger-scale circulation is controlled principally by longer-term thermohaline forcing. The currents generated by prevailing winds derive from a transfer of momentum (through friction) from the atmosphere to the ocean; the currents do not, however, follow the prevailing wind direction. The water, once moving, is deflected by the Coriolis force to the right of the wind in the northern hemisphere, and to the left of the wind in the southern hemisphere. The resulting direction of the current is at approximately 45° to the right of the wind in the northern hemisphere (to the left in the southern hemisphere) and is called the Ekman drift, after the Swedish mathematician who first formulated the problem in 1905.

The general circulation of the two largest ocean basins is very similar; there is a clockwise gyre in the northern hemisphere of both the Pacific and the Atlantic and an anticlockwise gyre in the southern hemisphere. In both hemispheres the gyres are anticyclonic (i.e. opposite to the rotation of the Earth) and are especially strong in the mid-latitude regions. The circulation of the Indian Ocean is slightly more complex because of the seasonal wind changes which accompany the monsoon. In this region from May to September the winds typically blow from the south-west, whereas from November to March they reverse to blow from the north-east. The seasonal change in wind direction results in a seasonal change in the surface currents. The high-latitude currents of the polar seas are discussed in the entry on high-latitude ocean currents.

Another important observation is that there are strong narrow currents at the western edge of all the ocean basins. This western intensification drives some of the strongest currents, and these were among the first currents of the oceans to be surveyed accurately. By far the most studied of these western boundary currents is the Gulf Stream in the North Atlantic. The prevailing west to south-west winds across the North Atlantic are responsible for extending the Gulf Stream and giving the British Isles a temperate maritime climate. The other major western boundary currents are the Agulhas on the east coast of South Africa, which was so important to the tea clippers of the nineteenth century, the Kuroshio on the east coast of Japan, and the East Australia Current. In contrast, eastern boundary currents on the east of the ocean basins tend to be broad and slow.

Changes in the atmospheric windfield can be responsible for rapid and dramatic changes in the ocean currents. An example of this is the periodic extreme climate event known as El Niño.

Mark A. Brandon

Bibliography

Pickard, G. L. and and Emery, W. J. (1982) Descriptive physical oceanography. Pergamon Press, Oxford.