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Ocean Currents

Earthis a water planet and the world ocean is always in large-scalemotion, slowly

rearranging its waters over large extents oflatitude and longitude. Whereas tides

and waves are vertical motions, ocean currents are horizontal. Currents can be

directly forced by surface wind or occur more slowly and deeply because of gra-

dients oftemperature, pressure, and salinity. Humans have long been knowledge-

able about currents because they are large and readily associated with fishing

conditions and navigation. More recently, science has connected changes in

currents with global changes in weather and long-term changes inclimate.

There is a known geography to the major currents of the world. Because of the

wind emanating outward from the subtropical highs of the global wind and pres-

sure belts, kinetic energy from the fluid that is theatmosphereis imparted to the

much denser fluid of the ocean. In that the subtropical highs are semi-permanent

features of the atmosphere, this means that air blows from preferred directions

over large stretches of oceans for days and weeks at a time. The ocean water starts

to move in response though not as fast or in exactly the same direction because of

the increased force of friction and the correspondingly lesser Coriolis effect.

The main geographic regularity that can be observed is the presence of oceanic

gyres, immense circulations around ocean basins. These gyres have a clockwise

flow of water in the Northern Hemisphere and a counterclockwise flow of water

in the Southern Hemisphere. This means that there is considerable water being

transported from tropical to polar latitudes and vice versa. The main subtropical

gyres are joined by Equatorial Countercurrents in the Pacific and Indian oceans.

The strong trade winds on the equatorward side of the gyres cause the westward

motion of the water and the piling up of water level in the eastern parts of these

ocean basins. The higher water in the east literally runs downhill into the linear

area of light and variable Intertropical Convergence Zone winds, and it is this

eastward-moving water that is the Equatorial Countercurrent.

There is the tendency for poleward transport of water near east coasts of conti-

nents and the equatorward transport of water along west coasts. When water tem-

peratures are studied it is apparent that east coast currents are “warm” and west

coast currents are “cold” compared to the average of the ocean temperatures of

each latitude. These temperature adjectives do not have absolute meaning. For

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