The ocean is an enormous body of about 317,000,000 cubic miles (1,321,000,000 km 3) of water, all of which is more or less in motion. Chief among the sea's movements are the currents. They occur as seasonal and permanent streams flowing horizontally at the surface and at deep levels; they also occur as vertical movements, with water upwelling to the surface or surface water sinking toward the ocean bottom. The currents best understood are the surface currents, many of which have long been studied and used, especially in navigation.
Many interacting forces cause currents. One of the prime causes particularly of surface currents, is wind.
As the wind blows over the sea, part of its energy is transferred to the water, which is dragged along as a current. As a result of the earth's rotation, however, such a current does not move directly downwind. The earth's rotation produces an effect known as the Coriolis force, which deflects winds, ocean currents, and moving bodies to the right of a straight-line course in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect is greatest at high latitudes and weakest in the tropics.
Ocean currents are generally deflected more than the winds that cause them, because the ocean currents have less speed. Thus the paths taken by the surface currents are only roughly similar to those of the prevailing winds.
Unequal heating of the sea by the sun is another major cause of the ocean's circulation, particularly in the deep layers. In polar regions, especially during winter, extremely cold, salty, and dense water sinks far below the surface and moves toward the Equator in very slowly moving layers. At the same time, warm and less dense tropical water in the ocean's upper layers moves toward the poles. Other factors also influence the ocean's circulation. Among them are the shape and relative positions of the continents; the presence of barrier-like island chains; and local winds.
For such varying reasons, some of the currents are strong and relatively steady, while others are weak and erratic. To varying degrees, they all wax and wane with the seasons, mainly because of the shifting of the winds. Currents also meander and shift their courses from year to year.
Except in polar areas, the sea's main surface currents flow in giant swirls north and south of the Equator. (See map, Major Surface Currents.) They turn clockwise in the Northern Hemisphere and counterclockwise in the Southern. The waters near Antarctica move clockwise in a wide band around the ice-covered continent as the West Wind Drift. In the Arctic Ocean, the water swirls and moves mainly into the North Atlantic around Greenland after crossing from the Siberian coast.
Numerous currents make up the swirls; only the major ones are considered here. In the equatorial waters of the Pacific, Atlantic, and Indian oceans are two westward-moving currents—the North Equatorial and the South Equatorial currents. Between them and flowing in the opposite direction is the Equatorial Countercurrent. Although they differ greatly, all three currents are relatively strong in the Atlantic and particularly strong in the Pacific. In the Indian Ocean, they are seasonally disrupted by monsoons.
At the western edge of each of the oceans, the equatorial currents turn poleward and flow along adjacent continents. Three of the flows intensify and become exceptionally strong, often attaining speeds up to four miles per hour (6 km/h). They are the Gulf Stream in the Atlantic, the Kuroshio (Japan Current) in the Pacific, and the Agulhas Current in the Indian Ocean. The others, the Brazil and East Australia currents, are relatively weak.
The currents veer from the coasts and move eastward into the open ocean. Here, all of the flows become sluggish and are often called drifts rather than currents. In the north, these slow-moving flows include the North Atlantic and North Pacific currents and their branches, the Norwegian and Alaska currents. Converging with them are cold currents from the Arctic, notably the East Greenland and Labrador currents in the Atlantic and the Oyashio (Okhotsk, or Kamchatka, Current) in the Pacific. In the Southern Hemisphere, which at comparable latitudes is nearly all water, the currents merge with the broad West Wind Drift and have no individual names.
The currents veer again and return to the Equator by flowing along the west coasts of continents. In the Atlantic are the Canaries and Benguela currents; in the Pacific, the California and Peru (Humboldt) currents; and in the Indian Ocean is the West Australia Current.
Because of the circular flows of the currents, large eddies of relatively quiet water, such as the Sargasso Sea in the Atlantic, exist at the center of the loops. Beneath the surface currents, at varying depths and moving opposite to them, are various subsurface currents. Examples include the Cromwell Current in the Pacific and the Gulf Stream Countercurrent in the Atlantic.
By their constant movement of great masses of water, both warm and cold, the currents play a major role in determining the climate of coastal areas. Warm currents off the coasts of western Europe, southern Alaska, and Japan, for example, temper what would otherwise be a harsh, inhospitable climate. Cold currents off the coasts of Peru, Ecuador, and the western United States have a definite cooling effect on shore.
Aridity, heavy precipitation, and much foggy weather are in some areas consequences of offshore currents. Currents also bring icebergs from polar regions into shipping lanes, where they menace ships.
Besides influencing climate, currents affect the fertility of the sea—its ability to support marine life. The basic food of fish is plankton, which depends mainly on nutrients from the ocean's depths. Regions of strongly upwelling water, such as those off the western coast of South America, abound in fish. Where waters are relatively quiet and warm, there is little marine life.