current (oceanography)

Flowing water in the ocean. The surface currents depicted on atlases of the seas are long-term averages of the direction of water motion at the sea surface, driven primarily by the winds. Because most do not extend deeper than 300–500 m/1000–1600 ft, they can be conveniently separated from the circulation of intermediate and deep waters, driven by varying densities of sea water caused by differences in temperature and salinity. (The situation is more complex, in fact, because the Antarctic Circumpolar Current and even parts of the Gulf Stream system do extend all the way to the sea floor.)

The surface circulation has certain similarities in each of the major ocean basins. The strong, persistent trade winds, blowing out of the NE in the N hemisphere and out of the SE in the S hemisphere, produce major W-flowing equatorial currents. These currents flow along bands of latitude until, deflected by continents, they form boundary currents flowing N or S. In the Atlantic and Pacific Oceans these equatorial and boundary currents are parts of semi-enclosed circulation cells called gyres. The gyres are oval-shaped, elongated E–W, and centred in the subtropics (30ºN and S). Their centres are displaced towards the W side of ocean basins because of the rotation of the Earth, which results in an intensifying of boundary currents on the W side and a weakening of E boundary currents.

In general, a subtropical gyre has four components. (1) An equatorial current travelling W in response to the Trade Winds. (2) A W boundary current flowing towards the Poles; these currents are important in transporting heat from lower to higher latitudes; they are narrow, swift, and deep. (3) An E-flowing current pushed along by the bands of W winds at temperate latitudes. (4) An E boundary current flowing towards the Equator; these currents transport cool water from higher to lower latitudes, and are broad, shallow, and weak. Smaller subpolar gyres, turning in the opposite direction to the subtropical gyres, are present especially in the N hemisphere where continents deflect current flow.

The W-flowing N and S equatorial currents of the N and S subtropical gyres are separated by an E-flowing equatorial counter-current. Counter-currents are developed at the inter-tropical convergence (ITC), the area of weak and variable winds known as the Doldrums. The ITC is not found at the geographic equator, but is shifted to c.5°N in the Atlantic and Pacific and to c.7°S in the Indian Ocean. The shift of this ‘climatic equator’ away from the geographic equator is the result of the unequal distribution of land and sea between the N and S hemispheres.

Current may refer to:

Current affairs Electric current Current (fluid) Ocean current Current (mathematics), geometrical current in differential topology The conserved current or Noether current in physics and mathematics, as a result of Emmy Noether's theorem. Present (time) Current (band) Current, Incorporated, a stationery manufacturer Media Current Magazine, a student newsmagazine Current (newspaper), a newspaper about the public broadcasting industry Current Publishing puts out six newspapers in southern Maine.