Ocean Circulation and currents

Ocean circulation is the large, connected system of water movements in the oceans, including not only the surface movement, but also the slow, deep-water circulations. While the surface currents are caused by the winds and a geographically uneven solar energy distribution, the deep ocean currents are the result of sinking and upwelling water, and termohaline (temperature and salinity) differences. The atmospheric and oceanic circulations are linked together, and this global ocean circulation system transfers heat from low to higher latitudes, making the oceans responsible for about 40% of the global heat transport. Although there are similarities between the atmospheric and oceanic circulations, ocean currents move slower than winds, with a speed of about several kilometers per day to a few kilometers per hour.

The system of surface currents resembles the major wind patterns of the atmosphere. However, the surface currents do not move exactly in the same direction, but at a 45-degree angle to wind direction, with a water speed of less than 3% of the original wind speed. It turns to right on the Northern Hemisphere, and to left on the Southern Hemisphere. Going down to a deeper layer, it tends to turn even further compared to the surface, and going even deeper in the water (until about 109 yd [100 m], where the depth of no motion is reached), eventually, the angle between the surface winds and the deep movements reaches 90 degrees. This is called the Ekman spiral.

The surface currents in each ocean can be described with a simplified scheme (although the actual currents differ in depth, size or exact location). The dominant features are the subtropical gyres (semi-closed circles of the currents), and the boundary currents, both in the east and west ocean basins. The gyres are centered around 30 degrees latitude in the major ocean basins, and rotate clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere because of the earth's rotation and the change in wind direction with latitude. The basic ocean circulation looks like a westward flow near the equator, which moves warm water to higher latitudes at the western ocean basins. At about 35 degrees of both North and South latitudes, the major currents turn in an eastern direction, to carry warm water to higher latitudes. To balance this poleward movement at the eastern ocean basins, cold water has to return in the currents. In the Southern Hemisphere, the gyre turns in the opposite direction, and because there is no land between 35 and 60 degrees latitude, a strong, cold current around Antarctica completes the system.

The clockwise rotating gyre in the northern part of the Atlantic Ocean consists of the warm North Equatorial Current flowing near the equator, joining the western boundary of the warm Gulf Stream, which turns towards east and becomes the warm North Atlantic Drift, while the cold Labrador Current, West Greenland Drift, and East Greenland Drift return cold water from the North. The eastern boundary, the southward Canary Current close to West Africa also brings cold water towards the equator, closing the North Atlantic Ocean gyre. In the southern part of the Atlantic Ocean, in the counterclockwise gyre, the warm South Equatorial Current joins the warm Brazil Current going south towards Antarctica at the east coast of South America, meeting the cold Falkland Current, and the cold, West Wind Drift going east around Antarctica. The gyre is closed by the cold Benguela Current going north at the southwestern coast of Africa, while the eastward flowing warm Equatorial Countercurrent connects the gyres from the north and South Atlantic Oceans.

The system of currents in the Pacific Ocean is very similar to the Atlantic currents. The clockwise rotating gyre in the northern part of the Pacific Ocean consists of the warm North Equatorial Current flowing westward north of the equator, joining the western boundary warm Kuroshio Current, which turns towards the East and becomes the warm North Pacific Drift, while the cold Oyashio Current returns cold water from the North. At the coast of Alaska loops the warm Alaska Current. The eastern boundary California Current, close to the coast of California, also brings cold water towards the equator, closing the North Atlantic Ocean gyre. This is where upwelling, the rising of cold water replacing surface water that drifts away due to the winds, occurs. Although it brings cold water, low clouds, and sometimes fog in summer, the nutrientrich, cold water helps the fishing industry. South of this gyre but still in the Northern Hemisphere, the eastward flowing warm North Equatorial Countercurrent can be found. In the southern part of the Pacific Ocean, the warm, westward South Equatorial Current flows opposite of the eastward warm South Equatorial Countercurrent, and continues towards the South at the east coast of Australia. While the West Wind Drift is heading east around the Antarctica, the cold Peru or Humboldt Current going northward at the southwestern coast of South America closes this gyre.

The system of surface ocean currents is simpler in the Indian Ocean, because instead of the "8-shaped" two gyres, only one is present, influenced by seasonally changing winds. It consists of the warm North Equatorial Current, the South Equatorial Countercurrent, the South Equatorial Current, and the eastward, circumpolar current around Antarctica, the cold West Wind Drift.

See also Gulf stream; Oceans and seas