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Distinctive white lines trace the flow of surface currents around the world.
Animation of circulation around ice shelves of Antarctica.
An ocean current is a continuous, directed movement of seawater generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences, with tides caused by the gravitational pull of the Moon and the Sun. Depth contours, shoreline configurations and interaction with other currents influence a current’s direction and strength. A deep current is any ocean current at a depth of greater than 100m. A part of oceanography is the science studying ocean currents.
Ocean currents can flow for great distances, and together they create the great flow of the global conveyor belt which plays a dominant part in determining the climate of many of the Earth’s regions. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude. Another example is Lima, Peru where the climate is cooler (sub-tropical) than the tropical latitudes in which the area is located, due to the effect of the Humboldt Current.
Major ocean surface currents, (Source: NOAA)
A recording current meter
Surface ocean currents are generally wind driven and develop their typical clockwise spirals in the northern hemisphere and counter clockwise rotation in the southern hemisphere because of the imposed wind stresses. In wind driven current, the Ekman spiral effect results in the currents flowing at an angle to the driving winds. The areas of surface ocean currents move somewhat with the seasons; this is most notable in equatorial currents.
Ocean basins generally have a non-symmetric surface current, in that the eastern equatorward-flowing branch is broad and diffuse whereas the western poleward flowing branch is very narrow. These western boundary currents (of which the Gulf Stream is an example) are a consequence of basic fluid dynamics.
Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation, also known as the ocean’s conveyor belt which refers to the deep ocean density driven ocean basin currents. These currents, that flow under the surface of the ocean and are thus hidden from immediate detection, are called submarine rivers. These are currently being researched using a fleet of underwater robots called Argo. Upwelling and downwelling areas in the oceans are areas where significant vertical movement of ocean water is observed.
The South Equatorial Currents of the Atlantic and Pacific straddle the equator. Though the Coriolis effect is weak near the equator (and absent at the equator), water moving in the currents on either side of the equator is deflected slightly poleward and replaced by deeper water. Thus, equatorial upwelling occurs in these westward flowing equatorial surface currents. Upwelling is an important process because this water from within and below the pycnocline is often rich in the nutrients needed by marine organisms for growth. By contrast, generally poor conditions for growth prevail in most of the open tropical ocean, because strong layering isolates deep, nutrient rich water from the sunlit ocean surface.
Surface currents make up about 8% of all the water in the ocean. Surface currents are generally restricted to the upper 400 m (1,300 ft) of the ocean. The movement of deep water in the ocean basins is by density driven forces and gravity. The density difference is a function of different temperatures and salinity. Deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
Ocean currents are measured in sverdrup (sv), where 1 sv is equivalent to a volume flow rate of 1,000,000 m3 (35,000,000 cu ft) per second.
About 10% of the water in the world ocean is involved in surface currents, which are water flowing horizontally in the uppermost 400 meters (1,300 feet) of the ocean surface, driven mainly by wind friction. Winds drive currents that are at or near the ocean’s surface. These currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). surface currents move water above the pycnocline, the zone of rapid density change with depth. The primary force responsible for surface currents is wind. Most of Earth’s surface wind energy is concentrated in each hemisphere’s trade winds (easterlies) and westerlies. Waves on the sea surface transfer some of the energy from the moving air to the water by friction. This tug of wind on the ocean surface begins a mass flow of water. The water flowing beneath the wind forms a surface current. The moving water “piles up” in the direction the wind is blowing. Water pressure is higher on the “piled up” side, and the force of gravity pulls the water down the slope – against the pressure gradient- in the direction from which it came. But the Coriolis effect intervenes. Northern Hemisphere surface currents flow to the right of the wind direction, because of the Coriolis effect. Southern Hemisphere currents flow to the left. Continents and basin topography often block continuous flow and help deflect the moving water into a circular pattern. This flow around the periphery of an ocean basin is called a gyre.
Seawater flows in six great surface circuits
Gyres in balance between the pressure gradient and the Coriolis effect are called geostrophic gyres, and their currents are called geostrophic currents. The geostrophic gyres are largely independent of one another in each hemisphere, because of the patterns of driving winds and the present positions of continents. There are six great current circuits in the world ocean: two in the Northern Hemisphere and four in the Southern Hemisphere. Five are geostrophic gyres: the North Atlantic gyre, the South Atlantic gyre, the North Pacific gyre, the South Pacific gyre, and the Indian Ocean gyre. Though it is a closed circuit, the sixth and largest current, known as the West Wind Drift (or Antarctic Circumpolar Current), is technically not a gyre since it does not flow around the periphery of an ocean basin. The West Wind Drift flows endlessly eastward (i.e., never deflected by a continent) around Antarctica, driven by powerful, nearly ceaseless westerly winds. While it might be assumed that the two gyres in the North and South Pacific (and the two gyres in the North and South Atlantic) converge exactly at the geographical equator, instead the junction of equatorial currents (referred to as the meteorological equator) lies a few degrees north of the geographical equator. The meteorological equator and the Intertropical Convergence Zone (the band at which the trade winds converge) are displaced 5º to 8º northward primarily because of the heat accumulated in the Northern Hemisphere’s greater tropical land surface area. Ocean circulation like atmospheric circulation, is balanced around the meteorological equator.
Classification for geostrophic currents
Boundary currents have different characteristics. Because of the different factors that drive and shape them, the currents that form geostrophic gyres have different characteristics. Geostrophic currents may be classified by their position within the gyre as western boundary currents, eastern boundary currents, or transverse currents.
|Type of current
||Transport (millions of cubic meters per second)
|Western boundary currents
||Warm water; narrow (< 100 km); deep (substantial transport to depths of 2 km)
||Swift (hundreds of kilometers per day)
||Large (usually 50 sv or greater)
||Sharp boundary with coastal circulation system; little or no coastal upwelling; waters tend to be depleted in nutrients, unproductive; waters derived from trade-wind belts
|Eastern boundary currents
||Cold water; broad (~ 1,000 km); shallow (< 500 m)
||Slow (tens of kilometers per day)
||Small (typically 10–15 sv)
||Diffuse boundaries separating from coastal currents; coastal upwelling common; waters derived from mid-latitudes
- Western boundary currents
The western boundary currents are the fastest, deepest, and narrowest of all geostrophic currents, and they transport an extraordinary volume of water. They are found at the western boundaries of ocean basins (that is, off the east coast of continents). These currents move warm water poleward in each of the gyres. There are five large western boundary currents: the Gulf Stream (in the North Atlantic), the Japan or Kuroshio Current (in the North Pacific), the Brazil Current (in the South Atlantic), the Agulhas Current (in the Indian Ocean), and the East Australian Current (in the South Pacific). The Gulf Stream is the largest of the western boundary currents. The western boundary of the Gulf Stream is usually distinct, marked by abrupt changes in water temperature, speed, and direction. They can move for surprisingly long distances within well-defined boundaries, almost as if it were a river. However, long, straight edges are an exception, not a necessary property of western boundary currents. The western edge of these currents is often clearly visible. That is to say, the water within the current is usually warm, clear, and blue often depleted of nutrients and incapable of supporting a variety of ocean life. By contrast, water over the continental slope adjacent to the current, is often cold, green, and teeming with life. Western boundary currents meander as they flow poleward. The looping meanders sometimes connect to form turbulent rings, or eddies, that trap cold or warm water in their centers and then separate from the main flow. For example, cold-core eddies form in the Gulf Stream, meandering eastward upon leaving the coast of North America off Cape Hatteras. Warm-core eddies can form north of the Gulf Stream when the warm current loops into the cold water lying to the north. When the loops are cut off, they become freestanding spinning masses of water. Warm-core eddies rotate clockwise, whereas cold-core eddies rotate counterclockwise. The slowly rotating eddies move away from the current and are distributed across the North Atlantic. Some may be 1,000 kilometers (620 miles) in diameter and retain their identity for more than three years. In mid-latitudes, as much as one-fourth of the surface of the North Atlantic may consist of old, slow-moving, cold-core eddy remnants. Nutrients may be brought toward the surface by turbulence in eddies, which can stimulate the growth of tiny marine plantlike organisms. Recent research suggests that eddies may also influence the seafloor by slowly moving abyssal storms, which can be inferred from ripple marks that have been observed in deep sediments.
- Eastern boundary currents
Eastern boundary currents have properties that are nearly opposite of their western boundary counterparts. Eastern boundary currents carry cold water towards the equator; they are shallow and broad, sometimes more than 1,000 kilometers (620 miles) across; their boundaries are not well defined; and eddies tend not to form. Their total flow is less than that of their western counterparts. The current is so shallow and broad that sailors may not even notice it. There are five eastern boundary currents, each of which are at the eastern edge of ocean basins (that is, off the west coast of continents): the Canary Current (in the North Atlantic), the Benguela Current (in the South Atlantic), the California Current (in the North Pacific), the West Australian Current (in the Indian Ocean), and the Peru or Humboldt Current (in the South Pacific).
- Transverse currents
Transverse currents are currents that flow from east to west and west to east, linking the eastern and western boundary currents. They are derived from the trade winds at the fringes of the tropics and from the mid-latitude westerlies. The trade wind-driven North Equatorial Current and South Equatorial Current in the Atlantic and Pacific each transport ~30 sv westward, and are moderately shallow and broad. Since the Pacific has a greater expanse of water at the equator and stronger trade winds than the Atlantic, the Pacific develops more powerful westward flowing equatorial currents, which causes the height differential between the western and eastern Pacific to be as much as 1 meter (3.3 feet). Also, as a consequence of transverse currents, Atlantic water across the isthmus of Panama is usually 20 centimeters (8 inches) higher, on average, than water across the isthmus of Panama in the Pacific.
Countercurrents and undercurrents
Equatorial currents are typically accompanied by countercurrents, which flow on the surface in a direction opposite to that of the main current. At the meteorological equator, there is a continuous rising of air and a lack of presence of persistent trade winds across the boundary to drive water to the west. As a consequence, there is usually a backward (eastward) flow of water (referred to as a countercurrent) exactly at the meteorological equator, or a bit north or south of the meteorological equator. Countercurrents can sometimes be undercurrents, which are countercurrents that exist beneath the water surface. Undercurrents have been found under most major currents. The Pacific Equatorial Undercurrent, also known as the Cromwell Current, flows eastward beneath the North Equatorial Current with an average velocity of 5 kilometers (3 miles) per hour at a depth of 100–200 meters (330–660 feet). It is about 300 kilometers (190 miles) wide and carries a volume equivalent to about half the Gulf Stream. It has been traced for more than 14,000 kilometers (8,700 miles), from New Guinea to Ecuador. The first undercurrent was discovered in 1951 in the central Pacific by Townsend Cromwell, a researcher employed by the U.S. Fish and Wildlife Service.
Wind-induced vertical circulation
The wind-driven horizontal movement of water can sometimes induce the vertical movement of water from the deep, cold, nutrient-laden water toward the surface (a process known as upwelling). Equatorial upwelling occurs in these westward-flowing equatorial surface currents. Upwelling is an important process because water from within and below the pycnocline is often rich in the nutrients needed by marine organisms for growth. By contrast, generally poor conditions for growth prevail in most of the open tropical ocean, because strong layering isolates deep, nutrient rich water from the sunlit ocean surface. The South Equatorial Currents of the Atlantic and Pacific straddle the geographic equator. Though the Coriolis effect is weak near the equator (and absent at the equator), water moving in the currents on either side of the equator is deflected slightly poleward. Water north of the equator veers to the right (northward), and water to the south veers to the left (southward). Surface water therefore diverges, allowing deep water to replace surface water. Most of the upwelled water comes from the area above the equatorial undercurrent, at depths of 100 meters or less.
Winds that blow steadily across the ocean, and the small waves that such winds generate, can induce long sets of counter-rotating vortices (or cells) in the surface water, referred to as Langmuir circulation. These slowly twisting vortices align in the direction of the wind. It usually takes about an hour for a particle in a vortex to complete one revolution. Streaks of foam (or seaweed or debris), called windrows, collect in areas where adjacent vortices converge, while regions of divergence remain relatively clear. Langmuir circulation rarely disturbs the ocean below a depth of about 20 meters (66 feet). Langmuir circulation occurs within the surface layer, which is above the pycnocline, and thus does not cause upwelling.
Coupling data collected by NASA/JPL by several different satellite-borne sensors, researchers have been able to “break through” the ocean’s surface to detect “Meddies” — super-salty warm-water eddies that originate in the Mediterranean Sea and then sink more than a half-mile underwater in the Atlantic Ocean. The Meddies are shown in red in this scientific figure.
Horizontal and vertical currents also exist below the pycnocline in the ocean’s deeper waters. The movement of water due to differences in density as a function of water temperature and salinity is called thermohaline circulation. The whole ocean is involved in slow thermohaline circulation. Ripple marks in sediments, scour lines, and the erosion of rocky outcrops on deep-ocean floors are evidence that relatively strong, localized bottom currents exist. Some of these currents may move as rapidly as 60 centimeters (24 inches) per second.
These currents are strongly influenced by bottom topography, since dense, bottom water must forcefully flow around seafloor projections. Thus, they are sometimes called contour currents. Bottom currents generally move equator-ward at or near the western boundaries of ocean basins (below the western boundary surface currents). The deep-water masses are not capable of moving water at speeds comparable to that of wind-driven surface currents. Water in some of these currents may move only 1 to 2 meters per day. Even at that slow speed, the Coriolis effect modifies their pattern of flow.
Downwelling of deep water in polar regions
Antarctic Bottom Water is the most distinctive of the deep-water masses. It is characterized by a salinity of 34.65‰, a temperature of -0.5°C (30°F), and a density of 1.0279 grams per cubic centimeter. This water is noted for its extreme density (the densest in the world ocean), for the great amount of it produced near Antarctic coasts, and for its ability to migrate north along the seafloor. Most Antarctic Bottom Water forms near the Antarctic coast south of South America during winter. Salt is concentrated in pockets between crystals of pure water and then squeezed out of the freezing mass to form a frigid brine. Between 20 million and 50 million cubic meters of this brine form every second. The water’s great density causes it to sink toward the continental shelf, where it mixes with nearly equal parts of water from the southern Antarctic Circumpolar Current. The mixture settles along the edge of Antarctica’s continental shelf, descends along the slope, and spreads along the deep-sea bed, creeping north in slow sheets. Antarctic Bottom Water flows many times as slowly as the water in surface currents: in the Pacific it may take a thousand years to reach the equator. Antarctic Bottom Water also flows into the Atlantic Ocean basin, where it flows north at a faster rate than in the Pacific. Antarctic Bottom Water has been identified as high as 40º N on the Atlantic floor.
A small amount of dense bottom water also forms in the northern polar ocean. Although, the topography of the Arctic Ocean basin prevents most of the bottom water from escaping, with the exception of deep channels formed in the submarine ridges between Scotland, Iceland, and Greenland. These channels allow the cold, dense water formed in the Arctic to flow into the North Atlantic to form North Atlantic Deep Water. North Atlantic Deep Water forms when the relatively warm and salty North Atlantic Ocean cools as cold winds from northern Canada sweep over it. Exposed to the chilled air, water at the latitude of Iceland releases heat, cools from 10°C to 2°C, and sinks. Gulf Stream water that sinks in the north is replaced by warm water flowing clockwise along the U.S. east coast in the North Atlantic gyre.
A 1943 map of the world’s ocean currents.
Knowledge of surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. In the sail-ship era knowledge was even more essential. A good example of this is the Agulhas Current, which long prevented Portuguese sailors from reaching India. Even today, the round-the-world sailing competitors employ surface currents to their benefit. Ocean currents are also very important in the dispersal of many life forms. An example is the life-cycle of the European Eel.
Ocean currents are important in the study of marine debris, and vice versa. These currents also affect temperatures throughout the world. For example, the current that brings warm water up the north Atlantic to northwest Europe stops ice from forming by the shores, which would block ships from entering and exiting ports, the currents have a decisive role in influencing the climate of the regions they flow through. The cold currents that flow from the polar and sub-polar regions, bring in a lot of plankton. Since this is the food of the fish you can find a lot of fish where these currents pass through.
OSCAR: Near-realtime global ocean surface current data set
The ‘OSCAR near-realtime global ocean circulation data set’ is based on NOAA and NASA satellite data (sea level altimetry, surface vector winds, and SST). The data set extends from 1993–present and is available at 1-degree and 1/3-degree resolution. The OSCAR data are continuously updated on an interactive website from which users can create customized graphics and download the data. A section of the website provides validation studies in the form of graphics comparing OSCAR data with moored buoys and global drifters.
OSCAR data are used extensively in climate studies. Monthly maps and anomalies have been published in the monthly Climate Diagnostic Bulletin since 2001, and are used routinely to monitor ENSO and to test prediction models. OSCAR currents are routinely used to evaluate the surface currents in Global Circulation Models (GCMs), for example in NCEP Global Ocean Data Assimilation System (GODAS) and European Centre for Medium-Range Weather Forecasts (ECMWF).
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