 | This article needs additional citations for verification .(December 2007) |
A subsurface current is an oceanic current that runs beneath surface currents. [1] Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, [2] and the Agulhas Undercurrent, [3] the deep thermohaline circulation in the Atlantic, and bottom gravity currents near Antarctica. The forcing mechanisms vary for these different types of subsurface currents.
The most common of these is the density current, epitomized by the Thermohaline current. The density current works on a basic principle: the denser water sinks to the bottom, separating from the less dense water, and causing an opposite reaction from it. There are numerous factors controlling density.
One is the salinity of water, a prime example of this being the Mediterranean/Atlantic exchange. The saltier waters of the Mediterranean sink to the bottom and flow along there, until they reach the ledge between the two bodies of water. At this point, they rush over the ledge into the Atlantic, pushing the less saline surface water into the Mediterranean.
Another factor of density is temperature. Thermohaline (literally meaning heat-salty) currents are very influenced by heat. Cold water from glaciers, icebergs, etc. descends to join the ultra-deep, cold section of the worldwide Thermohaline current. After spending an exceptionally long time in the depths, it eventually heats up, rising to join the higher Thermohaline current section. Because of the temperature and expansiveness of the Thermohaline current, it is substantially slower, taking nearly 1000 years to run its worldwide circuit.
One factor of density is so unique that it warrants its own current type. This is the turbidity current. Turbidity current is caused when the density of water is increased by sediment. This current is the underwater equivalent of a landslide. When sediment increases the density of the water, it falls to the bottom, and then follows the form of the land. In doing so, the sediment inside the current gathers more from the ocean bed, which in turn gathers more, and so on. As a limited amount of sediment can be carried by a certain amount of water, more water must become laded with sediment, until a huge, destructive current is washing down some marine hillside. It is theorized that submarine depths, such as the Marianas Trench have been caused in part by this action. There is one additional effect of turbidity currents: upwelling. All of the water rushing into ocean valleys displaces a significant amount of water. This water literally has nowhere to go but up. The upwelling current goes almost straight up. This spreads the nutrient rich ocean life to the surface, feeding some of the world’s largest fisheries. This current also helps Thermohaline currents return to the surface.
An entirely different class of subsurface current is caused by friction with surface currents and objects. When the wind or some other surface force compels surface currents into motion, some of this is translated into subsurface motion. The Ekman Spiral, named after Vagn Walfrid Ekman, is the standard for this transfer of energy. The Ekman Spiral works as follows: when the surface moves, the subsurface inherits some -but not all- of this motion. Due to the Coriolis Effect, however, the current moves at a 45˚ angle to the right of the first (left in the Southern Hemisphere). The current below is slower yet, and moves at a 45˚ angle to the right. This process continues in the same manner, until, at about 100 meters below the surface, the current is moving in the opposite direction of the surface current.
The final type of subsurface current is subsidence, caused when forces push water against some obstacle (like a rock), causing it to pile up there. The water at the bottom of the pileup flows away from it, causing a subsidence current.
Various subsurface currents conflict at times, causing bizarre wave patterns. One of the most noticeable of these is the Maelstrom. The word is derived from Nordic words meaning to grind and stream. Essentially, the maelstrom is a large, very powerful whirlpool, a large swirling body of water being drawn down and inward toward its center. This is usually the result of tidal currents.
Subsurface currents have a large effect on life on earth. They flow beneath the surface of the water, allowing them to be relatively free of external influence. Thus, they function like clockwork, providing nutrient transportation, water transfer, etc., as well as affecting the ocean floor and submarine processes.
The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.
North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation of the world's oceans involves the flow of warm surface waters from the southern hemisphere into the North Atlantic. Water flowing northward becomes modified through evaporation and mixing with other water masses, leading to increased salinity. When this water reaches the North Atlantic it cools and sinks through convection, due to its decreased temperature and increased salinity resulting in increased density. NADW is the outflow of this thick deep layer, which can be detected by its high salinity, high oxygen content, nutrient minima, high 14C/12C, and chlorofluorocarbons (CFCs).
Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation. The sinking of cold lithosphere at subduction zones is another example of downwelling in plate tectonics.
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll-a.
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.
Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.
The Agulhas Current is the western boundary current of the southwest Indian Ocean. It flows south along the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary current in the world ocean, with an estimated net transport of 70 sverdrups, as western boundary currents at comparable latitudes transport less — Brazil Current, Gulf Stream, Kuroshio.
The Benguela Current is the broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre. The current extends from roughly Cape Point in the south, to the position of the Angola-Benguela front in the north, at around 16°S. The current is driven by the prevailing south easterly trade winds. Inshore of the Benguela Current proper, the south easterly winds drive coastal upwelling, forming the Benguela Upwelling System. The cold, nutrient rich waters that upwell from around 200–300 m (656–984 ft) depth in turn fuel high rates of phytoplankton growth, and sustain the productive Benguela ecosystem.
The Ekman spiral is a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Walfrid Ekman. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen during the Fram expedition (1893–1896) and the effect was first physically explained by Vagn Walfrid Ekman.
The Agulhas Bank is a broad, shallow part of the southern African continental shelf which extends up to 250 km (160 mi) south of Cape Agulhas before falling steeply to the abyssal plain.
The Norwegian Current is one of two dominant arctic inflows of water. It can be traced from near Shetland, north of Scotland, otherwise from the eastern North Sea at depths of up to 100 metres. It finally passes the Opening into the Barents Sea, a large outcrop of the Arctic Ocean. Compared to its partial source the North Atlantic Current it is colder and less salty; the other sources are the less saline North and Baltic seas and the Norwegian fjords and rivers. It is considerably warmer and saltier than the Arctic Ocean, which is freshened by precipitation and ice in and around it. Winter temperatures in the flow are typically between 2 and 5 °C — the co-parent North Atlantic flow, a heat remnant of its Gulf Stream chief contributor, exceeds 6 °C.
Carbonate compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite lags behind the rate of solvation, such that no calcite is preserved. Shells of animals therefore dissolve and carbonate particles may not accumulate in the sediments on the sea floor below this depth. Aragonite compensation depth describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporation or sea ice formation which result in an increase in salinity. The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height. The depth of the atmospheric mixed layer is known as the mixing height. Turbulence typically plays a role in the formation of fluid mixed layers.
The Atlantic meridional overturning circulation (AMOC) is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.
Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman Transport is a component of wind-driven ocean current. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at 90° anticlockwise. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s. Ekman transport has significant impacts on the biogeochemical properties of the world's oceans. This is because it leads to upwelling and downwelling in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished. This can be done by either Ekman suction or Ekman pumping depending on wind patterns.
Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Antarctic Polar Front zone. This convergence zone is normally located between 50°S and 60°S, hence this is where almost all of the AAIW is formed.
The Somali Current is a cold ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean. This current is heavily influenced by the monsoons and is the only major upwelling system that occurs on a western boundary of an ocean. The water that is upwelled by the current merges with another upwelling system, creating one of the most productive ecosystems in the ocean.
A Wind generated current is a flow in a body of water that is generated by wind friction on its surface. Wind can generate surface currents on water bodies of any size. The depth and strength of the current depend on the wind strength and duration, and on friction and viscosity losses, but are limited to about 400 m depth by the mechanism, and to lesser depths where the water is shallower. The direction of flow is influenced by the Coriolis effect, and is offset to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. A wind current can induce secondary water flow in the form of upwelling and downwelling, geostrophic flow, and western boundary currents.
Ring shedding is a phenomenon in ocean currents where circle or ring-shaped eddies separate from the current. The rings are independent water current systems that can persist for several months and occur in most ocean basins. The separated rings can have both warm or cold cores and play a role in the thermohaline circulation, interocean mixing, and nutrient supply for algae and bacteria. The physical processes behind ring shedding are not fully understood yet and are thus an active subject of research.
