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A subsurface ocean current is an
oceanic current 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 contour ...
that runs beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep thermohaline circulation in the Atlantic, and bottom gravity currents near Antarctica. The forcing mechanisms vary for these different types of subsurface currents.


Density current

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.


Salinity

One is the
salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal t ...
of water, a prime example of this being the
Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...
/
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe a ...
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.


Temperature

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.


Turbidity current

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 Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, san ...
. 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.


Ekman Spiral

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 The oceanic, wind driven Ekman spiral is the result of a force balance created by a shear stress force, Coriolis force and the water drag. This force balance gives a resulting current of the water different from the winds. In the ocean, there are ...
, 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.


Subsidence

The final type of subsurface current is
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
, 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.


Wave Patterns

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.


Effect

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.


See also

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Oceanography Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dyna ...


References

{{DEFAULTSORT:Subsurface Currents Ocean currents Oceanography