Subsurface currents
Encyclopedia
A subsurface current is an oceanic current
that 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.
, 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.
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.
. 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.
, 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.
, 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.
. 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.
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
that 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 currentGravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...
, 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 salinitySalinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
of water, a prime example of this being the Mediterranean
Mediterranean sea (oceanography)
In oceanography, a mediterranean sea is a mostly enclosed sea that has limited exchange of water with outer oceans and where the water circulation is dominated by salinity and temperature differences rather than winds....
/Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
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 substanitally 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 sedimentSediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
. 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 SpiralEkman spiral
The Ekman spiral refers to 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...
, named after Vagn Walfrid Ekman
Vagn Walfrid Ekman
Vagn Walfrid Ekman was a Swedish oceanographer.Born in Stockholm to Fredrik Laurentz Ekman, himself an oceanographer, he became committed to oceanography while studying physics at the University of Uppsala and, in particular, on hearing Vilhelm Bjerknes lecture on fluid dynamics.During the...
, 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
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
, 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 subsidenceSubsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
, 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 MaelstromMaelstrom
A maelstrom is a very powerful whirlpool; a large, swirling body of water. A free vortex, it has considerable downdraft. The power of tidal whirlpools tends to be exaggerated by laymen. There are virtually no stories of large ships ever being sucked into a maelstrom, although smaller craft are in...
. 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.