Ocean current - Wikipedia
An ocean current is a continuous, directed movement of sea water generated by a number Surface currents make up only 8% of all water in the ocean, are generally restricted to the upper m (1, ft) of ocean water, and are See also . one-dimensional Saint-Venant equations · shallow water equations. A powerful, warm, surface current in the North Atlantic Ocean , east of of water per second, an amount greater than that carried by all of the world's rivers combined. North America's oldest city, St. Augustine, sits on the coast of eastern Florida . Where the warm surface waters of the Gulf Stream meet the cold winds. We aim to provide continuous water supply to our customers, however on You may also like to speak to your plumber about options to meet your needs.
Ekman convergences have the effect of accumulating less dense surface water. This water floats above the surrounding water, forming a hill in sea level and driving an anticyclonic geostrophic current that extends well below the Ekman layer.
Divergences do the opposite: This induces a depression in sea level with a cyclonic geostrophic current. The ocean current pattern produced by the wind-induced Ekman transport is called the Sverdrup transportafter the Norwegian oceanographer H.
Sverdrupwho formulated the basic theory in Several years later the American geophysicist and oceanographer Walter H. Two types of ocean circulation Ocean circulation derives its energy at the sea surface from two sources that define two circulation types: These two circulation types are not fully independent, since the sea-air buoyancy and momentum exchange are dependent on wind speed.
The wind-driven circulation is the more vigorous of the two and is configured as gyres that dominate an ocean region. The wind-driven circulation is strongest in the surface layer. The thermohaline circulation is more sluggish, with a typical speed of 1 cm 0. Wind-driven circulation Wind stress induces a circulation pattern that is similar for each ocean. In each case, the wind-driven circulation is divided into gyres that stretch across the entire ocean: The depth penetration of the wind-driven currents depends on the intensity of ocean stratification: Near the thermal equator, where the warmest surface water is found, there occurs the eastward-flowing Equatorial Counter Current.
At the geographic Equator a jetlike current is found just below the sea surface, flowing toward the east counter to the surface current. This is called the Equatorial Undercurrent.
It attains speeds of more than 1 metre per second at a depth of nearly metres. It is driven by higher sea level in the western margins of the tropical ocean, producing a pressure gradient, which in the absence of a horizontal Coriolis force drives a west-to-east current along the Equator. The wind field reverses the flow within the surface layer, inducing the South Equatorial Current. Equatorial circulation undergoes variations following the irregular periods of roughly three to eight years of the Southern Oscillation i.
Weakening of the east-to-west wind during a phase of the Southern Oscillation allows warm water in the western margin to slip back to the east by increasing the flow of the Equatorial Counter Current. Surface water temperatures and sea level decrease in the west and increase in the east. In the tropical Indian Ocean the strong seasonal winds of the monsoons induce a similarly strong seasonal circulation pattern.
The subtropical gyres The subtropical gyres are anticyclonic circulation features.
The centre of the subtropical gyre is shifted to the west. This westward intensification of ocean currents was explained by the American meteorologist and oceanographer Henry M.
Stommel as resulting from the fact that the horizontal Coriolis force increases with latitude. This causes the poleward-flowing western boundary current to be a jetlike current that attains speeds of 2 to 4 metres 6.
This current transports the excess heat of the low latitudes to higher latitudes. The flow within the equatorward-flowing interior and eastern boundary of the subtropical gyres is quite different. It is more of a slow drift of cooler water that rarely exceeds 10 cm about 4 inches per second. Associated with these currents is coastal upwelling that results from offshore Ekman transport.
It carries about 30 million cubic metres 1 billion cubic feet of ocean water per second through the Straits of Florida and roughly 80 million cubic metres 2. Responding to the large-scale wind field over the North Atlantic, the Gulf Stream separates from the continental margin at Cape Hatteras.
After separation it forms waves or meanders that eventually generate many eddies of warm and cold water. The warm eddies, composed of thermocline water normally found south of the Gulf Stream, are injected into the waters of the continental slope off the coast of the northeastern United States. They drift to the southwest at rates of approximately 5 to 8 cm about 2 to 3 inches per second, and after a year they rejoin the Gulf Stream north of Cape Hatteras.
Cold eddies of slope water are injected into the region south of the Gulf Stream and drift to the southwest. After roughly two years they reenter the Gulf Stream just north of the Antilles islands.
The path that they follow defines a clockwise-flowing recirculation gyre seaward of the Gulf Stream. Among the other western boundary currents, the Kuroshio of the North Pacific is perhaps the most like the Gulf Stream, having a similar transport and array of eddies.
The Agulhas Current has a transport close to that of the Gulf Stream. It remains in contact with the margin of Africa around the southern rim of the continent. It then separates from the margin and curls back to the Indian Ocean in what is called the Agulhas Retroflection. Not all the water carried by the Agulhas Current returns to the east; about 10 to 20 percent is injected into the South Atlantic Ocean as large eddies that slowly migrate across it.
The subpolar gyres The subpolar gyres are cyclonic circulation features.
The Florida Current
The Ekman transport within these features forces upwelling and surface water divergence. In the North Atlantic the subpolar gyre consists of the North Atlantic Current at its equatorward side and the Norwegian Current that carries relatively warm water northward along the coast of Norway.
The heat released from the Norwegian Current into the atmosphere maintains a moderate climate in northern Europe. Along the east coast of Greenland is the southward-flowing cold East Greenland Current. It loops around the southern tip of Greenland and continues flowing into the Labrador Sea.
The southward flow that continues off the coast of Canada is called the Labrador Current. This current separates for the most part from the coast near Newfoundland to complete the subpolar gyre of the North Atlantic. Some of the cold water of the Labrador Current, however, extends farther south.
In the North Pacific the subpolar gyre is composed of the northward-flowing Alaska Currentthe Aleutian Currentand the southward-flowing cold Oyashio Current. The North Pacific Current forms the separation between the subpolar and subtropical gyres of the North Pacific.
In the Southern Hemisphere the subpolar gyres are less defined. Large cyclonic flowing gyres lie poleward of the Antarctic Circumpolar Current and can be considered counterparts to the Northern Hemispheric subpolar gyres. The Antarctic coastal current flows toward the west. The northward-flowing current off the east coast of the Antarctic Peninsula carries cold Antarctic coastal water into the circumpolar belt.
Another cyclonic gyre occurs north of the Ross Sea. In this belt flows the Antarctic Circumpolar Current from west to east, encircling the globe at high latitudes.
It transports million cubic metres 4. The Antarctic Circumpolar Current is not a well-defined single-axis current but rather consists of a series of individual currents separated by frontal zones. It reaches the seafloor and is guided along its course by the irregular bottom topography. Large meanders and eddies develop in the current as it flows. These features induce poleward transfer of heat, which may be significant in balancing the oceanic heat loss to the atmosphere above the Antarctic region farther south.
Thermohaline circulation The general circulation of the oceans consists primarily of the wind-driven currents. These, however, are superimposed on the much more sluggish circulation driven by horizontal differences in temperature and salinity—namely, the thermohaline circulation. The thermohaline circulation reaches down to the seafloor and is often referred to as the deep, or abyssal, ocean circulation. Measuring seawater temperature and salinity distribution is the chief method of studying the deep-flow patterns.
Other properties also are examined; for example, the concentrations of oxygencarbon, and such synthetically produced compounds as chlorofluorocarbons are measured to obtain resident times and spreading rates of deep water. Thermohaline circulation transports and mixes the water of the oceans. In the process it transports heat, which influences regional climate patterns.
The density of seawater is determined by the temperature and salinity of a volume of seawater at a particular location. The difference in density between one location and another drives the thermohaline circulation.
In some areas of the ocean, generally during the winter seasoncooling or net evaporation causes surface water to become dense enough to sink. Convection penetrates to a level where the density of the sinking water matches that of the surrounding water. It then spreads slowly into the rest of the ocean. Other water must replace the surface water that sinks.
This sets up the thermohaline circulation.
The basic thermohaline circulation is one of sinking of cold water in the polar regions, chiefly in the northern North Atlantic and near Antarctica. These dense water masses spread into the full extent of the ocean and gradually upwell to feed a slow return flow to the sinking regions.
A theory for the thermohaline circulation pattern was proposed by Stommel and Arnold Arons in In the Northern Hemisphere the primary region of deep water formation is the North Atlantic; minor amounts of deep water are formed in the Red Sea and Persian Gulf.
A variety of water types contribute to the so-called North Atlantic Deep Water. North Atlantic Deep Water is primarily formed in the Greenland and Norwegian seaswhere cooling of the salty water introduced by the Norwegian Current induces sinking.
This water spills over the rim of the ridge that stretches from Greenland to Scotland, extending to the seafloor to the south as a convective plume. It then flows southward, pressed against the western edge of the North Atlantic. Additional deep water is formed in the Labrador Sea. This water, somewhat less dense than the overflow water from the Greenland and Norwegian seas, has been observed sinking to a depth of 3, metres about 9, feet within convective features referred to as chimneys.
Vertical velocities as high as 10 cm per second have been observed within these convective features. This draws surface water into the Mediterranean through the Strait of Gibraltar. The mass of salty water formed within the Mediterranean exits as a deeper stream.
It descends to depths of approximately 1, metres in the North Atlantic Ocean, forming the uppermost layer of North Atlantic Deep Water. The outflow in the Strait of Gibraltar reaches as high as 2 metres per second, but its total transport amounts to only 5 percent of the total North Atlantic Deep Water formed. The outflow of the Mediterranean plays a significant role in boosting the salinity of North Atlantic Deep Water.
The blend of North Atlantic Deep Water, with a total formation rate of 15 to 20 million cubic metres to million cubic feet per second, quickly ventilates the Atlantic Ocean, resulting in a residence time of less than years. The deep water spreads away from its source along the western side of the Atlantic Ocean and, on reaching the Antarctic Circumpolar Current, spreads into the Indian and Pacific oceans. The sinking of North Atlantic Deep Water is compensated for by the slow upwelling of deep water, mainly in the Southern Oceanto replenish the upper stratum of water that has descended as North Atlantic Deep Water.
North Atlantic Deep Water exported to the other oceans must be balanced by the inflow of upper-layer water into the Atlantic. Some water returns as cold, low-salinity Pacific water through the Drake Passage in the form of what is known as Antarctic Intermediate Waterand some returns as warm salty thermocline water from the Indian Ocean around the southern rim of Africa.
Here it upwells to a level of 2,—3, metres about 6,—9, feet and returns to the south lower in salinity and oxygen but higher in nutrient concentrations as North Pacific Deep Water. Modification of deep water in the North Pacific is the direct consequence of vertical mixingwhich carries into the deep ocean the low salinity properties of North Pacific Intermediate Water. Click here for more information.
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