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. 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
than the tropical latitudes in which the area is located, due to the effect of the Humboldt
Current.
Function
Surface ocean currents are generally wind driven and develop their typical clockwise
spirals in the northern hemisphere 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 are a consequence of the rotation of the Earth.
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, 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 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, where 1 sv is equivalent to a volume flow rate of
1,000,000 m3 per second.
Surface currents Thermohaline circulation
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. 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
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. 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, and a density of 1.0279 grams per
cubic centimeter. This water is noted for its extreme density, 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.
Importance
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. 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, for example in NCEP Global Ocean Data
Assimilation System and European Centre for Medium-Range Weather Forecasts.
See also
Deep ocean water Currentology
Thermohaline circulation Fish migration
List of ocean circulation models Oceanic gyres
Physical oceanography Marine current power
Latitude of the Gulf Stream and the Gulf Stream north wall index
References ^ OSCAR website
Hansen, B.; Østerhus, S; Quadfasel, D; Turrell, W. "Already the day after tomorrow?". Science
305: 953–954. doi:10.1126/science.1100085. PMID 15310882.
Kerr, Richard A.. "A slowing cog in the North Atlantic ocean's climate machine". Science
304: 371–372. doi:10.1126/science.304.5669.371a. PMID 15087513.
Munday, Phillip L.; Jones, Geoffrey P.; Pratchett, Morgan S.; Williams, Ashley J.. "Climate change
and the future for coral reef fishes". Fish and Fisheries 9: 261–285. doi:10.1111/j.1467-2979.2008.00281.x.
Rahmstorf, S.. "Thermohaline circulation: The current climate". Nature 421: 699–699.
doi:10.1038/421699a. PMID 12610602. Roemmich, D.. "Physical oceanography: Super
spin in the southern seas". Nature 449: 34–35. doi:10.1038/449034a. PMID 17805284.
External links NOAA Ocean Surface Current Analyses - Realtime
Near-realtime Global Ocean Surface Currents derived from satellite altimeter and scatterometer
data. RSMAS Ocean Surface Currents
Coastal Ocean Current Monitoring Program Ocean Motion and Surface Currents
Data Visualizer from OceanMotion.org Changes in Ocean Circulation - Cluster of
Excellence "Future Ocean", Kiel