CausesOcean dynamics define and describe the motion of water within the oceans. Ocean temperature and motion fields can be separated into three distinct layers: mixed (surface) layer, upper ocean (above the thermocline), and deep ocean. Ocean currents are measured in Sverdrup, sverdrup (sv), where 1 sv is equivalent to a volume flow rate of per second. Surface currents, which make up only 8% of all water in the ocean, are generally restricted to the upper of ocean water, and are separated from lower regions by varying temperatures and which affect the density of the water, which in turn, defines each oceanic region. Because the movement of deep water in ocean basins is caused by density-driven forces and gravity, deep waters sink into deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
Wind driven circulationSurface oceanic currents are driven by wind currents, the large scale prevailing winds drive major persistent ocean currents, and seasonal or occasional winds drive currents of similar persistence to the winds that drive them, and the Coriolis effect plays a major role in their development. The Ekman spiral velocity distribution results in the currents flowing at an angle to the driving winds, and they develop typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere. In addition, the areas of surface ocean currents move somewhat with the seasons; this is most notable in equatorial currents. Deep ocean basins generally have a non-symmetric surface current, in that the eastern equator-ward flowing branch is broad and diffuse whereas the pole-ward flowing western boundary current is relatively narrow.
Thermohaline circulationDeep ocean currents are driven by density and temperature gradients. This thermohaline circulation is also known as the ocean's conveyor belt. These currents, sometimes called submarine rivers, flow deep below the surface of the ocean and are hidden from immediate detection. Where significant vertical movement of ocean currents is observed, this is known as upwelling and downwelling. Deep ocean currents are currently being researched using a fleet of underwater robots called Argo (oceanography), Argo. The thermohaline circulation 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 ''wikt:thermo-, thermo-'' referring to and ' referring to salinity, salt content, factors which together determine the Water (molecule)#Density of saltwater and ice, density of sea water. Wind-driven surface currents (such as the ) travel Polar regions of Earth, polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high s (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwelling, upwells in the Southern Ocean, the oldest waters (with a transit time of around 1000 years) 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. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is imprecisely used to refer to the Zonal and meridional, meridional overturning circulation, ''MOC''. File:Recording Current Meter.jpg, left, 70px, A recording current meter, alt=Device to record ocean currents
DistributionImage:Ocean currents 1943 (borderless)3.png, 250px, A 1943 map of the world's ocean currents Currents of the Arctic Ocean * * * * * * * * * * Currents of the Atlantic Ocean * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Currents of the Indian Ocean * * * * * * * * * * * * * * Currents of the Pacific Ocean * * * * * * * * * * * * * * * * * * * * * * * Currents of the Southern Ocean * * * Ocean gyre, Oceanic gyres * * * * * * * *
Effects on climate and ecologyOcean currents are important in the study of marine debris, and vice versa. These currents also affect temperatures throughout the world. For example, the ocean current that brings warm water up the north Atlantic to northwest Europe also cumulatively and slowly blocks ice from forming along the seashores, which would also block ships from entering and exiting inland waterways and seaports, hence ocean currents play a decisive role in influencing the climates of regions through which they flow. Cold ocean water currents flowing from polar and sub-polar regions bring in a lot of plankton that are crucial to the continued survival of several key sea creature species in marine ecosystems. Since plankton are the food of fish, abundant fish populations often live where these currents prevail. Ocean currents are also very important in the dispersal of many life forms. An example is the Eel life history, life-cycle of the European Eel.
Economic importanceKnowledge of surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. In the wind powered sailing-ship era, knowledge of wind patterns and ocean currents was even more essential. A good example of this is the Agulhas Current (down along eastern Africa), which long prevented sailors from reaching India. In recent times, around-the-world sailing competitors make good use of surface currents to build and maintain speed. Ocean currents can also be used for Marine current power, marine power generation, with areas of Japan, Florida and Hawaii being considered for test projects.
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