OCEAN CURRENT

horizontal and vertical circulation system of ocean waters produced by gravity, wind friction, and water density variation in different parts of the ocean. The direction and form of oceanic currents is governed by a number of natural forces, including principally horizontal pressure gradient forces; forces generated by variable density of seawater, which is a product of temperature and salinity variables; the Coriolis forces, exerted by the rotating Earth on all moving objects at or near the Earth's surface; and friction, caused by winds blowing over the ocean's surface as well as the friction between different layers of water. The Coriolis forces cause ocean currents to move clockwise (anticyclonically) in the Northern Hemisphere and counterclockwise (cyclonically) in the Southern Hemisphere and deflect them about 45 from the wind direction. This movement creates distinctive current cells called gyres. The rotational pattern causes the anticyclonic gyres to displace their centres westward, forming strong western boundary currents against the eastern coasts of the continents, such as the Gulf StreamNorth AtlanticNorway Current in the Atlantic Ocean and the KuroshioNorth Pacific Current in the Pacific Ocean. In the Southern Hemisphere the counterclockwise circulation creates strong eastern boundary currents against the western coasts of continents, such as the Peru (Humboldt) Current off South America, the Benguela Current off western Africa, and the Western Australia Current. The Southern Hemisphere currents are influenced by the powerful eastward flowing, circumpolar Antarctic Current. It is a very deep, cold, and relatively slow-flowing current, but it carries a vast mass of water, about twice the volume of the Gulf Stream current. The Peru and Benguela currents draw water from this Antarctic current and, hence, are cold. The Northern Hemisphere lacks continuous open water bordering the Arctic and so has no corresponding powerful circumpolar current, but there are small, cold currents flowing south through the Bering Strait to form the Oya and Anadyr currents off eastern Russia and the California Current off western North America; others flow south around Greenland to form the cold Labrador and East Greenland currents. The KuroshioNorth Pacific and Gulf StreamNorth AtlanticNorway currents move warmer water into the Arctic Ocean via the Bering, Cape, and West Spitsbergen currents. In the tropics the great clockwise and counterclockwise gyres flow westward as the Pacific North and South Equatorial currents, Atlantic North and South Equatorial currents, and the Indian South equatorial current. Because of the alternating monsoon climate of the northern Indian Ocean, the current in the northern Indian Ocean and Arabian Sea alternates. Between these massive currents are narrow eastward flowing countercurrents. Vertical oceanic circulation is far less dramatic but is important because it brings up deep ocean waters and moves down surface waters. The wind-driven currents are confined to the Ekman Layer, the upper 100 m (330 ft) of the ocean; below this the deep currents are much slower. They are generated by the convection pattern caused by the surface currents. Where the surface currents converge because of meeting coastlines or encountering winds from the opposite direction, water tends to pile up but is also pulled down by gravity. Where waters diverge, the sea surface loses water, and deep ocean waters well up to the surface to replace the diverging waters. This pattern generates the convection that powers the undercurrents. Vertical circulation also occurs because of variations in salinity, as saline water is denser than less saline water, and because warm water tends to rise and cold water tends to sink. Saline input comes from the Mediterranean Sea and to a lesser extent from the Red Sea and Persian Gulf. These seas lie in regions where evaporation exceeds precipitation and inflow. The more highly saline water produced by the net evaporation sinks in the eastern Mediterranean and flows in a deep bottom current, termed the Upper Deep Water, through the Straits of Gibraltar westward into the Atlantic Ocean, while at the surface, less saline Atlantic water flows eastward into the Mediterranean. A similar principle operates in the Red Sea and Persian Gulf. The Pacific Ocean lacks a similar input but acquires its salinity from the Upper Deep Water, which enters the Antarctic circumpolar current and thereby gradually reaches the Pacific Ocean. Other deep currents are generated by the cold Greenland and Bering Strait currents, which sink to form the Middle and Lower Deep water masses; in the Antarctic, very cold water from near the ice cap sinks to form the Antarctic Intermediate Water, which spreads northward at depths of 7001,000 m. These Arctic and Antarctic currents are very cold and much less saline because of the input of melting fresh ice-shelf and glacial water. Besides the large oceanic gyres, there are smaller current systems found in certain enclosed seas or ocean areas. Their circulation patterns are influenced more by the direction of water inflow than by the Coriolis forces. Such currents are found in the Tasmanian Sea, where the southward flowing East Australian Current generates counterclockwise circulation, in the northwestern Pacific, where the eastward flowing KuroshioNorth Pacific current causes counterclockwise circulation in the Alaska and Subarctic currents, in the Bay of Bengal, and in the Arabian Sea. The study of oceanic currents developed as oceanic voyages became common in the 18th century, in the interests of navigation. Subsequent studies occurred because ocean currents were found to profoundly affect weather and climate. Thus, the Gulf StreamNorth AtlanticNorway Current brings warm tropical waters northward, warming the climates of eastern North America, the British Isles and Ireland, and the Atlantic coast of Norway in winter, and the KuroshioNorth Pacific Current does the same for Japan and western North America, where warmer winter climates also occur. The warmer waters also evaporate more readily in the warmer temperatures they help generate, increasing precipitation along these coasts. In the Southern Hemisphere, by contrast, the cold Peru and Benguela currents hinder evaporation and, as they flow along the warmer coasts of South America and southwest Africa, generate fogs but no precipitation, thereby generating the hyper-arid deserts of Peru, Chile, and Namibia; yet as these cold currents also well up from the deep ocean, they are rich in nutrients, and some of the world's best fishing grounds are found in them. Ocean currents and atmospheric circulation influence each other. For example, when periodically the warm, humid wind circulation above the western Pacific is displaced eastward, the eastern Pacific waters are warmed, creating the El Nio effect, which widely affects climate and weather, bringing drought to Australia, storms to California, and a warm winter to central North America and upsetting the fishing industry in Peru and Chile. Density currents in the oceans General observations Density currents are currents that are kept in motion by the force of gravity acting on a relatively small density difference caused by variations in salinity, temperature, or sediment concentration. As noted above, salinity and temperature variations produce stratification in oceans. Below the surface layer, which is disturbed by waves and is lighter than the deeper waters because it is warmer or less saline, the oceans are composed of layers of water that have distinctive chemical and physical characteristics, which move more or less independently of each other and which do not lose their individuality by mixing even after they have flowed for hundreds of kilometres from their point of origin. An example of this type of density current, or stratified flow, is provided by the water of the Mediterranean Sea as it flows through the Strait of Gibraltar out into the Atlantic. Because the Mediterranean is enclosed in a basin that is relatively small compared with the ocean basins and because it is located in a relatively arid climate, evaporation exceeds the supply of fresh water from rivers. The result is that the Mediterranean contains water that is both warmer and more saline than normal deep-sea water, the temperature ranging from 12.7 to 14.5 C and the salinity from 38.4 to 39.0 parts per thousand. Because of these characteristics, the Mediterranean water is considerably denser than the water in the upper parts of the North Atlantic, which has a salinity of about 36 parts per thousand and a temperature of about 13 C. The density contrast causes the lighter Atlantic water to flow into the Mediterranean in the upper part of the Strait of Gibraltar (down to a depth of about 200 metres) and the denser Mediterranean water to flow out into the Atlantic in the lower part of the strait (from about 200 metres to the top of the sill separating the Mediterranean from the Atlantic at a depth of 320 metres). Because the strait is only about 20 kilometres wide, both inflow and outflow achieve relatively high speeds. Near the surface the inflow may have speeds as high as two metres per second, and the outflow reaches speeds of more than one metre per second at a depth of about 275 metres. One result of the high current speeds in the strait is that there is a considerable amount of mixing, which reduces the salinity of the outflowing Mediterranean water to about 37 parts per thousand. The outflowing water sinks to a depth of about 1,500 metres or more, where it encounters colder, denser Atlantic water. It then spreads out as a layer of more saline water between two Atlantic water masses. Turbidity currents Density currents caused by suspended sediment concentrations in the oceans are called turbidity currents. They appear to be relatively short-lived, transient phenomena that occur at great depths. Turbidity currents are thought to be caused by the slumping of sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons (see below Continental margins: Submarine canyons). Slumping of large masses of sediment creates a dense sediment-water mixture, or slurry, which then flows down the canyon to spread out over the ocean floor and deposit a layer of sand in deep water. Repeated deposition forms submarine fans, which are analogous to the alluvial fans found at the mouths of many river canyons. Sedimentary rocks that are thought to have originated from ancient turbidity currents are called turbidites. Although large-scale turbidity underflows have never been directly observed in the oceans, there is much evidence supporting their occurrence. This evidence may be briefly summarized: (1) Telegraph cables have been broken in the deep ocean in a sequence that indicates some disturbance at the bottom moving from shallow to deep water at speeds on the order of 20 to 75 kilometres per hour, or 10 to 40 knots. The trigger for this phenomenon is commonly, though not exclusively, an earthquake near the edge of the continental slope. The only disturbance that seems capable of being transmitted downslope at the required speed is a large turbidity current. The best-known example of such a series of cable breaks took place in the North Atlantic following the 1929 earthquake under the Grand Banks of Newfoundland, but other examples have been described from the Magdalena River delta (Colombia), the Congo delta, the Mediterranean Sea north of Orlansville and south of the Straits of Messina, and Kandavu Passage, Fiji. (2) Cores taken from the ocean bottom in the area downslope from cable breaks reveal layers of sand interbedded with normal deep-sea pelagic or hemipelagic oozes (sediments formed in the deep sea by quiet settling of fine particles). In the case of the cable breaks south of the Grand Banks, a large-diameter core taken from the axis of a submarine canyon in the continental slope contained 1 centimetre of gray clay underlain by at least 20 centimetres of gray pebble and cobble gravel. Cores farther south showed a graded layer about one metre thick of coarse silt and fine sand. The presence of these gravel and sand layers is consistent with the hypothesis that they were deposited by the turbidity current that broke the cables. (3) Coring has revealed layers of fine-grained sand or coarse silt at many other localities in the abyssal plains of the oceans. These layers are generally moderately well sorted and contain microfossils characteristic of shallow water that are also size-sorted. In some cases the layers are laminated and arranged in a definite sequence. It is clear that the sand forming these layers has been moved down from shallow water, and in many cases the only plausible mechanism appears to be a turbidity current. (4) At the base of many submarine canyons there occur very large submarine fans. Deep-sea channels on the fan surfaces extend for many tens of kilometres and have depths of more than 100 metres and widths of one kilometre or more. Submarine levees are a prominent feature, and these project above the surrounding fan surface to elevations of 50 metres or higher. The gross characteristics of such channels suggest that they were formed by a combination of erosion and deposition by turbidity currents. (5) Thick deposits of interbedded graded sandstones and fine-grained shales are common in the geologic record. In some cases there is good fossil evidence that the shales were deposited in relatively deep water, perhaps as much as several thousand metres deep. Relatively deepwater deposition is also suggested by the absence of sedimentary structures characteristic of shallow water. The interbedded sandstones, however, contain shallow-water fossils that are sorted by size, have a sharp basal contact with the shale below and a transitional contact with the shale above, and display a characteristic sequence of sedimentary structures. The structures include erosional marks made originally on the mud surface but now preserved as casts on the base of the sandstone bed (sole marks) and internal structures including some or all of the following: massive graded unit, parallel lamination, ripple cross-lamination or convolute lamination, and an upper unit of parallel lamination. This combination of textural and structural features can be explained by deposition from a current that slightly erodes the bottom and then deposits sand that becomes finer grained as the velocity gradually wanes. The properties inferred from these ancient sandstone deposits are consistent with the properties of turbidity currents inferred from laboratory experiments. In spite of the convincing nature of the evidence, there are still some objections to the turbidity current hypothesis. Most geologists and oceanographers accept that such currents exist and that the currents are important agents of erosion and sediment deposition, in both modern and ancient seas, but researchers believe that the turbidity current hypothesis has been overworked. There is evidence, for example, which suggests that currents flowing parallel to submarine contours exist in many ocean basins. These bottom currents have been observed in a few cases, and velocities as high as 20 to 50 centimetres per second have been recorded. These currents can produce some of the features that previously had been attributed to turbidity current action. Moreover, nearly all features of sands that are produced by turbidity currents can be formed by shallow-water action, such as fluvial processes. Hence the problem of discriminating between deposits formed by turbidity currents and deposits formed by other current types is quite complex and requires a careful assessment of all lines of evidence in each case. Some ancient sandstones have been interpreted as fluxoturbidites because the sedimentary structures and other properties suggest a transporting agent intermediate between turbidity currents and large-scale slumping and sliding of sediment. Gerard V. Middleton The Editors of the Encyclopdia Britannica Economic aspects of the oceans The sea is generally accepted by scientists as the place where life began on Earth. Without the sea, life as it is known today could not exist. Among other functions, it acts as a great heat reservoir, leveling the temperature extremes that would otherwise prevail over the Earth and expand the desert areas. The oceans provide the least expensive form of transportation known, and the coasts serve as a major recreational site. More importantly, the sea is a valuable source of food and a potentially important source of energy and minerals, all of which are required in ever-increasing quantities by industrialized and developing nations alike. Medium for transportation and communications From the beginning of recorded history, people have used the sea as a means of transporting themselves and their goods. The bulk of the tonnage of products transported throughout the world today continues to be moved in ocean vessels. The size of these vessels ranges from small boats capable of carrying a few tons to bulk carriers (e.g., supertankers) capable of transporting more than 500,000 tons of oil. The cost of transporting goods on the ocean depends on the product, the form of shipment, and the type of vessel. As the per capita consumption of materials increases, the outlook for marine transportation is one of ever-increasing tonnages and size of carrying vessels. Since the laying of the transatlantic cable in the 19th century, the oceans have served as a major means of communication between continents and islands. Hundreds of seafloor cables connect many large centres of world population. With the development of satellite communications, seafloor cables as a means of communication have decreased somewhat in importance, but they will continue to carry information for many decades to come. In addition to communications, cable and pipes laid on the seafloor carry electrical energy, oil, and other commodities in many parts of the world. Impact of ocean-atmosphere interactions on weather and climate Seasonal and interannual ocean-atmosphere interactions General considerations The notion of a connection between the temperature of the surface layers of the oceans and the circulation of the lowest layer of the atmosphere, the troposphere, is a familiar one. The surface mixed layer of the ocean is a huge reservoir of heat when compared to the overlying atmosphere. The heat capacity of an atmospheric column of unit area cross-section extending from the ocean surface to the outermost layers of the atmosphere is equivalent to the heat capacity of a column of seawater of 2.6-metre depth. The surface layer of the oceans is continuously being stirred by the overlying winds and waves, and thus a surface mixed layer is formed that has vertically uniform properties in temperature and salinity. This mixed layer, which is in direct contact with the atmosphere, has a minimum depth of 20 metres in summer and a maximum depth exceeding 100 metres in late winter in the mid-latitudes. In lower latitudes the seasonal variation in the mixed layer is less marked than at higher latitudes, except in regions such as the Arabian Sea where the onset of the southwestern Indian monsoon may produce large changes in the depth of the mixed layer. Temperature anomalies (i.e., deviations from the normal seasonal temperature) in the surface mixed layer have a long residence time compared with those of the overlying turbulent atmosphere. Hence they may persist for a number of consecutive seasons and even for years. Observational studies to investigate the relationship between anomalies in ocean surface temperature and the tropospheric circulation have been undertaken primarily in the Pacific and Atlantic. They have identified large-scale ocean surface temperature anomalies that have similar spatial scales to monthly and seasonal anomalies in atmospheric circulation. The longevity of the ocean surface temperature anomalies, as compared with the shorter dynamical and thermodynamical memory of the atmosphere, has suggested that they may be an important predictor for seasonal and interannual climate anomalies. Link between ocean surface temperature and climate anomalies First, it is useful to consider some examples of the association between anomalies in ocean surface temperature and irregular changes in climate. The Sahel, a region that borders the southern fringe of the Sahara in Africa, experienced a number of devastating droughts during the 1970s and '80s, which can be compared with a much wetter period during the 1950s. Data was obtained that showed the difference in ocean surface temperature during the period from July to September between the driest and wettest rainfall seasons in the Sahel after 1950. Of particular note were the higher-than-normal surface temperatures in the tropical South Atlantic, Indian, and Southeast Pacific oceans and the lower-than-normal temperatures in the North Atlantic and Pacific oceans. This example illustrates that climate anomalies in one region of the world may be linked to ocean surface temperature changes on a global scale. Global atmospheric modeling studies undertaken during the mid-1980s have indicated that the positions of the main rainfall zones in the tropics are sensitive to anomalies in ocean surface temperature. Shorter-lived climate anomalies, on time scales of months to one or two years, also have been related to ocean surface temperature anomalies. The equatorial oceans have the largest influence on these climate anomalies because of the evaporation of water. A relatively small change in ocean surface temperature, say, of 1 C, may result in a large change in the evaporation of water into the atmosphere. The increased water vapour in the lower atmosphere is condensed in regions of upward motion known as convergence zones. This process liberates latent heat of condensation, which in turn provides a major fraction of the energy to drive tropical circulation and is one of the mechanisms responsible for the El Nio/Southern Oscillation phenomenon discussed later in this article. Given the sensitivity of the tropical atmosphere to variations in tropical sea surface temperature, there also has been considerable interest in their influence on extratropical circulation. The sensitivity of the tropospheric circulation to surface temperature in both the tropical Pacific and Atlantic oceans has been shown in theoretical and observational studies alike. Figures were prepared to demonstrate the correlation between the equatorial ocean surface temperature in the east Pacific (the location of El Nio) and the atmospheric circulation in the middle troposphere during winter. The atmospheric pattern was a characteristic circulation type known as the Pacific-North American (PNA) mode. Such patterns are intrinsic modes of the atmosphere, which may be forced by thermal anomalies in the tropical atmosphere and which in their turn are forced by tropical ocean surface temperature anomalies. As noted earlier, enhanced tropical sea surface temperatures increase evaporation into the atmosphere. In the 198283 El Nio event a pattern of circulation anomalies occurred throughout the Northern Hemisphere during winter. These modes of the atmosphere, however, account for much less than 50 percent of the variability of the circulation in mid-latitudes, though in certain regions (northern Japan, southern Canada, and the southern United States), they may have sufficient amplitude for them to be used for predicting seasonal surface temperature perhaps up to two seasons in advance. The response of the atmosphere to mid-latitude ocean surface anomalies has been difficult to detect unambiguously because of the complexity of the turbulent westerly flow between 20 and 60 latitude in both hemispheres. This flow has many properties of nonlinear chaotic systems and thus exhibits behaviour that is difficult to predict beyond a couple of weeks. The atmosphere alone can exhibit large fluctuations on seasonal and longer time scales without any change in external forcing conditions, such as ocean surface temperature. Notwithstanding this inherent problem, some effects of ocean surface temperature anomalies on the atmosphere have been observed and modeled. The influence of the oceans on the atmosphere in the mid-latitudes is greatest during autumn and early winter when the ocean mixed layer releases to the atmosphere the large quantities of heat that it has stored up over the previous summer. Anomalies in ocean surface temperature are indicative of either a surplus or a deficiency of heat available to the atmosphere. The response of the atmosphere to ocean surface temperature, however, is not random geographically. The circulation over the North Atlantic and northern Europe during early winter has been found to be sensitive to large ocean surface temperature anomalies south of Newfoundland. When a warm positive anomaly exists in this region, an anomalous surface anticyclone occurs in the central Atlantic at a similar latitude to the temperature anomaly, and an anomalous cyclonic circulation is located over the North Sea, Scandinavia, and central Europe. With colder than normal water south of Newfoundland, the circulation patterns are reversed, producing cyclonic circulation over the central Atlantic and anticyclonic circulation over Europe. The sensitivity of the atmosphere to ocean surface temperature anomalies in this particular region is thought to be related to the position of the overlying storm tracks and jet stream. The region is the most active in the Northern Hemisphere for the growth of storms associated with very large heat fluxes from the surface layer of the ocean. Another example of a similar type of air-sea interaction event has been documented over the North Pacific Ocean. A statistical seasonal relationship exists between the summer ocean temperature anomaly in the Gulf of Alaska and the atmospheric circulation over the Pacific and North America during the following autumn and winter. The presence of warmer-than-normal ocean surface temperature in the Gulf of Alaska results in increased cyclone development during the subsequent autumn and winter. The relationship has been established by means of monthly sea surface temperature and atmospheric pressure data collected over 30 years in the North Pacific Ocean. The air-sea interaction events in both the North Pacific and North Atlantic oceans discussed above raise questions as to how the anomalies in ocean surface temperature in these areas are initiated, how they are maintained, and whether they yield useful information for atmospheric prediction beyond the normal time scales of weather forecasting (i.e., one to two weeks). Statistical analysis of previous case studies have shown that ocean surface temperature anomalies initially develop in response to anomalous atmospheric forcing. Once developed, however, the temperature anomaly of the ocean surface tends to reinforce and thereby maintain the anomalous atmospheric circulation. The mechanisms thought to be responsible for this behaviour in the ocean are the surface wind drift, wind mixing, and the interchange of heat between the ocean and atmosphere. The question of prediction is therefore difficult to answer, as these events depend on a synchronous and interconnected behaviour between the atmosphere and the surface layer of the ocean, which allows for positive feedback between the two systems.