Of the major units that comprise the world ocean, three—the Atlantic, Indian, and Pacific oceans—extend northward from Antarctica as huge "gulfs" separating the continents. The fourth, the Arctic Ocean, nearly landlocked by Eurasia and North America and nearly circular in outline, caps the north polar region. The Southern Ocean (also called the Antarctic Ocean) is now often considered a fifth, separate ocean, extending from the shores of Antarctica northward to about 60°S. The major oceans are further subdivided into smaller regions loosely called seas, gulfs, or bays. Some of these seas, such as the Sargasso Sea of the North Atlantic Ocean, are only vaguely defined, while others, such as the Mediterranean Sea or the Black Sea, are almost totally surrounded by land areas. Large and totally landlocked saltwater bodies such as the Caspian Sea are actually salt lakes.
The boundaries between oceans are usually designated by the continental land masses bordering them or by ridges in the ocean floor, which also serve as geographic boundaries. Where these features are absent (such as the ill-defined northern boundary of the Antarctic Ocean), the boundary is somewhat arbitrarily fixed by fluctuating zones of opposing currents that act as partial barriers to the mixing of waters between the two adjacent oceans.
The oceans are not uniformly distributed on the face of the earth. Continents and ocean basins tend to be antipodal, or diametrically opposed to one another, i.e., continents are found on the opposite side of the earth from ocean basins. For example, Antarctica is antipodal to the Arctic Ocean; Europe is opposed by the South Pacific Ocean. Furthermore, over two thirds of the earth's land area is found in the Northern Hemisphere, while the oceans comprise over 80% of the Southern Hemisphere.
The world ocean has an area of about 361 million sq km (139,400,000 sq mi), an average depth of about 3,730 m (12,230 ft), and a total volume of about 1,347,000,000 cu km (322,280,000 cu mi). Each cubic mile of seawater weighs approximately 4.7 billion tons and holds 166 million tons of dissolved solids. One of the most unique and intriguing aspects of ocean water is its salinity, or dissolved salt content. The measurement of salinity is essentially the determination of the amount of dissolved salts in 1 kg of ocean water and is expressed in parts per thousand (‰). Ocean salinities commonly range between 33 ‰ to 38 ‰, with an average of about 35 ‰. Thirty-five parts per thousand salinity is equivalent to 3.5% by weight. Six elements (chlorine, sodium, magnesium, sulfur, calcium, and potassium) constitute over 90% of the total salts dissolved in the oceans. Pressure in the ocean waters increases with increasing depth due to the weight of the overlying water. The pressure increases at the rate of 1 atmosphere for every 10 m (33 ft) of depth (1 atm=15 lb per sq in. or 1,016 dynes per sq cm). The average temperature of the oceans is 3.9°C; (39°F;).
It now appears that the waters making up the present oceans (and the gases that make up the present atmosphere) were not of cosmic origin, i.e., were not present in the primordial atmosphere. Instead, they were derived from the interior of the earth sometime in the first one or two billion years after the earth's formation. It is now also generally accepted that a new ocean crust has been forming more or less continuously for at least the past 200 million years through a process of volcanic activity along the midocean ridge system (see seafloor spreading), which consists of a series of underwater mountains. On the basis of present knowledge it seems highly probable that all ocean waters and atmospheric gases were gradually released by the separation of these volatile components from the silicate rocks of the crust and upper mantle through volcanic activity. (Molten lava is known to contain appreciable amounts of water and other volatiles that are released upon solidification.) With the passage of time, water released by volcanic activity gradually filled oceanic depressions.
Virtually all continents are surrounded by a gently sloping submerged plain called the continental shelf, which is an underwater extension of the coastal plain. The continental shelves are the regions of the oceans best known and the most exploited commercially. It is this region where virtually all of the petroleum, commercial sand and gravel deposits, and fishery resources are found. It is also the locus of waste dumping. Changes in sea level have alternatingly exposed and inundated portions of the continental shelf. Continental shelves vary in width from almost zero up to the 1,500-km-wide (930-mi) Siberian shelf in the Arctic Ocean. They average 78 km (48 mi) in width. The edge of the shelf occurs at a depth that ranges from 20 to 550 m (66 to 1,800 ft), averaging 130 m (430 ft). The shelves consist of vast deposits of sands, muds, and gravels, overlying crystalline rocks or vast thicknesses of consolidated sedimentary rocks. Although there is a great variation in shelf features, nonglaciated shelves are usually exceptionally flat, with seaward slopes averaging on the order of 205 m per km (10 ft per mi), or less than 1° of slope. The edge of the shelf, called the shelf break, is marked by an abrupt increase in slope to an average of about 4°.
The continental slopes begin at the shelf break and plunge downward to the great depths of the ocean basin proper. Deep submarine canyons, some comparable in size to the Grand Canyon of the Colorado River, are sometimes found cutting across the shelf and slope, often extending from the mouths of terrestrial rivers. The Congo, Amazon, Ganges, and Hudson rivers all have submarine canyon extensions. It is assumed that submarine canyons on the continental shelf were initially carved during periods of lower sea level in the course of the ice ages. Their continental slope extensions were carved and more recently modified by turbidity currents—subsea "landslides" of a dense slurry of water and sediment.
Many continental slopes end in gently sloping, smooth-surfaced features called continental rises. The continental rises usually have an inclination of less than 1/2°. They have been found to consist of thick deposits of sediment, presumably deposited as a result of slumping and turbidity currents carrying sediment off the shelf and slope. The continental shelf, slope, and rise together are called the continental margin.
One of the most surprising findings of the early oceanographers was that the deepest parts of the oceans were not in the centers, as they had expected, but were in fact quite close to the margins of continents, particularly in the Pacific Ocean. Further exploration showed that these deeps were located in long V-shaped trenches bordering the seaward edge of volcanic island arcs. These trenches are one of the most striking features of the Pacific floor. Trenches virtually encircle the rim of the Pacific basin. The trenches have lengths of thousands of kilometers, are generally hundreds of kilometers wide, and extend 3 to 4 km (1.9-2.5 mi) deeper than the surrounding ocean floor. The greatest ocean depth has been sounded in the Challenger Deep of the Marianas Trench, a distance of 10,911 m (35,798 ft) below sea level.
The deep ocean floor begins at the seaward edge of the continental rise or marginal trench, if one is present, and extends seaward to the base of the underwater midocean mountains. Many relief features of great importance are present in this region. Vast abyssal plains cover significant portions of the deep ocean basin. Such plains are occasionally broken by low, oval-shaped abyssal hills. The abyssal plains cover about 30% of the Atlantic and nearly 75% of the Pacific ocean floors. They are among the flattest portions of the earth's crust and appear to be formed by the deposition of fine sediment carried by turbidity currents that have covered and smoothed out irregularities in the ocean floor.
One of the most significant features of the ocean basins is the midocean ridge. First discovered in the Atlantic Ocean on the Challenger expedition, its relief features were further investigated during the German Meteor expedition of 1925-26. By the early 1960s it had been confirmed that the Mid-Atlantic Ridge was only part of a continuous feature that extended 55,000 km (34,000 mi) through the Atlantic, Indian, South Pacific, and Arctic oceans. The ridge is a broad bulge in the ocean floor that rises 1 to 3 km (0.6-2 mi) above the adjacent abyssal plains. It has a variable width averaging more than 1,500 km (c.900 mi). It is crossed by a number of fracture zones (transform faults) and displays a deep rift 37 to 48 km (23-30 mi) wide and about 1.6 km (1 mi) deep at its very crest.
The atmosphere affects the oceans and is in turn influenced by them. The action of winds blowing over the ocean surface creates waves and the great current systems of the oceans. When winds are strong enough to produce spray and whitecaps, tiny droplets of ocean water are thrown up into the atmosphere where some evaporate, leaving microscopic grains of salt buoyed by the turbulence of the air. These tiny particles may become nuclei for the condensation of water vapor to form fogs and clouds.
In turn, the oceans act upon the atmosphere—in ways not clearly understood—to influence and modify the world's climate and weather systems. When water evaporates, heat is removed from the oceans and stored in the atmosphere by the molecules of water vapor. When condensation occurs, this stored heat is released to the atmosphere to develop the mechanical energy of its motion. The atmosphere obtains nearly half of its energy for circulation from the condensation of evaporated ocean water.
Because the oceans have an extremely high thermal capacity when compared to the atmosphere, the ocean temperatures fluctuate seasonally much less than the atmospheric temperature. For the same reason, when air blows over the water, its temperature tends to come to the temperature of the water rather than vice versa. Thus maritime climates are generally less variable than regions in the interiors of the continents.
The relationships are not simple. The pattern of atmospheric circulation largely determines the pattern of oceanic surface circulation, which in turn determines the location and amount of heat that is released to the atmosphere. Also, the pattern of atmospheric circulation determines in part the location of clouds, which influences the locations of heating of the ocean surface.
The surface circulation of the oceans is intimately tied to the prevailing wind circulation of the atmosphere (see wind). As the planetary winds flow across the water, frictional stresses are set up which push huge rivers of water in their path. The general pattern of these surface currents is a nearly closed system of currents, called gyres, which are approximately centered on the horse latitudes (about 30° latitude in both hemispheres). Major circulation of water in these gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In the North Pacific and North Atlantic oceans, smaller counterclockwise gyres are developed partly due to the presence of the continents. These are centered on about 50°N lat. The most dominant current in the Southern Ocean is the West Wind Drift, which circles Antarctica in an easterly direction. The northern and southern hemispheric gyres are divided by an eastward flowing equatorial countercurrent, which essentially follows the belt of the doldrums. This countercurrent is caused by the return flow of water piled up along the eastward portion of the equatorial seas, and its return flow is uninhibited by the weak and erratic winds of the doldrums. Analysis of current records shows that a number of major currents, such as the Gulf Stream, have strong fast-moving currents beneath them trending in the opposite direction to the surface current. Such undercurrents, or countercurrents, appear to be as important and pervasive as the surface currents. In 1952 the Cromwell current was found flowing eastward beneath the south equatorial current of the Pacific. In 1961 a similar current was discovered in the Atlantic. See also tide.
Thermohaline CirculationThermohaline circulation refers to the deepwater circulation of the oceans and is primarily caused by differences in density between the waters of different regions. It is mainly a convection process where cold, dense water formed in the polar regions sinks and flows slowly toward the equator. Most of the deep water acquires its characteristics in the Antarctic region and in the Norwegian Sea. Antarctic bottom water is the densest and coldest water in the ocean depths. It forms and sinks just off the continental slope of Antarctica and drifts slowly along the bottom as far as the middle North Atlantic Ocean, where it merges with other water. The circulation of ocean waters is vitally important in dispersing heat energy around the globe. In general, heat flows toward the poles in the surface currents, while the displaced cold water flows toward the equator in deeper ocean layers.
The oceans hold the answers to many important questions about the development of the earth and the history of life on earth. For instance, within the rocks and sediment of the ocean floors the geological history of the earth is recorded. Fossils in this sediment record a portion of the biological history of the earth at least back to the Jurassic period, which ended about 140,000,000 years ago. The first appearance of life on the earth is thought to have occurred in the oceans 2 or 3 billion years ago. The modern marine environment is divided into two major realms, the benthic and the pelagic, based upon the ecological characteristics and marine life associated with them. See also marine biology.
The Benthic RealmThe benthic realm refers to the floor of the oceans, extending from the high tide line to the greatest ocean depths. The organisms that live in or on the bottom are called benthos. The benthic realm is subdivided on the basis of depth into the littoral zone, which extends from high tide to a depth of about 200 m (660 ft), and the deep-sea realm. The benthic life forms are both sessile (attached) and motile (mobile). They are distributed from near-shore littoral regions to the ocean depths and play an important role in the food chain. Some benthonic life forms live by predation, others sift organic matter from the water, and others scavenge the bottom for organic debris that has settled there. Benthonic plants can live only in the euphotic zone, the uppermost 100-200 m (330-660 ft) of the ocean, where sunlight penetrates. Benthonic animals that live below the euphotic zone often must depend on the rain of organic debris from above to supply their food needs, and thus the deep regions of the benthic realm are not highly populated except in the areas around hydrothermal vents where chemosynthesis provides an alternative food source.
The Pelagic RealmThe pelagic realm consists of all of the ocean water covering the benthic realm. It is divided horizontally into the neritic, or fertile near-shore, province and the oceanic province. Vertically it is divided into the euphotic, or photic, zone and the aphotic (without sunlight) zone. Drifting, free-floating organisms, called plankton, and organisms with poor mobile ability populate the euphotic zone. Most plankton are microscopic or near-microscopic in size. Phytoplankton are photosynthetic bacteria (cyanbacteria) and floating algae, such as diatoms, dinoflagellates, and coccolithopores. Heterotrophic plankton (zooplankton) are floating animals and protozoans of the sea and rely on the phytoplankton as food sources. Foraminifera and radiolaria are the dominant protozoan zooplankton that secrete tests (shells), which become incorporated into the sediment of the ocean floor. Many juvenile forms of swimmers (such as shrimp) or bottom dwellers (such as barnacles) pass through a planktonic phase. Marine organisms capable of self-locomotion are called nektonic life forms. Fish, squid, and whales are examples of marine nekton.
Throughout history humans have been directly or indirectly influenced by the oceans. Ocean waters serve as a source of food and valuable minerals, as a vast highway for commerce, and provide a place for both recreation and waste disposal. Increasingly, people are turning to the oceans for their food supply either by direct consumption or indirectly by harvesting fish that is then processed for livestock feed. It has been estimated that as much as 10% of human protein intake comes from the oceans. Nevertheless, the food-producing potential of the oceans is only partly realized. Other biological products of the oceans are also commercially used. For example, pearls taken from oysters are used in jewelry, and shells and coral have been widely used as a source of building material.
Ocean water is processed to extract commercially valuable minerals such as salt, bromine, and magnesium. Although nearly 60 valuable chemical elements have been found dissolved in ocean water, most are in such dilute concentrations that commercial extraction is not profitable. In a few arid regions of the world, such as Ascension Island, Kuwait, and Israel, ocean water is desalinated to produce freshwater.
The shallow continental shelves have been exploited as a source of sands and gravels. In addition, extensive deposits of petroleum-bearing sands have been exploited in offshore areas, particularly along the Gulf and California coasts of the United States and in the Persian Gulf. On the deep ocean floor manganese nodules, formed by the precipitation of manganese oxides and other metallic salts around a nucleus of rock or shell, represent a potentially rich and extensive resource. Research is currently being conducted to explore nodule mining and metallic extraction techniques. Ocean water itself could prove to be a limitless source of energy in the event that nuclear fusion reactors are developed, since the oceans contain great quantities of deuterium.
The oceans also have become more important for recreational use, as each year more people are attracted to the sports of swimming, fishing, scuba diving, boating, and waterskiing. Ocean pollution, meantime, has escalated dramatically as those who use the oceans for recreational and commercial purposes, as well as those who live nearby, have disposed of more and more wastes there (see water pollution).
See also R. Carson, The Sea Around Us (1961); J. Bardach, Harvest of the Sea (1968); J. R. Moore, ed., Oceanography (1971); R. Perry, The Unknown Ocean (1972).
The Pacific Ocean extends from the arctic to antarctic regions between North and South America on the east and Asia and Australia on the west. The international date line passes through it. It is connected with the Arctic Ocean by the Bering Strait; with the Atlantic Ocean by the Drake Passage, Straits of Magellan, and the Panama Canal; and with the Indian Ocean by passages in the Malay Archipelago and between Australia and Antarctica. Its maximum length is c.9,000 mi (14,500 km), and its greatest width c.11,000 mi (17,700 km), between the Isthmus of Panama and the Malay Peninsula. The principal arms of the Pacific Ocean are (in the north) the Bering Sea; (in the east) the Gulf of California; (in the south) Ross Sea; and (in the west) the Sea of Okhotsk, the Sea of Japan, and the Yellow, East China, South China, Philippine, Coral, and Tasman seas. Few large rivers drain into the Pacific Ocean; the largest are the Columbia of North America and the Huang He and Chang (Yangtze) of China.
Coastline and IslandsAlong the E Pacific shore, generally, the coast rises abruptly from a deep seafloor to mountain heights on land, and there is a narrow continental shelf. The Asian coast is generally low and indented and is fringed with islands rising from a wide continental shelf. A series of volcanoes, the Circum-Pacific Ring of Fire, rims the Pacific basin.
The approximately 20,000 islands in the Pacific Ocean are concentrated in the south and west. Most of the larger islands are structurally part of the continent and rise from the continental shelf; these include the Japanese island arc, the Malay Archipelago, and the islands of NW North America and SW South America. Scattered around the Pacific and rising from the ocean floor are high volcanic islands (such as the Hawaiian Islands) and low coral islands (such as those of Oceania).
Ocean FloorThe floor of the Pacific Ocean, which has an average depth of c.14,000 ft (4,300 m), is largely a deep-sea plain. The greatest known depth (35,798.6 ft/10,911.5 m) is in the Challenger Deep in the Marianas trench c.250 mi (400 km) SW of Guam. Rising from the plain are swells (many of which are volcanic), seamounts, and guyots; the extensive Albatross Plateau covers most of the SE and E central Pacific basin.
Huge whirls, formed by the major ocean currents, are found roughly north and south of the equator; the Equatorial Counter Current separates them. The northern whirl is formed by the North Equatorial Current, Japan Current, North Pacific Drift, and California Current; the southern whirl is formed by the South Equatorial Current, East Australian Current, West Wind Drift, and Peruvian (or Humboldt) Current. There are many branch and feeder currents that help to constantly circulate ocean water of differing temperatures and salinities.
The principal commercial fishing areas in the Pacific are found in the shallower waters of the continental shelf; salmon, halibut, herring, sardines, and tuna are the chief catch. Most of the transpacific sea-lanes pass through the Hawaiian Islands; the chief Pacific ports are San Francisco, Los Angeles, Seattle, Tokyo-Yokohama, Guangzhou, Hong Kong, Shanghai, Manila, and Sydney. Since the 1950s many of the South Pacific islands have become tourist centers.
The Pacific islands of the south and west were populated by Asian migrants who crossed long distances of open sea in primitive boats. European travelers including Marco Polo had reported an ocean off Asia, and in the late 15th cent. trading ships had sailed around Africa to the western rim of the Pacific, but recognition of the Pacific as distinct from the Atlantic Ocean dates from Balboa's sighting of its eastern shore (1513). Magellan's crossing of the Philippines (1520-21) initiated a series of explorations, including those of Drake, Tasman, Dampier, Cook, Bering, and Vancouver, which by the end of the 18th cent. had disclosed the coastline and the major islands. In the 16th cent. supremacy in the Pacific area was shared by Spain and Portugal. The English and the Dutch established footholds in the 17th cent., France and Russia in the 18th, and Germany, Japan, and the United States in the 19th. Sealers and whalers sailed the Pacific from the late 18th cent., and Yankee clippers entered Pacific trade in the early 19th cent.
See G. Soule, The Greatest Depths (1970); E. S. Dodge, Beyond the Capes (1971); J. Gilbert, Charting the Vast Pacific (1971); V. S. Gorshkov, ed., Pacific Ocean (1976).
The Atlantic Ocean extends in an S shape from the arctic to the antarctic regions between North and South America on the west and Europe and Africa on the east. It is connected with the Arctic Ocean by the Greenland Sea and Smith Sound; with the Pacific Ocean by Drake Passage, the Straits of Magellan, and the Panama Canal; and with the Indian Ocean by the Suez Canal and the expanse between Africa and Antarctica. The shortest distance across the Atlantic Ocean (c.1,600 mi/2,575 km) is between SW Senegal, W Africa, and NE Brazil, E South America. The principal arms of the Atlantic Ocean are Hudson and Baffin bays, the Gulf of Mexico, and the Caribbean Sea in the west; the Baltic, North, Mediterranean, and Black seas in the east; and the Weddell Sea in the south. More large rivers, including the Mississippi, the Congo, and the Amazon, drain into the Atlantic than into any other ocean.
IslandsThe Atlantic has relatively few islands, with the greatest concentration found in the Caribbean region. Most of the islands are structurally part of the continents, such as the British Isles, Falkland Islands, Canary Islands, and Newfoundland. Iceland, the Azores, the islands of Cape Verde, Ascension, the South Sandwich Islands, the West Indies, and Bermuda are exposed tops of submarine ridges. The Bahamas are low coral islands that sit on the Blake Plateau, while the Madeiras are high volcanic islands.
Ocean FloorThe floor of the Atlantic has an average depth of c.12,000 ft (3,660 m). It is separated from that of the Arctic Ocean by a submarine ridge extending from SE Greenland to N Scotland; part of the floor (c.3,000 ft/910 m deep) is known as "telegraph plateau" because of the network of cables laid there. A shallow submarine ridge across the Strait of Gibraltar separates the Mediterranean basin from the Atlantic and limits the exchange of water between the two bodies. The Mid-Atlantic Ridge (c.300-600 mi/480-970 km wide), a submarine mountain range extending c.10,000 mi (16,100 km) from Iceland to near the Antarctic Circle, generally follows the trend of the coastlines of the continents. It rises to an average height of c.10,000 ft (3,050 m), and a few peaks emerge as islands. The ridge, which is the center of volcanic activity and earthquakes, has a great rift that is constantly widening (see seafloor spreading) and filling with molten rock from the earth's interior. As a result the Western Hemisphere and Europe and Africa are moving away from each other. The Mid-Atlantic Ridge divides the floor of the Atlantic Ocean into eastern and western sections that are composed of a series of deep-sea basins (abyssal plains). The greatest depth (c.28,000 ft/8,530 m) is the Milwaukee Deep, in the Puerto Rico Trench, N of Puerto Rico. Scientific knowledge of the ocean floor dates from the Challenger expedition (1872-76).
Because of its shape, the Atlantic may be divided into two basins—North Atlantic Ocean and South Atlantic Ocean—each with a distinct circulation system. The clockwise-moving currents of the North Atlantic (North Equatorial Current, Antilles Current, Gulf Stream, North Atlantic Drift, Canaries Current) and the counterclockwise-moving currents of the South Atlantic (South Equatorial Current, Brazil Current, West Wind Drift, Benguela Current) are separated from each other by the Equatorial Counter Current; the Guinea Current off W Africa is a link between the two systems. At the Grand Banks off Newfoundland heavy fogs form along the front where the warm Gulf Stream meets the cold Labrador Current. The surface waters in the Atlantic's trade wind belts attain the highest salinity known in ocean water.
The North Atlantic Ocean has some of the world's busiest shipping lanes; the northern lanes are patrolled for icebergs. Commerce between the Mediterranean Sea and the NE Atlantic Ocean was initiated by the Carthaginians. From the 7th cent. A.D., Scandinavians navigated the Atlantic; they probably reached North America c.1000. Trade routes along the coast of Africa were opened by Portugal in the 15th cent. and to the Western Hemisphere by Spain after the voyages of Columbus. The Grand Banks have traditionally contained some of the world's best commercial fishing grounds, but by the early 1990s the area had been overfished, and many species were depleted.
See V. H. Cassidy, The Sea Around Them: The Atlantic Ocean, A.D. 1250 (1968); K. F. George, The Atlantic Ocean (1977); K. O. Emery and E. Uchupi, The Geology of the Atlantic Ocean (1984).
Nearly landlocked, the Arctic Ocean is bordered by Greenland, Canada, Alaska, Russia, and Norway. The Bering Strait connects it with the Pacific Ocean and the Greenland Sea is the chief link with the Atlantic Ocean. The principal arms of the Arctic Ocean are the Beaufort, Chukchi, East Siberian, Laptev, Kara, Barents, and Greenland seas. The floor of the Arctic Ocean is divided by three submarine ridges—Alpha Ridge, Lomonosov Ridge, and the Arctic Mid-Oceanic Ridge; other submarine ridges, such as the Faeroe-Icelandic Ridge, act to separate the Arctic Ocean from the Atlantic.
The Arctic Ocean has the widest continental shelf of all the oceans; it extends c.750 mi (1,210 km) seaward from Siberia. From the shelf rise numerous islands, including the Arctic Archipelago, Novaya Zemlya, the New Siberian Islands, and Wrangel Island. The continental shelf encloses a deep oval basin (average depth 12,000 ft/3,658 m) that stretches between Svalbard and Alaska; E of Greenland the ring of the continental shelf is broken by the Greenland Sea. The greatest depth (17,850 ft/5,441 m) in the Arctic Ocean is found just N of the Chukchi Sea. Since the Arctic's connection with the Pacific Ocean is narrow and very shallow, its principal exchange of water is with the Atlantic Ocean through the Greenland Sea. Even there, though surface waters communicate freely and a strong subsurface current brings warm water from the Atlantic into the Arctic basin, exchange of deeper waters is barred by submarine ridges. Thus a near stagnant pool of very cold water is found at the bottom of the Arctic basin.
Because several major rivers in Siberia (Lena, Yenisei, Ob) and Canada (Mackenzie) bring in much water, and because evaporation is only slight, the outflow through the Greenland Sea is important. It creates the cold East Greenland Current, which flows south along the coast of E Greenland. A weaker current goes through Smith Sound and Baffin Bay and is known as the Labrador Current. Another weak current flows out of Bering Strait. The water that does not flow out by the Greenland Sea seems to be deflected by N Greenland and forms the current that gives rise to a circular current in the Arctic basin itself. This circular current causes the relatively light ice of the Siberian seas, which contrasts with the heavy-pressure ice phenomenon off Greenland and Ellesmere Island (in the Arctic Archipelago). The drift of ice southward and westward has been noted and utilized by explorers.
Once called the Frozen Ocean, the Arctic Ocean is covered with ice (2-14 ft/.6-4 m thick) throughout the year in most of its central and western portions. Some of the ice pack remains in the Arctic basin, and some, carried out by the East Greenland Current, melts before going far enough south to reach the regular Atlantic shipping lanes; the icebergs that harass ships are generally brought from the fjords of W Greenland by the Labrador Current. It was long thought that no non-oceanic life could exist in the Arctic; however, despite drifting ice, ice packs, vast ice floes, and winter temperatures to -60°F; (-51°C;), there are hares, polar bears, seals, gulls, and guillemots as far north as 88°.
The cold Arctic currents give the shores of NE North America and NE Asia a much colder climate than the northwest shores of Europe and North America, which are warmed by the North Atlantic Drift and the Japan Current. The Arctic currents are also less saline and lighter than these warmer currents, and therefore the Arctic water is at the surface and the Atlantic current beneath, where they are exchanged in the Greenland Sea.
The Arctic basin was almost wholly unexplored until the Amundsen-Ellsworth flight over it in 1926. Arctic research was stimulated when it was recognized that the shortest air routes between the great cities of the Northern Hemisphere cross the Arctic Ocean. Improved technology has also facilitated research, with the development of aerial and satellite photography and photogrammetry for precise mapping, the sonic echo sounder for measuring ocean depths, and radio to maintain contact with the rest of the world. Detailed knowledge of drifts and ice floes, water depths, and the ocean floor has vastly increased. Soviet polar scientists investigated (1948-49) the Lomonosov Ridge, an undersea mountain range that influences the pattern of ice drift and the circulation and exchange of water in the Arctic Ocean. American scientists in 1959 discovered the existence of a submarine plateau rising 8,100 ft (2,469 m) from the ocean floor. In 1995 the U.S. navy agreed to lend its force of nuclear attack submarines for a series of civilian expeditions to the Arctic.
One fact of great potential importance is now being studied—the Arctic Ocean is warming. Recorded temperatures, glacial regressions, and the appearance of observed species of fish in larger numbers, at higher latitudes, at earlier seasons, and for long periods prove that over the decades a "climatic improvement" has taken place. Similar changes have been reported in sub-Arctic latitudes. Whether the warming is a phase in a cycle or a permanent development cannot yet be said. The warming may be affecting wind patterns above the region, amplifying the depletion of the ozone layer and possibly increasing precipitation. The area of the Arctic Ocean covered by year-round ice has decreased since the late 1970s, and an increased amount of fresh water is entering the ocean from bordering rivers.
For an account of exploration and for bibliography, see Arctic, the.
Any long, narrow, steep-sided depression in the ocean bottom in which maximum oceanic depths (24,000–36,000 ft, or 7,000–11,000 m) occur. The deepest known depression of this kind is the Mariana Trench. Most trenches occur at subduction zones, where one tectonic plate is thrust under another.
Learn more about deep-sea trench with a free trial on Britannica.com.
Continuous, submarine mountain chain extending approximately 50,000 mi (80,000 km) through all the world's oceans, separating them into distinct basins. The main ridge extends down the middle of the Atlantic Ocean, passes between Africa and Antarctica, turns north to the Indian Ocean, then continues between Australia, New Zealand, and Antarctica and across the Pacific basin to the mouth of the Gulf of California. Lateral ridges extend from islands on the axis of the oceanic ridge to coasts of adjacent continents. The oceanic ridge system is the largest feature of the Earth's surface after the continents and the ocean basins themselves; it is explained by the theory of plate tectonics as a boundary between diverging plates where molten rock is brought up from deep beneath the Earth's crust. Seealso subduction zone.
Learn more about oceanic ridge with a free trial on Britannica.com.
Commercially important food fish (Sebastes marinus) of the scorpion fish family (Scorpaenidae), found in the Atlantic along European and North American coasts. It has a large mouth, large eyes, and spines on its head and cheeks. It grows to about 40 in. (1 m) long. Related species include S. owstoni, a food fish of the Orient, and the Norway haddock (S. viviparus) of Europe. Both are red and grow to about 10 in. (25 cm) long.
Learn more about redfish with a free trial on Britannica.com.
Large merchant ship that visits designated ports on a regular schedule, carrying whatever cargo and passengers are available on the date of sailing. The first liners were operated in the North Atlantic, notably by Samuel Cunard of Britain, beginning in 1840. Their heyday lasted from the late 19th to the mid-20th century. Many were extraordinarily luxurious. Among the most famous were Cunarders such as the Mauretania and the Queen Mary; the German Vaterland (later renamed Leviathan), for many years the largest ship afloat; the ill-fated
Learn more about ocean liner with a free trial on Britannica.com.
Major surface currents of the world's oceans. Subsurface currents also move vast amounts of water, elipsis
Learn more about ocean current with a free trial on Britannica.com.
Large, continuous body of salt water. Ocean covers nearly 71percnt of the Earth's surface and is divided into major oceans and smaller seas. The three principal oceans, the Pacific, Atlantic, and Indian, are largely delimited by land and submarine topographic boundaries. All are connected to what is sometimes called the Southern Ocean, the waters encircling Antarctica. Important marginal seas, primarily in the Northern Hemisphere, are partially enclosed by landmasses or island arcs. The largest are the Arctic Ocean and adjacent seas, Caribbean and adjacent waters, Mediterranean, Bering Sea, Sea of Okhotsk, Yellow and China Seas, and Sea of Japan.
Learn more about ocean with a free trial on Britannica.com.
Body of salt water extending from the Antarctic region in the south to the Arctic circle in the north and lying between the continents of Asia and Australia on the west and North and South America on the east. It occupies about one-third of the surface of the earth and is by far the largest of the world's oceans. Its area, excluding adjacent seas, is approximately 63,800,000 sq mi (165,250,000 sq km), twice that of the Atlantic Ocean and more than the whole land area of the globe. Its mean depth is 14,040 ft (4,280 m). The western Pacific is noted for its many peripheral seas.
Learn more about Pacific Ocean with a free trial on Britannica.com.
Body of salt water stretching between Africa in the west, Australia in the east, Asia in the north, and Antarctica in the south. With an area of 28,360,000 sq mi (73,440,000 sq km), it covers approximately one-seventh of the Earth's surface, and it is the smallest of the world's three major oceans (see Atlantic Ocean; Pacific Ocean). Its greatest depth (24,442 ft [7,450 m]) is in the Java Trench. Its chief marginal seas include the Red Sea, Arabian Sea, Persian Gulf, Andaman Sea, Bay of Bengal, and the Great Australian Bight. Its major islands and island groups include Madagascar, Sri Lanka, and the Mascarenes.
Learn more about Indian Ocean with a free trial on Britannica.com.
Ocean separating North and South America from Europe and Africa. The second largest of the world's oceans, the Atlantic has an area of 31,830,000 sq mi (82,440,000 sq km). With its marginal seas, including the Baltic, North, Black, and Mediterranean to the east, and Baffin Bay, Hudson Bay, the Gulf of St. Lawrence, Gulf of Mexico, and Caribbean Sea to the west, it covers some 41,100,000 sq mi (106,450,000 sq km). Including these latter bodies of water, its average depth is 10,925 ft (3,330 m); its maximum depth is 27,493 feet (8,380 m) in the Puerto Rico Trench. Its most powerful current is the Gulf Stream.
Learn more about Atlantic Ocean with a free trial on Britannica.com.
Learn more about Arctic Ocean with a free trial on Britannica.com.
|
|
|
|
|
|
|
|
|
|