Atlantic Ocean

The Atlantic Ocean, second largest of the world's oceans, occupies an elongated, S-shaped basin extending in a north-south direction and is divided into the North Atlantic and South Atlantic by EQUATORIAL COUNTERCURRENTS at about 8 deg north latitude. Bounded by North and South America on the west and Europe and Africa on the east, the Atlantic is linked to the Pacific Ocean by the Arctic Ocean on the north and the Drake Passage on the south. An artificial connection between the Atlantic and Pacific is also provided by the Panama Canal. On the east, the dividing line between the Atlantic and the Indian oceans is the 20 deg E meridian. The Atlantic is separated from the Arctic Ocean by a line from Greenland to southernmost Spitsbergen to northern Norway.

Covering approximately 20% of the Earth's surface, the Atlantic Ocean is second only to the Pacific in size. The name is derived from Greek mythology and means "Sea of Atlas." With its adjacent seas it occupies an area of about 106,450,000 sq km (41,100,000 sq mi); without them, it has an area of 82,362,000 sq km (31,800,000 sq mi). The land area that drains into the Atlantic is four times that of either the Pacific or Indian oceans. The volume of the Atlantic Ocean with its adjacent seas is 354,700,000 km(3) (85,093,000 mi(3)) and without them 323,600,000 km(3) (77,632,000 mi(3).

The average depth of the Atlantic, with its adjacent seas, is 3,332 m (10,932 ft); without them it is 3,926 m (12,877 ft). The greatest depth, 8,381 m (27,498 ft), is in the Puerto Rico Trench. The width of the Atlantic varies from 2,848 km (1,769 mi) between Brazil and Liberia to about 4,830 km (3,000 mi) between the United States and northern Africa.

The Atlantic Ocean has irregular coasts indented by numerous bays, gulfs, and seas. These include the Caribbean Sea, Gulf of Mexico, Gulf of St. Lawrence, Hudson Bay, Baffin Bay, Mediterranean Sea, Black Sea, North Sea, Baltic Sea, Barents Sea, Norwegian-Greenland Sea, and Weddell Sea. Another characteristic feature is its relatively small number of islands. These include Svalbard, Greenland, Iceland, the British Isles, the Azores, the Madeira Islands, the Canaries, the Cape Verde Islands, Bermuda, the West Indies Ascension, St. Helena, Tristan da Cunha, the Falkland Islands, and the South Georgia Islands.


The principal feature of the bottom topography of the Atlantic Ocean is a great submarine mountain range called the Mid-Atlantic Ridge. It extends from Iceland in the north to approximately 58 deg south latitude, reaching a maximum width of about 1,600 km (1,000 mi). A great RIFT VALLEY also extends along the ridge over most of its length. The depth of water over the ridge is less than 2,700 m (8,900 ft) in most places, and several mountain peaks rise above the water, forming islands. The South Atlantic Ocean has an additional submarine ridge, the Walvis Ridge.

The Mid-Atlantic Ridge separates the Atlantic Ocean into two large troughs with depths averaging between 3,660 and 5,485 m (12,000 and 18,000 ft). Transverse ridges running between the continents and the Mid-Atlantic Ridge divide the ocean floor into numerous basins. Some of the larger basins are the Guiana, North American, Cape Verde, and Canaries basins in the North Atlantic. The largest South Atlantic basins are the Angola, Cape, Argentina, and Brazil basins.

The deep ocean floor is thought to be fairly flat, although numerous SEAMOUNTS and some guyots exist. Several deeps or trenches are also found on the ocean floor. The Puerto Rico Trench, in the North Atlantic, is the deepest. In the south Atlantic, the South Sandwich Trench, reaches a depth of 8,428 m (27,651 ft). A third major trench, the Romanche Trench, is located near the equator and reaches a depth of about 7,760 m (24,455 ft). The shelves along the margins of the continents constitute about 11% of the bottom topography. In addition, a number of deep channels cut across the continental rise.

Ocean sediments are composed of terrigenous, pelagic, and authigenic material. Terrigenous deposits consist of sand, mud, and rock particles formed by erosion, weathering, and volcanic activity on land and then washed to sea. These materials are largely found on the CONTINENTAL SHELVES and are thickest off the mouths of large rivers or off desert coasts. Pelagic deposits, which contain the remains of organisms that sink to the ocean floor, include red clays and Globigerina, pteropod, and siliceous OOZES. Covering most of the ocean floor and ranging in thickness from 60 m (200 ft) to 3,300 m (10,900 ft), they are thickest in the convergence belts and in the zones of UPWELLING. Authigenic deposits consist of such materials as MANGANESE NODULES. They occur where sedimentation proceeds slowly or where currents sort the deposits.


The salinity of the surface waters in the open ocean ranges from 33 to 37 parts per thousand and varies with latitude and season. Although the minimum salinity values are found just north of the equator, in general the lowest values are in the high latitudes and along coasts where large rivers flow into the ocean. Maximum salinity values occur at about 25 deg north latitude. Surface salinity values are influenced by evaporation, precipitation, river inflow, and melting of sea ice.

Surface water temperatures, which vary with latitude, current systems, and season and reflect the latitudinal distribution of solar energy, range from less than 2 deg to 29 deg C (28 deg to 84 deg F). Maximum temperatures occur north of the equator, and minimum values are found in the polar regions. In the middle latitudes the area of maximum temperature variations, values may vary by 7 deg to 8 deg C (12.6 deg to 14.4 deg F).

The Atlantic Ocean consists of four major water masses. The North and South Atlantic central waters constitute the surface waters. The sub-Antarctic intermediate water extends to depths of 1,000 m (3,300 ft). The North Atlantic deep water reaches depths of as much as 4,000 m (13,200 ft). The Antarctic bottom water occupies ocean basins at depths greater than 4,000 m (13,200 ft).

Water in the North Atlantic circulates in a clockwise direction, whereas water circulation in the South Atlantic is counterclockwise--a reflection of the Coriolis force. The South TIDES in the Atlantic Ocean are semidiurnal; that is, two high tides occur during each 24 lunar hours. The tides are a general wave that moves from south to north. In latitudes above 40 deg north some east-west oscillation occurs.


The climate of the Atlantic Ocean and adjacent land areas is influenced by the temperatures of the surface waters and water currents as well as the winds blowing across the waters. Because of the oceans' great capacity for retaining heat, MARITIME CLIMATES are moderate and free of extreme seasonal variations. Precipitation can be approximated from coastal weather data and air temperature from the water temperatures. The oceans are the major source of the atmospheric moisture that is obtained through evaporation. Climatic zones vary with latitude; the warmest climatic zones stretch across the Atlantic north of the equator. The coldest zones are in the high latitudes, with the coldest regions corresponding to the areas covered by sea ice. Ocean currents contribute to climatic control by transporting warm and cold waters to other regions. Adjacent land areas are affected by the winds that are cooled or warmed when blowing over these currents. The Gulf Stream, for example, warms the atmosphere of the British Isles and northwestern Europe, and the cold water currents contribute to heavy fog off the coast of northeastern Canada (the Grand Banks area) and the northwestern coast of Africa. In general winds tend to transport moisture and warm or cool air over land areas. Hurricanes develop in the southwestern North Atlantic Ocean.


The Atlantic Ocean appears to be the youngest of the world's oceans. Evidence indicates that it did not exist prior to 100 million years ago, when the continents that formed from the breakup of the ancestral supercontinent, Pangaea, were being rafted apart by the process of SEAFLOOR SPREADING. The Atlantic has been extensively explored since the earliest settlements were established along its shores. The Vikings, Portuguese, and Christopher Columbus were the most famous among its early explorers. After Columbus, European exploration rapidly accelerated, and many new trade routes were established. As a result, the Atlantic became and remains the major artery between Europe and the Americas. Numerous scientific explorations have been undertaken, including those by the German Meteor expedition, Columbia University's Lamont Geological Observatory, and the U.S. Navy Hydrographic Office.

The ocean has also contributed significantly to the development and economy of the countries around it. Besides its major transportation and communication routes, the Atlantic offers abundant petroleum deposits in the sedimentary rocks of the continental shelves and the world's richest fishing resources, especially in the waters covering the shelves. The major species of fish caught are cod, haddock, hake, herring, and mackerel. The most productive areas include the GRAND BANKS off Newfoundland, the shelf area off Nova Scotia, Georges Bank off Cape Cod, the Bahama Banks, the waters around Iceland, the Irish Sea, the DOGGER BANK of the North Sea, and the Falkland Banks. Eel, lobster, and whales are also taken in great quantities. All these factors, taken together, tremendously enhance the Atlantic's great commercial value. Because of the threats to the ocean environment presented by OIL SPILLS, plastic debris, and the incineration of toxic wastes at sea, various international treaties exist to reduce some forms of POLLUTION.

Bibliography: Barton, Robert, Atlas of the Sea (1974); Cassidy, Vincent H., The Sea Around Them: The Atlantic Ocean, A.D. 1250 (1968); Emery, K. O., and Uchupi, E., The Geology of the Atlantic Ocean (1984); Fairbridge, Rhodes W., ed., Encyclopedia of Oceanography (1975); George, Kay F., The Atlantic Ocean: History and Oceanography of the Bridge between Two Worlds (1977); Harvey, J. G., The Ocean and Atmosphere: Our Fluid Environment (1976); Huxley, Anthony, ed., Standard Encyclopedia of the World's Oceans and Islands (1962); Klenova, M. V., ed., Oceanographic Research in the Atlantic (1967).

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