Atlantic Ocean is the world’s second largest ocean. Only the Pacific Ocean is bigger. The Atlantic makes up about 26 percent of Earth’s ocean area and 17 percent of its total surface area. The Atlantic Ocean contains many special habitats for marine life. It is a rich source of seafood as well as other valuable resources. The Atlantic has been a major avenue for commerce and an active site for exploration for hundreds of years. The islands and coastal regions of the Atlantic are valuable sites for marine recreation and tourism.
The ancient Romans named the Atlantic after the Atlas Mountains. Those mountains rise at the western end of the Mediterranean Sea and, in ancient times, marked one of the limits of the known world. Atlantic probably referred to the fact that the ocean lies beyond the Atlas range.
Boundaries and size
The Atlantic Ocean is bordered by North and South America on the west and by Europe and Africa on the east. It connects to the Arctic Ocean, the Greenland Sea, and the Norwegian Sea through the Davis Strait. It joins the Southern Ocean between 40° and 60° south latitude.
The Atlantic Ocean is separated into the North Atlantic and the South Atlantic by the equator. The North Atlantic has a number of seas, called marginal seas, opening into it. These include the Caribbean, Labrador, Mediterranean, and North seas and the Gulf of Mexico. Some scientists regard the Arctic Ocean as a marginal sea of the North Atlantic. The South Atlantic has no marginal seas.
The widest part of the Atlantic, between Spain and Mexico, extends about 5,500 miles (8,800 kilometers). The narrowest part spans about 1,800 miles (2,900 kilometers), between Brazil and western Africa. The Atlantic is about 9,000 miles (14,500 kilometers) long from north to south, and its area is about 34 million square miles (88 million square kilometers).
The Atlantic seafloor
The continental margin
is the part of the Atlantic seafloor that borders the continents. It consists of (1) the continental shelf, (2) the continental slope, and (3) the continental rise. The continental shelf slopes gently from the coastline to a depth of about 330 feet (100 meters). The width of the shelf varies from a few miles or kilometers to about 250 miles (400 kilometers). Beyond the continental shelf, the seafloor drops steeply to about 9,800 feet (3,000 meters). This region is called the continental slope. At the base of this slope is the continental rise, a region of sediments that have washed from the continents. From there, the seafloor drops gradually to the abyssal depths of about 13,100 feet (4,000 meters).
At some places along the continental shelf, continental slope, and continental rise, erosion has formed submarine canyons and channels. Some of these features occur off rivers. Others may represent ancient patterns of drainage from the continents.
The ocean’s large islands lie close to the mainland of the continents and are called continental islands. They include Cuba, Great Britain, Ireland, and Newfoundland in the North Atlantic, and the Falkland Islands in the South Atlantic. The marginal seas have many more islands than the ocean itself.
The Mid-Atlantic Ridge,
the most striking feature on the floor of the Atlantic, is a submarine mountain range that extends from north of Iceland to the Southern Ocean. The ridge forms the boundary between the tectonic plates, rigid slabs of Earth’s rocky outer shell that support the continents and make up the seafloor. New seafloor is created in the gap between the plates, called a rift, by a process known as seafloor spreading. In this process, the plates move away from the rift at about 1 inch (2.5 centimeters) per year. Melted rock rises into the rift and hardens to form the new seafloor..
The Mid-Atlantic Ridge is part of a mid-ocean ridge system that circles Earth. This great mountain range is the site of submarine earthquakes, volcanos, and hydrothermal vents. These vents occur when water seeps through cracks in the seafloor and is then heated by underlying volcanic molten rock. The water then rises back to the ocean floor as mineral-rich hot water springs.
The Mid-Atlantic Ridge rises above the ocean surface in several places, forming islands. These islands include Ascension, the Azores, Iceland, and St. Helena. Volcanic islands—that is, those created by the eruption of volcanoes—include Bermuda, the Canary Islands, the Cape Verde islands, Iceland, the Madeira Islands, the South Sandwich Islands, St. Helena, and Tristan da Cunha.
The seafloor slopes gradually away from the Mid-Atlantic Ridge to the abyssal plains, which lie mainly at depths of about 14,000 to 18,000 feet (4,300 to 5,500 meters) below sea level. Trenches form the deepest parts of the Atlantic. A small number of trenches occur near the Caribbean Islands and off the southern tip of South America. The ocean’s deepest point is in the Puerto Rico Trench, 28,232 feet (8,605 meters) below the surface.
Climate
The far northern and southern parts of the Atlantic Ocean have long, cold winters and short, cool summers. In these regions, the winter air may be much colder than the water. The variation between surface and air temperature causes low fog, sometimes called sea smoke, to form. In the Grand Banks and other areas, warm, moist air moves over cold water, producing heavy fog that may be dangerous for ships. Icebergs float in the far north and south, and are hazards to navigation. Near the equator, the climate stays hot all year.
Cyclical events also affect weather patterns and climate. For example, El Niño, a periodic pattern of interaction between the atmosphere and the tropical waters of the Pacific Ocean, affects the formation of hurricanes in the Atlantic (see El Niño). Another cyclical event, the North Atlantic Oscillation, takes place about every 10 years. It may affect temperature, rainfall, drought, and storm conditions in Europe and Africa.
Water temperatures
at the Atlantic’s surface range from about 86 °F (30 °C) at and near the equator in summer to about 28 °F (–2 °C) at and near the boundary with the Southern Ocean in winter. In warm and moderate regions, the temperature may vary little from the surface to a depth of about 330 feet (100 meters). In these regions, the temperature drops off quickly below 330 feet. At a depth of about 3,300 feet (1,000 meters), the temperature is about 41 °F (5 °C) and varies little to the ocean floor. In cold regions, temperatures change little between surface and deep waters.
Winds.
Wind belts in the Atlantic Ocean include the trade winds and the prevailing westerlies. The trade winds occur on each side of the equator. The northeast trades blow steadily from Africa to the Caribbean at 5° to 30° north latitude. The southeast trades blow from Africa toward South America at 5° to 30° south latitude. An area of light and variable winds called the doldrums occurs between the two trade wind belts.
The westerlies extend from 35° to 60° north latitude and 35° to 60° south latitude. The westerlies in the Northern Hemisphere influence weather patterns across North America and the Atlantic to Europe. From July through October, tropical storms develop in the area of 10° to 20° north latitude between Africa and Central America. These storms sometimes intensify into powerful hurricanes that can cause great destruction.
Moving ocean waters
Currents
along the surface of the Atlantic move in enormous circular patterns called gyres. The trade winds and the westerlies drive the gyres, which center in the subtropics. The North Atlantic Gyre is north of the equator, and it circulates clockwise. The South Atlantic Gyre, south of the equator, circulates counterclockwise.
The movement of the trade winds and the westerlies determines the direction of currents in the Atlantic. The trades drive the North and South Equatorial currents from Africa westward. The South Equatorial Current splits at the eastern tip of South America. One portion travels southward, and the other flows into the Caribbean, where it joins the North Equatorial Current. This water enters the Gulf of Mexico, resulting in the Gulf Stream, a strong current that sweeps up the North American continent. The current then flows eastward, becoming the North Atlantic Current. This current splits into several routes carrying waters toward the United Kingdom, Iceland, the Norwegian Sea, and Portugal. The southernmost branch continues southward off the coast of Africa as the Canary Current. The South Atlantic Gyre consists of the South Equatorial Current, the Brazil Current, the South Atlantic Current, and the Benguela Current.
Other major surface currents in the Atlantic include the Labrador Current, which brings cold water down the east side of North America to about Cape Hatteras; the North Equatorial Countercurrent, which flows toward Africa near 5° north latitude; and the Falkland Current, which brings cold waters into the South Atlantic Gyre circulation.
Subsurface currents
of the Atlantic Ocean originate from water forced down from the surface by wintertime cooling, and by inflows of water from other oceans and seas. The layers of water at the lowest depths are called deep waters and bottom waters. The densest and coldest water, Antarctic Bottom Water, is formed near Antarctica and flows below around 13,000 feet (4,000 meters) northward to about 45° north latitude. North Atlantic Deep Water lies above this layer from depths of 3,000 to 13,000 feet (1,000 to 4,000 meters). This deep water is formed by overflows from the Norwegian, Greenland, and Labrador seas. In addition, salty water from the Mediterranean flows out through the Straits of Gibraltar, sinks, and spreads across the Atlantic at depths of 3,000 to 10,000 feet (1,000 to 3,000 meters). Lying above these deep waters is Subantarctic Intermediate Water, which spreads northward from the South Atlantic at a depth of about 2,300 feet (700 meters).
North Atlantic Deep Water flows from its northern origin southward along the western boundary of the Atlantic and across the equator into the South Atlantic and Southern Ocean. The southward flow of this water is balanced by northward flows of upper-layer water above 3,000 feet (1,000 meters) and also by Antarctic Bottom Water.
The warm upper layer of water flows northward, sinks into cold deep water in the north, and flows southward as cold deep water. This movement of water from the surface to the ocean depths makes up an ocean “conveyor belt” known as the Atlantic meridional overturning circulation. The warm water going north and cold water going south causes a northward flow of heat through the Atlantic Ocean. Scientists believe the meridional overturning circulation and northward heat flow play important roles in climate regulation and change.
Tides.
Along most of the Atlantic coast, tides reach their maximum height twice each day. The Atlantic’s—and the world’s—largest difference in high and low tides takes place in eastern Canada. There, in both the Bay of Fundy and Ungava Bay, the daily tidal range can exceed 50 feet (15 meters). The Gulf of Mexico has only one high and low tide each day. The average range between high and low tides in the gulf is only about 2 feet (0.6 meters). Some places in the Gulf have tidal ranges of only a few inches or centimeters. Tides are also small in the Mediterranean.
Ocean life
A wide variety of plant and animal life thrives in the Atlantic Ocean. Plants and plantlike organisms can live only in the sunlit surface waters, to a depth of about 330 feet (100 meters). Animals live throughout the Atlantic.
The surface waters of the continental shelf support one-celled plantlike organisms called phytoplankton. These organisms are a major food for marine life. During winter months, surface waters are mixed by strong winds to greater depths. This process increases nutrient concentrations in the upper waters. When spring begins to warm the surface waters, the ocean experiences the year’s largest growth of phytoplankton. This growth is called the spring bloom.
The hydrothermal vents on the seafloor support vast communities of marine life. Near these vents, bacteria thrive by using mineral substances from the vents as food. The bacteria, in turn, are consumed by other forms of marine life.
Coral communities live only in warm waters. They are common in the Caribbean and in Bermuda.
Whales migrate over long distances between the warm waters of the tropics and the cooler, plankton-rich waters in coastal and high-latitude areas. In the North Atlantic, whales live in the Caribbean and in the waters around Greenland and the Norwegian Sea. In the South Atlantic, they swim along the coasts of Africa and Brazil and in the waters of the Antarctic Ocean.
Ocean resources
The Atlantic Ocean provides about a fourth of the world’s annual catch of fish and shellfish. People also raise fish and shellfish on coastal fish farms using a process called aquaculture.
Petroleum and natural gas are the Atlantic’s most valuable mineral resources. About a fourth of the world’s petroleum comes from sediments in the Atlantic’s continental shelf. About 70 percent of this petroleum comes from marginal seas, especially the Gulf of Mexico and the North Sea. Other valuable mineral products of the Atlantic include gems, sand and gravel, and sulfur.
Pollution
Pollutants enter the ocean from many sources. Rivers and runoff from land bring many wastes to the sea. Many coastal cities discharge treated and untreated wastes into the ocean. Some pollutants are transported through the atmosphere to the ocean. Some wastes enter the ocean from ships. Spills of petroleum and hazardous wastes occur when ships run aground or are damaged by storms. Before the 1980’s, factories and power plants commonly discarded industrial and radioactive wastes into the ocean. Ships also incinerated hazardous wastes at sea.
Phytoplankton thrives on nutrients produced by the decay of organic matter. Thus, organic pollution can lead to a huge increase in the population of phytoplankton off the continental shelf. The phytoplankton eventually decomposes, consuming more oxygen than normal in the process. The increase in the loss of oxygen can make the waters unfit for marine animals. Under some conditions, the populations of certain species (kinds) of phytoplankton can increase rapidly within a few days. These thick growths of phytoplankton, called blooms, can kill fish. Some species produce toxins that are absorbed by the fish and shellfish that eat them. Such contaminated seafood can cause poisoning and paralysis in human beings.
Since the 1970’s, governments have begun to regulate most uses of the ocean for waste disposal. Governments have also taken steps to ban the use of certain pollutants, such as lead, which was once used as an additive in gasoline, and the pesticide DDT.
Exploration
Early explorers.
Phoenician traders began to explore the Atlantic by the 700’s B.C. They probably reached the islands of Great Britain and Ireland and the southern part of the continent of Africa by about 600 B.C. Vikings started to explore the North Atlantic in the A.D. 800’s. During the next 200 years, they colonized Greenland and Iceland and sailed as far as North America.
During the 1300’s and 1400’s, exploration of the Atlantic increased rapidly. Many merchants sought new routes from Europe to the Spice Islands, a group of Indonesian islands in the East Indies. People learned to build better ships, and navigation methods improved. Europeans explored most of the west coast of Africa by the late 1400’s. The Portuguese navigator Bartolomeu Dias sailed around the southern tip of Africa in 1488. In 1492, Christopher Columbus, an Italian navigator in the service of Spain, made the first of his historic voyages from Europe to the Americas. His voyages raised great interest in exploration, brought knowledge of the New World to Europe, and proved that ships could safely travel the western Atlantic. During the next 50 years, explorers from such nations as England, France, Portugal, and Spain sailed through most of the Atlantic Ocean.
The beginnings of scientific oceanography.
Oceanographers aboard the British research ship Challenger made the first major study of the Atlantic floor. In 1873 and 1876, they obtained samples of the Atlantic seabed, of the ocean’s water, and of many deep-sea animals and plants.
During the German expedition of the Meteor from 1925 to 1927, scientists performed the first extensive survey of the South Atlantic Ocean below the surface. They gathered conclusive evidence for the existence of the Mid-Atlantic Ridge and charted the surface and deep currents of the South Atlantic Ocean.
New techniques.
By the mid-1960’s, sonar and other electronic devices enabled geologists to chart much of the Atlantic floor. Beginning in 1968, deep sea drilling of sediments and rocks from the ocean floor provided new information about the ocean basins. Scientists found evidence to support the theory that the Atlantic formed by seafloor spreading over the past 180 million years. They also learned more about the geology and mineral resources of the continental shelf.
During the late 1900’s and early 2000’s, oceanographic studies of the Atlantic assembled large teams of scientists from many countries. Important scientific studies included the Geochemical Ocean Section Studies (GEOSECS), an international project begun in 1971 to investigate the chemical properties of ocean water. From 1987 to 2003, the Joint Global Ocean Flux Study (JGOFS) analyzed the role of carbon in the world’s oceans. The World Ocean Circulation Experiment (WOCE), which ran from 1990 to 2002, was a 40-nation effort to understand ocean circulation.
The technologies developed for oceanic studies created new opportunities in the area of submarine archaeology. Explorers located many shipwrecks at the bottom of the Atlantic, including the Titanic, in the late 1900’s. Underwater archaeologists also explored sites from Roman times in the Mediterranean, offering new opportunities to learn about the past.
