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The Atlantic Ocean forms a broad s-shape from the Arctic Sea to the north and from Antarctica to the south. North America and South America are to the west; Europe and Africa are to the east. It is about half the size of the Pacific Ocean and slightly larger than the Indian Ocean. It covers 31,800,000 square miles (36,000,000 square kilometers) or 16 percent of the Earth’s surface. If marginal seas are included, the coverage is nearly 20 percent. The ocean’s principle marginal seas are the Caribbean Sea, the Gulf of Mexico, and the Hudson and Baffin bays to the west; the Arctic, Greenland, and Norwegian seas to the North; the Baltic, North, Mediterranean, and Black Seas to the east; and the Weddell Sea to the south. The Atlantic Ocean proper refers to the ocean minus its marginal seas.
The equator divides the Atlantic Ocean into the North Atlantic and the South Atlantic. Drake Passage (between the island of Tierra del Fuego and Antarctica) and the Magellan Strait (between Tierra del Fuego and South America) connects the South Atlantic to the Pacific Ocean. A broad stretch of water separating Africa and Antarctica connects the South Atlantic to the Indian Ocean. The North Atlantic’s connection to the Pacific Ocean follows a circuitous series of straits among the northern Canadian islands to the Arctic Sea and thence to the Bering Strait.
Plate tectonics have given rise to the general topography of the seafloor. The gigantic north-south trending Mid-Atlantic spreading ridge makes up about one-third of the sea bottom and divides the Atlantic Ocean rather evenly into western and eastern halves. The ridge in most places rises to within about 1.5 miles (2.5 kilometers) of the surface and occasionally breaches the surface to form prominent oceanic islands: Iceland, the Azores, Ascension, St. Helena, and Tristan da Cuhna. On either side of the ridge are abyssal plains. The plains extend from the base of the mid-ocean ridge to the base of adjoining continents. The name “plain” implies that this part of the seafloor is a monotonous, uninteresting place. Actually, abyssal plains are remarkable for their deep sediments and life forms. On the landward sides of the plains, the sea bottom rises gently landward as continental shelves (submerged portions of continents). The average depth of abyssal plains is about 4 miles (6.5 kilometers). The ocean’s average depth (without its marginal seas) is about 2.5 miles (4 kilometers), owing mainly to the Atlantic Ocean’s broad, shallow continental shelves, which make up 13 percent of the Atlantic Ocean proper. The greatest depth (28,224 feet or 8,605 meters) is Milwaukee Deep, in the Puerto Rico Trench, north of Puerto Rico.
Available solar energy, which decreases with increasing latitude, affects the ocean’s climate as well as its temperature and salinity levels. The high sun angles of the equatorial latitudes create warm tropical waters, a belt of low surface pressure, convergent trade winds, and convective thunderstorms. Salinity levels in this zone are especially low where large rivers such as the Amazon, Orinoco, Niger and Congo Rivers empty freshwater from the heavy rains into the sea. Moreover, the broad extent of tropical waters in the North Atlantic serve as the repository of heat energy that feeds an annual supply of tropical storms and hurricanes there.
Poleward of the equatorial low-pressure belt are subtropical high-pressure cells in each hemisphere. The high pressure inhibits cloud formation, so the annual rates of evaporation and salinity levels are high there. In the middle and subpolar latitudes, cyclonic storms form over the oceans. The storms generate above-average wave heights that are hazardous to ships and coastlines.
In polar latitudes, the air is formidably cold due to low sun angles and long winter days. The cold air pulls prodigious amounts of heat from the ocean, causing water temperatures to drop at or near the freezing mark. Salinity levels at these latitudes are high due to sea ice formation, which leaves salts concentrated in the remaining seawater. Icebergs shed from glaciers on Greenland and Baffin Island in the North Atlantic; and from Antarctica in the South Atlantic, venture into the middle latitudes as far as 40 degrees N and 50 degrees S latitudes before melting.
Due to the Coriolis effect on wind-driven surface currents, major currents in the North Atlantic flow clockwise, whereas those in the South Atlantic travel counterclockwise. Each of these broad circulating loops (gyres) has an equatorial current component that flows parallel to the equator, a warm current section that carries tropical heat to polar latitudes, and a cold current that returns to the equator to store more heat from the tropical sun. The circulating water moderates the global temperatures by absorbing and transferring surplus tropical heat to polar regions. The Atlantic Ocean is also part of a global conveyor belt of Thermohaline (vertical) circulation that affects the global climate.
The Atlantic Ocean has continental, oceanic, and coral islands. The largest continental islands are made of bedrock exposures, such as Great Britain, Ireland, Greenland, and Newfoundland. Glacial deposits form smaller islands, such as Long Island (New York) and Martha’s Vineyard (Massachusetts); barrier island deposits make up Hilton Head (North Carolina), Chincoteague (Virginia), and Fire (New York) islands.
Oceanic islands rise from the deep ocean floor rather than a continental shelf and are usually of volcanic origin. Atlantic examples of oceanic islands include the Azores Islands, the Canary Islands, the Cape Verde Islands, Iceland, and the Lesser Antilles. Coral islands can be of either the continental or the oceanic type. Most of those in the Atlantic Ocean are the high parts of a large limestone platform situated on continental shelves. The Bahamas and Florida Keys are examples of this type. Oceanic coral islands sit atop submerged volcanoes and are most typical of the western Pacific Ocean; Bermuda is an example of this island type in the Atlantic Ocean.
- Cornelia Dean, Against the Tide: The Battle for America’s Beaches (Columbia University Press, 1999);
- Robert E. Gabler, James F. Peterson, and Michael Trapasso, Essentials of Physical Geography (Brooks/Cole, 2004);
- Open University, The Ocean Basins: Their Structure and Evolution (Butterworth-Heinemann, 1998);
- Thomson and P. P. E. Weaver, eds., The Geology and Geochemistry of Abyssal Plains, Geological Society Special Publication No. 31 (Oxford: Blackwell Scientific Publishers for Geological Society of London, 1987);
- Harold V. Thurman and Allan P. Trujillo, The Essentials of Oceanography (Prentice Hall, 2001).