Atlantic Ocean

• 1. Introduction

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  The Atlantic Ocean covers about 85,133,000 square kilometers, the second largest of the world's five oceanic divisions. It accounts for roughly 17 percent of Earth's surface and about 24 percent of its water surface area. For centuries it sat between two worlds. During the Age of Discovery, it was the barrier that separated the New World of the Americas from the Old World of Afro-Eurasia. But the same water that divided also connected. Why did the oldest known mentions of an Atlantic sea come from a Greek poet in the mid-sixth century BC? How did an ocean shaped like a stretched letter S come to carry the weight of slavery, empire, and the first computer-assisted reconstructions of moving continents? And what happens when a barrier this vast begins, very slowly, to close? The Atlantic occupies an elongated, S-shaped basin running between Europe and Africa to the east and the Americas to the west. It is divided into northern and southern halves by the Equator. The stories below begin where the water meets myth, geology, and human history.

• 2. What's In A Name

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  Stesichorus, around the mid-sixth century BC, left the oldest known mention of an Atlantic sea, writing of the Atlantikoi pelagei. Around 450 BC, Herodotus used the phrase Atlantis thalassa in his Histories, naming the sea beyond the pillars of Hercules. That phrase pointed past the Strait of Gibraltar, beyond the Atlas Mountains in Morocco and off the West African coast. The name itself reaches back to Atlas, the Titan of Greek mythology who supported the heavens. Atlas would later appear as a frontispiece in medieval maps and atlases. Early Greek sailors believed the Atlantic to be part of the Oceanus, the great sea or river that surrounds all land, mentioned in the Iliad and the Odyssey. This set it apart from the enclosed seas the Greeks knew well, the Mediterranean and the Black Sea. Because the great ocean was thought to wrap around Africa, the term Aethiopian Ocean, derived from Ancient Ethiopia, was applied to the southern Atlantic as late as the mid-19th century. English cartographers of the Age of Discovery had their own label, the Great Western Ocean. There is also a humorous name. British and American speakers call the northern Atlantic the pond, an ironic understatement. The phrase dates to 1640, first appearing in a pamphlet released during the reign of Charles I. It was reproduced in 1869 in Nehemiah Wallington's Historical Notices of Events Occurring Chiefly in The Reign of Charles I, where great Pond is used by Francis Windebank, Charles I's Secretary of State.

• 3. Measuring The Deep

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  The International Hydrographic Organization defined the limits of the oceans and seas in 1953, yet some of those definitions have been revised, and some are not recognized by various authorities, such as the CIA World Factbook. As a result, the Atlantic's exact extent and the number of its seas vary by source. Including its marginal seas, the Atlantic covers 106,460,000 square kilometers, or 23.5 percent of the global ocean, and holds a volume of 310,410,900 cubic kilometers. Excluding those marginal seas, it covers 81,760,000 square kilometers. The North Atlantic spans 41,490,000 square kilometers and the South Atlantic 40,270,000 square kilometers. Its irregular coasts are indented by numerous bays, gulfs, and seas. These include the Baltic Sea, Black Sea, Caribbean Sea, Gulf of Mexico, Labrador Sea, Mediterranean Sea, North Sea, and Norwegian Sea, among others. With these marginal seas included, the Atlantic coastline measures 111,866 kilometers, compared with 135,663 kilometers for the Pacific. The average depth is 3,646 meters. The deepest point, the Milwaukee Deep in the Puerto Rico Trench, plunges to 8,376 meters. That trench is one of the few places where an active margin breaks the Atlantic's otherwise passive edges.

• 4. The Ridge That Made Continents Move

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  A submarine mountain range called the Mid-Atlantic Ridge dominates the floor of the ocean. It runs from 87 degrees North, about 300 kilometers south of the North Pole, to the subantarctic Bouvet Island at 54 degrees South. The ridge rises 2 to 3 kilometers above the surrounding ocean floor, and its rift valley marks the divergent boundary between the North American and Eurasian plates in the north and the South American and African plates in the south. Eyjafjallajokull in Iceland sits along this seam, and pillow lava forms on the ocean floor below. In the 1870s, the Challenger expedition discovered parts of what is now known as the Mid-Atlantic Ridge, describing an elevated ridge traversing the basins of the North and South Atlantic. The remainder of the ridge was found in the 1920s by the German Meteor expedition using echo-sounding equipment. The exploration of the ridge in the 1950s led to the general acceptance of seafloor spreading and plate tectonics. Where the ridge breaks the surface, it raises volcanic islands. Nine of these have collectively been nominated a World Heritage Site for their geological value. Four are considered of Outstanding Universal Value: Thingvellir in Iceland; the Landscape of the Pico Island Vineyard Culture in Portugal; Gough and Inaccessible Islands in the United Kingdom; and the Brazilian Atlantic Islands of Fernando de Noronha and Atol das Rocas Reserves. The ridge is a barrier for bottom water, except at two transform faults, the Romanche Trench near the Equator and the Gibbs fracture zone at 53 degrees North, where deep currents slip from one side to the other.

• 5. Currents And Salt

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  On average, the Atlantic is the saltiest major ocean, with surface salinity ranging from 33 to 37 parts per thousand. The highest values occur at about 25 degrees north and south, in subtropical regions of low rainfall and high evaporation. The lowest values appear just north of the equator, where heavy tropical rainfall dilutes the surface. This high salinity sustains the Atlantic thermohaline circulation, and two processes keep it high. The Agulhas Leakage brings salty Indian Ocean waters into the South Atlantic, while the Atmospheric Bridge evaporates subtropical Atlantic waters and exports them to the Pacific. The Coriolis effect spins North Atlantic water clockwise and South Atlantic water counter-clockwise. In the North Atlantic, three connected currents dominate the surface: the Gulf Stream, which flows north-east from Cape Hatteras; the North Atlantic Current, a branch flowing northward from the Grand Banks; and the Subpolar Front beyond it. Without this system carrying warm water north, temperatures in the North Atlantic and Europe would plunge dramatically. Beneath it all moves the North Atlantic Deep Water, a complex of four water masses. Some form by deep convection in the open ocean, others from dense water spilling across the Greenland-Iceland-Scotland sill. Since the 1960s, the path of that deep water can be traced by measuring tritium and radiocarbon from nuclear weapon tests, along with CFCs.

• 6. A Sea Within The Sea

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  The Sargasso Sea sits in the western North Atlantic, an area roughly 4,000 kilometers wide where two species of Sargassum float free. It is encircled by the Gulf Stream, the North Atlantic Drift, and the North Equatorial Current, with no land border at all. This drifting seaweed probably descended from Tertiary ancestors on the European shores of the former Tethys Ocean. If so, it has maintained itself by vegetative growth, floating in the ocean for millions of years. Among its endemic residents is the sargassum fish, a predator with algae-like appendages that hovers motionless among the weed. Fossils of similar fishes have been found in the Carpathian region, in fossil bays of the former Tethys. That so-called quasi-Sargasso assemblage, found in the mid-20th century, traced the lineage from the Carpathian Basin over Sicily to the central Atlantic. The Sargasso Sea hides another secret. For decades, the spawning ground of European eels was unknown. In the early 19th century, the southern Sargasso Sea was identified as the spawning ground for both the European and American eel. The European eel migrates more than 5,000 kilometers, the American eel 2,000 kilometers. Ocean currents like the Gulf Stream carry eel larvae from the Sargasso Sea to foraging areas in North America, Europe, and northern Africa. Recent but disputed research suggests eels may use Earth's magnetic field to navigate, both as larvae and as adults.

• 7. The Atlantic World

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  Christopher Columbus reached the Americas in 1492, sailing under the Spanish flag, and his expedition proved the most consequential crossing of all, though the Norse were the first known humans to cross the Atlantic. Six years later, Vasco da Gama reached India under the Portuguese flag, rounding the Cape of Good Hope and proving the Atlantic and Indian Oceans are connected. In 1500, Pedro Alvares Cabral reached Brazil, carried by the currents of the South Atlantic Gyre on his own voyage to India. Spain and Portugal swiftly conquered and colonized large New World territories, forcing the Amerindian population into slavery to extract silver and gold. The two powers monopolized the trade, and conflicting interests led to a series of Spanish-Portuguese wars. A peace treaty mediated by the Pope split the conquered territories into Spanish and Portuguese sectors. England, France, and the Dutch Republic watched enviously and allied with pirates such as Henry Mainwaring and Alexandre Exquemelin. As disease and depredation devastated the indigenous population, colonists introduced the Atlantic slave trade to replace them. Between the 15th century and 1888, when Brazil became the last part of the Americas to end the trade, an estimated 9.5 million enslaved Africans were shipped into the New World, most destined for agricultural labor. The slave trade was officially abolished in the British Empire and the United States in 1808. Slavery itself ended in the British Empire in 1838 and in the United States in 1865 after the Civil War. The trade reshaped Europe too. In Atlantic-facing countries, urbanization grew from 8 percent in 1300 to 24.5 percent by 1850, and GDP doubled, while the rest of Europe rose by only 30 percent. By the end of the 17th century, the volume of trans-Atlantic trade had surpassed that of the Mediterranean.

• 8. The Closing Ocean

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  An embryonic subduction margin is potentially developing west of Gibraltar, a hint that the Atlantic may not widen forever. The Gibraltar Arc in the western Mediterranean is migrating westward into the central Atlantic, where it joins the converging African and Eurasian plates. Together these three forces are slowly building a new subduction system in the eastern Atlantic Basin. Across the ocean, the Scotia Arc and Caribbean plate form eastward-propagating subduction systems in the western Atlantic Basin. Together with the Gibraltar system, they might mark the beginning of the closure of the Atlantic and the final stage of the Atlantic Wilson cycle. Not everyone agrees on this fate. Other theories propose that the Atlantic will keep widening while the Pacific Ocean closes first, creating the future supercontinent Amasia. Under that view, dominant subduction in the Pacific would close that ocean over the next 200 to 300 million years while the Atlantic continues to grow. The pressures of the present are easier to measure. A 2005 report indicated the Atlantic meridional overturning circulation slowed by 30 percent between 1957 and 2004. In 2024, research highlighted a significant weakening of about 12 percent over the past two decades. The warm shallow flow that feeds the North Atlantic Deep Water is the same flow responsible for Europe's anomalously mild climate, which means its weakening is a change the people on both sides of the pond will feel.