One of the major subdivisions of the interconnected body of salt water that occupies almost three-quarters of the Earth’s surface. Earth is the only planet in the solar system whose surface is covered with significant quantities of water (Fig. 1). Of the nearly 1.4 billion km3 of water found either on the surface or in relatively accessible underground supplies, about 96% is in the oceans; most of the rest is in glacier-covered Greenland and Antarctica. During much of Earth’s history, the oceans have been a difficult, if not quite impassable, barrier for the movement of land-based plants and animals from one continent or island to another. This article describes and compares some of the major oceans and their features. See also: Glaciology; Greenland; Indian Ocean; Oceanography
Oceans cover 71% of the Earth’s surface, with a mean depth of 3729 m. More than 70% of the oceans have a depth between 3000 and 6000 m. Less than 0.2% of the oceans have depths as great as 7000 m.
Temperature, salinity, and density
The oceans are cold and salty. Some 50% of the water in the ocean has a temperature between 0 and 2°C and a salinity between 34.0 and 35 g/kg. A salinity of 34 g/kg is the equivalent of 34 g of salt in a kilogram of seawater or 34 ppt. Water with a temperature above a few degrees Celsius is confined to a relatively thin surface layer of the ocean. Figure 2 shows the distribution of both temperature and salinity in the world’s oceans. See also: Seawater
Nearly all known elements have been found dissolved in seawater, and those that have not are assumed to be present. However, all but a few are found in very small amounts. Sodium chloride accounts for some 85% of the dissolved salts, and an additional four ions (sulfate, magnesium, calcium, and potassium) bring the total to more than 99.3%. The ratio of ions is remarkably constant from one ocean to another and from top to bottom of each.
Ocean salinity is controlled primarily by the balance of precipitation, river runoff, and evaporation of water at the sea surface. The highest salinities are found in major evaporation basins with little rainfall or river runoff, such as the Red Sea. The lowest salinities are found near the mouths of major rivers such as the Amazon. See also: Red Sea
Temperature and salinity, as well as pressure, determine the density of ocean water through a relationship called the equation of state. The density of freshwater is approximately 1000 kg/m3, and salt water densities range from 1020 to 1050 kg/m3. In general, cold water is denser than warm water, and because of its greater density it resides in deeper layers of the ocean than warm water. Saltier water is denser than fresher water and will also tend to remain below freshwater. When sea ice forms, salt is unable to remain within the crystalline structure of the ice. Dense brine is rejected from the ice and can form a dense water that mixes readily with the water below it and sinks toward the ocean floor. In contrast, melted sea ice is fresh and low in density, which allows it to float at the ocean surface. Some of the lowest open-ocean salinities are found in the relatively small and isolated Arctic Ocean, which contains only 1% of the total volume of the oceans but has freshwater supplied by summertime ice melt. The Arctic Ocean also receives freshwater from several of the world’s largest river systems, the Mackenzie from Canada and the Lena, Yenisei, and Ob from Russia. See also: Arctic Ocean; Sea ice
The mass of the ocean is about 270 times greater than the mass of the atmosphere, and water has approximately four times greater heat capacity than air per unit mass. This means that the ocean can absorb significant amounts of energy from the Sun with comparatively small change in temperature. Since 1970, the ocean has taken up more than 90% of the heat gained by the Earth and has warmed by an average of about 0.045°C.
Most of the Sun’s energy strikes the Earth in the tropics. However, the average temperature gradient between high and low latitudes is less than might be expected based on the distribution of incoming energy from the Sun. This is because the ocean and atmosphere together transport excess heat from the tropics toward the poles. In the tropics (equatorward of about 20° latitude), the atmosphere and ocean contribute roughly equally to the poleward transport of excess heat. At mid-latitudes (poleward of about 35°), the atmospheric transports more heat than does the ocean, although the ocean’s contribution to heat transport remains significant. A significant part of the ocean heat-exchange process is carried out by the major ocean currents that are located along the western boundaries of the major ocean basins. These “named” currents (Fig. 3) include the Gulf Stream (in the North Atlantic), the Brazil Current (in the South Atlantic), the Kuroshio (in the North Pacific), the Agulhas (in the Indian Ocean), and the East Australia Current (in the South Pacific). These currents are driven primarily by winds, and they flow faster at the ocean surface than at depth. There is considerable similarity in their pattern from one ocean basin to another. See also: Atlantic Ocean; Gulf Stream; Indian Ocean; Kuroshio; Ocean circulation; Pacific Ocean
Persistent ocean currents also occur along the eastern boundaries of ocean basins (Fig. 3), including the California Current (in the North Pacific), the Canary Current (in the North Atlantic), the Peru Current (in the South Pacific), the Benguela Current (in the South Atlantic), and the West Australia Current (in the Indian Ocean). Eastern boundary currents tend to carry cold water equatorward but can reverse direction depending on seasonal changes in wind forcing.
The average winds over the North and South Atlantic as well as the North and South Pacific oceans come out of the west (westerlies) at the middle latitudes (around 40–50°) and from the east at the lower latitudes (trade winds, 30°S–30°N). The frictional drag of these winds on the surface of the water imparts a spin or torque to the surface of the ocean, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (Fig. 4). The resulting basin-wide current gyres are apparent in the diagram of the major ocean currents in Fig. 3. The gyres link the north–south-flowing western and eastern boundary currents with east–west currents that flow in the same direction as the prevailing winds. The major exception is the Indian Ocean north of the Equator, where the circulation is strongly influenced by the winds of the seasonal monsoon. See also: Antarctic Ocean; Coriolis acceleration; Equatorial currents
Below the surface, the strongest deep-ocean currents flow along the western boundaries of ocean basins and can bring cold, deep water toward the Equator, cooling the tropics and contributing to the overall redistribution of heat. The Meridional Overturning Circulation describes the large-scale pattern of ocean circulation that links wind-driven surface currents and deep currents that are not directly wind-forced. Water that travels poleward at the surface of the North Atlantic cools and becomes denser in the seas around Greenland, where it sinks to form North Atlantic Deep Water. This deep water ultimately travels southward at depth and spreads through the global ocean and can rise to the surface in the Southern Ocean. Antarctic Bottom Water forms on the continental shelf around Antarctica and travels northward along the sea floor.