Southern Ocean

The global ocean influences the Earth’s climate by storing and transporting vast amounts of heat, moisture, and carbon dioxide. Huge quantities of carbon are cycled annually among the biosphere (forests, grasslands, and marine plankton), the atmosphere, and the oceans. The oceans are the largest active reservoir of carbon, containing 50 times more carbon than the atmosphere. Of the 6 to 7 billion tons of carbon currently released into the atmosphere by human activities, approximately 3 billion tons remain in the atmosphere, 1 to 3 billion are absorbed by the oceans, and up to 2 billion appear to be absorbed by the terrestrial biosphere.

Oceanographers commonly refer to the oceanic region that surrounds the continent of Antarctica as the Southern Ocean. The northern boundary of the Southern Ocean is not well defined, but it coincides approximately with a broad zone of transition between the warm, saline surface waters of the subtropical regime and colder, fresher subantarctic waters, called the Subtropical Front, which occur between 40 degrees S and 45 degrees S. Using this definition, the surface area encompassed by the Southern Ocean represents approximately 29.7 million sq. mi. (77 million sq. km), or 22 percent of the global surface ocean. Its unique geography makes it a key player in global climate.

The Southern Ocean is the only ocean that encircles the globe unimpeded by a land mass. It is home to the largest of the world’s ocean currents: the Antarctic Circumpolar Current (ACC). The ACC carries between 135 and 145 million cu. m of water per second from west to east along a 12,427-mi. (20,000-km) path around Antarctica, thus transporting 150 times more water around the globe than the total flow of all the world’s rivers. By connecting the Atlantic, Pacific, and Indian Oceans, the ACC redistributes heat around the Earth and so exerts a powerful influence on global climate.

Near the Antarctic continent, the Southern Ocean is a source of cold, dense water that is an essential driving force in the large-scale circulation of the world’s oceans. The cooling of the ocean and the formation of sea ice during winter increases the density of the water, which sinks from the sea surface into the deep sea. This cold, high-salinity water includes Antarctic Bottom Water and Antarctic Intermediate Water. Antarctic Bottom Water originates on the continental shelf close to Antarctica, spills off the continental shelf, and travels slowly northward, hugging the seafloor beneath other water masses, moving as far as the North Atlantic and North Pacific. Antarctic Intermediate Water is less saline and forms farther north, when cold surface waters sink beneath warmer sub-Antarctic waters at the Antarctic Convergence at about 55 degrees S. Together, these motions form a complex, three-dimensional pattern of ocean currents that extends around the globe, known as the thermohaline circulation, or “great ocean conveyor.” The thermohaline circulation has a critical influence on climate by transporting heat efficiently around the globe and by controlling how much dissolved inorganic carbon is stored in the ocean.

At the sea surface, seawater exchanges gases such as oxygen and carbon dioxide with the atmosphere at the same time that it is being cooled. As a result, sinking water efficiently transfers changes in temperature, freshwater, and dissolved gases into the deep ocean 2.5 to 3 mi. (4 to 5 km) beneath the sea surface; in terms of carbon sequestration, this is called the solubility pump. Biological processes also play a role in the surface layer, where photosynthesis by single-celled marine phytoplankton can sequester carbon dioxide in the surface water and, through the process of sedimentation, transfer this organic carbon to deeper waters—the biological pump.