Antarctic bottom water is formed in the
Weddell and
Ross Seas, off the
Adélie Coast and by
Cape Darnley from surface water cooling in
polynyas and below the
ice shelf. An important factor enabling the formation of Antarctic bottom water is the cold surface wind blowing off the Antarctic continent. The surface winds
advect sea ice away from the coast, creating polynyas which opens up the water surface to a cold atmosphere during winter, which further helps form more sea ice. Antarctic coastal polynyas form as much as 10% of the overall Southern Ocean sea ice during a single season, amounting to about 2,000 km3 of sea ice. Surface water is enriched in salt from sea ice formation and cooled due to being exposed to a cold atmosphere during winter, which increases the density of this water mass. Due to its increased density, it forms overflows down the Antarctic
continental slope and continues north along the bottom. It is the densest water in the open ocean, and underlies other bottom and intermediate waters throughout most of the southern hemisphere. The
Weddell Sea Bottom Water is the densest component of the Antarctic bottom water. A major source water for the formation of AABW is the warm offshore watermass known as the
circumpolar deep water (CDW; salinity > 35 g/kg and potential temperature > 0°C). These warm watermasses are cooled by coastal polynyas to form the denser AABW. Coastal polynyas that form AABW help prevent the intruding warm CDW water masses from gaining access to the base of ice shelves, hence acting to protect ice shelves from enhanced basal melting due to oceanic warming. In areas like the
Amundsen Sea, where coastal polynya activity has diminished to the point where dense water formation is hindered, the neighboring ice shelves have started to retreat and may be on the brink of collapse. Evidence indicates that Antarctic bottom water production through the Holocene (last 10,000 years) is not in a steady-state condition; that is, bottom water production sites shift along the Antarctic margin over decade-to-century timescales as conditions for the existence of
polynyas change. For example, the calving of the
Mertz Glacier, which occurred on 12–13 February 2010, dramatically changed the environment for producing bottom water, reducing export by up to 23% in the region of
Adélie Land. Evidence from sediment cores, containing layers of
cross-bedded sediments indicating phases of stronger bottom currents, collected on the
Mac. Robertson shelf and
Adélie Land suggests that they have switched "on" and "off" again as important bottom water production sites over the last several thousand years.
Atlantic Ocean The Vema Channel, a deep trough in the
Rio Grande Rise of the South Atlantic at , is an important conduit for Antarctic Bottom Water and
Weddell Sea Bottom Water migrating north. Upon reaching the
equator, about one-third of the northward flowing Antarctic bottom water enters the
Guiana Basin, mainly through the southern half of the Equatorial Channel at 35°W. The other part recirculates and some of it flows through the
Romanche fracture zone into the eastern Atlantic. In the Guiana Basin, west of 40°W, the sloping topography and the strong, eastward flowing deep
western boundary current might prevent the Antarctic bottom water from flowing west: thus it has to turn north at the eastern slope of the
Ceará Rise. At 44°W, north of the Ceará Rise, Antarctic bottom water flows west in the interior of the basin. A large fraction of the Antarctic bottom water enters the eastern
Atlantic through the
Vema fracture zone. South of Africa, Antarctic bottom water flows northwards through the
Agulhas Basin and then east through the
Agulhas Passage and over the southern margins of the
Agulhas Plateau and then into the
Mozambique Basin. == Climate change ==