Antarctic Bottom Water

AABW is formed in the Southern Ocean from surface water cooling in polynyas

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from -0.8 to 2 °C (31 °F), salinities from 34.6 to 34.7 psu. Being the densest water mass of the World Ocean, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level. The major significance of Antarctic bottom water is that it is the coldest bottom water, giving it a significant influence on the movement of the world's oceans. Antarctic bottom water also has a high oxygen content relative to the rest of the oceans' deep waters. This, thanks to the oxidation of deteriorating organic content in rest of the deep oceans. Antarctic bottom water has been considered the ventilation of the deep ocean.^{[citation needed]}

Formation and circulation

Antarctic bottom water is created in part due to the major overturning of ocean water.

Antarctic bottom water is formed in the Weddell and Ross Seas from surface water cooling in polynyas and below the ice shelf. A unique feature of Antarctic bottom water is the cold surface wind blowing off the Antarctic continent. The surface wind creates the polynyas which opens up the water surface to more wind. This Antarctic wind is stronger during the winter months and thus the Antarctic bottom water is more pronounced during the Antarctic winter season. Surface water is enriched in salt from sea ice formation. Due to its increased density, the water is flowing down the Antarctic continental margin and on the bottom further north. It is the densest water in the free ocean and is overlain by the waters of the Antarctic Circumpolar Current at a depth of 1000 to 2000 m (3,300 to 6,600 feet) and overlies Weddell Sea bottom water in some locations.

AABW flow in the Equatorial Atlantic

Atlantic Ocean

The Vema Channel, a deep trough in the Rio Grande Rise of the South Atlantic at 31.3°S 39.4°W / -31.3; -39.4, 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.

Indian Ocean

On the Indian Ocean side, Antarctic bottom water cannot reach the northern waters due to the various deep basins of the Indian Ocean topography. There is a gap called the Crozet-Kerguelen Gap that allows Antarctic bottom water to move toward the equator beneath the Indian Ocean. This northward movement amounts to 2.5 Sv. It takes the Antarctic Bottom Water 23 years to reach the Crozet-Kerguelen Gap

References

• Glossary of Physical Oceanography
• Steele, John H., Steve A. Thorpe and Karl K. Turekian, editors, Ocean Currents: A derivative of the Encyclopedia of Ocean Sciences, Academic Press, 1st ed., 2010 ISBN 978-0080964867
• Seabrooke, James M., Hufford, Gary L., and Elder, Robert B., “Formation of Antarctic Bottom Water in the Weddell Sea,” Journal of Geophysical Research, Vol. 76, No. 9, pp. 2164-2178, 1971
• Fahrbach, E., G. Rohardt, N. Scheele, M. Schroder, V. Strass, and A. Wisotzki, “Formation and discharge of deep and bottom water in the northwestern Weddell Sea”, Journal of Maritime Research, 53(4), pp. 515-538, 1995