Antarctic Intermediate Water

Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Antarctic Polar Front zone. This convergence zone is normally located between 50°S and 60°S, hence this is where almost all of the AAIW is formed.

Properties

The AAIW is unique water mass in that it is a sinking water mass with a moderately low salinity, unlike most sinking water masses which have a relatively high salinity. This salinity minimum, unique to the AAIW, can be recognized throughout the Southern Ocean at depths ranging from 700 to 1200 meters. Typical temperature values for the AAIW are 3-7°C, and a salinity of 34.2-34.4 psu upon initial formation. Due to vertical mixing at intermediate depths in the Southern Ocean, the salinity slowly rises as it moves northward. Typical density of AAIW water is between 1026.82 kg/m³ and 1027.43 kg/m³.[1] The thickness of the AAIW ranges greatly between where it forms and its most northern extent.

Formation

The formation of AAIW can be explained very simply through the Ekman transport process and the divergence and convergence of water masses. The winds over Antarctica are called the polar easterlies where winds blow from the east to the west. This creates a counter-clockwise surface current near the coast of Antarctica, called the Antarctic Coastal Current. Ekman transport causes the water to push towards the left of the surface motion in the Southern Hemisphere. Thus, this westward directed coastal current in Antarctica will push the water towards Antarctica.[2]

At the same time there is a strong current north of the Antarctic Coastal Current, called the Antarctic Circumpolar Current (ACC) created by the strong westerlies in this region which flows clockwise around Antarctica. Again, Ekman transport will push this water to the left of the surface motion, meaning away from Antarctica. Because water just offshore of Antarctica is being pushed away and into Antarctica, it leads to the Antarctic Divergence region. Here, upwelling of North Atlantic Deep Water (NADW) takes place. NADW is cold and quite saline. Once the NADW is upwelled to the surface some of it diverges towards Antarctica, gets colder, and sinks back down as Antarctic Bottom Water.[2]

The NADW water also diverges away from Antarctica when it is upwelled. This diverged water moves northward (equatorward), and at the same time persistent precipitation (location is near the polar lows ~60°S) along with an influx of melt water decreases the salinity of the original NADW. Because the salinity of the NADW has changed by so much and it has essentially lost all its unique characteristics to be NADW, this northward propagating surface water is now called Antarctic Surface Water (AASW). Also, the AASW movement northward has gained some heat from the atmosphere, thereby increasing the temperature slightly.[2][3]

When this water reaches between 50°S and 60°S it encounters the Antarctic Convergence zone. At this point the Subantarctic waters, which are characterized as being much warmer than the Antarctic waters, are just north of the Antarctic Polar Front and the Antarctic waters are just south of the Antarctic Polar Front. This region is referred to as the Antarctic Convergence Zone/Antarctic Polar Front because of the sharp gradients in both temperature and salinity (esp. temperature) between the Antarctic waters and the Subantarctic waters. It is also a region of strong vertical mixing.[2][4] It is important to note that this convergence zone does not occur simply because the Subantarctic water is flowing southward and the AASW is flowing northward, but due to Ekman convergence.

Once the northward propagating Antarctic Surface Water reaches the Antarctic Convergence zone it begins to sink because it is more dense than the Subantarctic water to its north, but less dense than the Antarctic water to its south. This water is then referred to as AAIW. The sinking AAIW becomes sandwiched between the Subantarctic water (above) which is much warmer, but more saline and the NADW (below) which is cold and quite salty.[5][6]

For many years the aforementioned formation of AAIW was thought to be the only formation process. Recent studies have found that there exists some evidence that some Subantarctic mode water is able to penetrate through the Subantarctic front (frontal region separating the Polar frontal zone from the Subantarctic zone) and become the dominant source of AAIW, rather than the AASW. Because of the difficulty of getting observations in this very treacherous area, this research on Subantarctic mode water mixing theory is still being worked out, but a lot of evidence exists for its inclusion in the formation of AAIW.[7][8] It is important to note that the biggest source of AAIW formation is just southwest of the southern tip of South America.

Areal extent and movement

The interesting characteristic of AAIW is how far it extends northward. The salinity minima associated with the AAIW can be seen in intermediate waters (~1000m) as far north as 20°N, with trace amounts as far as 60°N. It is by far the largest spreading intermediate water of all the ocean intermediate water masses. It continues northward until it encounters other intermediate water masses (e.g.AIW).[9] The movement of the AAIW is predominantly northward due to the Ekman volume transport mostly directed in that way. When the AAIW is initially formed, the ACC is able to transport the AAIW into all ocean basins because the ACC flows clockwise around Antarctica with no land based boundaries.

References

  1. ^ Wallace, Gary Ernst (2000). Earth Systems: processes and issues. ISBN 0-521-47895-2. pp 170-180
  2. ^ a b c d Tomczak, Matthias & J Stuart Godfrey(2003). Regional Oceanography: an Introduction 2nd edn. pp.63-82,ISBN 81-7035-306-8
  3. ^ Reddy, M. (2001). Descriptive Physical Oceanography. pp.273-327 ISBN 90-5410-706-5
  4. ^ Reddy, M. (2001). Descriptive Physical Oceanography. pp.273-327 ISBN 90-5410-706-5
  5. ^ National Research Council (U.S.). Ad Hoc Committee on Antarctic Physical and Chemical Oceanography (1988). Physical oceanography and tracer chemistry of the Southern Ocean. pp. 40-50
  6. ^ Fabio, F. et al. (2008). Antarctic Climate Evolution. pp. 86-92. ISBN 0-444-52847-4
  7. ^ National Research Council (U.S.). Ad Hoc Committee on Antarctic Physical and Chemical Oceanography (1988). Physical oceanography and tracer chemistry of the Southern Ocean. pp. 40-50
  8. ^ Fabio, F. et al. (2008). Antarctic Climate Evolution. pp. 86-92. ISBN 0-444-52847-4
  9. ^ Talley, L. D., 1999. Some aspects of ocean heat transport by the shallow, intermediate and deep overturning circulations. In Mechanisms of Global Climate Change at Millennial Time Scales, Geophys. Mono. Ser., 112, American Geophysical Union, ed. Clark, Webb and Keigwin, 1-22.

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Agulhas Basin

The Agulhas Basin is an oceanic basin located south of South Africa where the South Atlantic Ocean and south-western Indian Ocean meet. Part of the African Plate, it is bounded by the Agulhas Ridge (part of the Agulhas-Falkland Fracture Zone) to the north and the Southwest Indian Ridge to the south; by the Meteor Rise to the west and the Agulhas Plateau to the east. A large number of bathymetric anomalies hints at the basin's dynamic tectonic history.

Agulhas Current

The Agulhas Current is the western boundary current of the southwest Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary current in the world ocean, with an estimated net transport of 70 Sverdrups (Sv, millions m3/s), as western boundary currents at comparable latitudes transport less — Brazil Current (16.2 Sv), Gulf Stream (34 Sv), Kuroshio (42 Sv).

Agulhas Passage

The Agulhas Passage is an abyssal channel located south of South Africa between the Agulhas Bank and Agulhas Plateau. About 50 km (31 mi) wide, it connects the Natal Valley and Transkei Basin in the north to the Agulhas Basin in the south and is the only near-shore connection between the south-western Indian Ocean and South Atlantic Ocean.

Agulhas Plateau

The Agulhas Plateau is an oceanic plateau located in the south-western Indian Ocean about 500 km (310 mi) south of South Africa. It is a remainder of a large igneous province (LIP), the Southeast African LIP, that formed 140 to 95 million years ago (Ma) at or near the triple junction where Gondwana broke-up into Antarctica, South America, and Africa. The plateau formed 100 to 94 Ma together with Northeast Georgia Rise and Maud Rise (now located near the Falkland Island and Antarctica respectively) when the region passed over the Bouvet hotspot.

Atlantic Ocean

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers (41,100,000 square miles). It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and the Americas to the west. As one component of the interconnected global ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Equatorial Counter Current subdivides it into the North Atlantic Ocean and the South Atlantic Ocean at about 8°N.Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office.

Brazil Current

The Brazil Current is a warm water current that flows south along the Brazilian south coast to the mouth of the Río de la Plata.

Brazil–Malvinas Confluence

The Brazil–Malvinas Confluence Zone (also called the Brazil–Falkland Confluence Zone or the Brazil–Falklands Confluence Zone) is a very energetic region of water just off the coast of Argentina and Uruguay where the warm poleward flowing Brazil Current and the cold equatorward flowing Malvinas Current converge. The region oscillates latitudinally, but in general the region of confluence occurs between 35 and 45 degrees south latitude and 50 to 70 degrees west longitude. The confluence of these two currents causes a strong thermohaline to exist and causes numerous high energy eddies to form.

Circumpolar deep water

Circumpolar Deep Water (CDW) is a designation given to the water mass in the Pacific and Indian oceans that essentially characterizes a mixing of other water masses in the region. A distinguishing characteristic is the water is not formed at the surface, but rather by a blending of other water masses, including the North Atlantic Deep Water (NADW), the Antarctic Bottom Water (AABW), and the Pacific Intermediate Water Masses.

CDW, the greatest volume water mass in the SO, is a mixture of North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW), and Antarctic Intermediate Water (AAIW), as well as recirculated deep water from the Indian and Pacific Oceans (e.g., Wüst 1935; Callahan 1972; Georgi 1981; Mantyla and Reid 1983; Charles and Fairbanks 1992;You 2000)

Because the Circumpolar Deep Water is a mix of other water masses, its TS profile is simply the point where the TS lines of the other water masses converge. TS diagrams refer to temperature and salinity profiles, which are one of the major ways water masses are distinguished from each other. The convergence of the TS lines thus proves the mixing of the other water masses.

Circumpolar deep water is between 1 and 2 degrees Celsius and has a salinity between 34.62 and 34.73 practical salinity units.

In recent decades, hundreds of glaciers draining the Antarctic Peninsula (63° to 70°S) have undergone systematic and progressive change. These changes are widely attributed to rapid increases in regional surface air temperature, but it is now clear that this cannot be the sole driver. A strong correspondence has been discovered between mid-depth ocean temperatures and glacier-front changes along the approximately 1000-kilometer western coastline.In the south, glaciers that terminate in warm Circumpolar Deep Water have undergone considerable retreat, whereas those in the far northwest, which terminate in cooler waters, have not. Furthermore, a mid-ocean warming since the 1990s in the south is coincident with widespread acceleration of glacier retreat. The conclusion is that changes in ocean-induced melting are the primary cause of retreat for glaciers in this region.

Heinrich event

A Heinrich event is a natural phenomenon in which large armadas of icebergs break off from glaciers and traverse the North Atlantic. First described by marine geologist Hartmut Heinrich (Heinrich, H., 1988), they occurred during five of the last seven glacial periods or "ice ages" over the past 640,000 years (Hodell, et al., 2008). Heinrich events are particularly well documented for the last glacial period but notably absent from the penultimate glaciation (Obrochta et al., 2014). The icebergs contained rock mass, eroded by the glaciers, and as they melted, this material was dropped to the sea floor as ice rafted debris (abbreviated to "IRD").

The icebergs' melting caused extensive amounts of fresh water to be added to the North Atlantic. Such inputs of cold and fresh water may well have altered the density-driven, thermohaline circulation patterns of the ocean, and often coincide with indications of global climate fluctuations.

Various mechanisms have been proposed to explain the cause of Heinrich events, most of which imply instability of the massive Laurentide ice sheet, a continental glacier covering north eastern North America during the last glacial period. Other northern hemisphere ice sheets were potentially involved as well (Fennoscandic, Iceland/Greenland). However, the initial cause of this instability is still debated.

Lynne Talley

Lynne Talley (born May 18, 1954) is an American physical oceanographer.

Talley is Professor of Physical Oceanography at Scripps Institution of Oceanography and has spent many months on research ships serving as chief scientist and collecting oceanographic hydrography data. She has a strong record of continuous participation in international steering groups and oversight committees for collection and use of oceanographic data. Talley is a Fellow of the American Meteorological Society, the American Geophysical Union, The Oceanography Society, the American Association for the Advancement of Science, and the American Academy of Arts and Sciences.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.

Subantarctic

The Subantarctic is a region in the southern hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46° and 60° south of the Equator. The subantarctic region includes many islands in the southern parts of the Indian Ocean, Atlantic Ocean and Pacific Ocean, especially those situated north of the Antarctic Convergence. Subantarctic glaciers are, by definition, located on islands within the subantarctic region. All glaciers located on the continent of Antarctica are by definition considered to be Antarctic glaciers.

Subantarctic Mode Water

Subantarctic mode water (SAMW) is an important water mass in the Earth's oceans. It is formed near the Subantarctic Front on the northern flank of the Antarctic Circumpolar Current. The surface density of Subantarctic Mode Water ranges between about 1026.0 and 1027.0 kg/m3 and the core of this water mass is often identified as a region of particularly low stratification.

Another important facet of SAMW is that silicate (an important nutrient for diatoms) is depleted relative to nitrate. This depletion can be tracked over much of the globe, suggesting that SAMW helps set the blend of nutrients delivered to low-latitude ocean ecosystems, and thus determines the balance of species within these ecosystems.

SAMW is a very homogenous layer that forms north of the Subantarctic Front and is also referred to as a pycnostad. Its uniformity can be attributed to convective overturning that also serves to ventilate it resulting in the high dissolved oxygen value of >6ml/l.

It has slightly less dissolved oxygen than the surface water layer above it, but greater dissolved oxygen than the water masses below it. It has some variability in temperature, salinity and density in the Pacific Ocean. From west to east, the density increases from 1026.9 kg/m³ to 1027.1 kg/m³, the temperature decreases from 8.5 °C to 5.5 °C, and the salinity decreases from 34.62 ppt to 34.25 ppt (psu) In the region where the Peru-Chile Undercurrent flows above the SAMW, the SAMW can be distinguished as having locally-characteristic low phosphorus, silicate and other nutrient concentrations in comparison.

It moves by the transference of heat energy via the Subtropical anticyclonic gyre and retains its individuality as differentiated with the less-salty Antarctic Intermediate Water below it and the more highly oxygenated surface water above it. The oxygen maximum portion of SAMW sinks at 28˚S to 700m and lifts back to 500m around 15˚S after oxygen levels decreased.SAMW acts as an oxygenator for mid oceanic depths in the Southern oceans. Near the surface it picks up atmospheric oxygen and carbon dioxide and then sinks, or subducts near the Indian Ocean, contributing to the Indian subtropical gyre and cooling and contributing to the Antarctic Circumpolar Current (ACC).

Vitiaz Strait

Vitiaz Strait is a strait between New Britain and the Huon Peninsula, northern New Guinea.The Vitiaz Strait was so named by Nicholai Nicholaievich Mikluho-Maklai to commemorate the Russian corvette Vitiaz in which he sailed from October 1870 by way of South America and the Pacific Islands reaching Astrolabe Bay in September 1871.

Water mass

An oceanographic water mass is identifiable body of water with a common formation history which has physical properties distinct from surrounding water. Properties include temperature, salinity, chemical - isotopic ratios, and other physical quantities.

Water masses are generally distinguished not only by their respective tracers (see above) but also by their location in the Worlds' oceans. Water masses are also distinguished by their vertical position, so that there are surface water masses, intermediate water masses and deep water masses.

Common water masses in the world ocean are: Antarctic Bottom Water (AABW), North Atlantic Deep Water (NADW), Circumpolar Deep Water (CDW), Antarctic Intermediate Water (AAIW), Subantarctic Mode Water (SAMW), Arctic Intermediate Water (AIW), North Pacific Intermediate Water (NPIW), the central waters of various oceanic basins, and various ocean surface waters.

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