Antarctic Intermediate Water

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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.

Contents

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.

Related Research Articles

Antarctic Circumpolar Current Ocean current that flows clockwise from west to east around Antarctica

The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

North Atlantic Deep Water deep water mass formed in the North Atlantic Ocean

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation of the world's oceans involves the flow of warm surface waters from the southern hemisphere into the North Atlantic. Water flowing northward becomes modified through evaporation and mixing with other water masses, leading to increased salinity. When this water reaches the North Atlantic it cools and sinks through convection, due to its decreased temperature and increased salinity resulting in increased density. NADW is the outflow of this thick deep layer, which can be detected by its high salinity, high oxygen content, nutrient minima, high 14C/12C, and chlorofluorocarbons (CFCs).

Downwelling The process of accumulation and sinking of higher density material beneath lower density material

Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation. The sinking of cold lithosphere at subduction zones is another example of downwelling in plate tectonics.

Upwelling The replacement by deep water moving upwards of surface water driven offshore by wind

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll-a.

Ocean current Directional mass flow of oceanic water generated by external or internal forces

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

Physical oceanography The study of physical conditions and physical processes within the ocean

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Thermohaline circulation A part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes

Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

Agulhas Current Western boundary current of the southwest Indian Ocean that flows down the east coast of Africa

The Agulhas Current is the western boundary current of the southwest Indian Ocean. It flows south along 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, as western boundary currents at comparable latitudes transport less — Brazil Current, Gulf Stream, Kuroshio.

Water mass Identifiable body of water with a common formation history which has physical properties distinct from surrounding water

An oceanographic water mass is an 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 which are conservative flow tracers. Water mass is also identified by its non-conservative flow tracers such as silicate, nitrate, oxygen, and phosphate.

Brazil Current Warm current that flows south along the Brazilian south coast to the mouth of the Río de la Plata

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.

Antarctic Convergence Where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the subantarctic

The Antarctic Convergence or Antarctic Polar Front is a curve continuously encircling Antarctica, varying in latitude seasonally, where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the sub-Antarctic. Antarctic waters predominantly sink beneath the warmer subantarctic waters, while associated zones of mixing and upwelling create a zone very high in marine productivity, especially for Antarctic krill.

Atlantic meridional overturning circulation System of currents in the Atlantic Ocean

The Atlantic meridional overturning circulation (AMOC) is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.

Ekman transport Net transport of surface water perpendicular to wind direction

Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman Transport is a component of wind-driven ocean current. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at 90° anticlockwise. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s. Ekman transport has significant impacts on the biogeochemical properties of the world's oceans. This is because they lead to upwelling and downwelling in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished. This can be done by either Ekman suction or Ekman pumping depending on wind patterns.

Subantarctic Term describing the parts of the three largest oceans nearest the Southern Ocean

The sub-Antarctic zone 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 Atlantic, Indian, and Pacific oceans, especially those situated north of the Antarctic Convergence. Sub-Antarctic glaciers are, by definition, located on islands within the sub-Antarctic region. All glaciers located on the continent of Antarctica are by definition considered to be Antarctic glaciers.

Sub-Antarctic Mode Water (SAMW) is an important water mass in Earth's oceans. It is formed near the Sub-Antarctic Front on the northern flank of the Antarctic Circumpolar Current. The surface density of Sub-Antarctic 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.

Southern Ocean Ocean around Antarctica

The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the second-smallest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian oceans but larger than the Arctic Ocean. Over the past 30 years, the Southern Ocean has been subject to rapid climate change, which has led to changes in the marine ecosystem.

The Brazil–Malvinas 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 (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.

Mode water Type of water mass which is nearly vertically homogeneous

Mode water is defined as a particular type of water mass, which is nearly vertically homogeneous. Its vertical homogeneity is caused by the deep vertical convection in winter. The first term to describe this phenomenon is 18° water, which was used by Valentine Worthington to describe the isothermal layer in the northern Sargasso Sea cool to a temperature of about 18 °C each winter. Then Masuzawa introduced the subtropical mode water concept to describe the thick layer of temperature 16–18 °C in the northwestern North Pacific subtropical gyre, on the southern side of the Kuroshio Extension. The terminology mode water was extended to the thick near-surface layer north of the Subantarctic Front by McCartney, who identified and mapped the properties of the Subantarctic mode water (SAMW). After that, McCartney and Talley then applied the term subpolar mode water (SPMW) to the thick near-surface mixed layers in the North Atlantic’s subpolar gyre.

A Wind generated current is a flow in a body of water that is generated by wind friction on its surface. Wind can generate surface currents on water bodies of any size. The depth and strength of the current depend on the wind strength and duration, and on friction and viscosity losses, but are limited to about 400 m depth by the mechanism, and to lesser depths where the water is shallower. The direction of flow is influenced by the Coriolis effect, and is offset to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. A wind current can induce secondary water flow in the form of upwelling and downwelling, geostrophic flow, and western boundary currents.

References

  1. ↑ Wallace, Gary Ernst (2000). Earth Systems: processes and issues. ISBN   0-521-47895-2. pp 170-180
  2. 1 2 3 4 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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