Antarctic Intermediate Water
Encyclopedia
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.
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. The thickness of the AAIW ranges greatly between where it forms and its most northern extent.
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. 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
.
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. 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.. 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. For many years the aforementioned formation of AAIW was thought to be the only formation process, however 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. It is important to note that the biggest source of AAIW formation is just southwest of the southern tip of South America.
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
water mass
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
found mostly at intermediate depths in the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
. The AAIW is formed at the ocean surface in the Antarctic Convergence
Antarctic Convergence
The Antarctic Convergence is a curve continuously encircling Antarctica where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the subantarctic. Antarctic waters predominantly sink beneath subantarctic waters, while associated zones of mixing and upwelling create a zone...
zone or more commonly called the Antarctic Polar Front zone. This convergence
Convergence
-Mathematics:* Convergence , refers to the notion that some functions and sequences approach a limit under certain conditions* Convergence , the notion that a sequence of transformations come to the same conclusion, no matter what order they are performed in.-Natural sciences:*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 a unique water massWater mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
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. 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 transportEkman transport
Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman , is the term given for the 90 degree net transport of the surface layer due to wind forcings...
process and the divergence and convergence of water masses. The winds over Antarctica are called the polar easterlies
Polar easterlies
The polar easterlies are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and south poles towards low-pressure areas within the Westerlies at high latitudes...
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. At the same time there is a strong current north of the Antarctic Coastal Current, called the Antarctic Circumpolar Current
Antarctic Circumpolar Current
The Antarctic Circumpolar Current is an ocean current that flows 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, at approximately 125 Sverdrups, the largest ocean 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
Divergence
In vector calculus, divergence is a vector operator that measures the magnitude of a vector field's source or sink at a given point, in terms of a signed scalar. More technically, the divergence represents the volume density of the outward flux of a vector field from an infinitesimal volume around...
region. Here, upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
of North Atlantic Deep Water
North Atlantic Deep Water
North Atlantic Deep Water is a water mass that forms in the North Atlantic Ocean. It is largely formed in the Labrador Sea and in the Greenland Sea by the sinking of highly saline, dense overflow water from the Greenland Sea...
(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
Antarctic Bottom Water
The 'Antarctic Bottom Water' is a type of water mass in the seas surrounding Antarctica with temperatures ranging from 0 to -0.8◦ C, salinities from 34.6 to 34.7 psu, and a density near 27.88...
.
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
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of 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. When this water reaches between 50°S and 60°S it encounters the Antarctic Convergence
Antarctic Convergence
The Antarctic Convergence is a curve continuously encircling Antarctica where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the subantarctic. Antarctic waters predominantly sink beneath subantarctic waters, while associated zones of mixing and upwelling create a zone...
zone. At this point the Subantarctic
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° – 60° south of the Equator. The subantarctic region includes many islands in the southern parts of the Indian Ocean, Atlantic Ocean and...
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.. 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. For many years the aforementioned formation of AAIW was thought to be the only formation process, however recent studies have found that there exists some evidence that some Subantarctic mode water
Subantarctic Mode Water
Subantarctic mode water 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...
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. It is important to note that the biggest source of AAIW formation is just southwest of the southern tip of South America.