Antarctic Intermediate Water

Antarctic Intermediate Water
Name	Antarctic Intermediate Water
Depth	500–1500 m
Salinity	34.1–34.8 PSU
Potential temperature	~3–8 °C
Formation regions	Southern Ocean
Parent waters	Subantarctic Mode Water; Antarctic Surface Water

Antarctic Intermediate Water

Antarctic Intermediate Water is a water mass occupying intermediate depths in the Southern Ocean and adjacent basins. It is defined by a characteristic salinity minimum and potential temperature range, acting as a key conduit between surface waters and deep circulation. Its properties and pathways influence regional hydrography and global thermohaline transport.

Definition and Characteristics

Antarctic Intermediate Water is identified by a salinity minimum and potential temperature that distinguish it from North Atlantic Deep Water, Mediterranean Water, Subantarctic Mode Water, Antarctic Bottom Water, and Circumpolar Deep Water. Oceanographers trace it using temperature–salinity diagrams, WOCE sections, and tracer studies involving CFCs, tritium, and radiocarbon. It occupies depths typically between 500 and 1500 metres, forming a density layer that interleaves with Indian Ocean, South Atlantic Ocean, and South Pacific Ocean intermediate waters.

Formation and Sources

Formation occurs where surface waters modified by wind-driven mixing, convective processes, and subduction sink beneath fresher surface layers near the Antarctic Convergence and Subantarctic Front. Primary source regions include the northern flank of the Antarctic Circumpolar Current and areas influenced by Patagonian Shelf outflow and Weddell Sea processes. Contributions also arise from northern incursions of Antarctic Surface Water and from leakage of Subantarctic Mode Water via frontal interactions and eddy shedding associated with the ACC and Kerguelen Plateau bathymetry.

Distribution and Variability

AAIW spreads equatorward into the South Pacific Ocean, South Atlantic Ocean, and Indian Ocean basins via pathways constrained by the Antarctic Circumpolar Current, western boundary currents such as the Brazil Current and East Australian Current, and trench and ridge features like the Mid-Atlantic Ridge and Macquarie Ridge. Its core depth and salinity display seasonal to interannual variability linked to modes of climate variability including the Southern Annular Mode, El Niño–Southern Oscillation, and decadal oscillations like the Pacific Decadal Oscillation. Long-term changes have been inferred from repeat hydrographic programs such as GO-SHIP and historical voyages including Challenger expedition datasets.

Physical and Chemical Properties

AAIW is characterized by a salinity minimum typically between 34.1 and 34.8 PSU and potential temperatures usually from about 3 to 8 °C, with density anomalies that set it apart from overlaying and underlying water masses. Chemical tracers show distinct signals: low oxygen relative to surface waters due to aging along pathways, intermediate concentrations of nutrients such as nitrate and phosphate derived from remineralization, and measurable anthropogenic tracers like CFC-11 and CFC-12 indicating ventilation age. Isotopic markers such as dissolved inorganic carbon radiocarbon content connect AAIW to sources influenced by exchange with the atmosphere and mixing with Circumpolar Deep Water and Antarctic Bottom Water.

Role in Global Ocean Circulation

AAIW functions as an intermediate limb of the global meridional overturning circulation linking high-latitude exchange near the Southern Ocean with tropical and subtropical thermocline waters. By carrying freshwater and biogeochemical properties equatorward, it interacts with major currents including the South Equatorial Current, Agulhas Current, and Peru–Chile Current. Its subduction and return flow affect heat and salt redistribution relevant to the global transport budgets described in studies by institutions such as Scripps Institution of Oceanography, National Oceanic and Atmospheric Administration, and Bermuda Atlantic Time-series Study programs.

Ecological and Climatic Impacts

As a conduit for nutrients and oxygen, AAIW influences productivity and species distributions in pelagic ecosystems, affecting communities studied in regions like the Subantarctic Zone and Humboldt Current System. Changes in AAIW properties can alter habitat conditions for commercially important taxa managed by organizations such as the Commission for the Conservation of Antarctic Marine Living Resources. Climatically, variability in AAIW formation and pathways modulates regional heat uptake, contributes to sea level change via steric effects, and feeds back onto atmospheric patterns tied to the Southern Annular Mode and El Niño–Southern Oscillation. Observational and modelling efforts by groups including IPCC working groups and national oceanographic programs continue to assess its role in a warming climate.

Category:Oceanography Category:Water masses Category:Southern Ocean