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There is no universally accepted theory of AAIW formation. The classical view (Deacon 1933; Wu¨st 1935) ascribes the low salinity layer to sinking along constant density surfaces from the region near the Polar Front Zone, from where the water mass spreads northward. Sverdrup (1940) suggested that AAIW formation is associated to convergence and subse- quent sinking of upper layer waters induced by me- ridional structure of the wind field over the Southern Oceans. The sources for AAIW are mixtures of Ant- arctic Surface Water and Circumpolar Deep Water upwelled along the axis of the Antarctic Circumpolar Current. Alternatively, it has been suggested that AAIW is a product of relatively deep convection in the Sub-Antarctic Zone, north of the Sub-Antarctic Front. The latter water mass is referred to as Sub- Antarctic Mode Water (SAMW, McCartney 1977). Late winter convection forms relatively thick layers of quasi-homogeneous temperature, salinity, and density. These layers are referred to as thermostads (e.g. are quasi-homogeneous in temperature) or pyc- nostads (e.g. are quasi-homogeneous in density). Thus, from a volumetric point of view, the products of convection are modes, from which the water mass name is derived. Mode waters are also found in the subtropical and subpolar oceans in the Northern Hemisphere. After convection mode waters flow along constant density surfaces and are capped by the warm, salty upper layer waters characteristic of the midlatitude oceans. Mode water properties de- pend on the initial temperature and salinity of the surface layer and on the sea-air exchanges of heat and freshwater through the balance between evapora- tion and precipitation. The coldest, freshest, and densest variety of SAMW is found in the eastern South Pacific. Density progressively decreases east- ward as temperature and salinity increase. Because

the Smin observed in the western South Atlantic is

fresher and denser than the densest southeast Pacific SAMW, an Antarctic influence from the Polar Front Zone in the vicinity of Drake Passage and further modifications by sea-air interactions within the south- west Atlantic have been suggested. These AAIW transformations are also apparent in global ocean circulation models. Thus, southwest Atlantic AAIW appears to be formed by a complex combination of the mechanisms described above. Warmer and saltier SAMW is formed throughout the South Indian and western South Pacific oceans, but these waters are

significantly lighter than AAIW (26.8 kg m3

) and therefore ventilate density horizons shallower than the salinity minimum layer.

Circulation

The most vigorous element of the circulation at the core of AAIW is associated to the circumpolar flow within the Antarctic Circumpolar Current. This flow, primarily along the Sub-Antarctic Front, is a signifi- cant conduit of exchange between ocean basins. Far- ther north, within each ocean basin, AAIW describes a large-scale counterclockwise loop. These flow pat- terns, also known as gyres, are generated by the curl of the wind stress at midlatitudes, caused by the op- posing directions of the trades and the westerlies. AAIW flows northward on the eastern side of each

basin, turns eastward at about 20
S and southward

along the western boundary. The flow along this path is not uniform; velocity increases as the water approaches the western boundary and is most intense along the so-called western boundary currents that carry AAIW back southward. This asymmetry in the circulation is also reflected on the depth of the AAIW layer, which lies deeper in the western side of the oceans. In the Southern Hemisphere the western boundary currents in the Atlantic, Indian, and Pacific oceans are the Brazil, Agulhas, and East Australian currents, respectively. These are regions of enhanced turbulence associated to eddies and meanders where intense mixing of the different varieties of AAIW is observed. The recirculated AAIW observed at the poleward extensions of the western boundary currents are generally saltier and lower in dissolved oxygen than near the eastern boundary, since they have been subject to vertical mixing with the saltier layers above and below for a longer period of time. Leakage of Indian Ocean AAIW into the eastern South Atlan- tic via the Agulhas Current leads to increased sali-

nities at the Smin. In the western South Atlantic and

South Pacific oceans AAIW from the Sub-Antarctic Front flow northward along the western Argentine Basin (the Falkland/Malvinas Current) and the east- ern flank of the Campbell Plateau, respectively. The Falkland Current is a substantial source of nutrients that sustains the development of phytoplankton and fishery along the eastern edge of the Patagonian shelf. These waters mix with the recirculated varieties of AAIW flowing in the opposite direction but there is no evidence of northward penetrations of AAIW along the western boundary below the western boundary currents southward extensions as originally thought.

The marked northward penetration of AAIW in the Atlantic is presumably a response to the so-called Global Thermohaline Circulation. In this large-scale flow, dense, salty water sinks to 2000–3500 m depths in the northern North Atlantic, flows southward, and ANTARCTIC INTERMEDIATE WATER

is exported to the Indian and Pacific oceans. Export of deep waters is balanced by a return flow through the South Atlantic and into the North Atlantic, most- ly at shallower levels. This return flow is composed by AAIW and thermocline waters. Thus, northward flow of AAIW in the Atlantic is enhanced by this large- scale meridional cell.

Variability

Variability of AAIW can be attributed to a combina- tion of changes in the sea-air heat and freshwater exchanges at the regions of formation, variations in the thermal and haline characteristics of the source water, and variations in the circulation patterns. These processes can lead to changes in water mass properties in a variety of time and spatial scales. Once the water sinks its properties change by mixing with the surrounding waters, but mixing in the ocean inte- rior is weak and the water retains the signature of the formation conditions. Thus, away from the sea surface, water mass variability provides a record of changes at the sites of formation and may serve as indicators of climate variability. Time series are only available at few locations in the Southern Ocean, and knowledge of the variability is mostly based on comparison of the thermal and haline characteristics determined from hydrographic observations occupied at different times. Within these limitations, changes in the AAIW and SAMW properties in the Southern Ocean are now relatively well documented. In the period from the early 1960s to the late 1980s, AAIW and the denser varieties of SAMW throughout the South Indian Ocean and in the Tasman Sea have freshened and cooled. Freshening is consistent with an increased excess precipitation over evaporation in the region of AAIW formation and has been inter- preted as a signature of an increase in the strength or intensity of the hydrological cycle. Similarly, warming of the lightest varieties SAMW is associated to long- term warming of the surface waters in the formation region north of the Sub-Antarctic Front. This obser- vation is consistent with predictions of global warm-

ing associated to increased atmospheric CO2.

Observations in the South Indian Ocean, including data collected in 2002, suggest that after the late 1980s the warmer varieties of SAMW have become saltier, almost entirely reversing the effect of freshen- ing observed in the previous decades, while the salini- ty of the colder SAMW continued decreasing. Given

the few observations available in the Southern Hemi- sphere, it is not yet possible to determine whether the observed changes in water mass properties are related to climate trends or to decadal oscillations.

ALBERTOR. PIOLA

See also Antarctic Surface Water; Atmospheric Gas Concentrations from Air Bubbles; Circumpolar Current, Antarctic; Circumpolar Deep Water; Polar Front; Southern Ocean Circulation: Modeling; Southern Ocean: Climate Change and Variability; Southern Ocean: Fronts and Frontal Zones; Southern Ocean: Vertical Structure; Thermohaline and Wind-Driven Circulations in the Southern Ocean

References and Further Reading

Bryden, H. L., E. L. McDonagh, and B. A. King. ‘‘Changes in Water Mass Properties: Oscillations or Trends?’’ Science 300 (2003): 2086–2088.

Deacon, G. E. R. ‘‘The Hydrology of the Southern Ocean.’’ In Discovery Reports 7, Cambridge: Cambridge Univer- sity Press, 1933, pp. 171–238.

England, M. H., J. S. Godfrey, A. C. Hirst, and M. Tomczak. ‘‘The Mechanism for Antarctic Intermediate Water Renewal in a World Ocean Model.’’ Journal of Physical Oceanography 23 (1993): 1553–1560.

Johnson, G. C., and A. H. Orsi. ‘‘Southwest Pacific Ocean Water-Mass Changes Between 1968/69 and 1990/91.’’ Journal of Climate 10 (1997): 306–316.

Levitus, S., and T. Boyer. World Ocean Atlas 1994. NOAA Atlas NESDIS. Washington, DC: US Department of Commerce, 1994.

McCartney, M. S. ‘‘Subantarctic Mode Water.’’ In A Voy- age of Discovery, edited by M. V. Angel. Deep-Sea Re- search 24 (Supplement) (1977): 103–119.

Piola, A. R., and D. T. Georgi. ‘‘Circumpolar Properties of Antarctic Intermediate Water and Subantarctic Mode Water.’’ Deep-Sea Research 29 (1982): 687–711. Sabine, C. L., R. A. Feely, N. Gruber, R. M. Key, K. Lee, J.

L. Bullister, R. Wanninkhof, C. S. Wong, D. W. R. Wallace, B. Tilbrook, F. J. Millero, T-H. Peng, A. Kozyr, T. Ono and A. F. Rios. ‘‘The Oceanic Sink for Anthropogenic CO2.’’ Science 305 (2004): 367–371.

Sverdrup, H. U. ‘‘Hydrology, Report of B.A.N.Z. Antarctic Research Expedition, 1921–1931.’’ Oceanography Series A (3) (1940): 89–125.

Talley, L. D. ‘‘Antarctic Intermediate Water in the South Atlantic.’’ In The South Atlantic: Present and Past Circulation, edited by G. Wefer, W. H. Berger, G. Siedler, and D. J. Webb. Heidelberg: Springer, 1996, pp. 219–238. Tomczak, M., and S. Godfrey. Regional Oceanography: An

Introduction. London: Pergamon, 1994.

Wong, A. P. S., N. L. Bindoff, and J. Church. ‘‘Large-Scale Freshening of Intermediate Waters in the Pacifc and Indian Oceans.’’ Nature 400 (1999): 440–443.

Wu¨st, G. The Stratosphere of the Atlantic Ocean. English translation edited by W. Emery. New Delhi: Amerind Publishing, 1935.