In physical oceanography, a type of water mass in the Southern Ocean thought to originate mainly through convective overturning of surface waters during winter west of South America, after which it is injected into the subtropical gyre and fills the southern subtropics and tropics from the east.

In the Atlantic, the densest SAMW found in the Subantarctic Zone between the Subantarctic Front and the Subtropical Front is thought to be the primary precursor to AAIW, although some postulate substantial input across the Subantarctic Front. The AAIW in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop. AAIW from the Indian Ocean is added to the Atlantic AAIW via Agulhas Current leakage. The AAIW is recognized by a subsurface oxygen maximum and a salinity minimum north of about 50°S, although the oxygen maximum becomes weak north of 15°S. The oxygen maximum is found at a slightly lower density than the salinity minimum.

The salinity minimum is found at about 300 m near the Subantarctic Front at around 45°S, descends northward to 900 m at 30°S near the subtropical gyre center, and rises again to 700 m at the equator. The AAIW spreads to the North Atlantic, identified by a salinity minimum near the equator at a σθvalue of about 27. 3. This minimum has been found to 24°N, although traces of AAIW can be followed as far north as 60°N. AAIW is characterized by a temperature near 2. 2° C and a salinity around 33. 8 near its formation region, but erodes by the time it reaches the Subtropical Front to values closer to 3° C and 34. 3.