NGS Style Manual - S -

These place-names are not recognized by NGS but are sometimes used in the Southern Hemisphere and by scientists to designate those parts of the Indian, Atlantic, and Pacific Oceans surrounding Antarctica.

The following explanation appears on plate 96 of the National Geographic Atlas of the World: About 1,600 km off-shore is the Antarctic Convergence, where cold waters meet warmer waters from the north, enclosing a distinct ecosystem sometimes called the Southern Ocean.

Another explanation prepared by NG Maps says:
The Atlantic, Indian, and Pacific Oceans merge into ice waters around Antarctica. Some define this as an oceancalling it the Antarctic Ocean, Austral Ocean, or Southern Oceanbut there is no agreement on the name and the extent of a fifth ocean.

When using these terms in text, explain them: He sailed through what Australians call the Southern Ocean; the circumpolar sea, which some scientists call the Southern, or Antarctic, Ocean.