The Southern Ocean was officially designated to be the 5th ocean by the International Hydrographic Organization (IHO) in the spring of 2000. A note on the CIA World factbook website says that "It should be noted that inclusion of the Southern Ocean does not imply recognition of this feature as one of the world's primary oceans by the US Government."
The geography section of the About.com website gives some background on the designation of the Southern Ocean by the IHO after a vote by its member countries.
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