Like many people, while in school I learned there were four Oceans on Earth - Pacific, Atlantic, Indian, Arctic. In general I think it was described that the currents and the surrounding continents are what divided them into different oceans.
Now there is a Southern (Antarctic Ocean). I thought (when I was taught) that that area was just Southern Pacific and Southern Atlantic, but I'm sort of fuzzy.
How (or why) was it determined the Southern Ocean is an Ocean?