Why are there 5 oceans? 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?
The Southern Ocean does have its own current, the Antarctic Circunmpolar Current. 

It helps generate the Circumpolar Deep Water due to density gradients. Ice melts and thus salinity decreases. Differences in salinity are the reason for bulk water movements in the ocean.

Thus the Southern Ocean contributes to cool the Earth. 

Geographical criteria may be open for discussion, but oceanographically speaking it is a separate system from its neighbors, the Atlantic, Pacific and Indian.