Poppycock! Since when is there a "Southern Ocean"? All that down there is the combination of the Indian, Pacific and Atlantic. The only circumpolar "ocean" is the Arctic. not that the oceans are really made of anything different anyway . . .

Careful Jazzo, there are oceanographers aBoard! The "Southern Ocean" is not one of the Big 3 but it is important in an oceanographic sense. The number one reason (which brings me to your second statment) is that it is the only circumpolar ocean. The Arctic has some little problems with that where LAND is in the way - preventing a current from running in a circle all around the world at a given latitude (called a zonal current). On the other hand, with the way the Southern Hemisphere turned out, you actually have a zonal current running in a circle around the South Pole between about 40°S and 70°S. It's called - wait for it - the Antarctic Circumpolar Current! (South America forces it to narrow a bit but there's still plenty of room for a zonal current.)

And of course they are all made of different things! Different temperatures and different salinities. They even have names for the different water types. The different water types are important for global ocean circulation. And global ocean circulation is important for many people aBoard, for example, because the Gulf Stream keeps the Eastern US and England warmer than they really should be in the winter (given their latitudes).

Anyway, there are noticeable "fronts" between water types in the ocean (just like between air masses on weather maps). Here is some explanation from the most boring book ever written (Descriptive Physical Oceanography by Pickard and Emery)

"Going north from the Antarctic continent the average sea surface temperature increases slowly until a region is reached where a relatively rapid increase of 2 to 3K [Kelvin] occurs. The surface water from south of this region is moving north and sinks when it reaches the region, continuing north below the surface. At the surface therefore the water is converging to this region which...is now called the Antarctic Polar Front (APF)...Continuing north from this APF the temperature rises slowly to a second region where it rises rapidly by about 4K and the salinity by about 0.5 [PSU]. This is referred to as the Subtropical Convergence."

And on it goes like that!

Some ocean factoids so you realize not all seawater is the same:
(1) The mediterranean has the most saline water.
(2) The coldest, densest water on earth (Antarctic Bottom Water) is formed near the coastal shelf of Antarctica.
(3) The Pacific Ocean has slightly less saline water than the Atlantic Ocean.
(4) The second-coldest, densest water is formed in the Labrador Sea or the Greenland-Iceland-Norwegian (usually called the GIN ) seas and is called North Atlantic Deep Water.
(5) There is a so-called "conveyor belt" of ocean circulation in which water moves very slowly all over the world called the Thermohaline Circulation because the driving forces are heat (thermo) and salt (haline). Roughly, water sinks in the Southern Ocean, moves northward in the Pacific and rises near the west coast of North America, continues along the surface north of Australia, south of Africa, joins up with the Antarctic Circumpolar Current but also moves north along the east coast of South and North America, including the Gulf Stream, sinks near Newfoundland and Greenland, and moves along the bottom back to the Southern Ocean.

Man, you guys have been giving my oceanographic muscles a workout this week! Isn't it just the neatest subject?

Edit: Here's a link to a "cartoon" of thermohaline circulation (it's, of course, far more complicated than this looks). http://www.clivar.org/publications/other_pubs/clivar_transp/pdf_files/av_d3_992.pdf