The Southern Ocean refers to the ring of ocean that circles Antarctica. While the Antarctic continent provides a clear southern boundary, the northern limit of the Southern Ocean is not so clearly defined. Oceanographers usually consider the Subtropical Front — a transition zone between cool, fresh, nutrient-rich subantarctic waters and warm, salty, nutrient-poor subtropical waters — to indicate the northern extent of the Southern Ocean. Although the position of the Subtropical Front varies with longitude, it lies roughly along 40°S for much of the Southern Ocean. Defined in this way, the Southern Ocean occupies about 20% of the surface area of the global ocean.
The Southern Ocean is notorious for having some of the strongest winds and largest waves on the planet. It is also home to the largest current in the world ocean, the Antarctic Circumpolar Current. The Circumpolar Current carries between 135 and 145 million cubic meters of water per second from west to east along a 20,000km long path around Antarctica. While the speed of the current is not extraordinary (about 0.5m/sec, or 1 knot, at the surface), the great depth (4km) and breadth (100–200km) of the current results in a massive transport of water. The flow of the Circumpolar Current is equivalent to about 150 times the flow of all the world’s rivers combined, or 500 billion cans per second of your favourite cold beverage.