The Southern Ocean

The Southern Ocean is the body of water that wraps right around the continent of Antarctica at the bottom of the world. It is unusual because it does not have any land boundaries to the north, just open water connecting to the Pacific, Atlantic and Indian Oceans. The Southern Ocean is the newest officially recognised ocean, only added to the list in 2000.

• Surface area21.9 million km²Fourth biggest ocean
• Where is it?Around AntarcticaFrom the Antarctic coast up to 60°S
• Coldest tempAround -2 °CWhere the sea freezes
• Average depth4,500 mAmong the deepest oceans
• Famous currentAntarctic CircumpolarThe strongest current in the world
• Famous animalEmperor penguinLives and breeds on the Antarctic sea ice

Where the Southern Ocean ranks

Area in millions of km².

Area (million km²)

Pacific168.7

Atlantic85.1

Indian70.6

Southern21.9

Arctic15.6

The Southern Ocean is bigger than the Arctic but much smaller than the three big oceans. Its existence as a separate ocean was only officially recognised in 2000.

What is the Southern Ocean?

The Southern Ocean is the ring of water around Antarctica. Most geographers say its northern boundary is at 60 degrees south of the equator, which is where the cold Antarctic waters meet the warmer waters of the southern Pacific, Atlantic and Indian Oceans. The boundary follows the path of the Antarctic Circumpolar Current.

The National Geographic Society officially recognised the Southern Ocean as a separate ocean in 2000, though there is still some debate among scientists about whether it should be its own ocean or just a southern extension of the others.

The Antarctic Circumpolar Current

Running through the Southern Ocean is the most powerful current in the world: the Antarctic Circumpolar Current. It flows clockwise (when seen from above the South Pole) right around Antarctica, carrying approx. 100 times more water than all the rivers on Earth combined. Because there is no continent to block its path, it has been spinning around Antarctica unbroken for approx. 30 million years.

The current acts like a giant fence, keeping the warm waters of the rest of the world away from Antarctica. This is one of the main reasons Antarctica is so cold and so icy. Before the current formed, Antarctica had forests.

Fact The Southern Ocean is the windiest and stormiest sea on Earth. The "Roaring Forties", "Furious Fifties" and "Screaming Sixties" (named after the latitudes they blow in) are bands of constant strong winds that have terrified sailors for centuries.

Wildlife of the Southern Ocean

Despite the cold, the Southern Ocean is one of the richest seas on Earth for wildlife. The base of the food chain is Antarctic krill, a small shrimp-like animal that lives in enormous swarms. Krill feeds almost everything bigger: penguins, seals, whales, fish, sea birds.

Six species of penguin breed in the region, including the famous emperor penguin (which actually breeds on the sea ice during the dark Antarctic winter). Seals include the leopard seal, the crabeater seal and the huge southern elephant seal. Whales include the blue whale (the biggest animal ever to live), the orca, the humpback and the southern right whale.

The Southern Ocean and climate

The Southern Ocean plays a huge role in regulating Earth's climate. Its cold water absorbs more carbon dioxide than any other part of the ocean. Scientists estimate that the Southern Ocean alone takes up approx. 40% of all the carbon dioxide we humans are putting into the atmosphere.

Without the Southern Ocean, climate change would already be far worse. But there is also a worry: as the ocean warms, it may not be able to soak up as much carbon dioxide in future.

Did you know? Antarctica was completely uninhabited by humans for thousands of years. Today there are several research bases dotted around the continent, with approx. 1,000 to 5,000 scientists living there depending on the season. No country owns Antarctica.

Deeper dive: the Polar Front, krill ecology and the Antarctic Treaty

The northern boundary of the Southern Ocean is marked by the Antarctic Polar Front (sometimes called the Antarctic Convergence). This is the line where cold, dense Antarctic surface water meets warmer sub-Antarctic water and sinks beneath it. The front is a real biological boundary: the species on either side are dramatically different. Penguins, krill and the most cold-adapted seals live south of it; albatrosses, tuna and many other open-ocean species range up to it but rarely cross.

Antarctic krill (Euphausia superba) is one of the most numerous animal species on Earth by biomass. A single swarm can contain billions of individuals and weigh hundreds of millions of tonnes. Krill feed on tiny algae called phytoplankton that grow on and under the sea ice; their populations rise and fall with the ice. Climate change may be a serious threat to the entire Southern Ocean food chain through its effects on krill. There is also a small but growing commercial krill fishery for fish oil supplements and aquaculture feed, which has raised conservation concerns.

Antarctica and the seas around it are protected by an unusual international agreement called the Antarctic Treaty, signed in 1959 and in force since 1961. The treaty sets Antarctica aside for peaceful scientific research, bans military activity, suspends any country's territorial claims, and prohibits nuclear explosions or radioactive waste disposal. The treaty has 56 signatory countries today and is widely regarded as one of the most successful international agreements ever made. A separate convention (CCAMLR) regulates fishing in the Southern Ocean.

The other polar ocean is the Arctic at the top of the world. The biggest of all is the Pacific. The warmest is the Indian.