The Southern Ocean & Seas Of Antarctica

The Southern Ocean, sometimes called the Antarctic Ocean, is that great marine realm encircling Antarctica. Captain James Cook was the first to name it: In February 1775, musing about an as-yet unconfirmed polar continent at the bottom of the world, he wrote, “I firmly believe there is a tract of land near the Pole which is the source of most of the ice which is spread over this vast Southern Ocean.”

The Southern Ocean was officially recognized as a distinct ocean basin—as opposed to the murky southern sectors of the Atlantic, Pacific, and Indian oceans—by the National Geographic Society in 2021, though other authorities, including the U.S. National Oceanic & Atmospheric Administration, had formalized it earlier.

What’s in a Name? Defining the Southern Ocean

As Captain Cook’s account attests, the label “Southern Ocean” has long been applied, loosely or otherwise, to Antarctic and sub-Antarctic waters. “Antarctic Ocean” has been—and, to some extent, continues to be—widely used, but informally. Another name that’s occasionally been applied to this realm is “Austral Ocean.”

The International Hydrographic Organization (IHO), composed of nearly 100 coastal member countries, is an intergovernmental body that charts and labels the World Ocean. It formally recognized the Southern Ocean in 1937, but then retracted that recognition in 1953.

In 2000, the IHO again considered the Southern Ocean, with 28 countries responding to its request and all but one suggesting the ocean embracing Antarctica be given a unique and single label. The majority of those respondents preferred the name Southern Ocean over “Antarctic Ocean.” But the lack of consensus among the IHO’s membership means the organization hasn’t yet officially defined the Southern Ocean (in name or extent).

Yet it wasn’t until World Ocean Day on June 8, 2021 when National Geographic adjusted its map policy to formally recognize it as a distinct ocean basin that its new name was brought to the attention of the global media and adopted by the mainstream, albeit unofficially. Whilst the Society’s cartographers had long labeled it slightly differently and distinguished it typographically from the four historically recognized oceans—the Atlantic, Pacific, Indian, and Arctic—the decision to ‘create’ the world’s fifth ocean was not only a reflection of its ubiquitous usage in contemporary scientific and journalistic circles, but also made to further highlight its ecological uniqueness and focus public awareness onto a region in urgent need of the conservation spotlight.

Extent of the Southern Ocean

The northern limit of the Southern Ocean is not always exactly agreed-upon, but convention puts it in the belt of the Antarctic Convergence, a hugely significant boundary of ocean masses generally taken to mark the northern boundary of the Antarctic realm. The Antarctic Convergence—also called the Polar Front—reflects where cold polar waters from the south meet warmer waters from the southern Pacific, Atlantic, and Indian Ocean basins.

That convergence zone isn’t fixed—it meanders north or south depending on geography and time of year—but prevails roughly between 45 and 60 degrees South latitude. For convenience’s sake—and also because it roughly marks the heart of the great Antarctic Circumpolar Current, which we’ll get to shortly—that 60-degree latitude line is often taken to be the northern boundary of the Southern Ocean (as indeed the National Geographic Society recognizes as such).

Southern Ocean Snapshot

Before going further, how about a few Southern Ocean trivia nuggets?:

• Area: ~8.5 million square miles (~22 million square kilometers), accounting for about 10 or 15 percent of the area of the World Ocean
• Average Depth: ~11,000 feet (3,000 meters)
• Deepest Point: 24,383 feet (7,432 meters) in the South Sandwich Trench’s Factorian Deep
• Temperature: Between about 50 degrees F (10 degrees C) and 28 degrees F (-2.2 degrees C)

The Southern Ocean is the fourth-smallest ocean basin, smaller than the Indian but larger than the Arctic Ocean. Its Factorian Deep, set at the southern end of the South Sandwich Trench near the South Sandwich Islands, ranks third among the five World Ocean basins in depth. (For comparison, the Pacific Ocean includes the deepest seafloor of all in the 35,853-foot/10,928m Challenger Deep at the bottom of the Mariana Trench, while the Arctic Ocean has the shallowest deep point: the 18,209-foot/5,550m Molloy Deep.)

Surveying & Mapping the Southern Ocean

The Factorian Deep—visited via submersible for the first time in 2019 by Victor Vescovo on his Five Deeps Expedition—was only definitively recognized as the Southern Ocean’s deepest point in 2022, when a majorly expanded swath of the basin’s seafloor was mapped via the International Bathymetric Chart of the Southern Ocean (IBSCO). This map covers better than twice the area of the previous edition published in 2013, charting an enormous tract of ocean some five times the size of Canada.

It’s the latest in centuries of scientific exploration of this most remote and ferocious of ocean basins, starting with such early forays as Captain James Cook’s expedition south of the Antarctic Circle in the late 18th century. Many of the great early Antarctic explorations of the 19th and early 20th centuries yielded valuable information on the configuration and characteristics of the “Antarctic Ocean.” As recent discoveries such as the Factorian Deep demonstrate, this is still a great liquid frontier of exploration.

Formation of the Southern Ocean

The Southern Ocean was born in the same period as the White Continent, unsurprisingly. After all, this girding ocean is what separates Antarctica from the other Southern Hemisphere continents.

It formed between about 20 and 40 million years ago with the opening of the Tasmanian Seaway (between Australia and Antarctica) and the Drake Passage (between South America and Antarctica). This makes the Southern Ocean the youngest of the world’s great ocean basins.

Southern Ocean Winds & Currents

The Southern Ocean encircles the globe, joining the waters of the Pacific, Atlantic, and Indian oceans. Without any continental landmasses breaking up this circumpolar span of brine, westerly winds blow unimpeded, and are also powered up by the strong temperature and pressure gradient established between the temperate/subpolar and polar realms in the Southern Hemisphere by Antarctica’s sheer frigidity. This howling belt of westerlies explains the notorious gale latitudes of the “Roaring Forties,” “Furious Fifties,” and “Screaming Sixties.”

Those westerlies drag on the surface of the Southern Ocean, driving a east-flowing, globe-spanning current that—again, because no major landmasses get in the way of it—ranks as the largest and strongest of all ocean currents: the mighty Antarctic Circumpolar Current, or West Wind Drift.

This is an incredibly significant oceanic “river.” It and the Antarctic Convergence isolate Antarctica from the rest of the world and maintain that cold polar climate to the south. The current also has great importance as an upwelling center, as we’ll explain in a bit.

Which current flows around the coast of Antarctica? That would be the countercurrent to the West Wind Drift: the East Wind Drift, aka the Antarctic Coastal Current, which flows westward tight against the continent and its sea-ice extension. It ranks as the southernmost ocean current in the world, and another technically circumpolar one, though it’s warped by the protruding finger of the Antarctic Peninsula.

Rough Waters

The powerful circumpolar push of the West Wind Drift, the ferocious westerlies, and the extreme frontal boundaries of the Southern Ocean—all interrelated characteristics—make it a rough, wild, and stormy place indeed. This ocean is thought to be home to the tallest waves in the world, hence its nickname of the “liquid Himalayas.” In the especially stormy winter season, waves commonly exceed 33 feet (10m), and may be much taller. Data collected off the sub-Antarctic Kerguelen (or Desolation) Islands suggest maximum wave heights may sometimes reach 100 feet (30m).

Rough seas explain the label given to crossings of the Drake Passage—that 600-mile-wide portal by which the majority of travelers reach Antarctica—at their most tumultuous: the “Drake Shake.” Fortunately, modern-day cruise ships are well equipped to withstand big swells and dodge bad weather, so this fabled crossing is not so bad as it’s made out, and, for many is a thrilling rite of passage.

Southern Ocean Sea Ice

The Southern Ocean plays host to one of the most remarkable broad-scale seasonal cycles of any place on Earth: the yearly expansion and contraction of sea ice. In the austral winter, plunging temperatures cause seawater to start freezing, causing a great expansion of sea ice around Antarctica. Counting the halo of sea ice as an extension of Antarctica, the White Continent essentially doubles in size in winter, when the Southern Ocean’s ice covers roughly 11 million square miles (18 million square kilometers).

This maximum is typically achieved around September, in the early austral “spring,” when, on average, the northern limit of Southern Ocean sea ice effectively maps the Antarctic Convergence.

The sea ice retreats dramatically as summer unfolds, reaching a minimum extent in February and March. This summertime sea ice, which on average is most extensive in the Weddell, Bellingshausen, Amundsen, and Ross seas, is mainly fast ice: sea ice anchored either to the seafloor or to the continent, and thus more resilient to the surge of waves and currents.

The Southern Ocean & the Global System: Upwellings & Antarctic Bottom Water

The Southern Ocean has an outsized impact on the global environment. One of the great driving engines of the planetary-scale circulation of the World Ocean is Antarctic Bottom Water, a seafloor-hugging current that flows from the margins of Antarctica northward toward the Equator. It’s produced by the deep cold off the Antarctic continent, which chills coastal waters, and the freezing of sea ice, which increases the concentration of salt in the adjoining unfrozen seawaters. Colder water is denser than warmer water, saltier water denser than “fresher” water—thus the cold, salty water along the continent sinks into the depths.

The Antarctic Circumpolar Current serves as an incredibly important zone of oceanic mixing and overturning, where cold deepwater flow pulled in from the Atlantic, Pacific, and Indian oceans meets colder Antarctic waters and is shoved surfaceward, producing a major upwelling. Complex patterns of eddies in the Circumpolar Current as well as seafloor topography help drive this upwelling, which absorbs heat and carbon dioxide from the atmosphere. The upwelled water then flows back northward at the surface.

Much of the bottom water of the World Ocean seems to be upwelled by this process along the Antarctic Circumpolar Current, highlighting the huge significance of the Southern Ocean as a heat and CO2 sink and a crux of global oceanic circulation.

The Scotia Sea

The Scotia Sea forms a northern frontier of the Southern Ocean near its boundary with the South Atlantic. The sea comes edged on the north, east, and south by the Scotia Arc, which is the mostly submerged mountain range linking Tierra del Fuego and the Antarctic Peninsula.

Much of the Scotia Arc takes the form of drowned banks and seamounts, but the chain breaks the surface here and there in the form of seastacks, islets, and significant islands, including South Georgia, the South Sandwich Islands, the South Orkneys, and Elephant Island (where Shackleton’s shipwrecked Endurance party sheltered).

The western border of the Scotia Sea joins with the Drake Passage—known in Spanish-speaking countries as the Mar de Hoces (Hoces Sea)—separating the tip of South America from the South Shetland Islands and the Antarctic Peninsula. The Drake Passage forms the narrowest chokepoint of the Southern Ocean, with the powerful flow of the West Wind Drift surging through it into the Scotia Sea.

(It should be noted that not all definitions of the Southern Ocean include the Drake Passage and Scotia Sea. These waters are omitted, for example, from National Geographic’s definition, although the southern portions of both the Drake and the Scotia Sea lie below 60 degrees S latitude.)

Many Antarctic cruises voyage into the Scotia Sea by incorporating South Georgia—in many respects as remarkable a sightseeing destination as the White Continent itself—into their itineraries.

The Coastal Seas of Antarctica

The following is a list of the official coastal seas mapped along the margin of the Antarctic continent, beginning with the Weddell east of the Antarctic Peninsula and proceeding counterclockwise.

• Weddell Sea
• Bellingshausen Sea
• Amundsen Sea
• Ross Sea
• Samov Sea
• D’Orville
• Mawson
• Davis
• Cooperation
• Cosmonauts
• Riiser-Larsen
• Lazarev

The Weddell and Ross seas are by far the largest, forming the deepest embayments of the White Continent’s outline between West and East Antarctica. They’re also both coveted destinations for Antarctic voyagers, though seen by only a relative fraction of yearly sightseers to the White Continent.

The Ross Sea

The Ross Sea covers some 370,000 square miles (960,000 square kilometers), gated by Cape Colbeck in Edward VII Land to the east and Cape Adare in Victoria Land to the west. Its southern portion comes mantled by the enormous Ross Ice Shelf: at some 182,000 square miles (472,000 square kilometers), the world’s largest ice shelf.

While summertime sea ice typically extends into its eastern reaches, the Ross Sea is often otherwise quite accessible in that season. That accessibility, combined with the deep indentation it forms, made it a natural springboard for early Antarctic explorers aiming for the South Pole. Ross Island and McMurdo Sound come drenched in this history, with the historic huts of Ernest Shackleton’s and Robert Falcon Scott’s parties still viewable.

A globally significant Antarctic ocean sanctuary was established in the Ross Sea in 2016 with the creation of a 600,000-square-mile (1.6-million-square-kilometer) marine protected area where commercial fishing is banned.

The Weddell Sea

The Weddell is the largest Antarctic sea, covering about 1.08 million square miles (2.8 million square kilometers). It extends from the eastern coast of the Antarctic Peninsula to Coats Land of East Antarctica. LIke the Ross Sea, the Weddell Sea’s southern portion hosts a spectacular ice shelf: the Ronne-Filchner Ice Shelf, wrapped much of the way around the huge ice rise of Berkner Island. Sea ice historically has remained extensive in the Weddell Sea into summer, and it’s well known for its bountiful icebergs.

The Weddell is often claimed to be the world’s most pristine sea. It’s also thought to be the most significant source zone for Antarctic Bottom Water, lending it particular global significance when considering that deep current’s importance in the World Ocean’s circulation.

The Southern Ocean Ecosystem

From its major upwellings to the seasonal biomass spike produced by sea-ice melt, the Southern Ocean includes some of Earth’s most productive waters. Supported in large part by blooms of krill, the Antarctic marine ecosystem is mindbogglingly rich, reflected by the multitudes of seabirds, penguins, pinnipeds, and great whales serving as its defining higher-level consumers.

Given its remoteness and infamously treacherous waters, it’s little surprise the Southern Ocean has generated its fair share of sea-monster legends. Heck, as recently as 2016, the Internet lit up over an alleged Google Earth photograph of a gigantic kraken surfacing in this marine wilderness. As it happened, the photographed object was no tentacled beastie, but a lonesome, wave-hammered seastack south of Deception Island known as Sail Rock.

(Which, mind you, is not to say that oversized cephalopods don’t call the Southern Ocean home: The largest squid by mass, the aptly named colossal squid, is well documented in the deeps of these frigid waters. And there are other magnificently monstrous—in a good, biophilic kind of way—creatures in these depths, not least the giant Antarctic sea spider.)

The ecological magic of the Southern Ocean very much deserves its own treatment—and we’ve given it one in this article!

Conserving the Southern Ocean: Marine Protected Areas 

We mentioned above the establishment, in 2016, of the world’s biggest marine protected area (MPA) in the Ross Sea, based on an original proposal by the U.S. and New Zealand. But one other MPA currently exists in the Southern Ocean, the South Orkney Islands Southern Shelf MPA, which means that only about five percent of the ocean basin is fully protected.

In light of its uniqueness, productivity, relative pristineness, and fragility, numerous organizations advocate for the expansion of a MPA network in the Southern Ocean.

Among the specific areas that groups such as the Antarctic & Southern Ocean Coalition (ASOC), the Antarctic Ocean Alliance (AOA), and The Pew Charitable Trust have suggested for MPA status are the Weddell Sea and areas off the Antarctic Peninsula as well as the East Antarctica coastline.

New MPAs can be established through the agreement of all members of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). As of yet, the CCAMLR has reached no consensus on declaring the proposed Antarctic Peninsula, Weddell Sea, and East Antarctic MPAs.

Exploring the Southern Ocean & the Antarctic Seas

The majority of Antarctic tourism goes down by ship, and thus firsthand experience of the Southern Ocean is part and parcel of a vacation down here at the bottom of the world.

Whether you’re making the iconic crossing of the Drake Passage, venturing into the Scotia Sea to goggle at South Georgia’s rookeries and glaciers, seeking the emperor penguin amid the Weddell Sea bergs, or marveling at the skyward loom of the Ross Ice Shelf, much of the day-to-day magic of an Antarctic voyage transpires offshore.