Southern Ocean

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Map of the Southern Ocean

The Southern Ocean, also known as the Antarctic Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling the continent of Antarctica. As such, it is regarded as the fourth-largest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian Oceans but larger than the Arctic Ocean. This ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters.

By way of his voyages in the 1770s, Captain James Cook proved that waters encompassed the southern latitudes of the globe. Since then, geographers have disagreed on the Southern Ocean's northern boundary or even existence, considering the waters as various parts of the Pacific, Atlantic, and Indian Oceans, instead. However, according to Commodore John Leech of the International Hydrographic Organization (IHO), recent oceanographic research has discovered the importance of Southern Circulation, and the term "Southern Ocean" has been used to define the body of water which lies south of the northern limit of that circulation. This remains the current official policy of the IHO, since a 2000 revision of its definitions including the Southern Ocean as the waters south of the 60th parallel has not yet been adopted.

Geography

1906 map of the Antarctica showing the unknown boundries of the continent

The Southern Ocean, geologically the youngest of the oceans, was formed when Antarctica and South America moved apart, opening the Drake Passage, roughly 30 million years ago. The separation of the continents allowed the formation of the Antarctic Circumpolar Current.

With a northern limit at 60°S, the Southern Ocean differs from the other oceans in that its largest boundary, the northern boundary, does not abut a landmass (as it did with the first edition of Limits of Oceans and Seas). Instead, the northern limit is with the Atlantic, Indian and Pacific Oceans.

One reason for considering it as a separate ocean stems from the fact that much of the water of the Southern Ocean differs from the water in the other oceans. Water gets transported around the Southern Ocean fairly rapidly because of the Antarctic Circumpolar Current which circulates around Antarctica. Water in the Southern Ocean south of, for example, New Zealand, resembles the water in the Southern Ocean south of South America more closely than it resembles the water in the Pacific Ocean.

The Southern Ocean has typical depths of between 4,000 and 5,000 m (13,000 and 16,000 ft) over most of its extent with only limited areas of shallow water. The Southern Ocean's greatest depth of 7,236 m (23,740 ft) occurs at the southern end of the South Sandwich Trench, at 60°00'S, 024°W. The Antarctic continental shelf appears generally narrow and unusually deep, its edge lying at depths up to 800 m (2,600 ft), compared to a global mean of 133 m (436 ft).

Equinox to equinox in line with the sun's seasonal influence, the Antarctic ice pack fluctuates from an average minimum of 2.6 million square kilometers (1.0×106 sq mi) in March to about 18.8 million square kilometers (7.3×106 sq mi) in September, more than a sevenfold increase in area.

Natural resources

Manganese nodule

The Southern Ocean probably contains large, and possibly giant, oil and gas fields on the continental margin. Placer deposits, accumulation of valuable minerals such as gold, formed by gravity separation during sedimentary processes are also expected to exist in the Southern Ocean.

Manganese nodules are expected to exist in the Southern Ocean. Manganese nodules are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization. Interest in the potential exploitation of polymetallic nodules generated a great deal of activity among prospective mining consortia in the 1960s and 1970s.

The icebergs that form each year around in the Southern Ocean hold enough fresh water to meet the needs of every person on Earth for several months. For several decades there have been proposals, none yet to be feasible or successful, to tow Southern Ocean icebergs to more arid northern regions (such as Australia) where they can be harvested.

Natural hazards

Three ships moving an iceberg to clear shipping lanes in the Southern Ocean

Icebergs can occur at any time of year throughout the ocean. Some may have drafts up to several hundred meters; smaller icebergs, iceberg fragments and sea-ice (generally 0.5 to 1 m thick) also pose problems for ships. The deep continental shelf has a floor of glacial deposits varying widely over short distances.

Sailors know latitudes from 40 to 70 degrees south as the "Roaring Forties", "Furious Fifties" and "Shrieking Sixties" due to high winds and large waves that form as winds blow around the entire globe unimpeded by any land-mass. Icebergs, especially in May to October, make the area even more dangerous. The remoteness of the region makes sources of search and rescue scarce.

Physical oceanography

Antarctic Circumpolar Current and Antarctic Convergence

Antarctic Circumpolar Current

The Antarctic Circumpolar Current moves perpetually eastward – chasing and joining itself, and at 21,000 km (13,000 mi) in length – it comprises the world's longest ocean current, transporting 130 million cubic meters per second of water – 100 times the flow of all the world's rivers.

Several processes operate along the coast of Antarctica to produce, in the Southern Ocean, types of water masses not produced elsewhere in the oceans of the Southern Hemisphere. One of these is the Antarctic Bottom Water, a very cold, highly saline, dense water that forms under sea ice.

Associated with the Circumpolar Current is the Antarctic Convergence encircling Antarctica, where cold northward-flowing Antarctic waters meet the relatively warmer waters of the subantarctic, Antarctic waters predominantly sink beneath subantarctic waters, while associated zones of mixing and upwelling create a zone very high in nutrients. These nurture high levels of phytoplankton with associated copepods and Antarctic krill, and resultant foodchains supporting fish, whales, seals, penguins, albatrosses and a wealth of other species.

Upwelling

Water mass bodies of the Southern Ocean

Large-scale upwelling is found in the Southern Ocean. Strong westerly (eastward) winds blow around Antarctica, driving a significant flow of water northwards. This is actually a type of coastal upwelling. Since there are no continents in a band of open latitudes between South America and the tip of the Antarctic Peninsula, some of this water is drawn up from great depths. In many numerical models and observational syntheses, the Southern Ocean upwelling represents the primary means by which deep dense water is brought to the surface. Shallower, wind-driven upwelling is also found off the west coasts of North and South America, northwest and southwest Africa, and southwest and southeast Australia, all associated with oceanic subtropical high pressure circulations.

Some models of the ocean circulation suggest that broad-scale upwelling occurs in the tropics, as pressure driven flows converge water toward the low latitudes where it is diffusively warmed from above. The required diffusion coefficients, however, appear to be larger than are observed in the real ocean. Nonetheless, some diffusive upwelling does probably occur.

Ross and Weddell Gyres

Oceanographic frontal systems in the Southern Ocean

The Ross Gyre and Weddell Gyre are two gyres (a large system of circulating currents) that exist within the Southern Ocean. The gyres are located in the Ross Sea and Weddell Sea respectively, and both rotate clockwise. The gyres are formed by interactions between the Antarctic Circumpolar Current and the Antarctic Continental Shelf.

Sea ice has been noted to persist in the central area of the Ross Gyre. There is some evidence that global warming has resulted in some decrease of the salinity of the waters of the Ross Gyre since the 1950s.

Due to the Coriolis effect acting to the left in the Southern Hemisphere and the resulting Ekman transport away from the centers of the Weddell Gyre, these regions are very productive due to upwelling of cold, nutrient rich water.

Enviroment

Increased solar ultraviolet radiation resulting from the Antarctic ozone hole has reduced marine primary productivity (phytoplankton) by as much as 15% and has started damaging the DNA of some fish. Illegal, unreported, and unregulated fishing, especially the landing of an estimated five to six times more Patagonian toothfish than the regulated fishery, likely affects the sustainability of the stock. Long-line fishing for toothfish causes a high incidence of seabird mortality.

All international agreements regarding the world's oceans apply to the Southern Ocean. In addition, it is subject to additional agreements specific to the region such as the Southern Ocean Whale Sanctuary of the International Whaling Commission (IWC) which prohibits commercial whaling south of 40 degrees south (south of 60 degrees south between 50 degrees and 130 degrees west). Japan regularly does not recognize this provision, because the sanctuary violates IWC charter. Since the scope of the sanctuary is limited to commercial whaling, in regard to its whaling permit and whaling for scientific research, a Japanese fleet carried out an annual whale-hunt in the region. On 31 March 2014, the International Court of Justice ruled that Japan's whaling program, which Japan has long claimed is for scientific purposes, was a cloak for commercial whaling, and no further permits would be granted.

Many nations prohibit the exploration for and the exploitation of mineral resources south of the fluctuating Antarctic Convergence, which lies in the middle of the Antarctic Circumpolar Current and serves as the dividing line between the very cold polar surface waters to the south and the warmer waters to the north. The Antarctic Treaty covers the portion of the globe south of sixty degrees south, it prohibits new claims to Antarctica.

The Convention for the Conservation of Antarctic Marine Living Resources applies to the area south of 60° South latitude as well as the areas further north up to the limit of the Antarctic Convergence.

Economic activity

Antarctic krill

Between 1 July 1998 and 30 June 1999, fisheries landed 119,898 tonnes, of which 85% consisted of krill and 14% of Patagonian toothfish. International agreements came into force in late 1999 to reduce illegal, unreported, and unregulated fishing, which in the 1998–99 season landed five to six times more Patagonian toothfish than the regulated fishery.