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The Southern Ocean - a Description

The Southern Ocean was identified and named by Captain James Cook in the 1760s and its discovery predated that of Antarctica. This ocean constitutes about 15 per cent of the world’s total ocean surface (Miller 1991) and has a total area of about 28 million square kilometres (Stonehouse 2002). It comprises three major oceans: the Pacific, Atlantic and Indian Oceans. The Antarctic continent forms its southern boundary.

 

The Antarctic Convergence (or Antarctic Polar Front) surrounds the Southern Ocean at its northern boundary at between 50°S to 60°S, and it is a major oceanographic and biological boundary approximately 50 kilometres wide (Scott 1994).

 

Macquarie Island and the Kerguelen, Crozet and Prince Edward island groups lie in, or near, the Antarctic Convergence and they are usually considered to be part of the Southern Ocean.

 

Two important circumpolar currents affect the upper layers in the Southern Ocean, and mark large changes in temperature and salinity. The eastward-flowing Antarctic Circumpolar Current (West Wind Drift) connects the Pacific, Indian and Atlantic oceans. It flows more or less continuously in the north close to the Antarctic Convergence and is the largest ocean current in the world (CSIRO 2000). Near the Antarctic continent, the more discontinuous westward-moving East Wind Drift is broken into a series of clockwise gyres and local eddy systems close to the continental margin, such as the Weddell Sea Gyre (Miller 1991). These fronts mark large changes in temperature and salinity and they may also act as conveyers for transporting finfish around the Southern Ocean.

 

The Southern Ocean has great variations in depth although the mean depth is close to 4000 metres (Stonehouse 2002). The greatest depths (South Sandwich Trench) exceed 8000 metres.

 

This ocean significantly affects world climates in three ways (CSIRO 2000):

 

  • The Antarctic Circumpolar Current redistributes heat and other properties globally. This redistribution influences temperature and rainfall patterns.
  • Intermediate and deep waters renew the world’s other oceans. For example, the cooling of the Southern Ocean and formation of sea-ice during winter increase the water’s density, which sinks from the sea surface into the deep-sea to control the physical and chemical properties of these waters.
  • Gases such as oxygen and carbon dioxide are exchanged with the atmosphere at the sea surface, while being cooled. As this water sinks, it efficiently transfers heat, freshwater and absorbed gases from the atmosphere (including carbon released into the atmosphere by the burning of fossil fuels and deforestation) into the deep ocean.
Antarctic sea ice cover in February 1998 (above) and September 1998. Graphic derived special sensor Microwave Imager satellite data. (© Michael Van Woert, NOAA NESDIS, ORA)

Three major ecological zones, each with its own assemblage of species, can be distinguished in the Southern Ocean:

• an ice-free zone,

• a sea-ice zone and

• a permanent ice zone or high-latitude Antarctic zone.

 

The Southern Ocean is generally a region of low biological productivity. However, concentrations of high productivity are associated with the presence of sea-ice that surrounds Antarctica for most of the year, particularly at the ice edge (Miller 1991; El-Sayed 1978). In addition, the richness and diversity of benthic (or bottom dwelling) fauna are comparable to those of tropical regions, with a large number of long-lived and slow-growing forms present in the Southern Ocean (CCAMLR 2002).

 

 

References:

 

CCAMLR (2002). Understanding CCAMLR’s approach to management. Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), Hobart, Australia, Accessed 26 November 2002.

 

CSIRO (2000). The facts: Understanding our oceans. Southern Ocean and Antarctic circumpolar current. CSIRO Marine Research, Hobart Australia.

 

El-Sayed, S.Z. (1978). Primary production and estimates of potential yields of the Southern Ocean. In McWhinnie (ed.), Polar research to the present and the future, AAAS Selective Symposium, 7, 141-160.

 

Miller, D.G. (1991). Conservation of Antarctic marine living resources: A review and South African perspective. South African Journal of Antarctic Research, 21(2), 130-142.

 

Scott, J. (1994). Marine conservation at Macquarie Island. Tasmanian Parks and Wildlife Service, Hobart, Australia.

 

Stonehouse, B. (ed.) (2002). Encyclopaedia of Antarctica and the Southern Oceans. Scott Polar Research Institute, John Wiley and Sons Ltd, Chichester, UK.