Antarctica is located in what is commonly called the Southern Ocean, one of the richest marine ecosystems in the world. Three basic water masses comprise the Southern Ocean: Antarctic Surface Water, Circumpolar Deep Water, and Antarctic Bottom Water. The boundaries between the water masses tend to be sharp. On the continental shelf of Antarctica, there are two main water masses: Surface Shelf Water and a modified version of Circumpolar Deep Water.

Each water mass has different characteristics, and these differences drive circulation around the continent. As part of the global heat engine, the Antarctic has a major role in the world’s transfer of energy. Its ocean/atmosphere system is known to be both an indicator and a component of climate change. Oceanographers are attempting to improve our understanding of this oceanic environment, including global exchange of heat, salt, water, and trace elements, sea-ice dynamics, and marine biosphereresearch.

One of the most important ocean processes-one that is uniquely related to the Antarctic-is ocean ventilation, the process by which the deep ocean affects the atmosphere on the time scale of decades to centuries. If we could mark a cubic meter of sea water and follow its global meanderings through the various oceanic current systems, we would find that it spends most of its time isolated in the deep ocean, where it is dark and cold. Only occasionally-once every 600 years on the average-would it appear on the surface, and then only south of the Antarctic Circumpolar Current. This process is called overturning or ventilation of the ocean. Typically, when deep water reaches the surface, it gives up heat to the much colder atmosphere and picks up dissolved atmospheric gases, including carbon dioxide. The ventilation or overturning of the ocean can significantly affect climate change.

Changing Sea Levels

Three circumstances would result in sea level changes:

  • Expansion of the volume of the world’s oceans as they warm up
  • Melting of valley glaciers
  • Changes in Antarctic and Greenlandic ice sheets.

Existing data suggest that the average world sea level has risen by 10 cm in the last century. During that time, the Antarctic ice sheet is believed to have remained roughly stable. It appears that sea level will continue rising by 1 to 2mm per year, without any extra contribution from the Antarctic. Even minor melting of the Antarctic ice cap will significantly increase the rate of rise.

The Antarctic Convergence

  • The Antarctic Convergence, which encircles Antarctica roughly 1,500 kilometers off the coast, divides the cold southern water masses and warmer northern waters.
  • An ocean current, the world’s largest, moves eastward around the continent at an average speed of about half a knot, four times greater than the Gulf Stream.
  • Sea ice up to 3 meters thick forms outward from the continent every winter, making a belt 500 to 1,500 kilometers wide.
  • Even in summer the sea ice belt is 150 to 800 kilometers in most places.
  • The area of sea ice varies from 3 million square kilometers in summer to 20 million square kilometers in late winter.
  • This water comprises 10% of the world’s oceans; as well as connecting the Atlantic, Pacific and Indian oceans, it also isolates the continent from warmer waters.

Did You Know?

  • The seas south of the Antarctic Convergence contain the coldest and densest water in the world.
  • This water, called Antarctic bottom water, is formed as seawater sinks to the ocean floor when ice shelves melt.
  • It then moves along the ocean floor into the Northern Hemisphere, where it adds oxygen and reduces the temperature of these seas to less than 2°C.
  • This cooling effect on trop ical and temperate seas is an important feature of the world’s heat balance.

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