Australia: The Land Where Time Began

A biography of the Australian continent 

Abyssal Water Circulation                                                                                                       

Abyssal water circulation, or deep ocean circulation, is the movement of water along the ocean floor through the deepest parts of the oceans.

Sea ice around the margin of Antarctica melts, the very cold, salty water sinks to the sea floor, travelling north as an abyssal current along the floors of all the ocean basins. In contrast to the major surface currents that are mainly wind generated, the abyssal current is powered by temperature and salinity gradients. Icebergs melt at greater distances from the edge of Antarctica, the resulting water being less salty and not quite as cold. This water mass travels north at a shallower depth, and doesn't travel as far north. By the time the water of the abyssal current has reached Greenland it has warmed, returning south to Antarctica as a middle layer. It is joined on its southward journey by water of a similar temperature and salinity that has formed around Arctic ice cap. On reaching Antarctic latitudes the water of this middle layer upwells against the colder water around Antarctica. This leads to snowfalls on Antarctica.

Recently a new abyssal current has been discovered that is at a depth of at least 3 km, about 4,200 km southwest of Perth, Australia, that is very fast for a deep ocean current, travelling at speeds of up to 700 m/hr, and moving volumes up to about 30 million cu m per second. The Ross Sea, off the Adelie Land coast, the part of Antarctica facing Australia, is the source of the current, the wind blowing from Antarctica cooling the water that sinks to the bottom where it travels east along the coast of the continent. On reaching the Kerguelen Plateau it is pushed by the Coriolis force along the eastern margin of the plateau, resulting in a current about 50 km wide flowing north at high speed.

This current is allowing scientists to find evidence of changes in the climate and environment as they affect the Southern Ocean. One of the first discoveries made is that the abyssal water of this current is becoming less saline as the ice sheets melt.

Another 'natural experiment' could lead to further clarification of the effects of climate change. In the Australian Antarctic Territory, the Mertz Glacier, that flowed at the rate of about 10-12 Gigatonnes per year, extended more than 160 km on to the sea from the grounding line. The seaward extension of the glacier, the Mertz Glacier Tongue, protected the sea to the west from icebergs that would otherwise enter it from the east. Scientists had been studying the tongue, following the development of 2 rifts in the tongue, on opposite sides. The rifts had begun to heal but in February 2010, the southern summer, A 98 km long iceberg, B-9B, rammed it from the east. The tongue broke along the rifts, and a 78 km long section became completely free.

While the sea to the west of the tongue was ice free the freezing wind form the Antarctic interior cooled the water rapidly causing it to sink and become part of the cold abyssal current system. Following the removal of the large section of the tongue the icebergs from the east will have easier access to the previously iceberg free sea and it is now being studied to see what effect the reduction of the open ocean water available for chilling by the wind from the Antarctic interior will have on the generation of deep ocean currents.

Since the collision, both huge icebergs have drifted westward towards a coastal basin, the Adelie Depression, between the Mertz Glacier and Dumont D'Urville, the French Antarctic station. This depression is a major site of formation of the dense water that drives the deep oceanic circulation. The position of the 2 icebergs in the future is expected to affect the local oceanic circulation, formation of sea ice in the area and the biodiversity. One of the places of possible future problems for wildlife as the conditions change in the Adelie Depression is the effect it will have on the emperor penguin colony at Pointe Geologie adjacent to the Dumont D'Urville station. It was this colony that featured in the movie March of the Penguins. The emperors depend for their existence on the food resources of the presently rich waters off the coast in the Adelie Depression.

Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003


Author: M. H. Monroe
Last updated 19/04/2011


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