The Southern Ocean and global climate
Key text
This topic is sponsored by the Cooperative Research Centre for Antarctica and the Southern Ocean and the Australian Government's National Innovation Awareness Strategy.
Research shows that the Southern Ocean is crucial to the world's climate system.
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The waters of the Southern Ocean circle Antarctica and wash the shores of southern Australia, New Zealand, South Africa and South America.
Since explorers first began venturing south of the known land masses in search of a great southern land it has been clear that the Southern Ocean is a unique – and hostile – environment. These early explorers met a barrage of storm-force westerly winds, huge seas, vast expanses of sea-ice, and mountainous icebergs.
The Southern Ocean circles the globe
These and other characteristics of the Southern Ocean distinguish it from other oceans. The most important, from an oceanographic point of view, is the fact that the Southern Ocean is the only ocean that circles the globe without being blocked by land.
The Southern Ocean is home to the largest of the world's ocean currents: the Antarctic Circumpolar Current. Because it connects the Indian, Atlantic and Pacific Ocean basins, the Circumpolar Current has a powerful influence on much of the Earth's climate. Indeed, the current is so vast it carries 150 times more water around Antarctica than the flow of all the world's rivers combined (Box 1: The Antarctic Circumpolar Current).
The Southern Ocean and climate
The Southern Ocean controls climate in a number of ways:
- The strong flow of the Circumpolar Current from west to east
around Antarctica connects the Pacific, Indian and Atlantic ocean
basins and their currents. The resulting global circulation redistributes
heat and other properties, influencing patterns of temperature
and rainfall.
- The Southern Ocean is a source of cold, dense water that is an essential driving force in the circulation of the world's oceans. The cooling of the ocean and the formation of sea-ice during winter increases the density of the water, which sinks from the sea surface into the deep sea. This cold, very salty water includes Antarctic Bottom Water and Antarctic Intermediate Water. Antarctic Bottom Water originates on the continental shelf close to Antarctica, spills off the continental shelf and travels northwards hugging the sea floor beneath other water masses. It travels far from its source, even as far as the North Atlantic and North Pacific.
Antarctic Intermediate Water is less saline, and forms further north, when cold surface waters sink beneath warmer ones at the Antarctic Convergence.
- At the sea surface, water exchanges gases such as oxygen and
carbon dioxide with the atmosphere at the same time that it is
being cooled. As a result, sinking water efficiently transfers
changes in temperature, fresh water and gases into the deep ocean 4 to 5 kilometres
beneath the sea surface. Biological processes also play a role
here, by influencing the content of carbon dioxide in the surface
water.
- The extent and thickness of sea-ice has a major influence on the Earth's climate. Its formation is the largest single seasonal phenomenon on Earth. The size of Antarctica doubles each year with the freezing of ice around the continent. It has a profound effect on climate processes. Because of its whiteness, it reflects the sun's heat back into space, intensifying the cold. But it can also act as a patchy blanket, limiting heat loss from the ocean to the atmosphere. Its yearly formation injects salt into the ocean, making the water denser and causing it to flow downwards as part of the deep circulation.
The ocean is a reservoir of carbon
Huge quantities of carbon are cycled between the biosphere (forests, grasslands and marine plankton), the atmosphere and the ocean. The ocean is the largest active reservoir of carbon, containing 50 times more than the atmosphere. Of the 6 to 7 billion tonnes of carbon released into the atmosphere by the burning of fossil fuels and deforestation, 3 billion remain in the atmosphere, 1 to 3 billion are absorbed by the ocean and up to 2 billion appear to be absorbed by the terrestrial biosphere.
The Southern Ocean is one of the few areas of the world's oceans where surface waters are dense enough to sink into the deep sea. These waters absorb carbon dioxide from the atmosphere, and by sinking into the deep they effectively pump it out of the atmosphere. Without this process, the buildup of carbon dioxide in the atmosphere would be much faster.
Understanding the global circulation and conditions under which surface waters sink into the deep ocean is therefore critical for scientists estimating the timing and magnitude of climate change.
Australian research
Researchers at CSIRO and the Cooperative Research Centre for the Antarctic and Southern Ocean Environment, together with US colleagues, have been studying the ocean between Australia and Antarctica since 1991 (Box 2: Observing the Southern Ocean).
The general pattern of the circulation of the Southern Ocean has been known for decades. However, observations collected by Australian and international researchers in recent years are enabling them to quantify and understand ocean currents for the first time.
Boxes
1. The Antarctic Circumpolar Current
2. Observing the Southern Ocean
Related Nova topic
Astronomy in the deep freeze
Page updated June 2010.