The Southern Ocean and global climate

Key text

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:

1. 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.

2. 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.

3. 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.

4. 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 Marine Research 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 for the last 7 years (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.

Related Nova topic:

Astronomy in the deep freeze

Box 1. The Antarctic Circumpolar Current

The Circumpolar Current mixes the oceans

The Circumpolar Current is not a single mass of water flowing around and around Antarctica, but a series of linked flows which follow the uneven shape of the sea bed. And just like water flowing along a creek, there are ups and downs and twists and turns. In some places it is shallower, and because more water has to get through a smaller space it flows faster – but where the sea is deep it tends to move more slowly.

Because the Circumpolar Current flows around the bottom of three of the world's great oceans – the Atlantic, Pacific and Indian - it mixes the waters. This means that water from, say, the Atlantic may get dragged into the Circumpolar Current and then flow out into the Pacific. The Circumpolar Current has a major effect on the way the world's oceans behave.

Antarctic Bottom Water

Cold water (which is denser) tends to flow under the warmer water at the surface of the ocean. In the ocean around Antarctica there is a great deal of heavy and cold water which sinks to the bottom of the sea and is called Antarctic Bottom Water. Antarctic Bottom Water flows downwards and outwards until it spills off the edge of the shallower continental shelf and 'falls' into the deep ocean and moves towards the equator. Scientists have estimated that this cold heavy water flows north at the rate of more than 10 million cubic metres per second. This huge amount of water pushes the warmer water out of the way, usually by flowing underneath it – and this causes new flows and currents in other directions. In fact the masses of cold water flowing from Antarctica literally have a flow-on right around the planet. The Antarctic Bottom Water flowing along the bottom of the oceans and away from Antarctica has to be replaced by other water, so the warmer waters in the north tend to flow southward to fill in the gap. Then they cool down and the cycle keeps going.

Related sites

• Antarctic circumpolar current (Tasmanian Parks and Wildlife Service, Australia)

• Antarctic surface water (Rice University, Texas, USA)

Box 2. Observing the Southern Ocean

1. Measuring how much water, heat and salt is being carried from the Indian Ocean to the Pacific Ocean, south of Australia.

   If scientists can work out how the transport of the currents varies today in response to winds and cooling by the atmosphere, they should be able to predict how the current might vary if the climate changes. Because ocean currents influence the overlying atmosphere, changes in the strength of the current may in turn drive further changes in the climate. An important issue is the role of ocean eddies, or 'turbulence'. Eddies transport heat and momentum from one place to another, and are likely to play an important role in controlling the strength of the Circumpolar Current and in supplying heat to high latitudes where the atmosphere strongly cools the ocean.

   The centrepiece of this observational program is a transect from Tasmania to Antarctica. The Australian research icebreaker Aurora Australis repeats the transect to measure variations in the transport between the Indian and Pacific Oceans. Upper ocean temperature observations collected by the French supply ship L'Astrolabe along the same route complements the less frequent but more comprehensive measurements collected on the Aurora Australis.

2. Measuring the rate at which water sinks from the sea surface.

   Near the axis of the Circumpolar Current (midway between Tasmania and Antarctica), surface water sinks to depths of up to 1 kilometre. Extreme cooling and ice formation over the Antarctic continental shelf can force surface water to sink all the way to the sea floor, as much as 5 kilometres below the surface of the sea.

   The sinking rate determines how much heat the ocean can store and how much carbon dioxide and oxygen reach the deep sea. Scientists now believe that about 40-50 per cent of carbon dioxide entering the atmosphere from the burning of fossil fuels ultimately enters the ocean to be contained in water masses. Thus, to estimate the amount of carbon dioxide transferred from the atmosphere to the ocean, oceanographers need to know the rate at which water sinks from the sea surface in the Southern Ocean, how deep it sinks and its initial carbon dioxide content which is influenced by biological activity.

   To measure the rate at which water sinks from the sea surface researchers use chemical 'tracers' such as chlorofluorocarbons (CFCs). CFCs were first produced for use as refrigerants in the 1930s. Water at the sea surface picks up CFCs from the atmosphere: when the water sinks, the CFC signal is carried with the sinking water. Scientists use CFC tracers to calculate how recently a water mass was at the surface, providing a 'clock' with which to measure the motion of water masses.

3. Understanding the role of ocean circulation in controlling the biological productivity of Southern Ocean surface waters.

   The Southern Ocean is not as productive as expected, given the abundant nutrients. Deep mixing associated with the high winds of the region, low light levels because of persistent cloudiness, and low levels of iron – an essential micronutrient - may all play a role.

   A variety of biological and chemical measurements are made during expeditions on the Aurora Australis and the Southern Surveyor to investigate the factors controlling the biological productivity of the Southern Ocean. Properties measured include primary productivity, carbon dioxide concentrations, organic matter, fluorescence, nutrients (including trace nutrients such as iron), and the light available for phytoplankton growth.

Observational research tools

Ships provide an opportunity to make observations over a wide area at a particular time. To obtain measurements at particular sites over a long period, moorings are used. Australian and US scientists have recently deployed the most comprehensive array of moorings ever deployed in the Southern Ocean along the repeat transect between Tasmania and Antarctica. The mooring array will address two questions: how does the strength of the Antarctic Circumpolar Current vary with time, and how much heat and momentum is carried by ocean eddies?

To answer these questions, three types of instruments are being used. Conventional current meters, which use a rotor and a vane to measure current speed and direction, have been deployed in the centre of the array. The other instruments are being used for the first time in the Southern Ocean. Inverted echo sounders measure changes in the time it takes a sound pulse to travel from the sea floor to the sea surface and return. Changes in the travel time are related to changes in density of the overlying water, which are in turn related to changes in ocean currents. Sea floor electrometers measure the average speed of an ocean current by sensing the electric field created by salty seawater moving through the Earth's magnetic field.

Satellites are an invaluable tool for oceanographers because they can provide regular observations of the entire globe. The satellite instrument of most importance to oceanographers is the altimeter. Satellite altimeters measure the height of the sea surface with an accuracy of a few centimetres. Because ocean currents cause the sea surface to slope (eg, the sea surface is about a metre higher near Tasmania than it is near Antarctica), the altimeter provides a means of monitoring ocean currents from space.

Computer models

These (and other) observations are used to test and improve computer models that simulate ocean circulation. The ocean models are combined with models of the atmosphere and sea-ice to create comprehensive models of the global climate system that can predict how climate may change in the future. For these climate predictions to be reliable, ocean currents must be accurately reproduced. Ocean modellers are developing state-of-the-art models of the Southern Ocean using powerful supercomputers.

Understanding the circulation of the Southern Ocean and its interaction with the atmosphere and sea-ice lies at the heart of reliable predictions of climate change.

Related sites

• Our programs (Antarctic Climate and Ecosystems Cooperative Research Centre, Australia)

• Antarctic circumpolar current (Tasmanian Parks and Wildlife Service, Australia)

• Robotic floats: ocean sentinels of the future (CSIRO Marine Research, Australia)

• Satellite altimeters (CSIRO Wealth from Oceans Flagship, Australia)

• Ocean currents (Tasmanian Department of Environment, Parks, Heritage and the Arts, Australia)

• Sounding out the ocean's secrets (Beyond Discovery, National Academy of Sciences, USA)

• Drifting buoys (photograph and diagram from the National Ocean and Atmospheric Administration, USA)

Activities

• Exploring oceans (Australian Science Teachers Association, 1998)
  • Sea water versus fresh water (pages 20-23)
  • How and why does the sea move? (pages 30-33)

• Australian Science Teachers Journal
  • Climate change: An activity – use foraminifera data to discover the relationship between water temperature and sea level change (Vol. 41, no. 4, 1995, pages 74-79)

• Discovery Channel School
  • Understanding: Oceans

• Australian Antarctic Division
  • Deep freeze – provides nine activities that explore the global force of Antarctica and the Southern Ocean (eg, 'Ice cycle' and 'Ice melt').

Activity 1. Thermal expansion of water

Materials (for each small group)

• round-bottomed flask
• clamp and stand
• 2-holed bung
• water coloured with food colouring
• thermometer
• graduated capillary tube, or capillary tube with a paper scale taped to it
• Bunsen burner
• matches

Procedure

1. Fill the round-bottomed flask with coloured water.

2. Close it with the 2-holed bung.

3. In one hole place the thermometer, in the other the graduated capillary tube (see diagram).

4. To show thermal expansion, heat the bottom of the flask.

1. Why is cooler water more dense than warmer water?

2. Thermal expansion of water is one reason why a rise in sea level could be a consequence of global warming. Suggest another reason.

Teachers notes

Remind students that sea temperature increases due to global warming would only be in the order of 1-2°C and would take place primarily in the surface layers. In this activity the effect of thermal expansion is greatly exaggerated because the volume of the capillary tube is very small compared to the volume of the flask

1. Water doesn't behave in quite the same way as other liquids. When a substance is heated it usually expands and becomes less dense, and cooling has the opposite effect, causing it to contract.

   As water cools, it contracts and its density increases, as expected, until it reaches about 4°C. But then, further cooling causes it to become less dense and to actually expand. At 0°C, pure water turns into ice. Ice takes up more space and so is less dense than water at 4°C. The consequence of this anomaly is that ice floats on top of water. If, instead of floating, ice sank, much more of the water in the cold regions of the world would be frozen. This is because water, cooled to 0°C by the air above it, would turn to ice and sink, and more ice would then be formed on top and sink. But a layer of floating ice acts to protect the water beneath it from being frozen by the air.

   For more information about water density, its relation to Antarctic waters and diagrams of the water masses around Antarctica:

   • Antarctic surface water (Rice University, Texas, USA)

   • What does water do below the surface? (Rice University, Texas, USA)

2. The melting of snow and ice on Antarctica and Greenland (as well as non-polar glaciers) is usually mentioned as a reason for the sea level rising as a result of global warming.

   For more information about research projects to measure changes in sea level:

   • Sea level rise (Antarctic Climate and Ecosystems CRC, Australia)

Further reading

• Mysteries of the ocean,
• What is happening in the North Atlantic?
• The unique relationship between the sea and CO₂
• The strange world of oceanic methane

Useful sites

Describes the Antarctic circumpolar current and its effect on climate change
http://www1.parks.tas.gov.au/fahan_mi_shipwrecks/infohut/acc.htm

Sea level rise (CSIRO Marine and Atmospheric Research, Australia)

A website with information on sea level rise, its causes and estimates of global and regional sea levels.
http://www.cmar.csiro.au/sealevel/index.html

Australian Antarctic Division

• The global jigsaw
  Discusses the role of Antarctica in global processes.
  http://www.aad.gov.au/Asset/looksouth/pdfs/c5.pdf

• Ice shelves
  Provides information on Ice cores and how they are used to measure climate change and volcanic eruptions that alter the atmosphere.
  http://www.aad.gov.au/default.asp?casid=1708

• The Southern Ocean's global reach
  Describes the Southern ocean storage and transport of heat, moisture and carbon dioxide and its influence on the Earth's climate system.
  http://www.aad.gov.au/default.asp?casid=4267

Australian Broadcasting Corporation (transcripts)

• Indian Ocean – influence on Australia's climate (The Science Show, 30 September 2006)
  Discusses how the Indian Ocean influences the climate over the whole of Australia.
  http://www.abc.net.au/rn/scienceshow/stories/2006/1751859.htm#

• Deep ocean currents (The Science Show, 20 May 2000)
  Describes how the ocean acts as a conveyer belt and explains how deep water currents are formed.
  http://www.abc.net.au/rn/science/ss/stories/s131912.htm

• Catching up with us: Global warming and the oceans (Quantum, 26 March 1998)
  Features the role of oceans, particularly the Southern Ocean, in climate change.
  http://www.abc.net.au/quantum/scripts98/9802/feature.htm

Stories in the ice – nature's time machine (Public Broadcasting Service, USA)

Documents events in the Earth's history using an ice core timeline.
http://www.pbs.org/wgbh/nova/warnings/stories/

Antarctica – in from the cold? (On Line Opinion, 15 January 2002, Australia)

Dr Michael Stoddart, from the Australian Antarctic Division, explains how Antarctica influences climate and ocean circulation.
http://www.onlineopinion.com.au/view.asp?article=1201

Antarctic Region (University of Texas, USA)

A clear map of the Antarctic region. Shows the location of the major ice shelves and the average minimum extent of sea-ice.
http://www.lib.utexas.edu/Libs/PCL/Map_collection/islands_oceans_poles/antarctic_pol97.jpg

A disintegrating glacier (Science@NASA, USA)

An Australian researcher has discovered that a large glacier tongue on the Antarctic coast has disintegrated.
http://science.nasa.gov/headlines/y2000/ast05dec_1.htm?list153136

Glossary

Antarctic Bottom Water and Antarctic Intermediate Water. Water does not have the same composition throughout the ocean. The different masses of water can be described by their chemical and physical properties – temperature and salinity are used most frequently. These two properties affect the density of water.

Antarctic Bottom Water forms close to Antarctica and is the most dense of the water masses. (Its high density is a result of its coldness and high levels of salinity.) It flows northwards from Antarctica under other water masses, hugging the sea floor.

Antarctic Intermediate Water also forms in the Antarctic region then sinks and spreads northwards. Antarctic Intermediate Water is less saline than Antarctic Bottom Water because it receives fresh water from melting ice shelves and glaciers.

More information about Antarctic water masses can be found at Antarctic circumpolar current (Tasmanian Parks and Wildlife Service).

micronutrient. A chemical element that is essential for plants to grow and reproduce but is only needed in very small amounts. There are seven micronutrients: iron, chlorine, copper, manganese, zinc, molybdenum, and boron.

sea-ice. The sea around Antarctica begins to freeze in March and the area covered by floating sea-ice increases until September or October when it reaches a maximum of about 19 million square kilometres. This sea-ice 'blanket' affects sea temperatures and sea currents by shielding the ocean surface from the strong winds that blow in the high latitudes. Sea-ice is also important because it is white and reflects back to space most of the sun's radiation that falls on it. The presence of more sea-ice cools the earth. For more information see Sea ice (National Snow and Ice Data Center, USA).

transect. An imaginary line drawn through an area in order to help scientists sample and monitor organisms or conditions along the line. The results obtained from samples along the line give an indication of the organisms or conditions in the entire area.