The Southern Ocean and global climate
Box 2 | Observing the Southern Ocean
Despite the importance of the Southern Ocean to the world's climate system, many aspects of its circulation remain unknown, primarily due to the lack of ocean observations. Since 1991, Australian scientists from the Antarctic Cooperative Research Centre and CSIRO Marine Research, together with their international collaborators, have been using a variety of observational tools and computer models to study the ocean currents south of Australia. They are focussing on three scientific goals.
- 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.
- 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.
- 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.
Box
Box 1. The Antarctic Circumpolar Current
Related sites
Argo floats (Catalyst, Australian Broadcasting Corporation)
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)
Page updated July 2009.