If you look at a map of the world or a globe you will see that the Southern Ocean circles the world in the Southern Hemisphere between latitudes 40 degrees and 60 degrees South. Unlike the Northern Hemisphere, there are no land masses to break up this great continuous stretch of sea water.

Due to the earths rotation and the prevailing weather conditions in these southern regions, this ocean has developed over millions of years a current flow which is predominantly from west to east. This strong current was first reported by James Cook during his voyages of exploration in 1775. Sailors have repeatedly found that the Southern Ocean was a unique but hostile environment with continuous storm-force westerly winds, mountainous seas, huge expanses of ice and icebergs.

Scientists examining the world?s climate, particularly in relation to global warming, have realised that this current is an extremely important factor in their studies. The Antarctic Circumpolar Current (ACC), sometimes called the "great ocean conveyor", connects the three great ocean basins, Atlantic, Indian and Pacific, allowing water, heat, salt and other properties to flow from one to the other.

 

 ACC flows around the World

While the circumpolar current is the dominant feature of the Southern Ocean, there are other important flows in the north-south direction and in the vertical plane. The average depth of the ocean ranges from 2,000 to 4,000 metres. Some water is made lighter by warming and freshening due to rainfall and melting ice while some is made more dense by cooling and the addition of salt rejected during the freezing of sea ice along the edge of the Antarctic Continent.

The temperature of the water in the Southern Ocean has a powerful influence on the climate of much of the earth. The current is not always stable. It can develop disturbances called waves or eddies which involve the entire water column. These can pinch off into a ring and move away from the main current flow taking either warm or cold water into other parts of the oceans and affecting the weather in those areas.

 

At the narrowest part of the ocean, south of the southern tip of South America, the International Southern Ocean Studies Group spent 6 years measuring the flow of water through Drakes Passage. It found that the current carried about 134 x 106 cu. metres per second of ocean water through the passage. Such a huge volume is difficult to visualise; so perhaps one can relate it to all the water in Sydney Harbour passing you in 3 seconds! The flow is equivalent to a current of approximately 10 -18 km per day. Some of the sea water in the ACC travels the 24,000 km. around the world in about 8 years.

 

The ACC is also well confined by land between Tasmania and Antarctica. Since 1991, the CSIRO?s Division of Marine Research and the Antarctic Cooperative Research Centre have taken six opportunities to measure the flow through this section of the ocean. Their research shows the mean net total flow of 143 x 106 cu.m/s passes eastwards through this channel. The major flow of 87 x 106 cu. m/s runs south-east past Macquarie Island while a relatively minor irregular flow swings north towards the Tasman Sea.

Scientists have been studying the ACC and its effect on climate. They find it controls climate in three ways:-

1. By connecting the world?s oceans, the ACC redistributes heat and other properties influencing the patterns of temperature and rainfall.

2. The vertical movement of water, caused by antarctic freezing during the winter and warming during summer, controls the renewal of deep water in the worlds oceans.

3. There is an exchange of gases, such as oxygen and carbon dioxide, with the atmosphere at the sea surface. The ocean contains 50 times more carbon than the atmosphere, so the rate at which carbon dioxide is absorbed by the Southern Ocean can directly affect climate change.

Diagram of ACC flow past Australia
Diagram of ACC flow past Australia. After Rintoul

 

Antarctic Circumpolar Wave
Antarctic Circumpolar Wave circulating around Antarctica.
Warm pools in red; cold pools in dark blue.      After Baines
Scientists have recently noticed another strange fact about the ACC. Using the data from the latest satellites that measure the sea surface temperature and height, scientists have noticed that the temperature of the water in the current varies, some parts are 2-3 degrees C warmer (shown in red) while other parts are 2-3 degrees C colder (shown in blue)than the average. There are two warmer and two colder pools and each one is several thousands of kilometres long and thousands of metres deep. They appear to be due to interactions between the atmosphere and the ocean. The regions seem to move eastwards with the ACC and take about 8-9 years to travel around the globe in the southern latitudes. This phenomena has been christened the "Antarctic Circumpolar Wave" (ACW). Researchers believe that the ACW may have a considerable on influence the weather patterns in southern Australia, South America and southern Africa.

 

CSIRO research continues in these important areas to help our understanding of the link between the Antarctic Circumpolar Current and climate. It is presently focusing on three important goals; to measure the heat and salts being carried from the Indian Ocean into the Pacific, to measure the rate at which the water sinks from the sea surface and to understand the exact role of the ocean circulation including the eddies.

 

Glyn Roberts.
May 2000.

References:

Rintoul S. R. 1998, ?Southern Ocean Currents and Climate? CSIRO Research Paper.

Baines P, 1998 "Australia's Climate Cerberus", Ecos, December 1998.

CSIRO Marine Research Pamphlet, 1997

 

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