New Window To The Deeps Found In Antarctica

Dr Steve Rintoul, a Southern Ocean specialist at CSIRO Marine Research and the Antarctic Cooperative Research Centre (CRC) in Hobart, has identified a section of the Antarctic coast directly south of Tasmania as the source of one quarter of the Antarctic Bottom Water formed.

Cold air near Antarctica cools the ocean to the point that sea ice begins to form. As ice forms, salt is released to the water beneath the new ice. Both the low temperatures and higher salinity make the surface water heavy enough to sink 4 kilometres to the bottom of the ocean, hence the name Antarctic Bottom Water.

As dense bottom water sinks, it carries oxygen to the deep ocean, says Dr Rintoul.

“If these ‘windows’ to the deep sea were closed, oxygen levels would gradually decrease, causing fundamental changes to the chemistry and biology of the deep ocean.

“Near Antarctica, the ocean is delicately poised - small changes in salinity determine if water sinks or remains at the sea surface.

“If precipitation increases, the formation of bottom water may be slowed or shut down and some climate models suggest this may occur as the result of an enhanced greenhouse effect,” Dr Rintoul says.

Antarctic Bottom Water is also an important player in the global pattern of ocean currents which acts as a ‘conveyor belt’ to carry water, heat and salt around the Earth. Heat carried by the conveyor has a major impact on Earth's climate. Changes in bottom water formation may influence climate in the Antarctic and in more distant locations.

Dr Rintoul says there are only a few places in the ocean where conditions are right to make surface water heavy enough to sink.

“Somewhat to our surprise, the Adelie Land coast appears to be a much more substantial source of bottom water than we previously thought,” said Dr Rintoul.

Earlier studies suggested that most bottom water was formed in the Atlantic sector of the Southern Ocean, with the remainder formed in the Pacific sector. Early measurements suggested the Adelie area might also produce bottom water, but it was largely dismissed as an insignificant source. “Our new observations suggest that about a quarter of the total volume of bottom water in the world ocean is formed there.”

“The fact that a lot of bottom water is formed on the Adelie coast is intriguing, because it doesn't really fit our ideas about what is required to form bottom water,” said Dr Rintoul.

One important factor is the presence of a large ‘polynya’, an area of ocean within the sea ice pack which remains ice-free all winter. Because the air temperature can be up to 30 degrees Celsius colder than the water, the ocean is cooled very strongly.

The water freezes to form sea ice, but the ice is moved off-shore by very strong winds, maintaining the ice-free polynya. The polynya acts as an ‘ice factory,’ which in turn means that salt is released to the water below the ice. The input of salt is the key to making light surface water heavy enough to sink to the deep ocean.

“The Adelie polynya is an unusually productive source of bottom water. We think that ocean currents are probably the reason for this, but the evidence so far is largely circumstantial,” says Dr Rintoul.

“The next step is to go there in the middle of winter and make measurements.”

Dr Rintoul and Antarctic CRC colleagues Dr Nathan Bindoff and Dr Ian Allison will visit the region this winter to test the hypothesized link between ocean currents, sea ice and bottom water formation. The ice breaker Aurora Australis will need to penetrate through hundreds of kilometres of pack ice to reach the polynya.

“This is an exciting opportunity to observe how the ocean, atmosphere and sea ice interact in an Antarctic polynya in winter, a time of year when we have almost no measurements.”

Dr Rintoul’s findings have just been published by the American Geophysical Union, in a paper Oceanography of the Antarctic Continental Shelf.

More information from: Dr Steve Rintoul, steve.rintoul@marine.csiro.au