Comparing detailed measurements taken during the Australian Antarctic program’s 2012 Southern Ocean marine science voyage, to historical data dating back to 1970, scientists estimate there has been as much as a 60% reduction in the volume of Antarctic Bottom Water – the cold dense water that drives global ocean currents.
In an intensive 25-day observing program in January this year, temperature and salinity samples were collected at 77 sites between Antarctica and Fremantle (Western Australia). Such ship transects provide the only means to detect changes in the deep ocean.
The new measurements suggest the densest waters in the world ocean are gradually disappearing and being replaced by less dense waters. These measurements concur with a recent study by US-based researchers which showed that Antarctic Bottom Water is contracting over much of the global ocean. The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.
‘It’s a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions,’ Voyage Leader and Chief Scientist, Dr Steve Rintoul, said.
‘The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important.’
Dr Rintoul, an oceanographer with the CSIRO and the Antarctic Climate and Ecosystems Cooperative Research Centre, led the scientific voyage from Hobart, south to Commonwealth Bay, before turning west along the Antarctic coast and returning to Fremantle. The ship visited Commonwealth Bay as part of a celebration of the centenary of Sir Douglas Mawson’s Australasian Antarctic Expedition. Dr Rintoul’s team had the opportunity to repeat oceanographic measurements made by Mawson’s team 100 years ago, obtaining one of the few century-long records obtained anywhere in the ocean.
‘Our measurements collected in 2012 are quite different to those collected by Mawson in 1912,’ Dr Rintoul said.
‘The water is cooler now than it was then, indicating a change in ocean currents that may be related to a reduction in the amount of dense water formed near Antarctica – as less dense water is exported, less warm water flows into the region to replace it.
‘When we speak of global warming, we really mean ocean warming: more than 90% of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean.
‘The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change. A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future.’
The causes of the observed changes in the Southern Ocean are not yet fully understood. Changes in winds, sea ice, precipitation, or melt of floating glacial ice around the edge of Antarctica, may be responsible. Data collected on the latest voyage will help unravel this mystery.
A major challenge is the lack of observations at high latitude, where much of the ocean is covered by sea ice in winter. During the voyage, scientists deployed nine drifting profilers, called Argo floats, which will transmit profiles of temperature and salinity every 10 days for the next five years.
‘The Argo floats have revolutionised our ability to measure the ocean, particularly in winter when ship observations are very rare,’ Dr Rintoul said.
‘On this voyage we deployed a new kind of float designed to survive encounters with the sea ice. These floats will allow us to see how dense water forms in winter for the first time.’