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Scientists have discovered a link between winds that circle Antarctica, and changes in the depth of an important ocean layer.

But researchers say it's still too early to know whether climate change is affecting these circumpolar winds, which could in turn further impact the rate of climate change.

The report, which is published online today in Nature Geoscience, is based on seven years of data collected from a global array of free-drifting ARGO floats.

CSIRO Marine and Atmospheric Research scientist Dr Steve Rintoul says the data shown the role of the ocean in climate change depends on how the oceans and atmosphere exchange heat and carbon dioxide.
Surface-mixed layer

Rintoul says a region known as the surface-mixed layer forms a crucial link between the atmosphere and the deeper parts of the ocean.

"The depth of the layer affects the amount of air mixing into the water, as well as ocean carbon and heat storage," he says.

"It also affects the availability of light and nutrients to support the growth of phytoplankton.

"When the winds strengthen and contract closer to Antarctica, the surface-mixed layer deepens in the eastern Indian and central Pacific oceans, and shallows in the western part of these basins.

The reverse is seen when the winds weaken and migrate north.

Called the Southern Annular Mode, this wind pattern is the major source of variability for the southern hemisphere's atmosphere.

The asymmetry can be explained by small deviations in the generally west-to-east winds and their effect on heat exchange between ocean and atmosphere: Cold winds blowing from the south, cause heat loss from the ocean and deeper mixed layers.
Shallow or deep

Rintoul says as the surface-mixed layer becomes shallower in the direction the current is flowing, there's is increased heat exchange between the atmosphere and the deeper interior of the ocean.

"A shallow mixed layer also means phytoplankton stay mostly within the well-lit zone, where they thrive," he says.

"But when the mixed layer deepens, heat exchange is worse and phytoplankton are distributed over a larger volume of water, often sinking to depths too dark to sustain them.

Rintoul says, "Phytoplankton are a major conduit for the sequestration of carbon from the atmosphere, so any cut in their numbers impacts climate change".
Detailed ocean

Rintoul says the ARGO floats have allowed scientists to measure the ocean in immense detail and to measure places where ships normally don't go.

"There are 3400 of these free-drifting scientific floats in the program," he says.

"Each is a metre long, equipped with an array of scientific instruments measuring ocean temperature and salinity to depths of two kilometres."

The floats use satellite communication to transmit their data to scientists and carry enough battery life to last four years.

"Australia has the third largest number of ARGO floats in the international program, well above what's expected for a country of our size," says Rintoul.

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