Andy Hogg

Study unlocks secrets of world’s strongest current

Research at the Australian National University challenges the view that the powerful roaring forties and furious fifties winds alone drive the world’s strongest ocean current.

Andrew Hogg, of the Research School of Earth Sciences, has discovered that a temperature gradient across the Southern Ocean, combined with the Earth’s spin, also could power the Antarctic Circumpolar Current.

The current transports the equivalent of 700 Amazon Rivers west to east around the ice continent.

It was established some 30 million years ago when the Drake Passage between South America and Antarctica opened up, severing the last landbridge to the southern continent.

Previously, it was thought that the westerly winds drove the current as they lashed the surface water.

But Hogg’s research suggests that another mechanism, dubbed surface buoyancy forcing, could play a bigger part.

"Heating of the surface waters on the northern side of the Southern Ocean and cooling on the southern side set up a pressure gradient, which, in combination with the Earth’s spin, might drive the current," he says.

Hogg used a sophisticated mathematical model to compare the effects on the current of the world’s strongest winds and surface buoyancy forcing.

The model, run on a supercomputer at the National Computational Infrastructure facility, based at the ANU, divided the Southern Ocean into a grid.

A set of equations in the model related water transport to parameters affecting surface buoyancy and wind stress. The program was run repeatedly, stepping forward in time, with the output of each run forming the initial condition of the next one.

By varying the input values, Hogg could see which parameters generated virtual Antarctic Circumpolar Currents closest to the real one.

"In most cases, switching on the wind forcing does nothing at all" he says, adding that the model was more finely resolved than those used in previous studies.

"In this model, we can drive the full Antarctic Circumpolar Current with buoyancy forcing alone."

His results have been accepted for publication in Geophysical Research Letters.

The current is one of the main drivers of global climate.

It isolates Antarctica from warm subtropical waters to the north, and it is thought to have caused the continent to freeze. However, some scientists argue that a fall in atmospheric carbon dioxide levels coincided with the onset of the current. This, they say, could also have played a role in making Antarctica the coldest place on Earth – by damping the natural greenhouse effect.

Hogg’s results come as scientists sharpen their focus on the ice continent in a bid to predict its response to global warming. The behaviour of the Antarctic ice sheet is one of the big unknowns of the greenhouse future. The complete melting of the 3-kilometre-thick layer of ice would raise sea level by about 60 metres.

The warming of the Southern Ocean due to the greenhouse effect would lessen the pressure gradient because there would be preferential warming on the southern, colder side, Hogg says.

However, it is too early to gauge the extent to which the Antarctic Circumpolar Current would be weakened, and the impact on the ice sheet.

Hogg says more research is needed to pin down the relative contributions of buoyancy forcing and wind.

"We can’t understand one without understanding the other" he says.