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Understanding what drives our Southern Ocean

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As it girdles the planet linking the main oceanic basins, the Southern Ocean is a potent influence over the Earth’s energy distribution system: this vast and still-enigmatic water body holds one of the main keys to understanding how climate change will unfold.

The Southern Ocean is warming much faster than predicted, carrying heat towards the Antarctic continent with implications for climate, currents and sea levels. In the forefront of global efforts to understand the processes that govern this vast water body and assess their significance for humanity is Dr Andy Hogg at the Australian National University.

Dr Hogg uses the NCI’s supercomputer to model the dynamics of the Southern Ocean at a particularly fine scale – that of the eddies and jets of water which can easily be glimpsed from satellites high above, but which existing large-scale climate models cannot yet encompass. These turbulent swirls of water, from tens to hundreds of kilometres in size, are the dominant feature of the Southern Ocean and its currents, he says.

“We can see these eddies and gyres in satellite imagery of sea surface temperature, sea surface height and chlorophyll distribution. In fact they are the first thing you notice,” Dr Hogg explains. “We think they may be the most important aspect of the Southern Ocean, the main transporters of its heat energy and critical to the circumpolar currents.”

The Antarctic Circumpolar Current is the largest ‘river’ of water on the planet, with a flow equivalent to 700 Amazon rivers, gushing in five to ten braided currents which shift 130 million cubic metres of water every second. The eddies within this current are a major transporter of heat energy and hence, an engine of our climate.

The eddies dominate this system not only at the surface but also vertically, turning over water a kilometre or more deep. This churning is thought to bring up trapped carbon dioxide from depth, releasing it back into the atmosphere with obvious implications for the global climate, but ones that are, as yet, not clearly defined. “It could be a plus or a minus, depending on whether this system is stable or not. The eddies may act to prevent this CO2 outgassing – or they may accelerate it: these are issues to which we urgently need answers,” Dr Hogg says.

The eddies may also affect the rate at which cold water pours off the Antarctic continental shelf to form the great bottom currents that creep along the floor of the Atlantic and Pacific ocean systems. These bottom currents are regarded by many scientists as fundamental to the stability of the Earth’s present climate system.

To understand how heat energy is distributed around the Earth, an insight into ocean turbulence is essential, Dr Hogg contends. “The problem is that global climate models operate at a very large scale, whereas these eddies occur at a much smaller scale: we are trying to integrate the two within the climate models so we can have even greater confidence in what they say about the changes in store for us.”

To create a model of how the Southern Ocean will evolve in coming decades requires projecting millions of grid points into the future every few minutes for a century or more – a vast calculation that can only be performed using the ultra-powerful NCI facility. “The challenge is to find enough computer power to model at a sufficiently fine resolution so we can really understand what’s going on, and so make better climate predictions,” he says

Major advances in the research achieved so far include establishing the importance of fine-scale processes like eddies to large-scale climate models, demonstrating how eddies interact with coastlines and the seabed, the extension of ocean models into three dimensions and, significantly, confirming that stronger winds have contributed to a warmer Southern Ocean.

Key questions under investigation include whether the eddy field in the Southern Ocean has grown in recent years and is transporting more heat, and whether the ocean warming is due to climate patterns shifting southwards or to generalised global warming.

The research combines three different climate models and will, as it is perfected, feed into more accurate global and Australian climate and weather forecasting with immediate relevance to the lives, health, jobs and industries of Australians.

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