The Southern Ocean is wild and dynamic. It experiences Earth’s . It is home to city-sized and the on the globe, as well as tiny turbulent flows that fit inside a teacup.
Authors
Luke Bennetts
Lecturer in applied mathematics, University of Adelaide
Callum Shakespeare
Senior Lecturer in Climate and Fluid Physics, Australian ³Ô¹ÏÍøÕ¾ University
Catherine Vreugdenhil
ARC DECRA Research Fellow in Fluid Dynamics, The University of Melbourne
The Southern Ocean is also crucial to Earth’s natural systems. It forms the dense water that fills the world’s deep oceans. It stores heat and carbon resulting from human-caused global warming, and controls the flux of heat to the huge ice sheet of Antarctica – the to runaway global sea-level rise.
The scale and complexity of the Southern Ocean can be hard to comprehend. But our may help. It summarises the present state of understanding of the Southern Ocean, how it is changing, and where the knowledge gaps lie.
Scientists and others regularly voyage to the Southern Ocean’s furthermost icy reaches – but more research is needed. The scientific and broader community must join together to advance Southern Ocean science and protect this vital natural asset.
Masses of ice at risk
The Antarctic Ice Sheet is the largest ice mass on Earth, equivalent to of the global sea level.
The ice sheet flows onto the Southern Ocean surface in the form of giant ice shelves. Many of these ice shelves are being eaten away from below by a warmer ocean, or and becoming icebergs at a faster rate than before.
Beyond the ice shelves, millions of square kilometres of the Southern Ocean surface is frozen into a layer of sea ice. This acts as a giant solar reflector and from powerful .
After decades of seemingly defying warming temperatures, the Southern Ocean’s sea ice has . This puts ice shelves and the ice sheet under even greater stress.
Filling Earth’s oceans
Much of the sea ice is produced in small regions of open water, called “polynyas”, formed by strong and cold winds blowing off Antarctica. These winds cool the ocean surface below the freezing point, causing ice to form.
As the ice forms it ejects salt into the ocean surface. This extra salt, in addition to cooling effects from the atmosphere, makes the surface seawater heavier, or more “dense”.
The dense water sinks in turbulent plumes (imagine an upside-down volcano) and cascades through underwater canyons into the deep ocean, while mixing with overlying waters.
The resulting dense water mass, produced in only a few relatively small regions of Antarctica, accounts for an extraordinary 40% of the global ocean volume. It is ultimately lifted back to the ocean surface by centimetre-scale turbulent eddies – of the type you see when mixing milk into your tea.
In the deep ocean, this mixing is largely driven by ocean tides that slosh over the rough seafloor and produce .
Climate system at risk
It takes many for the ocean water to cycle from the surface Southern Ocean to the deep and back. Water returning to the surface today is like a time capsule, reflecting the cooler, pre-industrial climate when it first sank to the ocean depths.
The water that sinks today has absorbed more carbon to store in the deep ocean, helping to limit global warming.
However, models and observations reductions in sea ice and ice shelves are weakening this crucial climate system. They are making the water warmer, less salty and more buoyant, so less prone to sinking. This means less carbon storage and a warmer atmosphere in the years ahead.
So much we don’t know
Making measurements in the Southern Ocean is immensely challenging due to its remote location and hostile conditions. This means in many cases data is sparse, and so scientists don’t know exactly how quickly changes are occurring.
Our identified several areas as a key priority for future Southern Ocean research. They include observations of ocean temperatures and melting beneath ice shelves, as well as long term measurements of dense water formation.
More data is needed to monitor changes and provide early warning of significant climate events, such as ice sheet collapse. Crucially, more data is also needed on which government, industry and society rely to predict future climate.
Unfortunately, ocean observations are expensive. For example, Australia’s premier research vessel, the RV Investigator, costs to run. And the new – a joint project of the European Union and United States to measure the ocean surface at unprecedented resolution – .
These costs also highlight the need for enhanced national and international collaboration. This would make the best use of available resources, and promote technological innovation to develop more cost-effective observing systems such as drones and .
The federal government’s and funding decisions should reflect the crucial importance of Southern Ocean science.
We are currently in the , which aims to improve predictions of ocean and climate change. Improved understanding of the Southern Ocean is vital to this effort.
Luke Bennetts receives funding from the Australian Research Council.
Callum Shakespeare receives funding from the Australian Research Council
Catherine Vreugdenhil receives funding from the Australian Research Council.
