Bowing
Editors’ Vox is a blog from AGU’s Publications Department.
The Southern Ocean is a unique, dynamic environment and is considered the central hub of the global ocean. Due to its profound influence on Earth’s present and future climates, an improved understanding of the Southern Ocean is crucial.
A new article in Reviews of Geophysics explores the complex dynamics of the Southern Ocean, connecting the smallest scales of ocean mixing to the global circulation and climate. Here, we asked the authors to give an overview of the Southern Ocean, how scientists study it, and what questions remain.
In simple terms, what makes the Southern Ocean unique?
The Southern Ocean is the only part of the ocean that is circumpolar, with no continental boundaries. This property results in the extreme features of the Southern Ocean, such as the world’s biggest ocean waves, strongest ocean current, and most intense ocean turbulence. At its northern boundary, the Southern Ocean connects all major oceans to the north and is the source for 40% of the global ocean water mass. Towards the south, it is the gateway to Antarctica, and its southern region is characterized by the presence of a frozen surface (sea ice) and the giant floating extensions of the Antarctic Ice Sheet (ice shelves) that enclose water cavities shut off from direct contact with the atmosphere.
Why is it important to understand Southern Ocean dynamics and interactions?
The Southern Ocean is the central hub of the global ocean and a crucial component of Earth’s climate. Its dynamic properties dictate water mass transformation, which exerts a dominant control on global air–sea carbon and heat exchange, how heat is absorbed by the ocean and transported towards Antarctica, where it melts Antarctic ice and contributes to global sea level rise, and oceanic nutrient cycling. The complex interplay between the dynamic processes remains poorly understood, limiting our ability to understand, model, and, crucially, predict changes to the Southern Ocean in a warming world. This undermines our resilience to the impacts of climate change.
What are the main components of the Southern Ocean dynamic system?
For the review article, we classified the Southern Ocean dynamic system into four categories:
- Large-scale circulation, which includes the Antarctic Circumpolar Current and polar gyres, as well as the ‘overturning’ circulation, which moves water between the surface and deep ocean.
- The cryosphere, which includes the permanent Antarctic ice sheets where they meet the ocean (‘ice shelves’), and the frozen ocean surface (‘sea ice’) that grows and retreats annually.
- Turbulence, which includes smaller scale ocean flows from mesoscale eddies—known as the ‘storms of the sea’—down to tiny overturning circulations that would fit inside a teacup.
- Waves, which include both the familiar ocean waves that propagate along the surface, as well as the ocean tides, and internal waves that propagate through the ocean interior.
What are some of the ways that these components interact with each other?
There are many examples, and they are (almost) all connected! For instance, the formation of sea ice at the ocean surface results in the ejection of salty brine from the ice. This brine is denser than the surrounding water and therefore sinks. As it does so, it drives intense turbulence, mixing with the surrounding water and forming a new dense water mass which flows northward along the seafloor as part of the large-scale circulation.
Farther to the north, ocean tides slosh this water back and forth over the rough seafloor, radiating internal waves. These waves break in the deep ocean, driving turbulent mixing and gradually lifting the dense Antarctic-sourced water back to the surface, closing the large-scale overturning circulation.
How do scientists observe and gather data on Southern Ocean dynamics?
Scientists use a range of methods to gather data on the Southern Ocean depending on the process and location of interest. Many different kinds of observational data are collected, including shipboard observations from voyages, autonomous floats, remotely driven submarines or gliders, moorings anchored to the seafloor, sensors attached to seals, and, increasingly, observations made via satellites.
Satellite data is of increasing importance as it provides a map of data over the entire Southern Ocean in real time, as opposed to earth-bound observations at specific locations and times. However, the drawback of satellite observation is that it can only see the ocean surface and often doesn’t directly measure the property of interest. Therefore, direct observations remain vitally important to observe sub-surface processes, as well as calibrate satellite measurements.
How is the Southern Ocean changing in response to global warming? Are these changes projected to continue?
The Southern Ocean, like the rest of the world, is warming, although at a somewhat slower rate than most parts of the ocean.
The Southern Ocean, like the rest of the world, is warming, although at a somewhat slower rate than most parts of the ocean. We are observing increased melting and mass loss from ice shelves, as well as alarming reductions in Antarctic sea ice cover in recent years. These factors have global impacts through how they affect the formation of deep water in the Southern Ocean, which flows north to fill 40% of the ocean’s volume. This water mass is becoming fresher and warmer over time. Some studies suggest this process may cause the ocean overturning to significantly reduce or turn off entirely.
In addition, the loss of sea ice causes an acceleration of global warming as more radiation is absorbed by the ocean, rather than being reflected by the ice, which can lead to the ice–albedo feedback loop. These changes are predicted to continue, and speed up, as the climate warms and melt rates increase further.
What are some of the unresolved questions or knowledge gaps where additional research, data, or modeling are needed?
There are many unresolved questions in Southern Ocean dynamics, both in understanding recent changes and being able to predict future changes.
There are many unresolved questions in Southern Ocean dynamics, both in understanding recent changes, such as the sudden shift from the baffling record sea ice extents around 10 years ago to the record lows in recent years, and being able to predict future changes. These questions need a broad community effort across many fields of science to better observe the Southern Ocean, especially under ice shelves (‘sub-ice shelf water cavities’) and sea ice, and better resourcing of fundamental science to understand turbulent mixing processes and ice–ocean interactions. Both of these aspects are crucial if we are to build more accurate numerical models, which are the sole means for assessing future ocean and climate scenarios.
—Luke G. Bennetts (luke.bennetts@adelaide.edu.au, 
0000-0001-9386-7882), University of Adelaide, Australia; Callum J. Shakespeare (0000-0002-8109-0751), Australian National University, Australia; and Catherine A. Vreugdenhil (0000-0002-1808-6274), University of Melbourne, Australia
