Source: Pellichero, V., J. Sallee, C. C. Chapman, and S. M. Downes (2018), The southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxes. Nature Communications, doi:10.1038/s41467-018-04101-2.
What is the Overturning Circulation?
The ocean plays a crucial role in regulating the earth’s climate by taking up excess heat and carbon dioxide from the atmosphere. The ocean surrounding Antarctica, called the Southern Ocean, absorbs a particularly large amount – nearly 30% of the carbon and 90% of the heat associated with anthropogenic climate change.
The Southern Ocean is so good at trapping heat because it is one of the only places on earth where the atmosphere can communicate directly with the deep ocean. This is due to the global current system called the overturning circulation. The image below shows the approximate path traveled by waters around the world’s oceans – red indicates transport near the surface and blue represents deep transport. In the Antarctic, the overturning circulation moves water between the surface and the ocean abyss. Scientists are still working to understand the processes that drive this global system. Determining these processes is important because changes in the circulation could affect the rate of heat uptake by the ocean and thus climate.
Ice in Charge?
One of the reasons that it’s difficult for scientists to understand the overturning circulation is because there aren’t very many measurements taken in the Antarctic due to its remote location and harsh weather. Because observational data is so sparse, many past studies that examined the overturning circulation in the Southern Ocean used models. One of these studies concluded that melting and freezing of sea ice is one of the most important factors controlling Antarctic overturning. Since this result was based on a model alone, a new paper led by Violaine Pellichero at Sorbonne Universités in France, investigates whether observational data collected in the Southern Ocean supports this same result.
The study uses data collected on research cruises, from autonomous floats that drift around the Southern Ocean, and even measurements collected by seals! Elephant seals outfitted with data-collecting instruments, like the one in the picture on the left, are an important source of data in ice-covered regions that are difficult to reach by ship.
By compiling data from many different sources, Pellichero was able to estimate the rate of overturning across the entire Southern Ocean. Similar to past modeling studies, the results showed that sea ice is one of the major controls on transport from the surface to the deep ocean.
Why is sea ice so important? Melting of sea ice adds freshwater to the ocean, while sea ice formation injects salt. This is key because the density of seawater depends on the amount of salt in it. When sea ice forms, the surface ocean become saltier and therefore denser than the water below it, causing it to sink. This sinking of surface waters in the Southern Ocean is how the atmosphere communicates with the deep ocean. Furthermore, the formation of dense waters by sea ice processes also helps to maintain the overturning circulation shown in the first diagram.
Pellichero et al. showed using observational data that sea ice processes are crucial to sustaining the global overturning circulation. This result suggests that changes to sea ice extent and concentration in the Antarctic could impact global circulation as well as the ocean’s uptake of heat and carbon dioxide. In addition to ice dynamics, carbon uptake is affected by a complex combination of interaction with the atmosphere, mixing, and biological processes. Because of this, scientists have different hypotheses about how the Southern Ocean’s ability to absorb carbon will change in the future. By identifying the mechanisms important to the overturning circulation in the Southern Ocean, Pellichero et al. can help us understand and predict changes in the ocean’s ability to regulate climate.
I’m a physical oceanography PhD student at Scripps Institution of Oceanography in La Jolla, California. I use a combination of numerical models, observations, and remote sensing to investigate the role of the ocean in climate. I’m particularly interested in Southern Ocean dynamics, including air-sea-ice interactions and physical controls on biogeochemistry.