Long-term measurements of the salinity in the Southern Ocean have revealed a strong freshening signal over the past decades. In fact, these salinity changes are among the most pronounced in the global ocean. To date, the source of these changes has remained a conundrum.
A newly published study in the scientific journal Nature by researchers from ETH Zurich, the GEOMAR Helmholtz Centre for Ocean Research Kiel and the University of Hamburg now describes a possible reason for this freshening of the Southern Ocean waters.
In their study, the scientists describe for the first time that the sea-ice conveyor belt around Antarctica has strengthened substantially, and that the associated freshwater fluxes to the open Southern Ocean can explain the Southern Ocean salinity distribution and its recent changes.
This conveyor belt consists of the formation of sea ice along the Antarctic coast, thereby fixing water in ice, the northward transport of this frozen water, and the subsequent release of this water upon melting at the sea-ice edge.
The drifting Antarctic sea ice
It may seem paradoxical, but while the sea ice in the Arctic is rapidly shrinking, the sea ice surrounding the Antarctic continent is actually expanding, despite global warming. Satellite observations show that the maximum ice cover in the Southern Ocean now extends further north than it did 30 years ago. This expansion is mainly due to a stronger transport of sea ice that has pushed the sea-ice edge further to the north.
The new study unveils the substantial consequences for the ocean's salinity: Antarctic sea ice forms and melts every year. At the time of its maximum extent, it covers an area of 18 million square kilometres - roughly the combined land area of the USA and Canada.
During the freezing process, the salt in the sea water is rejected, thereby increasing the salinity of the remaining sea water. When the ice melts, the fresh water is released back into the ocean, thus freshening it. The ice mostly forms close to the coast of Antarctica.
Strong winds and ocean currents then drive the sea ice more than 1,000 km northwards across the open sea. The northernmost edge of the sea ice is located roughly at 60 degrees south. This is where the ice starts to melt in the spring, releasing freshwater into the ocean.
Meltwater freshens Antarctic Intermediate Water
The cold meltwater that enters the sea water cools and freshens the ocean surface. Driven by the winds and other factors, this modified water mass then sinks below the warmer surface waters to form what is known as the Antarctic Intermediate Water, which has a comparatively low salinity. At depths of about 600 to 1,500 metres, this water spreads as a tongue to the north with its tip stretching as far as the Equator, and in the eastern Atlantic even as far as the coast of the Iberian peninsula.
"Our research demonstrates that the low salinity of the Antarctic Intermediate Water can be mostly explained by the freshwater released from the sea ice," says Dr Matthias Muennich, who is lecturer in physical oceanography at ETH Zurich and has been deeply involved in the study.
Continuous rise in freshwater input
"The amount of freshwater released from the sea ice into the open ocean surface waters and the Antarctic Intermediate Water has increased significantly in past decades. For the first time, we have been able to quantify these changes, which are presumably caused by stronger southerly winds during this period," says the lead author of the study, Alex Haumann, a doctoral student in the Environmental Physics group and the Center for Climate Systems Modeling at ETH Zurich.
According to their calculations, Alex Haumann and his colleagues estimated that the transport of freshwater by the sea ice has increased by up to 20 percent over the period 1982 to 2008. This would have caused a freshening of the sea water in the melt zone by as much as 0.02 grams per kilogramme of sea water per decade. "This figure is compatible with long-term records," says Nicolas Gruber, Professor of Environmental Physics at ETH Zurich and Alex Haumann's Ph.D. advisor.
"Research conducted over many years has shown that the Antarctic Intermediate Water has been freshening strongly," he explains. Scientists had assumed, however, that this phenomenon was due to the increased rainfall over the Southern Ocean. "But the changes in rainfall reconstructed in the weather and climate models are far too small to be able to explain the observed freshening." The ETH professor is therefore confident: "It must be the increased northward transport of freshwater by the sea ice that is largely responsible for this change."
Impact on global climate
The sea ice affects not only the salinity of the sea water, but also its stratification. Water with a low salinity is lighter than more salty waters, and therefore floats at the surface. So if the surface water becomes fresher and thus lighter, it is more difficult for the saltier and heavier deep water to rise to the surface.
This makes the vertical stratification of the water masses more stable. In turn, the stratification determines how the different water masses interact with each other and with the atmosphere to take up greenhouse gases, such as carbon dioxide, and heat.
"A more stable stratification could theoretically lead to a stronger uptake of carbon dioxide by the Southern Ocean, because less deep water that is rich in CO2, rises to the surface, where it releases carbon dioxide to the atmosphere," Professor Gruber explains. In the case of heat, the reverse situation would apply: a more stable ocean would actually absorb less heat.
For a long time, researchers assumed that the exchange of heat and carbon dioxide was controlled mainly by changes in the strong winds that are typical of this region. However, the research conducted by Professor Gruber's group shows that the system is far more complicated. Changes to the sea ice around the Antarctic could play a much more important role than previously thought.
"In the past we have given far more attention to the sea ice changes in the Arctic because it is shrinking so dramatically. In the long term, however, changes in the Antarctic could be far more important for our climate, as they have a major influence on the planet's surface heat balance and the atmospheric carbon dioxide levels," says Alex Haumann.
It is not clear yet whether the southerly winds have strengthened due to anthropogenic climate change or whether these are simply natural variations. "If these changes were man-made, this would be a dramatic consequence of human activity on the climate and ecosystem in one of the most remote and, so far, most pristine regions of the Earth."
So far, the Southern Ocean has acted as a climate regulator and carbon sink: climate models show that this ocean has absorbed around three quarters of the excess heat. The Southern Ocean has also taken up around half the total amount of anthropogenic carbon dioxide absorbed by the world's oceans.
Haumann FA, Gruber N, Muennich M, Frenger I, Kern S. Sea-ice transport driving Southern Ocean salinity and its recent trends. Nature, published online 31th August 2016. DOI: 10.1038/nature19101
Nicolas Gruber | EurekAlert!
Greenland ice flow likely to speed up: New data assert glaciers move over sediment, which gets more slippery as it gets wetter
17.08.2017 | Swansea University
Climate change: In their old age, trees still accumulate large quantities of carbon
17.08.2017 | Universität Hamburg
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
17.08.2017 | Physics and Astronomy
17.08.2017 | Earth Sciences
17.08.2017 | Physics and Astronomy