Is melting ice lowering the salinity of the Southern Ocean?
Over the past decades, the northward drift of sea ice surrounding Antarctica has strengthened. This not only has increased the extent of the sea ice, but also has freshened the sea water around the sea-ice edge. How this will affect the global climate system and Antarctica’s ecosystem isn’t clear.
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. But scientists couldn’t pinpoint the source of the fresh water.
The transport of freshwater by the sea ice has increased by up to 20 percent over the period 1982 to 2008.
A newly published study in Nature offers an explanation.
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 water due to melting at the sea-ice edge.
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.
melting ice – Deep, cold water
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 kilometers–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.
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 meters (1,968 to 4,921 feet), this water spreads 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 Matthias Münnich, who is lecturer in physical oceanography at ETH Zurich.
“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 at ETH Zurich.
melting ice – Don’t blame it on the rain
According to their calculations, Haumann and his colleagues estimated 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 kilogram of sea water per decade.
“This figure is compatible with long-term records,” says Nicolas Gruber, a professor of environmental physics.
Water with a low salinity is lighter than more salty waters, and therefore floats at the surface
“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,” says Gruber. “It must be the increased northward transport of freshwater by the sea ice that is largely responsible for this change.”
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,” explains Gruber. 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, work 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 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.
Researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel and the University of Hamburg also contributed to the research.
Source: ETH Zurich