“These salinity changes are among the most pronounced in the global ocean,” Alex Haumann of ETH Zurich told environmentalresearchweb. “To date, the source of these changes has remained a conundrum.”

Sea ice, which in Antarctica mainly forms near the coastline, alters salinity because as it freezes the ice rejects salt into the remaining seawater. Once winds and currents drive this ice more than 1000 km to the north, to a latitude of about 60°, it melts in the spring, decreasing the salinity of the water at its new location, Haumann and colleagues believe.

“Sea ice is, compared to the land ice on Antarctica, very thin (0.5–1 m) but its area is enormous,” said Haumann. “At the time of its maximum extent, it covers an area of 18 million square km – roughly the combined land area of the US and Canada.”

According to recent satellite observations, the maximum ice cover in the Southern Ocean extends further north than 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,” said Haumann. “So, we thought that this should also change the ocean’s salinity substantially.”

To come up with the results, Haumann and colleagues reconstructed time series from 1982 to 2008 for ice thickness, ice drift rate and ice concentration – the area of the ocean covered in ice. They combined satellite data and more direct measurements, where they were available, to try and reduce the uncertainty in the data. Then they were able to estimate the amount of freshwater transport and its effects on salinity.

Going deep

Water at the ocean surface that’s been freshened and cooled by summer sea-ice melt subsequently sinks to form Antarctic Intermediate Water. “Our research demonstrates that the low salinity of the Antarctic Intermediate Water can be mostly explained by the freshwater released from the sea ice,” said Haumann. “Our data shows that the freshwater fluxes that are related with sea-ice formation and melting are much larger than freshwater fluxes that are related to precipitation, evaporation or melting of icebergs and glaciers from Antarctica. Consequently, sea ice is largely responsible for the salinity distribution in the Southern Ocean.”

The team believes it is the first to quantify this strengthening of the sea-ice conveyor belt around Antarctica, which is presumably caused by stronger southerly winds. From 1982 to 2008, up to 20% more freshwater was released from sea ice into the open ocean surface waters and Antarctic Intermediate Water, causing freshening of as much as 0.02 grams per kg of sea water per decade.

Decreasing salinity also makes water lighter and less likely to sink. As a result, it’s more difficult for the saltier and heavier deep water to rise to the surface, and the vertical stratification becomes more stable.

“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,” said Haumann. “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 carbon dioxide rises to the surface, where it is released to the atmosphere.”

Researchers had previously assumed that this exchange of carbon dioxide was controlled mainly by changes in the strong winds that are typical of this region. “Changing sea ice around the Antarctic could play a much more important role than previously thought,” said Haumann. “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 atmospheric carbon dioxide levels.”

To date, the Southern Ocean has absorbed around three quarters of the excess heat due to climate change, and taken up around half the total amount of anthropogenic carbon dioxide absorbed by the world’s oceans, Haumann says.

Coastal paradox?

Along the coast of Antarctica the picture is slightly different. Here, waters have also freshened in recent decades but the transport of sea ice to the north should actually make them saltier.

“This only seems a paradox at a first sight,” Haumann explained. “One also has to consider that in this coastal region Antarctic glaciers have been melting much more rapidly in recent years, adding large amounts of extra freshwater to the ocean.”

Earlier research revealed that the amount of glacial meltwater entering coastal waters should lead to a much greater freshening than is occurring. “Our study now shows that part of this freshening is compensated for by the sea ice,” said Haumann. “Thus, in the coastal ocean the observed freshening over recent decades is most likely due to increasing meltwater from Antarctica, and sea ice acted to reduce this freshening.”

Next the team plans to assess the influence of the sea-ice conveyor belt on the exchange of carbon dioxide and heat between the atmosphere and the ocean. “This understanding will be important to understand future changes in the climate system,” said Haumann.

“Current global climate models still have major difficulties in accurately simulating the processes in the Southern Ocean. Now, we can use our data set to improve these simulations, which will reduce uncertainties in future projections.” The findings could also help understand past variations in global climate.

And new satellites will provide more information on the sea ice and ocean. “The Southern Ocean is currently the focus of a lot of research because it is widely under-sampled despite its major importance for our climate,” said Haumann. “During the next austral summer season, I will participate in a field project…on a research ship that will go once around the Southern Ocean. During this cruise we will take many samples of surface seawater to measure the contribution of freshwater from sea ice, land ice and the atmosphere. This will show if we can confirm the major importance of sea ice in the system that we now inferred mostly from satellite data.”

The team reported the results in Nature.

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