Salt water behaves strangely.  As a result, ice can form in polar regions

Since warm salt water rises, ice formation in the sea should be impossible. Researchers have shown with model calculations how ice can still form in polar regions. Due to the unique properties of salt water, rising warm water cannot get past a kind of ‘shield’ in the sea. In this way, cold and fresher water remains unmixed on the surface, which can freeze more easily. The publication appeared in scientific journal Science Advances.

Almost all liquids expand when heated – and thus become lighter. But water is different. Water expands between zero and four degrees Celsius cooling. For example, water is at its lightest at zero degrees Celsius. As a result, ice always forms on the surface of a lake, because the warmer water sinks. This is because especially cold water molecules form a mutual pattern, with a lot of space between the water molecules. By adopting this pattern, cold water expands, is lighter, and remains on the surface.

There is a way to counter this property of water: with salt. Salt settles between the water molecules and disrupts the pattern. Salt water therefore behaves much more like a ‘normal’ liquid: warm salt water expands and rises. For that reason, seawater should hardly freeze at the top.

“The main question of the study was why in polar regions warm salt water from the depths does not mix with water at the surface, while outside polar regions it does,” says Fabien Roquet. He is an oceanographer at the University of Gothenburg and author of the study. “Many people thought that it was enough for fresh water to float to the surface from precipitation, but there would still have to be mixing.”

Warm salt water rises and collides with a shield in the polar regions

In polar regions, it is precisely the interaction between water and salt that determines ice formation, the researchers write. Different layers of seawater are created in the sea based on the salinity. Saltier water has a high density and sinks. Due to rainfall, the water at the top is relatively fresher, which is colder and can freeze more easily. Due to the differences in salt, the layers of seawater are so different that they cannot mix. “It’s like oil floating on water,” says Roquet. Warm rising salt water collides with a “shield”.

“The study highlights an important property of the climate system,” says Wilbert Weijer, a climate scientist at the Los Alamos National Laboratory in the United States. According to him, the importance of the study is “purely academic”, because the advanced models already accurately take into account the properties of seawater.

Kial Stewart thinks otherwise. He is a physical oceanographer at the National University in Australia. “The study is both fundamental and practically relevant. It is a fundamental property of seawater that allows it to form sea ice. The study shows that it is essential that climate and sea ice models include a correct description of sea water. That already happens with the most advanced models.”

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