(ORDO NEWS) — With the onset of cold weather, the polar seas are covered with ice. However, low temperatures alone would not be enough for this.
An important role in the formation of the ice cover is played by an increased salt content at depth. It does not allow warm water to rise to the surface.
Winter cold cools the surface layers of water. But this does not mean that they will automatically turn into ice.
Cold water is heavier than warm water and tends to sink to a depth, being replaced by not so cold water that rises to the surface.
These processes could prevent the polar seas from freezing if not for the difference in salt content.
The density of a normal liquid increases as its temperature decreases. However, due to the special geometry and strong polarity of the molecules, water behaves in an unusual way. Its density is complexly dependent on temperature and solute content.
Pure distilled water reaches its maximum density at about 4 °C. With a further decrease in temperature, it expands again, and freezing into ice at 0 ° C, it turns out to be even less dense, remaining floating on the surface of the liquid. This is how fresh water bodies are covered with ice.
But in the seas things are different. Salts dissolved in water lower the freezing point of water and at the same time increase its density. With the amount that is contained in sea water, it turns into ice at about minus 2 ° C.
Its density at this temperature is higher than that of a warmer one. Due to this, she tends to go down, and not so cold water rises to the surface from the depth. This leads to mixing, which should prevent the formation of an ice crust.
However, melting ice, flowing rivers and precipitation all bring in large amounts of fresh water, reducing the salinity of the near-surface layers of the seas.
The authors of the new work showed that due to this, the density of such water is less than in deeper and saltier parts, albeit warmer ones.
As a result, it forms a kind of “lid” that interferes with the mixing of the upper and lower layers, preventing the flow of water from below to the surface.
“The difference in salinity between water at the surface and at depth serves as an important factor in the formation of ice at low temperatures in the region of the poles.
Without a difference in salinity, the water would not be stratified and would be constantly mixed, preventing ice from forming, ” explained Professor Fabien Roquet, one of the authors of the work.
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