Adding salt to water lowers the freezing temperature. Yet, despite the saltiness of the ocean, sea ice contains very little salt, about a tenth of the amount of salt that sea water has. This is because ice will not incorporate sea salt into its crystal structure. Therefore, sea ice is actually drinkable. The temperature and salinity of the sea water also help determine its density.
As the temperature of sea water decreases the density also increases. Also, as the salt content of sea water increases , so does its density. This makes the density of sea water, unlike fresh water, below the freezing point. So, in situations of sea ice formation, the salinity, and therefore the density of the underlying water continues to increase well after an area is iced over. In the "Average Salinity" map right below , it shows the lowest salinity in the polar regions. Bear in mind, this image depicts surface salinity only.
The surface salinity is lower in the polar regions than in the tropical regions due to melting each summer. However, each winter below the ocean surface, the increased salinity in the water due to ice formation, causes the water below the ice to sink and that sinking motion governs the motion of the ocean's deep-water currents.
NASA image. The Mediterranean Sea, however, has very high salinity — 38 ppt or more. It is almost closed from the Atlantic Ocean, and there is three times more evaporation than there is rain or freshwater flowing into it from rivers. Additionally, because of the high temperatures in the Mediterranean region, evaporation in the Mediterranean Sea occurs much more rapidly than in other bodies of water, leaving more salt behind as fresh water molecules rise from it and enter the atmosphere.
The warm, dense, salty water of the Mediterranean is replaced by the much less salty Atlantic water that flows in through the Strait of Gibraltar. Water that enters the Mediterranean from the Atlantic usually remains in the Sea for anywhere from 80 to years before returning to the Atlantic Ocean, according to researchers.
The Mediterranean loses three times more fresh water from evaporation than it takes in from its many tributaries — even including gigantic rivers such as the Nile , and the somewhat smaller Po, Rhone, Ebros, Tiber, Ceyhan, Seyhan, Adige, Neretva, and Drin-Bojana rivers. Scientists have discovered that the salt and sediments at the bottom of the Mediterranean Sea prove that on several occasions over the course of history, the Mediterranean Sea has indeed dried up, leaving a large layer of salt behind it.
At that time the Strait of Gibraltar itself closed up, stopping any water from flowing back and forth between the Atlantic Ocean and the Mediterranean. During what researchers call the Messinian Salinity Crisis, a geological event occurring from 5. This enabled the migration of mammals , including camels, into Europe. The basin then filled up again with water from the Atlantic during what is called the Zanclean Flood.
As can be seen from the NASA satellite image above, the Mediterranean is still clearly among the saltiest bodies of water anywhere on the planet. The data depicted shows average salinity from May 27 to June 2, , in a range from 30 to 40 grams per kilogram, with 35 grams being the average. Lower values are represented in purples and blues; higher values are shown in shades of orange and red. Black areas occur where no data was available, either due to the orbit of the satellite or because the ocean was covered by ice, which Aquarius cannot see through.
NASA created an animation, clickable here, of how salinity patterns changed week by week in all the major bodies of water on the Earth, including the Mediterranean, over the year View unit. Science Ages Frozen Oceans The Frozen Oceans Science resources introduce working scientifically concepts and skills to year-olds through enquiry-based lessons which replicate work done by field scientists in the Arctic.
You might also be interested in:. How warm is the ocean? Learn more about ocean surface temperatures with this map. Why is thermohaline circulation so important? Learn more about thermohaline circulation and why it is important. Which creatures live in the seas around the UK?
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