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Cold water sinks. Salty water sinks. Warm water floats. Fresh water floats.
The average temperature of the sea is about 2°C. The average surface temperature is about 17°C.
Cold water is heavier or more dense than warm water. That means that cold water will sink below warm water.
Watch this video demonstration that shows cold water sinking below warm water.
Salinity is a measure of how much salt there is in water. Ocean water has a salinity of about 3.5% salt. That means that in a 100gm solution of salty water, there is 3.5 grams of salt. Or, put another way, one litre of seawater contains 35 grams of salt - that is approximately 6 teaspoons.
About 90% of sea salt is sodium chloride or table salt. The other salts are made up of chlorine, sodium, magnesium, sulphur, calcium, and potassium. (see image)
Salt water has more stuff in it than fresh water. That stuff would be the salt, which makes the mass of water greater, without adding much to its volume.
Density = mass/volume. Increasing the mass by adding salt increases the density.
Seawater is a little bit more dense than fresh water so it sinks beneath freshwater. This means that when rivers flow out into the sea the river freshwater floats on top of the sea water. However when the wind blows and the sea becomes rough, the two waters get all mixed up.
The winters in the Arctic and the Antarctic are extremely cold. The surface waters of the Arctic and Antarctic oceans become very cold and finally freeze.
When seawater freezes to form sea ice, salt is squeezed out and moves down into the water. This means that sea ice is made up of mostly fresh water. The water just below the sea ice is saltier and more dense than the water below. Because it is more dense, it sinks. The water that sinks is replaced by surface water that comes from areas closer to the equator where it is warmer.
So, ocean currents are formed because dense water (cold and salty) sinks.