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Temperature

• The sea water around the Antarctic continent is always very cold. It can be as cold as -2°C.
• The sea water around the equator can be very warm. For example the sea temperature around the Persian Gulf can be as warm as 36°C.

The average temperature of the sea is about 2°C. The average surface temperature is about 17°C.

Cold water is more dense than warm water

Cold water is heavier than warm water and sinks below warm water.

Watch this video demonstration that shows cold water sinking below warm water.

Salinity

Salinity is a measure of how much salt there is in water. Ocean water has a salinity of about 3.5% salt. One litre of seawater contains 35 grams of salt - that is approximately 6 teaspoons.

Salt water is more dense than fresh water

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. 

Seawater is 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.

What happens when seawater freezes?

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. 

• The cold, salty water sinks to form an ocean current that moves deep underwater toward the equator. 
• The surface water that replaces the sinking water forms an ocean current that moves from the equator toward the Arctic and Antarctic.

So, ocean currents are formed because dense water (cold and salty) sinks.