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The World’s Oceans

The air and the sea exchange gases. Ocean currents are caused by tides, winds and changes in water temperature and salinity.

The sea is the salty water that includes the Arctic, Atlantic, Indian, Pacific and Southern Oceans. The sea covers 70% of planet Earth. 

The air and sea are very closely connected

  • Many gases in the air are able to move from the air into sea water (this is called dissolving)
  • Many gases in sea water are able to move from sea water into the air (this is called evaporation)

Gases move backwards and forwards between the sea surface and the air. 

How easily gases are able to move between water and air depends on the sea conditions:

  • The calmer the sea surface, the more difficult it is for gases to move from one to another
  • The rougher the sea surface, the easier it is for gases to move from one to another

Ocean currents

The sea is always moving. When huge masses of sea water move they cause currents. Here are the main reasons for why there are currents in the oceans.

  1. The moon’s gravity pulls the oceans towards it as it circles the earth. This causes the tides
  2. Winds blow across the sea surface and push the water along
  3. Cold water is heavier so it sinks causing warmer water to take its place
  4. Warm water is lighter so it rises causing colder water to take its place
  5. Salty water is heavier so it sinks causing less salty water to take its place


The surface can be very different from the rest of the ocean below. Colder water and salty water are both more dense and sink causing ocean currents. Winds stir up the surface water.


Sea ice forms on top of sea water. Salt is squeezed out of the ice as it freezes and collects under the ice. This makes the sea water more dense. Image LEARNZ

Next step learning: find out about a large ocean current; its name and where it starts from and goes to