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The World’s Oceans

The air and the sea exchange gases such as carbon dioxide. 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 sea moderates the earth's climate and has important roles in the water cycle, the carbon cycle, and the nitrogen cycle.

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. Sometimes the movement of these gases is part of a cycle, for example the carbon cycle.

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

One gas in the air that is easily dissolved in sea water is carbon dioxide. Because carbon dioxide easily dissolves in water we say that carbon dioxide is soluble. Carbon dioxide is also harmless to humans so that is why it is used as the gas in carbonated drinks such as Coca Cola and lemonade. Carbon dioxide in the sea is used by marine plants called phytoplankton. These tiny plants are the start of all marine food chains. 

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

Surface ocean currents are usually caused by wind. This is why surface water in the northern hemisphere flows in a clockwise direction, and in the southern hemisphere surface water flows counter clockwise.

Deep ocean currents are caused by changes in water temperature and salinity. These change the water’s density. Deep ocean currents are sometimes called submarine rivers and cannot be detected easily except where they come to the surface. When the current comes to the surface it's called an upwelling. When the water sinks it is called a downwelling. Argo floats that stay at 1000m for 9 days at a time are shifted around by these deep currents.

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.

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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, where it starts from and goes to and what effect it has on the weather