The ocean is always moving. The ocean moves water, heat, salt and nutrients around the world.
What causes this movement?
Ocean currents are caused by:
- changes in temperature
- changes in salinity
- wind
- tides
There a two main types of ocean current:
- Surface currents in the top 400m of the ocean, caused by wind and tides.
- Deeper currents, caused by changes in water density.
Both surface and deep ocean currents affect the Earth’s climate.
Surface currents
There are five main oceans, these are the:
- North Atlantic Ocean
- South Atlantic Ocean
- North Pacific Ocean
- South Pacific Ocean
- Indian Ocean
Around each of these oceans is a huge circular current, called a gyre. These circular currents are driven by:
- the wind
- heating at the surface
- the spin of the Earth (the Coriolis Effect).
These large-scale surface currents are important because they carry heat away from the tropics and move it towards the poles.
New Zealand is affected by warm subtropical currents coming from Australia and the cold Antarctic circumpolar current.
Wind can also create much smaller, local currents that can change overtime.
Deep Ocean Currents
Deep in the ocean there are slower moving currents created by changes in:
- temperature
- salinity.
Dense water sinks below less dense water. This drives the Global Ocean Conveyor, a system of deep currents that carry heat, salt and nutrients around the world. The Global Ocean Conveyor is a cycle of currents that flow under the main surface currents.
In the North Atlantic, cold winds from the Arctic cool the surface water. Sea ice forms and removes water but leaves the salt behind, so the water becomes more salty. The cold, salty water is denser or heavier than the water below, so it sinks. This sinking water flows south like a very large, slow river in the ocean.
The cold, salty water slowly flows past Antarctica, then north. As the water flows north it warms and mixes with other water to become less salty, so it rises to the surface.
The water rises only a few metres a year. Water that sinks in the North Atlantic may take a thousand years to make it back to the surface.
Tides
Tides create a current in the oceans. Water moves up and down over a long period of time. These tidal currents are strongest near the shore, and in bays and estuaries along the coast.
Tidal currents change in a very regular pattern. Tides are driven by the gravitational pull of the moon and sun.
By studying ocean currents scientists can gain a better understanding of how heat, nutrients and pollutants such as plastic are spread throughout the world.
