Professor Keith Hunter, of the University of Otago, describes how the North Atlantic current, and others like it, flush out and ventilate the world’s oceans. Because of the size of the ocean, and the slow speed of the currents, water that sinks in the North Atlantic may not see the surface again for a thousand years.
PROFESSOR KEITH HUNTER
There are lots of currents in the ocean but there are some big ones that are responsible for the main behaviour of the ocean, and one of them is the current we call the North Atlantic Current, which is formed in the Norwegian Sea, in the north of the Atlantic Ocean.
And that current is formed by cooling down of seawater, and when you cool seawater down, it becomes heavier - if you cool anything down, it becomes heavier, and when it becomes heavier the force of gravity becomes more important, and so the cold water begins to sink down and it forms a river. This cold seawater flows down the sea floor into the bottom of the ocean, so it’s a river flowing inside water.
Now the size of the North Atlantic Current is about 50 times the size of all rivers in the world added together, and there are quite a number of currents like the North Atlantic Current. Ironically, although they are huge, it takes a very long time for those currents to move the water around, but that is because the ocean is so big, and people don't often realised how huge the ocean is.
And if you had a current moving, or if you were in a boat moving at 1 kilometre an hour, which would be a pretty good ocean current. It takes you 75 years to go round the Earth at that speed. Because of the huge size of the ocean even though we have these gigantic submarine rivers, they take between 1 and 2 thousand years to ventilate the whole ocean.
The ocean currents that sink down into the deep ocean flush out the deep ocean. They take the materials that have accumulated in the deep ocean and bring them back to the surface. And replace it with fresh seawater, if you like, that has come from the surface, and so it makes sure that the chemical ingredients of the ocean get mixed up properly.
NASA/Goddard Space Flight Centre Scientific Visualization Studio/courtesy of nasaimages.org.