Lyndon Gray, of Ardgowan School, asks :-
What makes the water currents in the ocean?
Ralph Marrett, a physicist with the Defence Research Establishment, Auckland, responded.
The permanent currents are one of the most majestic features of the ocean.
They are created and maintained by global scale forces - heat from the sun, forces from the rotation of the earth, global wind patterns, and tidal forces from the sun and moon - and always of course they are controlled by the shape of the ocean floor. The sun heats the sea much more in the tropics than in polar regions, and warmer water is lighter and less dense than colder water. So the warm water rises and the cold water flows down in its place. A circulation is set up which is then deflected and enhanced by the other forces. Great eddies form in the oceans. In the northern hemisphere, the earth's rotation causes them to turn generally clockwise. The Gulf Stream forms along the northern edge of a giant North Atlantic eddy. In the southern hemisphere the eddies are anti-clockwise. At the equator where these northern and southern eddies meet there will be westward flowing equatorial currents.Although the many mechanisms which produce these currents are fairly well understood, putting them together into a complete picture (for example, a computer model) of ocean circulation which can show exactly how a current like the Gulf Stream is created has yet to be accomplished.
Rachel Carson's book 'The Sea Around Us', while originally published in the 50s and 60s, is very interesting, and there is a special edition for young readers with many fascinating photos and illustrations.