Ebbysemeyer says currents are like giant rivers in the ocean. They are found both at the oceans surface and several thousands feet down on the seafloor.
Surface currents are driven mainly by the wind and by earths rotation, through a force called the Coriolis effect. As the wind pushes the water forward, the Coriolis effect nudges it slightly sideways. The two influences combine to make surface waters move in great loops.
Deep ocean currents are created as seawater approaches the North and South Poles. As the water cools, its molecules draw closer together, making each gallon denser. Heavier than warm water, the cold water sinks to the ocean floor, miles beneath the surface flows. The deep currents then drift toward the equator, where they are gradually heated by the sun. The water molecules spread out again, and the lighter, less dense fluid rises to the surface.
That is not the whole story, Ebbysemeyer says. Before you can accurately predict where or when a floating object will reach a particular shore, you must also consider certain details. One detail is windage. To calculate windage, Ebbysemeyer floats various itemscans, bottles, shoesin a tank, then blasts each one with the breeze from a powerful fan.
Some things sit on the water and just scoot right along, said Ebbysemeyer. Others are fairly well submerged and are not exposed to the wind much at all. A rubber bathtub toy might move at a rate of around 48 kilometers per day, compared with an athletic shoe, which will cover only 32 kilometers in the same period.
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