Ted Bystrom, age 14, of Los Angeles, Calif., for his question:
How are deep underwater currents formed in the ocean?
The Japan Current ‑ which sweeps down the coast of California ‑ and the Gulf Stream ‑ which eddies around the North Atlantic ‑ are surface currents no deeper than a few hundred yards. Below this surface region there are deep ocean currents which reach down two or three miles. Down here, as oil the surface, the watery ocean is in restless turmoil and the deep water too follows definite patterns in obedience to the laws of nature. There are deep ocean currants and, more important to us, there are areas where the deep ocean waters well up the surface.
It is no easy job to track the course of a current which runs a mile or more below the surface and most of this fascinating story is still unknown. We know of two deep currents in the Atlantic. Each streams ‑ from ore of the polar regions towards and perhaps beyond the equator.
The surface currents are started by wands, which are caused by the rotating earth, and take their shapes from the land masses. The deeper currents,, we thinks are caused by far more complex factors.
Cold water is heavier than warm water and the polar regions are constantly pouring icy waters into the oceans. Because of these facts a stream of cold heavy polar water would tend to under‑run the warmer surface water. But this is by no means the whole story. We believe now that the tides affect the deep ocean water perhaps more than they do the surface water. A great ocean basin behaves, on 4 grand scales much like the water in a jogging saucer. It rocks back and forth, with less motion towards the center and most motion at the rims. We know that certain deep ocean currents do respond strongly to the tidal pull from the moon and the sun. Tides, temperatures and perhaps the shape of the sea floor are but three of many complex factors which cause the deep undersea currents to flow as they do.
In certain areas, the deep cool waters of the ocean rise up to the surface. This happens when, for some reason, the surface of the ocean is lowered. When two surface streams part there is a kind of groove in the water. This happens off the coast of Japan. Cold water rushes up from below to keep the water surface level and this deep water is especially rich in minerals. Tiny diatoms and countless other one‑celled plants and animals thrive in these mineral‑rich waters. Larger and still larger sea dwellers come to feed on the plankton and on each other. The world's rich fishing grounds are centered where currents of deep sea water well up the surface.
The winds and the shape of the sea floor may also lower the surface and force deep, cold water up from below. These factors help cause the upwelling deep sea water off the coast of California and make it possible for the fisheries to take a billion pounds of sardines from the sea in a year.