Cathy Parker, age 12, of Prince Rupert, B. C., Canada, for her question:
How does the sea circulate?
The salty seas cover 70.3 per cent of our planet to depths varying from an inch to more than six miles. They hold 330 million cubic miles of crater, plus about 50 quadrillion tons of dissolved minerals. A whimsical math genius estimated the number o€ crater molecules in the whole thing. The fantastic figure was 60 plus a string of 45 zeros. And all of these molecules are busy circulating on global tours.
In the ocean deeps near Bermuda, waters from the Arctic and the Antarctic circulate with water from the Mediterranean Sea. Also present are mingling drops from the tropics and from every other part of the global ocean. The deep waters crawled there, taking centuries to travel from under the icy Antarctic shelf. The surface levels traveled somewhat faster, heaved by tossing tides and blown by stormy winds.
Most of the sea water is held in the deep basins in the Atlantic and Arctic, the Pacific and Indian Oceans. These waters are linked together in one big mixing and mingling ocean. Hater is fluid and keeps stirring.
However the waves and tides cause less circulating than we might suppose. The average waves bob up and doom but usually do not shift much water. However, wild storm winds stir great masses of water and hurricanes bash floods onto the shores.
The steady trade winds pile water along the equator and drive strong currents westward across the oceans. Other prevailing winds circulate these surface currents around the major oceans in circling eddies. The tides lift and drop great humps of water, twice a day on opposite sides of the globe. In certain estuaries, every high tide roars in and out, churning countless tons of water.
The grinds and tides help to circulate the surface ocean waters. At deep levels, circulation is stirred up mostly by temperature and saltiness. Heavier waters tend to sink and inch along the bottom. Cold water is denser and therefore heavier. Water that contains extra helpings of dissolved chemicals also is heavier.
Under the icy Antarctic shelf, the water is cold and also extra salty. It sinks and starts to inch away from home. Some of it may head north and cross the equator for a rendezvous near Bermuda. This tour may take 500 to 1,500 years. Here and at many other deep zones, cold weighty waters meet and circulate from far places in the global
Off the coast of Peru, the winds blow surface water out to sea. Deeper water wells up to replace the loss. Other up swellings in mid ocean also help to circulate surface and deep waters. Sometimes a quake in the seabed displaces enormous masses of water, causing a drastic tidal wave called a tsunami. This rare event creates a mighty stir but is not a regular part of the steady systems of ocean circulation.