Peter Mercaldi, age 12, of Bridgeport, Conn.

Why are ocean currents either warm or cool?

The great ocean currents sweep like mighty rivers through the surrounding waters of the vast seas. They are warm or cool in comparison with these surrounding waters. The world‑wide ocean, of course, is exposed to the torrid heat of the tropics, the temperate zones and the icy weather of the polar regions. Like the land masses, it has warm, cool and cold areas. Water, however, is fluid and tends to mix and merge.

Some of this mixing of waters is done by the sweeping ocean currents. Polar currents, bringing icy waters from the far north and south, sweep into temperate oceans. Compared with the surrounding waters, they are cold currents. Other currents sweep up from the torrid tropics bringing warmer waters to the temperate oceans north and south of the equator. These ocean rivers are warmer than the surrounding waters.