Ocean Currents
Over time, the waters of the world ocean circulate within each basin and from one basin to another. Ocean waters move in shallow currents, which are driven by the wind, and deep currents, which are produced by the force of gravity. Wind-driven currents usually move horizontally within the uppermost 100 to 200 meters (330 to 660 feet) of water. Along continental coasts, however, upwellings (movements of water from the deep sea to the surface) occur as winds cause surface waters to move farther offshore. Colder, deeper water then rises to the surface to replace the original surface waters.
Wind-driven surface currents move across ocean basins in huge circular patterns called gyres. In the Northern Hemisphere, gyres in subtropical regions move clockwise, while in the Southern Hemisphere, subtropical gyres move counter-clockwise. The direction of rotation is determined by a phenomenon called the Coriolis effect, which results from Earth's rotation. The Coriolis effect causes moving air masses to be deflected in a sideways direction--toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. The curved paths of the air masses, in turn, drive the circular motion of the gyres.
Ocean currents take on different characteristics depending on their position within the gyres. On the western edges of subtropical gyres, the currents, called western boundary currents, are warm and swiftly moving as they carry water from the tropics or subtropics poleward. The Gulf Stream is an example of a western boundary current. It flows from Florida to the North Atlantic Ocean, where it splits up into several currents, a few of which flow toward Europe.
On the eastern edges of subtropical gyres, water from subpolar regions circulates in cool, sluggish currents called eastern boundary currents. These currents carry water toward the tropics. One of the major Pacific eastern boundary currents is the California Current, which carries cool water southward along the west coast of the United States.
Deep currents, also called thermohaline currents, are movements of water from the surface to the bottom and back. These currents result from changes in water density, which is determined by the water's temperature and salinity (saltiness). Cold water in polar regions is dense, and thus heavier, than warm equatorial waters, partly because water molecules move closer together and become more compacted as water cools. Furthermore, the weight of polar water is increased by salt crystals, which are forced out of sea ice and into the surrounding water as ice forms in winter. This cold, heavy water sinks under the force of gravity. After the polar water sinks, it spreads out slowly toward the equator. As it warms along the way, the water becomes less dense and gradually rises back to the surface in some areas.
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