Currents, Ocean

Ocean currents are horizontal layers of seawater that move. There are three types of currents: coastal, surface layer, and deepwater. Coastal currents occur immediately adjacent to the shore. Wave action, gravity, and hydrostatic pressure generate such currents. Longshore currents, the ebb and flow of tidal currents, and dangerous rip current are common examples of coastal currents. Freshwater inflow from rivers, friction with the seafloor, and irregular coastlines add to their variability. Surface layer currents and deepwater currents occur farther offshore.

The sinking of cold, salty water in polar regions creates deepwater currents. These currents bring oxygen to marine life at great depths. The sinking of surface water and the upwelling of deepwater makes up a “conveyor belt” that includes both surface layer and deepwater currents. The belt slowly exchanges water between ocean basins. Climate experts believe that fits and starts in this conveyor belt may explain climate shifts over intervals of 1,000 years or more. Upwelling and downwelling also alter nutrient levels of the water that affect marine ecosystems and fishing patterns.

Surface layer currents are the most understood of the three currents. Persistent winds drive them. When winds blow across the ocean surface, friction transfers energy from the wind to the water. The transfer depends on the velocity of the wind, surface tension of the water, and roughness of the surface. Friction transfers kinetic energy into the water, and also transfers kinetic energy downward in progressively lesser amounts, so that wind-driven currents are usually restricted to the upper 1,300 feet (390 meters) of the oceans and generally to even shallower depths. The speed of the resulting current is about 3 to 4 percent of the wind speed.

The largest surface layer currents form gyres in subtropical latitudes. Gyres are large water circulation systems that flow around the peripheries of the oceans in the subtropical latitudes. The currents flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The trade winds and the westerlies create the gyres. Besides these winds, the Coriolis effect, configuration of landmasses, and higher sea levels near the centers of the gyres affect the flow of the currents. Separate subtropical gyres are present north and south of the equator in each ocean except in the Indian Ocean, which has only a southern gyre. Each gyre has an equatorial current, which absorbs energy from the tropical sun and flows parallel to the equator, a warm western current that delivers the tropical heat to polar latitudes, and a cold eastern current that returns to the equator.

The fastest and deepest currents are the warm western currents. There are five such currents: the [Page 395]Gulf Stream (in the North Atlantic), the Japan or Kuroshio Current (in the North Pacific), the Brazil Current (in the South Atlantic), the Agulhas Current (in the Indian Ocean), and the East Australian Current (in the South Pacific). There are also five cold eastern currents: the Canary Current (in the North Atlantic), the Benguela Current (in the South Atlantic), the California Current (in the North Pacific), the West Australian Current (in the Indian Ocean), and the Peru or Humboldt Current (in the South Pacific). The world's largest current is the Antarctic Circumpolar Current. This eastward flowing cold current encircles the Antarctica, but contributes cold water to the southern gyres.

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