A CuRRENT IS the ordered movement of a fluid in time. A number of different physical factors including tides, landslides, winds, horizontal pressure gradients, and changes in water density can put water into motion. Of these factors, regional winds, horizontal pressure gradients, and density changes can drive currents on basin-wide or global spatial scale. These moving water parcels, in turn, transport not only matter, but also heat. Therefore, changes in the strength or location of currents that cover sufficiently large distances can alter global climate.

Wind-driven currents are produced when winds transfer energy into the surface layer of the water column. Once the water is set into motion, the Coriolis force acts upon it, and the surface layer is deflected 45 degrees to the right of the wind stress in the Northern Hemisphere, and 45 degrees to the left of the wind stress in the Southern Hemisphere. Friction occurs between each of the infinite number of layers composing the water column. Therefore, as it moves deeper, the Coriolis force further deflects each layer, and its velocity is diminished. The depth at which the layer of water no longer is influenced by the wind is called the depth of frictional influence. The parcel of water from the surface to the depth of frictional influence is referred to as the surface Ekman layer. The surface Ekman layer flows 90 degrees to the right of the wind stress in the Northern Hemisphere, and 90 degrees to the left of the wind stress in the Southern Hemisphere. Movement of the surface Ekman layer can lead to a change in sea-surface height over a distance, producing the conditions favorable for geostrophic circulation.

Horizontal pressure gradients serve as another mechanism that causes and maintains large-scale ocean currents. The balance between the horizontal pressure gradient forces drives geostrophic currents, resulting from the change in sea-surface height and/or water density over a distance, and the Coriolis force. Variability in sea-surface height may be produced by winds, tides, or proximity to a coastal boundary. When a difference in sea-surface height occurs, water tends to flow "downhill," or move from areas of high to low pressure. Once in motion, the Coriolis force deflects the water until its motion directly opposes that of the horizontal pressure gradient. The net result is a geo-strophic current flowing 90 degrees to the right of the pressure gradient force in the Northern Hemisphere, and 90 degrees to the left of the pressure gradient force in the Southern Hemisphere.

A combination of wind-driven currents and geo-strophic flow sets up the circulation in ocean gyres, or ocean-basin scale (at least 621 mi. or 1,000 km.) circular currents. Subtropical gyres, or those that occur at mid-latitudes, are driven by a combination of westerlies and trade winds, which force them to flow anticy-clonically. The eastern boundary currents of subtropical gyres can bring cool water toward the equator, and are relatively slow and diffuse. In contrast, western boundary currents, such as the Gulf Stream, are concentrated and fast, quickly transporting warm, salty water from the equator toward the poles.

In addition to winds and pressure gradients, changes in water density can drive global-scale ocean circulation. Thermohaline circulation, or meridional overturning circulation, results from sinking of high-density surface water. This sinking process is usually localized in the north and south Atlantic oceans. High-density water in these regions is made from both seasonal cooling of saline water, and freezing of sea ice. This dense water sinks, and moves equatorward. Because water mass and heat must be conserved, the equatorward movement of cold, dense bottom waters is compensated for with poleward movement of warm surface waters. It has been suggested that melting of ice sheets caused by global warming has already reduced, and will continue to decrease, the intensity of global thermohaline circulation and cause climate to change significantly.

In addition to global-scale circulation patterns, regional currents, such as those driven by upwell-ing and downwelling circulation, can affect local climate. Upwelling circulation occurs when surface waters diverge from either a coastal boundary or another water mass. To conserve mass, deeper, cooler water moves in to take the place of the surface water. An example of this is seen off the coast of California, where surface waters flow offshore and are replaced with cooler deep water. This leads to a well-mixed water column near shore, and, subsequently, a highly-productive marine ecosystem. Conversely, downwelling circulation occurs when water piles up along a coast or converges with another water mass and sinks. Downwelling is observed in the centers of subtropical gyres, leading to low primary production, or low phytoplankton growth there.

Currents may be measured or studied using two different approaches: Eulerian and Lagrangian methods. In a Eulerian approach, a stationary instrument is placed in the water, and currents at that location are measured over time. While an Eulerian design, such as a moored array of current meters, provides a detailed description of flow in a specific location and may resolve vertical current shear, it lacks spatial coverage, and cannot track water flow over long distances. In contrast, a Lagrangian method follows water parcel trajectories by employing instruments such as drifting buoys. This approach works well for producing realistic water parcel tracks, but must be repeated numerous times to account for the sen sitivity of a particle track to its starting location. Historical changes in the strength and position of currents may be studied using paleoceanographic methods. One approach is to take sediment cores from regions throughout the world and compare the distribution of fossils of planktonic organisms over time. Variability of the ranges of species through time suggests long-term shifts in water temperatures or circulation, which may indicate changes in global climate.

SEE ALSO: Coriolis Force; Ekman Layer; Meridional Overturning Circulation; North Atlantic Oscillation; Southern Oscillation.

BIBLIOGRApHY. H.L. Bryden, H.R. Longworth, and S.A. Cunningham, "Slowing of the Atlantic Meridional Overturning Circulation at 25 Degrees N," Nature (v.438, 2005); James Kennett, Marine Geology (Prentice Hall, 1982); Stephen Pond and G.L. Pickard, Introductory Dynamical Oceanography (Pergamon Press, 1993).

Cecily Natunewicz Steppe U.S. Naval Academy

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