Western Boundary Currents

WESTERN BOUNDARY CURRENTS are intense jet currents at the western periphery of large-scale oceanic gyres in the World Ocean. As was shown in the pioneer paper of Henry Stommel in 1948, they are the result of two causes: the so-called p-effect (this term has arisen from traditional representation of Corio-lis force, f, in the following form: f=f + py, where f is a Coriolis parameter at a definite latitude; in other words, the p-effect is to the result of the spherical form of the Earth turning around its axis), and low conservation of absolute vortex for the oceanic motions.

Oceanic gyres are forced by horizontally inhomo-geneous large-scale wind fields (or wind vorticity). For instance, in the North Atlantic Ocean, anticy-clonic subtropical gyre is situated under northeastern trade wind and midlatitude westerly wind as a result of clockwise wind vorticity, whereas north tropical cyclonic gyre is a result of anticlockwise wind vorticity between the Intertropical Convergence Zone and the northeastern trade wind. Currents in the western part of each gyre are more intense than in the eastern part because this is dictated by low conservation of absolute (relative plus planetary) vortex.

Each particle moving northward (southward) gets an additional (loses) planetary vorticity as a result of the spherical form of the Earth. In the clockwise gyre, this should be compensated for by the increasing relative negative vorticity, that is, by the intensification of clockwise rotating. In the anticlockwise gyre, this should be compensated by the increasing of relative positive vorticity, that is, by the intensification of anticlockwise rotating. In both cases, this leads to intensification of currents in the western periphery of the basin. In the eastern part of gyres, all particles move in the opposite direction, in comparison with the western part. It leads to the weakening of circulation in the eastern gyres' end.

The p-effect may be also understood in terms of Rossby waves. Actually, long, nondispersive Rossby waves carry (kinetic) energy from the east to the west within each gyre. After their reflection from the western boundary of the basin, the short dispersive Rossby waves are generated and move to the east. However, the short Rossby waves are dissipated in the relatively narrow vicinity of the near-coastal zone as a result of their shortness and dispersive properties, which lead to more affective realization of dissipative processes. Thus, the kinetic energy of the planetary Rossby waves is accumulated in the vicinity of the western periphery of the gyres.

In fact, western boundary currents (especially in the Atlantic Ocean) are also controlled by thermohaline factors. The p-effect affects thermohaline circula tion and causes the intensification of the thermohaline currents in the western part of the basin. Deep thermohaline currents in the North Atlantic Ocean (generating in the region of sinking of Deep Atlantic Ocean Water and spreading at the depths between 1.5 and 2.5 mi., or 2.5 and 4 km.) are southward, whereas compensative thermohaline currents in the upper baroclinic layer (between the surface and 0.6 and 1.2 mi., or 1 to 2 km.) are northward. As a result, the wind-driven northward Western Boundary Currents in the clockwise gyres intensify, and southward ones in the anticlockwise gyres weaken as a result of meridional thermohaline circulation.

The most intense Western Boundary Currents in the Northern Hemisphere are the Gulf Stream, Labrador Current, North Brazil Current (Atlantic Ocean), Kuroshio (Pacific Ocean), and Somali Current (Indian Ocean). The velocity in these currents' axes reaches or even exceeds 6.5 ft. (2 m.) per second. Detailed analysis of the structure and origins of Western Boundary Currents (e.g., the Gulf Stream) was done by Henry Stommel in 1958 and 1966.

Western boundary currents in the North Atlantic Ocean carry up to about 100 Sv (1 Sverdrup = 106 cu. m. per second) of water in the upper baroclinic layer. The wind vorticity accounts for about 30 to 60 Sv (30-60 multiplied by 106 cu. m. per second). The average power of source of Deep Atlantic Ocean Water is about 20 Sv (20 multiplied by 106 cu. m. per second). So, the joint effect of wind vorticity and meridional thermohaline circulation can explain up to 80 percent of observed transport of the western boundary currents in the North Atlantic Ocean.

Western boundary currents are meandered jets that generate the intense mesoscale eddies, the so-called rings. The typical horizontal size of rings is about 62 mi. (100 km.), and orbital velocity is 3.3 to 6.5 ft. (1 to 2 m.) per second. Rings trap the water in their central part and carry it with a typical speed of about a few centimeters per second. The lifetime of the rings may reach 4 years. Thus, mesoscale eddies account for a significant portion of volume transport in the vicinity of Western Boundary Currents. Recirculation of the Gulf Stream is one of the integral manifestations of mesoscale effects.

sEE ALso: Coriolis Force; Gulf Stream; Intertropical Convergence Zone; Kuroshio Current; Stommel, Henry;

Thermohaline Circulation; Waves, Planetary; Waves, Rossby; Wind-Driven Circulation.

BIBLIOGRAPHY. Henry Stommel, "The Westward Intensification of the Wind-Driven Ocean Currents," Transactions of the American Geophysical Union, (v.29/2, 1948); Henry Stommel, The Gulfstream. A Physical and Dynamical Description (Cambridge University Press, 1958).

Alexander Boris Polonsky MARINE HYDROPHYSIcAL INSTITuTE, SEBASTOPOL

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