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 β-effect (this term has arisen from traditional representation of Corio-lis force, f, in the following form: f = f0 + βy, where f0 is a Coriolis parameter at a definite latitude; in other words, the β-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 β-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 thermoha-line factors. The β-effect affects thermohaline circulation [Page 1089]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.

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