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 following causes:

1. The β-effect, a term that has arisen from the traditional representation of the Coriolis force in the following formula: f = f0 + βy, where f0 is a Coriolis parameter at a definite latitude; in other words, the β-effect is due to spherical form of the Earth turning on its axis.

2. Low conservation of absolute vortex for oceanic motions.

Oceanic gyres are forced by horizontally inhomogeneous, large-scale wind fields (or wind vorticity). For instance, in the North Atlantic Ocean, the anticyclonic subtropical gyre is situated under the northeastern trade wind and midlatitude westerly wind as a result of clockwise wind vorticity, whereas the 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 they are dictated by the law of conservation of absolute (relative plus planetary) vortex.

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

The β-effect may be also understood in terms of Rossby waves. 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 just as a result of their shortness and dispersive properties, which leads 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, the western boundary currents (especially in the Atlantic Ocean) are also controlled by thermohaline factors. The β-effect impacts the thermohaline circulation 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 the sinking of deep Atlantic Ocean water and spreading at depths between 1.5 and 2.5 mi., or 2.5 and 4 km) are southward, while compensative thermohaline currents in the upper baroclinic layer (between the surface and 0.6 to 1.2 mi., or 1 to 2 km) are northward. As a result of superposition of the meridional thermohaline circulation, the wind-driven, northward western boundary currents in the clockwise gyres of the North Atlantic Ocean intensify, while southward currents in the counterclockwise gyres weaken.

The most intense western boundary currents in the Northern Hemisphere are the Gulf Stream, Labrador current, North Brazilian current (Atlantic Ocean), Kuroshio current (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 (such as the Gulf Stream) was conducted by Henry Stommel in 1958 and 1966.

Western boundary currents in the North Atlantic Ocean carry ~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 the source of deep Atlantic Ocean water is about 20 Sv (20 multiplied by 106 cu. m per second). Therefore, 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.

The remaining (at least) 20 percent of total transport is a result of the mesoscale eddies. In fact, the western boundary currents, which look like meandered jets, generate the intense mesoscale eddies, the “rings.” The typical horizontal size of rings is about 60 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 four years, after which time most of them are recirculated and feed the western boundary currents. Thus, the mesoscale eddies account for a significant portion of volume transport of the western boundary currents. Recirculation of the Gulf Stream is one of the integral manifestations of mesoscale effects.