10.2: Boundary Currents

The continental borders of the eastern and western halves of ocean basins define a set of important surface currents known as the boundary currents. Coined by American meteorologist Jule Gregory Charney (1917–1981; Charney 1955), “boundary current” has come to define the types of currents found at the edges—eastern and western—of the ocean basins. The western boundary currents flow along the east coasts of continents, while eastern boundary currents flow along the west coasts. The western and eastern boundaries refer to the ocean basins, not the continents. The western edge of an ocean basin is the eastern edge of a continent, and vice versa. Boundary currents represent the veins and arteries of the gyre circulation. They carry cold water toward the equator and warm water toward the poles. Western boundary currents move warm water poleward, while eastern boundary currents move colder water toward the equator. In this way, they act as the main transporters of heat in the world ocean and play an important role in atmospheric heat transport in the Northern Hemisphere (e.g., Palter 2015).

Whereas western boundary currents might be considered the mas macho of ocean currents—strong, fast, and intense—eastern boundary currents tend to be more chill—slow, broad, and diffuse. Western boundary currents gain their strength from global wind patterns and the Coriolis force, while eastern boundary currents profit from regional winds that blow seasonally along their length. Coastlines adjacent to western boundary currents experience warmer ocean temperatures and higher humidity. Eastern boundary current coastlines cope with colder water temperatures and abundant fog. Both boundary currents can enhance biological productivity and support active global and regional fisheries (e.g., Palomares and Pauly 2019).

Western Boundary Currents

Five major western boundary currents can be found in the subtropical gyres: the Kuroshio Current (KURO; pronounced cur-o-shee-o, with no syllable emphasized, as is common in Japanese), the Gulf Stream (GUST), the East Australian Current (EAUC), the Brazil Current (BC), and the Agulhas Current (AGUC). Several minor western boundary currents can be found at various locations—tropical, subtropical, and subpolar—around the world ocean. These minor currents exert considerable and important influence over local and, in some cases, global oceanographic processes. They may not be main arteries, but they feed important “organs” of the world ocean circulation.

Western boundary currents move volumes of water exceeding those of the world’s largest rivers. Two western boundary currents—the Kuroshio Current and the Gulf Stream—exhibit flows up to 140 Sverdrups (Sv, a unit equal to 1 million m³/second). That’s some 70 times greater than the Amazon River with a mean flow of 0.2 Sverdrups. In South Florida, where I grew up, the Gulf Stream approaches within a few miles of the coast, where it’s clearly visible against the eastern horizon. Standing on the beach, you can watch the massive undulating ripples of the current moving at speeds up to 4 knots (4.6 mph). Ships struggle against the flow, and scuba divers caught in the current haven’t got a chance. Many a planned reef dive ends up as a drift dive when the Gulf Stream is running close to shore.

In satellite images, western boundary currents appear like rivers winding back and forth along the edge of coastal seas. Their fluid “banks” meander, that is, they move in a snake-like motion. Occasionally, one of these meanders turns back on itself and pinches off, forming unusually large and coherent mesoscale eddies called rings. Think of them as giant, slow-moving whirlpools (minus the ship-endangering suction effect as seen in Pirates of the Caribbean: At World’s End; Verbinski 2007). Rings can be found throughout the world ocean, but they are best known in the Kuroshio and Gulf Stream where they have been studied for decades.

Oceanographers recognize two types of rings: cold-core rings that trap cold oceanic water inside a donut of warm boundary current water, and warm-core rings, which are the opposite—warm water trapped inside a ring of cold water. Rings are common to both currents but most prevalent in the Gulf Stream, which produces an average of 33 cold-core rings and 26 warm-core rings annually (e.g., Silver et al. 2021). Their structure differs markedly. Cold-core rings spin outward toward the middle of the gyre in a cyclonic rotation (counterclockwise in the Northern Hemisphere), while warm-core rings spin toward land in an anticyclonic rotation (e.g., The Ring Group 1981; Olson 1991; Faghmous et al. 2015; Gangopadhyay et al. 2020). The interior of cold-core rings traps nutrient-rich water that promotes phytoplankton blooms (e.g., Conway et al. 2018). Warm-core rings generate deep mixing that entrains nutrients and also supports blooms equal to or greater than those of cold-core rings (e.g., Dufois et al. 2016). Phytoplankton, of course, provide a food source in what is otherwise an oceanic desert—the central regions of the oceanic gyres. These features provide an excellent example of the interactions among the physics, chemistry, and biology of the ocean (e.g., McGillicuddy 2016; Gaube and McGillicuddy 2017; Xu et al. 2019).

The energy in eddies can also contribute to a recirculation of water within the main flow, known as a recirculation gyre. When averaged over time and space, these recirculation gyres appear as persistent features of a western boundary current and act to increase the volume of its flow. Both the Kuroshio and Gulf Stream current systems exhibit permanent recirculation gyres that loop back and reconnect downstream (e.g., Imawaki et al. 2013). Along the way, they interact with other currents and water masses. Ultimately, they feed modified water back into the main flow. Understanding recirculation in western boundary currents proves important for estimating heat transport from the tropics to the poles and for projections of the effects of climate change on ocean circulation (e.g., Delman and Lee 2021).

Eastern Boundary Currents

Five major eastern boundary currents can be found in the world ocean: the California Current System (CACS); the Peru–Chile Current (PECH), also known as the Peru Current or the Humboldt Current (HUMC); the Canary Current (CANC); the Benguela Current (BENC); and the longest boundary current—at more than 3,400 miles (approximately 5,500 km)—the poleward-flowing (instead of the usual equatorward-flowing) Leeuwin Current (LEEC; pronounced LOO-win). Now, scholars of boundary currents and alert students may note that most websites and textbooks still refer to the West Australian Current (WAUC) as the eastern boundary current in the South Indian Ocean. However, some decades ago, oceanographers recognized that a warm-water, southward-flowing current occurred along the West Australian coast. They named it for the first ship that explored the area in 1622—the Leeuwin, the Dutch word for lioness (Cressell and Golding 1980). No longer classified as an eastern boundary current, the West Australia Current refers to the broad northward flow of water in the eastern half of the South Indian Ocean. Update your charts.

Like western boundary currents, eastern boundary currents gain at least part of their strength from the gyre-scale wind patterns and the Coriolis force. But they also draw energy from seasonal winds that cause upwelling, the upward movement of subsurface waters toward the surface (e.g., Talley et al. 2011). If you look at eastern boundary currents in satellite images, you can often see jets of cold, blue water streaming along their paths. These pockets of cold water, drawn from depths between 164 and 984 feet (50–300 m), also bring with them an abundant supply of biologically important nutrients. In the days following an upwelling event, a green tint will begin to appear in the upwelled waters as phytoplankton divide and proliferate. Soon, meandering filaments of green take over the scene as the phytoplankton bloom reaches its peak. Eastern boundary currents are some of the most productive waters on the planet, supporting some 20 percent of the world’s coastal fisheries (e.g., Kämpf and Chapman 2016).