Chapter 17: The Surface Circulation

Though oceanographers and sailors give names to the surface currents that flow beside coastlines and along the equator—part of what’s known as the surface circulation—these are not, in fact, the only currents in the world ocean. An arguably more important, albeit weaker and slower, set of currents churn in the abyssal ocean—the deep circulation. Traditionally, oceanographers have treated surface and deep circulation as two independent systems. However, in recent decades, they have increasingly recognized that these are part of one global ocean circulation system—the world ocean circulation, what physical oceanographers formally refer to as the meridional (muh-RID-ee-un-ul) overturning circulation, or MOC (e.g., Knauss and Garfield 2017). 

While the emphasis on a world ocean circulation remains important, students of oceanography find it easier to tackle each part separately. Like Dr. Dolittle’s famous two-headed beast, the pushmi-pullyu, the world ocean circulation operates as a single system simultaneously driven by processes at the surface and within the deep. They’re parts of the same beast—the world ocean circulation. Nevertheless, in the interest of preventing your head from being pushed and pulled in two directions, the surface and deep circulation are presented here in separate chapters.

17.1 Main Features 

In this chapter, we largely follow the terminology and definitions adopted by Talley et al. (2011). Additional references are cited where helpful.

The surface circulation crosses the ocean like a vast network of oceanic rivers. However, unlike rivers, the currents that make up the surface circulation in each basin form an ensemble of currents known as oceanic gyres, or simply gyres. The word comes from the Greek word gyros, translated as “circle or ring.”  It describes a system of currents that rotate around a common center like spokes on a wheel. The centers of the oceanic gyres can be found in two general locations in the world ocean: the subtropical gyres, straddling the Tropic of Cancer and the Tropic of Capricorn; and the subpolar gyres, adjacent to the Arctic and Antarctic Circles. Their boundaries are fluid and may change seasonally, but these two gyre archetypes—subtropical and subpolar—help us simplify the complex patterns of the surface circulation and allow us to make some broad generalizations.

I should also note that oceanographers refer to currents that move along lines of latitude—east to west or west to east—as zonal currents. The equatorial currents and the west wind drift currents represent examples of zonal currents in a gyre. Currents that move along lines of longitude—toward the poles or the equator—are referred to as meridional currents. The California Current along the West Coast of the US and the Gulf Stream along the East Coast represent meridional currents in a gyre. Look for other examples in Figure 17.10.

17.1.1 Subtropical Gyres

As pointed out by Roemmich et al. (2016), most people in the world live under weather patterns influenced by the subtropical gyres. Five major subtropical gyres occupy the three major ocean basins: the North Pacific Gyre (NPG), South Pacific Gyre (SPG), North Atlantic Gyre (NAG), South Atlantic Gyre (SAG), and the South Indian Gyre (SIG)—one for each major ocean basin except the North Indian. Squeezed and shortened by the presence of the Indian subcontinent, the North Indian Ocean basin lacks a true gyre, but in its place lives a fascinating system of currents that change direction seasonally (see below). Note that the gyres in the Northern Hemisphere rotate clockwise, while the Southern Hemisphere gyres rotate counterclockwise. These rotations result, of course, from the Coriolis force (introduced in Chapter 15).

17.1.2 Polar and Subpolar Gyres

Polar gyres exist only in the Arctic Ocean whose waters reach the very highest latitudes of the world ocean. Here we find the Beaufort Gyre (BG) in the Beaufort Sea—named after Irish hydrographer Sir Francis Beaufort (1774–1857). This gyre spins clockwise in the Arctic Basin (when viewed from above). 

Subpolar gyres occur at latitudes above 60°. Generally, landmasses restrict their size in the Northern Hemisphere, while in the Southern Hemisphere, the Antarctic Circumpolar Current (described below) sweeps such a broad path that its gyres are confined to a few locations nestled against Antarctica.

Like their subtropical counterparts, subpolar gyres are named and feature zonal and meridional currents that mark their boundaries. They don’t rotate consistently because at these smaller scales, the momentum of the currents that feed them and the shape of the land that surrounds them govern their behavior. Nevertheless, they exert an important influence on regional oceanographic and meteorological conditions.

Notable in the North Pacific are the Alaskan Gyre (AG), where we find the northward-flowing coastal-hugging Alaska Current; and the Western Subarctic Gyre (WSAG), fed by the East Kamchatka and Oyashio Currents. In the Southern Ocean, two subpolar gyres can be found. The Ross Gyre (RG) acts as a gatekeeper for warm-water exchanges between the Antarctic Circumpolar Current and the Ross Sea—a broad, shallow sea located at the “mouth” of the elephant head–shaped continent. It occupies a region roughly between East and West Antarctica (e.g., Dotto et al. 2018). Across the “trunk” of the West Antarctic peninsula lies the Weddell Sea and the Weddell Gyre (WG), a site of bottom water formation thought to be critically important to world ocean circulation and climate change (e.g., Vernet et al. 2019).

17.1.3 Equatorial Currents

In all three major basins—Atlantic, Pacific, and Indian—currents flow along the equator from east to west in a complex of currents known as the equatorial currents. Occupying the tropical regions of our planet, this set of currents receives the most intense solar radiation. As their waters move from east to west, they warm up. By the time they travel the full width of the basin, their waters reach their highest temperatures. Indeed, the warmest waters in the world ocean can generally be found at the western terminus of the equatorial currents, places such as the Caribbean Sea and coastal Brazil, the Philippine Sea and Indonesian Seas, and north of Madagascar off the coast of Somalia, where modern-day pirates skulk.

The principal equatorial currents include the North Equatorial Current (NEC), the South Equatorial Current (SEC), the North Equatorial Countercurrent (NECC), the South Equatorial Countercurrent (SECC), and the Equatorial Undercurrent (EUC) in the Pacific, Atlantic, and Indian Oceans. Note that in the Pacific Ocean, the Equatorial Undercurrent is also known as the Cromwell Current, named after the man who discovered it, Scripps oceanographer Townsend Cromwell (1922–1958). In the Pacific Ocean, the South Equatorial Current exists as a broad, westward-flowing current that flows along the equator in both hemispheres. The South Equatorial Countercurrent develops seasonally as an eastward flow in the western part of the basin (e.g., Chen and Qiu 2004). In the Atlantic Ocean, the South Equatorial Current develops two additional branches: the North South Equatorial Current (NSEC) and the Central South Equatorial Current (CSEC). In the Indian Ocean, the South Equatorial Current and Equatorial Undercurrent only develop seasonally (e.g., Phillips et al. 2021). 

Equatorial currents move waters from the eastern sides of ocean basins to their western sides. Indeed, oceanographers use the terms eastern and western to distinguish the eastern and western halves of ocean basins. For example, the US West Coast lies along the eastern North Pacific while the US East Coast lies along the western North Atlantic. It can get a bit confusing, but it helps to remember that eastern refers to the right-hand side of the basin while western refers to the left-hand side. Just remember western US (left-hand side of the continent) and eastern US (right-hand side of the continent) and apply the same reasoning to figure out which side of an ocean basin you’re on.

17.2 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).

17.2.1 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). 

17.2.2 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).

17.3 West Wind Drift Currents

The currents that flow along the northern boundary (in the Northern Hemisphere) or southern boundary (in the Southern Hemisphere) of the subtropical gyres move water left to right—that is, from west to east, or zonally, just like the equatorial currents. Some oceanographers refer to these eastward-flowing currents as the west wind drift currents, after the winds that generate them—the westerlies (e.g., Talley et al. 2011, 308). This terminology has not gained widespread use, but it provides a convenient means for students to distinguish the four parts of a subtropical gyre, so I’ve adopted it here. 

The westerlies blow in broad bands west to east at middle latitudes in each hemisphere. These winds drive the northern and southern limbs of the subtropical gyres in the Northern and Southern Hemispheres, respectively. In the Northern Hemisphere, the broad, slow-moving west wind drift currents flow at middle latitudes—around 40°N in the North Pacific and 50°N in the North Atlantic. They transport warm water zonally across the basin, where the currents encounter the North American or Eurasian continents. In the Southern Hemisphere, the west wind drift currents interact with the Antarctic Circumpolar Current (discussed below).

The principal west wind drift currents include the North Pacific Current (NPC), the North Atlantic Current (NAC), the South Pacific Current (SPC), the South Atlantic Current (SAC), and the South Indian Current (SIC). Until recently, the South Pacific Current, South Atlantic Current, and the South Indian Current were thought to be part of the Antarctic Circumpolar Current. Oceanographers now recognize these as distinct currents flowing along the southern boundaries of their respective gyes (e.g., Talley et al. 2011). 

17.4 Polar Ocean Currents 

The polar oceans occupy unique positions in the world ocean circulation. Both contribute mightily to the deep circulation, but their role in the surface circulation has gained increased attention as the linkages between the surface and deep circulation become better understood.

Central to the surface and deep circulation is the Antarctic Circumpolar Current, or ACC. The Antarctic Circumpolar Current occupies the southernmost limb of all three subtropical gyres in the Southern Hemisphere. From this vantage point, it controls flows of surface and deep water into the major ocean basins and influences their properties (as we shall see in Chapter 18). In this way, the Antarctic Circumpolar Current acts as the Grand Mix Master of the world ocean. Like a DJ, it samples, blends, and invents unique combinations of water masses that it distributes throughout the ocean. (Cue DJ Spooky, who actually visited Antarctica and composed a musical work in tribute to this extraordinary continent.)

The Arctic Ocean basin—first observed by Norwegian oceanographer Fridtjof Nansen (1861–1930)—hosts two wind- and buoyancy-driven current systems that deserve our attention for their role in sea ice dynamics, climate change, and the general circulation of the Atlantic Ocean. Along the eastern (southward-flowing) extension of the Beaufort Gyre, we find the Transpolar Drift Stream (TPD). This current flows off the Siberian Shelf and transports water and sea ice into the Norwegian, Greenland, and Iceland Seas. Dramatic warming and changes in sea ice and freshwater flows in this region have generated concern among polar oceanographers for disruptions to Arctic ecosystems, Arctic peoples, and global climate (e.g., Proshutinsky et al. 2015; Timmermans and Marshall 2020). An ice-free, summertime Arctic has major implications for maritime, political, and military interests. The Transpolar Sea Route, a hypothesized shipping lane from the Atlantic to the Pacific across the Arctic Ocean, may become a reality by the 2040s (e.g., Bennett et al. 2020; Crawford et al. 2021). 

17.5 Understanding Ocean Motions

Before taking a closer look at individual currents, it’s helpful to learn a little about the planetary, atmospheric, and oceanic factors that generate flows in the ocean. While this is a bit of an advanced topic—and one which students of physical oceanography learn about in great detail—a brief summary for the introductory students provides insights into the machinery that makes the ocean move.

17.5.1 Ekman Transport

Fridtjof Nansen was a Norwegian explorer and oceanographer. He was also a great humanitarian and helped half a million former prisoners of World War I repatriate in countries around the world. For this he was awarded the Nobel Peace Prize in 1922, the only oceanographer ever to receive a Nobel Prize. Nansen’s contributions remain relevant today. He invented the Nansen bottle (still in use for water sampling) and the Nansen closing net, a zooplankton net that opens and closes at depth (still sold today). He is also known for having built a round-bottom ship, the Fram, that could not be crushed if the ocean froze around it. It’s here where we pick up his story.

Nansen dreamed of being the first person to reach the North Pole. He had already achieved fame as the first person to cross the interior of Greenland on skis. But instead of crossing the frozen ocean on skis, he came up with the idea of letting a ship, the Fram, freeze into the sea ice, the movements of which would take him across the North Pole. It was a great idea, in theory. Alas, it was not to be. The drift of the ice and the motions of the underlying currents did not favor a journey over the pole. The Fram only made it to 84°4’N. Still determined to make the pole, Nansen set off on dogsled with the athletic Hjalmer Johansen (1867–1913), shipmate and fellow explorer. They made it as far as 86°13.6’N, the farthest north of any men at the time. Rugged ice and lack of food forced them to head west to Franz Josef Land. Conditions forced them to spend the winter in a small hut, where polar bears, walrus, and seals kept them fed. The following June, they were found and rescued by British explorer Frederick Jackson (1860–1938). 

Among Nansen’s many valuable contributions to our knowledge of Arctic waters, one in particular changed our understanding of ocean circulation. While adrift in the Fram, he noted something curious about the interaction of the wind with the ice. The ice appeared to move to the right of the direction of the wind, which he hypothesized was due to Earth’s rotation (Nansen 1902, 369). That conclusion, however, would require mathematical proof. 

He mentioned this observation to Norwegian meteorologist and oceanographer Vilhelm Bjerknes (1862–1951), who handed the problem to a Swedish graduate student, V. Walfrid Ekman (1874–1954). The son of a physical oceanographer and well versed in mathematics, Ekman had no trouble deriving the solution (Eliassen 1982; Jenkins and Bye 2006). The rotation of the Earth and the associated Coriolis force explained the drift of the ice to the right of the wind in the Northern Hemisphere. What’s more, the right-directed surface layer dragged the layer beneath it, causing it to move and also deviate to the right, as did the layer beneath that one, and so on. Ekman likened the effect to a spiral staircase. He published his results in 1902 (in Norwegian) and 1905 (in English), and the phenomenon came to be known as the Ekman spiral. Soon it became apparent that the Ekman spiral could explain other water motions as well. And, indeed, the motions of the ocean as driven by the wind-generated Ekman spiral underlie the surface circulation of the world ocean.

The Ekman spiral can be best envisioned as a spiral stack of books. In the Northern Hemisphere, as the wind blows over the surface layer of the ocean—the top book—it slowly moves forward and turns, adopting an angle of 20°–45° to the right of the wind. (In the Southern Hemisphere, the angle is to the left of the wind.) Due to friction, the surface layer drags the layer underneath it—the second book in the stack—and that layer begins to move and turn, adopting an even greater angle relative to the direction of the wind. It also moves more slowly than the surface layer, which moves more slowly than the wind, because the transfer of momentum is never 100 percent efficient. Each successive layer moves a bit more to the right and a bit more slowly. At some point, water layers may even move in the opposite direction of the wind. At the Ekman depth—defined as the depth where the current flow is 37 percent of the surface current flow and in a direction 180° opposite the direction of the wind—the Ekman spiral runs out of steam, so to speak, and the motion ceases. You’ve reached the bottom book.

If we take the average direction of all the water layers from the surface to the Ekman depth, we find that the average direction of water movement is 90° to the right of the wind. So the net transport of water, the net motion caused by a blowing wind, is 90° to the right (or left) of the wind (depending on the hemisphere). The 90° net movement of the ocean by the wind-driven Ekman spiral is called Ekman transport. By way of example, consider that a sustained 20 mph wind will generate a surface current of less than 0.05 mph, or about 236 feet per hour. Slow. That surface current moves in a direction that is 45° to the right of the wind. At depths of roughly 330–500 feet (100–150 m), the much, much slower current moves in the opposite direction—at an angle of 180° to the wind. If we take the average speed and direction of all the layers between the surface and 500 feet (150 m), we find that the net water movement is 90° to the direction of the wind.

17.5.2 Geostrophic Flow

The importance of Ekman transport becomes clear when you consider the effects of the global surface winds on the ocean. You may recall that trade winds blow generally east to west in a region about 20° north and south of the equator. At the same time, the westerlies blow west to east at middle latitudes (30° to 60°). If you consider the flows of water generated by Ekman transport (i.e., 90° to the right or left of the wind, depending on the hemisphere), you might predict that the trade winds will move water toward the poles and the westerlies will move water toward the equator. In theory, water should pile up in the center of the gyres. 

Though humans can’t discern changes in the height of the sea surface, satellites can. Indeed, scientists using satellite altimeters observe hills of water—from 3 to 6 feet (0.9–1.8 m) high—in the central gyres. You might say that the ocean surface resembles the bumpy appearance of a pan de muerto (Mexican bread of the dead)—albeit one stretched over thousands of miles. The bumps represent changes in sea surface height, the changes in elevation of the sea surface. Measurements of sea surface height produce maps of sea surface topography—the shape of the sea surface. 

As discussed in Chapter 4, satellite altimeters send out a pulse of microwave energy and measure quite precisely the time it takes for that pulse to return to the satellite. In principle satellite altimeters work just like sonar or the laser rangefinders used in golfing, construction, or the military. Knowing the speed of the microwave pulse and dividing the time of pulse and return by two (because the pulse goes down and back up, traveling twice the distance), computers aboard the satellite can calculate the height of the sea surface relative to the height of the satellite. These highly sophisticated instruments can actually detect variations in sea surface height on the order of a few millimeters, which is pretty impressive considering they’re being done in space. Satellite altimetry reveals a great deal about the circulation of the ocean, tides, climate change, El Niño and La Niña, and even the seafloor, because the ridges and trenches and seamounts create bumps and wiggles on the sea surface. 

The increased elevation of the sea surface in the center of the gyres creates a pressure imbalance at the surface of the ocean. Just like water poured on a tabletop tends to spread out, the ocean surface tries to remain level. Pressure imbalances, or pressure gradients—differences in pressure between two locations—in the ocean cause currents, just like pressure gradients in the atmosphere cause winds. Where elevations exist, water begins to flow downhill. But the flow of water is subject to the Coriolis force, just like any current in the ocean. As a result, the water will turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Though initially water moves uphill by Ekman transport, it begins to flow back down as a result of the pressure gradient. And as it moves downhill, it is deflected toward the right or left. Eventually, the pressure gradient force and the Coriolis force come in to balance with each other. As a result, the balanced current flows at right angles to the directions of the pressure gradient and the Coriolis force and flows around the sea surface elevation. This balanced current is called a geostrophic current, one under the influence of Earth’s rotation. 

The rotation of geostrophic ocean currents around the sea surface elevations in gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The balance between the wind-driven pressure gradient (caused by Ekman transport) and the Coriolis force explains this pattern. Put another way, the major ocean currents are geostrophic. The geostrophic balance provides a satisfactory explanation for the broad surface circulation patterns that we observe in the world ocean.

17.5.3 Western Intensification

Now, there’s one more physical explanation required to explain a difference observed in the surface currents. Don’t worry, it’s a short explanation, and it doesn’t even require you to think. 

Along the western boundaries of the gyres—that is, on the western sides of the ocean basins—the surface currents move much more quickly than currents along the eastern sides of the ocean basins. As mentioned earlier, western boundary currents are faster, narrower, and more energetic than eastern boundary currents, which are slower, wider, and less energetic. The difference is explained by something called western intensification, which is caused by latitudinal differences in the Coriolis force. That’s it. That’s the explanation. The Coriolis force, which governs the behavior of geostrophic currents, gets stronger from the equator to the poles. The result is a narrowing and intensification of western boundary currents.

17.6 Ocean Currents Up Close

Now that we know something about the overall patterns of currents and the forces that generate those patterns, let’s take a look at some specific examples of individual current systems, especially those that play important roles in human affairs.

17.6.1 The Kuroshio Current: Where Sushi Is Born

Along the western edge of the North Pacific near the equator, the North Equatorial Current splits into southward and northward flows. The southward flow forms the lesser-known Mindanao Current (MINC), which originates in the vicinity of the second-largest island in the Philippines, Mindanao, the Land of Promise (Jhian 2016). The northward flow becomes the well-known Kuroshio Current, or “black current” (presumably for its dark blue waters). The Kuroshio officially starts along the coast of Taiwan, then flows north along the coast of southern Japan and splits eastward as the Kuroshio Extension. Off the island of Hokkaido, the Kuroshio meets the southward-flowing Oyashio Current (OYAC) (o-ya-shee-o; “parental tide”), forming a massive confluence of currents that flow toward the northwest. Though ranked as the second-largest western boundary current in the ocean, the Kuroshio’s influence over people and cultures is unmatched. Its highly productive waters boast dozens of commercially important species and provide a spawning grounds and nursery for Pacific bluefin tuna (Oshima et al. 2017; Ashida et al. 2022). The warm waters of the Kuroshio even allow coral reefs to grow along the coast of Japan, the northernmost coral reefs in the world. But the Kuroshio’s use as a “highway” connecting islands and mainland all along its length—the Kuroshio Road, as it is called—has inspired exchanges of resources and cultures since ancient times. As Hyun (2018) expresses it: “The sea in which the Kuroshio Current flows is not just a geographical unit but also an integration of trajectories for those who live and have lived in the area.” The Kuroshio maritime culture extends well beyond Japan. You could even argue that sushi’s popularity in the US has brought the Kuroshio culture to our shores as well.

17.6.2 The Gulf Stream: The Ocean’s First Freeway

Along the East Coast of the United States lies a familiar western boundary current—the Gulf Stream. To the early Spanish explorers, this fast-moving current seemed to flow out of the Gulf of Mexico, which indeed it does. Naturally it was first known as la Corriente del Golfo, the Spanish term for Gulf Stream. Had the Gulf Stream been any old current, it would have remained unremarkable in human history. But Spanish sailors in the 1500s soon learned to use the current to quicken their journey home. They managed to keep this knowledge secret for a few decades, but soon other seafaring peoples, especially whalers, fishers, and merchant sailors, learned to use the current to their advantage. In this way the Gulf Stream became known as the most favorable route from the New World to the Old World (e.g., Gaskell 1972; MacLeish 1989; Ulanski 2008).

Historically, the Gulf Stream was first noted in 1513 by Spanish explorer Juan Ponce de Leon (1474–1521) and his skilled navigator, Antonio de Alaminos (1475–1520; Pillsbury 1891). It was first accurately charted in 1733 by British captain and tobacco merchant Walter Hoxton (1699–1741). Though his chart was largely ignored, he clearly understood important details about the Gulf Stream and rightfully deserves credit for the first description (Richardson 1982). Be that as it may, American Founder and inventor Benjamin Franklin (1706–1790) receives popular credit as the first person to systematically chart the Gulf Stream. Franklin served from 1753 to 1774 as the deputy postmaster general for North America (under British rule), and while visiting London in 1768 as part of these duties, he heard complaints about the longer delivery times for westbound (from Britain to the US) versus eastbound mail (US to Britain). Chatting about this problem with his cousin Timothy Folger, a Nantucket whaling captain, Franklin learned that whalers were quite familiar with the current. Folger sketched an outline of the strong current on a chart, and the rest is history, as they say. Franklin published three versions of the chart in 1769, 1780, and 1786. The first two printings were limited to avoid giving away secrets to the British, who were at war with the US from 1775 to 1783 (Lacouture 1995). The 1783 version, the most inaccurate of the three, remained the most popular until the 1978 discovery of the first version in Bibliothèque Nationale in Paris by oceanographer Philip Richardson (Richardson 1980). 

17.6.3 Other Western Boundary Currents

While the Kuroshio and Gulf Stream are the poster children of western boundary currents for oceanographers (at least in the Northern Hemisphere), among the movie-watching public, the most famous western boundary current has to be the East Australian Current, the EAC, as depicted in the Disney film Finding Nemo (Stanton and Unkrich 2003). This western boundary current in the South Pacific flows southward along the east coast of Australia (hence its name) and serves as a kind of superhighway for marine life, including fish and sea turtles. Unfortunately, and I really hate to break this to you, there’s no evidence that clownfish hitch rides on the backs of sea turtles on the EAC.

In the South Atlantic, the Brazil Current flows sluggishly southward until it meets the Malvinas Current (MC; also known as the Falkland Current) off the coasts of Uruguay and Argentina in a region known as the Brazil–Malvinas Confluence (e.g., Artana et al. 2019). This meeting of warm and cold water generates considerable dynamic complexity and a high productivity that supports one of the largest squid fisheries in the world (Alberto et al. 2020).

17.6.4 The California Current System: The Kelp Highway

Near and dear to those of us living on the West Coast of the United States is the eastern boundary current known as the California Current (sorry, Oregon and Washington). This shallow flow of water (surface to 1,000 ft) starts off the coast of British Columbia, Canada (around 48°N) and winds its way south past the tip of Baja and into equatorial waters. Around 15°N to 20°N, it joins the North Equatorial Current. The California Current varies seasonally, with its strongest flows during the upwelling season (July–August, when northerly winds are strongest) and weakest flows (sometimes no flow) during the winter. In general, the main flow of the current can be found offshore about 124 to 186 miles (200 to 300 km).

Most authors describe the California Current and similar eastern boundary currents as wide currents, on the order of 600 miles (1,000 km). Talley et al. (2011) describe the California Current and eastern boundary currents in general as relatively narrow, on the order of 60 miles (100 kilometers). They restrict their definition of eastern boundary currents to the intensified equatorward flows created by Ekman transport and upwelling (narrow in width) versus the general equatorward flow of the eastern Pacific (wide). Indeed, the narrower region defined by Talley corresponds to the “transition zone” identified by Lynn and Simpson (1987), which they describe as the “core” of the California Current. 

Indeed, the California Current is best described as a system of currents, and most authors refer to the boundary current of the eastern North Pacific as the California Current System (CCS). In fact, we find several named currents along the coast of California. From the shores of Orange County to the ocean side of the Channel Islands, we are more likely to see mean flows of water to the north rather than to the south. The true California Current flows outside of the Channel Islands. Because of the bathymetry of the California Borderland with its associated islands and the topography of the transverse mountain ranges of Southern California (San Gabriel and San Bernardino), which generate coastal atmospheric eddies, the current splits at the southern end of the Channel Islands. The main branch continues southward but another branch does a U-turn and flows northward along the coastlines of San Diego, Orange County, and Los Angeles. That northerly coastal flow is called the California Countercurrent. It is strongest in the fall and winter, when the California Current is weakest. Where the California Countercurrent flows north of Point Conception (just north of Santa Barbara)—along the West Coast to about 48°N—it is called the Davidson Current. This current was named after the British-born, American-raised geographer George Davidson (1825–1911), who, among other accomplishments, mapped the Pacific Coast, built the first West Coast astronomical observatory, and served for 17 years as president of the California Academy of Sciences (Lewis 1954). Underneath the Davidson Current is another northward-flowing current, the California Undercurrent, which flows at an average depth of about 656 feet (200 meters). The California Undercurrent can be traced from the eastern equatorial Pacific to the northernmost boundary of the California Current. Like other currents in the California Current System, it varies seasonally, reaching peak flows in the fall and winter. Who would expect anything different from a current named after California? We’re complicated.

Before we leave the California Current System, we need to pay homage to one of the reasons why the coastal waters of California, Oregon, and Washington support thriving communities of marine life: upwelling. When a northerly wind blows, Ekman transport drives surface waters offshore. The movement of surface water offshore sucks cold, nutrient-rich waters from 328 to 984 feet (100 to 300 meters) to the surface. In satellite images of ocean color, you can see the cold surface waters flowing like paint off the coastline. These upwelled waters bring abundant, dissolved, biologically important nutrients to waters that were previously lacking in them. The result: phytoplankton blooms all along the coast soon after upwelling occurs. Such images beautifully illustrate the link between a physical event—wind-driven upwelling—and a biological event—proliferation of phytoplankton in the presence of upwelled nutrients.

Upwelling also provides habitable temperatures and nutrients for marine seaweeds, such as the giant kelp, the largest seaweed in the world, reaching a length of nearly 200 feet (60 m; e.g., Schiel and Foster 2015). Kelps and similar seaweeds prefer colder waters, which upwelling and the southerly flow of the California Current System help to maintain. The “forests” they create and their tremendous productivity—the giant kelp can grow more than one foot (0.35 m) per day (e.g., Stewart et al. 2009)—attracts hundreds of species of marine organisms. Their abundance along the Pacific Rim coastline has led some anthropologists to speculate that humans migrated to the Americas from Northeast Asia by watercraft some 16,000 years ago. The so-called kelp highway—an ice-free, coastal corridor from Japan to Mexico—would have provided plenty of food, fur, and other materials to sustain their journey (e.g., Erlandson et al. 2015). The kelp highway offers an intriguing and perhaps more accessible alternative to the more commonly accepted land-bridge route, fraught with glaciers.

 17.6.5 Other Eastern Boundary Currents

The eastern boundary currents of other ocean basins exhibit features similar to the California Current System—seasonal flows that reverse, jets and eddies, and upwelling-driven productivity. The South Pacific basin is home to the famous Peru-Chile Current—also known as the Humboldt Current, named after Prussian explorer and biogeographer Alexander von Humboldt (1769–1859). This eastern boundary current and its upwellings drive the Peruvian anchoveta fishery, one of the most productive in the world (e.g., Bakun and Weeks 2008). In the North Atlantic basin, the Portugal–Canary Current System bathes the coastlines of Portugal and North Africa. The South Atlantic harbors the Benguela Current, an eastern boundary current that traces its northerly path along the western shores of South Africa. Shipwrecks litter the northern section of the current along coastal Namibia, earning this region the nickname of Skeleton Coast. The cold waters of the current and hot desert air generate dangerous fogs here. In the South Indian basin, the Leeuwin Current joins a complex of other currents along the western and southern shores of Australia. Unlike other eastern boundary currents, the Leeuwin Current is generally nutrient-poor and lacks upwelling, though the potential exists for enhanced productivity due to eddies that spin off the current (Waite et al. 2007).

17.6.6 Currents and Winds along the Equator

At the southern terminus of the California Current in the North Pacific, we find the beginnings of the North Equatorial Current, a broad, westward-flowing surface current generally found between latitudes 10°N and 30°N. The North Equatorial Current forms the lower limb of the North Pacific Gyre. The North Equatorial Current is strongly influenced by processes associated with the convergence of trade winds at the Intertropical Convergence Zone (ITCZ). It also interacts with the eastward-flowing Equatorial Countercurrent—sometimes divided into the North and South Equatorial Countercurrents—whose existence is associated with the dynamics of the ITCZ. Of course, this is where the world famous El Niño and La Niña occur. Though not well understood, both El Niño and La Niña affect the flow of the North Equatorial Current, which in turn affects the flow of the Kuroshio and Mindanao (e.g., Wang et al. 2020). This in turn influences fisheries and weather in these regions. One thing leads to another. And while we’re here, I have to mention the South Equatorial Current, which you might think (and others have claimed) exists in the Southern Hemisphere. Due to the larger surface area of land in the Northern Hemisphere and larger surface area of ocean in the Southern Hemisphere, the trade winds and ITCZ shift northward, so that the westward-flowing South Equatorial Current flows from about 30°S to 5°N. (Yes, that means part of the South Equatorial Current actually flows into the Northern Hemisphere.) 

17.6.7 West Wind Currents: The Weather-Makers

Along the northern boundaries of the North Pacific and North Atlantic subtropical gyres, we find the North Pacific Current and North Atlantic Current, respectively. Though complicated, their flows—highly dependent on the boundary currents to which they connect—exert a great influence on weather patterns in the US and Europe.

From its starting point off the Grand Banks of Newfoundland, the North Atlantic Current immediately splits into two branches. One branch dips southward along the eastern edge of the Gulf Stream Recirculation Gyre, then swings eastward across the Azores, where it is known as the Azores Current. A northerly branch takes a bit of a sightseeing tour along the edge of the continental shelf off Newfoundland. This branch swings northeast and northward, following the contours of Flemish Cap, which you may remember from the book The Perfect Storm (Junger 2009). Here it acts as a western boundary current along the coast of Newfoundland. This branch splits off the coast of Ireland. A northerly flow becomes the Norwegian Current, and a southerly flow moves along the Bay of Biscay toward Portugal. Finally, another branch of the North Atlantic Current follows the traditional route of westerly flows. This branch heads almost due east from the Grand Banks until it reaches the coast of Portugal, where it joins water from its northerly sister branch and a bit of flow from the Azores Current. Together these flows contribute to the surface waters of the Mediterranean Sea as they enter through the Straits of Gibraltar. Suffice it to say that the North Atlantic Current takes more turns than a trip to Grandma’s house at Thanksgiving.

The twists and turns of the North Atlantic Current aside, there’s a larger principle at work here that gets to the heart of one of the reasons why ocean currents are so important: they affect large-scale weather patterns. Broadly speaking, we can think of the North Atlantic Current as a distribution pipeline for the Gulf Stream. The flows of the North Atlantic Current distribute water, heat, dissolved substances, life, and manmade materials from the Gulf Stream to the east, north, and northeast. The northerly flows of the North Atlantic Current bring warm water farther north than in any other ocean basin. The citizens of the United Kingdom—and much of Europe—owe their damp but temperate weather to the western boundary portion of the North Atlantic Current, which heats air masses moving eastward with the polar jet stream (e.g., Palter 2015).

Another key aspect of the multidirectional flows of the North Atlantic Current is their impact on the circulation of the deep ocean. The flows of the North Atlantic Current influence the formation of deep water masses that are formed in the subarctic North Atlantic. These masses sink and spread throughout the world ocean. The North Atlantic Deep Water may play a role in abrupt climate change, a hypothesis that gained notoriety (albeit in a nonscientific fashion) from the movie The Day after Tomorrow (Emmerich 2004). The take-home message here is that surface currents interact with the atmosphere, and these interactions influence Earth’s weather and climate. Temporal and spatial variability in surface currents, especially in their energy and mass transport, contribute to temporal and spatial variability in weather and climate. The atmosphere and ocean form a linked system: as one goes, the other goes. A great deal of scientific effort is being spent to understand how the atmosphere and ocean interact.

The same message—coupled air–sea interactions—applies to the North Pacific Ocean. You can blame the weather along the West Coast of the United States—and the Midwest, for that matter—on conditions in the North Pacific Ocean. Just as the Gulf Stream feeds the North Atlantic Current, so too the Kuroshio Current feeds the North Pacific Current. And so temporal and spatial variability in the Kuroshio and the Kuroshio Extension, and their contributions to variability in the North Pacific Current, will influence weather and climate in the western United States (Schulte and Lee 2017). Pretty amazing, huh?

17.7 Monsoonal Circulation

The Encyclopedia Britannica calls the Indian Ocean “the smallest, geologically youngest, and physically most complex of the world’s three major oceans.” To that I would add that the Indian Ocean is the least known scientifically, a cruel irony given that it’s the one on which humans have sailed since sailing became a thing. Our lack of knowledge of the Indian Ocean strikes home for a more urgent reason: the Indian Ocean strongly influences weather and climate patterns around the world. It connects with the Pacific Ocean through a well-known ocean choke point, the Indonesian Throughflow, that acts as a kind of control valve for exchanges of waters between the Pacific, Indian, and even the Atlantic Oceans. And scientists now recognize that the Indian Ocean spawns the Madden–Julian Oscillation, an intraseasonal (i.e., within a season), eastward-traveling, globe-trotting atmospheric “wave” that influences weather. As described by Gottschalck (2014):

The MJO can modulate the timing and strength of the monsoons, influence tropical cyclone numbers and strength in nearly all ocean basins, and result in jet stream changes that can lead to cold air outbreaks, extreme heat events, and flooding rains over the United States and North America.

At a time when we need as much knowledge about the climate system as we can muster, our incomplete knowledge of the Indian Ocean remains a hindrance. We cannot protect what we don’t understand.

Be that as it may, there is one phenomenon of the Indian Ocean that we understand fairly well: the seasonally reversing wind pattern known as the mausin (alternatively, mausam) in Arabic, better known as the monsoon in English (e.g., Tripati 2017). Characterized by strong winds from the southwest in summer and strong northeast winds in winter, the monsoon brings seasonal changes in rainfall. Given that agricultural productivity in this region feeds a third of the world’s population, the monsoon rains are literally a matter of life and death for billions of people (McPhaden et al. 2009). The monsoon winds also bring seasonal changes in ocean currents. In fact, like the winds, the currents in the Indian Ocean change direction in accordance with the seasonally changing winds. 

17.7.1  Left Wind, Right Wind, Left Wind

Since the time of the Harappan Civilization, which spanned 3300 to 1700 BCE in the Indus Valley in modern-day Pakistan (e.g., Shinde 2016), people have known about monsoon weather. This should come as no surprise: the monsoon brings seasonal changes in rainfall, and the tens of thousands of people living in cities in this region at that time depended on the rains to produce their food. It appears that by at least 1 CE and perhaps as early as 2500 BCE, Indian Ocean sailors used the monsoon winds to carry out trading voyages (e.g., Tripati 2017). Whether headed east across the Bay of Bengal and Andaman Sea to Indonesia or west across the Arabian Sea to Africa or the Middle East, these early sailors timed their voyages to coincide with the prevailing winds. 

During the summer season, heating of the Indian subcontinent results in a kind of giant sea breeze. Hot air rises above the subcontinent, and cooler air from the ocean rushes in to take its place. Because of the Coriolis force, the predominant direction of these winds is from the southwest. This tropical ocean air holds vast amounts of water vapor. So as it rises above the subcontinent and cools, that water vapor is released as torrential rain. The combination of southwest winds and heavy rains characterizes the summer monsoon, also known as the southwest monsoon.

In the winter the situation reverses. The warmer ocean water (as compared to land) generates a circulation similar to a land breeze (albeit a continental-sized one). Winds blow now from the northeast, from the land to the ocean. The winter monsoon, or northeast monsoon, brings dry weather to the Indian subcontinent and beyond.

These seasonally reversing winds meant that sailors could venture from India to the west toward Africa and the Middle East during the winter (northeast) monsoon and return under favorable winds during the summer (southwest) monsoon. Though scholars still debate the timing of the origins of monsoonal sailing, a considerable body of archaeological and cultural evidence appears to support the earlier dates. It makes for a fascinating study and a rich opportunity to learn about the cultural and economic development of India. 

17.7.2 Monsoonal Currents

Changes in wind direction cause changes in ocean currents. During the winter monsoon (November–March), the wind patterns in the equatorial Indian Ocean resemble those in the equatorial Atlantic and Pacific. Winds north of the equator blow from the northeast and winds south of the equator blow from the southeast—a typical trade wind pattern. In this case the currents in the tropical Indian Ocean move toward the west. 

Now, the geography of the Indian Ocean and the nature of the currents cause some crossing of the equator by these equatorial currents, but by and large they behave similarly to the equatorial currents in the other ocean basins. A westward-flowing North Equatorial Current is present, though it also goes by the name Northeast Monsoon Current. (Do you ever wonder if these currents are in some kind of witness protection program with all their different names?) A South Equatorial Current and Equatorial Countercurrent also exist.

Things get weird in the summer. The onset of the summer monsoon (June–October) winds blowing from the southwest cause a complete reversal of the ocean currents. The North Equatorial Current and the Equatorial Countercurrent disappear and become the Southwest Monsoon Current—with current speeds in excess of 2 knots (strong for a current). This strong eastward-flowing current sends branches into the Arabian Sea and Bay of Bengal as well. The South Equatorial Current crawls up the northern coast of Africa and southeastern Arabia, where it’s called the Somali Current.

Though our tidy description here makes it all seem quite simple, the circulation of the Indian Ocean is—like some people—complicated. For a deeper look into the details of this circulation, consult Phillips et al. (2021).

17.8 Final Thoughts

Since humans first ventured into the sea on primitive boats, the surface currents have carried us in search of new opportunities. Even now, the surface currents serve as convenient ocean highways for a myriad of human industries. Unfortunately, the surface currents also carry with them the debris of humanity. As we learned in Chapter 6, “The ocean is downhill from everywhere” (Moore 2023). Indeed, anything tossed onto the ground—even thousands of miles from the ocean—will under the power of winds or moving water make its way into the ocean. Pollutants and toxins, too, released in one part of the ocean make their way to other parts, near and far. It’s an extraordinary journey with a lesson for humans: What happens in one part of the ocean can affect the entire ocean. In other words, what happens here doesn’t always stay here. Our wastes and byproducts of modern industrial civilization—no matter where they originate—reach all parts of the ocean and every shore. 

The world ocean circulation lies at the heart of the concept of a world ocean discussed in Chapter 1: it’s all one ocean. Waters that flow across the surface of the ocean lose buoyancy in polar regions and sink to the abyss. Winds and other mixing processes pull these waters back to the surface. Surface waters that sink in one part of the ocean—say, off the coast of Greenland in the North Atlantic—may travel southward toward the Antarctic, bend eastward below the southern tip of Africa, and be pulled to the surface of the South Pacific a thousand years later. For better or for worse, we are all connected to the same world ocean system. 

Our understanding of the surface circulation provides a context for appreciating the interconnectedness of the ocean. To the extent that the ocean connects our activities and our lives, we should always remember it’s all one ocean, our world ocean.

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