10.6: 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.

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.

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).

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).

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.

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).

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.)

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?