Ocean Currents: _{Mapping and explaining the ocean’s gyres}

Introduction

Sitka is a small coastal town on Baranof Island in southern Alaska. You can’t drive to Sitka from the mainland. Instead, the town hosts a large harbor facing the Gulf of Alaska, so most people and products arrive in Sitka from the ocean. Many other things come in from the ocean around Sitka, too, and many residents are beachcombers, walking the shoreline looking for items that have washed ashore. On November 16, 1992, one beachcomber found six small plastic bath toys—rubber duckies—on a rocky beach town. Twelve days later, another beachcomber found 20 toys on a beach 85 km further north. By the summer of 1993, hundreds of the same kind of toys had been found and reported by beachcombers over 850 km of the Alaskan coastline.

Eben Punderson, a part-time journalist for the Sitka Daily Sentinel, thought the situation looked familiar. He recalled a national news story from just a few years before: two oceanographers based in Seattle had sought out beachcomber reports of Nike shoes that had washed ashore after a freighter carrying them encountered a storm and lost several containers overboard. The oceanographers were Curtis Ebbesmeyer, who worked for an environmental consulting firm, and James Ingraham, a scientist in the Marine Fisheries Division of the National Oceanic and Atmospheric Administration (NOAA). The two had been purposefully releasing drift bottles containing records of the time and place at which they were set adrift in the ocean (like a message in a bottle), to map and model ocean currents. They saw the opportunity that the release of as many as 80,000 floating objects—Nike running shoes—presented to them. 

Ebbesmeyer and Ingraham compared the distribution of shoes with the paths of their drift bottles. They determined that the shoes were distributed over a much larger region than the drift bottles. They then incorporated these data into a computer program, the Ocean Surface Current Simulations (OSCURS) numerical model, which combined large-scale ocean currents and wind speed and direction to model the trajectory of drift buoys. They used OSCURS to hindcast the toys’ path—meaning, they wanted to see if the model could accurately recreate the path and amount of time the toys took to go from the spill site to the Sitka region. Based on their modeling, Ebbesmeyer and Ingraham concluded that wind and near-shore currents must have influenced the shoes in addition to major ocean currents, and they published their results in 1992 (Ebbesmeyer and Ingraham 1992) with a map showing the recovery locations and the results of their modeling (Figure 1).

When Ebbesmeyer and Ingraham published their work, it also gained a lot of media attention. They were seeking reports from beachcombers, which is how Punderson had heard about the incident. The shoes had mostly been found from southern Oregon in the United States to Vancouver Island in Canada (see Figure 1), but the toys were appearing much further north. Punderson thought the oceanographers might be interested in this second accidental release of floating objects, so he reached out to them in the summer of 1993 with his beachcomber reports. Punderson had also tracked down the company that manufactured the bath toys, called “Friendly Floatees.” He learned that on January 10, 1992, the cargo ship carrying the toys from Hong Kong to the port of Tacoma, Washington, encountered a storm in the middle of the Pacific Ocean. Twelve of its shipping containers snapped loose from their moorings and washed overboard. At least one of those containers burst open, releasing boxes filled with nearly 30,000 floating toys. 

Ebbesmeyer and Ingraham were intrigued by Punderson’s information and started to work with him and other newspapers to solicit reports from beachcombers and lighthouse keepers. To better determine the time the toys were released and could float freely, they purchased an unopened package and let it sit in a bucket of seawater. Within a day, the glue holding the box together deteriorated. They used this result to make the assumption that the toys had begun drifting individually shortly after the container had fallen overboard. 

Hindcasting the distribution of the bath toys required modifying OSCURS to increase the effect of wind. Since toys were designed to float, they rode much higher in the water and caught more wind than the shoes or the drift bottles. By comparing the toys’ distribution with that of the shoes and the drift bottles, Ebbesmeyer and Ingraham were able to determine the influence of winds and ocean currents on each type of object. This enabled them to forecast the trajectories of different floating objects more precisely. In 1994, they published their modeling results and their predictions for future landfall of the toys, and they requested “help in reporting any subsequent finds of the toys or other interesting drifting objects” (Ebbesmeyer and Ingraham 1994). 

For more than a decade, Ebbesmeyer and Ingraham collected data on the times and locations where Friendly Floatees (and Nike shoes) washed ashore. Beachcombers found them in the Aleutian Islands of Alaska in 1995, Washington state in 1996, Hawaii in 1997, along the coasts of Maine and Scotland in 2003, and on the Devon coast in the UK in 2007. A few toys would show up on the beaches of Sitka every two or three years. By incorporating these data into OSCURS, Ebbesmeyer and Ingraham started to see larger patterns. Some toys were making a circuit and returning to the same place at a regular interval, where a few would get pushed close enough to shore that wave action (the motion of the ocean due to wind) could wash them onto the beach. Others appeared to escape this circuit and get caught by different currents. The toys' regular appearances near Sitka, reported by beachcombers over 15 years, allowed oceanographers to determine the time it took for surface water in the ocean (and the objects it carries) to travel around that circular current. Moreover, the oceanographers could show how wind could influence the “escape” of some toys into other large currents (Ebbesmeyer et al. 2007). The summary of their findings is shown in Figure 2. 

Using the data they accumulated, Ebbesmeyer and Ingraham were able to determine the velocity of the Pacific Subarctic Gyre, and determine that it takes just over three years for an object to complete this circuit. Until the accidental release of the toys, no one had been able to put all the data together to determine the velocity of these large currents. The team of oceanographers wouldn’t have been able to do it either, without help from all the beachcombers making observations and reporting their findings. 

Mapping surface currents

Ebbesmeyer and Ingraham used numerical models to refine our understanding of one particular current. But the idea that the ocean’s surface currents move in predictable ways has been known for millennia. Humans have been navigating the oceans for at least 30,000 years for the purposes of exploration, trade, and sustenance. And for as long as they have been navigating, people have recorded and shared their observations of the currents and winds that helped and hindered their travels. These records helped them and other voyagers travel with the wind and currents to take the safest and fastest routes. 

The Polynesians explored and settled islands of the South Pacific by traveling in 18-meter-long, double-hull canoes and living on the open ocean for days or weeks as they moved between islands. They used observations of the stars to help them navigate across the ocean and developed a keen sense of the ocean currents and how winds and islands influenced them. Polynesian navigators shared their knowledge orally and through the construction of stick charts, a form of navigational map (Figure 3). In a stick chart, shells and knots would be connected by a framework of coconut fibers or wood to illustrate the relative positioning of islands. Smaller, curved fibers indicated the dominant currents, waves, and wind patterns of the waters in between.

In the 15th and 16th centuries, many European nations, including Portugal, Spain, the Netherlands, England, and France, took to the sea to explore regions across the globe, driven by religion, scientific and cultural curiosity, and a desire for imperial dominance. These European mariners kept detailed logbooks (official records of voyage events), recording distances traveled, as well as speed and position fixes on navigation charts. In the mid-1800s, Matthew Fontaine Maury, who led the US Navy’s Department of Charts and Instruments, began compiling records from individual ships’ logbooks. Ultimately, Maury published a series of wind and current charts as part of a comprehensive book titled The Physical Geography of the Sea (Figure 4).

Unfortunately, one of Maury’s goals in creating these detailed charts was to expand slavery in the United States to South America, thus leaving behind a complicated legacy. His maps and logs, as well as his standardization of data collection techniques (see our Data Analysis and Interpretation module), were important in that they built a foundation for how we evaluate weather, climate, and physical ocean processes. Yet his focus was on making detailed maps of the currents and winds that facilitated the slave trade across the Atlantic Ocean. He called this “The Great Equatorial Current,” which carries water (and ships filled with slaves) from western Africa to South America and up the east coast of North America (Figure 4).

Maury’s map centers the Sargasso Sea and depicts it as a place filled with seaweed. Seafarers had documented this as a quiet zone. Christopher Columbus thought the seaweed was a sign that the ship was close to shore, but he could not have been more wrong. This quiet zone within the roughly circular currents in the North Atlantic is among the furthest places from any coastline. The lack of both currents and winds makes travel to any shore slow and arduous. 

Understanding the currents

Our collective navigational knowledge allowed us to map the major surface currents of the ocean by the mid-1800s. However, explaining the currents took several more decades of research in what became a new branch of science: oceanography, or the scientific study of the ocean, including its physical, chemical, and biological features. 

One key advance came from the Arctic, where another naturally floating object—ice—helped provide insight into the motion of the water. During an 1893 to 1896 expedition that was an attempt to reach the North Pole, Norwegian explorer Fridtjof Nansen made detailed observations of the ocean and icebergs. Before the expedition, zoology had been Nansen’s primary interest. But when he returned after three years in the Arctic, he had shifted his attention to oceanography and published a six-volume record of his expedition (Nansen 1902). In his report, Nansen described his observation that icebergs did not drift in the direction of the wind as he had expected. Instead, they drifted in a direction that was 20° to 40° to the right of the wind direction. 

Based on his observations, Nansen hypothesized that the consistent difference between the direction of the wind and the direction of the currents resulted from the Earth’s rotation. This was another visible impact of the Coriolis effect, which causes particles moving northward or southward on the Earth’s surface to apparently deviate from straight-line paths (see our Factors that Control Regional Climate module). Because water is denser than air, Nansen thought the Coriolis effect must have a stronger influence on the ocean than it did on the wind.  

Nansen’s observations and hypothesis caught the attention of Vilhelm Bjerknes, a mathematician at the University of Uppsala well-known for his work on using mathematical equations to describe atmospheric circulation. Bjerknes asked one of his students, Vagn Walfrid Ekman, to explore Nansen’s hypothesis mathematically. Ekman developed an equation to account for the forces acting upon the uppermost layer of seawater and identified two sources: 1) friction between the water surface and the wind blowing over it, which causes the uppermost layer of water to be dragged along with the overlying air (see our Factors that Control Regional Climate module), and 2) the speed of Earth’s rotation at the latitude of the seawater (Ekman 1905). He noted that he left out the location of the continents and different water densities that would also influence currents.

Ekman’s equations did account for the viscosity of water, which is much greater than the viscosity of air. When the wind blows over the ocean, it drags the uppermost layer along with it and in the same direction because of the water’s surface friction. Because water is more viscous than air, it moves more slowly and gets deflected eastward or westward by the Coriolis effect a greater amount in a shorter span of time than the air moving above. Additionally, as that uppermost layer of the ocean moves along, it drags the layer below it. The lower layer moves more slowly because only some of the upper layer’s energy is transferred, so it deflects even more due to the Coriolis effect. This layer then drags on the layer below that, and so forth, down the water column until all the energy absorbed by the uppermost ocean layer from the wind has completely dissipated, usually within the upper 200 m. When the movement of the layers in that 200 m is summed, the net direction of motion is roughly perpendicular to the wind direction, and the surface current direction is about 45° from the wind direction (Figure 5). This complicated phenomenon is called Ekman transport, named after the scientist who developed the predictive mathematical explanation for Nansen’s observation. 

Putting the currents together

By the mid-20th century, the patterns in surface currents and the reasons for them were apparent. Large-scale currents link together to form gyres, which are ocean-spanning, roughly circular currents thousands of miles in diameter (Figure 6). The major gyres north of the equator circulate in a clockwise direction, and the gyres south of the equator circulate counter-clockwise. Because of the distribution of the continents, smaller gyres north of 60° latitude also rotate counter-clockwise. In the northern Pacific Ocean, where the Friendly Floatees spilled, they were taken up in the North Pacific Drift (Figure 6) and continued northward into the Alaska Current—one of the northern gyres.

The calm zones within gyres that encompass thousands of square kilometers were once known for seaweed and slow travel. Now, these zones are better known for their garbage patches—collections of trash and abandoned or lost fishing gear mixed with natural ocean debris. These patches include microplastics (extremely small plastic debris formed from the breakdown of consumer products) and maybe even a few Friendly Floatees.

Ekman’s equations also explain gyres and their calm interiors. Due to Ekman transport, water below the surface tends to be pushed inward towards the center of the circular flow. This creates differences in the height of the ocean—the interior of the ocean is a few meters higher than the edges near continents. This inner ocean water making up the mound pushes outward, causing the circular currents to move along the boundaries of the oceans, producing the ocean-scale gyres that we observe in every major ocean. 

Ocean currents and climate

The map in Figure 6 includes more than just the direction of the moving water: It also shows the water’s temperature. While surface water spends time traveling across the ocean near the equator, such as in the South Equatorial Current in the Pacific Ocean, it is warmed. In the western Pacific, where the current turns south, it brings warm water south to Australia and New Zealand. The water then cools as it travels across the southern Atlantic and brings cold waters up the western coast of South America in the Humboldt current. A similar process happens in every major gyre.

Importantly for our climate, moving water carries more than stuff like ships, icebergs, and bath toys. The currents also carry energy in the form of heat around the globe. Warm currents bring warm water to the colder regions, influencing regional climate. For example, the Gulf Stream is a warm current moving northward from the Gulf of Mexico along the eastern coast of North America (Figure 6). The warm current increases regional air temperatures in coastal northern Atlantic regions by as much as 10 degrees C. The current extends all the way up the eastern coast of the United States and Canada. Due to the warm water at the surface, the current eventually brings milder-than-expected weather conditions (for the latitude) to northern Europe. 

In contrast, the Antarctic Circumpolar Current flows clockwise (with a perspective from the South Pole) around Antarctica. It is the strongest ocean current on our planet and extends from the sea’s surface to the bottom of the ocean. Many sailors dread making the voyage from the tip of South America to the tip of Antarctica, also known as the Drake Passage. The passage is known for its fierce winds and fierce waves. Many of the sailors who travel across it call it the passage “Drake Shake.” The Antarctic Circumpolar Current keeps Antarctica climatically isolated, as it is a cold current whose waters never travel further north to warm up. Additionally, it blocks the passage of warmer water from equatorial regions. 

The study of surface currents today

Today, satellites allow us to map and track ocean surface currents in real-time, at high resolution. We use this for weather forecasting, climate modeling, and contaminant tracing. In 2010, the Deepwater Horizon offshore oil rig in the Gulf of Mexico failed, releasing millions of barrels of oil into the deep sea. Many agencies and scientists monitored the sea surface to trace the dispersal of oil into the Gulf to delineate “no fishing” zones and organize efforts to capture the released oil. Monty Graham of the Dauphin Island Sea Lab documented the aftermath of the rig failure along the coast of Alabama. There was no trace of the oil at the ocean’s surface where Graham searched. However, he observed small pieces of white, foam-like flotsam (ship or cargo wreckage found floating in the ocean) mixed with seaweed. The flotsam extended for nearly six miles in an east-west direction. 

Like Punderson had done to track the origin of the Friendly Floatees, Graham and his colleagues sought out the origin of the white foam. With the help of clean-up crews, they found additional, larger pieces of the same material, and one of those pieces had a manufacturer’s name on it. Graham and his colleagues called the manufacturer and confirmed that the white foam was from the oil rig, consisting of material used to help maintain buoyancy for the pipe bringing up the oil from the seafloor. This was exciting news: it meant the foam had been released at the same time and from the same place as the oil but moved to and along the ocean surface faster than the oil. The foam moved faster than the oil for the same reason the bath toys did: The foam pieces floated higher than the oil and were more strongly influenced by the wind. Graham and his colleagues used a model similar to OSCURS to determine that the flotsam could be used as an indicator for the oil slick’s (a layer of oil on the ocean’s surface) future path, providing an advance warning that could prevent damaging spills from reaching ecologically sensitive areas (Carmichael et al. 2012).

Models other than OSCURS have been developed and used to visualize global ocean currents. Figure 7 shows the output from one of these models, showing both sea surface temperature (the colors) and water velocity (the length and orientation of the lines). In studying the models, many details emerge, including small circular currents called eddies, faster currents along the east coasts of continents, slower currents along the west coasts, and more. 

Models such as these are helping us learn more about how the ocean currents work and how they are changing as the climate changes. Studying such models can help us prepare for and mitigate the impact of those changes.