Ocean Currents: Forces, Concepts, Terminology, Maps | Sea Life, Islands and Oceania

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OCEAN CURRENTS

Gulf Stream map from 1943
The term "current" describes the motion of something. Ocean.currents describe the movement of water from one location to another in the sea. Currents are vital for: 1) moving nutrients and food sources in the sea; 2) cooling, heating and providing water to terrestrial areas, and 3) transferring heat from the Equator to the Poles.

According to the University of Hawaiʻi at Mānoa, ocean currents behave much like rivers within larger bodies of water. These currents can range in size from small ones near beaches to ocean-spanning flows like the enormous gyres that snake between continents. The North Atlantic Gyre, For instance, consists of water that flows west along the equator, north past the U.S. East Coast as the Gulf Stream, east along the Arctic, and then south past Europe and Africa as the Canary Current.[Source: Michael Dhar, Live Science, November 26, 2022]

Currents are generally measured in meters per second or in knots (1 knot = 1.85 kilometers per hour or 1.15 miles per hour). Currents affect the Earth's climate by driving warm water from the Equator and cold water from the poles around the Earth. The warm Gulf Stream, for instance, brings milder winter weather to Bergen, Norway, than to New York, much further south.

Ocean currents are driven mainly by prevailing winds and differences in water density, which changes with temperature and salinity of the seawater. The overall pattern of ocean circulation appears to driven by thermohaline circulation (“thermo” for heat and “haline” for salinity”) — a conveyor belt driven by the sinking of dense cold water in North Atlantic. This draws warm surface water (the Gulf Stream) from the south and sets in motion currents that affect all the world’s oceans.

Currents can also be influenced by salt and and influxed of fresh water. Salt water is heavier than fresh water, As fresh water enters the systems it rides over ocean water and pulls some of the salt water into it, creating a sort of vacuum that draws in more deep ocean water. Saline water that flows from warm areas to cold areas loses heat and becomes more saline as water evaporates. The coldness and high salinity makes the water more dense. It sinks into the oceans. As it does surface water moves in to displace it.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org

Important Terms for Ocean Currents and Circulation

Ocean currents off Panama

According to the “Introduction to Physical Oceanography”: 1) General Circulation is the permanent, time-averaged circulation. 2) Abyssal also called the Deep Circulation is the circulation of mass, in the meridional plane, in the deep ocean, driven by mixing. 3) Wind-Driven Circulation is the circulation in the upper kilometer of the ocean forced by the wind. The circulation can be caused by local winds or by winds in other regions. 4) Gyres are wind-driven cyclonic or anticyclonic currents with dimensions nearly that of ocean basins. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

5) Boundary Currents are currents flowing parallel to coasts. Two types of boundary currents are important: A) Western boundary currents on the western edge of the ocean tend to be fast, narrow jets such as the Gulf Stream and Kuroshio Current off Japan; B) Eastern boundary currents are weak, e.g. the California Current. 6) Squirts or Jets are long narrow currents, with dimensions of a few hundred kilometers, that are nearly perpendicular to west coasts. 7) Mesoscale Eddies are turbulent or spinning flows on scales of a few hundred kilometers.

In addition to flow due to currents, there are many types of oscillatory flows due to waves. Normally, when we think of waves in the ocean, we visualize waves breaking on the beach or the surface waves influencing ships at sea. But many other types of waves occur in the ocean. A) Planetary Waves depend on the rotation of the earth for a restoring force, and they including Rossby, Kelvin, Equatorial, and Yanai waves. B) Surface Waves sometimes called gravity waves, are the waves that eventually break on the beach. The restoring force is due to the large density contrast between air and water at the sea surface. C) Internal Waves are sub-sea wave similar in some respects to surface waves. The restoring force is due to change in density with depth. D) Tsunamis are surface waves with periods near 15 minutes generated by earthquakes. E) Tidal Currents are horizontal currents and currents associated with internal waves driven by the tidal potential. F) Edge Waves are surface waves with periods of a few minutes confined to shallow regions near shore. The amplitude of the waves drops off exponentially with distance from shore.

Forces Behind Ocean Currents

Oceanic currents are driven by three main factors: 1) winds, 2) thermohaline circulation and 3) tides. Differences in temperature and saltiness between the equator and Earth's poles power deep-water currents known as thermohaline (for "heat" plus "salt") circulation. Tides create smaller currents. [Source: Michael Dhar, Live Science, November 26, 2022]

1) Winds, powered by solar energy, direct surface currents, like those in gyres. Winds drive currents that are at or near the ocean's surface. Near coastal areas winds tend to drive currents on a localized scale and can result in phenomena like coastal upwelling. On a more global scale, in the open ocean, winds drive currents that circulate water for thousands of miles throughout the ocean basins. Earth's spin pushes gyres clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (the so-called Coriolis effect). [Source: NOAA]

2) Thermohaline circulation. This is a process driven by density differences in water due to temperature (thermo) and salinity (haline) variations in different parts of the ocean. Currents driven by thermohaline circulation occur at both deep and shallow ocean levels and move much slower than tidal or surface currents. It can take a thousand years to complete a global thermohaline cycle, James Potemra, a professor at the University of Hawaiʻi at Mānoa Institute of Geophysics and Planetology, told Live Science.

3) The rise and fall of the tides. Tides create a current in the oceans, which are strongest near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents change in a very regular pattern and can be predicted for future dates. In some locations, strong tidal currents can travel at speeds of eight knots or more.

Prevailing winds both push water and create circulation by displacing water at the surface that allows upwelling — the upward flow of cold water from deep in the ocean to the surface of the sea. The cold oxygen-poor water is often rich in nutrients such as nitrates, phosphate and silicate that are consumed by creatures on the bottom of the food chain beginning with phytoplankton that in turn feed marine life further up the food chain.

Scientists think ocean currents began as soon as Earth had oceans — roughly 3.8 to 4.4 billion years ago. The same forces that drive currents today (winds, tides, temperature and salinity differences, and Earth’s rotation) were already in place, so early oceans would have circulated much like modern ones in principle. However, the currents themselves looked completely different. Continents were arranged in shifting supercontinents, creating very different ocean basins and pathways. With landmasses in unfamiliar positions, surface and deep currents would have flowed along routes nothing like today’s North Atlantic Gyre or Gulf Stream. [Source: Michael Dhar, Live Science, November 26, 2022]

Important Concepts in the Study of Ocean Currents

According to the “Introduction to Physical Oceanography”: To begin our study of currents near the sea surface, let’s consider first the response of the ocean to an impulse that sets the water in motion. For example, the impulse can be a strong wind blowing for a few hours. The water then moves only under the influence of Coriolis force (force resulting from the earth's rotation that deflects movement to the right in the northern hemisphere and to the left in the southern hemisphere). Such motions are said to be inertial. The mass of water continues to move due to its inertia. If the water were in space, it would move in a straight line according to Newton’s second law. But on a rotating earth, the motion is much different. The following are some important things to consider when discussing currents. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

Ocean Currents

1) Changes in wind stress produce transient oscillations in the ocean called inertial currents : A) Inertial currents are very common in the ocean. B) The period of the current is (2π)/f. 2) Because of the Coriolis Affect, movement can be 90 degrees to the right of the wind in the northern hemisphere and 90 degrees to the left of the wind in the southern hemisphere.

3) Steady winds produce a thin boundary layer, the Ekman layer, at the top of the ocean. Ekman boundary layers also exist at the bottom of the ocean and the atmosphere. The Ekman layer in the atmosphere above the sea surface is called the planetary boundary layer. 4) The Ekman layer at the sea surface has the following characteristics: A ) Direction: 45 degrees to the right of the wind looking downwind in the Northern Hemisphere. B) Surface Speed: 1–2.5 percent of wind speed depending on latitude. C) Depth: approximately 40–300 meters depending on latitude and wind velocity.

4) Careful measurements of currents near the sea surface show that: A) Inertial oscillations are the largest component of the current in the mixed layer. B) The flow is nearly independent of depth within the mixed layer for periods near the inertial period. Thus the mixed layer moves like a slab at the inertial period. C) An Ekman layer exists in the atmosphere just above the sea (and land) surface. Surface drifters tend to drift parallel to lines of constant atmospheric pressure at the sea surface. This is consistent with Ekman’s theory. E) The flow averaged over many inertial periods is almost exactly that calculated from Ekman’s theory.

5) Spatial variability of Ekman transport, due to spatial variability of winds over distances of hundreds of kilometers and days, leads to convergence and divergence of the transport such as producing upwelling far away from where the winds that generate them. 6) Ekman pumping, which is driven by spatial variability of winds, drives a vertical current, which drives the interior geostrophic circulation of the ocean.

Tides and Currents

Tides go up and down; currents move left and right. Tides create a current in the oceans, near the shore, and in bays and estuaries along the coast but currents in the open ocean are generally influenced more other factors. [Source: NOAA]

Tides are driven by the gravitational force of the moon and sun. Tides are characterized by water moving up and down over a long period of time. Oceanic currents are driven by several factors. One is the rise and fall of the tides. Tides create a current in the oceans, near the shore, and in bays and estuaries along the coast. These are called "tidal currents." Tidal currents are the only type of currents that change in a very regular pattern and can be predicted for future dates. Currents are generally influenced more by winds and thermohaline circulation.

Ocean current map from 1943, made in World War II

Currents in coastal areas can be caused the ebb and flow of tides. In bays and straits where the water narrows the tides and currents can be quite strong. In some places such as the Bay of Fundy water levels can range 50 feet between low and high tide. In places where there are narrow straight currents can reach 15 knots when the tide ebbs and then lie still for 15 minutes or so and then start flowing with equal velocity in the other direction.

Wind Driven Circulation

You would think that currents on the surface of the ocean follow the prevailing winds. While this often true it isn’t always the case. Robert Stewart wrote in the “Introduction to Physical Oceanography”: Sometimes “strong currents, such as the North Equatorial Countercurrents in the Atlantic and Pacific Ocean go upwind. Spanish navigators in the 16th century noticed strong northward currents along the Florida coast that seemed to be unrelated to the wind. How can this happen? And, why are strong currents found offshore of east coasts but not offshore of west coasts? Answers to the questions can be found in a series of three remarkable papers published from 1947 to 1951. In the first, Harald Sverdrup (1947) showed that the circulation in the upper kilometer or so of the ocean is directly related to the curl of the wind stress if the Coriolis force varies with latitude. Henry Stommel (1948) showed that the circulation in oceanic gyres is asymmetric also because the Coriolis force varies with latitude. Finally, Walter Munk (1950) added eddy viscosity and calculated the circulation of the upper layers of the Pacific. Together the three oceanographers laid the foundations for a modern theory of ocean circulation. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

Sverdrup’s Theory of the Oceanic Circulation: While Sverdrup was analyzing observations of equatorial currents, he came upon the equation below relating the curl of the wind stress to mass transport within the upper ocean. To derive the relationship, Sverdrup assumed that the flow is stationary, that lateral friction and molecular viscosity are small, that non-linear terms such as u ∂u/∂x are small, and that turbulence near the sea surface can be described using a vertical eddy viscosity. He also assumed that the wind-driven circulation vanishes at some depth of no motion.

click once and the click the Wikicommons image

Global Ocean Conveyor Belt and the Oceanic Transport of Heat

The oceans move roughly half of the heat that must escape the tropics to keep Earth’s climate in balance. In the Atlantic, the Gulf Stream and the North Atlantic Drift carry warm water far north, keeping the high-latitude Atlantic ice-free and giving Europe its unusually mild climate. Norway at 60°N is far warmer than southern Greenland or northern Labrador at the same latitude, and palm trees grow on Ireland’s west coast — something impossible in Newfoundland, which sits farther south. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

In 1987, Wally Broecker of Columbia University’s Lamont-Doherty Earth Observatory coined the term Global Conveyor Belt to describe this vast system of oceanic heat transport. Warm surface currents move north, releasing heat and moisture into the atmosphere. As the water cools, becomes saltier, and grows denser, it sinks in the Norwegian and Greenland Seas to form deep North Atlantic waters. These cold bottom currents flow south, while some surface waters also return south via cooler currents like the Labrador and Portugal Currents.

The conveyor belt is powered by thermohaline circulation — the global movement of water driven by differences in temperature and salinity, combined with wind-driven surface flow. Dense, cold, salty water sinks; warmer, fresher water stays near the surface. As NOAA notes, the system begins in the Norwegian Sea, where heat loss to the atmosphere causes surface waters to become heavy enough to plunge to the ocean floor. That deep water then travels south past the equator to Antarctica.

Over time, mixing and wind-driven upwelling bring this deep water back to the surface in other ocean basins. It eventually returns to the Atlantic and joins the northward-flowing currents feeding the Gulf Stream. To replace the water that sinks in the far North Atlantic, warm water from the South Atlantic must flow north — pulling so much heat with it that the Atlantic transports heat northward even in the Southern Hemisphere. The tropical Atlantic absorbs enormous amounts of solar energy, much of which is ultimately exported to warm Europe and the northern half of the planet.

Monitoring Currents

To measure currents, you need three basic tools: 1) an observer, 2) a floating object or a drifter, and 3) a timing device. An observer stands on a ship, throws the drifter into the water, and then measures the time that it takes that object to move along the side of a ship. As technology improved over time, oceanographers began using mechanical current meters. A ship would deploy a meter and usually some sort of rotor would turn and measure the currents. This is still the basic process today; however there are more accurate and sophisticated instruments. [Source: NOAA]

Currents are observed at depths throughout the water column and are measured over a period of time. Today in the open ocean, a drifter is similar to a buoy in the water that may be equipped with global positioning system technology or satellite communications that would relay data and information. Drifters can also submerge for long periods of time to measure ocean currents at a particular depth. The drifter would then resurface occasionally to send a signal with its data and position to observers on the land.

In addition to buoys, there are other tools that are used to monitor currents. Acoustic Doppler Current Profiler (ADCP) is an instrument that measures the currents by emitting beams of sound, which reflect off of particles in the water and back to the ADCP. Commonly used to measure currents, it is normally deployed on the seafloor or attached to the bottom of a boat. It sends an acoustic signal into the water column and that sound bounces off particles in the water. The instrument can calculate the speed and direction of the current by knowing the frequency of the return signal, the distance it traveled, and the time it took for the signal to travel.

Many oceanographers also use radio antennas and high frequency Radio Detecting and Ranging systems (radar) to measure surface ocean currents. Similar to the Acoustic Doppler Current Profiler, these shore-based instruments use the Doppler effect to determine when currents are moving toward or away from the shore or to measure the velocity of a current. At NOAA, oceanographers use knots to measure current speed. The term knot is defined as one nautical mile per hour. One nautical mile is equal to 1.85 kilometers. One knot is also 51.44 centimeters per second.

After the ocean current measurements are collected, oceanographers download the data and then analyze it through a computer program. A statistical process called harmonic analysis determines the part of the current caused by the tides. This “tidal current” can then be predicted at that location for many years into the future. Other factors that influence the current, such as wind, cannot be forecast for more than a few days and are not included in the prediction.

NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) is primarily responsible for predicting and measuring water levels and currents and disseminating this information. CO-OPS collects, analyzes, and distributes such data to maintain safe maritime navigation and waterborne commerce. Tide and current data is available from CO-OPS Products and Services website. Among the things that are offered are: 1) Sea Levels, a global map depicting regional trends in sea level, with arrows representing the direction and magnitude of change; 2) Real-Time Current Data, collected by NOAA current meters around the U.S.; 3) Historic Current Data.; 4) Tidal Current Predictions, predictions for more than 2700 tidal current stations nationwide; 5) Storm QuickLook, real-time oceanographic and meteorological observations at locations affected by a tropical cyclone.

Using Swept Away Lego Toys To Study Ocean Currents

In June 2024, marine biologist Hayley Hardstaff discovered a black Lego dragon on Portwrinkle Beach in Cornwall, part of a decades-long aftermath of the 1997 Great Lego Spill. Nearly 5 million Lego pieces, including 33,427 dragons, fell off the cargo ship Tokio Express after a rogue wave, scattering toys across the oceans. [Source Aimee Ortiz, The New York Times, September 8, 2024]

The spill has become a unique case study in ocean currents and plastic pollution. Many pieces continue to wash ashore or are found by fishermen decades later, tracked by the Lego Lost at Sea project, founded by Tracey Williams. The project maps where pieces turn up across Europe and encourages enthusiasts to report their finds. Rare pieces, like green dragons or black octopuses, have become prized discoveries.

Scientists, including oceanographer Curtis Ebbesmeyer, note that floating plastic moves through ocean currents like a “subway system,” explaining why so many toys remain missing. Researchers also see the spill as a public-facing example of ocean plastic pollution.

Why the Study of Currents Is Important

Current data is critical for supporting safe and efficient shipping and marine transportation. With predicted, real-time, and forecasted currents, people can safely dock and undock ships, maneuver them in confined waterways, and safely navigate through coastal waters. This helps to avoid ship collisions or delay the arrival of goods. In addition, current measurements are important for search and rescue operations, environmental disasters, and coastal engineering projects. [Source: NOAA]

When supporting search and rescue operations, understanding the speed and direction of the currents in an area helps to narrow down the rescue and recovery effort. Current prediction information can help scientists clean up after a hazardous oil spill by helping them understand the direction and movement of the oil. Engineers also use currents information to help build marine structures such as bridges or docks and piers. Current observations are also used to develop and evaluate coastal nowcast or forecast model products that are provided online.

An Operational Forecast System provides a nowcast and forecast (up to 48 hours) of water levels, currents, salinity, water temperatures, and winds for a given area. These systems are located in coastal waters and the Great Lakes in critical ports, harbors, and estuaries. NOAA periodically conducts current surveys in areas around the nation to ensure the accuracy of tidal current predictions. Commercial and recreational mariners depend on this information for safe navigation.

Ocean Currents Are Moving Faster

Ocean currents are now moving faster than they were around the year 2000, and the trend is global. A study published in Science Advances in February 2020 reported that currents are accelerating not just at the surface but down to 2,000 meters (6,560 feet). Co-author Janet Sprintall of Scripps Oceanography said the scale of this speedup was “surprising,” exceeding what would normally be expected from stronger winds alone — a sign that climate change is likely driving the increase. [Source: Stephanie Pappas, Live Science, February 7, 2020]

Since 2000 ocean winds have strengthened by about 1.9 percent per decade, transferring more energy into the water. As a result, roughly 76 percent of the upper 2,000 meters of the global ocean has gained kinetic energy, and current speeds overall have climbed about 5 percent per decade since the early 1990s.

The team, led by Shijian Hu of the Institute of Oceanology in Qingdao, China, combined historical current measurements with new data from the Argo network of autonomous floats. Their analysis helps clarify earlier conflicting results: while some subtropical currents have strengthened, others — like the Kuroshio — show little long-term change.

Because currents move slowly, their acceleration can be hard to notice. For instance, the South Equatorial Current flows at about 1 mile per hour, so even a decade of acceleration only adds around 0.05 mph. But pushing such massive volumes of water requires enormous energy, far more than natural variability can explain. Many uncertainties remain — especially deeper in the ocean, where measurements are sparse. But understanding how circulation is changing is crucial, the researchers note, because currents redistribute heat, shape marine ecosystems, and influence weather and regional climate.

Antarctic Currents Supplying Much of World's Deep Ocean with Nutrients and Oxygen Slowing Dramatically

Deep ocean currents surrounding Antarctica, crucial for sustaining marine life, slowed by 30 percent between the 1990s and 2020s and may be at risk of stopping entirely, a study published May 25, 2023 in Nature Climate Change, reveals. These currents, known as Antarctic bottom waters, are driven by dense, cold water that forms along the Antarctic continental shelf and sinks to depths exceeding 10,000 feet (3,000 meters). From there, the water flows northward into the Pacific and eastern Indian oceans, fueling the global meridional overturning circulation and supplying roughly 40 percent of the world’s deep oceans with oxygen and nutrients. [Source: Sascha Pare, Live Science May 26, 2023]

Warming temperatures are melting Antarctica’s ice shelves, releasing less-dense freshwater that disrupts this circulation. “If the oceans had lungs, this would be one of them,” said Matthew England, a professor of ocean and climate dynamics at the University of New South Wales and co-author of the study. Earlier research predicted a 40 percent decline in Antarctic bottom water strength by 2050, with a potential long-term collapse.

England and colleagues confirmed these predictions using observations from the Australian Antarctic Basin. Examining the flow of bottom water between 1994 and 2017, they recorded a 30 percent decrease in velocity, signaling early stagnation in these abyssal currents. Slower circulation could trap nutrients and oxygen in the deep ocean, reducing the supply of essential elements to surface ecosystems. “All of the marine life at the surface eventually sinks to the bottom, creating nutrient-rich waters,” England explained. “Slowing this circulation cuts off a key pathway for nutrients to return to the surface and support marine life.”

Each year, about 276 trillion tons of cold, salty, oxygen-rich water sink around Antarctica. Warming climates reduce the density of this water, leaving more in upper layers and disrupting circulation across the Pacific and eastern Indian oceans. The study warns that as freshwater influx continues and accelerates, these vital currents could collapse, profoundly affecting the ocean’s transport of heat, carbon, nutrients, and oxygen for centuries. Independent experts, including Ariaan Purich from Monash University, emphasized the significance of the findings. Observational evidence now complements earlier modeling studies, confirming that Antarctic ice melt is likely to have major, long-term impacts on global ocean circulation and the ocean’s role in regulating climate.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated November 2025