Surface Ocean Currents

Ocean currents represent a dynamic system that, along with atmospheric circulation, helps to distribute heat evenly across the planet. Responding to the seasons, ocean currents play important roles in climate, marine life, and ocean transportation.

Overview

The “heat budget” of Earth results in a temperature range that makes life on the planet possible. Ocean currents play a vital role in the heat budget. These currents are major determinants of climates and strongly influence the distribution of marine life. Ocean currents must be studied in relation to other aspects of the environment with which they interact. The currents are, for example, closely associated with atmospheric circulation because the planetary winds are the prime movers of the currents. The friction of the wind blowing over the ocean surface began the slow, shallow movement of surface waters, eventually becoming a global circulation of immense volumes of seawater. There are also deeper ocean currents that are much slower moving and are difficult to monitor, whose significance is less well understood. In part, the deep currents derive from the physical fact that water is continuous in structure, such that water moved from one location must be replaced by water from a different location. The deep currents, however, are primarily caused by thermohaline circulation, driven by slight differences in seawater densities resulting from differences in temperature and salinities. Although the shallow, wind-driven currents affecting the surface waters may be hundreds of meters deep, they are effectively independent of such deep ocean currents.

The most significant features of ocean currents are their geographic locations and their directions of flow. It is helpful to recognize overall patterns. There are large-circulation gyres in each of the major ocean basins, discernible as an apparent overall circular movement of surface waters. These gyres, or gyrals, move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The North Central Atlantic Gyre located east of the United States, is one of the best-known and most-studied. The Florida Current (part of the Gulf Stream system) is on the west side of the gyre and is a warm current flowing generally northward. The Canaries Current, on the east side of the gyre, is a cold current that flows generally southward. The North Atlantic Drift and the North Equatorial Current form the eastward and westward components of the gyre, respectively. The result of circulation in the gyre is that warm water from the equatorial region is transported poleward to heat-deficient areas. Simultaneously, the Canaries Current transports colder water back toward the equator. The ocean currents thus help distribute surface and atmospheric heat more evenly worldwide. In the Atlantic Ocean south of the equator, a large gyre moves counterclockwise. The warm Brazil Current on the west side of the gyre flows southward, transporting heat away from the equator. The Benguela Current on the east side of the gyre moves colder water toward the equator.

In the Pacific Ocean, similar patterns of clockwise and counterclockwise gyres are apparent. North of the equator, the Japan Current (also known as the Kuroshio) transports warm water toward the pole, and the California Current moves colder water toward the equator. In the Pacific Ocean south of the equator, the cold, nutrient-rich Humboldt Current flows northward off the west coast of South America and is renowned historically as one of the most fertile commercial marine fishery areas in the world. The Indian Ocean possesses similar gyres, although the attenuated portion north of the equator presents some special features.

Wind and

The forces that drive the oceanic current circulations are the planetary winds. The planetary winds are, in turn, driven by solar energy. The ocean currents, therefore, are Sun-driven as sunshine energizes the Gulf Stream and the other currents. Some general principles about Earth’s heat budget can be stated. The Sun heats Earth, its atmosphere, oceans, and land, but each portion heats differently. The atmosphere, the most fluid and responsive of the three, has developed large bands of alternating pressure and wind belts, such as the Northeast Trade Winds and the Prevailing Westerlies. The Northeast Trade Winds lie between 5- and 25-degrees north latitude. The winds flow predominantly from the northeast and form one of the most constant of the wind belts. The friction of the wind moving over the ocean surface causes the surface waters to move with the wind. Still, because of the Coriolis effect, caused by Earth’s rotation, the movement of the water current in the Northern Hemisphere tends to be about 45 degrees to the right of the winds that cause the current. The resultant North Equatorial Current is fragmented into different oceans because of the intervening continents. Largely due to the Coriolis effect, the current deflects to its right and, in the Atlantic Ocean, eventually becomes the Gulf Stream. In the Pacific Ocean, the comparable current is the Japan Current. Thus, one can thus see the origins of the clockwise gyrals in the Northern Hemisphere. Another wind belt in the Northern Hemisphere, the prevailing westerlies, is located between 35- and 55-degrees north latitude. These winds are not as constant as the northeast trade winds and flow prevalently from the west. The correlation of the westerlies' latitudes and the gyres' west-to-east-moving currents is apparent. The currents slow down and become more widespread, shallower, and less distinguishable but are urged toward the east by the Westerlies. The North Atlantic Drift and the North Pacific Current result from this relationship.

Northern Hemisphere

Analyzing the North Central Atlantic Gyre, the blocking position of the Iberian Peninsula causes the North Atlantic Drift to split, part moving southward toward the equator as the cold Canaries Current and part moving poleward into the Arctic Ocean as the warm Norwegian Current. The Canaries Current merges into the North Equatorial Current to complete the gyre. The temperature characterizations of currents and drifts as “warm” or “cold” are relative. There are no absolute temperature divisions. Some warm currents are actually lower in temperature than some cold currents. For example, the Norwegian Current is considered a warm current only because it is warmer than the Arctic water it enters. Only a few degrees above freezing in winter, the Norwegian Current nevertheless transfers significant amounts of heat into these high latitudes and moderates the winter temperatures in Western and Northern Europe. A compensating movement of cold water out of the Arctic is accomplished by the southward-flowing Labrador Current between Greenland and North America.

The Gulf Stream is the world’s greatest ocean current. There is, however, some confusion about what constitutes the Gulf Stream. The Gulf Stream generally includes the entire warm-water transport system from Florida to the point at which the warm water is lost by diffusion into the Arctic Ocean. It would thus include both the North Atlantic Drift and the Norwegian Current. Technically, the Gulf Stream is a smaller segment of that transport system—the portion off the northeast coast of the United States. The Gulf Stream system thus includes the Florida Current, the Gulf Stream, the North Atlantic Drift, and the Norwegian Current.

Southern Hemisphere

Ocean current patterns in the Southern Hemisphere are almost a mirror image of those in the Northern Hemisphere, adjusted for differences in the configuration of the continents. The southeast trade winds drive the South Equatorial Current, while the Coriolis effect causes it to deflect to the left. Resultant gyres are counterclockwise, but again, the poleward-moving currents transfer the heat away from the equator, and the equatorward-moving currents return colder water. In general, cold currents are richer in nutrients, have higher oxygen content, and support greater life than warm currents. Cold waters yield most products of the world’s commercial fisheries. In contrast, cold currents offshore are associated with desert climates onshore. The atmospheric circulations that drive the ocean currents also create conditions not conducive to precipitation in the latitudes of these cold currents. Examples are the Sahara Desert adjacent to the Canaries Current, the Atacama Desert adjacent to the Humboldt Current, the Sonoran Desert adjacent to the California Current, and the Kalahari Desert adjacent to the Benguela Current.

Other currents are sporadic in occurrence, such as the warm El Niño current that periodically develops off the west coast of northwestern South America for reasons that are not well understood. Tides, storms, and local weather conditions also cause numerous small local currents.

Study of Ocean Currents

The study of ocean currents has acquired new significance as scientists have discerned the role of the currents in climate and marine life. Information comes from many sources. Benjamin Franklin made one of the earliest attempts to identify and chart ocean currents when he was postmaster general of colonial America. His map of the Gulf Stream was published in 1770 and has proved to be remarkably accurate when one considers his sources of information. Franklin noted that vessels sailing westward from England to America in the midlatitudes of the Atlantic Ocean were taking longer than ships moving eastward and longer than ships moving westward but in lower latitudes. He correctly concluded that the vessels were moving against a slow, eastward-moving current.

Since then, vast amounts of data have been acquired to detect, measure, and chart the currents. One of the early methods still employed is the use of drift bottles. Sealed bottles are introduced into the sea at various locations and dates and can float with the currents. Finders are requested to note the date and location of the bottle-find and to return the data to the address in the bottle. Ocean current data have also been obtained serendipitously by losing floating goods in midocean shipping accidents. The landfall points of such materials, as well as materials washed out to sea by some natural disaster such as a tsunami, provide important information about the surface movement of ocean waters. Various types of more technologically advanced current meters are also used. Some are moored to the sea bottom and can transmit results by radio. It is difficult for a ship at sea to measure currents because the ship itself is drifting with the current. Currents are generally very slow and difficult to measure. A few currents may be measured at 6 to 8 kilometers per hour, but those less than 1 kilometer per hour are much more common. The average surface velocity of the North Atlantic Drift is about 1.3 kilometers per hour. The currents also vary in width and depth. The Florida Current off Miami is about 32 kilometers wide, 300 meters deep, and moving at about 5 to 8 kilometers per hour. It transports more than 4 billion tons of water per minute. The volume of flow is more than one hundred times that of the Mississippi River. As the flow proceeds north and then east as the North Atlantic Drift, it spreads, thins, slows, and splits into individual meandering flows that are difficult to follow. Spin-off eddies, or “core rings,” occur that can persist for months.

One helpful method of tracking ocean currents is identifying water with slight temperature variations and salinity differences. When the flow movement is so slow as to be practically undetectable with current meters, the slight temperature and salinity differences can be used as “tracers” to identify current movements. This method is also used to identify even slower-moving deep-ocean currents. In the twenty-first century, advanced satellite imagery and high-altitude aerial photography have become extremely important in monitoring ocean currents. Using sensors that detect radiation at selected electromagnetic spectrum bands, satellites collect data on broad patterns of seawater temperatures and thus help scientists understand the movements and extent of the currents. This type of sea monitoring is also helpful in detecting any future changes that might occur in the oceans.

Significance

Ocean currents play a vital role in the environment. Along with atmospheric circulation, ocean currents serve to distribute the heat absorbed from the Sun to different parts of the world. Immense volumes of relatively warm seawater slowly move poleward, transporting heat from the heat-surplus equatorial regions to the heat-deficient regions nearer the poles. Cold-water currents, in turn, move colder water back toward the equator. Although neither solar energy nor rainfall is evenly distributed over the planet, the mixing actions of the ocean currents function to keep the global environment in a steady state. These moderating effects of the ocean currents affect the climates of coastal areas in the middle and high latitudes, especially in Europe. The densely populated nations of northwestern Europe experience much milder winters than expected for such high latitudes. Northwestern North America similarly benefits.

Life in the sea is also aided by this current-driven ocean water mixing. In addition to heat, ocean currents distribute oxygen and nutrients, forming certain areas in the oceans where very favorable life-supporting conditions occur. These fertile areas of mixing are concentrated sources of commercial marine fishery products. Where mixing is limited, nutrient-poor regions arise in the ocean, such as the Sargasso Sea, located in the center of the North Central Atlantic Gyre. Global climate change may alter ocean currents, creating a further need to study these currents and their effects on climate and marine life. Global climate change is expected to continue to affect ocean currents causing warmer waters and slowing currents, which could drastically affect already warming temperatures worldwide. Climate change can also change atmospheric temperatures, air flow patterns, and global winds, all of which can alter ocean currents.

Principal Terms

core ring or core eddy: a mass of water that is spun off of an ocean current by that current’s meandering motion

Coriolis effect: the apparent deflection of any moving body or object from its linear course, caused by Earth’s rotation

current: a sustained movement of seawater in the horizontal plane, usually wind-driven

drift: a movement similar to a current but more widespread, less distinct, slower, shallower, and less easily delineated

gyre or gyral: the very large, semiclosed surface circulation patterns of ocean currents in each of the major ocean basins

heat budget: the balance between the incoming solar radiation and the outgoing terrestrial reradiation

planetary winds: the large, relatively constant prevailing wind systems that result from Earth’s absorption of solar energy and that are affected by Earth’s rotation

thermohaline circulation: any circulation of ocean waters that is caused by variations in the density of seawater resulting from differences in the temperature or salinity of the water

Bibliography

Broecker, Wally. The Great Ocean Conveyor. Princeton, N.J.: Princeton University Press, 2010.

Clarke, Allan J. An Introduction to the Dynamics of El Niño and the Southern Oscillation. Burlington, Mass.: Academic Press, 2008.

Colling, Angela. Ocean Circulation. 2d ed., Oxford: Butterworth-Heinemann, 2001.

“Effects of Climate Change - Currents: NOAA's National Ocean Service Education.” NOAA's National Ocean Service, oceanservice.noaa.gov/education/tutorial‗currents/05conveyor3.html. Accessed 26 July 2024.

“How Does Climate Change Affect the Ocean?” Natural History Museum, www.nhm.ac.uk/discover/quick-questions/how-does-climate-change-affect-the-ocean.html. Accessed 26 July 2024.

Gross, M. Grant. Oceanography: A View of the Earth. 7th ed., Englewood Cliffs, N.J.: Prentice Hall, 1996.

Ingmanson, Dale E., and William J. Wallace. Oceanography: An Introduction. 5th ed., Belmont, Calif.: Wadsworth, 1995.

“Ocean Currents and Climate.” National Geographic Society, 2 Jan. 2024, education.nationalgeographic.org/resource/ocean-currents-and-climate. Accessed 26 July 2024.

Schwartz, M. Encyclopedia of Coastal Science. Dordrecht: Springer, 2005.

Stowe, Keith. Exploring Ocean Science. 2d ed., New York: John Wiley & Sons, 1996.

Sverdrup, Keith A., Alyn C. Duxbury, and Alison B. Duxbury. An Introduction to the World’s Oceans. 8th ed., Boston: McGraw-Hill, 2004.

Talley, Lynne D., et al. Descriptive Physical Oceanography: An Introduction. 6th ed., San Diego: Academic Press, 2011.

Thurman, Harold V., and Alan P. Trujillo. Introductory Oceanography. 10th ed., Upper Saddle River, N.J.: Prentice Hall, 2003.

Trujillo, Alan P., and Harold V. Thurman. Essentials of Oceanography. 10th ed., Upper Saddle River, N.J.: Prentice Hall, 2010.

Ulanski, Stan. The Gulf Stream: Tiny Plankton, Giant Bluefin, and the Amazing Story of the Powerful River in the Atlantic. Chapel Hill: University of North Carolina Press, 2008.

Voituriez, Bruno. The Gulf Stream. Paris: UNESCO, 2006.