Currents Tutorial

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OAA's National Ocean Service: Education Currents

N OS h o m e N OS e d u c a t io n h o m e s i t e ind ex

This site NOAA

Tu to r i a l

Roadmap

Lesson Plans

W e l c o me

Tidal Currents

Coastal Currents

Surface Ocean Currents

The Global Conveyor Belt

How Are Currents Measured

How Currents Affect Our Lives

References

Wel com e t o Cu r r en t s

When used in association with water, theterm "current" describes the motion of thewater. Some currents you may be familiarwith are the motion of rainwater as it flowsdown the street, or the motion of thewater in a creek, stream, or river flowingfrom higher elevation to lower elevation.This motion is caused by gravity. Thespeed and direction (velocity) of currentscan be measured and recorded.

Oceanic currents are driven by several

factors. One is the rise and fall of thetides, which is driven by the gravitationalattraction of the sun and moon on Earth'soceans. Tides create a current in theoceans, near the shore, and in bays andestuaries along the coast. These are called"tidal currents." Tidal currents are the only type of currents that change in a veryregular pattern and can be predicted for future dates.

A second factor that drives oceanic currents is wind. Winds drive currents that are ator near the ocean's surface. These currents are generally measured in meters persecond or in knots (1 knot = 1.15 miles per hour or 1.85 kilometers per hour).Winds drive currents near coastal areas on a localized scale, and also in the openocean on a global scale.

A third factor that drives currents is thermohaline circulation-a process driven bydensity differences in water due to temperature (thermo) and salinity (haline) indifferent parts of the ocean. Currents driven by thermohaline circulation occur atboth deep and shallow ocean levels and move much slower than tidal or surfacecurrents.

The Currents Tutorial is an overview of the types of currents, what causes them,how they are measured, and how they affect people's lives. The tutorial is composedof six primary "chapters," and includes many illustrations and animations.

The Roadmap to Resources directs you to online data and education offerings fromNOAA and other reliable resources.

The lesson plans integrate information from the tutorial with offerings from theroadmap. These lesson plans have been developed for students in grades 9-12, butare easily adaptable for middle school and undergraduate students.

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Oceanic currents are driven by tides, winds,and differences in water density. Currents areessential for maintaining the existing balanceof life on Earth, but they can be deadly as well.

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N OS h o m e N O S e d u c a t i o n h o m e s i t e ind ex

Cur ren ts

Tidal Cur ren ts 1 2

As the tides rise and fall, they create flood andebb currents. Click the image for a larger view.

The relationship between the masses of theEarth, moon, and sun and their distances toeach other play critical roles in affecting tidesand the currents they produce. Click theimage for a larger view.

Tidal currents occur in conjunctionwith the rise and fall of the tide. Thevertical motion of the tides near theshore causes the water to movehorizontally, creating currents. Whena tidal current moves toward the landand away from the sea, it floods.When it moves toward the sea awayfrom the land, it ebbs. These tidalcurrents that ebb and flood inopposite directions are called

rectilinear or reversing currents.

Rectilinear tidal currents, whichtypically are found in coastal riversand estuaries, experience a slackwater period of no velocity as theymove from the ebbing to floodingstage, and vice versa. After a briefslack period, which can range fromseconds to several minutes andgenerally coincides with high or lowtide, the current switches directionand increases in velocity.

Tidal currents are the only type ofcurrent affected by the interactions ofthe Earth, sun, and moon. Themoons force is much greater thanthat of the sun because it is 389times closer to the Earth than the sunis. Tidal currents, just like tides, areaffected by the different phases of themoon. When the moon is at full ornew phases, tidal current velocitiesare strong and are called springcurrents. When the moon is at first or third quarter phases, tidal current

velocities are weak and are called neap currents.

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Return to Currents: Tidal Currents

The relationship between the masses of the Earth, moon and sun and their distances to eachother play a critical role in affecting the Earth's tides. Although the sun is 27 million times moremassive than the moon, it is 390 times further away from the Earth than the moon. Tidalgenerating forces vary inversely as the cube of the distance from the tide-generating object. Thismeans that the suns tidal generating force is reduced by 3903 (about 59 million times) comparedto the tide-generating force of the moon. Therefore, the suns tide-generating force is about halfthat of the moon, and the moon is the dominant force affecting the Earths tides and the currentsthey produce.

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Cur ren t s

Tida l Cur r en ts 1 2

The elliptical orbits of the moon around theEarth and the Earth around the sun havesubstantial effects on the Earths tides and thecurrents they produce. Click the image for alarger view.

Also similar to tides, tidal currents areaffected by the relative positions of themoon and Earth. When the moon andEarth are positioned nearest to each other(perigee), the currents are stronger thanaverage and are called perigeancurrents. When the moon and Earth areat their farthest distance from each other(apogee), the currents are weaker and arecalled apogean currents.

The shape of bays and estuaries also canmagnify the intensity of tides and thecurrents they produce. Funnel-shapedbays in particular can dramatically altertidal current magnitude. The Bay of Fundyin Nova Scotia is a classic example of thiseffect, and has the highest tides in theworld - over 15 meters (Thurman, H.V.,1994).

Move your computer mouse over the image above to see the differences betweenhigh and low tides in the Bay of Fundy. Photos Scott Walking Adventures.

The daily tidal currents experienced by coastal areas can also have a dramatic effecton estuarine ecosystems. View a slide show of the remarkable daily rise of waters atthe Elkhorn Slough National Estuarine Research Reserve in California:http://oceanservice.noaa.gov/education/kits/estuaries/

media/supp_estuar01a_tide.html

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Tu to r i a l

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Welcome

Tida l Cur r en ts

Coastal Currents





References

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Back

Return to Currents: Tidal Currents

The elliptical orbits of the moon around the Earth and the Earth around the sun have substantialeffects on the Earths tides and the currents they produce. When the moon and Earth arepositioned nearest to each other (perigee), the currents are stronger than average and are calledperigean currents. When the moon and Earth are at their farthest distance from each other(apogee), the currents are weaker and are called apogean currents.

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OAA's National Ocean Service: Education Currents: Waves


Cur ren t s

Coasta l Cur ren ts 1 2 3 4

Anatomy of a wave. Click the image for largerview.

Wa ve s

Coastal currents are intricately tied towinds, waves, and land formations. Windsthat blow along the shorelinelongshorewindsaffect waves and, therefore,currents.

Before one can understand any type ofsurface current, one must understand howwind and waves operate. Wave height isaffected by wind speed, wind duration (orhow long the wind blows), and fetch,which is the distance over water that thewind blows in a single direction. If wind speed is slow, only small waves result,regardless of wind duration or fetch. If the wind speed is great but it only blows fora few minutes, no large waves will result even if the wind speed is strong and fetchis unlimited. Also, if strong winds blow for a long period of time but over a shortfetch, no large waves form. Large waves occur only when all three factors combine(Duxbury, et al, 2002.)

As wind-driven waves approach the shore, friction between the sea floor and thewater causes the water to form increasingly steep angles. Waves that become toosteep and unstable are termed breakers or breaking waves.

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Return to Coastal Currents: Waves

The highest surface part of a wave is called the crest, and the lowest part is the trough. Thevertical distance between the crest and the trough is the wave height. The horizontal distancebetween two adjacent crests or troughs is known as the wavelength.

Wave height is affected by wind speed, wind duration, or how long the wind blows, and fetch,which is the distance over water that the wind blows in a single direction. If wind speed is slow,only small waves result. If the wind speed is great but it only blows for a few minutes, no largewaves will occur. Also, if strong winds blow for a long period of time but over a short fetch, nolarge waves form. Large waves occur only when all three factors combine (Duxbury, et al, 2002.)

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OAA's National Ocean Service: Education Currents: Longshore


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Cur ren t s

Coasta l Cur r en ts 1 2 3 4

Longshore Cur rents

The speed at which waves approachthe shore depends on sea floor andshoreline features and the depth ofthe water. As a wave moves towardthe beach, different segments of thewave encounter the beach beforeothers, which slows these segmentsdown. As a result, the wave tends tobend and conform to the generalshape of the coastline. Also, waves

do not typically reach the beachperfectly parallel to the shoreline. Longshore currents are generated when a

"train" of waves reach the coastline andRather, they arrive at a slight angle,release bursts of energy.

called the angle of wave approach.

When a wave reaches a beach orcoastline, it releases a burst of energy that generates a current, which runs parallelto the shoreline. This type of current is called a longshore current.

Discover : How does an island disappear?

Longshore currents are affected by thevelocity and angle of a wave. When a wave

breaks at a more acute (steep) angle on abeach, encounters a steeper beach slope,or is very high, longshore currentsincrease in velocity. Conversely, a widerbreaking angle, gentler beach slope, andlower wave height slows a longshorecurrents velocity. In either case, the waterin a longshore current flows up onto thebeach, and back into the ocean, as itmoves in a sheet formation.

As this sheet of water moves on and offthe beach, it can capture and transport

beach sediment back out to sea. Thisprocess, known as longshore drift, cancause significant beach erosion.

Longshore drift can be very destructive tomanmade structures. Click the image to viewa slideshow and learn more.

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B ar r i e r I s lands

Longshore drift has a very powerful influence onthe shape and composition of the coastline. Itchanges the slopes of beaches and creates long,narrow shoals of land called spits, that extend outfrom shore. Longshore drift may also create ordestroy entire barrier islands along a shoreline.A barrier island is a long offshore deposit of sandsituated parallel to the coast. As longshore driftsdeposit, remove, and redeposit sand, barrierislands constantly change.

Tuckers Island, New Jersey, is a barrier island that

clearly illustrates how longshore drift and strongweather affect these transient sand deposits. Theisland was first settled in 1735. Since itssettlement, residents have had to move theislands lighthouse several times because thechannels shifted constantly as a result of longshoredrift. Eventually, they placed the lighthouse onhigh ground at the island's northern end.

Meanwhile, the inlet north of Tuckers IslandBeach Haven inletwas also effected by longshoredrift. At times, the inlet was narrow or nonexistentand Tuckers Island was attached to the nearbyLong Beach Island. At other times, the inlet waswide, and Tuckers Island was separated fromLong Beach Island.

In 1927, the lighthouse on Tucker's Island, NJ was

destroyed when powerful longshore currentswashed over 300 yards of the surrounding landout to sea.

Powerful longshore currents causing longshore drift washed Tucker's Island away. Click the images to view a seriesof maps from 1856-2005 that show how Tucker's Island changed shape and eventually disappeared.

In 1924, in an effort to stop the beach erosion that was occurring on Tuckers Island, experts

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installed jetties. They were initially successful in halting the erosion, but the jetties worked sowell that the currents of Beach Haven inlet began to wash in the other directiontowardTuckers Island. As the inlet began to widen, the island then began to erode very quickly. By1927, just three years after the jetties were installed, waves and longshore drift washed awaymost of the beach. Later that year, the remaining 300 yards of beach were washed away in aseries of storms. Then, in a final dramatic display, the lighthouse fell into the sea!

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OAA's National Ocean Service: Education Currents: Rip Currents


Cur ren t s


These images of dangerous rip currents weretaken at public swimming beaches. Click theimage to view a slideshow and learn m ore.

Rip Cur r ents

As longshore currents move on and off thebeach, rip currents may form around lowspots or breaks in sandbars, and also nearstructures such as jetties and piers. A ripcurrent, sometimes incorrectly called a riptide, is a localized current that flows awayfrom the shoreline toward the ocean,perpendicular or at an acute angle to theshoreline. It usually breaks up not far fromshore and is generally not more than 25

meters (80 feet) wide.

Rip currents typically reach speeds of 1 to2 feet per second. However, some ripcurrents have been measured at 8 feet persecondfaster than any Olympic swimmer ever recorded (NOAA, 2005b). If waveactivity is slight, several low rip currents can form, in various sizes and velocities.But in heavier wave action, fewer, more concentrated rip currents can form.

When waves travel from deep to shallow water, they break near the shoreline and generate currents. A ripcurrent forms when a narrow, fast-moving section of water travels in an offshore direction. Rip current speeds ashigh as 8 feet per second have been measured--faster than an Olympic swimmer can sprint! This makes ripcurrents especially dangerous to beachgoers as these currents can sweep even the strongest swimmer out to sea.

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13/49

OAA's National Ocean Service: Education Currents: Rip Currents

Because rip currents move perpendicular to shore and can be very strong, beachswimmers need to be careful. A person caught in a rip can be swept away fromshore very quickly. The best way to escape a rip current is by swimming parallel tothe shore instead of towards it, since most rip currents are less than 80 feet wide. Aswimmer can also let the current carry him or her out to sea until the forceweakens, because rip currents stay close to shore and usually dissipate just beyondthe line of breaking waves. Occasionally, however, a rip current can push someonehundreds of yards offshore. The most important thing to remember if you are evercaught in a rip current is not to panic. Continue to breathe, try to keep your headabove water, and dont exhaust yourself fighting against the force of the current.

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14/49


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Return to Coastal Currents: Rip Currents

These images of rip currents were taken at public swimming beaches. Rip currents are powerful, channeled currentsthat flow away from shore and can occur at any beach with breaking waves. Rip currents can be killers and account forover 80% of rescues performed by surf beach lifeguards.

The greatest precaution that you can take is to recognize the danger of rip currents and only swim at beaches withlifeguards on duty. If you get caught in a rip current, never fight against it. Remain calm to conserve your energy.Think of the rip current like a treadmill that can't be turned off, and which you need to step to the side of the treadmillto get off. To escape a rip current, swim in a direction parallel to the shoreline. When out of the current, swim at anangle--away from the current--towards shore. If you can't swim out of the rip current, float or calmly tread water.When out of the current, swim towards shore. For more information go to the NOAA Rip Current Safety Web site:

http://www.ripcurrents.noaa.gov.

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15/49

OAA's National Ocean Service: Education Currents: Upwelling


Cur ren t s


Upwe l l i ng

Winds blowing across the ocean surface often push water away from an area. Whenthis occurs, water rises up from beneath the surface to replace the diverging surfacewater. This process is known as upwelling.

Upwelling occurs when winds blowing across the ocean surface push water away from an area and subsurfacewater rises up to replace the diverging surface water.

Upwelling occurs in the open ocean and along coastlines. The reverse process, calleddownwelling, also occurs when wind causes surface water to build up along acoastline. The surface water eventually sinks toward the bottom.

Subsurface water that rises to the surface as a result of upwelling is typically colder,

rich in nutrients, and biologically productive. Therefore, good fishing groundstypically are found where upwelling is common. For example, the rich fishinggrounds along the west coasts of Africa and South America are supported by year-round coastal upwelling.

Seasonal upwelling and downwelling alsooccur along the West Coast of the UnitedStates. In winter, winds blow from thesouth to the north, resulting indownwelling. During the summer, windsblow from the north to the south, andwater moves offshore, resulting inupwelling along the coast. This summerupwelling produces cold coastal waters in

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OAA's National Ocean Service: Education Currents: Upwelling

the San Francisco area, contributing to thefrequent summer fogs. (Duxbury, et al,2002.)

Major upwelling areas along the world's coastsare highlighted in red. Click the image for alarger view.

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17/49


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Return to Coastal Currents: Rip Currents: Upwelling

This image highlights major upwelling areas along the world's coasts in red. Upwelling occurs when winds blowingacross the ocean surface push water away from an area and subsurface water rises up from beneath the surface toreplace the diverging surface water. These subsurface waters are typically colder, rich in nutrients, and biologicallyproductive. Therefore, good fishing grounds typically are found where upwelling is common. For example, the richfishing grounds along the west coasts of Africa and South America are supported by year-round coastal upwelling.

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18/49

OAA's National Ocean Service: Education Currents: Coriolis Effect

If the Earth did not rotate on its axis, theatmosphere would only circulate between thepoles and the equator in a simple back-andforth pattern. Click the image for a larger view.


Cur ren t s

Surface Ocean Cur ren ts 1 2 3 4

Coastal currents are affected by localwinds. Surface ocean currents, whichoccur on the open ocean, are driven by acomplex global wind system. Tounderstand the effects of winds on oceancurrents, one first needs to understand theCoriolis force and the Ekman spiral.

Cor io l is Ef fect

If the Earth did not rotate and remainedstationary, the atmosphere would circulatebetween the poles (high pressure areas)and the equator (a low pressure area) in asimple back-and-forth pattern. Butbecause the Earth rotates, circulating air isdeflected. Instead of circulating in astraight pattern, the air deflects towardthe right in the Northern Hemisphere andtoward the left in the SouthernHemisphere, resulting in curved paths.This deflection is called the Coriolis effect.It is named after the Frenchmathematician Gaspard Gustave deCoriolis (1792-1843), who studied thetransfer of energy in rotating systems likewaterwheels. (Ross, 1995).

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Because the Earth rotates on its axis,circulating air is deflected toward the right inthe Northern Hemisphere and toward the leftin the Southern Hemisphere. This deflection iscalled the Coriolis effect. Click the image for alarger view.
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19/49


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Return to Currents: Surface Ocean Currents: Coriolis Effect

If the Earth did not rotate on its axis and remained stationary, the atmosphere would onlycirculate between the Earth's polar regions (areas of high pressure) and the equator (a lowpressure area) in a simple back-and-forth pattern. Image B shows a "cutaway" view of thishypothetical circulation pattern.

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20/49


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Return to Currents: Surface Ocean Currents: Coriolis Effect

Cor iol is Ef fect

The rotation of the Earth on its axis deflects the atmosphere toward the right in the NorthernHemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. Thedeflection of the atmosphere sets up the complex global wind patterns which drive surface oceancurrents. This deflection is called the Coriolis effect. It is named after the French mathematicianGaspard Gustave de Coriolis (1792-1843), who studied the transfer of energy in rotating systemslike waterwheels. (Ross, 1995).

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21/49

OAA's National Ocean Service: Education Currents: Trade Winds


Cur ren t s


Atmospheric circulation and the Coriolis effectcreate global wind patterns including the tradewinds and westerlies. Click the image for alarger view.

Tr a d e W in d s

In the Northern Hemisphere, warm airaround the equator rises and flows northtoward the pole. As the air moves awayfrom the equator, the Coriolis effectdeflects it toward the right. It cools anddescends near 30 degrees North latitude.The descending air blows from thenortheast to the southwest, back towardthe equator (Ross, 1995). A similar windpattern occurs in the Southern

Hemisphere; these winds blow from thesoutheast toward the northwest anddescend near 30 degrees South latitude.

These prevailing winds, known as thetrade winds, meet at the IntertropicalConvergence Zone (also called thedoldrums) between 5 degrees North and 5 degrees South latitude, where the windsare calm. The remaining air (air that does not descend at 30 degrees North or Southlatitude) continues toward the poles and is known as the westerly winds, orwesterlies. The trade winds are so named because ships have historically takenadvantage of them to aid their journies between Europe and the Americas(Bowditch, 1995).

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22/49

OAA's National Ocean Service: Education Currents: Trade Winds

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Trade Wind s

Atmospheric circulation and the Coriolis effect create global wind patterns including the tradewinds and westerlies.

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23/49

OAA's National Ocean Service: Education Currents: Winds Drive Surface Ocean Currents


Cur ren t s


Win ds Dr ive Sur face Ocean Cur ren t s

Global winds drag on the waters surface, causing it to move and build up in thedirection that the wind is blowing. And just as the Coriolis effect deflects winds tothe right in the Northern Hemisphere and to the left in the Southern Hemisphere, italso results in the deflection of major surface ocean currents to the right in theNorthern Hemisphere (in a clockwise spiral) and to the left in the SouthernHemisphere (in a counter-clockwise spiral). These major spirals of ocean-circlingcurrents are called gyres and occur north and south of the equator. They do notoccur at the equator, where the Coriolis effect is not present (Ross, 1995).

There are five major ocean-wide gyresthe North Atlantic, South Atlantic, NorthPacific, South Pacific, and Indian Ocean gyres. Each is flanked by a strong andnarrow western boundary current, and a weak and broad eastern boundarycurrent (Ross, 1995).

One particularly powerful westernboundary current is the Gulf Stream. The

Gulf Stream, paired with the easternboundary Canary Current, flanks the NorthAtlantic gyre. The Gulf Stream, also calledthe North Atlantic Drift, originates in theGulf of Mexico, exits through the Strait ofFlorida, and follows the eastern coastlineof the United States and Newfoundland. Ittravels at speeds of 25 to 75 miles per dayat about one to three knots (1.15-3.45miles per hour or 1.85-5.55 kilometers perhour). It influences the climate of the eastcoast of Florida, keeping temperatureswarmer in the winter and cooler than theother southeastern states in the summer.

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OAA's National Ocean Service: Education Currents: Winds Drive Surface Ocean Currents

Since it also extends toward Europe, it The Gulf Stream is a powerful westernboundary current in the North Atlantic Oceanwarms western European countries as well.that strongly influences the climate of the EastCoast of the United States and many WesternEuropean countries. Click the image for alarger view.

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Return to Currents: Surface Ocean Currents: Winds Drive Surface Ocean Currents

The Gul f St ream Cur ren t

The Gulf Stream is a powerful western boundary current in the North Atlantic. Originating in theGulf of Mexico, waters in the Gulf Stream travel at speeds of about one to three knots (1.15-3.45miles per hour or 1.85-5.55 kilometers per hour). The Gulf Stream influences the climate of theeast coast of Florida, keeping temperatures warmer in the winter and cooler than the othersoutheastern states in the summer. Since it also extends toward Europe, it warms westernEuropean countries as well.

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OAA's National Ocean Service: Education Currents: Ekman spiral


Cur ren t s

Sur f ace Ocean Cur ren t s 1 2 3 4

The Ekman spiral occurs as a consequence ofthe Coriolis effect.Click the image for a largerview.

Ekm an sp i ra l

The Ekman spiral, named after Swedishscientist Vagn Walfrid Ekman (1874-1954)who first theorized it in 1902, is aconsequence of the Coriolis effect. Whensurface water molecules move by the forceof the wind, they, in turn, drag deeperlayers of water molecules below them.Each layer of water molecules is moved byfriction from the shallower layer, and eachdeeper layer moves more slowly than the

layer above it, until the movement ceasesat a depth of about 100 meters (330 feet).Like the surface water, however, thedeeper water is deflected by the Corioliseffectto the right in the NorthernHemisphere and to the left in the SouthernHemisphere. As a result, each successivelydeeper layer of water moves more slowly to the right or left, creating a spiral effect.Because the deeper layers of water move more slowly than the shallower layers,they tend to twist around and flow opposite to the surface current.

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OAA's National Ocean Service: Education Currents: Ekman spiral

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Ekm an sp i ra l

The Ekman spiral occurs as a consequence of the Coriolis effect. When surface water moleculesare moved by the wind, they drag deeper layers of water molecules below them. Like surfacewater, the deeper water is deflected by the Coriolis effectto the right in the NorthernHemisphere and to the left in the Southern Hemisphere. As a result, each successively deeperlayer of water moves more slowly to the right or left, creating a spiral effect.

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OAA's National Ocean Service: Education Currents: The Global Conveyor Belt


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Cur ren t s

The Global Conveyo r Bel t 1 2 3

This animation shows the path of the global conveyer belt. The blue arrows indicate the path of deep, cold, densewater currents. The red arrows indicate the path of warmer, less dense surface waters. It is estimated that it cantake 1,000 years for a "parcel" of water to complete the journey along the global conveyor belt.

Thermohaline circulation drives a global-scale system of currents called the global conveyor belt. The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current.This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge ofAntarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.

Cold, salty, dense water sinks at the Earth'snorthern polar region and heads south alongthe western Atlantic basin.

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These two sections that split off warm upand become less dense as they travelnorthward toward the equator, so thatthey rise to the surface (upwelling). Theythen loop back southward and westward tothe South Atlantic, eventually returning tothe North Atlantic, where the cycle beginsagain.

The conveyor belt moves at much slowerspeeds (a few centimeters per second)

than wind-driven or tidal currents (tens tohundreds of centimeters per second). It isestimated that any given cubic meter ofwater takes about 1,000 years to completethe journey along the global conveyor belt.In addition, the conveyor moves animmense volume of watermore than 100times the flow of the Amazon River (Ross,1995).

The conveyor belt is also a vitalcomponent of the global ocean nutrientand carbon dioxide cycles. Warm surfacewaters are depleted of nutrients andcarbon dioxide, but they are enrichedagain as they travel through the conveyorbelt as deep or bottom layers. The base ofthe worlds food chain depends on thecool, nutrient-rich waters that support thegrowth of algae and seaweed.

The current is "recharged" as it travels alongthe coast of Antarctica and picks up morecold, salty, dense water.

The main current splits into two sections, onetraveling northward into the Indian Ocean,while the other heads up into the westernPacific.

The two branches of the current warm andrise as they travel northward, then loop backaround southward and westward.

The now-warmed surface waters continuecirculating around the globe. They eventuallyreturn to the North Atlantic where the cyclebegins again.

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Cur ren t s

The Global Conveyo r Bel t 1 2 3

Global climate change could disrupt the globalconveyer belt, causing potentially drastictemperature changes in Europe and evenworldwide.

The global conveyor belt is a strong, buteasily disrupted process. Researchsuggests that the conveyor belt may beaffected by climate change. If globalwarming results in increased rainfall in theNorth Atlantic, and the melting of glaciersand sea ice, the influx of warm freshwateronto the sea surface could block theformation of sea ice, disrupting the sinkingof cold, salty water. This sequence ofevents could slow or even stop theconveyor belt, which could result in

potentially drastic temperature changes inEurope.

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OAA's National Ocean Service: Education Currents: How Are Currents Measured?

Cur ren t s

How Are Cur r en ts Measured? 1 2 3 4 5 6

Since the age of exploration, marinershave needed to know the speed anddirection (velocity) of ocean currents tosteer their ships within harbors and alongtrade and exploration routes. A marinerneeds to be able to measure the velocityof currents by observing distance, time,and direction.

The simplest method of determining thevelocity of a current involves an observer,a floating object or drifter, and a timing

device. The observer stands on ananchored ship with a timer. He or she thenplaces the drifter (such as a piece of wood)into the water and measures the amountof time the drifter takes to move along thelength of the ship. He or she then stopsthe timer after the object has traveledsome distance, and measures thatdistance, noting the direction in which theobject moved.

The observer then divides the distance theobject traveled by the time it took theobject to travel that distance, which equalsthe speed of the current. By combining thespeed of the object with the direction inwhich it moved, the observer can thendetermine the currents velocity. Oceancurrents typically are measured in knots.

Although they still follow the sameessential concept to measure oceancurrents, mariners today use moreaccurate and sophisticated instruments.Today, drifters are often elaborate buoysequipped with multiple oceanographicinstruments. Some are equipped with

global positioning system technology andsatellite communications to relay theirposition in the ocean back to observers onland. Other drifters submerge for longperiods of time to measure the oceancurrents at depth. The drifter occasionallyrises to the surface to send a signal thatrelays its position.

All drifter measurements are termedLagrangian measurements, named aftermathematician Joseph Louis Lagrange(1736-1813), who first described the pathfollowed by fluids. But current velocities

An oceanographer deploys a current meter inthe 1920s while working in Alaska.

Joseph Louis Lagrange (1736-1813) was thefirst mathematician to describe the pathfollowed by fluids. To this day, all drifter buoymeasurements are referred to as "Lagrangianmeasurements."

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can be measured another way as wellusing Eulerian measurements. Namedafter Swiss mathematician Leonhard Euler(1707-1783), Eulerian measurementsinvolve describing fluid flow by measuringthe speed and direction of the fluid at onepoint only. In this method, an instrumentis anchored in the ocean at a givenlocation, and the water movement ismeasured as it flows past the instrument.

Measuring currents by Eulerian methods isbecoming increasingly more common. Onereason is that it is easier to retrieve theseexpensive but stationary instruments thanit is to locate floating drifters.

(top)Cont in ue t o 2 3 4 5 6

Leonhard Euler (1707-1783) was the firstmathematician to describe the speed anddirection of a liquid's flow as it passes a singlepoint in space.



http://oceanservice.noaa.gov/education/kits/currents/07measure1.html


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Cur ren t s

How Are Cur r en ts Measured? 1 2 3 4 5 6

W h a t i s a k n o t ?

The term knot, in reference to currents, is defined as one nautical mile per hourand is used to measure speed. A nautical mile is slightly more than a standard mile.

1 nautical mile = 1.15 miles = 1.85 kilometers1 knot = 1.15 miles per hour = 1.85 kilometers per hour

1 knot = 20.251969 feet per second = 51.44 centimeters per second

The term knot dates from the 17th Century, when sailors measured the speed oftheir ship by the use of a device called a common log. This device was a coil ofrope with uniformly spaced knots tied in it, attached to a piece of wood shaped likea slice of pie. The piece of wood was lowered from the back of the ship and allowedto float behind it. The line was allowed to pay out freely from the coil as the piece ofwood fell behind the ship for a specific amount of time. When the specified time hadpassed, the line was pulled in and the number of knots on the rope between the shipand the wood were counted. The speed of the ship was said to be the number ofknots counted (Bowditch, 1984).

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