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CHAPTER 31

OCEAN CURRENTS

TYPES AND CAUSES OF CURRENTS

3100. Definitions

The movement of ocean water is one of the twoprincipal sources of discrepancy between dead reckonedand actual positions of vessels. Water in motion is called acurrent; the direction toward which it moves is calledset,and its speed is calleddrift . Modern shipping speeds havelessened the impact of currents on a typical voyage, andsince electronic navigation allows continuous adjustmentof course, there is less need to estimate current set and driftbefore setting the course to be steered. Nevertheless, aknowledge of ocean currents can be used in cruise planningto reduce transit times, and current models are an integralpart of ship routing systems.

Oceanographers have developed a number of methodsof classifying currents in order to facilitate descriptions oftheir physics and geography. Currents may be referred toaccording to their forcing mechanism as eitherwind drivenor thermohaline. Alternatively, they may be classifiedaccording to their depth (surface, intermediate, deep orbottom). The surface circulation of the world’s oceans ismostly wind driven. Thermohaline currents are driven bydifferences in heat and salt and are associated with thesinking of dense water at high latitudes; the currents drivenby thermohaline forces are typically subsurface. Note thatthis classification scheme is not unambiguous; thecircumpolar current, which is wind driven, extends fromthe surface to the bottom.

A periodic current is one for which the speed ordirection changes cyclically at somewhat regular intervals,such as a tidal current. Aseasonal currentis one whichchanges in speed or direction due to seasonal winds. Themean circulation of the ocean consists of semi-permanentcurrents which experience relatively little periodic orseasonal change.

A coastal current flows roughly parallel to a coast,outside the surf zone, while alongshore current is oneparallel to a shore, inside the surf zone, generated by wavesstriking the beach at an angle. Any current some distancefrom the shore may be called anoffshore current, and oneclose to the shore aninshore current.

General information on ocean currents is availablefrom NOAA’s National Ocean Data Center at:http://www.nodc.noaa.gov. Satellite graphics and otherdata can be found at: http://wwwo2c.nesdis.noaa.gov.

3101. Causes of Ocean Currents

The primary generating forces are wind and differencin water density caused by variations in heat and salinCurrents generated by these forces are modified by sufactors as depth of water, underwater topography includishape of the basin in which the current is running, extent alocation of land, and deflection by the rotation of the Ear

3102. Wind Driven Currents

The stress of wind blowing across the sea causesurface layer of water to move. Due to the low viscosity owater, this stress is not directly communicated to the oceinterior, but is balanced by the Coriolis force within arelatively thin surface layer, 10-200m thick. This layer icalled theEkman layer and the motion of this layer iscalled theEkman transport . Because of the deflection bythe Coriolis force, the Ekman transport is not in thdirection of the wind, but is 90° to the right in the NorthernHemisphere and 90° toward the left in the SouthernHemisphere. The amount of water flowing in this layedepends only upon the wind and the Coriolis force andindependent of the depth of the Ekman layer and tviscosity of the water.

The large scale convergence or divergence of Ekmtransport serves to drive the general ocean circulatioConsider the case of the Northern Hemisphere subtropTo the south lie easterly winds with associated northwaEkman transport. To the north lie westerly winds witsouthward Ekman transport. The convergence of theEkman transports is calledEkman pumping and results ina thickening of the upper ocean and a increase in the deof the thermocline. The resulting subsurface pressugradients, balanced by the Coriolis force, give rise to tanticyclonic subtropical gyres found at mid latitudes ieach ocean basin. In subpolar regions, Ekman suctproduces cyclonic gyres.

These wind driven gyres are not symmetrical. Alonthe western boundary of the oceans, currents are narrowstronger, and deeper, often following a meandering courThese currents are sometimes called astream. In contrast,currents in mid-ocean and at the eastern boundary, are obroad, shallow and slow-moving. Sometimes these acalleddrift currents .

Within the Ekman layer, the currents actually form

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spiral. At the surface, the difference between wind directionand surface wind-current direction varies from about 15°along shallow coastal areas to a maximum of 45° in thedeep oceans. As the motion is transmitted to successivelydeep layers, the Coriolis force continues to deflect thecurrent. At the bottom of the Ekman layer, the current flowsin the opposite direction to the surface current. This shift ofcurrent directions with depth, combined with the decreasein velocity with depth, is called theEkman spiral.

The velocity of the surface current is the sum of thevelocities of the Ekman, geostrophic, tidal, and othercurrents. The Ekman surface current or wind drift currentdepends upon the speed of the wind, its constancy, thelength of time it has blown, and other factors. In general,however, wind drift current is about 2 percent of the windspeed, or a little less, in deep water where the wind has beenblowing steadily for at least 12 hours.

3103. Currents Related to Density Differences

The density of water varies with salinity, temperature,

and pressure. At any given depth, the differences in densare due only to differences in temperature and salinity. Wsufficient data, maps showing geographical density distbution at a certain depth can be drawn, with lineconnecting points of equal density. These lines wouldsimilar to isobars on a weather map and serve an analogpurpose, showing areas of high density and those of ldensity. In an area of high density, the water surfacelower than in an area of low density, the maximumdifference in height being about 1 meter in 100 kmBecause of this difference, water tends to flow from an arof higher water (low density) to one of lower water (higdensity). But due to rotation of the Earth, it is deflected bthe Coriolis force or toward the right in the NorthernHemisphere, and toward the left in the SoutheHemisphere. This balance, between subsurface pressfields and the Coriolis force, is calledgeostrophicequilibrium . At a given latitude, the greater the densitgradient (rate of change with distance), the faster tgeostrophic current.

OCEANIC CIRCULATION

3104. Introduction

A number of ocean currents flow with greatpersistence, setting up a circulation that continues withrelatively little change throughout the year. Because of theinfluence of wind in creating current, there is a relationshipbetween this oceanic circulation and the general circulationof the atmosphere. The oceanic circulation is shown on thechart following this page (winter N. hemisphere), with thenames of the major ocean currents. Some differences inopinion exist regarding the names and limits of some of thecurrents, but those shown are representative. Speed mayvary somewhat with the season. This is particularlynoticeable in the Indian Ocean and along the South Chinacoast, where currents are influenced to a marked degree bythe monsoons.

3105. Southern Ocean Currents

The Southern Ocean has no meridional boundaries andits waters are free to circulate around the world. It serves asa conveyor belt for the other oceans, exchanging watersbetween them. The northern boundary of the SouthernOcean is marked by the Subtropical Convergence zone.This zone marks the transition from the temperate region ofthe ocean to the polar region and is associated with thesurfacing of the main thermocline. This zone is typicallyfound at 40°S but varies with longitude and season.

In the Antarctic, the circulation is generally from westto east in a broad, slow-moving current extendingcompletely around Antarctica. This is called theAntarcticCircumpolar Current or theWest Wind Drift , and it is

formed partly by the strong westerly wind in this area, anpartly by density differences. This current is augmentedthe Brazil and Falkland Currents in the Atlantic, the EaAustralia Current in the Pacific, and the Agulhas Currentthe Indian Ocean. In return, part of it curves northwardform the Cape Horn, Falkland, and most of the BengueCurrents in the Atlantic, and the Peru Current in the Pacif

In a narrow zone next to the Antarctic continent,westward flowing coastal current is usually found. Thcurrent is called theEast Wind Drift because it isattributed to the prevailing easterly winds which occuthere.

3106. Atlantic Ocean Currents

The trade winds set up a system of equatorial currewhich at times extends over as much as 50° of latitude ormore. There are two westerly flowing currents conformingenerally with the areas of trade winds, separated byweaker, easterly flowing countercurrent.

The North Equatorial Current originates to thenorthward of the Cape Verde Islands and flows almost dwest at an average speed of about 0.7 knot.

TheSouth Equatorial Current is more extensive. Itstarts off the west coast of Africa, south of the Gulf oGuinea, and flows in a generally westerly direction at aaverage speed of about 0.6 knot. However, the spegradually increases until it may reach a value of 2.5 knoor more, off the east coast of South America. As thcurrent approaches Cabo de Sao Roque, the easextremity of South America, it divides, the southern pacurving toward the south along the coast of Brazil, and t

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northern part being deflected northward by the continentof South America.

Between the North and South Equatorial Currents, theweakerNorth Equatorial Countercurrent sets toward theeast in the general vicinity of the doldrums. This is fed bywater from the two westerly flowing equatorial currents,particularly the South Equatorial Current. The extent andstrength of the Equatorial Countercurrent changes with theseasonal variations of the wind. It reaches a maximumduring July and August, when it extends from about 50°west longitude to the Gulf of Guinea. During its minimum,in December and January, it is of very limited extent, thewestern portion disappearing altogether.

That part of the South Equatorial Current flowing alongthe northern coast of South America which does not feed theEquatorial Countercurrent unites with the North EquatorialCurrent at a point west of the Equatorial Countercurrent. Alarge part of the combined current flows through variouspassages between the Windward Islands and into theCaribbean Sea. It sets toward the west, and then somewhatnorth of west, finally arriving off the Yucatan peninsula.From there, the water enters the Gulf of Mexico and formstheLoop Current ; the path of the Loop Current is variablewith a 13-month period. It begins by flowing directly fromYucatan to the Florida Straits, but gradually grows to flowanticyclonically around the entire Eastern Gulf; it thencollapses, again following the direct path from Yucatan tothe Florida Straits, with the loop in the Eastern Gulfbecoming a separate eddy which slowly flows into theWestern Gulf.

Within the Straits of Florida, the Loop Current feedsthe beginnings of the most remarkable of American oceancurrents, theGulf Stream. Off the southeast coast ofFlorida this current is augmented by theAntilles Currentwhich flows along the northern coasts of Puerto Rico,Hispaniola, and Cuba. Another current flowing eastward ofthe Bahamas joins the stream north of these islands.

The Gulf Stream follows generally along the east coastof North America, flowing around Florida, northward andthen northeastward toward Cape Hatteras, and then curvingtoward the east and becoming broader and slower. Afterpassing the Grand Banks, it turns more toward the north andbecomes a broad drift current flowing across the NorthAtlantic. The part in the Straits of Florida is sometimescalled theFlorida Current .

A tremendous volume of water flows northward in theGulf Stream. It can be distinguished by its deep indigo-bluecolor, which contrasts sharply with the dull green of thesurrounding water. It is accompanied by frequent squalls.When the Gulf Stream encounters the cold water of theLabrador Current, principally in the vicinity of the GrandBanks, there is little mixing of the waters. Instead, thejunction is marked by a sharp change in temperature. Theline or surface along which this occurs is called thecoldwall. When the warm Gulf Stream water encounters coldair, evaporation is so rapid that the rising vapor may be

visible as frost smoke.Investigations have shown that the current itself

much narrower and faster than previously supposed, aconsiderably more variable in its position and speed. Tmaximum current off Florida ranges from about 2 toknots. Northward, the speed is generally less, anddecreases further after the current passes Cape Hatterasthe stream meanders and shifts position, eddies sometimbreak off and continue as separate, circular flows until thdissipate. Boats in the Newport-Bermuda sailing yacht rahave been known to be within sight of each other andcarried in opposite directions by different parts of the samcurrent. This race is generally won by the boat whiccatches an eddy just right. As the current shifts position,extent does not always coincide with the area of warm, blwater. When the sea is relatively smooth, the edges ofcurrent are marked by ripples.

A recirculation region exists adjacent to and southeaof the Gulf Stream. The flow of water in the recirculationregion is opposite to that in the Gulf Stream and surfacurrents are much weaker, generally less than half a kn

As the Gulf Stream continues eastward annortheastward beyond the Grand Banks, it graduawidens and decreases speed until it becomes a vast, slmoving current known as theNorth Atlantic Current , inthe general vicinity of the prevailing westerlies. In theastern part of the Atlantic it divides into theNortheastDrift Current and theSoutheast Drift Current.

The Northeast Drift Current continues in a generalnortheasterly direction toward the Norwegian Sea. Asdoes so, it continues to widen and decrease speed. SouIceland it branches to form theIrminger Current and theNorway Current . The Irminger Current curves toward thenorth and northwest to join the East Greenland Curresouthwest of Iceland. The Norway Current continues innortheasterly direction along the coast of Norway. Part ofthe North Cape Current , rounds North Cape into theBarents Sea. The other part curves toward the north abecomes known as theSpitsbergen Current. Beforereaching Svalbard (Spitsbergen), it curves toward the wand joins the coldEast Greenland Current flowingsouthward in the Greenland Sea. As this current flows pIceland, it is further augmented by the Irminger Current.

Off Kap Farvel, at the southern tip of Greenland, the EaGreenland Current curves sharply to the northwest followithe coastline. As it does so, it becomes known as theWestGreenland Current, and its character changes from that of aintense western boundary current to a weaker eastboundary current. This current continues along the west coof Greenland, through Davis Strait, and into Baffin Bay.

In Baffin Bay the West Greenland Current generalfollows the coast, curving westward off Kap York to formthe southerly flowing Labrador Current . This coldcurrent flows southward off the coast of Baffin Islandthrough Davis Strait, along the coast of Labrador anNewfoundland, to the Grand Banks, carrying with it larg

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quantities of ice. Here it encounters the warm water of theGulf Stream, creating the cold wall. Some of the cold waterflows southward along the east coast of North America,inshore of the Gulf Stream, as far as Cape Hatteras. Theremainder curves toward the east and flows along thenorthern edge of the North Atlantic and Northeast DriftCurrents, gradually merging with them.

The Southeast Drift Current curves toward the east,southeast, and then south as it is deflected by the coast ofEurope. It flows past the Bay of Biscay, toward southeasternEurope and the Canary Islands, where it continues as theCanary Current . In the vicinity of the Cape Verde Islands,this current divides, part of it curving toward the west to helpform theNorth Equatorial Current , and part of it curvingtoward the east to follow the coast of Africa into the Gulf ofGuinea, where it is known as theGuinea Current. Thiscurrent is augmented by theNorth Equatorial Counter-current and, in summer, it is strengthened by monsoon winds.It flows in close proximity to the South Equatorial Current, butin the opposite direction. As it curves toward the south, stillfollowing the African coast, it merges with the SouthEquatorial Current.

The clockwise circulation of the North Atlantic leavesa large central area between the recirculation region and theCanary Current which has no well-defined currents. Thisarea is known as theSargasso Sea, from the largequantities of sargasso or gulfweed encountered there.

That branch of the South Equatorial Current whichcurves toward the south off the east coast of South America,follows the coast as the warm, highly-salineBrazilCurrent , which in some respects resembles a weak GulfStream. Off Uruguay it encounters the colder, less-saltyFalkland or Malvinas Current forming a sharpmeandering front in which eddies may form. The twocurrents curve toward the east to form the broad, slow-moving,South Atlantic Current in the general vicinity ofthe prevailing westerlies and the front dissipates somewhat.This current flows eastward to a point west of the Cape ofGood Hope, where it curves northward to follow the westcoast of Africa as the strongBenguela Current,augmented somewhat by part of theAgulhas Currentflowing around the southern part of Africa from the IndianOcean. As it continues northward, the current graduallywidens and slows. At a point east of St. Helena Island itcurves westward to continue as part of the South EquatorialCurrent, thus completing the counterclockwise circulationof the South Atlantic. The Benguela Current is alsoaugmented somewhat by the West Wind Drift, a currentwhich flows easterly around Antarctica. As the West WindDrift flows past Cape Horn, that part in the immediatevicinity of the cape is called theCape Horn Current . Thiscurrent rounds the cape and flows in a northerly andnortheasterly direction along the coast of South America asthe Falkland or Malvinas Current.

3107. Pacific Ocean Currents

Pacific Ocean currents follow the general patternthose in the Atlantic. TheNorth Equatorial Current flowswestward in the general area of the northeast trades, andSouth Equatorial Current follows a similar path in theregion of the southeast trades. Between these two,weakerNorth Equatorial Countercurrent sets toward theeast, just north of the equator.

After passing the Mariana Islands, the major part of thNorth Equatorial Current curves somewhat toward thnorthwest, past the Philippines and Taiwan. Here itdeflected further toward the north, where it becomes knowas theKuroshio, and then toward the northeast past thNansei Shoto and Japan, and on in a more easterly directPart of the Kuroshio, called theTsushima Current, flowsthrough Tsushima Strait, between Japan and Korea, andSea of Japan, following generally the northwest coastJapan. North of Japan it curves eastward and thsoutheastward to rejoin the main part of the Kuroshio. Tlimits and volume of the Kuroshio are influenced by thmonsoons, being augmented during the seasonsouthwesterly winds, and diminished when the northeastewinds are prevalent.

The Kuroshio (Japanese for “Black Stream”) is snamed because of the dark color of its water. Itsometimes called theJapan Current. In many respects it issimilar to the Gulf Stream of the Atlantic. Like that currenit carries large quantities of warm tropical water to highelatitudes, and then curves toward the east as a major pathe general clockwise circulation in the NortherHemisphere. As it does so, it widens and slows, continuion between the Aleutians and the Hawaiian Islands, whit becomes known as theNorth Pacific Current .

As this current approaches the North Americacontinent, most of it is deflected toward the right to formclockwise circulation between the west coast of NorAmerica and the Hawaiian Islands called theCaliforniaCurrent . This part of the current has become so broad ththe circulation is generally weak. Near the coast, tsoutheastward flow intensifies and average speeds are a0.8 knot. But the flow pattern is complex, with offshordirected jets often found near more prominent capes, apoleward flow often found over the upper slope and outcontinental shelf. It is strongest near land. Near the southend of Baja California, this current curves sharply to the weand broadens to form the major portion of the NorEquatorial Current.

During the winter, a weak countercurrent flownorthwestward, inshore of the southeastward flowinCalifornia Current, along the west coast of North Americfrom Baja California to Vancouver Island. This is called thDavidson Current.

Off the west coast of Mexico, south of Baja Californithe current flows southeastward during the winter ascontinuation of part of the California Current. During th

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summer, the current in this area is northwestward as acontinuation of the North Equatorial Countercurrent.

As in the Atlantic, there is in the Pacific a counter-clockwise circulation to the north of the clockwisecirculation. Cold water flowing southward through thewestern part of Bering Strait between Alaska and Siberia, isjoined by water circulating counterclockwise in the BeringSea to form theOyashio. As the current leaves the strait, itcurves toward the right and flows southwesterly along thecoast of Siberia and the Kuril Islands. This current bringsquantities of sea ice, but no icebergs. When it encounters theKuroshio, the Oyashio curves southward and then eastward,the greater portion joining the Kuroshio and North PacificCurrent.

The northern branch of the North Pacific Current curvesin a counterclockwise direction to form theAlaskaCurrent , which generally follows the coast of Canada andAlaska. When the Alaska Current turns to the southwest andflows along the Kodiak Island and the Alaska Peninsula, itscharacter changes to that of a western boundary current andit is called the Alaska Stream. When this westward flowarrives off the Aleutian Islands, it is less intense andbecomes known as theAleutian Current . Part of it flowsalong the southern side of these islands to about the 180thmeridian, where it curves in a counterclockwise directionand becomes an easterly flowing current, being augmentedby the northern part of the Oyashio. The other part of theAleutian Current flows through various openings betweenthe Aleutian Islands, into the Bering Sea. Here it flows in ageneral counterclockwise direction. The southward flowalong the Kamchatka peninsula is called theKamchatkaCurrent which feeds the southerly flowing Oyashio. Somewater flows northward from the Bering Sea through theeastern side of the Bering Strait, into the Arctic Ocean.

The South Equatorial Current , extending in widthbetween about 4°N latitude and 10°S, flows westward fromSouth America to the western Pacific. After this currentcrosses the 180th meridian, the major part curves in acounterclockwise direction, entering the Coral Sea, and thencurving more sharply toward the south along the east coastof Australia, where it is known as theEast AustralianCurrent . The East Australian Current is the weakest of thesubtropical western boundary currents and separates fromthe Australian coast near 34°S. The path of the current fromAustralia to New Zealand is known as theTasman Front,which marks the boundary between the warm water of theCoral Sea and the colder water of the Tasman Sea. Thecontinuation of the East Australian Current east of NewZealand is theEast Auckland Current . The East AucklandCurrent varies seasonally: in winter, it separates from theshelf and flows eastward, merging with the West WindDrift, while in winter it follows the New Zealand shelfsouthward as theEast Cape Current until it reachesChatham Rise where it turns eastward, thence merging withthe West Wind Drift.

Near the southern extremity of South America, most of

this current flows eastward into the Atlantic, but part ofcurves toward the left and flows generally northward alonthe west coast of South America as thePeru Current orHumboldt Current . Occasionally a set directly towardland is encountered. At about Cabo Blanco, where the cofalls away to the right, the current curves toward the lepast the Galapagos Islands, where it takes a westerly setconstitutes the major portion of the South EquatoriCurrent, thus completing the counterclockwise circulatioof the South Pacific.

During the northern hemisphere summer, a wenorthern branch of the South Equatorial Current, knowntheNew Guinea Coastal Current, continues on toward thewest and northwest along both the southern anortheastern coasts of New Guinea. The southern part flothrough Torres Strait, between New Guinea and Australinto the Arafura Sea. Here, it gradually loses its identitpart of it flowing on toward the west as part of the SoutEquatorial Current of the Indian Ocean, and part offollowing the coast of Australia and finally joining theeasterly flowing West Wind Drift. The northern part oNew Guinea Coastal Current both curves in a clockwidirection to help form the Pacific Equatorial Countercurreand off Mindanao turns southward to form a southwaflowing boundary current called theMindanao Current .During the northern hemisphere winter, the New GuineCoastal Current may reverse direction for a few months

3108. Indian Ocean Currents

Indian Ocean currents follow generally the pattern of thAtlantic and Pacific but with differences caused principalby the monsoons, the more limited extent of water in thNorthern Hemisphere, and by limited communication withe Pacific Ocean along the eastern boundary. Duringnorthern hemisphere winter, theNorth Equatorial CurrentandSouth Equatorial Current flow toward the west, withthe weaker, eastwardEquatorial Countercurrent flowingbetween them, as in the Atlantic and Pacific (but somewhsouth of the equator). But during the northern hemisphesummer, both the North Equatorial Current and thEquatorial Countercurrent are replaced by theSouthwestMonsoon Current, which flows eastward andsoutheastward across the Arabian Sea and the Bay of BenNear Sumatra, this current curves in a clockwise directiand flows westward, augmenting the South EquatorCurrent, and setting up a clockwise circulation in thnorthern part of the Indian Ocean. Off the coast of Somalthe Somali Current reverses direction during the northerhemisphere summer with northward currents reachispeeds of 5 knots or more. Twice a year, around May aNovember, westerly winds along the equator result ineastwardEquatorial Jet which feeds warm water towardsSumatra.

As the South Equatorial Current approaches the coasAfrica, it curves toward the southwest, part of it flowing

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through the Mozambique Channel between Madagascar andthe mainland, and part flowing along the east coast ofMadagascar. At the southern end of this island the two jointo form the strongAgulhas Current, which is analogous tothe Gulf Stream. This current, when opposed by strongwinds from Southern Ocean storms, creates dangerouslylarge seas.

South of South Africa, the Agulhas Current retroflects,and most of the flow curves sharply southward and theneastward to join the West Wind Drift; this junction is oftenmarked by a broken and confused sea, made much worse bywesterly storms. A small part of the Agulhas Current roundsthe southern end of Africa and helps form theBenguelaCurrent ; occasionally, strong eddies are formed in the retrof-lection region and these too move into the SoutheasternAtlantic.

The eastern boundary currents in the Indian Ocean arequite different from those found in the Atlantic and Pacific.The seasonally reversingSouth Java Current hasstrongest westward flow during August when monsoonwinds are easterly and the Equatorial jet is inactive. Alongthe coast of Australia, a vigorous poleward flow, the

Leeuwin Current, runs against the prevailing winds.

3109. Arctic Currents

The waters of the North Atlantic enter the Arctic Oceabetween Norway and Svalbard. The currents flow easternorth of Siberia, to the region of the NovosibirskiyOstrova, where they turn northerly across the North Poand continue down the Greenland coast to form theEastGreenland Current. On the American side of the Arcticbasin, there is a weak, continuous clockwise flow centerin the vicinity of 80°N, 150°W. A current north throughBering Strait along the American coast is balanced byoutward southerly flow along the Siberian coast, whiceventually becomes part of theKamchatka Current . Eachof the main islands or island groups in the Arctic, as faris known, seems to have a clockwise nearshore circulataround it. The Barents Sea, Kara Sea, and Laptev Sea ehave a weak counterclockwise circulation. A similar buweaker counterclockwise current system appears to existhe East Siberian Sea.

OCEANIC CURRENT PHENOMENA

3110. Ocean Eddies and Rings

Eddies with horizontal diameters varying from 50-150km have their own pattern of surface currents. Thesefeatures may have either a warm or a cold core and currentsflow around this core, either cyclonically for cold cores oranticyclonically for warm cores. The most intense of thesefeatures are calledrings and are formed by the pinching offof meanders of western boundary currents such as the GulfStream. Maximum speed associated with these features isabout 2 knots. Rings have also been observed to pinch offfrom the Agulhas retroflexion and to then drift to thenorthwest into the South Atlantic. Similarly, strong anticy-clonic eddies are occasionally spawned by the loop currentinto the Western Gulf Mexico.

In general, mesoscale variability is strongest in theregion of western boundary currents and in the CircumpolarCurrent. The strength of mesoscale eddies is greatlyreduced at distances of 200-400 km from these strongboundary currents, because mean currents are generallyweaker in these regions. The eddies may be sufficientlystrong to reverse the direction of the surface currents.

3111. Undercurrents

At the equator and along some ocean boundaries,shallow undercurrents exist, flowing in a direction counterto that at the surface. These currents may affect theoperation of submarines or trawlers. The most intense ofthese flows, called the PacificEquatorial Undercurrent ,is found at the equator in the Pacific. It is centered at a depth

of 150m to the west of the Galapagos, is about 4 km widand eastward speeds of up to 1.5 m/s have been obserEquatorial Undercurrents are also observed in the Atlanand Indian Ocean, but they are somewhat weaker. InAtlantic, the Equatorial Undercurrent is found to the east24°W and in the Indian Ocean, it appears to be seasona

Undercurrents also exist along ocean boundaries. Thseem to be most ubiquitous at the eastern boundaryoceans. Here they are found at depths of 100-200m, may100 km wide, and have maximum speeds of 0.5 m/s.

3112. Ocean Currents and Climate

Many of the ocean currents exert a marked influenupon the climate of the coastal regions along which thflow. Thus, warm water from the Gulf Stream, continuinas the North Atlantic, Northeast Drift, and IrmingeCurrents, arrives off the southwest coast of Icelanwarming it to the extent that Reykjavik has a higher averawinter temperature than New York City, far to the southGreat Britain and Labrador are about the same latitude,the climate of Great Britain is much milder because of threlatively warm currents. The west coast of the UniteStates is cooled in the summer by the California Curreand warmed in the winter by the Davidson Current. Paras a result of this circulation, the range of monthly averatemperature is comparatively small.

Currents exercise other influences besides thosetemperature. The pressure pattern is affected materiallyair over a cold current contracts as it is cooled, and that oa warm current expands. As air cools above a cold oce

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current, fog is likely to form. Frost smoke occurs over awarm current which flows into a colder region. Evaporationis greater from warm water than from cold water, adding toatmospheric moisture.

3113. Ocean Current Observations

Historically, our views of the surface circulation of theocean have been shaped by reports of ocean currentsprovided by mariners. As mentioned at the start of thischapter, these observations consist of reports of thedifference between the dead reckoning and the observedposition of the vessel. These observations were routinelycollected until the start of World War II.

Today, two observation systems are generally used for

surface current studies. The first utilizes autonomous fredrifting buoys which are tracked by satellite or relay theposition via satellite. These buoys consist of eitherspherical or cylindrical surface float which is about 0.5mdiameter with a drogue at a depth of about 35m. The secosystem utilizesacoustic Doppler current profilers. Theseprofilers utilize hull mounted transducers, operate atfrequency of 150 kHz, and have pulse repetition ratesabout 1 second. They can penetrate to about 300m, awhere water is shallower than this depth, track the bottoMerchant and naval vessels are increasingly being outfitwith acoustic Doppler current profilers which, whenoperated with the Global Positioning System, providaccurate observations of currents.

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