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CHAPTERTHREE
INTRODUCTIONTOOCEANOGRAPHY
3.1 OCEANBOTTOMCHARACTERISTICSLearningObjectives
Namethefivemajoroceanicprovincesandtherelieffeaturesassociatedwiththeoceanbottom.
Describethefivemajoroceanicprovincesandtherelieffeaturesassociatedwiththeoceanbottom.
Nameanddescribethevarioustypesofbottomsediments.
Earth'stopographyhasadefiniteandimportanteffectontheelementsandcharacteristicsofits
surroundingatmosphere. Thesamerelationshipexistsbetweentheoceanfloorandtheoceans. The
irregularterrainoftheoceanflooraffectsthemovementofoceanwater,temperaturegradientsin
areasofchanneling,navaloperations,andsubmarine/antisubmarine(ASW)tactics. Manyrelief
featuresandbottomtypesareusedbysubmarinerstoconcealtheirsubmarinesanddecreasetheir
probabilityofdetectionbysurfacesonar. Sonartransmissionsthatimpactthebottomareaffectedby
thebottomtopographyandbottomtypes(sand,mud,etc.). Sonarperformancemaybeimprovedor
hinderedbythebottom;therefore,submarinersusethebottomtotheirbestadvantage. Thesurface
fleetmustalsobeawareofbottomrelieffeaturesandbottomtypesinordertoimprovethe
effectivenessoftheirsearchsonar.
3.1.1 BOTTOMTOPOGRAPHY
Fromsealeveltothedeepestdepthsbeneaththesea,therearefivemajorbottomprovinces. These
provincesincludethecontinentalshelf,thecontinentalslopeandrise,andthemidoceanridges. See
Figure31.
3.1.1.1 ContinentalShelf
TheContinentalShelfisthefirstprovinceoffshore. Theaveragewidthoftheshelfisapproximately40
miles,butinsomeplacessuchasthewestcoastofSouthAmerica,theshelfisnonexistent. Thewidest
shelfisfoundalongtheglaciatedcoastofSiberia,whereitextendsoutwardfromshoreroughly800
miles. Continentalshelvescompriseabout7.5percentofthetotaloceanbottom.
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Thecontinentalshelfhasaverygradualslope. Itdeclinesatanaveragerateof2fathomspermile.
Eventhoughtheaverageslopeoftheshelfisgradual,terraces,ridges,hills,depressions,anddeep
canyonsarefoundwithinitsboundaries. Theshelfregionisatransitionzonebetweenfreshwater
runofffromlandandthemoresalinewaterofthesea;consequently,itisanareaofgreatmixingwith
generallyunstablewaterconditions. Currentsnormallyrunparalleltotheshoreinthisregion.
3.1.1.2 ContinentalSlope
Attheseawardedgeofthecontinentalshelf,theslopebecomesmuchsteeperandthedropoffisvery
rapid. Onaverage,theslantratioisroughly20timesgreaterthanthatofthecontinentalshelf. The
ratioisgreateroffmountainouscoaststhanoffwide,welldrainedplains.
Thecontinentalsloperesemblesasteepcliffthathasbeenerodedbyheavyrains. Itsmoststriking
featuresarethesubmarinecanyonsthatareprevalentalongtheslopeface. Thesecanyonsarethought
tohavebeenformedbyturbiditycurrents,whicharedense,sedimentladencurrentsthatflowalongthe
oceanfloor. Attheseawardendofthesecanyons,largeamountsofsedimentaredepositedandspread
outinafanlikemannertoformthecontinentalrise.
3.1.1.3 ContinentalRise
Thecontinentalriseisfoundseawardofthecontinentalslope,inapproximately500fathomsofwater.
Itismadeupofthicksedimentdepositsthatcoverirregularrelieffeatures. Thesedepositsslopegently
seawardformingtheabyssalplainsofthedeepoceanbasins. Attheseawardedgeofthecontinental
rise,thewaterdepthisabout1,500fathoms.
Figure31.BottomTopography(Source:NAVMETOCCOM)
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3.1.1.4 OceanBasins
Theoceanbasinsaccountfor76percent
oftheoceanfloor,withdepthsranging
from1,500to3,000fathoms. Ocean
basins,onaverage,haveveryslight
inclinesofnomorethan1:90miles. For
every90milesseawardthebottom
slopesnomorethan1mile.
Superimposedonthisveryflatplainare
manyruggedrelieffeatures,suchas
seamounts,guyots,atolls,sills,and
trenches.
3.1.1.4.1 Seamounts
Seamountsaresubmerged,isolated,pinnacledmountainsrising3,000feetormoreabovetheseafloor.
3.1.1.4.2 GuyotsorTablemounts
Guyotsaresubmerged,isolated,flattoppedmountainsthatrise3,000feetormoreabovetheseafloor.
3.1.1.4.3Atolls
Atollsareseamountsorguyotsthathavebrokentheseasurface,andcoraldepositshavebuiltup
aroundtherim. Thecoralformsareefaroundashallowbodyofwater. SeeFigure32.
3.1.1.4.4 VolcanicIslands
Theseislandscanoccurindividuallyandingroups. Approximately10,000volcanoesarelocatedacross
theoceanfloorandtheyareespeciallyabundantinthewesternPacificbasin. TheHawaiianIslandsare
probablythebestknownexampleofvolcanicislands.
3.1.1.4.5 Sills
Sillsareelevatedpartsoftheoceanfloorthatpartiallyseparateoceanbasins. Asillrestrictsthe
movementofbottomwatermassesandresultsintheirpartial,andinsomecasesnearlytotal,isolation.
Figure32. Atoll
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3.1.1.4.6 Trenches
Trenchesarelong,narrow,andrelativelysteepsideddepressionsandtheycomprisethedeepest
portionsoftheoceans. TrenchesaremorenumerousinthePacificthaninanyotherocean. Pacific
Oceantrenchesaverage2,500milesinlengthandareofgreaterdepththanallothertrenchesaround
theworld. Forexample,theMarianaTrenchis35,600feetdeep;theTongaTrenchis35,430;andthe
MindanaoTrenchis34,428feetdeep. Trenchesarenormallyfoundontheseawardsideofislandarcs,
whicharelong,curvedchainsofoceanicislandsassociatedwithintensevolcanicandseismicactivity,
whilerelativelyshallowseasexistonthecontinentalside.
3.1.1.5 Ridges
Ridgesarethelastoftheoceanprovinces. TheMidAtlanticRidgeextendssouthwardfromIceland
acrosstheequatortonear55S,dividingthe Atlanticintoaneasternandwesternbasin. The Mid
AtlanticRidgerisesfromadepthof2,500fathomsand iscontinuous atdepthsoflessthan1,500
fathomsoverthe greaterpartofits length. Inseveralplaces,thisridgerisestoabovesea level
toformislandssuchasthe Azoresand AscensionIslands.
3.2 BOTTOMCOMPOSITION
Theoceanbottomiscoveredbyvarioustypesofbottomsedimentsmixedwithdissolvedshellsand
bonesofmarineorganisms. Sedimentdepositsarethinorabsentonthenewlyformedcrustofmid
oceanridgesandarethickestontheoldercrustandnearcontinents. Thefourmajorclassificationsof
sedimentsareterrigenous,pelagic,glacialmarine,andvolcanic.
3.2.1 TERRIGENOUSSEDIMENTS
Terrigenoussedimentsarelandderivedsiltsandclaysthatarecarriedtoseabyrivers. Windsalso
carrydustandsandouttoseaanddepositthemonthesurface,wheretheyeventuallysinktothe
bottom. Terrigenousdepositsaremostlyfoundintheregionofthecontinentalshelf.
3.2.2
PELAGICSEDIMENTS
Thesesedimentsarealsoknownasoozebecauseoftheirappearance. Theyformindeepwaterandare
mostcommonlycomposedofshellsandskeletalremainsofmarineplantsandanimals.
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Identifythosepropertiesofseawaterthatdensityisdependenton.
Recognizehowdensityaffectsseawaterstability.
Recognizethedegreetowhichseawateriscompressible,andtheimportanceofthisproperty.
Definespecificheatandrecognizetheeffectsalinityhasonthespecificheatofseawater.
Defineviscosity.
Recognizetheotherpropertiesofseawaterthatcontrolit.
Recognizetheeffectstemperature,pressure,andsalinityhaveonthethermalexpansionofseawater.
Identifyoneofthemajorrolesofthermalexpansioninthesea.
Identifythepropertiesthatcontrolsoundvelocityintheocean.
Recognizehoweachcontrolsthespeedanddirectionofasoundwave.
Identifythethreelayersofthethreelayeredoceanmodel.
Differentiatebetweenmechanicalandconvectivemixing.
Definewatertypeandwatermass.
Identifythepropertiesusedintheclassificationofwatertypesandwatermasses.
Recognize
the
oceans'
basic
vertical
structure
with
regard
to
latitudinal
distribution.
Recognizewatermasssourceregionsandhowtheyareformed.
Recognizehowdeepoceancirculationdiffersfromsurfacecirculationandhowthecirculationpatternismaintained.
JustastheairofoneregionofEarthcandifferinitsmakeupfromthatofanotherregion,socan
seawater. Forexample,thewateraroundAntarcticadiffersfromthatofthemidlatitudesandthe
tropics,andwaterfoundattheoceansurfacediffersfromthatfoundatornearthebottom. The
differencesfoundinseawaterarerelatedtoseawaterproperties. Itistheseawaterpropertiesthatare
usedtoclassifywatermasses. Inthislesson,wewillcovervariousseawaterproperties,thethreelayer
oceanmodelasitrelatestothepropertyoftemperature,andthewatermassesoftheworld'soceans.
3.3.1PROPERTIESOFSEAWATER
Temperature,pressure,andsalinityarethethreemostimportantpropertiesofseawater,andthey
determinetheotherphysicalpropertiesassociatedwithseawater. Thisdiffersfrompurewater,where
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onlytemperatureandpressuredeterminethephysicalproperties. Wavemotionandthepresenceof
smallsuspendedparticlesinseawaterarealsoimportantvariablesthataffectthepropertiesofseawater.
Wavemotioncausesachangeintheprocessesofchemicaldiffusion,heatconduction,andtransferof
momentumfromonelayertoanother. Wavemotionalsoincreasestheamountofsuspendedparticles,
causingincreasedscatteringofincomingsolarradiationandahigherabsorptionrateinseawaterthana
similarlayer(thickness)ofpurewater. Thevariablesofwavemotionandsuspendedparticles,although
important,cannotbemeasured. Inadditiontotemperature,pressure,andsalinity,othercommon
physicalpropertiesofseawaterarewatercolor,transparency,ice,andsoundvelocity.
3.3.1.1Temperature
Theocean,liketheatmosphere,isheatedthroughabsorptionoftheSun'sincomingradiation. Atall
latitudes,theicefreeportionsofoceansreceiveasurplusofradiation. Someofthisheatisgivenupto
theatmospherethroughthereleaseoflongwaveradiation,andsomeofitisretained. Becausethesea
retainsaportionofthisheat,theseasurfacetemperatureisnormallyhigherthantheairtemperature.
However,thisistrueonlywhenaverageconditionsareconsidered. Whethertheseasurfaceiswarmer
orcolderthantheairaboveitatanyparticularmomentisdependentuponthelocality,theseasonof
theyear,thecharacteroftheatmosphericcirculation,andthecharacteroftheoceancurrents.
Theaveragetemperatureoftheoceanrangesfromabout 2Cto30C. Oceanwaterthatisnearly
surroundedbylandmayhavehighertemperatures,buttheopensea,wherethewaterisfreetomove
about,rarelyexceedstemperaturesabove30C. Here,theoceancurrentsdistributetheheatandtend
toequalizethetemperature. Deepandbottomwatertemperaturesarealwayslow,varyingbetween
4Cand1C.
Theannualseasurfacetemperaturevariationinanyregiondependsuponthevariationofincoming
radiation,thecharacteroftheoceancurrents,andthecharacteroftheatmosphericcirculation. The
annualrangeofsurfacetemperatureismuchgreateroveroceansoftheNorthernHemispherethan
thoseoftheSouthernHemisphere. Thiswiderrangeoftemperaturesappearstobeassociatedwiththe
large
difference
in
land/sea
distribution
between
the
Northern
and
Southern
Hemispheres,
and
the
characteroftheprevailingwinds,particularlythecoldwindsblowingfromthecontinents. Themore
consistentannualtemperaturesintheSouthernHemispherearerelatedtothefairlyequivalentaccess
toincomingsolarradiationduetotheabsenceoflargelandmassessouthof45S. Here,theprevailing
windstravelalmostentirelyoverwaterwhichbringsaboutafargreaterdegreeofconsistencyand
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moderationintheannualseasurfacetemperaturepatterns,andamuchsmallerannualtemperature
rangewhencomparedtotheNorthernHemisphere.
Watertemperaturesneartheequatorexperienceasemiannualvariationthatcorrespondstothetwice
yearlypassageoftheSun'smostdirectraysacrosstheequator.
Seasurfacetemperatureschangefromdaytonightjustlikethoseoftheatmosphere,buttoamuch
lesserdegree. Thediurnalvariationofseasurfacetemperaturesintheopenoceanis,onaverage,only
0.2Cto0.3C. Thegreatestdiurnalvariationtakesplaceinthetropics,withlesservariationathigher
latitudes. Therangeofdiurnalvariationisdependentontheamountofcloudinessandthedirection
andspeedofthewind. Thetropicshaveconsiderablylesswindspeedandcloudcoveragethan
locationsinhigherlatitudes;thereforethereisagreaterdiurnalvariationinseasurfacetemperatures.
Theannualvariationoftemperatureinsubsurfacelayersdependsonseveraladditionalfactors:
Thevariationintheamountofheatthatisdirectlyabsorbedatdifferentdepths.
Theeffectofheatconduction.
Thevariationincurrentsrelatedtolateraldisplacement.
Theeffectofverticalmotion.
Diurnaltemperaturevariationsinsubsurfacelayersarelargelyunknown. Whatwedoknowisthatthey
areextremelysmall.
3.3.1.2VerticalTemperatureStructure
Thebasicverticaltemperaturestructureoftheoceaninitssimplestformisbestdescribedusingthe
threelayeredoceanmodelwhichwillbediscussedlaterinsection3.3.2. Generally,thereislittle
temperaturechangewithdepththroughanupperormixedlayer,asharptemperaturedecrease
throughamainthermoclinelayer,andareturntogenerallyconstanttemperaturethroughadeepwater
coldlayer. AvisualexampleofthiscanbeseeninFigure33insection3.3.2.
3.3.1.3Pressure
Pressurebeneaththeseasurfaceismeasuredindecibars. Thepressureexertedby1meterofseawater
verynearlyequals1decibar(1/10ofabar)or100,000dynespersquarecentimeter. Greaterdepths
equalgreaterpressure,andsincepressureintheoceanisessentiallyafunctionofdepth,thenumerical
valueofpressureindecibarsapproximatestheoceandepthinmeters. Therefore,pressurevaluesrange
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fromzeroatthesurfacetoover10,000decibarsinthedeepestpartsoftheoceans. Thepressureis
createdbytheweightoftheseawaterabove. Theweightperunitvolumeofseawater,inturn,varies
withthetemperatureandsalinity. Inacolumnofwateratconstantdepth,thepressureincreasesas
temperaturesdecrease,orassalinityvaluesincrease,orboth.
3.3.1.4Salinity
Thetermsalinityisrelatedoftentotheamountofsaltinthewater. Inoceanography,salinityis
definedas"thetotalamountofdissolvedsolidsinseawater."Salinityismeasuredinpartsperthousand
byweight,andissymbolized(thepermillesymbol). Themeasurementgivesusthegramsof
dissolvedmaterialperkilogramofseawater. Thesalinityvaluesofoceanwaterrangebetween33
and37,withanaverageof35.
Intheopenocean,surfacesalinityisdecreasedbyprecipitation,increasedbyevaporation,andchanged
bytheverticalmixingandinflowofadjacentwater. Nearshore,salinityisgenerallyreducedbyriver
dischargeandfreshwaterrunofffromland. Inthecolderwatersthatfreezeandthaw,salinitygenerally
increasesduringperiodsoficeformationanddecreasesduringperiodsoficemelt.
Latitudinally,surfacesalinityvariesinasimilarmannerinalloceans. Maximumsalinityvaluesoccur
between20and23northandsouth,whereasminimumsalinityvaluesoccurneartheequatorand
towardhigherlatitudes. Thecontrollingfactorinaveragesurfacesalinitydistributionisthelatitudinal
differencesinevaporationandprecipitation. Exceptionstothisstatementdooccur,andlocalvariations
shouldbeexpected,especiallynearthemouthofthelargerriversystemsandintheAtlanticcoastal
wateroftheUnitedStates,Labrador,Spain,andScandinavia. Atlatitudespolewardof40northand
south,whereprecipitationgenerallyexceedsevaporation,salinityvaluestendtoincreasewithdepth.
Usuallyduringsummer,thesepositivesalinitygradientsareaccompaniedbystrongnegative
temperaturegradientsandresultinverystablewater,especiallyinthecoastalregions. Thesestrong,
shallowsalinity(andtemperature)gradientspersistthroughthesummer.
ThebestknownregionofstronghorizontalsalinitygradientsistheGrandBanksregion,wherewarm,
salineGulfStreamwatermixeswiththecolder,lesssalinewateroftheLabradorCurrent. Here,water
withasalinityvalueaslowas32maypossiblyoverrideorlieadjacenttowaterhavingasalinityvalue
greaterthan36. AsimilarsituationprevailsinthePacificOcean,wheretheKuroshioandOyashio
currentsmix.
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3.3.1.5Density
Thedensityofseawaterisalsodependentontemperature,pressure,andsalinity. Ataconstant
temperatureandpressure,densityvarieswithsalinity. Atemperatureof32Fandanatmospheric
pressureof1,013.2mbareconsideredstandardfordensitydetermination. Atothertemperaturesand
pressurestheeffectsofthermalexpansionandcompressibilityareusedtodeterminedensity. The
densityataparticularpressureaffectsthebuoyancyofvariousobjects,notablysubmarines. Densityis
definedasmassperunitvolume,andisexpressedingramspercubiccentimeter. Thegreatestchanges
indensityofseawateroccuratthesurface. Here,densityisdecreasedby:
Precipitation
Runofffromland
Meltingofice
Heating
Whenthesurfacewaterbecomeslessdense,ittendstofloatontopofthemoredensewater. Thereis
littletendencyforthewatertomix;therefore,theconditionisstable. Thedensityofsurfacewateris
increasedbyevaporation,theformationofseaice,andcooling. Ifthesurfacewaterbecomesdenser
thanthewaterbelow,itsinkstoalevelhavingthesamedensity. Here,itincreasesthethicknessofthe
layerandtendstospreadout. Asthemoredensewatersinks,thelessdensewaterrises,anda
convectivecirculationisestablished. Thecirculationcontinuesuntilthedensitybecomesuniformfrom
thesurfacetoadepthatwhichagreaterdensityoccurs. Ifthesurfacewaterbecomessufficientlydense,
itsinksallthewaytothebottom. Ifthisoccursinanareawherehorizontalflowisunobstructed,the
waterthathasdescendedspreadstootherregions,creatingadensebottomlayer. Sincethegreatest
increaseindensityoccursinpolarregions,wheretheairiscoldandgreatquantitiesoficeform,thecold,
densepolarwatersinkstothebottomandthenspreadstolowerlatitudes. Thisprocesshascontinued
forsuchalongperiodoftimethattheentireoceanflooriscoveredwiththisdensepolarwater. This
explainsthelayerofcoldwateratgreatdepthsintheocean
3.3.1.6Compressibility
Seawaterisnearlyincompressible. Thecompressibilityofseawaterchangesslightlywithchangesin
temperatureorsalinity. Theeffectofcompressionistoforcethemoleculesofthesubstancecloser
together,causingthesubstancetobecomedenser. Eventhoughthecompressibilityofseawaterislow,
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thetotaleffectisconsiderablebecauseoftheamountofwaterinvolved. Ifcompressibilitywerezero,
sealevelwouldbeabout90feethigherthanitisnow.
3.3.1.7SpecificHeat
Inoceanography,specificheatisthenumberofcaloriesneededtoraisethetemperatureof1gramof
seawater1C. Thespecificheatofseawaterdecreasesslightlyassalinityincreases,whileconversely,
thespecificheatofseawaterincreasesassalinitydecreases. Thatbeingsaid,theratioofspecificheatto
seawaterataconstantpressureandconstantvolumehasadirectrelationshiptothespeedofsoundin
water.
3.3.1.8Viscosity
Viscosityisaliquidsabilitytoresistflow. Seawaterisslightlymoreviscousthanfreshwater,andthe
levelofresistanceiscontrolledbyitsthermalexpansion. Liquidsexpandandcontractwhen
temperaturechangestakeplace;somemorethanothers. Theresistancerateofseawaterisnot
uniform, viscosityincreaseswhensalinityincreasesorthewatertemperaturedecreases. However,the
effectofdecreasingtemperatureisgreaterthanthatofincreasingsalinity. Becauseoftheeffectof
temperatureonviscosity,anincompressibleobjectmightsinkatafasterrateinwarmsurfacewater
thanincoldersubsurfacewater. Formostcompressibleobjects,viscosityeffectsmaybemorethan
offsetbythecompressibilityoftheobject. Inrealitythisisaverysimpleexplanationtoacomplex
problem,sincetheactualrelationshipsexistingintheoceanareconsiderablymorecomplicatedthan
portrayedhere.
3.3.1.9ExpansionofSeaWater
Seawaterhasahighercoefficientofexpansionthanthatoffreshwater. Withinthesea,thecoefficient
ofthermalexpansionisaffectedbytemperature,pressure,andsalinity. Thecoefficientofthermal
expansionisgreaterinhighsalinitywater;greaterinwarmwaterthanincold(undersimilarsalinity
conditions);anditincreaseswithincreasingdepthunderconstanttemperatureandsalinityconditions.
Ofcourse,constancyisnotatrademarkofanyoftheseproperties;theyareallquitevariable. Inturn,
thethermalexpansionthattakesplaceintheseavariesandisdifficulttoassess. Amajorroleof
thermalexpansionisintheformationofice. Purewaterismostdenseat4C. Thermalexpansiontakes
placewhenwaterwarmsabove4C,butwateralsoexpandsevenmorewhenitcoolsbelow4Candas
itfreezes. Whenexpansiontakesplace,thevolumeisincreasedresultingindecreaseddensity. Ifwater
failedtoexpandduringthefreezingprocess,thedensityoficewouldbesuchthatitwouldsinktothe
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bottomuponformingandinthecoldofwinter,freshwaterlakeswouldeventuallybecomesolidblocks
ofice. Comesummer,onlytheupperfewfeetoficewouldmelt,leavingtheremainingicebeneaththe
meltedwater.
3.3.1.10Sound
Velocity
Velocitytakesintoaccountbothspeedanddirection. Thespeedofsoundinseawaterisgovernedby
temperature,pressure,andsalinity. Anincreaseintemperatureincreasesthespeedofsoundinwater,
whileadecreaseintemperaturedecreasesthespeedofsound. Thesamerelationshipappliesto
pressureandsalinity. Anincreaseinpressurecausesanincreaseinsoundspeed,asdoesanincreasein
salinity,andviceversa. Sincepressureisafunctionofdepthinthesea,ifweweretodiscounttheeffect
oftemperatureandsalinity,soundwouldtravelfasterattheoceanbottomthanitdoesatthesurface.
However,wecannotdiscounteitheroftheseothertwovariables,especiallytemperature. Temperature
isthemostimportantpropertycontrollingthespeedofsoundinwater. Asfarasdirectionisconcerned,
soundwavestravelinstraightlinesonlyinamediuminwhichthespeediseverywhereconstant. For
thistooccurinseawater,thetemperature,pressure,andsalinityvalueswouldhavetobeunchanging.
Changesinany,orallofthesevariablesdoesoccurwhich,inturnaffectsthespeedofsoundwavesand
thedirectionsuponwhichtheytravel. Soundwavesarebent(refracted)inthedirectionofslower
soundvelocities. Thedegreeofrefractionisproportionaltothevelocitygradient,orthechangein
soundvelocitywithdistance. Ifthevelocitygradientweresuchthatsoundspeedincreasedrapidlywith
depth,
sound
waves
would
refract
sharply
upward
toward
the
slower
sound
velocities
at
the
surface.
Ontheotherhand,ifthevelocitygradientweresuchthatsoundspeeddecreasedrapidlywithdepth,
soundwaveswouldrefractsharplydownwardtowardtheslowersoundspeedsattheoceanbottom.
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3.3.2THETHREELAYEREDOCEAN
Aconvenientmethodofvisualizingtheseaistodivideit
intolayersinmuchthesamewaythatwedothe
atmosphere. Usingbathythermographinformation
(temperatureversusdepthprofiles),asshowninFigure
33,theoceansdisplayabasicthreelayeredstructure:
themixedlayer,mainthermocline,anddeepwaterlayer.
3.3.2.1MixedLayer
Themixedlayeristheupperlayerofthethreelayered
oceanmodel. Themixedlayerconsistsofnearlyuniform,
orisothermalrelativelywarmertemperatureswith
depth,inmiddlelatitudes,andextendsfromthesurface
toamaximumdepthofabout450meters,or1,500feet. Thislayergetsitsnamefromthemixing
processesthatbringaboutitsfairlyconstantwarmtemperatures. Thetwomixingprocessesare
classifiedasmechanicalandconvective.
3.3.2.1.1MechanicalMixing
Thismixingprocessiscausedbywaveactionand/orsurfacestormsstirringupthewater. Warmer
surfacewaterisdrivendownward,whereitmixeswithcoldersubsurfacewater. Eventually,alayerof
waterwithafairlyconstant,orisothermal,temperatureisproduced. Thisprocessismoreimportantin
summerthaninwinter,becausesurfacewatersaremuchwarmerandlessdensethansubsurface
waters,therebyproducingastablewatercolumn. Themechanicalmixingprocessismorerapidand
irregularthantheconvectivemixingprocess.
3.3.2.1.2ConvectiveMixing
Thisprocessoccursasaresultofchangesinwaterstability. Whensurfacewatersbecomedenserthan
subsurfacewaters,anunstableconditionexists. Suchconditionscanoccurwhenthereisanincreasein
surfacesalinityduetoevaporation,theformationofice,orbyadecreaseinthesurfacewater
temperature. Atemperaturedecreaseof.01Corasalinityincreaseof0.01,issufficienttoinitiate
theconvectivemixingprocess. Intheformercase,forexample,acoldpolarorarcticairmassmoving
overwarmwatercoolsthesurfacewaterbeforeitcancoolthesubsurfacewater. Asthesurfacewaters
Figure
3
3.
Three
layered
ocean
mode.
(Source: PDC)
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Figure34. VerticalTemperatureProfileinSummer.SourcePDC
coolandbecomecolderthanthesubsurfacewaters,theybecomedenserandsink. Asthecolder
surfacewatersinks,thewarmerandlessdensesubsurfacewaterrisestothesurfacetoreplaceit. This
processcontinuesuntilthewateristhoroughlymixed,thedensitydifferenceeliminated,andthewater
columnstabilized. Eventhoughwindsandtheresultantwaveactionaregenerallystrongerduring
winter,convectivemixing,causedbythecolderwinterairtemperatures,producesadeepermixedlayer
thancanbeattainedbymechanicalmixing. Itisforthisreasonthatconvectivemixingisconsideredthe
moreimportantofthetwo,andthepredominantprocessofwinter.
Theconvectiveprocessisstrongestinnorthernwaterswhereverticaltemperatureandsalinitygradients
arenotextremeandsurfacewatersundergoahighdegreeofcooling. Convectivemixingattributedto
salinitychangesismostnoticeableintheMediterraneanandRedSeas,whereevaporationfarexceeds
precipitation.
Wehavelookedatbothprocessesindividually;however,thetwoprocessescan,andoftendo,take
placesimultaneously. Whenthisoccurs,themixedlayernormallyattainsagreaterdepththanwouldbe
attainedbyeitherprocessindividually.
3.3.2.2MainThermocline
Themainthermoclineisthecentrallayerofthethreelayeredoceanmodel. Themainthermoclineis
foundatthebaseofthemixedlayerandis
markedbyarapiddecreaseofwater
temperaturewithdepth. Athighlatitudes
thereisnomarkedchangeinwater
temperaturewiththeseasons,whileinthe
midlatitudes,aseasonalthermocline
developswiththeapproachofsummer(See
Figure34). Thisseasonalthermocline
comesaboutfromthegradualwarmingof
the
surface
waters.
The
warming
takes
place
intheupperfewhundredfeetofthesurface,
andresultsintheseasonalthermocline
becomingsuperimposedonthemain
thermocline. Figure34illustratesthedevelopmentoftheseasonalthermoclineinthemidlatitudes.
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Themidlatitudesummerthermoclineismorepronouncedthanthethermoclineofspringorautumn.
Bathythermographtracesofthesummerthermoclineshowthatitaffectsamuchbroaderrangeof
depththanatanyothertimeofyear. Theseasonalthermoclineisroughly35metersthick(90to125
metersdeep). Note,also,thatawintertemperatureprofilewillshownoseasonalthermocline(See
Figure35). Comespring,thesurfacewateriswarmedandaseasonalthermoclinedevelops. Inlow
latitudes,smallseasonaltemperaturechangesmakeitdifficulttodistinguishbetweentheseasonaland
thepermanentthermoclines.
3.3.2.3DeepWaterLayer
Thedeepwaterlayeristhebottomlayerof
water,whichinthemiddlelatitudesexists
below1,200meters. Thislayerischaracterized
byfairlyconstantcoldtemperatures,generally
lessthan4C. Tobetterunderstandthebasic
verticaltemperaturedistribution,lookonce
againatfigure35. Athighlatitudesinwinter,
thewateriscoldfromtoptobottom. The
verticaltemperatureprofileisessentially
isothermal(nochangeintemperaturewith
depth). Inlowlatitudes,themixedlayer
extendstoadepthofabout300feet. Here,
themainthermoclineisencounteredandthe
temperaturedropsabout8Cmorethanit
doesinthemidlatitudes. Thissharperdropisduetothehighersurfacetemperatureinthelower
latitudes. Thethermoclineextendstoanaverageof2,100feet,wherethedeeplayerisencountered.
3.3.3WATERTYPESANDMASSESS
Theconceptofvisualizingwatermassesaswedoairmassesispossiblebecausebotharebasedonthe
physicalpropertiesthatgointotheirmakeup. Thepropertiesoftemperatureandsalinityareusedto
classifybothwatertypesandwatermasses.
Awatertypehasasinglevalueofsalinityandasinglevalueoftemperatureassociatedwithit,For
example,RedSeawaterisawatertypecharacterizedbyatemperatureof9Candasalinityof35.5.
Figure35. VerticalTemperatureProfileinWinter.(SourcePDC)
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Awatermasstakesintoaccountarangeoftemperaturesandsalinitiesandmaybeconsideredtobe
madeupofacombinationoftwoormorewatertypes. Forexample,NorthAtlanticCentralWater(a
watermass)ischaracterizedbyarangeoftemperatures(4Cto17C)andsalinity(35.1to36.2).
3.3.3.1Water
Mass
Formation
Thevastmajorityofwatermassesareformedatthesurfaceoftheseainmiddleandhighlatitudes.
Cold,highlydensesurfacewatersinksuntilitreachesalevelhavingthesameconstantdensity. Here,it
spreadsouthorizontally. Themannerinwhichitspreadsoutdependsonitsdensityinrelationtothe
densityofthesurroundingwater. Thisistrueofnearlyallwatermasses,exceptthoseoflowlatitudes
inparticular,theequatorialwatermassesoftheIndianandPacificOceans. Thesewatermassesare
formedbythemixingofsubsurfacewaters.
3.3.3.2Distribution
Inlowandmiddlelatitudestheverticalarrangementofwaterissuchthatwecandistinguishasurface
layer,upperwater(centralandequatorial),intermediatewater,deepwater,andinsomelocalities,
bottomwater. Inhighlatitudes,thelayeredstructureallbutdisappearsbecausethesurfacewateris
similartothewateratornearthebottom.
3.3.3.2.1SurfaceLayer
Thesurfacelayerisnotclassifiedaswatertypeorwatermass,becauseitspropertiesvarywidelyfrom
oneareatoanother,dependingonoceancurrentvariations,ratesofevaporationorprecipitation,and
variousseasonalchanges,especiallyinthemiddlelatitudes. Inlowandmiddlelatitudesitisfound
abovecentraland/orequatorialwatertodepthsof100to200meters. Thesurfacelayerisseparated
fromdeeperwaterbyatransitionlayer(themainthermocline). Beneaththesurfacelayer,we
encounterthewatertypesandwatermasses. Likeairmasses,thewatertypesandwatermasseshave
sourceregionsinwhichtheyform.
3.3.3.2.2CentralWaterMasses
Centralwaterisnormallyfoundinrelativelylowlatitudesalthoughitssourceregionisintheregionof
thesubtropicalconvergence(betweenthe35thand40thparallelsineachhemisphere). Convergences
areregionsintheoceanwheresurfacewatersarebroughttogetherbythecurrents. Inthewestern
NorthAtlanticOcean,aregionofsubtropicalconvergenceexistswheretheGulfStreammeetsthe
colder,denserLabradorCurrent. Convergencesaremarkedbyrapidlyrisingseasurfacetemperatures.
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Centralwaterisnotusuallydiscernibleatthesurfaceandisgenerallyrelativelyshallow. Itsgreatest
thicknessisobservedalongitswesternboundaries. InthewesternNorthAtlanticintheregionofthe
SargassoSea,thethicknessmayreach900meters. Variationsinheatingandcooling,evaporationand
precipitation,oceancirculationpatterns,andmixingprocessesallcontributetothesalinityvaluesof
centralwaterbeingeitherquitesimilarorconsiderablydifferent. Forexample,centralwaterofthe
SouthAtlanticOcean,theIndianOcean,andthewesternSouthPacificOceanallhavesimilarsalinity
values,whilethesalinityvaluesofNorthAtlanticcentralwaterareconsiderablyhigherthanthecentral
wateroftheNorth Pacific Ocean. CentralwateroftheNorthandSouthAtlanticoceansisnot
separatedbyequatorialwaterlikethecentralwateroftheNorthandSouthPacificoceans. Instead,the
centralwateroftheNorthandSouthAtlanticcometogetherandmix,formingaregionoftransition
consistingofintermediateproperties.
3.3.3.2.3EquatorialWaterMasses
EquatorialwaterisfoundinthePacificandintheIndianOcean. InthePacificitisthoughttooriginate
onthesouthernsideoftheequator. Therearetworeasonsforthis:Itspropertiesaresimilartothoseof
thewatermassesoftheSouthPacific,anditssalinityvaluesarehigherthanthoseofthewatermasses
foundintheNorthPacificOcean.
EquatorialwaterisalsofoundinthenorthernpartoftheIndianOcean. Here,itshighersalinitiesare
probablyduetoitsmixingwiththewatersoftheRedSea. However,thisconclusionhasnotbeen
substantiated. Equatorialwater,likecentralwater,isnotdiscernibleatthesurface,becausethe
temperatureandsalinityvaluesusedtoisolateitcannotbeclearlyascertainedintheupper100to200
meters.
3.3.3.2.4IntermediateWater
Intermediatewaterisfoundbelowcentralwaterinalloceans. TheseincludeAntarcticintermediate
water,Arcticintermediatewater,Mediterraneanwater,andRedSeawater.
3.3.3.2.4.1AntarcticIntermediateWater
AntarcticintermediatewaterencirclestheAntarcticcontinentandisthemostwidespreadofallthe
intermediatewatermasses. ItformsinthevicinityoftheAntarcticconvergence,whereitsinks. Asit
sinks,itflowsnorthandmixeswiththewatermassesthatlieimmediatelyaboveandbelowit. Inthe
Atlantic,theabsenceofequatorialwaterallowsAntarcticintermediatewatertoflowacrosstheequator
andreachroughly20Nto35Nlatitude. IntheSouthPacificandIndianoceans,whereequatorialwater
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doesexist,Antarcticintermediatewaterfailstoreachtheequator. Itspreadsnorthtoabout10S
latitude. OneofthecharacteristicsofAntarcticintermediatewaterisitslowsalinity(34.1to34.6
). Incomparisontothewateraroundit,itdisplaysthelowestsalinityvalues.
3.3.3.2.4.2Arctic
Intermediate
Water
ArcticintermediatewaterandsubArcticwateraresimilar;however,intheNorthAtlanticOcean,Arctic
intermediatewaterformsonlyinsmallquantities,andinarelativelysmallareaeastoftheGrandBanks
ofNewfoundland. IntheNorthPacific,Arcticintermediatewaterformsduringwinteratthe
convergenceformedbytheOyashiocurrentandtheKuroshioExtension. Itexistsbetweenlatitude20N
and43N,exceptoffthewestcoastofNorthAmerica. Here,subArcticwaterextendstolowerlatitudes,
andthenorthernboundaryoftheintermediatewaterispushedmuchfarthersouth.
3.3.3.2.4.3Mediterranean
Water
ThiswatermassisformedbytheinteractionofdenseMediterraneanSeawaterwithwatersofthe
adjacentNorthAtlanticOcean. ThemoredenseMediterraneanwaterflowsoutthroughtheStraitof
Gibraltarandsinkstoadepthofabout1,000meters,whereitmixeswiththewateratthisdepth.
3.3.3.2.4.4RedSeaWater
ThiswatertypeisfoundoverlargepartsoftheequatorialandwesternregionsoftheIndianOcean.
Largequantitiesofwarm,highlysalinewaterfromtheRedSeaflowintotheIndianOcean,whereits
mixeswithAntarcticintermediatewatertoformtheRedSeawatermass. ThespreadingofRedSea
waterisnotaswelldefinedasMediterraneanwater.
3.3.3.2.5AntarcticCircumpolarorSubAntarcticWater
Thiswatermassisthoughttoformthroughacombinationofmixingandverticalcirculationintheregion
betweenthesubtropicalandAntarcticconvergence. Here,largequantitiesofAntarcticintermediate
waterandAntarcticbottomwatermixwithNorthAtlanticdeepwatertoformAntarcticcircumpolar
water. Thephysicalpropertiesofthiswatermassarequiteconservative,andasitsnameimplies,it
extendscompletelyaroundtheAntarcticcontinentandtheSouthPole. BecauseAntarcticcircumpolar
waterformsinthedeeperwatersoftheAntarcticOcean,itisoftenreferredtoassubAntarcticwater.
3.3.3.2.6SubArcticWaterMasses
SubArcticwaterismuchlikeAntarcticcircumpolarorsubAntarcticwater;however,thereare
differences. Thedifferencesareattributedtothelandandseadistributioninthetwohemispheres. In
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theSouthernHemisphere,theAntarcticconvergenceextendsaroundthecontinentofAntarctica,butin
theNorthernHemisphere,theArcticconvergenceisfoundonlyinthewesternportionsofoceans.
However,evenintheseareastheconvergenceisnotalwayswelldefined. IntheNorthAtlanticOcean,
subArcticwatercoversarelativelysmallarea,anditpossessesahighersalinitythansurroundingwaters.
Ontheotherhand,thesubArcticwateroftheNorthPacificismuchmoreextensive,anditssalinity
valuesarelowerthansurroundingwaters.
3.3.3.2.7DeepandBottomWaterMasses
Inthedeepoceanbasinsbelowintermediatewater,highdensitydeepandbottomwaterexists. These
watermassesforminbothhemispheres. IntheSouthernHemisphere,Antarcticbottomwaterforms
neartheAntarcticcontinent,whileintheNorthernHemisphere,Arcticdeepandbottomwaterformsin
northwesternLabradorBasinandinasmallareaoffthesoutheastcoastofGreenland. Thesewater
massesformatthesurface,sink,andspreadouttofillthedeepoceanbasins. Deepandbottomwaters
aredetectableinareasfarremovedfromtheirsourceregions. Moreinformationonthespreadingof
deepandbottomwaterispresentedinthefollowingdiscussionondeepoceancirculation.
3.3.3.2.8DeepOceanCirculation
Methodsdevisedtodeterminedeepoceancirculationhavemetwithvaryingsuccess,butallpointtoa
quitecomplexpatternofsubsurfacecurrents. Thedeepoceancurrentsdifferfromsurfacecurrentsin
thattheyare:
Densitydriven.
Muchslower.
Moveinapredominantlynorthsouthdirection.
Crosstheequator.
Thedeepoceancirculationisoftenreferredtoasathermohalinecirculation,becausethecirculationis
controlledbydifferencesintemperatureandsalinity. Varyingcombinationsoftemperatureandsalinity
producewaterofvaryingdensities,anditisthesedensitydifferencesthatproducedeepocean
circulations.
Sincethemajorityoftheworld'swatermassesareformedatthesurface,ourdiscussionofthedeep
oceancirculationmuststartthere. Wewillmovethroughthecirculatorypattern,beginningandending
withthesurfacewatersaroundAntarctica. AsthehighdensitysurfacewateraroundAntarcticasinks,it
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mixeswiththewarmer,moresalinecircumpolarwatertoformAntarcticbottomwater. Because
Antarcticbottomwateristhedensestwaterfoundintheocean,itsinkstotheoceanfloorandspreads,
orflows,northwardintothedeepoceanbasinsoftheAtlantic,Pacific,andIndianOceans. Thiswater
masshasbeentrackedasfarnorthasthe35thparalleloftheNorthernHemisphere.
InthesubArcticregionsoftheNorthernHemisphere,thesametypeofprocessoccurs. Thecold,dense
surfacewatersinksandformsNorthAtlanticdeepandbottomwater. Thiswatermassspreads
southwardandisincontactwiththebottom,exceptwhereitencountersAntarcticbottomwater.
BeinglessdensethanAntarcticbottomwater,itisfoundaboveAntarcticbottomwaterwhereverthe
twocoexist.
TheNorthAtlanticdeepandbottomwatereventuallymakesitswaybacktotheAntarcticOcean,where
itmixeswithintermediatewatermassesandAntarcticbottomwatertoformAntarcticcircumpolar
water. Here,thecyclebeginsagainasthecold,densesurfacewaterofAntarcticasinksandmixeswith
thecircumpolarwater. Abovethedeepandbottomwaters,theintermediatewatermassesalsoshowa
basicequatorwardmovement. Antarcticintermediatewateractuallycrossestheequatorandmovesas
farnorthas20to35N. ItsNorthernHemispherecounterpart,Arcticintermediatewater,movessouth
butdoesnotcrosstheequator. MediterraneanandRedSeawaterbothcrosstheequator,andhave
beenidentifiedfarintotheSouthernHemisphere. TheCentralandEquatorialwateroflowandmiddle
latitudesmovepolewardintheirrespectivehemispheres,whileinhighlatitudesthenearsurfacewaters
move
toward
the
equator.
The
Atlantic
circulation
is
considered
much
more
vigorous
than
that
of
the
Pacific,becausesurfacedensitycontrastsaremuchgreater. However,evenwiththegreatersurface
densitycontrasts,thecirculationisSLOWVERYSLOW. Thedeepseacurrentsassociatedwiththe
deepoceancirculationflowatarateofafewcentimeterspersecondorless. Ifwewereabletofree
floatabottleatadesignateddepth,thisrateofspeedwouldequatetothebottlemovinglessthan2
degreesoflatitude(120nauticalmiles)inayear,or0.0137knots.
Wecansaythedeepoceancirculationconsistsprimarilyof(1)equatorwardflowingsubsurfacewater
whichmovesatanextremelyslowrateofspeedand(2)themuchfasterpolewardflowingsurfacewater.
3.4 HYDROACOUSTICS
LearningObjectives
Identifythevariouspropertiesofsoundwaves.
Defineenergylossorspreadinglossasitpertainstosoundwaves.
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DefineDopplerEffect.
RecognizehowtheDopplerEffectaffectsthepitchandfrequencyofsound.
Definesoundvelocity.
Describetheeffectoftemperature,pressure,andsalinityonsound.
Explainwhysoundpropagatesalongmoreorlesscurvedpaths.
Describethefivebasicsoundraypatternsandtheirattendanttemperatureandsoundvelocityprofiles.
Differentiatebetweenactiveandpassivesonar;definethemodesofactivesonarsearch.
Describethepropagationpathsusedwitheachmodeofactivesonar.
Defineanddifferentiatebetweentheelementsusedintheactiveandpassivesonarequations.
Hydroacousticsisthestudyofsoundinwater. InthecaseoftheNavy,itisthestudyofsoundenergyin
seawater.
TheNavy'sgreatestinterestinhydroacousticsisrelatedtosubmarineandantisubmarinewarfareor
morepreciselytheeffectofseawateronsonar. Certainpropertiesofseawatercontrolsoundasit
propagatesthroughthewater. Theireffectmayaidorhindersonaroperations.
3.4.1 PROPERTIESOFSOUND
Soundin
the
world
of
oceanography
has
asignificant
meaning.
It
is
necessary
to
be
familiar
with
some
ofthefundamentalconceptsconcerningthepropertiesofsound.
3.4.1.1 SoundProduction
Soundisthephysicalcauseofhearing.
Beforesoundcanbeproduced,three
basicelementsmustbepresent:asound
source,amedium,andadetector.
(RefertoFigure36)
3.4.1.1.1 Source
Anyobjectthatvibratesordisturbsthe
mediumarounditmaybecomeasoundFigure36 PropertiesofSound. (Source:PDC)
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source. Thesoundsourceistheinitialrequirementintheproductionofsound.
3.4.1.1.2 Medium
Themediumistheelementthatcarriessound. Airactsasamediumintheatmosphere. Particlesinthe
aircarrythesoundsthatyouheareveryday. Noisescanalsobeheardunderwaterwhereparticlesinthe
watercarrysound.
Themediumisthecontrollerofsound. Itcontrolshowfarandhowfastsoundtravels. Soundtravels
faster,farther,andwithmoreeasethroughmediumsofhighelasticityanddensity. Solidsarebetter
transmittersofsoundthaneitherliquidsorgases.
3.4.1.1.3 Detector
Adetectoractsasareceiverofsound. Thedetectorallowsustotellwhethersoundhasbeenproduced.Soundtravelsinwavesthatmoveradially(360)fromtheirsource,andonlyasmallpartofawaves
energyreachesadetector. Therefore,detectorsoftencontainamplifierstoboostasignalsenergy
permittingreceptionofweaksignals.
3.4.1.2 SpeedofSound
Thespeedofsoundinairisapproximately331.5m/secat0C. Soundspeeddecreasesatlower
temperaturesandincreasesathighertemperatures. Soundspeedincreasesatarateof
approximately3.2m/secforevery1Cincreaseintemperature.
Thespeedofsoundinwaterisabout4timesgreaterthanthespeedofsoundinair. Seawateris
denserthanfreshwater;therefore,atthesametemperature,thespeedofsoundinseawaterwillbe
slightlygreaterthanthespeedofsoundinfreshwater.
Insteel,soundspeedisabout15timesgreaterthaninair. Soundtravelsatapproximately5,200
m/secthroughathinsteelrod.
3.4.1.3
Sound
Waves
Soundtravelsintheformofwaves. Soundwavesarebroughtaboutbyvibrationswithinamedium.
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3.4.1.3.1 Wavelength
Thelengthofasoundwaveisthedistancebetweenanytwosuccessivecompressionsorrarefractions.
Onecompletewavelengthiscalledacycle. Wavelengthsvarydependingonthenumberofcyclesper
secondproducedbythesoundsource.
3.4.1.3.2 Frequency
Thenumberofcyclespersecondisameasureofasoundsfrequency,thehigherthefrequency,the
shorterthewavelengths. Theoppositealsoapplies,thelongerthewavelengththelowerthefrequency.
FrequenciesaremeasuredintheHertzsystem. 1hertz(Hz)isequalto1cyclepersecond. Frequencies
of1000Hzormorearemeasuredinkilohertz(kHz). Theaveragehumanhearssoundsbetween20Hz
and15kHz,whilesoundsbelow20Hzandabove15kHzarenormallybeyondthehumanrangeof
hearing.
3.4.1.3.3 Pitch
Thepitchofasounddependsonthefrequencyofthesoundasreceivedatadetector. Thehumanear
detectssoundsandclassifiesthembasedonthesoundquality. Somesoundsareharsh,whileothersare
pleasant. Pitchisasubjectivequalitydependentonthereceiver.
3.4.1.3.4 IntensityandLoudness
Intensityandloudnessareoftenmistakenashavingthesamemeaning. Althoughrelated,theyarenot
thesame. Intensityisameasureofasoundsenergy,whileloudnessistheeffectonthedetector. If
soundintensityisincreased,theloudnessisincreasedbutnotindirectproportion. Todoublethe
loudnessofsoundrequiresaboutatenfoldincreaseinthesoundsintensity.
Soundintensityismeasuredindecibels(dB). Adecibelistheunitusedtoexpressrelativeintensity
differencesbetweenacousticsounds. Decibellevelsareassignedbasedonasoundsintensity
comparedtoanestablishedstandard. Somecommonintensitylevelsareasfollows:awhisper,10to20
dB;heavystreettraffic,70to80dB;thunder,110dB.
3.4.1.4 EnergyLoss
Asasoundwavemovesawayfromitsource,itspreadsout. Theenergywithinthewavedecreasesas
thewavespreadsthroughanincreasinglylargearea. Thewaveenergyperunitareadecreasesasthe
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distancefromthesoundsourceincreases. Thislossofenergyduetodistanceisknownasspreading
loss.
3.4.1.5 DopplerEffect
TheDopplerEffectistheapparentchangeinasoundduetomotion. Itisachangeinpitchwithouta
frequencychangeoccurring. Thechangeinpitchisbroughtaboutbytherelativemotionofasound
sourceandadetector. Forexample,wehearthewhistleofanapproachingtrain. Thefrequencyofthe
whistledoesnotchangeasthetrainapproaches,butourearsdetectanincreaseinthepitch. The
increaseinpitchiscausedbythecompressionofsoundwaves. Thetrainactstopushthesound
wavestowardus. Thesoundwavesarriveatafasterratethantheywouldifthetrainwasnotmoving.
Then,asthetraingoesby,thesoundwavesarriveatamuchslowerrate. Thetrainisnowpushingthe
soundwavesawayfromus. Thesoundwavestotherearofthetrainspreadfartherapartasthetrain
movesfartherawayfromourposition,andtheeffectisoneoflowerpitch.
3.4.2 SOUNDPROPAGATIONINSEAWATER
Thewordpropagateistheactofsomethingmovingthroughamedium. Inthisinstance,themedium
iswaterandsoundmovesthroughit. Theseainfluencessoundinmanywaysasitmovesthroughthe
water.
3.4.2.1 SoundVelocity
SoundVelocitytakesintoaccountthespeedanddirectionofsoundrays. Thedirectionorpaththat
soundenergytakesasitmovesthroughthewaterisprimarilyafunctionofsoundspeed.
3.4.2.1.1 SoundSpeed
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Thespeedofsoundintheseaisafunctionofwatertemperature,pressure,andsalinity. Ofthesethree
variables,temperatureisthemostimportant. Itistheprimarycontrollerofsoundspeedanddirection,
intheupper300meters(1,000feet)ofseawater. Ingeneral,soundspeedincreases3.2m/secforevery
1Cincreaseintemperature.
Theeffectofpressureonsoundspeedisa
functionofdepth. Pressureincreaseswith
depthandsoundspeedincreaseswith
higherpressure. Soundspeedincreases
approximately1.7m/secper100metersof
depth. Pressureisthedominantsound
speedcontrollerbelow300meters,because
below300meters,thetemperatureis
relativelyconstant.
Theeffectofsalinityonsoundspeedisslight
intheopensea,becausesalinityvaluesare
nearlyconstant. Theaffectofsalinityon
soundspeedisgreatestwherethereisa
significantinfluxoffreshwaterorwheresurfaceevaporationcreateshighsalinity. Aonepartper
thousand
(1)
increase
in
salinity
increases
sound
speed
1.4
m/sec.
3.4.2.1.2 SoundVelocityProfile(SVP)
Asoundvelocityprofileisagraphicrepresentationofspeedversusdepth. (Seefigure37)SVPsprovide
surfacesoundspeed,depthofmaximumsoundspeed(soniclayerdepth),andlayerswheresound
travelsgreatdistances(ductsandsoundchannels).
3.4.2.1.3 SonicLayerDepth(SLD)
Thesoniclayerdepthisthedepthofmaximumnearsurfacesoundspeedabovethedeepsound
channelaxis. Anegativetemperaturegradient(temperaturedecreasingwithdepth),within
certainlimits,compensatesforanincreasein soundspeedwithdepthdueto pressure;this
resultsinaconstantsoundspeedwithdepth. The gradientlimitsper 30metersofdepthare as
follows:
0.1Cper30metersinwater4.4C
Figure37. SoundVelocityProfile.(Source: PDC)
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0.17Cper30 meters in water12.8C
0.22Cper30 meters in water18.3C
Temperaturegradients morenegative thanthoselistedaboveresultin decreasingsound
speedswith
depth.
Gradients
more
positive
than
those
listed
above
result
in
increasing
soundspeedswithdepth.
3.4.2.2 SoundPaths
Assoundenergyleavesasoundsource,ittravelsinwaves. Thesoundwavesexpandastheymove
awayfromthesource. Asoundwave'spathoftravelisdependentonitsspeedandanymatterinits
path. Sound,likelight,isrefracted,reflected,andscattered. Thesurfaceorobjectstruckdetermines
ifthesoundenergyisrefracted,reflected,scattered,orabsorbed.
3.4.2.2.1 Refraction
Asasoundwavemovesthroughthesea,ittravelsalongacurvedpath. Thepathiscurved,because
soundspeedvariesalongthewavefront. Soundwavesbend(arerefracted)inthedirectionofthe
slowersoundspeeds. Thisisthefundamentalprincipleofsonarrangepredictionandisderivedfrom
Snell'slaw. Snell'slawstatesthatasoundraypropagatingthrougharegionwithonesoundspeedwill
changedirection(berefracted)onenteringaregionhavingadifferentsoundspeed. Thedegreeof
refractionisproportionaltothesoundspeedgradient. Refractionincreaseswithagreaterchangein
speedoveragivendistanceordepth. Thegradientisafunctionofspeedversusdepthordistance.
Forexample,inalayerofwaterwheresoundspeeddecreasesrapidlywithdepth,soundwavesbend
sharplydownward.
3.4.2.2.1.1 StraightRays
Figure38illustrateshowsoundraystravelin
straightlinesonlywherethespeedisconstant
(isovelocity);nochangeinvelocitywithdepth.
Straightsoundraysoccurwhenthetemperature
profileisslightlynegative(adecreaseofabout1C
per30metersofdepth). Longsonarrangesare
possiblewhenthistypeofprofileexists.
Straight Rays
Figure38. StraightSoundRays.(Source:PDC)
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3.4.2.2.1.2 RaysCurvedDownward
Anegativetemperaturegradient
(temperaturedecreasingwithdepth)
producesanegative
velocity
gradient.
The
soundraysleavethesonarandarebent
downward(asshowninFigure39),thereby
limitingsonartoveryshortdetectionranges.
Forexample,adecreaseintemperature
of.56Cinthefirst10meterscausesthe
soundbeamtomissashallowtargetatarangeof1km. Thisisacommonoccurrenceinthenear
surfacelayer. Beyondtherangeofthedownwardbendingsoundrays,soundintensityisnegligible.
Thisareaisknownasashadowzone.
3.4.2.2.1.3 RaysCurvedUpward
Apositivetemperaturegradient(temperature
increasingwithdepth)producesapositivevelocity
gradient. Thesoundraysleavethesonarandare
bentupwardtowardtheseasurface.(Referto
Figure310.)
Longer
ranges
are
attained
with
this
typeofgradient,especiallyiftheseaisrelatively
smooth. Astheraysbendupwardandstrikethe
seasurface,theyarerepeatedlyreflectedbackintothelayerandfurtherrefractedupwardtowardthe
surfaceaslongastheyremainintheareaof
positivevelocitygradient.
3.4.2.2.1.4 SplitbeamPattern
Asdepicted
in
Figure
311,
asplit
beam
pattern
occurswhenthetemperaturegradientinthenear
surfacelayerisisothermal,andnegativebelow.
Soundraysfromasonarsplitatthedepthofthe
gradientchange. Partofthesoundraysare
Rays CurvedDownward
Figure39. SoundRaysCurvedDownward.(Source:PDC)
Rays CurvedUpward
Figure310.SoundRaysCurvedUpward.(Source:PDC)
Split-BeamPattern
Figure311.SplitbeamSoundRayPattern.(Source:PDC)
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refractedupwardtowardthesurface,andpartarerefracteddownwardtowardthebottom. Atthe
pointwheretherayssplit,ashadowzoneexists. Asubmarineoperatingatthesplitdepthimproves
itschancesofavoidingdetection.
3.4.2.2.1.5
SoundChannel
Asoundchanneloccurswhenanegativevelocitygradientexistsaboveanisovelocityorpositive
velocitygradient. Thedepthwherethevelocitygradientchangesfromnegativetopositiveistheaxis
ofthesoundchannel. Theaxisisthelevelofminimumsoundspeed. Thesoundraysonbothsidesof
theaxistravelfasterthantheraysinthecenter. Sincesoundrefractstowardslowersoundspeeds,
thefasterraysarecontinuallyrefractedtowardtheaxis.
3.4.2.2.2 Reflection
Soundwaves
that
strike
solid
surfaces
have
all
or
aportion
of
their
energy
redirected
or
absorbed.
Reflectedsoundenergycanbegoodorbad. Thetypeorqualityofreflectedsoundisdependenton
thesurfacefromwhichthesoundbounces. Bottomroughnesscanbeslightorgreat,andthe
wavelengthcomponentofthereflectedsoundcanrangefrommicronstomiles. Asmoothrockocean
bottomisperhapsthebestreflectorofsoundinthesea. Asmoothsandbottomalsoreflectssound
veryeffectively. Theseasurface,ifitiscalm,isalsoagoodreflector. Soundwavesbounceoffsuch
surfacesandloselittleoftheirenergy.
3.4.2.2.3
Scattering
Anirregularhardsurfaceisnotagoodreflector. Thesoundwavesarereflectedinmanydifferent
directionsandlosemostoftheirenergy. Thistypeofenergylossisknownasscattering.
Soundenergyintheseaisscatteredbytheseasurface,seafloor,andsuspendedmatter. Becausethe
seasurfaceisrarelysmooth,itismoreapttoscattersoundthantoreflectit. Aroughorrockybottom
alsodispersesorscatterssoundenergy.
3.4.2.2.4
Reverberation
Reverberationisnoiseorinterferenceatasonarreceiver,whichmakestargetdetectionverydifficult.
Thisinterferenceiscausedbyscatteredsoundenergybeingreflectedbacktothesonarreceiver.
Therearethreetypesofreverberation:surface,volume,andbottom.
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3.4.2.2.4.1 SurfaceReverberation
Surfacereverberationisaproductofsurfacewaveaction. Atshortranges,surfacescattering
increaseswithwindspeedsbetween7and18knots. Above18knots,afurtherincreaseinthe
surfacereverberationlevelispreventedbyasoundscreenofentrappedairbubbles. Theairbubbles
formnearthesurfaceandarecausedbythewaveaction.
3.4.2.2.4.2 VolumeReverberation
Volumereverberationiscausedbyscatterersorreflectorsinthewatersuchasfish,marineorganisms,
suspendedsolids,andbubbles. Volumescatterersarenotuniformlydistributedindepth,buttendto
beconcentratedinadiffuselayerknownasthedeepscatteringlayer.
Thedeepscatteringlayerisfoundintropicalwatersatdepthsbetween100and400fathoms. The
intensityof
the
scattering
is
afunction
of
sonar
frequency
and
the
density
of
the
organisms
in
the
layer. IntheNorthernHemisphere,themaximumvolumereverberationoccursinMarchandthe
minimuminNovember.
3.4.2.2.4.3 BottomReverberation
Bottomcompositionandroughnessgovernthedegreeofreverberationthatcontributestothe
maskingoftargetechoes. Intheory,theamountofbottomreverberationisdirectlyrelatedtothe
roughnessandcompositionoftheseafloor. However,theproblemofbottomreverberationisabit
morecomplicated.
Scientists
consider
the
ocean
floor
to
be
atwo
dimensional
volumetric
scattering
surface. Inotherwords,soundisnotonlyreflectedofftheseafloorbutalsofromformationsofrock
beneaththeseafloor.
3.4.2.2.4 Absorption
Absorptionisaproblemattheoceanbottom. Whenthebottomiscomposedofsoftmud,sound
energyisabsorbed. Absorptionalsooccursassoundpropagatesthroughthesea,andtheenergyis
convertedtoheat.
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3.4.2.2.5 Attenuation
Attenuationistheenergylossthatoccursinpropagatedsoundwavesduetoscatteringand
absorption. Itcanalsobedefinedastheconversionofthemechanicalenergyinasoundwavetoheat.
3.4.3Active
And
Passive
Sonar
Sonar(SoundNavigationandRanging)wasoriginallydesignedtoassistsurfaceshipswithnavigation
duringbadweather. Later,sonarwasemployedonsubmarinesforacousticlocationoftargets,and
today,itisourprimarymeansoflocatingsubmarines. Therearetwotypesofsonarsearches:activeand
passive. Activesonaremploysatransmittertosendoutsoundpulsesandareceivertorecordreturning
echoes. Passivesonarlistensforsoundsgeneratedbyothershipsandsubmarines.
3.4.3.1 ActiveSonar
Activesonarsearchisclassifiedintotwomodes:shallowwatertransmissionsanddeepwater
transmissions. Theoretically,theessentialdifferencebetweenshallow anddeepwatertransmissionsis
theinterferenceeffectsproducedbythemultiplereflectionsofsoundinshallowwater.
Shallowwaterisclassifiedaswaterlessthan100fathoms. Deepwaterisclassifiedaswater1,000
fathomsordeeper. Waterbetween100and1,000fathomsdeepismostcommonovercontinental
slopes. Itisnotconsideredoverlyimportantinactivesonaroperationsbecauseitexistsinsuchasmall
portionoftheworld'soceans.
3.4.3.1.1Shallow
Water
Transmissions
Shallowwaterpropagationpathsareclassifiedaseitherdirectpathorsurfaceduct.
3.4.3.1.1.1 DirectPath
DirectPathisthesimplestmode. Directpathsoundpropagationoccurswherethereisanapproximate
straightlinepathbetweenthesoundsourceandreceiver,withnoreflectionfromanyothersourceand
onlyonechangeofdirectionduetorefraction.
3.4.3.1.1.2Surface
Duct
Asurfaceductissimplyanearsurfacelayerthattrapssoundenergy. Surfaceductsexistintheoceanif
oneofthefollowingconditionsaremetwithinthemixedlayer: (1) temperatureincreaseswithdepth,
(2)temperaturethroughthemixedlayerisisothermal.
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Incondition1,soundvelocityincreasesasthetemperatureincreasesproducingalayerwithapositive
soundvelocitygradient. Incondition2,anisothermallayernearthesurface,pressurebecomesthe
dominantfactorandsinceanincreaseinpressurecausesanincreaseinsoundspeed,alayerwitha
positivesoundvelocitygradientisagainproduced. Thegreaterthedepthofaduct,themoresound
speedincreaseswithdepthproducinganevengreaterdifferencebetweenthesurfacevelocityandthe
velocityatdepth. Ductsthatextendtogreaterdepthstrapagreaternumberofsoundwavesandcan
extenddetectionrangestoverylongdistances. Theefficiencyofanysurfaceduct,nomatterthedepth,
ishighlydependantonthesmoothnessoftheseasurface. Waveactioncausesreverberationand
scattering,bothwhichreducetheefficiencyofasurfaceduct.
3.4.3.1.1.3 EnvironmentalControls
Thesuccessofactivesonarsearchesinshallowwaterdependsagreatdealonenvironmentalfactors.
Horizontalandverticaltemperaturegradients,waterdepth,andthephysicalcharacteristicsofthesea
surfaceandbottomallimpactshallowwatertransmissions. Ofthesecontrols,waterdepthisthemost
important.
3.4.3.1.1.3.1 TemperatureGradient
Variationsintheverticaltemperaturegradient,whichresultinsoundspeedvariations,areofutmost
importancewheresoundispropagatedthroughasurfaceduct. Achangeingradientof.2Cper30
meterscanbethedifferencebetweenanexcellentductwithgoodrangesandnoductandpoorranges.
Horizontalvelocitygradientsintheoceanarenotasgreatasthoseinthevertical;however,theycan
completelydestroyaductiftheyoccurbetweenthesoundsourceandthetarget.
3.4.3.1.1.3.2 WaterDepth
Waterdepthdeterminestherangeandangleatwhichsoundraysstrikethebottomandtosomeextent
thetypesoftransmissionpathsthatoccur.
Inshallowwater,asindeepwater,thesoundvelocityprofilecontrolsthedegreeofrefractionofsound
rays. Foranexampleofhowsimilarprofilesaffectshallow anddeepwatertransmissions,considerthe
following: Indeepwater,whereastrongnegativegradientexists,soundraysarerefracteddownward
andresultinshadowzones. Ontheotherhand,inshallowwaterthedownwardrefractedraysreflect
offthebottom,travelupward,andreflectofftheseasurface,andthentravelbacktowardthebottom.
Thisprocesscontinuesuntiltheshadowzoneiscompletelysaturatedwithenergy,resultinginvastly
improvedprobabilitiesofdetection.
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3.4.3.1.1.3.3 BottomComposition
Bottomcompositionandroughnesscontrol,toalargeextent,thereflectiveandabsorbentcapabilities
ofthebottom. Shallowwaterbottomcompositionandtopographycontrolthereflectivecapabilitiesof
thebottomandtheattenuationofsoundenergy. Shallowwatersedimentsarequitediversewithareas
ofmud,sand,mudsand,gravel,rock,andcoralnotuncommonovershelfregions. Topographyand
bottomcompositionalsocontrolthedegreeofreverberationthatcanmasktargetechoes.
3.4.3.1.2 DeepWaterTransmissions
Indeepwater,soundcantravelfromandtothesonarviasurfaceduct,convergencezone,bottom
bounce,andsoundchanneltransmissionpaths.
3.4.3.1.2.1 SurfaceDucts
Surfaceductsoccurindeepwaterjustastheydoinshallowwater.
3.4.3.1.2.2 SoundChannels
Asoundchannelisformedwhenanegativevelocitygradientexistsaboveapositivevelocitygradient.
Thethermalgradientnecessarytoproduceasoundchannelisnegativeoverisothermalornegativeover
positive. Thesoundchannelaxisisfoundatthepointofminimumsoundvelocity,wherethesound
velocityprofilechangesfromnegativetopositive. Asoundchanneltrapssoundraysandprovides
extremelylongranges. Theverticaltemperatureprofilesthatproducesoundchannelscanbefoundin
bothshallowanddeepwater.
3.4.3.1.2.2.1 ShallowSoundChannels
Shallowsoundchannelsarefoundinthenearsurfacelayer. Theyarerareandtransitory,butcanoccur
inshallowwaterduetotheintermixingofwatersdifferingintemperatureand/orsalinity. Asthese
watersintermingleandmixaccordingtotheirdensitycharacteristics,weakshortlivedsoundchannels
result. Theseshallowsoundchannelsareseldomofsufficientextentorpersistencetobetactically
usefulinunderseawarfareoperations.
InthePacificOcean,shallowsoundchannelsaremostcommonintheareanorthof40Nbetween
HawaiiandthecontinentalUnitedStates. IntheAtlantic,theyaremostfrequentlyobservedinthe
vicinityoftheGulfStream.
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3.4.3.1.2.2.2 DeepSoundChannels
Deepsoundchannelsarefarmoreprevalentthantheirshallowcounterpart. Inthedeepocean,
temperaturegenerallydecreaseswithdepththroughthemainthermocline. Thisproducesanegative
velocitygradientandsoundraysrefractdownward.
IntheAtlantic,suchgradientsexisttoadepthofapproximately700fathoms. Below700fathoms,the
gradientbecomesisothermal,whileinthePacific,theisothermallayerbeginsaround500fathoms.
Belowthesedepths,increasingpressurebecomesthedominantfactoraffectingsoundspeedanda
positivevelocitygradientisproducedrefractingsoundraysupward. Thedeepsoundchannelaxisexists
attheminimumdeepsoundspeedwherethevelocitygradientchangesfromnegative(throughthe
mainthermocline)topositive(throughthedeeplayer). Thedepthofthedeepsoundchannelaxis
variesfrom1,225metersinthemidlatitudestonearthesurfaceinthePolarRegions.
Extremelylongsonarranges(ontheorderofthousandsofmiles)arepossiblewithinadeepsound
channel.
3.4.3.1.2.3 ConvergenceZone
Thissoundtransmissionpathisbasedontheprinciplethatsoundenergyfromashallowsourcetravels
downwardinthedeepoceanandisrefractedandupwardtowardthesurfaceatadepthwherethe
deepwatersoundvelocityequalsthenearsurfacemaximumsoundvelocity. Thesoundraysare
refractedandfocusedupwardandreflectoffthesurfaceabout30milesfromthesoundsource. The
reflectedraysthentraveldownward,andthepatternrepeatsitself. Thesoundraysreappearinthe
surfacelayeratsuccessiveintervalsofabout30milesandmaycontinueforseveralhundredmiles.
Therearetwoconditionsnecessaryforconvergencezonetransmission: (1)Thesoundvelocityindeep
watermustbeequaltoorgreaterthanthenearsurfacemaximumsoundvelocity(thisiscalledCritical
Depth)and(2)waterdepthbelowthecriticaldepthmustbegreatenoughtopermittherefractedsound
raystoconvergeinasmallareaatthesurface,thisiscalledDepthExcess.
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3.4.3.1.2.4 BottomBounce
Bottombouncetransmissionusessharplyangledraypathstoovercomevelocitygradientchanges. The
soundenergyisdirecteddownwardatahighangleofincidencetothebottom(Figure312). With
steeplyinclinedrays,transmission
isrelativelyfreefromthermal
effectsatthesurface,andthe
majorpartofthesoundpathisin
nearlystablewater. Thesound
energyisaffectedtoalesser
degreebyvelocitychangesthan
themorehorizontalraypathsof
other
transmission
modes.
Longrangescanoccurinwater
deeperthan1,000fathoms,dependingonthebottomslopeandbottomcomposition. Itisestimated
that85%oftheoceanisdeeperthan1,000fathoms,andbottomslopesaregenerallylessthanorequal
to1degree. Onthisbasis,relativelysteepanglescanbeusedforsinglebottomreflectiontoarangeof
approximately20,000yards. Atshallowerdepths,multiplebouncepathsdevelopwhichproduce
scatteringanditshighintensityenergyloss.
The
geologic
composition
of
the
ocean
bottom
has
an
extremely
significant
effect
upon
the
final
strengthofbottomreflectedsound. Dependingoncomposition,suchinterrelatedeffectsasreflection,
absorption,scattering,attenuation,andreverberationcomeintoplay. Factorsthatincreasethesound
reflectivityofthebottomare:(1)anincreaseinthecalciumcarbonatecontentofthesediments,(2)a
decreaseinporoussedimentandcompaction,(3)anincreaseinthemeandiameterofsediment
particles,(4)anincreaseinthedegreeofcementationorrigidity,(5)anincreaseinthetemperatureof
thesediments. Energylossintobottomsedimentsdependsprimarilyuponbottomcomposition. The
NavalOceanographicOffice,StennisSpaceCenterproducesbottomlosschartsandbottomtypelosses
forvariousbottomtypesarediscussedinNavalMeteorologyandOceanographyCommandInstructions.
3.4.3.1.2.5 ArcticandHalfChannelPropagation
IntheArcticOceanregion,thelackofsolarheatingpreventstheformationofthemainthermocline
evidentinthelowerlatitudeoceans. Apositivesoundspeedgradientextendsuptoshallowdepthsin
thesummerandallthewaytotheiceboundaryinthewinter.
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Inthesummer,inopenwater,athinsurfaceduct(normally 100feet)canoccur. Strongsalinity
generatedpositivesoundspeedgradientscanoccurinthesurfaceregionduetomeltingiceorfresh
waterrunofffromriversnearcoastalregions;therebyremovinganysolargeneratednegative gradients.
Theinteractionofupwardrefractedenergywiththeundericesurfaceisdependantupontheroughness
oftheice,whichservesasthemajorcauseofattenuation. Duetotheupwardrefractionoftheenergy
andthedominanteffectoftheicecoveronattenuation,bottombounce,orinteractionwiththe
seafloorisaminorsourceofpropagationlossintheArcticRegion.
3.4.3.2 ActiveSonarEquationSE=SL+TSRDNL+DI2PL,
WhereSE=Signalorechoexcess,SL=Sourcelevel,TS=Targetstrength,RD=Recognitiondifferential,
NL=Noiselevel,DI=Directivityindex,2PL=Twowaypropagationloss.
Whenreverberationdominates,theequationmaybewrittenSE=SL+TSRDRL2PL,whereSE=
Signalexcess,SL=Sourcelevel,TS=Targetstrength,RD=Recognitiondifferential,RL=Reverberation
level,and2PL=Twowaypropagationloss.
3.4.3.2.1 SignalExcess(SE)
Signalexcessistheamountofsoundenergyreceivedfromatargetoverandabovetheamount
requiredtodetectit. Signalexcessisbasedonprobabilityconditions. Whenthesignalexcessiszero,
theprobabilityoftargetdetectionisconsideredtoberoughly50%. Signalexcess,likealloftheother
factorsoftheequation,isexpressedindecibels.
3.4.3.2.2 SourceLevel(SL)
Sourcelevelofthesonarprojectorpertainstotheintensityoftheradiatedsound,indecibels,relative
toareferencedintensity. Sourceleveliscontrolledbythedesign,maintenance,andsonarmodeof
operation.
3.4.3.2.3Recognition
Differential
(RD)
Recognitiondifferentialpertainstotheabilitytodifferentiatetargetnoisefrombackgroundnoise. Itis
afunctionoftargetdesign,maintenance,atarget'smodeofoperation,andtheexperienceofthesonar
operatortodetectatargetthroughthebackgroundnoise. Recognitiondifferentialwasoriginally
definedasthesignaltobackgroundnoiseratiorequiredatthereceivertorecognizeatarget50%ofthe
time. However,usingthe50%probabilityresultedintoomanysignalsbeingclassifiedastargetsthat
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werenottargets. TheinordinateamountoffalsealarmsledtoamorespecificqualificationofRD.
Today,RDcanapplytoaspecificprobabilityofdetectionandaspecifiedprobabilityofafalsealarm.
3.4.3.2.4 TargetStrength(TS)
Thetargetstrengthofareflectingobjectistheamountbywhichtheapparentintensityofsound
scatteredbytheobjectexceedstheintensityoftheincidentsound. Thisvaluedependsonthesize,
shape,construction,typeofmaterial,roughness,andaspectofatarget,aswellastheangle,frequency,
andwaveformoftheincidentsoundenergy.
3.4.3.2.5 NoiseLevel(NL)
Noiselevelpertainstoambientnoiseandselfmadenoiseatthelocationofthesonar. Noiselevelisa
functionoftheenvironmentandship'sspeed.
3.4.3.2.6 PropagationLoss(PL)
Propagationlossisthelossofsignalstrengthbetweenthesonarandthetarget. Intheactivesonar
equation,PLisatwowaylossofenergysincesoundenergytravelsoutfromthetransmitterthen
reflectsoffthetargetbacktothereceiver. Propagationlossinwaterdependsonthefollowingfactors:
Spreadingofthesoundwavefront.Thefartherthesoundwavemovesfromthesource,thegreaterthesizeof
thewavefrontandthespreadingofthesoundenergy.
Conversionofthemechanicalenergyinasoundwavetoheat(attenuation).
Scatteringduetosurface,bottom,andsuspendedparticulatereflections.
Diffractionloss,whichistheLeakageofsoundenergyfromlayersoftrappedsound(ductsandsoundchannels)
andleakageofenergyintoareaswhereitisabsorbedornotcapableofdetection(shadowzones).
Multiplepathinterferencethatoccurswhenoneormoresoundpathschangewithtimeandintensity
fluctuationsoccur.
3.4.3.2.7 DirectivityIndex(DI)
Directivityisafunctionofthereceiversmechanicalabilitytofocusalongacertainazimuthandisbased
onthedimensionsofthesonarshydrophone(receiver)array,thenumberandspacingofthe
hydrophones,andthefrequencyofthereceivedacousticenergy. Thesefunctionsenablethedirection
ofareceivedsignaltobedetermined. Directivityalsoreducesnoisearrivingfromdirectionsotherthan
thatofthetarget. Thedirectivityindexpertainstoasonar'sabilitytodiscriminateagainstnoise. Itis
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definedasthesignaltonoiseratio(indecibels)attheterminalsofahydrophonearrayoradirectional
hydrophone,relativetothesignaltonoiseratioofanondirectionalhydrophone. Thusdefined,DIis
alwaysapositivequantityintheequation.
3.4.3.2.8Reverberation
Level
(RL)
Reverberationisobservedatthesonarreceiver. Thelevelofreverberationisafunctionofsourcelevel;
range;andsurface,volume,andbottomreverberation. Whenactivesonarisreverberationlimited,RLis
usedintheequationinplaceofNLandDI.
3.4.3.3 PassiveSonarEquation
Inpassivesonaroperations,thehydrophonesreceivesoundsgeneratedbyamultitudeofsoundsources.
Individualsmustdifferentiatebetweensoundsgeneratedbythetargetandinterferingbackgroundnoise
calledAmbientNoise. Thisprocessisbestdescribedinwhatisknownasthepassivesonarequation.
Thepassiveformofthesonarequation,liketheactiveform,iswrittenusingseveraldifferentsymbolsto
representtheequationparameters. Oneformoftheequationisasfollows:SE=SLRDNL+DIPL,
whereSE=Signalexcess,SL=Sourcelevel,RD=Recognitiondifferential,NL=Noiselevel,DI=
Directivityindex,andPL=Propagationloss. Notethatpropagationlossinthepassivesonarequationis
onlyonewaysinceallsignals(sounds)arereceivedpassively.
3.4.3.3.1 SignalExcess(SE)
Signalexcesshasthesamemeaninginthepassiveequationthatitdoesintheactiveequation.
3.4.3.3.2 SourceLevel(SL)
Sourcelevelpertainstotargetradiatednoise. Itistheamountofsoundenergygeneratedbyatarget.
Thelevelofenergyreachingthesonarreceiverdependsonthetypeoftargetanditsmodeofoperation.
Sourcelevelisafunctionoffrequency,speed,depth,andtargetaspect. Thelatterreferstoatarget's
orientationinrelationtothesonarreceiver.
3.4.3.3.3 RecognitionDifferential(RD)
RDhasthesamemeaningasintheactivesonarequation.
3.4.3.3.4 NoiseLevel(NL)
ThedefinitionforNLinthepassiveequationisthesameasintheactiveequation. Passivesonarsmay
beselfnoiseorambientnoiselimited. Thesesonarscanusebroadbandtolistenforawiderangeof
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frequenciesorcanbetunedtolistenforanarrowrangeoffrequencies,thusfilteringoutcertain
background,orambientnoise.
Theprimarygoalinunderwateracousticsistodistinguishspecificsoundsfromthetotalbackground
noise.
Selfnoiseisthatpartofthetotalbackgroundnoiseattributabletothesonarequipment,theplatform
onwhichitismounted,orthenoisecausedbythemotionoftheplatform. Themajorclassesofself
noisearemachinerynoise,propellernoise,andhydrodynamicnoise. Thelatterresultsfromtheflowof
waterpasthydrophones,supports,andthehullstructureoftheplatform.
Ambientnoiseisthatpartofthetotalnoisebackgroundnotduetosomeidentifiablelocalizedsource.
Ambientnoiseiscreatedbyseveraldifferentsourcesincludingsurfaceshiptraffic,waveaction,
precipitation,ice,andcertainformsofmarinelife.
3.4.3.3.4.1 SurfaceShipTrafficNoise
Atthelowerfrequencies,thedominantsourceofambientnoiseisthecumulativeeffectofshipsthat
aretoofarawaytobeheardindividually. Theradiatednoisespectrumofmerchantshipspeaksat
approximately60Hz,afrequencythatcorrespondstothemaximuminthecavitationfrequency
spectrumoftypicalmerchantships.
3.4.3.3.4.2 SeaStateNoise(WaveAction)
Seastateisacriticalfactorinbothactiveandpassivedetection. Forshipboardsonarsystems,the
locationofthesonardome,shipsspeed,course,andrelationtotheseaallhaveaneffect. Thelimiting
situationforactivesonaroperationsisgenerally612feet(seastate4or5). Forpassivedetection,the
noiselevelcreatedbywindwavesof10feetorgreaterwillresultinaminimumofunderseawarfare
operationaleffectiveness. Ambientnoisegeneratedbywaveactionusuallyvariesinrangefrom300Hz
to5kHz.
3.4.3.3.4.3 Precipitation
Rainandhailwillincreaseambientnoiselevelsatsomefrequencies. Significantnoiseisproducedby
rainsquallsoverarangeoffrequenciesfrom500Hzto15kHz. Largestormscangeneratenoiseat
frequenciesaslowas100Hzandcansubstantiallyaffectsonarconditionsataconsiderabledistance
fromthestormcenter.
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3.4.3.3.4.4 Ice
InPolarRegions,seaiceinfluenceswaternoise
levelsdependingonthestateoftheice,whether
itisforming,coversthesurface,orisbreakingup.
Noiselevelsaregenerallylowduringice
formationbutcanbecomeextremelynoisyif
entrappedaircausesdeformationandthe
temporarybreakupoficeduringiceformation.
3.4.3.3.4.5 MarineLife
Alsoreferredtoasbiologics,marinelifemay
contributesignificantlytoambientnoiseinmany
areasoftheocean. Becauseofthehabits,distribution,andsoniccharacteristicsofthevarioussound
producers,certainareasoftheoceanaremoreintensethanothers. Theeffectofbiologicalnoiseon
overallnoiselevelsismorepronouncedinshallowcoastalwatersthanintheopensea,andmore
pronouncedinthetropicsandintemperatezonesthanincolderwaters. Muchmoreinformationis
availableintheNavalOceanographicOffices,FleetOceanographicandAcousticReferenceManual,RP33.
3.4.3.3.5 DirectivityIndex(DI)
DIhasthesamemeaningasintheactivesonarequation.
3.4.3.3.6 PropagationLoss(PL)
PLhasthesamemeaningasintheactivesonarequationexceptthatwithpassivesonar,theenergyloss
isonewaysincethesourcetargetgeneratednoiseratherthanthesonar.
3.5OCEANOGRAPHICSUPPORTPRODUCTS
3.5.1 NAVALOCEANOGRAPHICOFFICE
TheNavalOceanographicOffice(NAVOCEANO)providesoperationaloceanographicsupporttotheFleet
throughtailoredanalysis,realtimedata,climatologicalproductsandoperationaloceanmodels.
3.5.1.1 NavyLayeredOceanModel(NLOM)
Globalcoverage
Figure313. ExampleofaNavyLayeredOceanModel(Source:NavalOceanographicOffice)
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1/16degreeresolution
Seawardof200mdepth
Sixverticallayers
Forecastsfrontandeddypositions
dailyfrom0to48hours
Forecastslayeredseasurface
temperature(SST)andseasurface
height(SSH)
AnexampleoftheproductispresentedinFigure313.
3.5.1.2 GlobalNavyCoastalOceanModel(GNCOM)
1/8degreeresolution
42verticallayers
Willprovideboundaryconditionsfor
higherresolutionnests
AssimilatesNLOMSSH
UnderwentvalidationtestinginFall2003
Forecasts3Dtemperature,salinityand
currentstructurefrom0to96hours
Anexampleoftheproductispresentedin
Figure314.
3.5.1.3 ShallowWaterAnalysisandForecast
System(SWAFS)
3Dcoastalcirculationmodel
BasedonPrincetonOceanModel(POM)
Resolutionvariesbyregion(1/2to24km)
Assimilatesobservationsfromsatellite(SST,SSH)andinsitu(ExpendableBathythermograph
(XBT);Conductivity,Temperature,andDepth(CTD);andprofilingfloat)
Figure314. ExampleProductofGNCOM.(Source:NavalResearchLaboratory)
Figure315. ExampleofaSWAFSProduct(Source: NavalOceanographicOffice)
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Figure316. ExampleofMODASDataSetSource: Naval Oceano ra hic Office
ForcedbytidesandFleetNumericalMeteorologyandOceanographyCenter
(FLENUMMETOCCEN)windsandfluxes
Providesdaily3Dforecastsofcurrents,tides,temperature,salinityfrom0to48hours
AnexampleoftheproductispresentedinFigure315.
3.5.1.4 ModularOceanDataAssimilationSystem(MODAS)
Statisticalanalysismodelfor:
Temperature
Salinity
Derivedquantities(soundspeed,etc.)
Relocatable,variableresolution. UsesOptimumInterpolationschemestocombine:
o SatelliteDerivedSeaSurfaceAltimetry
o Griddedclimatology(temperature,salinity)
o NearrealtimeXBT,CTD,floatandbuoydata
Provides3Dtemperatureandsalinitygrids
Usedforacousticpredictionmodels
FoundationforMODAS,runatNMOCregionalcentersanddeployedNavyships.
Providesinitializationfieldsfor3Dmodels
AproductexampleispresentedinFigure316.
3.5.1.5 2DTidalElevation/CirculationModels
RMA2Riverineandestuarymodel
ADCIRCCoastalcirculationmodel
WQMAPEstuarineandcoastalcirculation
model
PCTidesCoastalandsmallbasintidalmodel
Delft3DIntegratednearshorecirculation,wave
andsurfmodelingsystem
Relocatablemodelswithhighresolutiondomains
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3.5.1.6 WaveModel(WAM)
Areacoverage
Globallyrelocatable
Currently
running
many
domains
Variableresolution(1/4to1/12degree)
Deepwaterwavemodel(>20m)
Analysisandforecaststo48/72hours(twicedaily)
SurfacewindforcingusingFNMOC'sNavyOperationalGlobalAtmosphericPredictionSystem
(NOGAPS)andCoupledOcean/AtmosphereMesoscalePredictionSystem(COAMPS)models
Producesgraphicsandgriddedsetofwaveparameters
Predominantwavedirection
ProductsInclude:
o Significantwaveheight
o Swelldirection,period,andheight
o Windwaveheight
o Averagewaveperiod
3.5.1.7 SteadyStateSpectralWaveModel(STWAVE)
Areacoverage:~25kmalongcoast
Relocatable,variableresolution(100to400m)
Shallowwatermodel(
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o Peakwaveperiod
o DeepwaterinputprovidedbyWAM
3.5.2 GEOPHYSICSFLEETMISSIONPROGRAMLIBRARY(GFMPL)
GFMPLisasoftwarelibraryestablishedbyCommander,NavalMeteorologyandOceanography
Command(COMNAVMETOCCOM). GFMPLprovidesenvironmental,meteorological,electromagnetic,
oceanographic,hazardavoidance,acousticandweaponsystemsupportsoftwareforfleetair,surface,
amphibious,andantisubmarinewarfareoperationsandplanningpurposes.
GFMPLutilizesinsituandhistoricalenvironmentaldatarunthroughspecificalgorithms. These
algorithmsaresupplied,ifnecessary,withforce,threat,sensorandweaponcharacteristicstoprovide
environmental,sensor,orweaponperformancepredictions.
3.5.3 MODULAROCEANOGRAPHICDATAASSIMILATIONSYSTEM(MODAS)
MODAS2.05hasaGraphicalUserInterfacethatenablesausertoedittheBTdata,runMODASon
demand,andbuildgraphicalproducts. ItishostedonahighendUNIXworkstationattheRegional
METOCCenters.
UndertheMODASConceptofOperations,NAVOCEANOreceivesandprocessessatelliteandBTdata
andgeneratesanoceandepictionusingMODASHeavy. TheresultinganalysesaresenttotheRegional
METOCCenters.
TheRegionalCentersupdatetheseanalysesusingtheMODAS2.05toolandrecentBTdatathatmight
nothavereachedNAVOCEANO. Theseupdatedanalysesarethendeliveredtocustomersatsea.
Finally,thereisMODASLite. Thisthirdversionacceptsafirstguessfield(eitherastaticclimatological
fieldorapreviousanalysis)andupdatesitwithBTdata. MODASLiterunsonaPersonalComputer(PC)
and,likeMODAS2.05,doesnotexploitDynamicClimatology.
MODASLiteenablesthesecustomerstoupdatetheMODASfieldsagainusinganyonsceneBTs.
MODASLiteisintegratedwiththePCInteractiveMultisensorAnalysisTraining(PCIMAT)system. PCIMATwasoriginallyanaidfortrainingtheacousticsoftheocean. However,ithasbeensowellaccepted
thatithasbecomeaTDA.
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3.5.4 PERSONALCOMPUTERBASEDINTERACTIVEMULTISENSORANALYSISTRAININGSYSTEM(PC
IMAT)
ThePersonalComputerBasedInteractiveMultisensorAnalysisTrainingSystem(PCIMAT)isthe
premieroceanacousticanalysisandplanningtoolavailableonallASWplatforms,includingsurface
ships,aircraft,andsubmarines. TheSpaceandNavalWarfareSystemsCommandoriginallydeveloped
thissystemasatrainingtool,butithassinceevolvedintoatacticaldecisionaidforASWplatforms.
Oceanconditionscanberetrievedfromaworldwide,historicaldatabasethatusesuptodatesatellite
imagery. PCIMATalsoprovidesoceantemperatureestimates,whichaffectsensorperformance,aswell
asavisualrepresentationofestimatedsoundpropagationpaths(i.e.,raytracing),whichenables
MeteorologyandOceanographyprofessionalstoprovideactionablerecommendationstothe
Warfighteronthemosteffectivesearchpatterns,sensorplacements,andsonaroperationalmodes
basedonrealtimeenvironmentaldataintheoperationalarena. ThenewestversionofPCIMATaddsa
missionplanningmodulethatanalyzestheacousticconditionsatauserdesignatedgeographiclocation,
andprovidesagraphicalrepresentationofaplatformseffectivesearchrangesanditsvulnerabilityto
counterdetectionbyanenemy.
3.6 OCEANOGRAPHICOBSERVATIONS
LearningObjectives
Recognizeandexplainthenearshorecirculationsystem.
Figure317. ExampleProductofPCIMAT. (Source:SPAWARSYSCOM)
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Identifythemajoroceancurrentsandtheirlocations.
Recognizethemajoroceancurrentsand
theireffectsontheweather.
Explaintheimportanceofseaconditions
tonavaloperations.
Definedurationlimitedseasandfetch
limitedseas.
Definewaveheight,wavelength,wave
period,andwavedirection.
Defineanddistinguishthedifference
betweenseawavesandswellwaves.
Describetheformationoficeonthesurfaceofthesea.
Differentiatebetweenseaiceandlandice.
3.6.1 COASTALANDNEARSHORECIRCULATIONS
Twointerrelatedcurrentsystemsmayappearneartheshore. Theyarethecoastalcurrentsystemand
the
nearshore
current
system.
The
coastal
current
system
is
a
relatively
uniform
drift
that
flows
roughly
paralleltoshore. Itcanbecomposedoftidal,winddriven,orlocaldensitydrivencurrents. The
nearshorecurrentsystemismorecomplexandiscomposedofshorewardmovingwaterintheformof
wavesatthesurface,areturnflowalongthebottominthesurfzone,nearshorecurrentsthatparallel
thebeach,andripcurrents.
3.6.1.1 LongshoreCurrents
Longshoreorlittoralcurrentsoccurinthesurfzoneandarecausedbywavesapproachingthebeachat
an
angle
(Figure
3
18).
At
times
the
current
is
almost
imperceptible,
but
at
other
times,
it
can
be
quite
strong. Longshorecurrentsincreaseinvelocitywithincreasingbreakerheight,increasingbreakercrest
speed,increasinganglebetweenbreakercrestsandbottomcontours,anddecreasingwaveperiod. A
steepbeachwillhaveastrongerlongshorecurrentthanamoregentlyslopingbeach.
Figure318. LongshoreCurrent.Source: Naval Oceano ra hic Office
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3.6.1.2 RipCurrents
RipCurrentsareoftenerroneouslycalledriptides,buttheyarenotassociatedwithtides. Theyare
causedbyreturnflowofwaterfromthebeach. Thecurrentresemblesasmalljetinthebreakerzone,
whichfansoutbehindthebreakersandbecomequitediffuse. Thisstrongcurrentextendsfromthe
surfacetothebottom. Thestrengthofripcurrentsisnotpredictable,butisdeterminedusingthesame
factorsthatcontrollongshorecurrents. Ripcurrentsmayormaynotoccur,butwhentheydo,theycan
beirregularlyspacedorspacedatlongorshortintervals. Theycommonlyformatthedowncurrentend
ofabeachwhereaheadland(apointwherethelandjutsoutintothewater)deflectsthelongshore
currentseaward.
3.6.2 MAJOROCEANCURRENTS
Themajoroceancurrentsareestablished,andmaintainedbythestressesexertedbytheprevailing
winds. TheoceaniccirculationpatternroughlycorrespondstoEarthsatmosphericcirculationpattern.
Inthemiddleandlowerlatitudes,theoceaniccirculationismainlyanticyclonic. Warmcurrents(Ex.
GulfStreamandKuroshio)flowpolewardalongtheeasterncoastofcontinentsandcoldcurrents(Ex.
CanaryandCalifornia)flowequatorwardalongthewesterncoastofcontinents. Thisistrueforboth
Figure319Themajoroceancurrentsoftheworld.(Source:AmericanMeteorologicalSociety)
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hemispheres.
Athigherlatitudes,thewindflowisprincipallycyclonicandtheoceaniccirculationfollowsthispattern.
Coldcurrentsflowequatorwardalongtheeastcoastofcontinentsandwarmcurrentsflowpoleward
alongthewestcoastofcontinentsintheNorthernHemisphere. Inregionsofpronouncedmonsoonal
flow,themonsoonwindscontrolthecurrentsandvarywiththeseasons. Irregularcoastlinescan
causedeviationsinthegeneraldistributionofoceancurrents.
Theoceaniccirculationpatternactstotransportheatfromonelatitudebelttoanotherinamanner
similartotheheattransportedbytheprimarycirculationoftheatmosphere. Thecoldwatersofthe
ArcticandAntarcticmoveequatorwardtowardwarmerwater,whilethewarmwatersofthelower
latitudesmovepoleward. Theeffectonclimateisseeninthecomparatively,mildclimatethatexistsin
theareaofnorthwestEurope. Figure319displaysgeoloc
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