introductory oceanography (ocng 251) study guide: part 1ocng251... · 2012-07-16 · introductory...

IntroductoryOceanography(OCNG251)

StudyGuide:Part1ThishalfsessiondealtwiththeconstructionofallconditionsresponsiblefortheobservedglobalcirculationpatternsintheWorldOcean.Inasense,westartedthecoursefromtheveryend,tryingtobuildanoceanandunderstandingitsphysicalstructure(thewaterpartinthiscase).TheobjectiveoftheentiresessionistounderstandtheconceptbehindFigure1below:

Figure1:Generalcirculationpatternoftheocean.Surfacecurrentsareindicatedinredwhiledeepcurrentsarepresentedinblue.InFigure1,onecanseethatthereisalinkbetweensurfacecirculation(red)anddeepcirculation(blue).Ofcourse,toconservemass,theremustbealinkbetweenthesewocirculationpatterns.Areasof“deepwaterformation”willtransferwaterfromthesurfacetothedeepoceanwhereaswaterreturnstothesurfaceviazonesofupwelling.Transferfromthesurfacetothedeepoceanwilloccurduetodensification(increaseddensity)ofsurfacewater(mostlythroughcoolingbutalsothroughsomeincreasedsalinityduringiceformationandsaltconcentrationinseawater).Upwellingwilloccurthroughphysicaltransferfromcurrentformation(Ekmancirculationineasternoceanbasins)andaswaterispushedupcontinentalslope(likewhentheNorthAtlanticDeepWaterispusheduptheslopeoftheAntarcticcontinent).ThesefeaturesareallshowninFigure1withareasofdeepwaterformationaspurpledots(NorthandSouthAtlantic),andareasofupwellingwithblue‐to‐redarrows(easternregionsofoceanbasins).Alsonotethatsurfacecurrentsarecharacterizedbycircularpatterns,calledgyres,ineachoceanicbasinforeachhemisphere(AtlanticandPacificeachhave2gyres,whereastheIndianhasonly1).TheentirepurposeofthefirstsessionwasthustobringalltheelementsnecessarytocomprehendtheprocessesresponsiblefortheoceancirculationillustratedinFigure1.Theseelementsare:

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‐ Systemsandcycles.Specifically,howmassandenergycyclethroughdifferentsectionofasystem(fromthemicro‐tomacro‐scales).Inthissectionweemphasizednotionsofreservoir,flux,source/sink,residencetime,steadystate,aswellaspositiveandnegativefeedbackmechanisms.

‐ Physicalpropertiesofwaterand,inparticular,howtemperatureandsalinityaffectthedensityofseawater.Wealsofocusedonheatcapacitytoexplainthetemperaturechangesdifferentmediaexperience(i.e.atmospherevs.ocean,continents,vs.oceans,etc)whensubjectedtoagainorlossofheat.

‐ Heatbudgetoftheearth,particularlywithrespecttotheunbalanceinincomingshortwaveradiationsandoutgoinglongwaveradiationsthatisobservedininter‐tropicalvs.highlatitudezones.

‐ Atmosphericcirculation,asitisdrivenbythatsameunbalanceintheearthheatbudgetandaffectedbytheearth’srotation(Coriolis).Theinterplayoftheseprocessesthenleadstoglobalaswellasseasonalwindpatterns(e.g.easterlies/westerliesandmonsoons,respectively).

‐ Surfaceoceancirculation,drivenitselfbythewinddragofconstantwindsandaffectedbycoriolis,vorticity,andgeostrophicforces.Exceptfortheeffectoflocalwinds,thegeneralsurfaceoceancirculationfollowstheatmosphericHigh/Lowdistributionpatternwithcircularmotion(gyres)ineachoceanbasin.Thecirculationisclockwiseinthenorthhemisphere,andcounterclockwiseinthesouthhemisphere.

‐ Deepoceancirculation,drivenbydensityformationinhighlatitudezones.Surfacewatercanundergolargeincreasesindensityduetoaninterplayofsalinityandtemperaturechanges.Whenwarmwatercools,itsdensityincreasesmarkedly.Similarly,whenwaterincreasesinsalinity,itsdensityincreasesaswell.Thecoolingofsurfaceseawaterinnorthernlatitudes(e.g.sub‐ArcticseasandaroundAntartica)leadstoanincreaseinitsdensityandthusverticaltransferofwatertowardsthedeepocean.Similarly,duringseaiceformation,theexpulsionofsaltsfromtheformingiceresultsinbrineformation(increaseinsalinityinseawaters)andthusanincreaseinthewaterdensity.Theseprocessesleadtodeepwater‐massformation,eachwithspecificdensityconditionsthathelporpreventtheirmixinginthedeepocean.

‐ Globaloceancirculation.Thesurfaceanddeepoceancirculationsaretiedatboth“ends”wheresurfacewatercools(athighlatitudes)toformdeepwaters,andwheredeepwatersareupwelledtowardstosurface(mostlyoneasternboundariesofoceans)toreintegratethesurfacecirculationloopsandeventuallyreachthecoolingsitesforanothercycle.Onaverage,afulloceancirculationcycletakesseveralhundredyearstocomplete(~500yrs)butthis“mixingspeed”isvariableandcanaccelerateordeceleratedependingontherateofdeepwaterformation(cooling,salinitychanges)andupwelling(windstrength,atmosphericpressureoscillation).

‐ Earthclimatebalance.Therelationshipbetweenatmosphericandoceancirculation,helpredistributeheatfromzonesofsurplusradiation(inter‐tropicalzones)tozonesofdeficit(highlatitudes).Inlowlatitudes,themajorityoftheheattransferoccursthroughoceancirculation,whereasatmosphericcirculationisresponsibleformostoftheheattransferinmid‐tohighlatitudes.Eventsuchashurricanesarerapidandnatural“pressurevalve”processesthattransferlargeamountsofheatfromtheinter‐tropicalzonestomid‐latituderegions.

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1) SystemsandcyclesSomeDefinitions

Transportandtransformationprocesseswithindefinitereservoirs:Carbon,Rock,WaterCyclesReservoir:(box,compartment:Minmassunitsormoles)Anamountofmaterialdefinedbycertainphysical,chemical,orbiologicalcharacteristicsthatcanbeconsideredhomogeneous:O2intheatmosphere;carboninlivingorganicmatterintheOcean;oceanwaterinsurfacewatermasses.Flux:(F)Theamountofmaterialtransferredfromonereservoirtoanotherperunittime(perunitarea):Therateofevaporationofwaterfromthesurfaceocean;therateofdepositionofinorganiccarbon(carbonatesinmarinesediments);therateofcontaminantinputtoalakeorabaySource:(Q)AfluxofmaterialintoareservoirSink:(S)AfluxofmaterialoutofareservoirBudget:Abalancesheetofallsourcesandsinksofareservoir.Ifsourcesandsinksbalanceeachotheranddonotchangewithtime,thereservoirisinsteady‐state(Mdoesnotchangewithtime).Ifsteady‐stateprevails,thenafluxthatisunknowncanbeestimatedbyitsdifferencefromtheotherfluxesTurnovertime:Theratioofthecontent(M)ofthereservoirtothesumofitssinks(S)orsources(Q).Thetimeitwilltaketoemptythereservoiriftherearen’tanysources.Itisalsoameasureoftheaveragetimeanatom/moleculespendsinthereservoir.Cycle:Asystemconsistingoftwoormoreconnectedreservoir,wherealargepartofthematerialistransferredthroughthesysteminacyclicfashionFeedback:Allclosedandopensystemsrespondtoinputsandhaveoutputs.Afeedbackisaspecificoutputthatservesasaninputtothesystem.NegativeFeedback(stabilizing):Thesystem’sresponseisintheoppositedirectionasthatoftheoutput.Anexamplegiveninclassistheincreasedreflectionofsolarradiation(albedo)fromupperlevelclouds.Increasedheatevaporationcloudsincreasedalbedoloweredincomingradiationdecreasedoverallheat.PositiveFeedback(destabilizing):Thesystem’sresponseisinthesamedirectionasthatoftheoutput.Anexamplegiveninclassistheincreasedtrappingofinfraredradiationfromlowerlevelclouds.IncreasedheatevaporationcloudsincreasedI.R.trappingincreasedoverallheat.Wealsosentsometimeontheconceptofresidencetime(aconceptwewillbeusingalsointhesecondsectionofthiscoursetoexplainthesaltcompositionofseawaterandbiogeochemicalcycles).ResidenceTimeisahighprobabilitythatacertainfractionofasubstance(atomsormolecules)formingthereservoir(M)willbeofacertainage(meanageoftheelementwhenitleavesthereservoir).Theresidencetimeofwaterintheatmosphereisveryshort(~10‐20days).TheresidencetimeofwaterintheOceansismuchlonger(~4000years).However,theresidencetimeindifferentcomponentsoftheatmosphereandoceans,andthereforethetimeofexchangebetweenthesedifferentreservoirs,varywidely.

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Figure2:Timeexchangeforexchangeofairandwaterbetweentheatmosphereandocean.AdvantagesofCycleApproach

• Providesoverviewoffluxes,reservoircontents,andturnovertime• Givesabasisforquantitativemodeling• Helpstoestimatetherelativemagnitudesofnaturalandanthropogenicfluxes• Stimulatesquestionssuchas:Whereisthematerialcomingfrom?,whereisitgoingnext?• Helpsidentifygapsinknowledge

DisadvantagesofCycleApproach

• Analysis,bynecessity,superficial.Littleornoinsightintowhatgoesinsidethereservoir(“blackbox”)

• Givesfalseimpressionofcertainty.Often,atleastoneofthefluxesisderivedfrombalanceconsiderations(maybeerroneous!)

• Analysisbasedonaveragequantitiesthatcannotalwaysbeeasilymeasuredbecauseofspatialandtemporalvariations,aswellasotherfactors.

2) PhysicalpropertiesofwaterWatermolecule:DipoleUnevenchargeHydrogenbonds!(DNAanyone?)Higherenergyrequirementforchangeofstate(solidtoliquid,liquidtogaseous)thansimilarmolecules.Makesureyoucanexplainthefigurebelow:

Figure3:Meltingandboilingtemperaturesforwaterandaseriesofmoleculeswithsimilarchemicalcomposition.

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Thestructureofthewatermoleculethusleadstoveryhighenergyrequirementsforchangesofstate(LatentHeat),inparticularforchangesbetweenliquidtogaseousstate.Inthefigurebelow,theheatrequiredtoforchangesinphase(state)areillustratedashorizontallines.Thisdemonstratesthatheathastoconstantlybesuppliedtowaterforhischangeofphasewithout,however,resultinginanychangeoftemperature.Latentheatisjustthat,andchangeinheatwithoutachangeintemperature.Notethemuchmoreimportantheatrequirementforvaporization(580cal/gram)thanforfusion(80cal/gram).Alsonotethatthisheattransferisreversible,meaningthat540calofheatisreleasedtotheatmospherewhen1gramofwatervaporcondensed(rain)and620calofheatisreleasedwhen1gramofvaporsolidifies(snow).Latentheatisthusanimportantcomponentoftheearthheatredistributionprocess(e.g.evaporationininter‐tropicalzonesandcondensationinmid‐tohighlatitudes).

Figure4:Heatandtemperaturechangesinwateracrossitsphasechangecontinuum.WealsospentsometimeontheconceptofHeatCapacity.Heatcapacityisdefinedasthequantityofheatrequiredtoraisethetemperatureof1gramofasubstanceby1°C.• Moreenergyisrequiredtoraisethetemperatureofasubstancewithhighheatcapacity• Atconstantenergyinputs,thesubstancewithlowerheatcapacitywillshowahigherincreaseintemperature

• Highheatcapacitysubstancescanstorelargeamountofenergy.

Weusedtheconceptofheatcapacitytoexplainmajordifferencesintemperatureobservedbetweencontinentsandoceans.Thiswasthenappliedtoexplainthethreefollowingfigures.

Figure5:SeasonaltemperaturecurvesatSanFrancisco(green)andNorfolk(blue).Bothcitiesarelocatedonthesamelatitude.Hence,differencesarenotduetosolarradiationdifference.

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Instead,themoistureintheairinSanFranciscotransportedfromthePacificthroughwesterliesmaintainstheairtemperaturemorestableoverwintertosummerseasonalchanges.Incontrast,thelackofmoistureintheairatNorfolk(windsblowaboutlandbeforereachingVirginia)isresponsibleformuchlargesseasonalchangesintemperature.

Figure6:Dailywindpatternsincoastalregions.Thewindsaregeneratedbypressuredifferencesintheatmosphere,whicharethemselvestheresultofheatcapacitydifferencesbetweenlandandwater.

Figure7:Seasonalwindpatternsinsomecoastalregions.Thewindsaregeneratedbypressuredifferencesintheatmosphere,whicharethemselvestheresultofheatcapacitydifferencesbetweenlandandwater.

ChangeinTemperaturebutnotinHeat

Adiabaticchange:Waterisslightlycompressible(inthedeepoceanwheretheweightofthewatercolumninducesahighpressure).Thisinducesfrictionandthushigherkineticenergyincreaseintemperature.Insitutemperature:temperaturemeasureonsite(insitu).Theutilizationofinsitutemperaturecangivethewrongimpressionthatthewatercolumninunstable(lighterwater–warmer–indeepwatermasses).Temperatureindeepwatermasses

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needstobecorrectedfortheeffectofpressure.TheresultingtemperatureiscalledPotentialTemperature.ChangeofPhase­Density

Duringthetransitionfromliquidtosolidstate,atthefreezingpoint,thebondanglebetweenoxygenandhydrogenatomsexpandsfromabout105°toabout109°.Thischangeallowsicetoformahexagonalcrystallattice.Thespacetakenby24moleculesinsolidstatecouldbeoccupiedby27intheliquidstateWaterexpandsabout9%!Icehasadensityof0.917vs.~1.000g/cm3.Freshwatermaximumdensityat~4°CDensity

Densityofseawaterisaffectedbyacombinationofparameters:temperature(densitywhentemperature),salinity(densitywhensalinity),pressure(densitywhenpressure).Wecan,however,removetheeffectofpressurebyusingpotentialtemperature(whichitselfiscorrectedforpressure).Hence,seawaterdensitycanbecalculatedasafunctionofbothtemperatureandsalinity:ItisthedensityofaparcelofwaterofspecificTandSthatisbroughtuptothesurface(nopressureeffect!)

σT=((1.02594/1.0000)–1)x1000=25.94(Nounits!)

Figure8:Sigma‐Tvaluesforwaterofdifferentsalinity(S)andtemperature(T).ToobtainthedensityofseawateryouneedtouseσTandintegrateitinthefollowingequation:Density=[(σT×10‐3)+1]g/cm3.NotealsothatyoucanobtainseawaterofsimilardensitiesbyvaryingSandT.Theexamplescircledinred,blue,andgreenshowseawaterofsimilardensities(withineachcolorcode)despitechangesinparameters(e.g.astemperatureincreases,salinityhastoincreaseforthewatertomaintainthesamedensity).

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Temperature­Salinity(TS)DiagramsInTSDiagrams(Figure9),salinity(S)isrepresentedonthex‐axiswhiletemperature(T)isrepresentedonthey‐axis.Thesigma‐T(σT)linesindicateconditionsofsimilardensities.Themovementfromtheupperlefttothelowerrightisadirectionofincreaseddensity(thelargestshiftindensityoccurswhenthemovementisperpendiculartotheσTlines).

Figure9:TSDiagram.Note:Adropof5°Cinwarmwater(25°C)generatesagreaterincreaseindensitythanasimilarcoolingincoldwater(5°C).Inshort,themoreperpendiculartotheσTlinesthechangeis,themoreintensethechangeindensity.PlottingactualvaluesofTandSonsuchaTSDiagramwillgiveyouthenumberofwatermassesinthewatercolumnandtheirspecificTandScharacteristics.

Figure10:DepthprofilesoftemperatureandsalinityforastationintheNorthAtlantic.

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Figure11:TSDiagramforthestationshowninFigure10.Surfacewateristhelightestandthusappearsatthetopofthediagram.Heaviestwaterappearsatthebottomofthediagram.Asstatedinthefigure,every“bend”inthecurvedenotesawatermassthatdoesnotmixwithwateraboveandbelow.Caballing:Whentwowatermassesofsimilardensitiesmerge(pointsaandbinFigure12below),thecombinationoftheirtemperaturesandsalinitiesresultsindensificationverticaladvectionofwater.

Figure12:TSDiagramshowingtheeffectofcabaling.Thewaterinc(a50:50mixtureofwateraandb)isdenserthanthetwooriginalwatermasses.

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Light

Autotrophs–withafewexceptions–dependonenergyfromsunlight.Asdolandplants,marineplantsusechlorophyllandotherpigmenttocapturethevisiblelightfromthesuntoperformphotosynthesis.AssolarradiationstrikesthesurfaceoftheOcean,alargefractionofitisreflectedbacktotheatmosphere(dependentontheangleofthesun’sraysandthesmoothnessofthewatersurface).Theamountthatentersisultimatelyabsorbedbywatermolecules(~65%ofvisiblelightisabsorbedwithin1mdepth!):Absorbedenergymanifestsitselfasheat(elevatingthetemperatureofthesurfacewater)

Figure13:Spectrumoflightabsorptionwithrespecttowaterdepth.Absorptionisgreatestatlongerwavelength.Inclearwater,only~1%ofsurfaceenergyremainsat100m(incoastalwaterswithlotsofparticles,lightdoesn’tpenetratemorethanafewmeters).3) HeatbudgetoftheearthBasedonthetemperatureofthesunandtheearth,andonphysicallawsofradiation(Stefan­Boltzmann’sandWien’slaws),thesunemitsradiationmostlyinthevisibletoultraviolet(UV),whereastheearthemitsmostlyintheinfrared(IR)spectrum.Thesedifferencesinradiationwavelengthsarecrucialtoexplaintheearthactualvs.theoreticalaveragetemperature.Basedonthesolarradiationthatearthreceivesperunitarea,thetheoreticaltemperatureoftheearthshouldbe‐18°C.Buttheactualtemperatureofearthismuchhigherthanthat(+15°C).The+33°differencecanbeexplainedbywhatiscalledthegreenhouseeffect.Inshort,gasesintheatmosphere(e.g.water,CO2,CH4,CFCs,N2O)arerelativelytransparenttoshortwaveradiation(meaningtheyletthesepassthrough),whereastheyabsorblongerwavelengthradiation.Figure14illustratesthisprocess.ThesubstantialabsorptionofIRbackradiationbytheearthatmospherethuspermitstheearthtoretainheatwithininfluidenvelopeandthuswarmbeyondthetheoreticalvaluedeterminedbysolarradiationalone(Figure15).

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Figure14:Proportionoflightabsorptionintheearthatmospherewithrespecttoradiationwavelength.Therearetwo“window”oftransparency.TheatmosphereisnearlytransparenttovisibleandnearUVradiation(majorityofsolarradiation),whereasitabsorbsstronglyintheUV(fromO3)andnearIR(water,CO2,CH4,CFCs,N2O).ThesecondwindowisintheIRandpermitssomelongwavelengthradiationtoescapetheearth’satmosphere.

Figure15:Warmingoftheearthbysolarradiationalone(left)andthroughthecombinationofsolarradiationandgreenhouseeffect(right).

Figure16.Althoughaveragetemperaturesvaryseasonallyandspatially,theearth’soverallTchangesonlyslightlyovertheyears“must”returntospacethesameamountofenergyitabsorbed.Totalenergyinput=100units(perunittime).Note:Albedoisthefractionofthesunradiationthatisreflectedbacktospacewithoutbeingincorporatedintheheatbudgetofearth.

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Twoconditions/processesaffecttheamountofsolarradiation(andthereforetheincomingenergysource)differentregionsoftheearthreceive:SphericityandSeasonality.Sphericity(below):IftheEarthwereadiskwithitssurfaceperpendiculartotheraysofsunlight,eachpointonitwouldreceivethesameamountofradiation.However,theEarthisasphereanditssurfacetiltswithrespecttotheincomingraysofenergywiththeregionsfurthestawayalignedinparalleltotheradiationandthusreceivingnoenergyatall.

Seasonality(right):TheEarth’saxisistiltedinrelationtotheplaneoftheecliptic(theSun‐Earthplanethatcutsthroughbothcenters).Higherlatitudesthusreceiveincomingradiationwithdifferentanglesthroughtheseasons(verylowangleinthewinterandcloseto90°inthesummer).

Theeffectofseasonalityandsphericityisillustratedinthefigurebelowwhereshortwaveincomingradiationismaximuminhighlatitudesummers(julyinthenorthhemisphereandJanuaryinthesouthhemisphere)andconstantattheequator.

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Becauseofthesegeographicalandtemporalvariationsinshortwaveincomingradiation,longwaveradiationalsoshowsimilarvariations(IR).Thefigurebelowillustratesthesegeographicalandtemporaldifferences.

Figure17.Longwaveradiationfortheearth.Lefthandpanel:January.Righthandpanel:June.ThesefiguresshowgraphicallythatthethereisanequatortopolechangeinincomingradiationΔHeatingaswellasaseasonalchangeinincomingradiationΔHeating.Theearthheatbudgetisthusapparentlyunbalancedwiththeinter‐tropicalregionapparentlycontinuouslygainingheatandthehighlatituderegionsapparentlycontinuouslylosingheat(Figure18).However,becausetheinter‐tropicalzonesdonotheatcontinuouslyorthehighlatituderegionsdonotfreezecontinuously,thenheattransferfromzonesofheatsurplustothezonesofheatdeficitmustoccur.Thisisthefoundingconditionthatleadstocirculationpatternsinthefluidenvelopesofearth(atmosphereandocean).

Figure18Shortwave(Qs)minuslongwave(Qb)radiationfortheearth.