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Meetingon thePhysicalOceanographyof SeaStraits,Villef ranche,15th-19thApril 2002

The current system of the Bosphorus Straitbased on recent measurements

EminOzsoy, MohammedA. Latif andSukru Besiktepe

Instituteof MarineSciences,Middle EastTechnicalUniversity, Erdemli- Icel 33731Turkey

Abstract. A seriesof measurementsin the BosphorusStrait usingADCP andCTD profiling, current-meterand sea-level recordingsare analysedto yieldthree-dimensionalmappingof hydrographyandcurrentsaswell asthestatisticalcharacteristicsof sea-levelandcurrentvariability. Theresultsshow time-dependentmeanderingof currents,with separatededdiesformedin shelteredareasalongthecoast. Spatialcorrelationalong the Strait reflectsthesechanges. Sea-leveldifferencesrespondrapidly to changesin flows, and suddenchangein winddirectionleadsto blockingconditions.

Introduction

The Bosphorusis a passageway of heavy marinetrafficbetweenthetwo seas,aswell asof landtraffic betweenthetwo continentsthatit connects.Informationon thenatureofthecurrentsin theBosphorusis thusessentialfor safenavi-gationandtransitthroughtheStrait.Furthermore,theTurk-ish StraitsSystem(consistingof the Dardanelles,Bospho-rus Straitsandthe Seaof Marmara)is sensitive to climaticchanges,andpotentiallycaninducesuchchangesin thead-jacentbasins[Ozsoy, 1999]. Acting asthelimiting elementof theTheTurkishStraitsSystem,theBosphorusStraitcon-trols the exchangesof massandpassive or active materials

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Figure 1. Bottom topographyof the Bosphorus,and thelocationsof sealevel (SL1, SL2) andcurrent-meter(CM1,CM2) stations.

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Figure 2. (a)GPSpositionsof theshipcollectingdataalongthe Bosphorus,and (b) interpolatedsurfacecurrentsfromcontinuousADCPmeasurements,on03September1998.

transportedbetweenthe Black andthe MediterraneanSeas[Ozsoyetal., 1995b;Polat andTugrul, 1995;Unluataetal.,1990].

Basedon budgetscalculatedfrom averagesalinity themassflux of the upperlayerflow is abouttwo timeslargerthan that of the lower layer, yielding a net flux of about������������ ��

fromtheBlackSeato theSeaof Marmara[Latifet al., 1991; Unluata et al., 1990]. Geometricalfeatures[Oguzet al., 1990;Unluataet al., 1990;Ozsoyet al., 1998]make the Bosphoruspredisposedto ’maximal exchange’,with contractionand sill controlsas in Farmer and Armi[1986]. Local topographicfeatureshave significant influ-enceon theflow, anddetermineits detailedstructure[Greggand Ozsoy, 2001]. The exchangeflows responddynam-

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Figure 3. Salinity, temperature,anddensitycrosssectionsobtainedfrom CTD measurementsalongthe Bosphoruson(a)15March1999and(b) 18March1999.

ically to time-dependentmeteorologicaland hydrologicalforcingin theadjacentbasins[Latif etal., 1991;Ozsoyetal.,1995a,1996,1998;OzsoyandUnluata, 1997,1998;Gregget al., 1999;Gregg andOzsoy, 2001]. Observationssuggestincreasedentrainmentsouthof thecontraction(upward)andpastthe northernsill (downward) in the Black Sea[Greggetal., 1999;Ozsoyetal., 2001].

The measurements and data processing

In thepresentstudywehavemadedetailedmeasurementsto observe the fine scalevariability in the Bosphoruscur-rentsandto determinetheir time andlengthscales.Thein-tensive experimentscarriedout in the BosphorusStrait in-cludedADCP andCTD datacollectionduring3-6 Septem-ber1998,4-22March1999and22July-3August1999.Thedatawerecollectedsimultaneouslyby two ships(the R/VBIL IM of theIMS-METU, andATMACA II, a commercialvessel)alongrepeatedcriss-crosspatternsacrossthe Straitattemptingto samplethe detailedstructureof currentsinthe main channeland in the variousbendsand small em-baymentsthat line its coasts.A Seabird911plusCTD pro-filer anda 150KhzRDI narrow-bandADCP wereusedonboardtheBIL IM while a Seabird25 SealoggerCTD anda300KHz RDI WorkhorseSentinelbroad-bandADCP wereusedonATMACA II. AccuratepositionfixeswereobtainedusingTrimble NT200GPSandEnsignXL GPSnavigationsystemson thetwo ships.In addition,sealevel andcurrent-metermeasurementswereperformedrespectively atstationsnearthetwo endsof theStraitFigure2, whereAanderaaIn-strumentswaterlevel recorderWLR7 andAanderaarecord-ing currentmeterRCM7 weredeployed. The currentandsealevel time seriesdatawererecordedat 5 min. interval.Absolutesealevel ateachstationwasdeterminedrelative toa commondatumusingleveling to benchmarks.

Oneof thegreatestproblemswasin processingthenav-igation datato obtainpositionfixesandgroundreferenced

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Figure 4. Interpolatedspeedof surfacecurrentsfromcontin-uousADCPmeasurementson(a)March16,1999,(b)March18,1999.

velocity from theGPSandADCP measurements.NoiseinGPSdataandmagneticdeviationscausedby theship’ssteelhull werefirst removed by applyingdespikingfilters, cor-relation techniquesand a numberof logical testsfor shipspeedand position. Independentmeasurementsof bottomtrackandGPSvelocity wereusedtogetherin thesecorrec-tions.

The ADCP measurementsobtainedby the broadbandADCP on board ATMACA II covered the Bosphorusingreatdetail, including the numeroussmall embaymentsonits banks,makinguseof themnaeouverabilityof thesmallervessel.Thecurrentvelocitydatafromthebroad-bandADCPstartedfrom adepthof 3m. For interpretationof surfacecur-rents,theADCPdatawerefirst averagedin the0-10mdepthrange,thenintepolatedona0.15� latitude-longitudegrid.

The current-meterandsealevel measurementswere in-terpretedbasedon standardtime-seriesanalyses,applyingcorrelationandspectralmethods.Thecurrentvelocity datawererotatedto maximizealong-channnelvariance.

Results

Surface currentsobtainedalong the ship track in Fig-ure 2a are shown in Figure 2b. In the numerousrealiza-tionsasshown in this example,the largercurrentsfollowedthe main channelof the Bosphorusand affectedby its to-pography(Figure1). Thesurfacecurrentsintensifiedin thenarrows in southernBosphorus,andfirst followedthedeepchannelon theeasternside,thencrossedto thewesternsidefollowing the main channel,forming a jet nearthe exit totheSeaof Marmara.Temporalchangesaswell asturbulentmeanderingaccountedfor day to day changesin the hori-zontalstructureof the main current. Therewerenumerouseddiesandreversalsin thecurrentsconsistentlyobservedin

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Figure 5. Time series displays and cross spectrabe-tweencurrentsatBeylerbeyi (CM1/BL) andthealong-Straitsealevel differencebetweenRumeli Kavagı (SL2/RK) andBesiktas (SL1/BS)stations.Dottedlines representspectralsmoothingwith a triple pointmoving averagefilter.

thevariousshallow banksandembayments.

DuringMarch1999measurementsthewindsweregener-ally calm, from thesouth,whentheBosphorushadits typ-ical two-layerstratiifiedflow structureshown in Figure3a.On 18thof Marchthewind suddenlychangeddirectionandincreasedin intensity, blowing at full force from the north.Theupperlayerflow increasedandthelower layerflow wasthenblockedandpushedsouthof thecontractionasshownin Figure 3b, leaving only a small traceof Mediterraneanwaternearthe bottomin the upperreachesof the Bospho-rus. Themagnitudeof surfacecurrentson March16 and18respectively areshown in Figures4a andb to illustratethegreatincreasein currentvelocity accompanying the flush-ing of theMediterraneanwaterfrom theBosphorusfollow-ing thenortherlywind event.Thegreatestincreasein surfacecurrentsoccursin thecontractionregionandin thesouthernBosphorus.

CorrelationbetweencurrentcomponentsalongtheStraitand thoseperpendicularto it were computed(not shown)as a function of distancebetweenpairs of data points toreveal structurerelationshipsof currents. The resultsindi-catedlengthwisecorrelationsondistancescomparableto thelengthof theStrait,asexpectedfor theconduittypeof flow.Therewerealsocorrelationsat scalescharacteristicof thecoastalfeaturesandbendsof theBosphoruschannel.

Thesealevel andcurrentmeasurementsaswell astem-peraturerecordedat current-metersites were individuallyanalysed(not shown), computingcorrelationsandspectra.Thecurrentvelocityobtainedfrom current-metersindicatedreversalsin direction,resultingfrom shelteringeffectsin thecoastalregion. Theanalysesrevealedoscillationsat diurnal(24h),inertial(18.3h)andsemidiurnal(12h)periods,aswellasthoseatperiodslongerthan2 days.

Thetime-seriesof currentsatcurrent-metersiteCM1 andthe sealevel differencebetweenstationsSL2 andSL1 fortheperiodstartingon06March1999areshown in Figure5,togetherwith the cross-spectralestimates,coherenceandpahsebetweenthe two time series.Correlatedpeaksoccurat diurnal, semi-diurnalperiodswith high coherence.Thephasedifferencesindicatethe sealevel differenceis totallyoutof phase(or hasnegativecorrelation)with thenortheast-wardcurrentsat thediurnalandlongerthan2d periods;i.e.the sealevel differenceis positively correlatedwith south-westwardcurrents.At semi-diurnalperiodsthephasediffer-enceis about90� , which is typical for tidal currents.

Conclusions

TheBosphoruscurrentshavefinefeaturesthatdependonthe topographyandcoastalfeatureson its banks,revealingeddiesandrevesringcurrentsin variousof its embayments,aswell asturbulentmeanderingof the maincurrent. Tran-sienceon varioustime scalesin additionto the spatialfea-turesmake theBosphoruscurrentshighly variable. Yet themain featuresof the currentsystemarefamiliar. BlockingeventscanflushtheStraitalmostcompletelypushingeither

180 Ozsoy, Latif andBesiktepe

the lower or the upperlayer back to its origin in the adja-centSeas.TheBosphoruscurrentshave a rapidresponsetosealevel differencesbetweentheBlackSeaandtheMediter-ranean.

Acknowledgments. Thisstudyhasbeenmadepossiblethrougha projectcarriedout for the Istanbul TechnicalUniversityFounda-tion / DefenseResearchCenter. Theauthorswish to expresstheirgratitudeto thecooperativeeffortsof thescientists,techniciansandgraduatestudentsof theIMS-METU whotookpartin thedatacol-lectionandprocessing,andthecaptainsandcrew of theshipswhowereinvolvedin variousstagesof thework.

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E.Ozsoy, M. A. Latif andS. Besiktepe,Instituteof MarineSciences,Middle EastTechnicalUniversityP.K. 28Erdemli- Icel 33731Turkey

This preprintwaspreparedwith AGU’s LATEX macrosv5.01,with theextensionpackage‘AGU��� ’ by P. W. Daly, version1.6bfrom 1999/08/19.