Wind driven cir cula t io n in Trinity a n d Co ncep t io n Bays

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Wind driven cir cula t io n in Trinity a n d Co ncep t io n Bays

by

©Frase rDavid so n

:\1.5(;.(Memo ria l Lnivcrsi t y. St.John's.Ca na d a ) 199.1 BoSe.(Ho n){LniversiryofOtt awa .Onawa ,Cana da)1992

Athesi s submittedtothe SchoolofGraduateStudies inpartialfulfilme nt ofthe requ irements for the degreeof

DoctorofPhilosop hy

Depa rt ment ofPhysics andPhysicalOceanography Memori alUniversi t y ofXewfoundla nd

(O ct obe r.19!J9)

St. John's Xewfoundland

Abstract

ln thisthes is the winddriven circu la t ionofTrini tyandConcept ion Baysis de- seribr-d. I make use of threenume rical mode ls. ra ngin gfrom alinear. single -layer.

rodncr-d-gr avity modeltothe3-Deddyreso lvin g CA:\DI E model. TheC.-\:\ DI E model is verifiedby app lyingit to a la keinit iallyat rest with horizon tallyunifon n stratification. aparaboli cbottomprofile. adia ba tic bou ndary cond itions and1:0ex- remul fordngof anykind"Overtime.verticul mixingofthe the rmoclinegivesriseto horizontaldensitygrad ientsandcyclo niccirculat ion in thelake.The model s are then appliedto inves tigate mech an ism s thatleadto observed asym metry in the respons e of Trinityand Conception Bays to ste ady wind for c ing direc tedoutofthehay . I ilL\"I>oitigatc the influenceoffa cto rs suchas cont inuousstratificarion.vert ica l mixi ng.

nonlinearityandrealistichotromtopo gra phy" Add ingnon-linearityallowsfo r signi f- k-antcross-bay transpo rtofupwelledwaterandleadstoan along-baypatte rnofthe surface isother ms evident inobservation s.and ca llalsoleadtothesepa rat ionof the ('oa>:ita ljetfrom theupwellingfavou ra blesho re.Theresp o nse tovariablewind isalso ill\"rstiga tedandthemode llingeffortis compa redtoavaila ble oceanog ra phicobsc r-

\1UitHlSof the two baysincluding ADCP.curr e nt mete rmoo rin gs.ther mis tor chains.

SST and CODARda ta.Overalltfindreason a bleto goodagreeme ntwit h the data"

Addingthe non-line a rtermstothe C.-\:\DIE mod el im proves there prod uctionof observedthermocl iuemoveme nt .All applica ti o ntoparticleadvect ionisund ert ak en . which suggests that theremaybelmt cexcha ngeoffishla rva e betweenTri nityand

Coucoprion Bays.

iii

DEDICATED TO THE MEMORY OF MY GRANDMOTHER CLA R A ELEANOR MOORE(1906- 1991 ) IN APPRECIATION OF HER LOVE OF LEARNING

AND ENCOURAGEMENT TO ALL HER GRANDCHILDREN IN THEIR PURSUIT OF KNOWLEDGE

iv

Contents

Abs tract List of Tables

Listof Figures Ack now ledgement s 1 Int r od u ction

1.1 Location . 1.2 Coas t alTrappedwaves

l.:~ localOc{'a nogr a p hyofTr inityand Con ceptionBays 1.-1Previousylodcliug Work.

1.5 BiologicalApplications 1.6 ResearchObjectives.

1.,

^{Ou tli ne.}

2 Model Appr oach and Observations 2.1 IntroductoryRem a r ks 2.2 Densit yStrat ifica tion. 2.3 :\.10([1:'1Descriptio n

2.3.1 Single-layer,Reduced Gra vity vlodcl 2.3.2 Stratifiedxtodet [Xormal :\Iod es) 2.3.3 C:\\"DIExlodel. 2.:3..1 BcundarvConditions. 2.-1 Model Setup . .

2.-1.1 xlodetDom ain 2.-t.2 WindFo rc ing . 2...1.3 ModelIn itializat io n.

2.5 ObsorvationsillConceptionandTrinityBays .

viii ix xiii

8 12 15 16 21 21 23 25 26 28 30

"

35 35

3.

38 38

2.;}.1 Curre nt Moorin gs. 39

2.5.2 AcousticDopplerCurrentProfi lcrData. -1.7

2,,1.3 CO DAR Data. 50

2..1.-1. SeaSurfaceTemperatureoyer the Avalcn.. 55

2.6 Sum m a ry 60

3 MeanCy clon ic Flow in Large Stratified La kes 81

3.1 Inrrod uctton. 81

3.2 Mod el Setupfor the La ke Problem 83

3.3 Result s 85

3.3.1 ParabolicLake 85

3.3. 2 Step La ke 89

3.-1. Discussion. . 90

4 TheRespon seToSteadyWind 102

-1..1 InrroductorvRemar ks 102

-1..2 Sing leLaye~^ReducedGravuv xlod cl 103

·L3 Sha llow WaterModelResult s:ContinuousStratifica tion 10-1.

-1..3. 1 Results. 105

-1..4 Linea rCA:\"D1Extod el Results 107

-1..5 Xon-I lnea rC..\SDl E :\Iodel Resul ts 109

-t.6 Rec tangularIdealizedBay Results.. II I

-t.6.1 Line a rizedxlode l Results. 112

-t.0.2 Xon-ltnear CA:\"DlE Resul ts: Const a nt \\'ind 113

-Hi-3 Xcn-LinearResu lts:Rela xationof \\'ind 117

-1..6.-1. Role of De nsi tyAdvection 118

-1..7 Realistic Coastli neG~metryResults 119

5 Circ u lation Respon setoVar iable\Vind Forcing 146

5.1 Int rodu ct ion 1-1.6

.j.2 Cha nges in Xlodc lSet up 1-1.7

.').3 Compariso n withcur rent mooringobservations in1990 1-1.9

5.3. 1 ModeledPycn oclineResp onse. 150

..:;.3.2 ComparisonofVerticalVclocittcs 152

.

1.3..3 ComparisonofIsopycnaland Isother m VerticalDisplacement, 153 .

5.3.-1. Sensitivityof xlodclcd Pyc noc line Respo nse l5-t

5.3 ..5 Compariso nsin the Frequen cy Domain. 156

.).-1. Compa riso nwithcur re nt mooringobservat ionsin199-1. 157

.j .5 Com pa risonofCur rents in1990 and 199-1. 159

5.5.1 Com paris o n withADCP Transects 159

5.5.2 Com pa riso nofXlcunCurr entswit hMoorin gs 163

;).6 Comparisonwith COD.-\R datain1991. 166

vii

.

5.6. 1 CQDARxteasurcdSurfaceCurrentsandtheEkmanlayer 166

<:;.6 .2 Results. 169

::1.6.3 ComparisonswiththeLinea r Model 171

.

'i.6 Ao Com parison of theXleanVclocitvField s 172

5.6.·j AbsenceofCoriolis Force . . 1.3

.

'i.6. 6 Summary I7-\.

5.7 Com pa riso n withSSTimages 175

s.s

Sum mar y 181

6 Bio logicalApp lic ation 209

G.l intr oduc toryRem a r ks 209

G.2 Veloc ityField Descriptio nandxtetbods. 211

G.3 Results 213

6.3.1 Gen eralParticleDrift 213

6...1 Summary 217

7 Con cl us ion 228

Ref erence s 234

List of Tables

:3.1 :\Iaxinmmvert icalaveraged azim ut ha lvelocity ill alak e.

ix

List of Figures

1.1 Xlnpof TrinityandConcept ionBays 1.2 Summe rSSTima ge ofXewfoundiand Shel f"

1.3 Circulationdiag ramofLab radorCurre nt

IS 19 20

2.t Summerdensityst ratificationand\"~. 61

2.2 Baroclintcmodest ructureforsummerdensi tvst ratification 61

2.3 1- DC-gridsherue. - 62

:!A \\"indstress projection ontothenor ma lmodes . 6:1

2.5 Cvgrtdsheme for 3-D C.-\\"DIE:\EODE L 63

2.v Squarebay topography 6-1

2.7 IdealizedTrinityBaymodeltop ogra phy 6-1

:1.8 Tri nity andConc ept ion Baymode l topograp hy 65

2.9 Observedwinds in1990 and199-1 66

2.10 \\"ind stresspower spectrum for1990and19!U

v7

2.11Mooring lo ca t io nsin 1990 and199-1 68

2.11 Time series of vertlca !temperatu re profile in1990. 69 2.13 Timese riesof ver tica l tem per a t ureprofileill199 -1. 70 1.1-1Time-mean vertical tem perature pro file at moorings \\"1.\\"-1and\\"7 71

2.15Observedme an velociticsin 1990 72

1.1vObservedmeanvclccities in199-1 73

2.17Observed vclocirv transr-crs from ADC?on Jutv19th199-1. 7-1 2. 18Observed \"cloci t;· transec ts fromAOCP011.lui; ' 22nd199-1. 7.) 2. 19 CO DARsurface velocit iesof Conce ptio nBay(2- 1O.Iuly199 1) 76 2.20COO.-\RsurfacevelocitiesofCo ncept ionBay (1l-19.Iuly 1991) 77 2.21 Observed CO DARbaseddeccrrc lationlengthscale 78 2."21 Observed CODAR bas ed decorrelationlengthscale(e ontiflued). 78 2.23ObservedSST imagesof theAva lonPeninsul a(1998) 79 2.2-1Observed SST IorTrini tyandConception Bay(1998 ). 80 3.1 Initialtemperaturefield illsrranfl cdlu kc.

:l.2 Cross-sect io ns of the parabo lic la ke model run at 60 days 95 3.:3 Vert ically averagedcirculation in paraboliclakemodelrun at 60 days 96 3..1 Cross-sec t ions of theste p lake modelrunat 60 days 97 3.5 Ste plake modelrun at 60 days:Surfacecirculation 98 .3.6 Diffusiononlymodelresu lts for Trinityand Concept ionBay 99 3.7 Time averaged cross-sectionof summercircu lationin Lake Ontario 100 .

'3.8 Timeaveragedcross-s ec tio n of wintercircula t ionin LakeOntario 101 -1.1 Observedsea surfacetemperatu rein Conceptionand TrinityBays on

Aug -Ith 1997 12-1

-1.2 Velocityand Interfacedisplacementfrom thesingle-layer. red uced-

gra vitymod el 125

-1.3 Thenor mal mode modelat day5with no friction ordampi ng 126

·t-l Asin Figure -1.3 but wit ha Pra ndtlnumber(Prjof100 127 -I..) The normalmodemodel atday5asin-1.3. but with windst ress mod-

elect as a bodv force 128

-1.6 AsinFigure "-1.3hutwithdlffusivi tiosfo rmo me nt um andpressu reof

OOm²s-^l• 1~

-1.7 Thelinea rized CA.XDIE modelatday 5 withidealizedgeometry 130

-1.8 De ns ity fieldforFigure -1.7 131

-I.D Pressure fieldco rres po nd ing toFigure-1.7 132

t.1OThe non-linear CAXDIEmodelatday .5 withidealized geome try 133

-1. 11Asin figure-I.lObutfor the density field 13-1

·tl 2 Pressurefield forFigu re-1.10 13.)

-1.1:3Surfaceveloclryfield {toplOm]from linear-izedCAXDIErunat 5 and LOdaysus ingidealiz ed 200Inflat bottom TrinityBay topography 136 -1.1-1Xon-IinearCAXDIEmodel with0.05Paconst a ntwind alongthex-axis

at~\"5 137

-1.1.)D('nsit y contoursforFigure-1.1-1 138

-1.16Xon- line arCA:'\DIEmodelrunatday 5 with constant0.1Pa wind 139 -1.17Same asfigure-1.16.butresults shownfordayto l-lO -1.18Sa me as figure-1.16.bu t with ano-slip boundarycondition appli ed.

inst ea dof a free-slipconditio n. l-ll

-1.19 Sameasfigur e-1.17but wind isrelaxed from 0.1Pa to zero (over2

days)<I..sof day 5 1-12

4.20Velocityat day5ofmode! run withlinearized momentum.yetincluding

de nsityadvection 1-13

-1.21 Sa me asFigure s-1.20but for density 1-13

-1.22Surfa ce vclocittcsatday 5wit h realisticgeometry. 1-1-1 -1.23Surfacefields at day 10 for the nonlinea rCA:'\DIE model 1-15

xi

.J.l Densitypro file fro mnon- line arCAXD IErunandobserved tempera ture

profilein1990 18~

.J.1 As infigu re5.1but forthe linea r model run 185

.5.:} Ap pare nt modeledvertica lvel oc itiesofnon- linearCA X D I E model 186

~A .-\Sin figure5.3.butforlinea ri zed C.-\SDI Emodel. 186

~.5 Verrtcal positionofthe mod eled 10"1.5.8kg m-^Jisopycna land the observedI"C isothcrm at H"1.H~and H6moorin g locations. 187 .

').6 Sha llowwaterInterfaceposit ioncom pared tothe posirion ofthecb-

servedI"CIsother m 188

5.7 De nsit yprofile com pa r isonbet weenrealistic botto mrun and fiat bot-

[~~In 1M

;j.8 Densitypro filecompar isonbetweenmode!runwit haPrand tlnumber

of I and100 190

;J.9 Powerspectr-aofappare ntve rticalvelociticsfromclay 150 to:WOin

1990 191

.).10De nsityprofile out p u t frontno n-Itnea rCAX DIE runandobse rvedrem- perat u re profile at 3cu r rentmoor ingloc a t ion sin199~ 192 .) .11Com pa r isonofobserved I°Cisotherm(thinline]with the 1025.7

kg111-3isopycnal(t h icklin e ] fromthe non- linear CAX DI E model on

thewest ernshore ofConce pt iallBay in199~ 193

~.12Asin Figure S.11 butusingthe linea r ized CA=, DIEmodelwithrealistic

bottomtopog raphy 19~

.:i.I:}Comparisonofobserved1"Cisother mwithiILt E'rfa(~positionofshal- lowwater modelon the westernshoreofConception Bay 19:')

~.1"Comparisonofnon-linea rC.-\X DIEmodel and.-\DCPcbserv-anonson

nJu~W~~ lm) . . 1%

.).I~Asin figu re 5.U butfora transec t located fu rth er intotheha yby

lOkrn 19T

.). 16 :\Iea ncurrentsat15mand-15m inthe1990and199~mod elruns 198 5.1,Cross-shore\~r ticaltransectof mea nmodeledcurrentandtheCOrTe-

spendi ng major and minoraxis ofstandarddevtarlons 199 .').18Voloc ujcsfrom CAXDIEmodel sho wnat12noonfrom.Inl y2^d to the

la

^h ~

.

').19.-\sinfigure 5.18.butcove ri ng.lulyl ith(0the 19^{t h} 2UI .

) .20Meansurface curren ts(topIm)Irom houri}'CO DARobserv -atlons

from .Iuly 2,,01tothe19^l1'of 199 1 202

:;.21~Ieansu rfacelayercu rrents(10 m)of lion-linearC.-\S D I Emodel6 hourlyoutputfrom.luly2,,01tothe19'hof 1991 202 .').'22Vctoctuc sfromCAXDIE mode lwit hf =0show nat12noo n from

.Iulv2".1tothelO"' 203

5.23 As ·inFigure5.22. butcoveri ng Jul ylI'htotlLC'19'h 20-1

xii

·3.2~:-'[eansurface laye r curre nts(10 m)ofnon-linea rCA:'\DIEmodelwith-

out theCceiolisfo rce[i.c.f=0) 205

.3.25 Surfa ce densityfrom non-li near CA:'\"DI E mod el using1998 win dfo re-

ing_ D

5.16Su rfa ce velocity fro m non-linearCA:'\ DIEmod e lusing 1998 windfore-

illg. Wi

5.2 7 Asin Ftgure5.25 butusingan uher n ar e topog ra phy filethatincludes

Bo na vis taBay 208

6.1 Su rfa ce velocityfieldIrom non-linea rC.-\:'\DIE mod elshownat day

201and~.8and 12 dayslat erin 1998 218

6.2 Su rfacevelocit yfield fromthelinearC.-\\" DIEmodel shownat day201

andL 8 and12 davla t erin 1998 219

6.:3 Drifterposit ion s~·days afterrelease(onday 201)in thesu rface laye r ofthe non-linearvelocity fieldin TrinityandConceptionBay 220 6A Positionat day 213 of dri ftersreleasedatday201 illthe surfacelayer

ofthe non-linearvelocityfieldinTr inityandConce p t io n Bay 221 6.'3 Drifterpositio ns~daysaft errelease(d ayWI.1998)fo r su rfacevetoc-

tri esfro m linearmodel .) .)0)

6.6 Drifterpositi on s12 daysaftcrrelease(d ay201.1998) for su r faceve-

loci t iesfrom linearmodel 223

6.7 Sa mp le of drifter tracks overaperiod of 20 daysstarting day201199B.

Thenon-lin ea r velocityfieldisused with particledlffuslonsetat 10 m²s-l2'2~

6.8 Sam ple of driftertra cksOH'raperiod of 20 daysstartingday 2011998

usingthe linear velocit yfield 225

6.9 Dis ta ncetr ave lledove r 20 daysbypar t iclesrel easedinnon-linearmodel

Olld ~ Wl

m

6.10 Dis tuncetravelle dover20daysbyparticlcsrele a sedill linearmodel on

d~ 2 0 1

n7

xii!

Acknowledgements

I wo uld liketotakethis oppor tunityto thankall thosewhoha n>mad econt ribu- tionstothisdisser tndon.The hos pitalit y oftheDepartmentofPhysicsand Physical On:'auogra phyatXlcmortalLnivers ity is gra tefullyacknowledged.

Iwish totha nk firstand fo rem ost my thesissuperv isor.Professor Brad de Youn g forme ntor ingmethro ug h all theseyearsdespiteadversity. He andProfessorRichard Greathat ch .whowas co-s uper viso rfo r partofthese studies.conrrtburedsu bst a nti a lly tolily knowled geand understandingofoceanography,

Ialii gra tefultoDrs. Pie rrePe pin.Dr. YakovAfanasavev..IiruHe lbig and

.Iinyu Sheng fordiscussionsand interaction s.IthankDrs.David Schwab.\\·illiam

O'Connorand George~rellorforthe irhelpfullco mme nts onCha pte r 3.Ithank Dr.

David D:euich forproddingrho originalsour ce code asastarti ngpoint forthe model used in thisstu dy.

Igrat efully acknowled ge the fina ncialassist ancebythe School of Crnduare St udies.

the Department of PhysicsandPhysicalOcean ography. theDepartmentof fisheries andOceansandthe~Iaritillj{'AwardSociety ofCanada whichhaveco ntributr-d to my educntion illthe formof fellowshipsand scholars hips.

Bre ndaBurkeis thanked for relayingnumerouscorr esp ondenceandfaxes.Len andLizZedclarcthankedfortheirfrien ds hip andmor a l support. Iwouldalsothank lilyfollow st ude nts YouyuLu.~IarkPepin.AnneParadisandHughesBe noit for the ir sharingof ideasaswell us the irfriendship. Iwould alsopraisethe positive

benefitsof theGordon Resear ch Confere ncein Coas ta lOcea nograp hy in prodding anop p ortun ity to discussscience inan infor mal andcons tructive setti ng.

filially.specia l thank s goto my par e nt s andfamily.who havealways beena source ofsupportforrne.Theirlove andencourage me ntisgreatlyapp reciat ed.Ithan k my father. Dr.\\-a1t erf. Davidson.for acriticalreviewofthis thesis.

Chapter 1

Introduction

1.1 Location

Alasting impressi onfor visitorstoSt..Ioh n's.Xcwfou ndla nd(F igu re 1.1).isthe vanahl hryin thewea ther ste mm ingfr o mthecity's loca ti on011theAvalon Penin sula inthoXorthwestAtl a ntic.

Till'AvalonPen insula lit's offthe west e rn edg eof theGra ndBanks. southof whichIiI'the eas twardflowingwarm waters ofthe GulfSt ream.\\-atersto the south of the Avalonpenins ulaarc relativelywarmin thesu m me r(Fig ur e1.2)with su r fa ce tetupornt ur cs abo ve16"C in August. Tothe north-east oftill' pe n insula isthe sout hwa rd flowingLabradorCu rr ent whichisresponaihle forcarr yingsea-ic c closer to theequ a t orthnu anywhe re ciS(' on theplanet.Eac hsp ring.sea-teeap pea rsoffthe eastcoas t ofXe wfouudla nd.sometimesexte nding well sout hof the isla ndtolatitudes of~~QX.Bylatesum me r,thesurface tempera tu res onthe Xcwfc undlandShelf reach rou~lll.\·8·12 QC (Fig u re1.2).coolertha nonthe Gra nd Bankstothe south.

windsin St. John's,par tic u la r ly in the early su mm e r,arenoticeab ly cooler when the winds blowfro mtheoceanbecauseofthe presence ofthe Labrado r Cu rr e nt[P rin- seubergetal..1997).\\"armsou t h- west erly wind sinfluencedby war mcont ine ntalair musses.arc oftenapleasant change afterthe Arc t icwindsfrom the nort h-cas t.This oseillation of windcontribu t es tothe widera ngeoftem pe ra t uresobservedinany given month at St. Jo hn'swhere thetwodom ina n t winddirect ion s are from the northeast (('0 01)andthesou r hwcs uwann}. TheLa b rad orCur re nt thus hasast ro ng inrlirectin flue nce ontheEast Co as t of Xcwfoundla ndandtheXewfo u ndlandShelf.

The cold wa te r"of the shelf have been spawninggro u nds forgene rat io nsof Xorth At lantic cod,precip ita t ingmanyfishingendeavoursandserrleme rn fromEurope since tlu-late15thccn t ury.

The sourcewaters ofthe LabradorCur rent arcout flowfro m HudsonBi\Y.Davis Strait.and thewar mer(_ 4°C ) Wes t Greenland Curr en t(S mithetal. (1937):

Koihncveretal.(1967):Cha p ma nand Beardsl ey ({989):and Mert zctat.(1993)) . ThoLabr ador Cu r re nt iscom pos edofthreebranch esat HamiltonDa n k.Theinshore branchcarries appro ximatel y1.GSv(1Sv

=

10⁶OIl ,,-1) ,theoffsho re branch sit uat ed ovr-rthr-shel f hrr-ak ca rr ies rough ly9S\'and the deep circula ti on(>1000 m)ofthe Labrado r Seiltransp o r ts an esrima red40S\' aspart oftheXort h Atlan ti c Sub po la r gyro(La zie r and Wrig ht.1993). Theinshore andoffs ho re branche sarebaroclin ic in nature whi le thedeep offshore branchisprimarilybaro tropic andassociatedwith thela rge sca lecircula tion in the Xo r chAtl ant ic.Colbo urneet al.(199G)show that

along the Bc na vist aTransect(Figure1.1)curr ents are sp rea dou tove rtheshel f with ,

:i- 1OemS-Isout heast wa rd flows insho re. incr easingtowar d s theshelf break.They (Colbourneetal.. 1996)meas uredthe totaltransp ortat 6S\" with3S\" acco unted for by geostr ophy(i.e. ther malwindequation s )alon g t!Lis transec t .AdjacenttoTri ni ty awlConceptio n Bays .theflowsplits(F ig ure 1.3)with asout hwa rdBowthro ughthe Avalon Chan nel[Petrie andAnd er so n.1983 ) and aneast war d flowalo ng thenort hern edge of the Grand Banks .

The seasonal strati fica ti onofthe wa te r colum n on the eas tcoastof Xe wfou ndland ca n b(' describedasfoll ows. Duri ng thewinter.sur fac ecooling ensures tha tthewate r colum nis\\"('1[mixed withtempe ra turesarou nd0DC.Analys is of datafro mStation 2,(cf.Figure1.1)showstha tduring thistimetht:' firstbaro c linicmod e phasespeed has a value of 0.3 mS-I. Theseas o naltempe ratu reandsalinitycycl esInfluen ce I!J<,for m a t ion of icealon g thelabrador CoastillJanua ryandFebruar y. an dit s subse quent advectiondown th eXewfou ndla ndShelfC\lye rsand Akcnhoad. 1990). Overthe course of thesum mer. once the packicehasbeenblow noffsh ore orhas melted[P r-inscnherget11.1..1997).the convectivel y mixed water colum nslowlygains strutificationfrom Apri lun rllllateAugustat whic h timesurfacewater s areattheir warmosr.withwate rlip tot.I°Cmeas uredat Stat io n 27(Pe tr ieetal.. 1992a).Sranon 27 islo ca tedjustafe wkilomet resfrom S1.John's(F ig ure1.1)andisassum ed toDC

<Igood proxy fortheseason a lvarta uousof tem pe ra turest ra t ificat ioninConception

and TrinityBays (deYoungand Sa nde rso n. 1(95).Str at ificat ioninflue ncesthe wind

forcedcirculatio nbymodifying the phase speed of inte rn a l gravity waves[Kundu.

1990)

\Yhill'wind sdoin fluen ce theacttvtetes of theresidentsofSt..jo hn's. thequestion arise showwind influences circ ulatio n andtemperaturedistrtbuno ninthe nearby Tr in it yand Concept ion Bays .Inth is thesis .Iexplo re thewind drivencircu la tio n andtemp era t ur e dis tributionin Trin ity andConce pt io n Bays(Figure1.1).\\'hile thcctlecr oftheLabradorCurrentisalwayspresen t.

r

foc us onthe wind fo rced compon ent ofthecurre nt. Animporta nt fea turenca r [hecoast is[he occurrenc eof upwelli ng ofcold deepwateranddow nwelling ofsurface wat er.duetowindforcin g and the sub sequentpropa gat ionof thislo calresp onsea.~wav es .

As anint roduc t ion.Ishall firs t reviewthetheoryof coastal tra p pedwaves(Sec- non 1.2),Ithendescribetheloc a l oce a nography and observa t ions illConceptio n and Trin it y Days(Scction1.1)and prese nt res ul t sfromprevio usmod ellin g stud ies (Sect ion1..t)inth isreg ion .InSectio n1.5.Itouch brieflyalllarval fishabundance work and theimporta nce ofphys ic al advectiontolarvalfish distr ibutio ns. Fina lly lformulatethethesisobj ec tive s in Sect ion1.6and describe thethesisout linein Section 1.7.

1.2 Coasta l TrappedWave s

InTri n ity andConce ptlonBays,windforcingappears tobethedom in ant contrt b..

urorro the circula ti o n and ther mo clin e displacemen t (de Youngct.al.. 19930).Over

time. on an f-planc[i.e.theCortolis forc eis co ns tan tinspace).class icEkmantheo ry shows that.for wind parallel toan infiniteshoreline.thewindpush es thesur faceof the watertotherightof the windtowards(or away) fromthe coas t(Gill. 198 2).This creates anupwell ing (or downwellin g)ofthe isotherms instrarlficd water (orthefree surface in thebarotropiccase) andis alocal effect of windforcing.

\\·ind canalso affectcirculat ionnearthecoast or011the shelf fro m some dista nce away.This isachieved by oneof two mecha nisms(orboth);theKelvinwave (Lord Kelvin.18,1)andthe Cont ine nt a lShe lfWevo (Robinson.t96-1).

Kelvin waves occ ur in arot a ti ngfluidinthepresen ce ofa vertical boundar y which docsnotallow wate rtofiowthrou g h it. \\-henthe wind pus heswatertowards oraway froma coast with some alongsho revariatio n.itcrea tesanalongshore pressure grad ienttha t propagatesas a gravityW3\·Cwith thecoastto therightinthenor thern hemisphere. Thiswave is calledit Ke lvinwave. InternalKelvinwa vescanoccur whcnovor isopycnals intersec ta coastl ine with a verticulwalldesp it ethepresen ce of variabletopography(\ \·a ngand :-'Iooers.19, 0).Therest oringfor cefor Kelvinwaves is gravit y while thegcostro phic balanceprod uces a free-surfaceslo pe(or sur face press uregradientin the rigid lid case]tobalancethe Ccriolisaccelerat ion.In this thr-sis.I concent rateonthe effect oftheinter nal Kolvln wave whosephasespeedis us ua lly lessthanI mS-I .hilt depen dsallstratification.

Over a variable ho tt om slope,howe ver.ContinentalShel f Waves occurwhose res tor ingforceisthe conservation ofpla ne taryvort.icity. \ra t('T mo vin g011oroff

tI ...shelfbecomesco mpressed orstre tched.respectivcly, fmelngit to spinsloweror faste rto conserveplanNaryvorticity .The vorticesinteractwuh cecb othe r making itho rizon tal oscilla to ry motion tha t ister med a Shdf\\-a\~_

Xumc nc a lstu diesof coastaltrap ped waves overvariable topog ra phyinthe pres-

('11("('of continuousstratjficat ionwere firstcarri edout hy\\-a ngand ylooers (1916).

endsubseq uently byHur hn an ce(19.6 ).They 5110w,-'\1thatforcontinuous srrenfica- nonovervaria ble topograp hycoastaltrap pedwaves behaveessentially as acomb tua- t.ionof thetwowave -t yp os.Forweak stratificationandstee p topography. thecoasta l ocenn respondslike cont inentalshelf waves while for st ro ngstr a ti fica t io n the response isdominatedhyinternal Kelvinwa ves.

A measureofthe strengthofstrati fica tio nreta uveto the bottomslope,illorder to<letermiu(' whichwavelimitapplies. is the stratificationparame ter5 [Huthna nce.

19.6 ):

s= _~:~1

^(LI)

where.Yisthe represent ativc buoyancyFreq uency.Hisr hema ximu m dept h.fistill' Coriolisparamet erandListhewldrhof the topographicalvariations. for5« I.

the she lfwave solutionappliesand for5» I.thl' Kelvin wave solut ionappl ies.

Inhetween.forSclose10unit y.hybridwaves occur. ForConcept ion Bar in the summer.thetyp ica lmille:'ofSis10(deYoung etal..199.1b).Thus.forConception Bay.Iexpecttha t bolt omtopog ra phy will exer tlitt leinfluenc e on wan'propa gat ion duua("(('rist ksaroundthe hayduringthe sum me r months.

Tho presenc eofvariablehaltomtopogr aphy near a coast also gin's risetoedge wan's.Edgewan's aresurfa cegravitywaveswithfrequency highe rthantheinertial frequencyfandare not affectedby theCor io lis force.TheseWI\H'Smaypropa ga t e withthecoasteit he rtotheleft orrig ht. Edgewaves aretrappedclosetoshore by thl'('ffpcts of variablebottom depthand can readilybe observed at beaches.In this study.however.Ishallap plythe rigi dlid ap p roximatio nandthe refo resurfacegravity wan'sarefiltered out ofthe modelsolution(;"Iysak. 1980) .As willbe explainedin Chapter2.therigidlidapproximatio nforces the barotropiccompo nent of the ocean torespondinstant lytoforcing.

1.3 Lo calOceanographyof Trinity an dCo n ceptio n Bays In thr-stud y of de Young andSanderson(199.'i).it was obse rved thatthest anda rd deviationof flowin Conce-ptionBayisfive timesgreaterthantile magnitudeof observed mean flo w.which is<2emS~Iat20mdr-pth. ThusImay inferthat theint rusio nof theIns ho re Brandl ofthe LabradorCurren t in to Conceptionand Trin it y Bays ispro ba bl yspo radic.Tides nrc weak with current amplitudesbetween 1 and2 emS- Iforthe ;"12and1\:1tidalcomponents(d e Youngand Sanderson.1995 ).

Spat ialccr rela nonscalesnrr- roughly .r.g km.approximatelyequivale nttothcRossb y rad iusof thefirst barocltntc mode for summerstraufi catlon.Dccorrelnriontimescales from cu rren t mooringsvary be t ween2 and.5days.Hori zont aldiffusi on estima tesfrom lagrang iandr-iftersare roughly 70-100m²S-I(d eYoun g andSanderso n. 199 3 ).

Suchdiffusio n include s effects ofed dies wh ichmayberesolve din high resolu t ion no n-ltuca r modelsandisprobabl yan upper estimatefo r the ditlus ivity.

Severalphysicalobse rva tio nst udi esinConce ption Bay haveta ke n placeinthe spri ngandsu mmer mo nth s .Drifterobserva t ions have sho wncyclo nic eddies (de You ng and Sanderson.1995). Ananticyclonic eddyfea turesin thedia gn os t ic out p utof de YOUIlj!; N al (199301) basedon de nsit y observations. Th e appearance of anticy- do n ie andcyclonicmotion further illus t rates theva riab ilit yofthe circu la t io n pattern intheBay. At afixed point in time..\DCP(Aco ustic Curre n t Dopple rProfiler )tra n- sects across the mout hofthehay showtha t the surface flowreve rsesdirectiontwoor three times between outflo wandinflow(Pepinet al..1995).Sur face drift er observa - nons of circulurton pa t t ern s furt her indi ca t e the prese nce ofstrongspa t ial\<triability inthe centre oftheha y.whe re drif ters release d closetoge ther (:2 km)pursue differe nt paths.deYoungandSand e rso n(199.')) alsonoted that nea r bottom cu rre nts(100m) showed sometid al effects aswell asindk-atious of aseichemovement at11periodbe- twee n :Vi days. Atthesur fa ce . however.tidesarc negligi blecomparedtowin ddr iven current variability.flowat.lcprh nearth e mou t hisusually op positein direc ti onto that ncarthe su rface(deYo un g andSanderson.1995)_

1.4 PreviousModeli ng Work

Pr evio usphys ical ocea nograp hy modeling st udiesfor TrinityandConceptio n Oay have focusedontill.'wind driven responseusinglinea rredu ced-gravity sha llo w wate r

models (Yao(W86).Otte rson (199 2) and deYounget<11.(1993 b)),Thesemod els han' a lightla yer of waterof agivendep th(-10IIIforexample ) overlyi ng a much deeperlaye rcons ide redtobeinfinitely deepandat rest. The rigid lidapproxima tion ismade wheredisplacementat the air-seainte rfaceisassumed tobe muchsma ller thanthedis plu remcnt ofthe interfacebetweentwo wa te rlaye rsof differingdensity (Gill.1982),

The imposit io n of acoas tal boundary im plies diver genceorco nve rge nce ofwind drive n horizo nt alsurface flow .Thisdrives the interfacebetween tile twolayersup or downgen er ati ng alongshorepressure grad ients(a t disco ntinuitiesinthecoastl ine) thatpropagate asinternalKelvinwa n 's,Thereareno edge waves orshe lf wavessince bott o mtop ography is absentfrom the model.Since the modelsarelinear.theydo not allowforhorizontaltrans portof water properties{i.e.temp era t ure.sali nityor

Thestr eng t hof a 2layer redu cedgravuvmodelwhe ncompa ring wit hobserva - tions liesinthedisp lacem entof thepyc nocline.The 2lay('rmodelis solved for the displacem e nt oftheinterfacebetweentilt'upper and lower layers. Thisprocedure is simil artothe displa cem e ntoftheobse rved pycnoclinewhichis direc tlyarrr tbutodto thediver gen ce ofthevertica l integratedhorizontalcurrent s0100\·('thethe rmocline,

Otter so n(199 2) appliedawind - force d.single-layer.reduced-g ra vity,sha llo w water model10Co nce pt io n Bay alone. Agr eementof modeled thermoclinedisplacement with mooringdataatthe headof the baywasfoundinthe 2to.')daylime scalewith

to

11cohe re nce squaredof arou nd 0.9 for 6 degrees of freed om. However , agreementwith observedthe rm o clin e displ ace mentat themouth of thebar was poor. .-\1 the mouth circulationmay be st ro ngly influenc ed bymean currentsontheXewfoun d lan dShe lf as measured atStation

Z,

(?).She lfcurrents are probablybarodinicallyunstable (Anders on.1986 ) whichmardrivevariable flow in and outof Trin it yand Conce p ti on Gays.Such forcing wouldbe mostno t icea b le at themou t h of the bays.Themodel re p ro d uced 4ma jo r upwelli ngeventsobse rved inthe bar during a 60 dar per iod in 1989.The mod el do mai nincl ud ed onlyConception Bar withmode l bound a ries extending 20 kru beyond themou t h of thebay.Shallo wwatermudd compar iso ns with observations show thatthesemod els dono tre pr oduce velocitiesaswell[Yao (1986). d eYoung et al.(1993b))as comparisonswit hthermocl ineobservatio ns.

deYeunget at. (l99 3b)then app liedthesamemo del as Otterson(1992) to Tri n it y and Conce ptionBays.Incl uding Tri nityBayin themodel domainincreased till'explained variance between themodel'sin terface displacementand theobserved r.hcrmochnc disp lace mentinConroptlo nBay. Themodelre prod uced.the r mocline dlsplacr-mcnr best at thehea d ofthe hay,AddingTr inityBaytothemodeldom a in increasedthr-am plitu d e of [hemodeled inte rfacedisplaceme ntinConceptionBay, such tlw titbetterma tch ed the amplitudeofrho observedthermocli nedisplaceme nt.

par'ticulnr-lynear themouthonthe wes tern shore(deYoungetal.. 1993b). These findings support the idea thatupstreamwindforcingdirec tlyinflue ncescircu lat io n illCono-ptlonBay,

11 For time-de pe ndent win d-drivenmodels.boun d a ryconditio ns needtoheimple- mentedca refu lly.:\Iod el ge nerat ed effectsshould occur duetowind forcin gwit hi n themo de ldomain bounda ries[Great batch andOtterson.(991 )and notby spurio us dfensarisingfrom the op enbound ary conditions.Kel vi n waves sho uldbe ge nera t ed onlyduetothe presence of land andwindinside thedom ain. One couldincl ude generat ionof Kelvinwave satthebo und aryifadd iti o nalInforma t ionwere available.

Groat batc handOtt e rson (199 1)chose theboundary of the irmodel do m ainsothatthe upstreamop enbounda ry(in the senseofKelvin wave propagat ion]isacontin ua tio n of thecoast.Theyabo usedabo undary cond ition ofnonor ma lgradient(Xcumann) atthe inflowbou nda ry- onbothvelociti esand interfacedis p lacemen t. Thisensu res no sp ur iousupwellingge nerated at theupst rea m bo und a ry occ u rs. Theboundary trearrucnt of Grca t barc h andOt t e rson(1991)was also used inOn crscn(1991) and de Youngoral. (1993 b).

A diagnosticmodelhasalsobeenappliedto Concep tionDay (deYoungctal..

1993a)" Diagnosticcalc ul at ionsassessthecu rrents foragiven fixeddensi tyfiel d usua llyprescribedfromobserva t ions.Effectsofwindfo rcing and wavepropag a tion arcim p licit illthe presc ribeddensityfield.Thercsult ingveloc ityfieldsshowoneor two gyr cspresentin Concep tion Day.albeitwithsomo meande ringcurrents .Howeve r.

evident inthemodelledvelocity fields is theabse nceofacoasta ljet (t he rearc1i0 tuncdependenttermsinthis mo del)consist entwithlineartirue depen dent theory"

It isnotew orthytha tinthe surface laye r,at 2tildep t h"velocitydistributionsare

12

primarilyaffectedby wind fo rc in g andgeostrophy.ratherthan from the density field;

resultingina stro ng hor izont a llyho mogen eo us velocity fieldrela t ive to thatat 20III or .30 m of dep th[d eYoun getal..H>!).3a).

.-\ linearreduced- gravit ymodelandadiagnos t icmod elcons titu teusefulandver- sarllc tools asfirst approachesto modeling curr e nt s ill a bay.Ther e are.however.

limit at ionstosuchoverlysim p lifiedapp roa ches. The reduced-gravitymerle]docsnot producea 3D struct u re for thebay, itswaveprop ag ationchar acterist icsremainfixed.

andit does not incl udethe non-linea rte r ms ofthe Xavler-Srokoseq uations.Fur- thermcrc .thismodelign oresbot tom topog ra phy.On the otherhand.thediag nost ic modelgivesa3Dpict u re ofthe circula t io n fieldwh ichisa "snapshot". geostroph- icaltyFrozenintime. ofthecirculati on field . Circulationinthe ocean.ontheshelf andillthebaysisdj-na rnicandthree dimens io nal. Duetothe variable natu reof thecirc ulation and wa terdensi ty distributionin Conceptio nandTrin ityBays.a .3 dime nsion a lprognost ic eddy-resolving mod el isasensible approach.

1.5 Biol ogicalApplications

Theinflue nceofphysical circu la t ion andtempera turedistr ib utiononbiological organis ms isunde n iab leandakno wled ge ofthe oceanogra p hicenvir on men tca nhe use fulill understa ndi ngbio logicalobservationssuchas thoseoffisheggand lar val uhundancos(HelbigandPepin.1998 b).

St u d ieshan'h~1Icond uc te dprevious lyonla rvaladvect io ninthisregion. In

13

particularDavidsonand de Youn g (1995) demon s t ratedthat itisunlikely thateggs spa wn ed onthe she lf break (200 killoffsho re)will drif t into the hays of Xewfo un d- land as larvae. This conclus ion is ccrro bo ra rcd by genet ic differen cesfound between cod fishwinte r ingoffshore and those wint erin ginsh or e intheXewfoun dland Bays (Ruzza nt c ctal.. 19%).

\\'ithrespecttoConcep tio n Bay.man y studies have been carriedoutonfacto rs influencing la r val ab u nda nceobscrvanons. (0particu lar(to na mea few).effects ofsizedep e nd enc e ofpred atorand prey {Paradisctal..19%), larvalgrowthwith rcm pr-ra ture(P epin. t99l).adve cri veInfluences oflar va e intoan dout ofthebay (Pepin<'tal.. 1995) andturb ule nce(Doweretal..1998) have been investigated.

The irnpo rtnnceofadvecrt ve effectsonobserved la r val abundancevariesas the square root ofthe studyarea(Pep in ct al.. 1995 ),Howeve r,even in the theoret- ica l limitwhenphysica l vuriabillryiswell determined.currentbio logicalsaillp l i n~

pro t ocolsca nno tad equatelyresolvela rva l ab u ndancevariability {l.e.mortality and gro wt h) (Hel bigand Pepin.1998a .b).

Thediag nos ti c circ u la tio n field s ofdeYoungctnl.(1993a )wereapplied tomo d el- ingthe dis pe rsion ofca pe linlarvae (MullottllSl)illOSll.~)in Concep t ion Bay (deYou ng ct al..199-1).They concl udedth at la rva lca peliu residencetimewasreduced on the eas te rn shore of the hay where larvae werecarried outint otheAvalon Chan nel.Xot surprising ly, theyfoun d tha tresidence timewas higllcs t at thehea d ofthebay.

"' h ileadvecnon andtem peratureaffec t fish atthe lar valstage. they mayalso

influe nce adultfish.Templeman(1966) surm ised th a t codfishrespondedto near- shoreupwellingeven tsbymovi ng int o shallo werwaters wher e they canbe caughtby thetrap fishery" This was observedby Roseand Leggett(1988 ) whonot edhighe r ca tc h ra tesofcodunder fav ou rabl eupwelling condit ionson theQuebecsho reof the Gulfof St.Lawr ence.

\\·!tt'thercod fishres pond to coastalupwelli ngeventsby moving to sha llo wer10- cariouscloserto sho rehasnot yet been decisivelydeterm ined . Sch ne id e r(L99.t) . Schneider andxte rbven(ID88).andIngs et al.(19D,)prop oseamod ificat io n ofTom- plemans (I DGG)idea "suggestingtha tcodres pon d onlyto extre meupwellingeve nts (whe re su rfacetempe raturesdecreasebymorethan.t°C).and"moveinsho re only afte rupwellingsubsides.Theobser vationsofRoseandLegge tt(1( 88 )ofincrea sed r-archesof cod by tra psonthe northshoreof theSt. Lawre nceestuarymayhaveheen due to such extremeupwellingevents.

Capellndis t r ib u t io ns withres pect to upwellingeventsinthis region have also bee n considorodby Schneid e r(l99..J.).SchneiderandMethvcn(lD88)andln gset al.(I W ,) "

ClllikC'cod.the near-shore prese nce of capcllnincreasesduring as wellas preceding strung upwellingevents.

The biologicalgoat ofth isworkcentres011prodd ing arealis t ic timedep enden t doscripuon ofsur facecurre ntsand surfacetempera t ure dist r ib ut ioninresponseto wind forcingforTr init y andConception Ba ys. Knowled ge of these currentfields r-antln-nbe ap p liedtodet erm ine mea nreside nce timesandtrajec tor ies.toim prove

thedesi gn of sa m p lingsche mes forlarval abundance in the bay (HelbigandPep in . 1998b ) andtopro videinsi ghts asto whe re (andwhy ) upwell ing is mostprevalentin the region.

1.6 Res e ar chObjective s

This work has severalobjectives.allofwh ich rely on the crea tion andimplemen- tationof a prog nosticeddyresolvin gmod el based on theC.-\:,\OrEmode l(She ng etal.,1998)[basedonthe Dic C.-\STmodel (D iet richct al.. 1987)). Theworkcall lu- divided into technical andscie nt ific objecti ves.The techn ical obj cc t ivcs of this thesis.are theas..sessmcntofthevalidityoftheC.-\:'\DIEmode l versio ncreat ed at

~Iellioria[Un iver xit y aswellas the appl ied bou ndarycond itio ns.This objective is achieved by app ly ing themodeltoasuccessio nofproblems ofincreasingcomp lex ity and comparingthe mod el solu t ionwith pub lishedstudi osaswellas linea rsh a llow wutor modelsolutio ns .

The scientificobjcctivcs incl udean unde rs tandingoftherole ofver-tica ldiffus io n in gcucratingmea n circulat io nin acoasta len vir o n ment. Thepr im a ryobject iveis to uuders tandthe winddri ve nres pon seof Trtnir yandConcep tion Bay sundersteady and variable wind co nd it io ns. Theapp roachtak enhere is to build upgra dua lly all nndr-rst andingofthewind fo rced circula t io n through theapplica ti onofsimple.

shallow-water modelsto simplifi edem ba y me nt topo gr aphiesbeforeaddin g physica l complexity aile step atalim e.

16

Thlsthes ispullstoget he ra mult itude of published and unpub lis heddatainde- sc-rjhing the response of thesebaystowin d forcing. One specific objectiveis the compariso nof model resu lt s wit ha varietyofsynopticobservation datasets suchas remo te lysensed observations (see Chapter2).Thisobj ec tiveinclu des an understa nd- ingofobservedtem pera tu re distr ibutionsas well as the thermocl inedisplacement.

Theinflue nceof theno n-linea rte rmson these mod elsolutionsis explored.

To date.therehave been onlya handfulof physical oceanogra p hicresearch pu b- licatiunsdev oted to Conceptio n Bay.andeve n fewertoTrinityBay. Recentcbse rva- ricnsofcod spawningandaggre gationsinTrini ty Bay (c.g.Smcdbol and Wroblewski (1997))may sen-etoencour a ge futurephys icaland bio logicalworkin Tr inityBay.

This thesis pro vides newunderstandingof wind drivencirculatio nin TrinityBa:,r.

TIH'bio log ica l objecrlveofth isstudyistodescribeparticleretentionpatterns in TnuitvandConcept ion Ba ys .A questio n Iparticularlywis h toansweris whetherthe larvaldrift betweenTrinityandConce ptionBay is signifi cant..-\ secondaryobjective relatestothemapping oftheretention pa t t er nsof larvae inboth bays.

1.7 Outline

Thiswo r k is presentedas follo ws.InChapter2.Ipresent thenumericalmo d els used and discusstheir irnplomenratiou. further mor e.1 describea variety of cbscr- vauouat da t a towhich mo del resultssh alltwco m pared in Chapter .5.IIIChapt er3.

rho C.-\\"DIE model isappl ied to a parabolicstrat ified la ke withadia baticboundary

17

conditionsin the absen ce ofany windfo rcin g.In Cha pte r~.the resp on se of bays to 11steadyoutof the baywindstressispresen ted.In Chapter 5.the response of the baystolime dependent windforcingis st udied.In Cha pter6. modeledvelocitiesare usedina track ing modelforanalysisof particleadvectioninTrinityandConce ption Bays.FinallyinCha pte r7. a brief sum ma ryanddisc ussion onthe import an t results of thisthesis arc given.

18

Eigutc1.1:Theregion of inter estfo r thisst ud}'includ es Trinity and Conception Days,Xowfo un dlan d.Regularocean ographic observa tio ns havebeen mad eon' [the past50yea rsatstation27.just a few kilom ete rs offsho re fromSt. John's.

19

Summer

Surface

Temperature

-62 -59 -56 -53 -50

Longnude -47

-44

-41

20 18 16 14

12

10 8

2

Figure1.2:Sea Surface Temperatur e (SST)imagefrom anopt imalest imatio nofa climatedatabase ofallavailable temperatur edata takeninAugust fortheeast coast (roughly 15millionindividu al observat ions).Theimageis courtesyof DougGregory (Head,Data ManagementandServices)at the Depart ment of Fisheries andOceans, BedfordInstitute ofOceanography.

20

Figur e1.3:Figure rep roducedfromxlc rtzctul. (1993) showing thelocation ofthe Lab ra do r Curre nt. Xo tcthe inshorebran ch of theLabradorCurrent neartile:'\ew- foundland Coastandthe offshore branch along the shelfbrea k.Flowis splitaround theGra nd Banks wit hlittleflow overthebanks(GreenbergandPetr ie,1988 ).

C hapter 2

Mode l A p proach a n d O b s ervations

2.1 Introduct oryRemark s

IIIthischapterI describethe modelingapproach for Trini t yand Conce p t ionBays andthe datausedtoverifymodel results .Idescribea hie rarchy of models ranging from fairly sim ple2-D linearmod elstoaso ph ist ica ted 3-Deddyresolving model . The primarygoalistomod el thewinddri vencirculanon inTrinityan dCon ceptio n Bays.

[ focus on the effec t oflo cal wi nd forcinginthe absen ce ofexplici text e rn alforci ng at the modelledope n boundari e s .Thus.the reisno explicitinfluence ofthe Labrador Curr en t inthemodeldomain.Implici tl y,however,some influen ce ispresen tinthe init ialconditions forde nsi ty stra tifica t io n. Theonlyexplicitforcin gin themodelis localwind forcingoverthemod el domai n,

Three stand ar docea nog raphicap p roxi mationstotheequations of motionareap- pliN[ here.The firstisthe rigid lidapprox im a t ion.in wh ichtheverticaldisplace me nt ofthr-air-seasur fac e interfaceisignored.as suming thatitis muchsmal lerthanthe

vcrrtcatinte rn alinterface displace ment bet wee n two la ye rs ofwat er of diffe rin g den- sity.The rigid lid app roxi m at io n implies tha tpressu revariesatthe su r face(.::~0) inste adofallow ing thesurfacetomoveup and down(Gill.1982).Inthe rigidlid ap prox imati on. the ba ro tro p ic mode solution is distorte dsince thephasesp eedis higher thanth a t in thefreesurfacecase.Th is distor tionofthebarotrop ic wnvcspeed isdue to thesho rt pressure ad just me nt tirnescale oftherigid lidco mpa red to tha t of rho freesu rface. Thefreesu r fa cetakes a finiteamo un t oftim etomoveupand dowuandthepress u re adjust saccordingl y.Intherigidlid ap p roxim a tio nhowever, pressurebuildsupnea rl y insta nta neouslyat the surface.since by de finit io nthereare no vertlealvelocitiesat therigid lidsu rface. Therigid lid approximationwo u ld not I){' appropriatefor surface tides where spa surfaceheigh tvar-ies overrune.andwould ovcresrimarebya factorof 100 (or mor e ) thebaro trop ic mod e phas espee d.

Thesecondassumption istheBo uss inesqapp roxi m at io n(Spiegel andVeronts . 19GO):here spa t ialdpnsity vur iu rionsare ignoredintheco n se r va tionofmome ntu m equat io ns with the exceptio n ofthegravitationa l term s.TheBo ussln csq ap proxi ma- tio n is validwhe-n relativedens itycnria rio ns~are sma ll.Furthermore.under the Bousslnesqap proxi mat io n.the cont inuityequationsimp li fiesto zero diverge ncein velocity(v.it~O) as for an inco m pressi ble fluid.

Thethird simplification is thehyd rost a t icapproxim a ti onin whichthe vertlealruo- mentu mcquntionis takena.s a bala nce between pressu re andgravityand thewater column is approxima tedtobeatrest inthevcrtical axis.The hydrost a t ic app rox ima-

23 tionis validwhen horizontalscalesofmoricn arcmuchbigge r thanvcrticalscalesof motion(G ill. UI82).Verticalvelocitiesarepresent.but calc u late dfrom the continuity equation.The rela ti ve errorinvertle a l velo ci ties islarge sincevert icalvelocit iesarc derived fromthe horizontalgradie ntsofhorizon t a lveloc it ies han'relat ively smallval- lies anticonseq uently largeerrors (comp aredto the errorsfo rIIandv}.Thevert ica l velocityfieldsare thereforeexpectedtobe inhere ntlynoisy.

2.2 Dens it yStr atification

In this section.Idescribehowthe initialdensitystratificatio nformodel runs of Trinityand Conce ptio nBays is derived.Themo stfreque nted oceanogra ph icsrnricn in thisregionis Station17(Figur e1.1).which has beensam pledoverthelast 50 yearsrou ghlytwice amo nt h(Petrieetal..1991 a). withhigh e r sam pling ra tes in the sum me r andfull.Station 27is situ atedabout 20 kmtothe southof the mouthuf ConceptionBay.andisconsid e reda reasonable proxyfordens it ystratificafionin Couccptiouand Trinity Days(d eYo un g andSan d e rso n.19(5).The density profile at St a t io n27isobt a in edfrom densityobservationsat standard dept hsO.10.20.30.50.

7::;.100.11::;.150and1/.5III(Petrie ctnl..199 1a)..-\.11densit y observat ionstakenin themonthof June overa-10 year observationper iodare usedto extractanaver age .Junc densityprofi le[Figure 2.1a ).I thencalcu lat ea rep reseutativevertical pro file of tln-squa reof theBrunt\·iiisi!lii frequencyS:l(Figure2.1h}whichisa measu re of the

"2-1

Htab ilit yofthe watercolumn.

(2.l)

Here y is gr av it at io n a l acceleration in ms->. Poistherepresentativefluidden sit y.

andPis thefluiddensity.Thelinea r densityequnrion can he approximated hy

P,+U'pz

=

O. (Z.2)

wherethesubs c r ip tstand::repres ent partlul differentiati onwith respect to tim e audthe vcrticalcoo rd in a tec:u:is the verttcalvelocity.Ihave neglectedho ri zon t a l densitv transportter ms anddiffusjvity,The onl yfacto r affectin gdensit yisadvccricn iiithe vertlealaxis. Su bs t it utingforPoin equation 2.2 one ge ts:

PI- po.\'2u:

=

0 9

Sl'pa rati ngthe aI10\'cequanonusing st an d a rdsepa rat io n of variables

(u.I:.'!!"' )

= t

(u,,(r. y .t).r,,(r.y.t).p,,(r.Y.t) )(',,(::) Po "",0

(2. 3)

(2.-1)

yield san equa ti onin Sturm-liouv illeform WhOH(,solutionsarc orthogon al(K und u . 1990).

(V i)

25

HI'!I'rnisthe phasespeed ofawan.'associated wit h thevertic almod efunc t ion l'n'Equation 2.5issolvedfo rafla t bott om ocean with200m depthand a vertical rcsolutlon ofImusingmatrix inversion.Thisverti calresolurionof 1 myield s :?OO modes,[spl'cify arigidlidandno flux cond it ionatthesu rfa ceandbotto mof the water column(~

=

0at z

=

0 and

==

-H).Thereis onebaro tropic mod!"for whichthe verticalstructure functi onisuniformatalldepths (LI'. t'o=1) andthe phasespeedislarge (...000 m:;-1)asaco nse q ue nce oftherigidlidapprox i ma ti o n . Lsiug the.v:lprofile give nin Figur e 2.1b ,Isolveforth ever ticnlmodefunctions andthe ir respec ti ve phase speeds(f igur e 2.2).Thewavespeed of thefirstba roclin ic modofor themonthof June is 0.51 ms".givi nganint e rn a lRossbv radius ofdefor- mationof.') kill.fo rtherest of theyear.sim ilarly calcu la t ed monthlyphasespee ds vary betweenCl

=

0.3ms-!in~[archandCl=0.8til05-1inSeptember for the firs t barocltnic mode.The abovecalculationofnorma lmodephas e speedsprov ides usefu l ins ig ht into the sing le luyr-r reducedgraviryshallo w Will ermodeldescrt br-dnext.

2.3 Mo de l Des cript ion

TIIrI'c differe ntnumericalmodelsareappliedill thiswork:(1)A sing le- laye r.

reducr-d- gravit vmodelin whichthewavespeedisspec ifie d:(2) aflat bottom.con- rlnnously strat ified.linear shallow wat er mo del with seve r almo de s:and(3)the 3·0.

ZccoordlnateC...\SDIEmodeliiilinea rized andno n-lineari zed form.

26 2.3.1 Single-Layer,Redu cedGravityModel

r

take averrtrn l averageofthe equationsofmotionfora uniformlayer ofwater of undis turbeddepthHanddensityPIoverlyinganinfinitelydeep quiescent bott o m laye rofdensity

os-

The rigid lidapproximat ion ismad e .

Thecorr es pond ing mcmeur um equations forthe toplayerare

Ul - II:

=

-a'l/z+

p~~

^{- eu,}

I:t+fu

= -g'l/~

⁺

p~~

^-£1:.

andthecorres po ndingcont inuityequa t ionis

(2.6)

(2.'1)

HN I'.y'is thered uced gravity.definedas(~)^g.IistheCo r iolis parameterset atIO-~s-I .uandt'arethevelocityco m pone ntsoftheupperlayerintherandy rlirr-ctions andTZand7¥arcthcra ndycomponentsofwindstr ess.

\\-eSC [the undisturbeddepth of thetip pe rlaye r at -tom,toapprox ima tethe vcrtica l profile of thefirstbarodin ie mod e forthe monthof.Iune(Fig ure 2.2).Fur- therruore. the wave propagationspeedc

= ..Ji1"

is setat0..53emS-1torepresent thcwave spee d ofthe firs tharoclimemode (F igure 2.2 ).Damping inthismodel is lin...ar and done using consta nt Rayleigh frict ion andXewtoniancooli ngparamet ers£ and~..The dampingparameterizationisan approximationfor theverticalinte grated diffusi onterms . Insteadof diffusingaway sharpgradie nts .vartabtcsarcda m ped ac- cordi ng totheir insir u ma gn it ude.ThePr a nd tl(P r) nu mbergivenby ;.Iiimost

cases{and~,are se ttoO.butIexpe rime ntwithrunsincludin g variousvalues of { and-,to study the effects ofda mp ing andof thePran dtlXumbe r.

The modelequat ionsarcfini t ediffere nced onanArakawa C-grid(Fig u re2.3) using theme t hodof Heap s (1971).Thefinite differ enc e equa t io nsarc comp utedin the followingorder: (1)Firs t the inte r face dtsplacc mcne via the comi nnityequation is updated

[ ( " " ." ." )] 1

I " .. I== "_H~t II •.} - 1l,_I.}+t"J -t,.)_[ _ _ ,

/ '1 ~.r ~y 1+~..~t ^(2.8)

(2 ) Theilthe bou ndarycond itionsare ap plied to'I:(3) ..\.fter this .Iupdat e theIIand v componentsofvelocity.

{2.9 }

en"[

⁼

[ en

⁺~t^(_/'~-J+_{U~_I .J}+11~-I .J "1+_{u~J + l}_g"I:~} _1^-IJ~J+_~)]_1_

.J. ~y PI H 1+f~t

(2.10) III the above equat ions.the damping ter ms [i.e.~,andu}are differencedbackwa rds inrime for uncond itio na l numerica l st a bili ty(of these te rms). .\11otherter msare differencedforwards in time.Thestability criterio nfor tilt> fo rwa rd differe ncing in time is~.r/~ t>CoSincetheph ase speedcis fixed.it isnecessa ryto ens u retha t

~f<~.r/cwherec==../ilH.Thusthe max imumtimestep forastabl.. solutionwith a phase speed of0.5IIIs-lam!a gr idspaeiug of·) 00Inis 1000s.

28

2.3.2 StratifiedModel(N o r mal Modes)

Her e the three dimensio na l linearequa tio nsof mot io n arcsolved by separation into standardvc rticalnormalmod es (sectio n 2.2)for1\flatbottomed coastalocean fo llowing :\fcCreary(1981).Theprod uc tofvert ical viscosity(o rdiffusivi ty] timesthe sq u a re of theBrunt\'iiisiilii freq ue ncy(.Yl) is assu medconsta nt. This simp lifica t ion allo ws for easy separ atio nofvariables: (vx X"J

=

'\,\f)and(Ii:x X"J

=

.\pl. The cons t a nt s ofpro portiuna lity.\.\/and.\pcallbeequalor di fferen t{i.e.ProF1).

\\'efirs t sepa ra tetheequatio nsof motionintover ncalnor ma l mudesandthen solve theequations fo r thehorizont alstructure ofeac h mode,givenbel o w. using the finite'differe ncemethodofHe aps (1971). as forthereduced- gr avity mode l

~

v ^-ft'"

=-~

v.l'

+ F"

^-

(~) ""

c"

D,'" up"

(0 ''')

-;:;-+f u,,=- ...,...- + C ,,- --:! t·"

vt v!/ c"

~_at+c",(~ax+

0:)

iJy -

__

(~)

c;.

p"

(2.11 )

ThevariableslI" .l'" .p".arcthevelocity compo nent s and pressu re foreachmode.

In thismo d el. the co nt inuity/densityequation isexpressedinter ms of press ur e .as opposedtoint er fa cedisplace me ntforthesinglelayermodel. \nt hv

= .\.

..,/X² and"

=

'\I';'\~2 ,viscosityanddiffush'ityareexpressedas Xewtoniancoolingand Rayleighfrictio n in theabove shallowwater equano ns:Po isthe refe re ncedensityfor tile model:F"andC" arctheprojec tion s oftheLa ndyco m po nentsofwindst ress

29

oneachmode(d iscussed below).Thedensityandthevertic almomentumequati on s an'easily der ived(d.Kundu(1990)).Till'pha se speedsc..arcde ter mined from the normal mode equ at io ns anddepe nd onthe.\'2profile select ed.

It sho uld he no ted that Ray le ighfrict ionand Xe wrc nta nda m ping dependallli e;.·

Since highe r modeshaveslower phase speeds.they areda mp edatagreaterrrnc and propagatesho rt e r dlstu nccs beforetheydec ay.

\\"indst ress isappliedat the ocea n- a t mos p hereinter face.\nndstress maybe impa rtedto the wat er colu m n either as(1)a bod y force over adepthH.s:.or(2) as the surfacebound ar ycond it io n. allowingmoment um to spread downwardthro ugh the wate-rcolum n byvert ica l diffusion.Forwindst ressap pliedasthe surface boundary condit ion,.\.Ifand.\"mustbenon-zeroforthe mod eltoha vephysicalmean in g.

Furthebodyro rcoimpleme nt atio n.windstress proje c tio nsF"andG"are:

(F". G nl

= (: : :) ji~~~~n:~:

^{(2.i 2)}

Her e.r" andrvarc thehori zon ta l co m po n e ntsof windstress.H",is the depthove r whk-hwind forci ngis applied asa bod yfor ce.D is thetotalwaterdepthandlo·nis rhovcrtical struct urefunctlon for thenIhmode.

Forwindstress applied as a su rface boundary condi t io n.the project io nsonto the norm almode saregil-enby:

(2. 13)

Themodelsolutio nisobtainedbj-su m m ingthe modal solut io ns(un.1.'" ,/1..)with

30

the ver ticalstruct ure fo r each mod e. The amplitudeof eachmodeisdep en d ent 011thestre ngthof thewindst ress projecti on{i.e.themagni tu desoftheF"and G n Iunctions]and the dampingcoefficie nts

.\,\1/2"

and.\ pjc;,.Th e windstressprojec tion s on thevr-rticalstruct ure1;:"ofeach mo d e(F..andG n) depen don thewindstress application method (see figure :2.4).Lowerbaroclinic modes aretheleastinflue nced bythe windst ress projectionmerhod. Forthe first andsecondbaroclinic mo des.only the.'i0iiihul k lay erwindstressproj ectionmeth od yields differ entresultsfro m the ot hermethods.particula rlyfor thesecondbaroclinicmod e(Fi gure2.-1)..Appl yin g windstr ess over a bul klayer of ;)0mroducostheimpor t ance of all hig he r mod esto neg lig ibleco nt rib ut ions . Forthefirs t 10mod es.the win dst ressprojec t ion forthe 1011\hu lk laye rmeth odand sur faceboundaryconditionmethod give almost ident ical res u lts.

2.3.3 CANDIEModel

Thethirdmodel Iusc. and theprimary mod el ofthis stu dy.isthe C:\:\DIE model(Sheng et al.. 1998)based ontheDicCASTmodel of Dietric hct al.(1987).

Thismodelsolves the 3--0 non-linear Xavier Stokesequationsonan f-pla nc usin g the hydros t atic .Boussinesq and rigid-lidapproxi mations.Densityisused asth emo d el trace rinste ad of tem pera t ureandsalinity for computa tio n a lfeasib ility.The r,y.and .:moment umequatio nsas wellas thedensity andcont in u ityequat ionsarc:

31

~

^{+Cu -ft·='}

- ~~

^+VmU+

~l\'/ /E

^(2.1.t)

~ + CL'+ fU = - ~~+ DmC + ~K:~

^(2.1.5)

o = -~ -

pg (2.16)

rJf

+Cp

=

DI'P+;;A/f/; (2.17)

I/ Z+t.'y+u:,=0 (2.18)

(2.19)

Hen'Cb;thend vec tion ope rator. Dm•andVI'arcthehor izo nt a lmome ntum oper-ator-and densitydiffusion ope ra to r resp ect ively.These operators aredefinedas:

(2.20) (2.21)

foragivenvariableq.A,,,,andAI'are respectivelythehor izontal viscosityand dif- fusivitycoefficients..-I.:andK:arcthe verti caldiffusiviryandviscosity coe fficie nts respectively.

Theaboveequa t ions are solvedon a3-0.Araka waC-grid [F tgurc 2.5)wit ha time S[C'pof.)min utes for111km horizont a l resolu tionru nand 100secondsfor;.500III ho r izonta lresolut io nmodel fun.IlISCquadraticbotto mfriction whe re thefrictional drag: on velocityisprc por t icnaltoth osquare orthe magnitudeofthevelocityinthe botto mlayer.

31

A free-sl iporano-s li p cond it ion L<;ap pliedonthelat e ral land bou nd a r ies.In the free-dip bounda ryconditio n.the gradientofthesho re parralle lcu rren tisset to 00\"('1' ulllandboundaries.In the no-s lip boundary'condition.theshore parr a lle lvelocities areset to0attheland bounda ry. Inallcases the reisnoflux ofmomentumor densirythroughthela nd bound aries.Thesen...ith-itytofree-slip andno-slipbo unda ry conditio nsis tested indlap!ef~.for allru ns . apartfromthose inchapt er :) (thelake domai n) and whereindic at edinchapter-I.afreeslip bou ndary cond itionisused .In thisthesisthe1I~ofthefree-slip ortheno-slip bound ary conditi o ndoesnotcha nge thr-model resul ts significantlyaswill beshewnin chapter-I-

"('rtic al and hori zo nt a l viscosityanddiffusivity aresetconst a n t. Thevert.ica]

rcsolunonis10mthrou gh ou tthe wa t ercolu m n.foran eddy viscosityof1O-~m^Js-".

III['uppermodellave rcon ta insallofthe Ekmantranspo rt.

Till'model solvesforvert tcal vr-loci t y atthr- intcrfaeeberwren modellaye rs(fig- IIH'2..5).At themodel surface (.:=0).urisze ro under the rigidlid approximati on. Atthebottomboundary.u-isalsozeroin ordertoeliminatemomentu mflux through the boucm.

The surface'boundary cond it ion fo r windstress is:

(2.22 )

whoreTZandT~aretIL("or andycomponent sofwin dstress.1\":isvertical diffusivit v and Poisarcprcsenta tive \1I1\lefot'thede nsityofwater."·illdstress is ap pliedtothe topmoo dlayer at ("wry rimeste p.Therodlsrrtbuucnofthisim pa rted mom e ntum

33

10 deepe rlavers takes place by verticalmixing.

The ope n boundaryco ndit io ns of the modelareappliedfollowi ngGreat.batchand Duerson(19 91) fora shallow wat ermod e l. The upstreamopenboundary(inthe senseofKelvin wave propagation)is an extensio nof thecoas t line (Figu re 2.6). The ba roclinic and baro tropiccompo nents of thevelocitynormalto the openboundary aretroatedseparately"III the firstinst ance. I set thenormalgradientto zero across allboundary pointsfor veloci ty (both uand v components ).densityandpressure.

Su bsequ entl y"Iremove the verticallyaveraged velocitynorm altotheboundar)"from lo calvelo cit yat each boundarygrid point.Inthe verti calaverage.thereisnoflow normalto the boundary.Thisercarcs divergenceat the boundary andim pos essma ll verticalvelocities attheboun d a ryfromthe cont inuit y equation.The zero verticalav- erugcvelocit y on thebounda ryis consis t e nt withthezero verticalaveragebaroctini c modo stru ctu res{l.e.J~D^l·..d:;

=

0).The Xcumannboundarycondition(Le.no nor- mul press uregra d ient) proved to he suffici e nt in allowingpropagationof disturbances outof the domain"

For non-l in ear runs thatcover longperiodsoftime [i.e. 10-20 daysor marc).

Imod ifythe vertlealdiffusiontermsin the CA:\"DIEmodel.Inthis case .vertical diffusion ofde nsityis appliedonly10 departuresfrom the init ial densit y field.This permits backgroundstratificationto be modifiedby theadvec uonte rms. butnot thediffusionterms. This approachis alsoaform of nudgingthe model solution tomai ntainthe initia lstratification.Thisavoidsthe necessityof includingsurface

heati ngtoma intainsum me rstratifica tion.

2.3.4 BoundaryCo ndi ti ons

Allthr eemode l top ogr aph iespossess threeope nboundaries(Figures2.6 . 2.7 and2.8 ) Atthenorthern andeas tern ope nboundaries Iapplya Xeu mann(no nor - mal gradient] bound a ry conditio non pressure(orsu rfaceelevation}.Onthe souther n boun d a ry,a Som merfeldrad ia tio ncond it ionis setfo rthesha llo w watermodel sto allowKelv-in wavesto leav e thedomai n,Inthe3-0.CA:\ D IEmodel.aXcu munn cond ition is appliedat allopenbou nd a ries . \\'iththesha llo wwate r mo d elsIsolve for the baroclmic mod eswhichim plies zero verticallyaveragedvelocity. Inthe3- D z-coordiuaremod e l. for comparison tothenorma l modemodel results.[set the ver ticallv aver ag edvelocityatthe open bound ar ies tozer o ateach modeltime step.

III general.open boun da riesprese nta challenge inoceanograp hicnumericalmod- piing. Thepro b lem isillposed (BennettandKlocden.I!),S ) andall bounda rycon- dition s requiresim pl ifica tionsof themo d eldyna m ics atthe boundaryofthemode l domain. Ope nbou ndary cond it ionsconsti tu te asource of ina ccuraciesinalmos t any model(Ch ap man.198.5).TIIf' boundaryconditio ns appliedher earepassive (l.e.

thereis noforcingapplied atthe bo un da ry ) save for zeroingthe vertleal aver a ged velocity normaltothebounda ryin the C:\S DlEmodel. The cho keof bou ndar ies worksreas o nablywell he re.Test cases witharadiat ion conrlitiononthe south ern openboundaryshowed nonotic eab leimp rove m ent.

2.4 ModelSetup

2.4. 1 ModelDoma in

III th is thes isIapply threedifferentmode lgeo me t ries.an idealizedflat bo tt o med MjUiUer-mbayme nt(Figure2.6 ).anIdealizedfla t bott omedrectangularemb aymen t (Figure2.i) .and therea listicgeome t ry ofTrinityandConceprtcn l3aY1i (F igu re 2.8).

In thc la tt er.rea hsuc geome trycase.Iuse eitherrealistic bottomtopograp hyora flat bonomedversion tha tretainsthereal ist iccoastlinegeomet ry.Forthosemodel runsthatusc aflat botto m.the depthis200 m.The averagedepthofConce p t ion Bay isabout 200malthoughthemaximumdep this roughl y290m.Therea ftsr tc ho ttomtopogr aphy (Fig u re 2.8 ) is applied forsomeofthcrunsusing the3-DCAX DIE model.In allcases. themodel'supstreamopenboundary(inthesense ofKelvin wave propuganonlisanextensionof thenorther ncoastline.as inGreat bat chandOtterson

noon.

Hsieh('tal.(198.1)show tha tif the model hortzoura lgrid spaci ngexceeds the Roe..h~·radiusof deformaucn.spuriousspatialoscillationsstartto occur.Th eUS("of 500mgr idspac ing ensuresproperresolutionoftherad ius of deform a t ion forthefirst 10 haro din ic mod es [Iro mS km1050)111)(S('("Figur e 2.2).xtodcsgreaterthan10 areassumedtohan'a negligible cont ri b u t ionto theoverall solution .

36 2.4.2 Wind Forci ng

\\"indforcinginthemod el is eit her specifiedforidea lizedstudiesorcomput ed from obser vedwindspeed.[ignoreorographic effect sand assum e tha tthewindis spa tia lly hom ogeneous over TrinityandConceptionBays asindeYou nget al.(1993h). \\"h ile theRossby rad iusofdefor m a t ion in the at mosp hereis around 1000 krn.orograp hic effectsdo affect windloca lly onsmalle rscale s(Smit hand~laI;Pll('rson,198;).How- ever.in theabs ence ofanysparial cove rage ofwindobse rva tions.Imakeuse ofthe mostcom p rehensi ve datasetforwindspeedand direc t ion inthisregion. Hour ly wind obse rvatio ns arema d e by Environ mentCanadaatSt.Joh n'sair port.which is situater]10 kmtotheeastofConce p rio n Bay(F igure1).

\\"in rlsrresscalculatio nsrequire wind sveloci ties10be specified10 mabovethe ocr-an-airsur face,whichIdeter mineinthefollo wingmann er .Observedwindspee d dat aatSt.Jo hn'saremultiplied oya correctionfactor of1.25toacco untfor thedif- fcrcncc inwindspee d overtheocea nrele ttvo tola nd(Smit h and),l acP hcrso n, 198 7) ,

\\"illd speer]isthe nconve rte dtowindstressaccording tothequadr a tic formulation ofLa rge andPond (1981),I low-pa ss filterwindstressto removesignalsatperiods shortcr th a n 32 hou rs ,The filterisrunforwardand backwa rd overthedatatoremove anyeffect s ofphase distortionhythefilter. The las tmod ifica t ioninvol vestheappli- eat ionof asmoot hrampingfunctio noverthe first two days oftheco mp utedwind stressda t a. Thisproced ur e avo idsexcitingPoin carewavesofnea r-inert ialfrequ ency when thewindstress timese ries isinlt.ialtyappliedtothemod el(P ollard and),[jllard.

WiD}.

\\'indstresscalculated followingtheabove approach is shown inFigu re 2.9 for theyear 1900and 190-l.They-axispoints30'"eas t of northsotha t itis aligned with the axis ofthe bay.The timeperiodshown cover s roug hly60 days sta rtingatJulian dayt-l·)(:-'lay :2-1th).This timepe riod coincideswithmooringde ployme nts during JuneandJuly ofthese twoyears.

\\'iudsshow simil archaracte ris t ics over hot h obser va t ion period s\Yindblows out of the hayforperi od s of;}' 6 dayswith weakwindsin bet ween.Inbot hyearsthereare a fewshortepisodes(1-3days )of light windsblowin g intothebaywithma g nitud es ofloss than0.1Pa .Thro ug ho utthe twoperi od s of obse rvations.pea k windstrengt h subsidesfrom 0.-1 Pain earlyJune to0.2Pa towardstheendof.Iuly(Figure2.9).

Analysisofthehistoricalreco rds ofwindspeedatSr.John'sair port from10.');}' 199:2 sho ws thatduringthe sum mer thefrequency ofsouthwesterly windIncr eases whilethe freque ncyof northeasterlywinds dec reases.:-'Ionthly binn edwindspeeds ove-rthe -10 year recordshowa dropin max im um windspeedsfro m-10mS-Iin Februarytoaminimum20mS-IillAugus t.:-'lea n.mo nt hlybinnedwindspeedsalso showa mi nimumin theslimm erof6 mS-I,comparedto 8IIIS-ISpcc<!Sinthe winter.

Xlost ofthe windene rgy occu rs at peri od s of :2to5 days .Figur e :2.10showsthe power spec t rum fo r theun filtered wind stressforthe 50-60 dayperiod ofinte rest (.IUIICand July)in1090 and 199-1.Iiibo t h cases. there is abroadpeak in energyat rollg;hly .)days(0.2cpd).Alsonot iceabl eisasmall peakatroug hly1 cycleper day.