A model of the plume front of the Pearl River Estu

ContinentalShelfResearch24(2004)1779–1795AmodeloftheplumefrontofthePearlRiverEstuary,ChinaandadjacentcoastalwatersinthewinterdryseasonL.A.Wonga,
,J.C.Chena,L.X.DongbaCenterforCoastalandAtmosphericResearch,HongKongUniversityofScienceandTechnology,ClearWaterBay,Kowloon,HongKongbStateKeyLabofChinaandSecondInstituteofOceanography,StateOceanicAdministration,P.O.Box1207,Hangzhou,ChinaReceived18July2002;accepted31March2004Availableonline10August2004AbstractThePrincetonOceanModelwasemployedforadetailedexaminationoftheplumefrontdynamicsofthePearlRiverEstuary(PRE),China,andthesmallscalecirculationassociatedwiththeplumefrontinthedryseasonduringthewinter.Ingeneral,theplumefrontinthePREisformedbythesalinitygradientbetweentheriverinewaterandthehighersalinityshelfwaters.AlthoughthebasicfeaturesinthedeeperwatersoftheplumefrontofthePREaresimilarwithrespecttotidalvariationsand,windandriverdischarge,boththetideandriverdischargeaffectthesubsurfacefront,whilethesedynamicfactorsmodifythesurfacefeaturesoftheplumefront.Atwo-layercirculationintheverticaloccurswithintheplumefront,whileatthesametimeahorizontalcirculationconvergesintheplumefrontalzone.Thedensity-drivencurrentthatisassociatedwiththefrontaldynamicsplaysadominantroleinmodifyingthedynamicbehavior(boththestrengthandthescale)ofthecirculation.Basically,thesmallscalecirculationassociatedwiththeplumefrontdynamicsdominatestheestuarinedynamicsaswellasthedynamicbehaviorofthewesterncoastalpartoftheplumefront.Moreover,mostofthedilutedwateristransportedalongtheplumefrontfromtheestuarytothewesterncoastalregionduetothefactthatmostofthewaterconvergesontwosidesoftheplumefront.Thesub-tidalcurrentplaysanimportantroleintransportingsuspendedsubstancesalongtheplumefrontduringaspringtideduetothenon-lineareffectofthetideontheheterogeneousdensityfieldandonthetopography.r2004ElsevierLtd.Allrightsreserved.Keywords:Estuary;Plumefront;Circulation;Mixing;Cross-fronttransport;PrincetonOceanModel1.IntroductionUda(1938)initiatedthestudyofoceanicfronts,butthisareaofresearchwasnotstudiedfurtheruntilthe1950s.ResearchduringthatperiodwasARTICLEINPRESS:10.1016/j.csr.2004.06.007
Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](L.A.Wong).mostlyfocusedonoceanicfrontssuchasplane-tary-scalefronts,westernboundarycurrentfronts,andshelfbreakfronts,etc.Thestudyonsmallscalefrontsinoffshoreareaswasnotcarriedoutuntilsomedecadeslater,andtheconceptsofashallowwatertidalfrontandplumefrontwereproposedbySimpsonandHunter(1974)andGarvineandMonk(1974),respectively,onlyinthe1970s(Wang,1992).SimpsonandJames(1986)pointedoutthat‘‘Fronts,especiallythosewithaconsistentposition,areoftenthoughtofasdividingtheoceanintodifferentregimeswithseparatewatermasses.Thisshouldnot,however,betakentoimplythatfrontsarenecessarilyabarriertohorizontalexchange.Theymay,onthecontrary,experiencestrongcross-frontalfluxesofpropertiesthroughtheoperationofhorizontalmixingprocessesonthelargegradientsthatoccurthere.Thevelocityfield,particularlytheverticalcirculation,mayalsobeimportantinbringingaboutaccumulationsofbuoyantormobileorgan-ismsinthefrontalzone.Thepresenceofafrontwillmodifythecirculationandmixingprocessesinwaysthatmayhaveasignificantinfluenceonthedispersalofpollutantsdischargedintocoastalwaters.Forthebuoyantpollutants,convergenceassociatedwiththefrontwillcertainlyinhibitdispersionandleadtoaconcentrationofpollu-tantsalongthelineofthefront.Suchaccumula-tions,whichfrequentlyindicatethepresenceofafront,mayevenproveuseful,aspreferredsam-plingsitesinthedetectionofincipientpollutionproblems’’.Ingeneral,thesmallscalefrontcanbethoughttoplayanimportantroleinmodifyingtheestuarinedynamics.ThePearlRiveristhethirdlargestriverinChinaandthePearlRiverEstuary(PRE)islocatedonthenorthernshelfoftheSouthChinaSea(SCS),nearHongKong.ThePREisadynamiccomplexestuarywitheightriverinletsonitswesternboundary(Fig.1a)(Zhao,1983).ThenamesoftheinletsareHumen,Jiaomen,Hongqili,Hengmen,Modaomen,Jitimen,Hutiao-men,andYamen.ThePearlRiverdischargeisabout3988m3s
1inwinterandpeakswith20076m3s
1insummer.TheannualaveragedischargepassingthroughModaomenisthelargestofalltheriverinlets.ThesecondlargestARTICLEINPRESSFig.1.TopographyofthePREandadjacentcoastalwaters(a)andthelocationofthecross-sectionsexamined(b).L.A.Wongetal./ContinentalShelfResearch24(2004)1779–17951780dischargeislocatedatHumen.Therearetwodeepchannels(WestChannelandEastChannel)withintheestuaryfromthemouthoftheestuaryuptotheheadoftheestuary,andtherearemanysmallislandsoutsidethemouthoftheestuary.Inthisregion,thereisanumberofdynamicalfactors,includingbottomtopography,freshwaterdis-charge,winds,tidesandcoastalcurrentsthatcontrolhydrodynamicprocesses,circulationanddistributionofthephysicalparameterssuchaswatertemperatureorsalinity.Thebottomtopo-graphyisastrongconstraintonflowpatternsinshallowcoastalregionsthroughvortexstretchingandsquashing.FreshwateroutflowfromthePearlRiversetsupastrongdensitygradientandresultsinawestwardflow