Manuscript prepared for Ocean Sci with version of the LATEX class copernicus cls Date: March

Marion Kersale

Description

Niveau: Supérieur, Doctorat, Bac+8
Manuscript prepared for Ocean Sci. with version 3.2 of the LATEX class copernicus.cls. Date: 9 March 2011 Sensitivity study of the generation of mesoscale eddies in a numerical model of Hawaii islands M. Kersalé1, A. M. Doglioli1, and A. A. Petrenko1 1Aix-Marseille Université; CNRS; IRD; LOPB-UMR 6535, Laboratoire d'Océanographie Physique et Biogéochimique, OSU/Centre d'Océanologie de Marseille, France. Abstract. The oceanic circulation around the Hawaiian archipelago is characterized by a complex circulation and the presence of mesoscale eddies west of the islands. These ed- dies typically develop and persist for weeks to several months in the area during persistent trade wind conditions. A series of numerical simulations on the Hawaiian region has been done in order to examine the relative importance of wind, inflow current and topographic forcing on the general cir- culation and the generation of eddies. Moreover, numerical cyclonic eddies are compared with the one observed during the cruise E-FLUX (Dickey et al., 2008). Our study demon- strates the need for all three forcings (wind, inflow current and topography) to reproduce the known oceanic circulation. In particular, the cumulative effect plays a key role on the generation of mesoscale eddies. The wind-stress-curl, via the Ekman pumping mechanism, has also been identified as an important mechanism upon the strength of the upwelling in the lee of the Big Island of Hawaii.

• eddy present


• comprehensive ocean-atmosphere


• quikscat wind


• eddies


• accelerates north


• performed numerical


• north hawaiian


• numerical results

Sujets

• Hawaii
• California
• Counter
• Karman
• Southern

Informations

• Publié par: profil-zyak-2012
• Nombre de lectures: 38
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

ManuscriptpreparedforOceanSci. withversion3.2oftheLAT E Xclasscopernicus.cls. Date:9March2011 Sensitivitystudyofthegenerationofmesoscaleeddiesina numericalmodelofHawaiiislands M.Kersalé 1 ,A.M.Doglioli 1 ,andA.A.Petrenko 1 1 Aix-MarseilleUniversité;CNRS;IRD;LOPB-UMR6535,Laboratoired’OcéanographiePhysiqueetBiogéochimique, OSU/Centred’OcéanologiedeMarseille,France. Abstract.TheoceaniccirculationaroundtheHawaiian archipelagoischaracterizedbyacomplexcirculationandthe presenceofmesoscaleeddieswestoftheislands.Theseed- diestypicallydevelopandpersistforweekstoseveralmonths intheareaduringpersistenttradewindconditions.Aseries ofnumericalsimulationsontheHawaiianregionhasbeen doneinordertoexaminetherelativeimportanceofwind, inflowcurrentandtopographicforcingonthegeneralcir- culationandthegenerationofeddies.Moreover,numerical cycloniceddiesarecomparedwiththeoneobservedduring thecruiseE-FLUX(Dickeyetal.,2008).Ourstudydemon- stratestheneedforallthreeforcings(wind,inflowcurrent andtopography)toreproducetheknownoceaniccirculation. Inparticular,thecumulativeeffectplaysakeyroleonthe generationofmesoscaleeddies.Thewind-stress-curl,via theEkmanpumpingmechanism,hasalsobeenidentifiedas animportantmechanismuponthestrengthoftheupwelling intheleeoftheBigIslandofHawaii.Inordertofindthe bestsetupofaregionaloceanmodel,wecomparemorepre- ciselynumericalresultsobtainedusingtwodifferentwind databases:COADSandQuikSCAT.Themainfeaturesof theoceancirculationintheareaarewellreproducedbyour modelforcedbybothCOADSandQuickSCATclimatolo- gies.Nevertheless,significantdifferencesappearinthelev- elsofkineticenergyandvorticity.Thewind-forcingspatial resolutionclearlyaffectsthewayinwhichthewindmomen- tumfeedsthemesoscalephenomena.Thehighertheresolu- tion,themorerealistictheoceancirculation.Inparticular, thesimulationforcedbyQuikSCATwinddatareproduces welltheobservedenergeticmesoscalestructuresandtheir hydrologicalcharacteristicsandbehaviors. Correspondenceto: M.Kersalé [e-mail] 1Introduction TheoceanicareaaroundtheHawaiianarchipelagoexhibits acomplexcirculationcharacterizedbythepresenceof mesoscaleeddieswestoftheislands.Thiscirculationis mainlyduetotheeffectsofthearchipelagotopographicforc- ingonboththeNorthEquatorialCurrent(NEC)andthetrade wind.Indeed,theblockingeffectoftheHawaiianislands bathymetryontheoceanicflowissimilartotheoneofthese islands’topographyonthewind. TheNECisabroadwestwardflow,constitutingthesouth- ernpartoftheNorthPacificsubtropicalgyre.WhentheNEC encounterstheislandofHawaii,itisdeflectedtowardsthe south,butalsogeneratesanorthernbranch(Lumpkin,1998). ThenorthernbranchisknownastheNorthHawaiianRidge Current(NHRC),flowingcoherentlyalongtheislandsatan averagespeedof0.10-0.15ms − 1 (Qiuetal.,1997).The wakegeneratedbytheHawaiianislandisthoughtresponsi- blefortheformationoftheHawaiianLeeCounterCurrent (HLCC). Byaclassicalmechanismofformationofeddiesinthe leeofanobstacle,thewakecanalsogeneratemesoscaleed- dies,asindicatedbytheobservationsreporteddownstream ofGranCanariaIsland(Aristeguietal.,1994,1997;Bar- tonetal.,2000).InthecaseofHawaii,south(north)of theHLCCtheseeddiesaretypicallyanticyclonic(cyclonic) (Lumpkin,1998). Asecondmechanismcouldalsoexplaintheformationof animportantcounter-current(heretheHLCC)andmesoscale eddies(whetherinHawaiiorGranCanaria).Lumpkin (1998)showedthatthereisaneddy-to-meankineticenergy conversionatthelatitudeoftheHLCCimmediatelywestof theIslandofHawaii.ByasimpleSverdrupbalance,Cha- vanneetal.(2002)predictanHLCCfromthewindstress curldipoleintheleeoftheislandofHawaii.Asexplained bySmithandGrubisic(1993),theHawaiianarchipelago presentsaseriesofhighverticalobstaclestothewind.The

2Kersaléetal.:SensitivitystudyofthegenerationofHawaiimesoscaleeddies islandsactasbarrierstothetradewinds,whichareconfined belowthetropopauseandconstrainedtoflowaroundtheto- pography(Chavanneetal.,2002).Thewindstressvariations intheleeoftheHawaiianIslandsdrivedivergentandconver- gentEkmantransportsintheupperlayeroftheocean.And, inparticular,theaccelerationofthepersistentnortheasterly tradewindsthroughtheAlenuihahaChannelseparatingthese islandsisthesecondmechanismgeneratingmesoscaleed- dies(Patzert,1969).Westoftheislands,thismechanism cancreatecycloniceddiessouthof20 ◦ Nandanticyclonic eddiesnorthof20 ◦ N. Historicalhydrographicandsatellitedatasets(Patzert, 1969;Lumpkin,1998;Chavanneetal.,2002;Sekietal., 2001,2002;Bidigareetal.,2003)indicatethatmesoscale eddiestypicallydevelopandpersistforweekstoseveral monthswestoftheHawaiianarchipelagoduringpersistent tradewindconditions.Moreover,interdisciplinaryobserva- tionsofmesoscaleeddieswererecentlymadewestofthe BigIslandofHawaii,combiningdifferentdatafromships, surfacedriftersandsatellitesensors(Dickeyetal.,2008, projectE-FLUX).DuringthecruisesE-FluxIandIII,two cold-corecyclonicmesoscaleeddies,NoahandOpal,respec- tively,havebeenfound. TosimulatetheoceaniccirculationaroundtheHawai- ianislands,weusedaversionofthemodelROMS(Re- gionalOceanicModelingSystem)providedwiththefollow- ingROMSTOOLS[ http://roms.mpl.ird.fr ].Thismodeling systemisincreasinglyused,sinceitsfunctionalityandro- bustnesshavebeendemonstrated.Differentphysicalcon- straintscanbeimplementedwithregardtoboundarycon- ditionsandbottomstress.Differentdatabasescanalsobe usedtoprovideatmosphericforcingtoROMSmodel,such asCOADS(ComprehensiveOcean-AtmosphereDataSet Project)anddatacollectedbyNASA’sSeaWindsScatterom- eteraboardQuikSCAT. DongandMcWilliams(2007)usedthemodelROMSto studyvortexsheddingbydeepwaterislandsandtopographic eddyforcingintheSouthernCaliforniaBight.Yoshida etal.(2010)studiedthewindforcingofHawaiianeddies. Theyfoundthat,intheimmediateleesouthwestofHawaii (18.9 ◦ N-20 ◦ N,158 ◦ W-156.7 ◦ W),eddysignalshaveapre- dominant60-dayperiodandashortlife-span.Theynoticed thattheobservededdiesoriginateinthesouthwestcornerof Hawaiiandareinducedbythelocalwindstresscurlvari- abilityassociatedwiththeblockingofthetradewindbythe islandofHawaii.Jiménezetal.(2008)performedseveral experimentstostudytherelativeimportanceoftopographic andwindforcingonoceaniceddysheddingbyanisolated, tall,deepwaterisland.Theyappliedthemodeltothecaseof eddiesshedbytheislandofGranCanaria.Theyfoundthat windforcingaloneisnotsufficienttoforceanoceanicVon Karmanvortexstreetintheleeoftheislandandthattopo- graphicforcingisnecessary.Intheircase,thewindforcing isnotthemainmechanismresponsibleforoceanicvortex shedding,itactsasatriggermechanismwhentheincident currentaloneisnotsufficientlyenergetictoproducevortex shedding. Asregardswindforcing,monthlymeanvaluesfrom COADSwerecomparedtomonthlymeanwindspeedanddi- rectionfromCanadianweatherbuoysinthenortheastPacific (CherniawskyandCrawford,1996).Windspeedmeasure- mentsfromQuikSCATscatterometerwerevalidatedbycom- parisonwithindependentdata.Forinstance,theywerecom- paredwithmeteorologicalanalyses(Renfrewetal.,2009) overtheDenmarkStrait,orwithwindspeedscomputedfrom RADARSAT-1syntheticapertureradar(SAR)intheGulfof Alaska(Monaldoetal.,2004).Concerningoceanmodel- ing,theinfluenceofCOADSversusQuickSCATwinddata onoceaniccirculationwasanalyzedintheCaliforniaCur- rentSystem(Penvenetal.,2002).Caliletal.(2008)have donenumericalmodelingsimulationsoftheoceanresponse torealisticwindforcingintheleeoftheHawaiianIsland chain.Sinceweusethesamemethodology,westress,inthis paper,thesimilaritiesanddifferencesbetweentheirandour results.OntheNorthequatorialcentralPacific,anocean modelforcedbyCOADSwindversusoneforcedbythe NCEP-NCAR(NationalCentersforEnvironmentalPredic- tion-NationalCenterforAtmosphericResearch)reanalysis windwereusedtocomparetheobservationsofoceanheat content(WuandXie,2003).Independentobservationsof oceaniccirculationwereinvestigatedwithanoceanmodel forcedbyQuikSCATwinddataindifferentregions:thePa- cificsystem(Xieetal.,2001)andtheSouthernBenguela upwellingsystem(Blankeetal.,2005). Themainpurposeofthisstudyistoanalyzetherelative importanceoftopographyandwindforcingontheHawaiian oceaniccirculationandeddiesgeneration,withparticularat- tentiontowind-drivenmesoscaleeddygenerationsimulated withbothCOADSandQuikSCATwinddata. Thepaperisorganizedasfollows:section2introduces brieflythenumericalmodel,themainsimulationandeddy trackingparameters.Section3presentsthemodelsensitiv- ityandadiscussionaboutthevariabilityinthesurfacewind stress.Thispartwillallowustoanalyzewhetherthemodel reproducesthegenerationofmesoscaleeddieswestofthe Hawaiianarchipelago.Insection4,weanalyzewhichsim- ulationisthemorerealisticandwhetheroneforcingispre- dominant.Ourconclusionsaredrawninsection6. 2MaterialsandMethods 2.1OceanModel OurcirculationmodelisbasedontheIRD(Institutde RecherchepourleDéveloppement)versionoftheRegional OceanModelingSystem(ROMS).Thereaderisreferredto ShchepetkinandMcWilliams(2003,2005)foramorecom- pletedescriptionofthenumericalcode.Themodeldomain