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

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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


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ManuscriptpreparedforOceanSci.withversion3.2oftheLATEXclasscopernicus.cls.Date:9March2011SensitivitystudyofthegenerationofmesoscaleeddiesinanumericalmodelofHawaiiislandsM.Kersalé1,A.M.Doglioli1,andA.A.Petrenko11Aix-MarseilleUniversité;CNRS;IRD;LOPB-UMR6535,Laboratoired’OcéanographiePhysiqueetBiogéochimique,OSU/Centred’OcéanologiedeMarseille,France.Abstract.TheoceaniccirculationaroundtheHawaiianarchipelagoischaracterizedbyacomplexcirculationandthepresenceofmesoscaleeddieswestoftheislands.Theseed-diestypicallydevelopandpersistforweekstoseveralmonthsintheareaduringpersistenttradewindconditions.AseriesofnumericalsimulationsontheHawaiianregionhasbeendoneinordertoexaminetherelativeimportanceofwind,inflowcurrentandtopographicforcingonthegeneralcir-culationandthegenerationofeddies.Moreover,numericalcycloniceddiesarecomparedwiththeoneobservedduringthecruiseE-FLUX(Dickeyetal.,2008).Ourstudydemon-stratestheneedforallthreeforcings(wind,inflowcurrentandtopography)toreproducetheknownoceaniccirculation.Inparticular,thecumulativeeffectplaysakeyroleonthegenerationofmesoscaleeddies.Thewind-stress-curl,viatheEkmanpumpingmechanism,hasalsobeenidentifiedasanimportantmechanismuponthestrengthoftheupwellingintheleeoftheBigIslandofHawaii.Inordertofindthebestsetupofaregionaloceanmodel,wecomparemorepre-ciselynumericalresultsobtainedusingtwodifferentwinddatabases:COADSandQuikSCAT.ThemainfeaturesoftheoceancirculationintheareaarewellreproducedbyourmodelforcedbybothCOADSandQuickSCATclimatolo-gies.Nevertheless,significantdifferencesappearinthelev-elsofkineticenergyandvorticity.Thewind-forcingspatialresolutionclearlyaffectsthewayinwhichthewindmomen-tumfeedsthemesoscalephenomena.Thehighertheresolu-tion,themorerealistictheoceancirculation.Inparticular,thesimulationforcedbyQuikSCATwinddatareproduceswelltheobservedenergeticmesoscalestructuresandtheirhydrologicalcharacteristicsandbehaviors.Correspondenceto:M.Kersalémarion.kersale@univmed.fr1IntroductionTheoceanicareaaroundtheHawaiianarchipelagoexhibitsacomplexcirculationcharacterizedbythepresenceofmesoscaleeddieswestoftheislands.Thiscirculationismainlyduetotheeffectsofthearchipelagotopographicforc-ingonboththeNorthEquatorialCurrent(NEC)andthetradewind.Indeed,theblockingeffectoftheHawaiianislandsbathymetryontheoceanicflowissimilartotheoneoftheseislands’topographyonthewind.TheNECisabroadwestwardflow,constitutingthesouth-ernpartoftheNorthPacificsubtropicalgyre.WhentheNECencounterstheislandofHawaii,itisdeflectedtowardsthesouth,butalsogeneratesanorthernbranch(Lumpkin,1998).ThenorthernbranchisknownastheNorthHawaiianRidgeCurrent(NHRC),flowingcoherentlyalongtheislandsatanaveragespeedof0.10-0.15ms1(Qiuetal.,1997).ThewakegeneratedbytheHawaiianislandisthoughtresponsi-blefortheformationoftheHawaiianLeeCounterCurrent(HLCC).Byaclassicalmechanismofformationofeddiesintheleeofanobstacle,thewakecanalsogeneratemesoscaleed-dies,asindicatedbytheobservationsreporteddownstreamofGranCanariaIsland(Aristeguietal.,1994,1997;Bar-tonetal.,2000).InthecaseofHawaii,south(north)oftheHLCCtheseeddiesaretypicallyanticyclonic(cyclonic)(Lumpkin,1998).Asecondmechanismcouldalsoexplaintheformationofanimportantcounter-current(heretheHLCC)andmesoscaleeddies(whetherinHawaiiorGranCanaria).Lumpkin(1998)showedthatthereisaneddy-to-meankineticenergyconversionatthelatitudeoftheHLCCimmediatelywestoftheIslandofHawaii.ByasimpleSverdrupbalance,Cha-vanneetal.(2002)predictanHLCCfromthewindstresscurldipoleintheleeoftheislandofHawaii.AsexplainedbySmithandGrubisic(1993),theHawaiianarchipelagopresentsaseriesofhighverticalobstaclestothewind.The
2Kersaléetal.:SensitivitystudyofthegenerationofHawaiimesoscaleeddiesislandsactasbarrierstothetradewinds,whichareconfinedbelowthetropopauseandconstrainedtoflowaroundtheto-pography(Chavanneetal.,2002).ThewindstressvariationsintheleeoftheHawaiianIslandsdrivedivergentandconver-gentEkmantransportsintheupperlayeroftheocean.And,inparticular,theaccelerationofthepersistentnortheasterlytradewindsthroughtheAlenuihahaChannelseparatingtheseislandsisthesecondmechanismgeneratingmesoscaleed-dies(Patzert,1969).Westoftheislands,thismechanismcancreatecycloniceddiessouthof20Nandanticycloniceddiesnorthof20N.Historicalhydrographicandsatellitedatasets(Patzert,1969;Lumpkin,1998;Chavanneetal.,2002;Sekietal.,2001,2002;Bidigareetal.,2003)indicatethatmesoscaleeddiestypicallydevelopandpersistforweekstoseveralmonthswestoftheHawaiianarchipelagoduringpersistenttradewindconditions.Moreover,interdisciplinaryobserva-tionsofmesoscaleeddieswererecentlymadewestoftheBigIslandofHawaii,combiningdifferentdatafromships,surfacedriftersandsatellitesensors(Dickeyetal.,2008,projectE-FLUX).DuringthecruisesE-FluxIandIII,twocold-corecyclonicmesoscaleeddies,NoahandOpal,respec-tively,havebeenfound.TosimulatetheoceaniccirculationaroundtheHawai-ianislands,weusedaversionofthemodelROMS(Re-gionalOceanicModelingSystem)providedwiththefollow-ingROMSTOOLS[http://roms.mpl.ird.fr].Thismodelingsystemisincreasinglyused,sinceitsfunctionalityandro-bustnesshavebeendemonstrated.Differentphysicalcon-straintscanbeimplementedwithregardtoboundarycon-ditionsandbottomstress.DifferentdatabasescanalsobeusedtoprovideatmosphericforcingtoROMSmodel,suchasCOADS(ComprehensiveOcean-AtmosphereDataSetProject)anddatacollectedbyNASA’sSeaWindsScatterom-eteraboardQuikSCAT.DongandMcWilliams(2007)usedthemodelROMStostudyvortexsheddingbydeepwaterislandsandtopographiceddyforcingintheSouthernCaliforniaBight.Yoshidaetal.(2010)studiedthewindforcingofHawaiianeddies.Theyfoundthat,intheimmediateleesouthwestofHawaii(18.9N-20N,158W-156.7W),eddysignalshaveapre-dominant60-dayperiodandashortlife-span.TheynoticedthattheobservededdiesoriginateinthesouthwestcornerofHawaiiandareinducedbythelocalwindstresscurlvari-abilityassociatedwiththeblockingofthetradewindbytheislandofHawaii.Jiménezetal.(2008)performedseveralexperimentstostudytherelativeimportanceoftopographicandwindforcingonoceaniceddysheddingbyanisolated,tall,deepwaterisland.TheyappliedthemodeltothecaseofeddiesshedbytheislandofGranCanaria.TheyfoundthatwindforcingaloneisnotsufficienttoforceanoceanicVonKarmanvortexstreetintheleeoftheislandandthattopo-graphicforcingisnecessary.Intheircase,thewindforcingisnotthemainmechanismresponsibleforoceanicvortexshedding,itactsasatriggermechanismwhentheincidentcurrentaloneisnotsufficientlyenergetictoproducevortexshedding.Asregardswindforcing,monthlymeanvaluesfromCOADSwerecomparedtomonthlymeanwindspeedanddi-rectionfromCanadianweatherbuoysinthenortheastPacific(CherniawskyandCrawford,1996).Windspeedmeasure-mentsfromQuikSCATscatterometerwerevalidatedbycom-parisonwithindependentdata.Forinstance,theywerecom-paredwithmeteorologicalanalyses(Renfrewetal.,2009)overtheDenmarkStrait,orwithwindspeedscomputedfromRADARSAT-1syntheticapertureradar(SAR)intheGulfofAlaska(Monaldoetal.,2004).Concerningoceanmodel-ing,theinfluenceofCOADSversusQuickSCATwinddataonoceaniccirculationwasanalyzedintheCaliforniaCur-rentSystem(Penvenetal.,2002).Caliletal.(2008)havedonenumericalmodelingsimulationsoftheoceanresponsetorealisticwindforcingintheleeoftheHawaiianIslandchain.Sinceweusethesamemethodology,westress,inthispaper,thesimilaritiesanddifferencesbetweentheirandourresults.OntheNorthequatorialcentralPacific,anoceanmodelforcedbyCOADSwindversusoneforcedbytheNCEP-NCAR(NationalCentersforEnvironmentalPredic-tion-NationalCenterforAtmosphericResearch)reanalysiswindwereusedtocomparetheobservationsofoceanheatcontent(WuandXie,2003).IndependentobservationsofoceaniccirculationwereinvestigatedwithanoceanmodelforcedbyQuikSCATwinddataindifferentregions:thePa-cificsystem(Xieetal.,2001)andtheSouthernBenguelaupwellingsystem(Blankeetal.,2005).ThemainpurposeofthisstudyistoanalyzetherelativeimportanceoftopographyandwindforcingontheHawaiianoceaniccirculationandeddiesgeneration,withparticularat-tentiontowind-drivenmesoscaleeddygenerationsimulatedwithbothCOADSandQuikSCATwinddata.Thepaperisorganizedasfollows:section2introducesbrieflythenumericalmodel,themainsimulationandeddytrackingparameters.Section3presentsthemodelsensitiv-ityandadiscussionaboutthevariabilityinthesurfacewindstress.ThispartwillallowustoanalyzewhetherthemodelreproducesthegenerationofmesoscaleeddieswestoftheHawaiianarchipelago.Insection4,weanalyzewhichsim-ulationisthemorerealisticandwhetheroneforcingispre-dominant.Ourconclusionsaredrawninsection6.2MaterialsandMethods2.1OceanModelOurcirculationmodelisbasedontheIRD(InstitutdeRecherchepourleDéveloppement)versionoftheRegionalOceanModelingSystem(ROMS).ThereaderisreferredtoShchepetkinandMcWilliams(2003,2005)foramorecom-pletedescriptionofthenumericalcode.Themodeldomain