20 Pages
English

Study of a mesoscale anticyclonic eddy in the western part of the Gulf of Lion

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Niveau: Supérieur, Doctorat, Bac+8
Study of a mesoscale anticyclonic eddy in the western part of the Gulf of Lion. Z. Y. Hu a,?, A. A. Petrenko a, A. M. Doglioli a, I. Dekeyser a aUniversite de la Mediterranee - Aix Marseille II, CNRS, Laboratoire d'Oceanographie Physique et Biogeochimique, UMR 6535, OSU/Centre d'Oceanologie de Marseille, Campus de Luminy case 901, 13288 Marseille Cedex 9, France. Abstract Acoustic Doppler Current Profiler (ADCP) and Expendable Bathy Thermograph (XBT) data were collected during the Latex08 cruise (September 1 - 5, 2008) throughout the western part of the gulf of Lion. These data, combined with Sea Surface Temperature (SST) satellite images and Lagrangian drifter trajectories, show the presence of an intense anticyclonic eddy in the western part of the gulf of Lion. The eddy is elliptic in shape and the estimated radii are 21.5 (15.5) km for its major (minor) axis. The vertical extent of the eddy reached about 35 m depth and was limited by the bottom of the seasonal mixed layer. The eddy interacts with the Northern Current at the end of the cruise, maybe leading to its deformation. More- over complementary drifter data suggest that this anticyclonic eddy was already present at the beginning of August 2008. Hence the eddy lasted around 50 days in the same region.

  • latex08 cruise

  • adcp

  • surface waters

  • khz adcp

  • drifter

  • waters inputs

  • can generate

  • sea state

  • structures could play


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StudyofamesoscaleanticycloniceddyinthewesternpartoftheGulfofLion.Z.Y.Hua,,A.A.Petrenkoa,A.M.Dogliolia,I.DekeyseraaUniversite´delaMe´diterrane´e-AixMarseilleII,CNRS,Laboratoired’Oce´anographiePhysiqueetBioge´ochimique,UMR6535,OSU/Centred’Oce´anologiedeMarseille,CampusdeLuminycase901,13288MarseilleCedex9,France.AbstractAcousticDopplerCurrentProfiler(ADCP)andExpendableBathyThermograph(XBT)datawerecollectedduringtheLatex08cruise(September1-5,2008)throughoutthewesternpartofthegulfofLion.Thesedata,combinedwithSeaSurfaceTemperature(SST)satelliteimagesandLagrangiandriftertrajectories,showthepresenceofanintenseanticycloniceddyinthewesternpartofthegulfofLion.Theeddyisellipticinshapeandtheestimatedradiiare21.5(15.5)kmforitsmajor(minor)axis.Theverticalextentoftheeddyreachedabout35mdepthandwaslimitedbythebottomoftheseasonalmixedlayer.TheeddyinteractswiththeNorthernCurrentattheendofthecruise,maybeleadingtoitsdeformation.More-overcomplementarydrifterdatasuggestthatthisanticycloniceddywasalreadypresentatthebeginningofAugust2008.Hencetheeddylastedaround50daysinthesameregion.Inlightofpreviousnumericalstudy,somehypothesesabouttheformationandbehaviouroftheeddyarealsodiscussed.Keywords:mesoscaleeddies;GulfofLion;northwesternMediterraneanSea;LATEXproject11Introduction2Oceaniceddiesmayhaveanimportantimpactontransferandredistributionof3heat,energyandmatter.Severalstudieshavepreviouslyfocusedonmesoscale4eddiesandtheirinfluencesonbiogeochemicaldistributions(e.g.Garonetal.,Correspondingauthor.Emailaddress:ziyuan.hu@univmed.fr(Z.Y.Hu).PreprintsubmittedtoElsevier27April2010
52001;Taupier-Letageetal.,2003;McGillicuddyetal.,2007;Benitez-Nelson6etal.,2007).Nevertheless,questionsremain,particularlyincoastalwaters7wheredynamicalprocessesareverycomplexandfluxesofnutrientsandsedi-8mentsarehigh.9TheGulfofLion(hereafterGoL)isalargecontinentalmarginwithasemi-10circleshapelocatedinthenorthwesternMediterraneanSea(Fig.1).The11hydrodynamicsoftheGoLarecomplexandhighlyvariable,asdescribedby12Millot(1990).Threemainforcingscoexist:13i)strongcontinentalwinds:theMistral(northerly)andtheTramontane(north-14westerly,sometimeswesterly).Theyarecoldanddryandcangeneratestrong15upwellingsanddownwellingsalongtheentirecoastoftheGoL(Millot,1979);16ii)aquasi-geostrophicalongslopecurrent:theNorthernCurrent(hereafter17NC).Itisthenorthernpartofthecyclonicgyreoccurringinthewestern18Mediterraneanbasin.TheNCflowsalongthecontinentalslopefromtheLig-19urianSeatotheCatalanSea(Millot,1990,1999).Asafirstapproximation,20theNCcanbeconsideredingeostrophicbalance,actingasadynamicalbar-21rierforcoastalwaters.Nevertheless,severalstudieshavedemonstratedthe22presenceoftransientageostrophicprocesses(Albe´rolaetal.,1995;Sammari23etal.,1995;Petrenko,2003);24iii)importantfreshwatersinputs,bringingnutrientsthatenhanceprimary25productivityontheshelf(e.g.MinasandMinas,1989;Ludwigetal.,2009).26TheRhoneriveronthenorth-easternpartofthegulfdeliversabout80%of27thetotalriverinewaterinputsandconstitutesthemainfreshwatersourceof28theGoL.TheplumeextensionoftheRhonecanextendoffshoretowardsthe29southwestpartoftheGoLundercertainmeteorologicalconditions(Estournel30etal.,1997).31Previousinsitustudiessuggestedthepresenceofananticycloniccirculationin32thewesternpartoftheGoL,inSeptemberduringstratifiedconditions(Millot,331979,1982).Estourneletal.(2003)havealsoshownwithnumericalmodelling34thatananticycloniccirculationcanbepresentinthisregioninwinter.These35coastaleddiescouldpotentiallybeinfluencedbytheRhonedistalplumewhich36couldbringnutrientsandphytoplanktoninsidetheeddy.Numericalmodelling37suggeststhattheeddycaninteractwiththeNC(Huetal.,2009).Suchin-38teractions,whetherwiththeriverplumeortheneighboringcurrent,indicate39thattheseeddystructurescouldplayanimportantroleinthecoastal-offshore40transportofnutrientsandphytoplankton,aswellasheatandenergy.Toour41knowledge,noexperimentalstudieshavebeendedicatedonthestudyofthe42eddydynamicsinthisareasincetheearlyhypothesisofMillot(1979,1982)43citedearlier.44Themainobjectiveofthispaperistoinvestigatethedynamicalcharacteris-45ticsofananticycloniceddyobservedduringtheLatex08cruise,addingcruise46datatothepreviousnumericalstudybyHuetal.(2009),tobetterunder-2