Spatial analysis of the wind field on the western

ContinentalShelfResearch25(2005)1728–1748SpatialanalysisofthewindfieldonthewesterncoastofGalicia(NWSpain)frominsitumeasurementsJ.L.Herrera
,S.Piedracoba,R.A.Varela,G.Roso´nGrupodeOceanografı´aFı´sicadelaUniversidadofVigo(GOFUVI),FacultaddeCienciasdelMar,36200Vigo,Pontevedra,SpainReceived7June2004;receivedinrevisedform3June2005;accepted5June2005Availableonline10August2005AbstractWeusedwinddatameasuredatsixmeteorologicalstationstostudytheirspatialrepresentativityonatmosphere–oceaninteractionsalongthewesternshelfofGaliciaandinsidetheRı´aofVigo.Thecorrelationasafunctionofdistanceandcomplexempiricalorthogonalfunctionsofthewindfieldwerecomputed,allowingustodividetheregionintothreesub-domains:theopenocean,theshelfandtheRı´aofVigo.Afterwards,windgradientsamongstationswerecomputedtodeterminespecificspatialwindfeatures:resultsshowedanexpectedwindchannellingeffectinsidetheRı´aofVigo,butalsoazonalonshorenegativefrictionalgradientontheshelfthatgeneratesEkmanpumpingduringupwellingfavourablewinds.Finally,theEkmantransportestimatedfrommeteorologicalstationswascomparedwithmeasuredtransportfromtwomooringslocatedinsidetheRı´aofVigoandontheshelf,andwedeterminedthatthemeteorologicalstationmostrepresentativeofthewindfieldintheshelfistheoneinstalledinamooredbuoyontheshelf.r2005ElsevierLtd.Allrightsreserved.Keywords:Windfield;Spatialvariations;Currentmeter;Upwellingindex;CEOF;Ekmanpumping;Galicianshelf;Rı´adeVigo;Spain1.IntroductionThewesterncoastofGalicia(NWSpain)constitutesthenorthernlimitofawind-drivenupwellingsystemthatextendsalongtheeasternboundaryoftheNorthAtlanticOceanbetweenlatitudes10
Nand44
N,withagapattheStraitofGibraltar(Woosteretal.,1976;Fiuzaetal.,1982).ManyauthorshaverelatedtheseasonalvariabilityoftheshelfdynamicsinthisregiontotheevolutionoftheAzoresHighandtheIcelandLow(Fiuzaetal.,1982;Frouinetal.,1990;Vitorinoetal.,2002;A´lvarez-Salgadoetal.,2003;Varelaetal.,2005)inacyclethatcanberoughlysummarizedasasummerseasondominatedbynortherlyupwellingfavourablewinds(Fraga,1981;Fiuza,1983)andathermallowovercentralIberia(Ferreira,1984),whilethewinterseasonshowshighervariabilityandratherfrequentARTICLEINPRESS:10.1016/j.csr.2005.06.001
Correspondingauthor.Fax:[phone].E-mailaddress:[e-mail](J.L.Herrera).downwelling-favourablewinds(Fiuzaetal.,1982;Vitorinoetal.,2002;A´lvarez-Salgadoetal.,2003).AlongthewesterncoastofGaliciatherearefourembaymentsnamed‘‘Rı´asBaixas’’,whosether-mohalineanddynamicalbehaviourisaffectedbytheshelfupwellingsystem.Afteranalyzingob-servationsandmodelresultsseveralauthorsconcludethattheRı´asbehavealternatingfrompositive(FragaandMargalef,1979)tonegativeestuarieslinked,respectively,totheupwellingordownwellingsituationsintheshelf(A´lvarez-Salgadoetal.,1993;Mı´guezetal.,2001;Torres-Lo´pezetal.,2001;Soutoetal.,2003;Piedracobaetal.,2005).Ontheotherhand,severalstudiessuggestedthatthewindfieldblowingalongthewesterncoastofGaliciaisfarfrombeingspatiallyhomoge-neous.BakunandNelson(1991)foundastrongcycloniccurlduringtheupwellingseasonnearthecoastofIberia,withamaximumadjacenttothenorth-westerncoast.McClainetal.(1986)ob-servedsmallanticyclonicwind-stresscurlfeaturesassociatedtotheRı´asBaixas.Recently,Torresetal.(2003)performedacomplexempiricalortho-gonalfunctions(CEOF)analysisofsatellitewindfieldmeasurementsandfoundamodeofvariationinwhichthewindisoppositeatbothsidesofalinejoiningCapeFisterraandthepointlocatedat42
N,12
W(Fig.1).Asaconsequence,windobservationsatasinglepointareunlikelytobefullyrepresentativeoftheoceanographiccondi-tionsfoundbothattheshelfandwithintheRı´asBaixas.Thisspatialinhomogeneityimpliesthatparti-cularcaremustbetakeninthisareatochooseawindtimeseriesinoceanographicstudies.Histori-cally,mostontheshelfandRı´asBaixasoceano-graphicalstudiesusedeitherwindmeasuredattheSpanishInstitutoNacionaldeMeteorologı´a(INM)CapeFisterrameteorologicalstation(42
520N9
150W),orgeostrophicwindsesti-matedfroma2
2
cellcentredat43
N11
W(Lavı´netal.,1990,2000),assumingthatthesepointsarerepresentativeoftheoverallGaliciancoast.Butthishypothesishasneverbeentested.Ontheotherhand,thesynopticmeasurementsofthewindfieldmadebymicrowaveradarinstalledonsatellitescouldbe,asafirstestimate,agoodsolutionfortheproblemofobtainingthelocationofwindmeasurementsatdifferentloca-tions.Inparticular,theSeaWindsinstrumentoftheQuickscatsatellitehasaspatialresolutionof25kmthatenablestheidentificationoffine-scalefeaturesinthewindfield.Unfortunately,theQuickscatdatahasa25kmoffshorecoastalmask,anditsdatacannotbereliablyusedattheRı´asBaixaswhoseinteriorwidthisonlyoffewkilometres.AlsoawkwardisthatQuickscatwindspeedmeasurementsrangefrom3to20ms1andhaveanaccuracyofabout2ms1(JPL,2001).TheimplicationsofthelowestthresholdoftheSeaWindsinstrumentwhenappliedtothewindsmeasuredinthestudiedareawillbediscussedlater.ThemainaimofthispaperistoquantifythesuitabilityofallavailablemeteorologicalstationsinthewesternGalicianshelftoadequatelyrepresentwinddataatsomewhatlargedistances.Inotherwor