alongshiptracksattheexpenseofaworsesynopticcoverageShipsalinityisalsomeasuredatafewmetersdepthPriortoSMOSlaunchH ID: 92433
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184J.Boutinetal.:SeasurfacefresheninginferredfromSMOSandARGOsalinity alongshiptracksattheexpenseofaworsesynopticcover-age.Shipsalinityisalsomeasuredatafewmetersdepth.PriortoSMOSlaunch,H´enocqetal.(2010)haveshownthatinrainyregions,insitusalinitymeasuredatvariousdepthsbetween1mand10mdifferby0.1to0.5in20%ofthecasesandthatcareshouldbetakenforvalidatingandinter-pretingsatelliteSSSinrainyregions.Actually,largefresh-enings(severaltenthsofsalinityunit)candevelopintheup-perfewmetersoftheoceanafteraheavyrainfall,asclearlyevidencedduringtheTropicalOceans-GlobalAtmosphereCoupledOcean-AtmosphereResponseExperiment(TOGACOARE)(SolovievandLukas,1996,1997).Theseasur-facesalinityfresheningbetween15cmand50cmdepthinthetropicaloceanshasbeenfurtherdocumentedusingsur-faceautonomousdrifters(Reverdinetal.,2012);onaverageoverallthesampledevents,thesalinitydrawdownofiso-latedrainfalleventsaveraged0.56psu,andwaterwasabout0.1fresherat15cmthanat45cmdepth,thisgradientdisap-pearingafter,typically,fourhours.TheSSSmeasuredfromspacebymicrowaveradiome-tersoperatingatL-band(1.4GHz),likeSMOSandAquar-ius,isrepresentativeoftherstcentimeteroftheseasur-face.Thegoalofthesemissionsistoprovideanaccuracyof0.2onSSSaveragedoverGODAE(GlobalOceanDataAs-similationExperiment)scales(typically200200km2and10daysor100100km2andonemonth).Theoceanha-lineskinlayer,duetosurfaceevaporation,isexpectedtobemuchthinner(60µm)thantheelectromagneticpene-trationdepthatL-bandsothattheeffectoftheseasurfacesalinitysublayeronsatelliteSSSisestimatedtobelessthan0.01,i.e.negligiblecomparedtoothererrorsourcesinSSSremotesensing(ZhangandZhang,2012).Hence,themaingeophysicalsourcesofvariationbetweensatelliteandinsitusalinityareexpectedtobelinkedtoverticalsalinitystrati-cationsespeciallyinrainyandriveroutowregionsandtosharpoceanicfronts.Inthispaper,westudytheobservedSMOSandARGOsalinitydifferences,asARGOSSSarewidelyusedforvalidatingsatelliteSSSandwefocusondif-ferencesobservedintherainyregionoftheInterTropicalConvergenceZone(ITCZ)ofthePacicOcean.Afteradescriptionofthemeasurementsandthemethods(Sect.2),avalidationofSMOSSSSovertheglobalocean,andinfourspecicregionsispresented(Sect.3).Then,wedetailtheSMOSminusARGOSSSdifferenceswithrespecttosatelliterainrate(Sect.4).WediscusstheseresultsinviewoferrorsourcesinSect.5;conclusionsandperspectivesaregiveninSect.6.2DataandmethodsOurstudyconcentratesontheperiodfrom1July2010to30September2010.ThisperiodfollowstheSMOScommis-sioningperiodduringwhichtheSMOSinstrumentcongu-rationwasvarying(Mecklenburgetal.,2012).Inaddition,theinuenceofthegalacticnoiseonSMOSmeasurementsduringascendingorbitsissmallerduringthatperiodthanduringpreviousmonths.2.1SMOSTheSMOSmission(Kerretal.,2010)waslaunchedinNovember2009,onasun-synchronouscircularorbitwithalocalequatorcrossingtimeat06:00a.m.onascendingnode.ItcarriesanL-bandinterferometricradiometer.Thisnewtechnologyallowsreconstructingbi-dimensionalmulti-angularimagesoftheL-bandbrightnesstemperatures,Tb,withameanspatialresolutionof40km.Individualmeasure-mentsareverynoisysothatretrievingaSSSwithanaccu-racyacceptableforoceanographicstudiesisonlypossibleowingtothecombinationofmultipleTbmeasurementsac-quiredatthesameoceangridpointatvariousincidencean-glesandpolarisationsoversuccessivesnapshots(Boutinetal.,2004).AsdescribedinBoutinetal.(2012),theretrievalschemeimplementedintheESA(EuropeanSpaceAgency)processingusestheLevenbergandMarcquardtiterativeal-gorithmforretrievingSSS,windspeed,seasurfacetemper-ature(SST)andtotalelectroncontent,fromthemultiangu-larandpolarisedinformationcontainedinSMOSTbsalongadwellline.PriorvaluesforwindspeedandseasurfacetemperaturearetakenfromECMWF(EuropeanCentreforMedium-RangeWeatherForecasts);intheversionconsid-eredinourstudy,errorsof2ms�1andof1ChavebeenputonwindcomponentsandSST,respectively.FirstassessmentofSMOSSSSretrievedwithESAprocessingjustafterthecommissioningphaseandmappedattypicalGODAEscales(Banksetal.,2012;Boutinetal.,2012)hasshowngoodgen-eralagreementwithrespecttoSSSclimatology,ARGOandoceanmodeloutput.Althoughthesestudiesareveryencour-aging,theyalsopointoutSMOSdatadeciencies:largeSSSbiasesrelatedtocontinentsvicinity,toradiofrequencyinter-ferencesandtoorbitorientation,thelatterbeinglikelyduetoimperfectionsofthethermalantennamodel(Kainulainenetal.,2012).Decienciesofdirectroughnessmodelsathighwindspeedsleadtothedevelopmentofnewempiricalorsemi-empiricalroughnessmodelsttedtoSMOSmeasure-ments(Guimbardetal.,2012;Yinetal.,2012).Usinganimprovedimagereconstructionalgorithm,Reuletal.(2012)foundanaccuracyonmonthly,11SSSof0.6globallyand0.4inthetropicsfor90%ofthedataandmuchlargerbiasesinthevicinityofland.Inthepresentstudy,level2SMOSSSSarefromtherstSMOS/ESAannualreprocessingcampaigninwhichESAlevel1v5.04andlevel2v5.50processorshavebeenused.Intheseversions,signicantimprovementswithrespecttotheawslistedabovehavebeenimplemented(seeacom-pletedescriptionintheAlgorithmTheoreticalBasisDocu-ment(ATBD)availableat: www.argans.co.uk ).ESAlevel2SMOSSSShaveameanspatialresolutionof40kmandareoversampledonanISEAgridat15kmresolution. OceanSci.,9,183– 192 ,2013www.ocean-sci.net/9/183/2013/ J.Boutinetal.:SeasurfacefresheninginferredfromSMOSandARGOsalinity185 SincethesenewSMOSSSShavebeendeliveredveryre-cently,noassessmentoftheiraccuracyhasbeenpublishedyet.Hence,werstpresentaglobalscalevalidation.WeusesimilarmethodsastheonesdescribedinBoutinetal.(2012).Theyaresummarizedbelow.WeuseESAlevel2SMOSSSSretrievedwithmodel1,whichmakesuseofthenew(Yinetal.,2012)roughnessmodel;onlyascendingorbitsareconsideredinordertomin-imizeuncertaintieslinkedtoFaradayrotationandtodiur-nalSSTcycle.WebuildSMOSSSSmapsat11reso-lutionbytakingtheaverageoftheESAlevel2SMOSSSSweightedbythevarianceoftheretrievedSSSerrorandbythemeanspatialresolutionofeachSSS.PrecisecollocationswithARGOmeasurementsareperformedbytakingSMOSSSSat50kmand5daysfromARGOoats.Whenaver-agedintheseradii,SMOSSSSareweightedthesamewayasinthemapproduction.ThelteringoftheESAlevel2SMOSSSSdiffersslightlyfrompreviousstudiesasthedenitionofagshaschanged(seeATBD).Weretaingridpointsaggedasvalid,furtherthan200kmfromthecoast,withsuccessfulretrieval,withagoodtbetweenmeasuredandmodelledTbs(testsonChi2andChi2 PasdenedinATBD),withlessthan20iterationsoftheLevenbergandMarquardtretrievalprocess,nosuspecticeandnotmanyoutliers;onlymeasurementswithlowsunglint,lowgalacticnoiseareusedintheSSSretrieval.Inad-dition,inorderto(1)avoidtoonoisyretrievalsattheedgeoftheswathand(2)imprecisionduetoloweraccuracyofECMWFforecastsandofourroughnessmodelatverylowandhighwindspeed,weonlyconsiderSMOSSSSretrievedingridpointswith(1)morethan130measurementscomingfromthealiasfreeeldofviewregion(roughlycorrespond-ingtoSSSretrievedat300kmfromthecentreofthetrack)and(2)ECMWFwindspeedbetween3and12ms�1.WhenaveragingESAlevel2SMOSSSS,onlyaveragesmadewithmorethan30individualSSSareretained.Withthesecriteria,agridpointisseenapproximatelyonceevery5days,duringascendingorbits.2.2ARGOWeusemeasurementsfromARGOoatsprovidedbytheCoriolisdatacentre( http://www.coriolis.eu.org/ ).Wedown-loaded23577proleswithmeasurementsbetween4mand10mdepthbetween1July2010and30September2010,asavailableontheCorioliswebsiteon5May2012:duringthestudiedperiod,14263proleshavebeenqualitychecked(delayedtimedata).WeonlyuseARGOmeasurementswithaqualityagequalto1,inagreementwithrealtimequalitychecksand,fordelayedtimedata,withstatisticalconsistencychecks(Carvaletal.,2012).Duringtheascentoftheoat,whentheoatisatabout5mdepthfromtheseasurface,thewaterpumpisstoppedinor-dertoavoiddamageslinkedtoairpumping.Thenopumpingleadstoinaccuratesalinitiesabovethatdepth.Somecom-parisonsperformedinsurfacelayersofawellmixedareabetweenpumpedandnonpumpedsalinityonanARVORoatindicateanoiseonthenonpumpedsalinity(at2mfromthelastpumpedmeasurement)of0.025withrespectto0.005onpumpedsalinities.Inaddition,incaseoftruesalin-itystraticationlinkedtorainevents,nonpumpedmeasure-mentswouldmissmostofthefreshening.Inordertomin-imizethiseffect,andgiventhatthepressuremeasurementsofsomeARGOoats(e.g.APEX)arenotveryprecise(sothatsomemeasurementsindicatedasmeasuredat4mdepthmaysometimesbepumped),forthepresentstudyweusetheclosestARGOsalinitytotheseasurface,provideditismeasuredbetween4mand10mdepth,withoutanyinterpo-lationtothesurface.WewilllaterrefertothismeasurementasARGOSSS.GlobalARGOSSSmaps(D2CA1S2re-analysisprod-uct)areproducedbyIFREMER/LPOLaboratoiredephysiquedesoceans(seeadescriptionat: www.ifremer.fr/lpo/SO-Argo-France ).Theyarederived,afterathoroughqualitycheckofARGOmeasurements,withtheoptimalin-terpolationmethoddescribedin(Gaillardetal.,2009);theyarenamedARGOOIinthefollowing.ThechoiceforthetimeandspacescalesusedinthatmethodresultsfromacompromisebetweenwhatisknownofoceantimeandspacescalesandwhatcanactuallyberesolvedwiththeARGOar-ray(3,10days);twoscalelengthsareconsidered:therstoneisisotropicandequalto300km,thesecondoneissetequalto4timestheaverageRossbyradiusofthearea.Asaresult,weexpectthesemapsbeingsmoother,especiallyintropicalareas,thanSMOSSSSmapsaveragedover11.ARGOsalinityareonthepracticalsalinityscalePSS-78,denedasaconductivityratio;followingUNESCO(1985)recommendations,salinityonpracticalsalinityscaleisunit-less.SinceL-bandbrightnesstemperaturesarerelatedtosalinityviawaterpermittivity,whichmostlydependsonseawaterconductivity,andsinceSMOSTbsarecalibratedeverytwoweekswithrespecttoclimatologicalsalinityonpracticalsalinityscaleandinordernottousedifferentconventionforsalinityfromvariousorigins,inthispaperweapplythisnounitconventiontobothinsituandsatellitesalinity.2.3SSM/IsrainrateInJuly–September2010,thelocalequatorcrossingtimeofthedescendingnodesofSSM/IsF17andSSM/IsF16missionsare,respectively,approximately20mnbeforeand40mnafterSMOSascendingnode.Hence,numerousSMOSlevel2arecollocatedwithSSM/Irainrates(RR)withinthisrangeoftime;onlytheSSM/IRRclosestintimewithSMOSSSSarekept.SSM/IF15isalsocloseintimetoSMOSpassesbutwedonotconsideritinourstatisticalanalysisbecauseofitsdegradationafterAugust2006.NeverthelessweuseSSM/IF15RRqualitatively.SSM/IsRRversion7aredownloadedfrom www.remss.com . www.ocean-sci.net/9/183/2013/OceanSci.,9,183– 192 ,2013