ObservedandModeledAir-SeaFluxintheNorthwestTropicalAtlan<c

Introduc<onAsurfacemooringsite,calledtheNorthwestTropicalAtlan<cSta<on(NTAS)hasbeenmaintainedwithNOAAsupportsince2001.SurfacemeteorologicalvariablesobservedontheNTASbuoyarewithheldfrommodelsandmadeavailableas independent reference data to explore air-sea interac<on processes, validate remote sensing products, anchorhybridair-seafluxfields,andinves<gatemodelperformance.

MethodsThe buoy meteorological measurements are a con<guous 11 year data set at 1 minute intervals from mooringdeploymentsbetweenApril2001andMarch2012.Forcomparisontomodels, thebuoydatawereaveragedtodailyvalues and adjusted to standard heights. Air-Sea fluxeswere es<mated using the COARE 3.0 bulk algorithm (Fairall1996;2003),whichtakesintoaccountairstability.

Thebuoyfluxeswereused todescribe the seasonal cycle, to iden<fy thepresenceof the ITCZ,and todiagnose thestrengthsandweaknessesofthreereanalysismodelproducts:

•  ERA-Interim(EuropeanCenterforMedium-RangeWeatherForecasts),•  MERRA-2(Na<onalAeronau<csandSpaceAdministra<on).

•  NCEP-2(Na<onalCentersforEnvironmentalPredic<ons)

Conclusions•  Na<vefluxesfromERA,NCEPandMERRAshowedlargedifferencesinindividualfluxcomponents

andnetheatflux(Qnet)rela<vetothebuoy.•  Usingacommonbulkfluxalgorithmbroughttheturbulentfluxcomponentsclosertothebuoy

valuesandreducedtheQnetdiscrepanciestoabout10W/m2.•  Radia<vefluxesfromthereanalysisproductswereinconsistentinreproducingtheobservedannual

cycle.ERAperformedwell,MERRAhadpersistentbiases,andNCEPshowedlargediscrepancies.•  Aregionalassessmentcomparingremotesensingproductstothemodelsindicatedthatseasonal

variabilityincloudcover(asdiagnosedfromlongwaveradia<on)isnotwellreproducedinsomeofthemodels,leadingtotheradia<vefluxdiscrepancies.

•  Precipita<onshowedrela<velygoodagreementamongobserva<ons,modelsandremotesensingproducts.Thus,totheextentthatradia<ondifferenceswereduetoclouds,precipita<ngcloudsdidnotappeartobethemainculprit.

Sebas<enBigorre,AlbertJ.PlueddemannandRobertA.WellerWoodsHoleOceanographicIns<tu<onWoodsHole,MA02543USA

Results–Buoyfluxvs.modelsComparing thena<vefluxes (as received in the reanalysismodels) to thebuoyfluxes showed significantdifferences(largerthanthebuoyerrors)inseveralfluxcomponents.Theeleven-yearmeanbuoyQnetshowedanoceanheatgainof42W/m2,whileNCEPandMERRAshowedheat lossofa fewW/m2.ApplyingtheCOAREalgorithmtothesurfacemeteorological variables from the models brought the turbulent flux components closer to the buoy values andreducedtheQnetdiscrepanciestoabout10W/m2.

InfluenceoftheITCZTrade winds from the Northern and Southern hemispheres converge at the Intertropical Convergence Zone (ITCZ),whichcanbeiden<fiedasaneast-westprecipita<onbandmigra<ngseasonallynorthandsouth.TheITCZhasameanloca<onnear6NintheAtlan<c.CloudcoverassociatedwiththeITCZinfluencestheregionalradia<veforcing.

The seasonal cycle is the dominant signal in heat flux at the NTAS site, but there are notable impacts from thenorthernmost excursion of the ITCZ in the fall (Sep-Nov): Wind speeds drop, precipita<on peaks, and a dis<nct“shoulder”isseeninlongwaveradia<on.

TheNTASsiteislocatedat51°W,15°N,roughly1,000kmeastofthelesserAn<lles,intheNorthAtlan<ctradewindregime.TheNTASbuoyisoueifedwithASIMETmeteorologicalinstrumenta<on(Hosometal.1995),measuringtenvariablesthatenablefluxesofmomentum,heatandmoisturetobecomputed.Mooringsarereplacedannually.Calibra<on,intercomparison,andcharacteriza<onoferrors(e.g.islandheateffect,flowdistor<on)improvemeasurementaccuracy.Heatfluxcomponenterrorsare2-4W/m2,whiletheerrorinnetheatfluxisabout8W/m2(ColboandWeller(2009)andBigorreetal.(2013).

DataAvailabilityTimeseriesdataofsurfacemeteorologyforindividualdeploymentsareavailablefromhfp://uop.whoi.edu/currentprojects/NTAS/ntasarchive.html

Concatenated,mul<-year<meseriesofmeteorologicaldataandfluxesareavailablefromhfp://uop.whoi.edu/ReferenceDataSets/ntasreference.html

Eleven-yearaveragenetheatfluxfromtheNTASbuoycomparedto(lej)thena<vefluxesfromthemodelsand(right)fluxescomputedusingtheCOAREalgorithmandsurfacemeteorologicalvariablesfromthemodels.Errorbarsof+/-8

W/m2areshownforthebuoyflux.

ThemajorityofthediscrepancyinnetheatfluxajerapplyingtheCOAREalgorithmwasfoundintheradia<vefluxes.

•  Shortwave radia<on (SWR): ERA in good agreement with buoy; MERRA overes<mates ocean heat gain; NCEPunderes<matesoceanheatgain,especiallyinwinter.

•  Longwaveradia<on(LWR): ERAingoodagreementwithbuoy;MERRAshowsapersistentlowbias;NCEPisapoormatchtotheobservedannualcycle.

The annual cycle of precipita<on was reproduced reasonably well by all three reanalysis products, although withnotabledifferencesinamplitude,especiallynearITCZ.ERAandNCEParehighcomparedtothebuoywhileMERRAissimilar.There is a good agreement between buoy measurements and satellite remote sensing products for precipita<on(TropicalRainfallMeasurementMission;TRMM),shortwaveandlongwaveradia<on(CloudsandEarth’sRadiantEnergySystem;CERES).

Eleven-yearmeanannualcycleof(lej)precipita<on,(middle)shortwaveradia<on,(right)longwaveradia<onattheNTASsitefromthebuoycomparedtothreereanalysismodels.Remotesensingobserva<onsfrom(lej)TRMM

(middleandright)CERESarealsoshown.The<meperiodinfluencedbytheITCZisshaded.

Results–Satellitefluxvs.modelsSatelliteremotesensing(e.g.CERES)andmodelproductsallowedregionalpafernsoftheannualcycletobeexamined:MERRAshowsapafernofLWRvariabilitycomparabletothatofCERES,althoughwithanotablelowbias.NCEPradia<onpafernsaredifferentinoutertropicsinwinter(similartodiscrepancywithNTASbuoy),andnearEquatorinsummer.Precipita<oniswellrepresentedinmodels,althoughitisenhancednearSouthAmericainNCEP.

Contoursofprecipita<onrate(mm/d)inthetopicalAtlan<cfor(lej)Januaryand(right)October.Theseare17-yearmonthlycompositesfromblendedsatelliteandraingaugeproducts(Chiangetal.,2002).TheNTAS

siteisindicatedbythegreendot.

AcknowledgmentThisresearchwassupportedbytheNa<onalOceanicandAtmosphericAdministra<on(NOAA),ClimateProgramOffice(CPOFundRefnumber(100007298),OceanObservingandMonitoringDivision,throughtheCoopera<veIns<tutefortheNorthAtlan<cRegion(CINAR)underCoopera<veAgreementNA14OAR4320158.NTASispartoftheOceanSITESnetwork(www.oceansites.org

Hovmollerdiagrams for: net longwaveradia<on (W/m2, lej),precipita<onrate (mm/d, right) showing theannualcycle vs. la<tude for eleven-yearmeans. Solid lines indicate themaximumvalue for eachmonth.Adashed line isshownatthela<tudeoftheNTASmooring.Basedonlongitudinalaveragesfrom51Wto44W.

Mapsofnetlongwave(lej)andshortwave(right)radia<on(W/m2),forwinterandsummer,usingdatafrom2001to2012.