Cruise Executive Final Report: Mixing Up the Tropical Pacific

R/VFalkorFK150728
MixinguptheTropicalPacific
FinalReport
30July–17August,2015
Majuro,MarshallIslandstoHonolulu,Hawaii
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1.Overview
Thecruiseextendedmeasurementsaimedatdeterminingandquantifyingthe
processesinthetropicaloceanthatgenerateturbulenceandassociatedmixingof
propertiesintheupperocean.Mixingofheat,saltandmomentumplaysalargerole
inshapingtheupperoceanstructureandthewaytheoceaninteractswiththe
atmosphere.PreviousmeasurementsinthewesternequatorialPacificidentified
smallverticalscaleflowfeatures(SVSs)asmajorcontributorstoshear-generated
turbulencewithinandabovethethermocline.TheSVSsthemselvesaregenerated
throughacombinationofwindforcingandflowinstabilities.Aprimaryaimofthe
presentcruisewastodetermineifsimilarflowfeaturesdominatetheverticalshear
inthecentralequatorialPacificandtodeterminethelevelsofturbulencegenerated
bythem.
ThecruisetookplaceduringadevelopingElNiño.Thishadconsequencesforthe
conditionsencountered.Figure1.1showsthetimehistoryofanomaliesinthe
eastwardwind,seasurfacetemperature(SST)andthedepthofthethermocline
(givenbythedepthofthe20oCisotherm)alongtheequatorinthePacificOcean.El
Niñoconditionsareseenintheslackeningorreversaloftheeasterlytradewinds,
increasingSSTinthecentralandeasternPacific,andthedeepeningofthe
thermoclineintheeast.
Figure1.1Timehistoryofanomaliesintheeastwardwind,seasurfacetemperature
(SST)andthedepthofthe20oCisotherm)alongtheequatorinthePacificOcean.
(fromPMELTAOwebsite)
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Inadditionanumberofstrongwesterlywindeventsoccurredinthewestern
equatorialPacific,startinginFebruary2015,continuingthroughtothetimeofthe
cruise,August2015.Theeastwardlimitofthesewindeventsincreaseswithtime.
Along170W(thelocationofthemeasurementstakenonthepresentcruise)two
strongwesterlywindeventsoccurredinJuly,extendingfromtheequatortoseveral
degreesnorth(Figure1.2).Thechangeinwindsledtosurfacecurrentsbeing
directedtowardstheeast.TheexpectationisthestrengthoftheEquatorialUnder
Currenthadbeenreduced(althoughnotadequatelymeasuredduringthecruise),
andthevariablewindsmayhavegeneratedstronginertia-gravitywavesleadingto
strongSVSs.
Figure1.2.Eastwardcomponentofthesurfacewindstressalong170W,Mayto
August,2015
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2Summaryofobservations
Allinsitumeasurements,apartfromunderwaymeteorologicalandoceansampling,
weretakenalong170W(seecruisetrack,Figure2.1).Primaryinstrumentationwas
aCTDanda600kHzADCPattachedtotheCTDframeoperatedinloweredmode
(LADCP).Castsweretypicallytakento500m.Watersamplesweretakenon6ofthe
CTDcastsfornutrientanalysis.Acombinationofmeridionalsections,timeseries
andtow-yo’sweremade:seeFigure2.2forthetimehistoryalong170W.Whenon
stationortow-yo’ingatslowspeeda300kHzADCPwasdeployedontheUSBLspar
tosamplecurrentsinthetop50-80mofthewatercolumn.Itwasplannedtotake
turbulencemeasurementswithaVMP500.Theinstrumenthadproblems
throughoutthecruisewithonlyacoupleofusableprofilesproduced.Bothofthe
ship’sADCPs,aOS75and300,didnotfunctionthroughoutthecruise.Amore
detaileddescriptionofthemeasurementsfromtheinstrumentationisgiveninthe
cruisereportavailableathttp://schmidtocean.org/wpcontent/uploads/FK150728cruise_report.pdf.
Figure2.1.Cruisetrack
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Figure2.2.Ship’spositionalong170Wasafunctionoftime.Circlesindicatecasts.
Numbersindicatecastnumbers.
3DataAvailability
TheCTD,LADCP,WH300ADCPandnutrientdataareavailablefromtheUniversity
ofHawaiiat
ftp://currents.soest.hawaii.edu/pub/outgoing/Falkor/clean/FK150728/
togetherwiththemetadatafilereadme_FK150728.txt
Additionaldata,includingunderway,swarthbathymetryandnavigationdatacanbe
foundattheRollingDecktoRepositoryandMarineGeoscienceDataSystem(see
http://schmidtocean.org/cruise/mixing-up-the-tropical-pacific/)
4InitialAnalysis
AsfoundinthewesternequatorialPacific,theverticalshearoftheoceancurrents
wasdominatedbyrelativelysmallverticalscale(~20m)features.Figures4.1and
4.2showtheverticalshearofthezonal(eastward)andmeridional(northward)
velocitycomponentsintheupper300metersfortheentirecruiseasafunctionof
time,respectively.Atimeserieswasperformedat1N,Aug4-7(casts010-044,see
Figure2.2).Wecanclearlyseeupwardphasepropagationataround150mdepth
whichisconsistentwiththedynamicsoftheinertia-gravitywaves(IGWs)radiated
fromthemixedlayerintotheoceaninterior.Furtherevidenceforthedownward
energypropagationwasgatheredbythehighhorizontalresolutionnorthward
section(Aug.7-9,casts045-074)inwhichthephaselinesbetween100and200m
depthsslopeupwardatasteeperanglesuggestiveofphaselinesslopinguptowards
thenorth.Thephaselinesduringasecondtimeseries(Aug9-11)andnorthward
section(Aug12-13)havesimilarcharacteristics.Theremainingportionsoftheplot
providefurtherdatathatwillbeusedinpostcruiseanalysisoftheflowfield.
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Figure4.1Verticalshearofthezonal(eastward)componentofvelocity,du/dz,as
functionoftime.Verticalprofileshavebeenalignedwithrespecttodensity.
Figure4.2Verticalshearofthemeridional(northward)componentofvelocity,
dv/dz,asfunctionoftime.Verticalprofileshavebeenalignedwithrespectto
density.
Figure4.3showsthepowerspectraoftheshearofthezonal(solidblueline)and
meridional(solidredline)componentsofvelocity.Bothspectrapeakata
wavelengthofaround50m.Tofurtherinvestigatethepropertiesoftheobserved
flowfeaturesandtheirsourcearegionalOceanModelhasbeenrun,withthe
necessaryhighverticalandhorizontalresolution,forcedwithobservedwindsfora
periodthatincludesthetimeofthecruise.Thepowerspectraoftheverticalshearof
themodelflowatthelocationoftheobservationsisalsoshowninFigure4.3
(dashedlines).Itispleasingtoseethemodelcapturesverysimilarsmallscale
featuresintheshearsuggestingthemodelwillbeausefultoolinelucidatingthe
physicsofflowfeatureandtheircontributiontoturbulenceandmixing.
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Figure4.3Powerspectraoftheverticalshearofthezonalandmeridional
componentsofvelocity(blueandredlines,respectively)around1oN,170oW,
August2015.Solidlines:Observations.Dashedlines:Modelresults.
Itisunfortunatethatbecauseofinstrumentfailureveryfewturbulence
measurementsweretakenduringthecruise.Wecanmakeuse,however,ofearlier
observationsthatshowastrongrelationshipbetweenturbulenceactivityandthe
measuredshearandstratification(Richardsetal,2015).Theverticaldiffusion
coefficient,κv,estimatedfromshearandstratificationmeasurementstakenaround
1oN,170oWonthepresentcruiseisshowninFigure4.4.Thereareelevatedvalues
ofκvdowntothebottomofthethermoclinewhichisatadepthofapproximately
200m.Theelevatedvaluescenteredon140mareaconsequenceofthedownward
propagatinginertia-gravitywave.
AlsoshowninFigure4.4isthetimeaveragedchlorophyllconcentrationestimated
fromtheobservedflorescenceandthenutrient(nitrateplusnitrite)concentrations
gotfrombottlesamples.Thesedatawillbeusedtoestimatenutrientfluxestobe
comparedwithdailyratesofchangeofbio-mass.
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Figure4.4Verticalprofilesoftheestimatedverticaldiffusioncoefficient(blueline),
chlorophyllconcentration(greenline)andnutrient(nitrateplusnitrite)
concentration(reddots)
5ExpectedOutcomes:
Analysisofthetheobservationsandmodelresultswillcontinueleadingtothe
preparationoftwopapers:
FactorsinfluencingmixinginthecentralequatorialPacific(fromobservationsand
modelresults)
Impactofmixingonbiologicalproductioninthedeepchlorophyllmaximuminthe
centralequatorialPacific
Reference:
RichardsK.J.,A.Natarov,E.Firing,Y.Kashino,S.M.Soares,M.Ishizu,G.S.Carter,J.H.
LeeandK.I.Chang,Shear--generatedturbulenceintheequatorialPacificproduced
bysmallverticalscaleflowfeatures.J.Geophys.Res.120,doi:
10.1002/2014JC010673,2015.
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