Coupling of Internal Waves on the Main Thermocline to the Diurnal

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. C6, PAGES 12,613-12,628, JUNE 15, 1998

Coupling of internal waveson the main therlnoclineto the diurnal surfacelayer and sea surfacetemperature during the Tropical Ocean-Global Atmosphere Coupled Ocean-AtmosphereResponse Experiment E. J. Walsh,•,2 R. Pinkel,3 D. E. Hagan,4 R. A. Weller,s C. W. Fairall,6 D. P. Rogers,3 S. P. Burns,7 and M. Baumgartner7

Abstract. Patternsin sea surfacetemperature (SST) on 5-km scaleswere observedfrom low-flyingresearch aircraft on a light wind day during the Tropical Ocean-Global AtmosphereCoupled Ocean-AtmosphereResponse Experiment. An inversetrend was observedbetween the SST and the sea surfacemean squareslope (mss). However, low correlationcoefficients indicate that the dominantprocess causing the spatialvariation of SST under these light wind conditionsis neither well controlledby the wind speednor well monitored by the mss.The SST spatialpattern persistedfor at least 1 hour and propagatedtoward the NE at about1 m s-•, a factorof 1.6faster than the speed of the surfacecurrent. Couplingbetween internal gravitywaves propagating on the seasonal thermoclineand the diurnal surfacelayer is examinedas a possibleexplanation for the observedSST variability in spaceand time.

1. Introduction spatialstructure in both the wind and the mixed layer depth can alsoproduce spatial patterns in SST. This paper dealswith data collectedon November28, 1992, The light westerlywind on November28 doesnot appearto duringthe TropicalOcean-Global Atmosphere (TOGA) Cou- be the originof the observedspatial variation in SST. Nor does pled Ocean-AtmosphereResponse Experiment (COARE) in spatial variability in the surface heat and freshwaterfluxes. the westernequatorial Pacific "warm pool" [Websterand Lu- Instead,we believethe observedpatterns to reflect the pres- kas, 1992].On that day and duringother periodsof light winds ence of internal waves. it wasobserved that spatialpatterns in seasurface temperature On November 28 a WP-3D aircraft of the National Oceanic (SST) on scalesof severalkilometers to 100 km persistedand and AtmosphericAdministration (NOAA) flew in formation were observedon successiveaircraft overflightswhich were as with the National Centerfor AtmosphericResearch (NCAR) much as 2 hours apart [Haganet at., 1997]. The anomalyam- Electra aircraft on a six-legtrack over the WoodsHole Ocean- plitudesin the SST associatedwith thesepatterns were of the ographicInstitution (WHOI) IMET buoy (Figure 1). The order of 0.5øC. Obukhovlength scale, -L, wasapproximately 7 m for this day SST variability resultslargely from the contributionsof the [Serraet at., 1997]. Since -L is a measureof the height at fluxes at and near the sea surface and entrainment of cooler whichbuoyant production of turbulentkinetic energyexceeds water acrossthe mixed layer base, as was demonstratedby shearproduction, the atmosphericboundary layer was under Price et at. [1986]. Spatial patternsin heat flux and rain can strong,free convection.Midlatitude valuesof -L typically imprint themselveson the seasurface. The wind-drivenveloc- exceed300 m. The wind stressmeasured at the IMET buoywas ity in the mixed layer is proportionalto the wind stressand about0.005 N m-2 toward80 ø. The meanair temperatureat inverselyproportional to the depth of the layer. As a result, 10 m was 29.07øC,and the air-seatemperature difference aver- increasedshear and mixing acrossthe basecould result from agedabout - 1.6øC.The mean specific humidity was 17.89 g kg-•. either higherwind stressor a reductionin layer thickness,and There were dominant,persistent spatial SST patternswhose translationspeed (0.8-1.2 m s-•) wastoo fast to be explained •ObservationalScience Branch, Laboratory for HydrosphericPro- as advectionby the surfacecurrent (0.4-0.7 m s-•) andtoo cesses,NASA Goddard SpaceFlight Center, Wallops Island, Virginia. slowto be explainedby the windspeed (2 m s-•, 1.6 m s-• 2Onassignment at NOAA EnvironmentalTechnology Laboratory, Boulder, Colorado. relativeto the surfacecurrent). Thus we wereled to investigate 3ScrippsInstitution of Oceanography,La Jolla,California. the possibilitythat internal gravitywaves [Ewing, 1950] prop- 4jet PropulsionLaboratory, Pasadena, California. agating in the ocean along the main thermoclinewere the SWoodsHole OceanographicInstitution, Woods Hole, Massachu- setts. causeof the observedspatial structure in SST. 6NOAA EnvironmentalTechnology Laboratory, Boulder, Colo- Section 2 discusses the semidiurnal tide internal wave in rado. evidencein the IMET mooring measurementswhich caused 7Departmentof MechanicalEngineering, University of California, the principalspatial variation of the near-surfacecurrent. Sec- Irvine. tion 3 discusses the measurement of SST from an aircraft and Copyright 1998 by the American GeophysicalUnion. comparesthe radiometer observationswith in situ measure- Paper number 98JC00894. ments. Section 4 examines the correlations of SST with wind 0148-0227/98/98JC-00894509.00 speedand seasurface mean square slope (mss) and eliminates

12,613 12,614 WALSH ET AL.' INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

, 1.0 cific is a particularlyenergetic region. Time seriesof current and temperaturemade during TOGA COARE indicate that large-amplitude(20 m) tidallyforced internal waves propagat- 1.2 ing towardthe northeastare common,apparently generated by topographyto the southwest[Pinkel et al., 1997].The profiles in Figure 2 indicate that the thermocline was significantly higher at 0655 than it was either 6 hoursearlier (0055) or 6 ,ES • 1.4,_, hourslater (1255).This vertical motion resulted from the propagationof a semidiurnaltidal internal wave of about3 m s-• phase speed and 130-km wavelength.The phase speed was determinedfrom observationsof the V•iis•il•ifrequency at R/V Vickers(2øS, 156.25øE) between 0700 and 1000 on November 28, 1992.The wavelengthfollows from the semidiurnalperiod. ••IMET•UOY 1.82.0 The top panel of Figure 3 showsa schematicrepresentation of the displacementof the thermoclineas an internal wave propagatesand indicatesthe currents induced near the sea !•JR/V Vickers 2.2 surface.Sufficiently nonlinear internal waves can take on a trochoidalappearance. When the water depthbelow the ther- LONGITUDE(øE) mocline(about 2000 m in the region shownin Figure 1) is Figure 1. NOAA WP-3D ground track on November 28, greaterthan the water heightabove it, the crestsof the internal 1992.The locationsof the WHOI IMET buoy(1.75øS, 156øW) wave are broad and the troughsare sharp, as is indicatedin and R/V Vickers(2øS, 156.25øE) are indicatedby the filled Figure 3 [LaFond and Cox, 1962; Apel et al., 1985]. Down- circles. welling regions occur following the passageof wave crests; upwellingregions precede their arrival. These are the sitesof maximalsea surface convergence and divergence,respectively. wind speedvariation as a possibleexplanation of the observed The maximumof the time integral of the convergenceis over spatialpatterns of SST. the trough,and the maximumintegrated updraft is overthe crest. Section 5 discusses the remarkable characteristics of the SST The secondpanel of Figure 3 showsfive isothermsfor a spatialpatterns observed by the aircraft and eliminatesadvec- 36-hourperiod surrounding the aircraftflight. The time axisin tion by the current as a possibleexplanation for their transla- Figure 3 hasbeen reversed to increasefrom right to left so the tion. Section 6 discusses the modulation of the diurnal surface IMET temporal observationswill correspondto the spatial layer by internal waveswith periodsshorter than the semidi- schematicrepresentation in the top panel. Reversingthe time urnal tide. Section7 discussesother experimentswhich suggest axisimplicitly incorporates the time to spaceconversion for a that internal waves modulate SST and demonstrates that the spatial structurepropagating from left to right past a fixed translationspeed of the SST spatialpattern is in the range of observationpoint. phase speedsfor mode 2 internal wavesof the appropriate The semidiurnalperiod dominatesthe isotherms,although wavelengthrange. All times cited in this paper are in coordi- nated universaltime unlessspecified otherwise.

2. IMET Mooring Measurements 5o The WoodsHole OceanographicInstitution (WHOI) 3-m discusbuoy [Weller and Anderson,1996] mooring (1.75øS, 156øE) was named IMET for its improved meteorological 100 - package,but it alsomeasured water propertiesat a variety of depths.IMET measuredwater temperatureevery 15 min using Brancker temperaturerecorders (TPOD) at six depthsin a 150- near-surfacearray between 0.45 and 2.5 m and also at ten depthsbetween 27 and 260 m. Measurementswere recorded 200 every 3.75 min with 14 SeaBird conductivityand temperature (SEACAT) sensorsbetween 2 and 108 m. SEACATs at 84- and 124-m depth were sampledat 5-min intervals.Except for 250 /'""'""' "•'"5"'•. • i • , I , , , , I , , , , I , the SEACAT at 2-m depth,there wasonly 15-minsampling at 10 2O 25 3O depthsabove 5 m or below 124 m. TEMPERATURE (oc) Figure2 showsthree temperatureprofiles made on Novem- ber 28, 1992.The diurnal surfacelayer, confinedto the top few Figure 2. IMET temperatureprofiles made at 0055(dotted), meters,will be discussedlater. The mixed layer extendeddown 0655 (solid),and 1255 (dashed)UTC on November28, 1992. Branckertemperature recorders (TPOD), sampledat 15-min to about 50-m depth. Below the mixed layer the temperature intervals,were at depthsof 0.45, 0.55, 1.1, 1.6,2.65, 4.65, 27, 35, decreasedapproximately linearly through the thermocline, 132, 164, 180, 196, 212, 228, 244, and 260 m. SeaBird conduc- whosebottom was at about 200- to 250-m depth. tivity and temperature(SEACAT) sensors,sampled every 3.75 Internal gravitywaves can propagatethrough stably strati- min, were at depthsof 2, 7.5, 11.5, 15.5, 19.5, 23.5, 31.5, 39.5, fied fluids.They are ubiquitousin the main thermoclineof the 45.5, 52.5, 76, 92, 100, and 108 m. SEACATs at 84- and 124-m world's oceans[Garrett and Munk, 1979]. The equatorialPa- depth were sampledat 5-min intervals. WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,615 higher-frequencyvariations are alsoapparent. The verticaldis- INTERNAL WAVE PROPAGATION DIRECTION placementsassociated with internal wavesdiminish above the SEA SURFACE - , thermocline,and there is almostno displacementsignature at • t --• • t "-•' • ' the surface.However, the inducedcurrents persist to the surface. The IMET mooring measuredcurrent vectorsat 24 depths, from 5 to 325 m. The two lower panels of Figure 3 show the total and the low-frequency(15.3-hour period or greater)component of the current speed and direction measuredat 5-m ß depth by a vectormeasuring current meter (VMCM) sampled at 3.75-min intervals.The current data in Figure 3 are plotted •oo at the 15-min resolutionof the isotherms.It is apparent that a semidiurnalperiod dominatesthe high-frequencycomponent of the current. The current speedhad local maxima in the vicinity of the .• •5o internal wave troughsat about 0000 and 1200, indicatingthat the semidiurnalcomponent was roughlyaligned with the mean current at those times. The current minimum occurred about midway between, when the semidiurnalcomponent opposed 200 the mean current. That suggeststhat the semidiurnaltide internal wave was propagatingtoward 60ø , the direction of the mean current. The alignmentwith the mean current is coinci- dental, but this direction of propagationis typical of internal 250 wavesin this region [Pinkelet al., 1997]. We will later have need of the current variation along leg 5 and will now obtain it through a spatial projection from the temporalmeasurements at the IMET mooring.The north and 0.7 eastcomponents of the currentobservations at 5-m depthwere separated into high- and low-frequencyintervals, with the •0.6 boundarybeing a period of 15.3 hourschosen on the basisof minimum spectralenergy. The high-frequencycurrent compo- nentswere assumedto be inducedby the internal wave that is •0.5 the semidiurnaltide in this region as it propagatedby IMET. Theywere projected spatially toward 60 ø, usingthe 3 m s-• phasespeed. The low-frequencycurrent components were as- sumed to exist simultaneouslythroughout the overflight re- 0.3 I gion. The low-frequencynorth and eastIMET componentsat each observationtime were added to the high-frequencycom- 80- ponentsprojected from earlier or later IMET measurements dependingon whetherthe observationpoint was NE or SW of the mooring. Figure 4 showsthe variation along leg 5 of the 60ø compo- F- 60- nent of the compositecurrent (solid curves)for two different time intervals separatedby about an hour. The components were nearlyequal to the currentitself sincethe currentand the c3 50- ground track were so closelyaligned. z It shouldbe noted that the low-high frequencydecomposi- n- 40 tion and spatialprojection of the current in no way underes- timated its value. The current at IMET never exceeded 45 cm 30 s-• duringour measurementinterval (indicated by the three I I I I I solidvertical lines in Figure 3), yet the maximum60 ø current 24 18 12 6 0 -6 -12 componentin ourprojection exceeds the 70 cms- • peakvalue TIME (UTC HOURS) on 28 NOV 92 recordedby IMET 4 hours before we began taking data. This is becausethe low-frequencycurrent componentwas increas- Figure 3. (top) Schematicrepresentation of the deflectionof ing. It took about 5.5 hours for the semidiurnal tide peak the thermocline and the currentsinduced by a nonlinear in- current observedby IMET at 0000 to propagateto the eastend ternal wave in deep water and five isothermsfrom IMET data, of leg 5, where its amplitudewas added to the larger low- with total (thin curve) and low-frequencycomponent (thick curve)of (middle) IMET 5-m depth currentspeed and (bot- frequencycurrent component occurring at that later time. tom) direction.The three verticalsolid lines indicate the starting time of leg 1 (Figure 1), the midpoint of leg 5, and the 3. Measurement of SST From Aircraft midpoint of leg 8 (nearly a reverse transit of leg 5). Time increasesfrom right to left to implicitlyincorporate the time to The SST data presentedin this paper were obtained from spaceconversion for a spatial structurepropagating from left the Barnes PRT-5 radiometer on the NOAA WP-3D aircraft to right past an observationpoint. 12,616 WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

140 - estimatesderived from radiative transfer calculationsusing a

/ \. standardtropical atmospheremodel. 120- • • _•..•\ Under some circumstances,specular reflections of the Sun in the FOV of a radiometer can contaminate its measure- / / ..... / • 100 ments.The analysisin the appendixof Saunders[1967] indi- E(D •../_._ / cates that the biggesteffect for a nadir directed radiometer under light winds occursfor a solar elevationangle of 90ø. The largest solar elevationangle during the present data acquisi- n- 60 tion was 55 ø. Since the aircraft attitude caused the radiometers o o to deviate from nadir by only 1ø-2ø during the flight lines, o 40 Saunders[1967] indicatesthat solar specularreflections were entirely negligible. Figure 5 showsa regionnear the IMET mooring,with segments of the six ground tracks shown in Figure 1 and two 155.9 156 156.1 156.2 156.3 156.4 156.5 additionalground tracks flown after the NCAR Electra aircraft LONGITUDE (OE) had departedthe area. The R/V Moana Wavemeasured fluxes Figure 4. Spatiall¾extrapolated values for the current com- and water propertiesabout 4 km north of IMET during this po•½m paraiM to the •rou•d track which were used to shift period. Its positionsduring the legs are also indicated. the SST transects.The thin solid curve is the extrapolated Figure 6 showsSST measurementsfor 12-km-longsegments currentcomponent for leg 5 (northeasttransect) and the thick from each leg, centered on the point of closestapproach to solidcurve is for leg 8 (southwesttransect). The dashedcurves IMET. No smoothingwas applied to these 1-s data. At zero are the solid curvesmultiplied by a factor of 1.68. along-trackdistance, the circle is the IMET SST determined from the TOGA COARE bulk algorithm[Fairall et al., 1996b], which includesboth the diurnal surfacelayer and the cool skin effects;the crossis the IMET seatemperature at 0.45-mdepth. N43RF and from the NASA Jet PropulsionLaboratory (JPL) At the minimum of the dottedcurve, the plus symbolis the sea SST radiometer on the NCAR Electra aircraft 308D. The temperaturemeasured at about5-cm depth at the R/V Moana PRT-5 radiometer was located toward the tail section of the Wave; the circle at the same position is the SST estimate NOAA aircraft and viewed the ocean through a small nadir arrived at by subtractingthe cool skin temperaturedifference port. It has a 2ø field of view (FOV) and operatesin the computedfrom the TOGA COARE bulk algorithm from the thermal IR regionbetween 8 and 12/xm. This spectralregion, 5-cm-depthtemperature measurement. The Moana Waveand often called the atmospheric"window" region, is commonly IMET SST estimatesare generallyin good agreementwith the usedfor measurementsof sea surfacebrightness temperature. aircraft measurementsconsidering the ground track locations The PRT-5 was calibratedbefore and after deploymentto the and the spatialgradients. SolomonIslands. For the rangeof temperaturesreported here the pre-missionand post-missioncalibration curves agreed to within 0.15øC,well within the nominal accuracyof the instrument (0.5øC). The JPL SST radiometer on the NCAR aircraft has a 1-mrad FOV and a collectingaperture of 20 cm. The radiometer telescopewas configuredwith an opticalfilter in the 10- to 1.715-I 4 2 + 11-/xm region. The instrument has a laboratory calibration accuracyof 0.1øCand a precisionof 0.002øCat 20øC(at 293 K 1.725 and 0.1 Hz). The radiometer was mounted in the forward baggagecompartment of the Electra and viewed the ocean 1.73 through a germaniumwindow. The window temperaturewas monitored to correct the detected radiance for the contribu- tion by the window emission[Hagan et al., 1997]. 1.735 Measurementsfrom the two radiometers agreed to within 1.74 about 0.2øC, when obtained wingtip to wingtip at the same 1.745 altitude.To comparethe radiometricdata setswith eachother and with the ocean in situ measurements, the data were cor- 1.75 rected for atmosphericattenuation and sea surfaceemissivity effects. Sky emissionwas measured by an upward looking PRT-5 radiometer on board the Electra. These measurements 1.76 were used to correct the JPL measurements for effects of 155.97 155.98 155.99 156 156.01 156.02 nonblacknessin the sea surfaceemissivity. The emissivityad- LONGITUDE (øE) justmentdetermined from the Electrameasurements (+0.3øC) Figure 5. Segments(bold numbers)of the sixground tracks was applied to the NOAA downwardlooking PRT-5 observa- shownin Figure 1 and two additionalground tracks flown after tions, sincesky radiation measurementswere not availableon the NCAR Electra aircraft left the area. The plus symbols the WP-3D. The signal attenuation due to absorptionpath indicatethe positions(regular numbers) of R/V Moana Wave length between the aircraft and surfacewas determined em- during each numbered leg. The IMET mooring position is pirically as 0.08øC per 30 m, in reasonableagreement with indicated by the circle. WALSHET AL.:INTERNAL WAVE COUPLING TO DIURNALSURFACE LAYER AND SST 12,617 31.5[ 31 ...... ß •.' .%' o ..' t. ..';...... ß :".: :x. ß'.,':; .";..-'.,ß

30.5

30 -5 0 5 3•t'...... o.• :..'...+...... t'.. ,...:...x...... ;.."t t-2.:'"'"'"'-.'. v. "'.-1 5, o 3 • oO "'"" -5 0 5 -5 0 5 • E •./'J31 j.-.&*, .. .', ..- - "'""<...' '. o .'":-':- •:: t ß;•';'-..:• o ...... :,. " ,'•...'.. 0'. ß,ß; "•-...... X + • .ß .•'.,...., ',.....o ;ß,,,,; ',=,,,o % ßß.-.:. % ß:...... o ß 7 • 30.5 ß"'•,•.'"•._... •' .... -"'5 '• LEG 6 z 30 • • -5 0 5 -5 o 5 !-- 31

ß ;"-....'.: ...,,.'... ..,, ;.,:'.,, ...... •'..'.•. .:. :.-.: :.'-+R : .... •97 30.5 , ß...... ø o..ß...'ß;•- o; ß :' ,'" LEG8 ...,5...... 1 I , I I ...... t 3O -5 0 5 -5 0 5 ALONG-TRACKDISTANCE (km) ALONG-TRACKDISTANCE (km) Figure6. SSTmeasurements for12-km-long segments from each leg, centered onthe point of closest approachtothe IMET mooring. Nosmoothing wasapplied tothese 1-s data. The abscissa isnegative for positionssouthwest ofthe point of closest approach toIMET and positive forpositions northeast, indepen- dentof the flight direction. Thecurves indicate thedistance ofeach measurement fromIMET (solid) and fromthe R/V Moana Wave (dotted). The symbols are described in section 3 of thetext.

4. SSTVariation With Wind Speedand Mean accurate(see Walsh et al. [thisissue] (hereinafter referred to as Square Slope paper2) fordetails of thewind speed measurement), butthe Figure7 showsdata from leg 1 of Figure1. Thegeneral high-low-high-lowvariation of thecorrelation coefficients on variationof theSST appears to be inverselycorrelated with thefirst four legs, where there are relatively small temporal mssover the leg, with SST resembling a W andmss resembling and spatial changes because the aircraft just keeps reversing an M. The sameis true to a lesserextent with SST andwind heading,emphasizes thetenuous nature of the relationship. speed.At 2.04øSthere is a 1 m s-• local minimumin wind Thetransient response toa fluctuatingwind speed would be speedthat correspondsto a localminimum in mssand a max- expectedto beless than is indicated in Figure8, sinceit takes imumin SST.In general,however, the variations in mss,wind timeto redistribute thenear-surface heatover a greater depth. speed,and SST on spatialscales of a few kilometersdo not However,the weak dependence of SST on wind speed also correlate well. suggestsprocesses other than just the interplay of surfaceforc- The SSTdependence on windspeed alone is indicatedin ingand entrainment encapsulated bythe Price et al. [1986] Figure8. The circlesindicate the nominalvariation with wind mixedlayer dynamics built into the COARE algorithm. This speedof thepeak afternoon temperature of thediurnal surface notionis reinforcedby the largernegative correlation found layerdetermined from the Fairall et al. [1996a]warm layer betweenmss and SST as seen in Figure7 butalso seen more model,while the stars show the wind speed dependence ob- generallyover all sixlegs in Figure10. servedby Solovievand Lukas [1997] during COARE. The Windspeed and mss are positively correlated [paper 2], but powerlaws fitted to Figure8 alsoappear in Figure9, which internalwaves can also give rise to patternsin mss.The con- showsthe SST data from flight lines 1-6 versuswind speed vergent currentsthey generatehave been describedas accu- relativeto thesurface. Both the SST and the wind speed are mulatingover the troughof the internalwave both warm sur- 12,618 WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

0.010 completed.An hour after the initial northeasttransit (leg 5), N43RF made a nearly overlyingsouthwest transit (leg 8, Fig- 0.005 ure 5). Figure 12 indicatesthe north-southdeviations of legs5 and 8 from a straightline passingthrough leg 5. The southwest 0 transit maintained a nearly constant400-m offset south of the northeastground track alongmost of its length. The top panel of Figure 13 showsSST from the N43RF PRT-5 radiometeron legs5 and 8. The correlationcoefficient for the SST measurementson leg 5 (solid curve, identicalto the solid curve in the bottom panel of Figure 11) and leg 8 (dots) was 0.73. Becausesimilar structurewas recognizedin the two SST transects,an attemptwas made to shift them into • + +-H,...... + +•+ + • 31.3 better alignment. O L+,_-• _•¾-m-• -h•'?m--¾ ½--•++ +•-•-H • •-•J 31.0 Legs 5 and 8 had orientationsclose to the mean current •_ + u) 30.7 direction,and the directionof propagationof the semidiurnal • • , , • + • • 30.4 tide internal wave. The first attemptwas an ad hoc assumption 2 1.8 1.6 1.4 1.2 LATITUDE (øS) that the SST structurewas rigidlyimbedded in the sea surface and advectedby the current. A referencetime was taken half- Figure 7. Mean squareslope, wind speedrelative to the sur- way betweenthe end of leg 5 and the start of leg 8. face, and SST versus latitude for leg 1. The vertical lines The overflightson November28 occurredin the afternoon throughthe msscircles indicate the lrr error bars on the mean values. under diminishingsolar heating. A linear regressionto the SST

face water and surfactants,which would tend to decrease mss 34 [LaFondand Cox, 1962;Fedorov and Ginsburg,1988; Kropfii et al., 1998]. Etkin et al. [1992] reported that slick regionsmea- •'33 sured off Kamchatkaunder light wind conditionsduring the dayhad surfacetemperatures measured with an IR radiometer :::) 32 that were 1.5ø-2øChigher than those of rough regions. I-,, n'- u,,i n 31 5. Spatial Variation of SST \ •,•,•d""Fairallet al. (1996a) On legs 1 and 5 of Figure 1, NOAA WP-3D aircraft N43RF • 0 flew wingtip to wingtip with NCAR Electra aircraft 308D at 3O 60-m height with about 100-m separationbetween their fuse- Soloviev and Lukas (19 lages. The top panel of Figure 11 showsan overlay of the O; simultaneousSST measurementson leg 1 from the PRT-5 radiometeron the NOAA WP-3D N43RF (solid curve) and the JPL radiometeron the NCAR Electra 308D (dots). The I11 5- bottom panel of Figure 11 is a similar overlay,but from leg 5. n- A constant bias of 0.2øC has been added to the JPL radiometer LLI >- data from leg 5 for this comparison.The latitudesand longi- •,.x II1.6 tudes indicated in this paper are from the N43RF GPS- m10- correctedInertial NavigationEquipment (INE) data (see ap- ' %'"•'• II 1.4 pendix). The relative SST variationsin both panelsof Figure 11 are in excellentagreement. These 1 Hz data were time shiftedwith respect to each other to determine the alignment with the z highestcorrelation. The 0.98 correlationfor the 60ø flight di- ko i i i i I I rection (Figure 11, bottom) was obtainedwith no time shift, 2O indicatingthat the long axis of these features was oriented 0 1 2 3 4 5 6 7 WIND SPEED (m/s) perpendicularto that groundtrack. The 0.96 maximumcorre- lation for the north flight direction(top) was obtainedwhen Figure 8. Expectedvariation with wind speed of (top) the the NCAR Electra data were shifted 1 s earlier in time. The peak afternoontemperature of the diurnal surfacelayer (not aircraftground speed on that leg was 111 m s-•, and the includingthe cool skin) and (bottom)the depthof the diurnal NCAR Electra was about 100 m to the right of the NOAA P-3. surface layer for the conditionsof the November 28 flight, This suggeststhat the long axis of the features was oriented computedfrom the Fairall et al. [1996a] warm layer model (circles).Results using Solovievand Lukas [1997] measure- perpendicularto 48ø , quite consistentwith the 60ø flight direc- mentsare indicatedby asterisks.The water at 5-m depthwas tion conclusion,considering the temporal quantization.The within 0.16øCof 29.3øCfrom November25 throughNovember minor differencesin the remarkableagreement of theseinde- 28. The plotted temperatureswere arrived at by adding the pendenttemperature measurements could be attributedto the cited temperature differencesacross the diurnal surfacelayer 100-m lateral displacementof the aircraft ground tracks. to 29.3øC. Curves show severalpower law dependencieson The NCAR Electra departedthe area soon after leg 6 was wind speedfor comparison. WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,619

\ o • 30.5 O • I I I • I I •. I • 31.5•LEG •,COR OEF. =-0.3034 •EG 4,COR EF. =-0.07341 • 30•oo o •co • o oo o • o o•o • 31 ho•

O 30.5- hx o • _

• 30

• 31.5

30.5 - øo oo O

0.5 1 1.5 2 2.5 0.5 1 1.5 2 2.5 WIND SPEED (m/s) Figure 9. SST data pointsfor each of fif6ht lines 1-6 versuswind speedrelative to the surface,with linear rc6rcssfonlines. The dashedand solid cu•cs are the sameas in the top panel of Ff6urc 8. data from flight lines 1-6 indicated SST cooling at a rate of curvein the bottom panel of Figure 13, the correlationcoef- 0.24øCh -• [paper2]. SSTvalues were extrapolated from the ficientwould increaseto 0.92. This may be becausethe NCAR two measuredtransects to the referencetime assumingthat all aircraftwas southof the NOAA aircraft on leg 5, so its ground the SSTvalues decreased at the 0.24øCh- • rate.The positions track was closerto that of leg 8. along the ground track changedby an amount determinedby Becauselegs 5 and 8 were traversedin the oppositedirec- the time interval between the measurement time and the ref- tions, the time interval was larger at the west end, where the erencetime multipliedby the componentof the currentvector speedof translation(Figure 4) was lower, and smallerat the parallel to the groundtrack at the measurementpoint (solid eastend, where the speedof translationwas higher.The result curvesin Figure 4). The resultis shownin the middlepanel of was an almostconstant relative shift of about3.5 km alongthe Figure 13. The agreementis considerablyimproved, and the groundtrack for the transectsshown in the bottom panel of correlation coefficient has increased to 0.81. Figure 13. The secondattempt to shift the SST patternsfrom the two Since the data on legs 5 and 8 were not collectedsimulta- flight legsinto better agreementwas done under the assump- neously,the standarddeviation of the positionalerrors on each tion that the patternstranslated at speedsfaster than the sur- leg would be expected to be about 50 m, and the relative face currentbut in the samedirection. If the translationspeeds positionalerror betweenlegs 5 and 8 would be expectedto be are increasedabove the surface currentsby a factor of 1.68 about 70 m. This estimateis quite reasonablefor the measure- (dashed curves in Figure 4), the correlation coefficientin- mentswe have presentedhere (see appendixfor more discus- creasesto 0.91 (bottompanel of Figure13). This is remarkable sion), and it is a negligibleuncertainty compared with the agreementconsidering not only the hour between the flight observed3.5-km displacementof the SST pattern between lines, but their 400 m lateral displacement.The aircraft sepa- those two transects.Even though a larger relative error is ration for the simultaneousdata of Figure 11 was only - 100 m. possible,it seemshighly improbablethat it would be exactly If the NCAR Electra JPL radiometercurve (dotted curvein alignedwith the 60ø internal wave propagationdirection. the bottom panel of Figure 11) were substitutedfor the solid Sinceleg 5 was at 60ø and leg 8 wasoffset about 400 m south, 12,620 WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

LEG 1, CORR. COEF. =-0.5465 LEG 2, CORR. COEF. =-0.4423 31.5[ oo o0 o• oo o oo •o o o• oo o 31

o ;%o • oo• o o o •oøO• •O 30.5 • LEG 4, CORR. COEF. = -0.1708 ::33

oo % o o - o_ •._o o o •

!-- o o LU o o • o 0 30.5

•) I I I I I I i I I i 03 30 LEG 5, CORR. COEF. = -0.22 LEG 6, CORR. COEF. =-0.2145 LLI 31.5

o o o 31 c•øø • o o

o o Ooo 30.5 - o 0 Oo%0% o I o•o Oo_ o 000 • 0 000 0 0 0 0 0

30 I I I i I I I I I 0 0.002 0.004 0.006 0.008 0.01 0 0.002 0.004 0.006 0.008 0.01 SEA SURFACE MEAN SQUARE SLOPE Figure 10. SST data Dointsfor each of flight lines 1-6 versusmss with linear regressionlines.

aligning the longitudevalues in Figure 13 actually shiftsthe Pacific,the gradientin the diurnal surfacelayer canbe as large relative positions200 m too far. The end of the southwest as4øC m -• [Ravier-Hayand Godfrey, 1993; Fairall et al., 1996a; transit (leg 8) occurred4440 s after the start of the northeast Solovievand Lukas, 1997], and the dotted curve in the top transit (leg 5), so aligningthe longitudeswould overstatethe panel of Figure 14 is an example.At 1302 LST the wind speed requiredcurrent by 4.5 cm s-• at the westend of leg5. Sim- averagedonly 0.5 m s- ] andthe highesttemperature gradient ilarly, the start of leg 8 occurred2870 s after the end of leg 5, occurred closest to the surface. At 1501 LST the wind was sothe currentrequired for alignmentwould be overstatedby 7 2.3m s-• anda shallowmixed layer had been generated right cm s- • at the eastend of leg 5. Thiswould reduce the multi- at the surface. The temperature increase in the layer was plicativefactor on the currentfrom the 1.68used in Figure 4 to higher and the depth more shallow than predicted by the about 1.58, still well above the observed current. TOGA COARE bulk algorithm,so the wind had probablyonly recentlyincreased to that level and the layer was still in tran- sition.At 1700LST, the wind averaged3.7 m s-] and the 6. Internal Wave Modulation of Diurnal diurnal surfacelayer had deepenedand its peak temperature Surface Layer had decreasedsignificantly. In this section we will look for evidence that the diurnal The bottom panel of Figure 14 showsthe IMET tempera- surfacelayer was modulatedby internal waves.In that context ture profiles on November 28 at 0240, 0440, and 0640 UTC, it is usefulto contrastthe diurnal surfacelayer temporalvari- which are within 4 min of the same three local solar times as ation at the IMET buoy on November 28, 1992, with that for the Solovievand Lukas [1997] profiles.The abscissafor the anotherlight wind day reportedby Solovievand Lukas [1997]. IMET profileshas been shiftedby 0.5øCrelative to the Solov- The top panel of Figure 14 showstemperature profiles made iev and Lukas data, sincethe mixed layer was coolerby about by Solovievand Lukas on May 3, 1994, within 25 km of 0øN, that amount.The wind speedrelative to the surfacemeasured 148.2øE,at 1302(dotted), 1501(solid), and 1700(dashed) local at 3.54-mheight at IMET averaged1.6 m s-• from0000 UTC solar times (LST). through0500 UTC, whichincluded the first two profiles.This During calm, sunny afternoons in the western equatorial agreeswith the aircraft wind speed measurements,which av- WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,621 eraged1.6 m s-• overthe experiment region and showed ,• o.3 negligibletemporal trend. The winddecreased to about1 m •o s-• bythe third profile, which was after the aircraft data con- • o.2 sideredhere were acquired. The IMET temperatureprofiles •: 0.1 showedmuch less temporal trend over the 4-hourperiod than •z the Solovievand Lukas data did becausethe wind showedlittle 2 0 G trend.This makes it easier toobserve transients caused by om -0.1 internal waves. u_ Figure15a shows the temporalvariation of the net short- •-0.2 wave radiation at both IMET and R/V Moana Wave, and the z totalheat flux measured byIMET. Figure 15b shows the •-0.3 Moana Wave sea temperature(ST) at 5-cm depth and the •-o.4 IMET measurementclosest to the surfacefrom a TPOD at •m 45-cmdepth. Also shown are the temperatureat 2-m depth o3• -0.5 from a SEACAT at IMET (solidline) and the TOGA COARE I I I I I I ø I bulk algorithm prediction of the peak diurnal surface layer 155.9 156 156.1 156.2 156.3 156.4 156.5 temperature(dashed line). LONGITUDE (OE) The top meter of the sea absorbsabout half of the incoming Figure 12. North-south deviation of legs 5 and 8 from a straightline.

31.6[ CORR.COEF. = 0.9554 solar radiation, and the warming responseis similar in the 5-

31.4 and 45-cm-depthtemperature increases. Under the light wind conditionsthe diurnal surface layer remained shallow, and

31.2 there was little temperatureincrease observed initially at 2-m depth. However, as the insolation decreased,the effect of wind-driven mixing became more apparent, and the near- 31 surfacetemperature of the diurnal surfacelayer began to decrease. There is a period in the afternoonwhen the net heat flux is øø.ø/ still positive but the layer is cooling and deepening.Shortly after 0500 on November28, the layer at the IMET mooring deepenedto approximately2.0 m for an hour, then restratified, 30.6[30.4 ' • • • • -2.2 -2 -1.8 -1.6 -1.4 -1.2 and finally deepenedpast 2.0 m again as the net heat flux LATITUDE (øS) becamenegative. At night, radiativecooling drives ocean con- vectionand a decreasein SST. The layer deepens,cooling the

CORR. COEF. = 0.9783 surfaceand warming the water below. By 1300, approaching 31.2•- local midnight, the warmed water had been mixed down, and the temperature differed very little between 0.45 and 5 m depth. Our interestin particular,however, is in the period in 31. • .• . the afternoon when transient deepening occurred at IMET. During this time the sensitivityof the shallow diurnal mixed

•- 30.8•!ß • layer to externalprocesses that modify the depth of the layer shouldbe great. Figure 15c showsthe wind speedrelative to the surfaceat both Moana Waveand IMET. Figure 15d shows26 ø, 27ø, and 28øC isothermsgenerated from IMET SEACAT data, which 0330.4•- had 4 timesthe temporalresolution of the TPOD data usedto 30.2[ . generate the isothermsof Figure 3. In addition to the dominant semidiurnalperiod, there are excursionswith about 20-m 30[ amplitudeand a 3- to 4-hour period, and 7- to 8-m excursions 155.9 156 156.1 156.2 156.3 156.4 156.5 LONGITUDE (OE) with half hour periods. Figure 16 showsTPOD temperatureprofiles for an 8-hour Figure 11. SST data on (top) leg 1 and (bottom) leg 5 from intervalwhich includes the aircraftdata acquisitionperiod. In the PRT-5 radiometer (solid curve) on the NOAA WP-3D the first 2 hours(0310-0510) there was little variationappar- aircraft N43RF, and the simultaneouslycollected JPL radiom- ent in these data taken at 15-min intervals. In the next hour eter (dots) on the NCAR Electra aircraft 308D which was (0510-0610) the diurnal surfacelayer deepenedsignificantly. flying wingtip-to-wingtipwith the NOAA aircraft. These 1-s In the next half hour (0610-0640) it movedback up, almostto data points have been smoothedwith a 9-point boxcarfilter. The fuselageof the Electra was about 100 rn to the right of the its startingposition. Then it startedmoving back down again. P-3 on both legs.For leg 1 comparisononly, the Electra data After 0700, vertical oscillationsare still apparent,but the total were shifted by 1 s becauseit encounteredthe SST features heat flux is negative(Figure 15a), and the diurnalsurface layer earlier than the P-3. For the leg 5 comparisononly, a constant is coolingand deepeningprogressively. bias of 0.2øCwas added to the JPL data points. The TPOD measurementsused to develop the profiles of 12,622 WALSHET AL.' INTERNAL WAVE COUPLINGTO DIURNAL SURFACELAYER AND SST

Thoseresults for the first31 profilesof Figure16 are shownin 31.2•CORR. COEF.=0.7326 Figure17 (profile32 did not reach30øC). The equationof the line in Figure 17 is d30-- 2 + 1.1(t2- 30), (1) whered30 is the depthin metersof the 30øCisotherm, and t 2 30.8 [ ;•:.' :.k. is the temperaturein degreesCelsius at 2-m depth. :j ': The solidcircles in Figure 17, in order of increasingdepth, correspondto the followingprofiles in Figure16: 1, 2, 6, 8, 15, 17, 24, 22, 11, and 12. Thoseprofiles are reproducedin Figure • •: : :... . 18, with profile 1 dotted,profile 2 dashed,profile 6 solid, profile8 dotted,etc. The simplelinear relationship (1) ac- countsfor both the changingthickness and corresponding changein temperaturegradient in the diurnalsurface layer. Someof the outliersare causedby apparentanomalies in the 29.830[ • • • • • • t temperatureprofiles (7 and 16, for example),while thosebe- CORR. COEF. = 0.8128 low 2.5 m are causedby the lack of observationsbetween that

31 depth and 4.65 m. Relationship(1) breaks down when the layer becomes

30.8 mixed. We can apply (1) to the 3.75-min data from the SEACAT at 2-m depthto obtaina highertemporal resolution for the 30øCisotherm than is providedby the TPOD 15-min 30.6 datainterval. The resultis presentedin Figure19a. Figure 19b is the 30øC isotherm at 15-min resolution determined from the •" 30.4 TPOD profilesof Figure16. Figure 19c shows the 28øC,27øC, and 26øCisotherms from the SEACAT data of Figure 15, and 30.2 - Figure 19d showsthe 24øC,21øC, 19øC, and 15øCisotherms from the TPOD data of Figure 3. The structureapparent in the SEACAT data suggeststhat the 15-min-resolutionTPOD data were aliased. There is gen- 29.8 eral agreementbetween the isothermsabove 25øC, but there CORR. COEF. = 0.9085 are alsoindications of the complexityof the internalwave field. 31 At the dotted vertical line there is no local displacementof ...: : either the 30øCor the 24øCisotherm, whereas the 28øC,27øC, 30.8

30.6 29.5 30 30.5 31 31.5 32 0 30.4 1302LST-•... •-- - .... "•

o lO 2o 3o "'"> I• •1501LST IJJ 30'21i .... • .... i • i • I ;,,'•/•'•--1700 LST ALONG-TRACK DISTANCE (km) :. • / / May3, 1994 w 2

I I I I I I 155.9 156 156.1 156.2 156.3 156.4 30 30.5 31 31.5 32 LONGITUDE(øE) 29.5 30 30.5 31 31.5 Figure 13. (top) N43RF PRT-5 datafrom leg 5 (northeast transect)(solid curve, repeated from the bottompanel of Fig- ure 11) andfrom leg 8 (southwesttransect) about 1 hourlater I- (dots).(Middle) Same two curves, but advectedby the current n , IJJ to a commonreference time halfwaybetween the two transects t n 2 and corrected for the measuredsea surface cooling rate of November 28, 1992 0.24øCh -]. (bottom)Same curves, but with the currentas- , (IMET buoy) sumedto be largerthan the observedcurrent by a factorof 3 1.68.The kilometerscale indicates the along-trackdistance for 29 29.5 30 30.5 31 31.5 the groundtrack orientedat 60ø . TEMPERATURE (oc) Figure 14. (top) Temperatureprofiles measured by Soloviev and Lukas [1997] on May 3, 1994, in the vicinityof 0øN, Figure16 were made only at 15-minintervals, and they showed 148.2øE,at 1302(dotted), 1501 (solid), and 1700 (dashed) local little variabilitybetween 0310 and 0455. To obtainhigher tem- solartimes (LST), with average wind speeds of0.5 m s-•, 2.3m poral resolution,the TPOD profileswere usedto relatethe s-] and 3.7 m s-], respectively.(bottom) IMET temperature temperatureat 2-m depthto the depthof the 30øCisotherm. profileson November28, 1992,for the samethree LSTs. WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,623 and 26øC isothermsare depressedand the 19øC and 15øC 1000 - isothermsare elevated,suggesting higher-mode motion. E Figure 20 is a blowupof the SEACAT data from Figure 19, • 500 with the additionof the current at 5-m depth.There is strong 0_.• agreementbetween the 20-min-periodoscillations in the iso- TOTAL HEAT FLUX therms on the main thermocline and in the diurnal surface layer. The excursionsare a significantlyhigher percentageof 31 the meanisotherm depth in the surfacelayer (about 15-30%) than they are on the main thermocline(about 5-10%). • 30

7. Internal Wave Modulation of SST Section5 indicated that the SST variabilitycan be success- fully interpretedas patterns propagating toward 60 ø . In section 3 6 we sawthat internal wavespropagating on the main thermocline can modulatethe thicknessof the diurnal surfacelayer and inducecurrents in it. What is not clear at thispoint is the exact nature of the physicalprocess that would result in the SST signatureswe have observed.We will examineour obser- vationsin the contextof other experimentsto developa better 60 senseof the attributesa proposedphysical mechanism should have. 70 Jessupand Hesany[1996] found that the downwindside of surfacegravity wave crestswas warmer than the upwindside wheneverthe wind was either alignedwith or opposedto the swellpropagation direction. They suggestedthat the effect was ao causedby disruption of the cool skin by small-scalewave lOO breaking.The 1-km averagesused in thispaper smoothed over any individualsurface gravity wave effects,but it seemslikely 11o that the SST spatialpatterns we observedwere more thanjust a surface manifestation. Were that the case, there should have 12o -5 0 5 10 15 20 been a muchhigher correlationbetween SST and mssthan was TIME (UTC HOURS) on 28 NOV 92 observed(Figure 10), and the sign of the correlationwould Figure 15. (a) Net shortwaveradiation at R/V Moana Wave havebeen positive. If small-scalewave breaking disrupted the (solidline) and IMET buoy(dots) and total heat flux (solid cool skin, then the warm areaswould have been rougher,not line). (b) Moana Wavesea temperature(ST) at 5-cm depth smoother. (circles)and IMET ST at 45-cmdepth (dots) and 2-m depth That the SST patternsreflect a bulk effect is suggestedby (solid),and predictionof the TOGA COARE bulk algorithm the R/V Moana Wave5-cm depth temperature (and, to a lesser (dashedline). (c) Wind speedrelative to the sea surfaceat extent,the 45-cmdepth IMET measurements)showing similar Moana Wave(circles) and IMET (dots). (d) The 26, 27, and magnitude excursionsrelative to the TOGA COARE bulk 28øCisotherms. The three verticallines indicatethe starting algorithmprediction (Figure 15) as occurredin the aircraft time of leg 1 (Figure 1), the midpointtime of leg 5, and the measurementsof SST (Figure 13). midpointtime of leg 8 (nearly a retraceof leg 5 flown in the oppositedirection) about 1 hour later. Sea surfaceslicks and roughnessvariation have long been associatedwith internal wave propagationin the thermocline [e.g.,LaFond and Cox, 1962;Liu et al., 1998].In the classical theory, however,internal wavesproduce no SST signature. wouldbe affectingthe water itself.Langmuir [1938] indicated Water initially at the surfacenever leavesit. Each incremental that the water in convergencezones had been at the surface subsurfacetemperature layer alternatelythins and thickensas longer than the water between them. the internalwave passesby, so that the temperaturegradient Fedorovand Ginsburg[1988] (see pp. 140-144of the English variesinversely as the depth of the layer, preservingthe SST. translation) cite several referencesindicating that internal McLeish [1968] studied the IR signature of wind slicks wavespenetrate into the diurnal surfacelayer and create a (Langmuircells). They are alignedin the downwinddirection systemof quasi-horizontalconvergent-divergent motions that and occur on a much smaller scalethan we are concernedwith, redistributethe heat accumulatedduring the day under light but hisinvestigation still providessome insight. McLeish found wind conditions.None of the measurementscited by Fedorov that the patternswere shownmore clearlyby an IR line scan- and Ginsburg[1988] includedSST. However, near-surfacesea ner than by visiblephotographs because the temperaturesig- temperaturemeasured with a sensoror multiplesensors towed naturewas stronger than the roughnessvariation that made the at fixed depthsshowed fluctuations with amplitudesof 0.5ø- slicksappear as dark lines on the photographs.This is similar 1.5øCand horizontalscales of about 1-5 km. During the 34th to the situationfound in Figure 10. Slicksform from the ac- cruiseof the R/V AkademikKurchatov, for example,a sensor cumulationof surfactants,which damp the capillarywaves. The towed at 0.15-m depth in the region of the Peruvian coastal reductionin msswould depend on the surfactantconcentra- upwelling[Zatsepin et al., 1984] demonstratedtemperatures tions availablein the area. However, the downwellingassoci- 0.3ø-0.6øChigher in the slickbands corresponding to areasof ated with convergencezones and the upwellingin between convergencelocated over the rear slopesof internal wave 12,624 WALSH ET AL.' INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

0 I i I I I I 29.5 30 30.5 3•1 29.5 30 30.5 311 1 x'"fi1 •04105 • /•' ,x,L0325 2 0425 6 I.t' 03403 '•0440 7 O455 8 I 29.5 30 30.5 31 29.5 30 30.5 31

I-

w n3 .."•/, /•'""/•'061013

'--- I I 29.5 30 30.5 31 29.5 30 30.5

I- n W n 3 ß'• 17 3- ./'•1 •0810 21 - 0725 18 Z/// • 082522 •,, 074019 •' r• 084023 - "-0755 20 I 29.5 30 30.5 31 29.5 30 30.5 3

1 -

I-

w • 3 if10 25 i • 101029 4 L 094027 4 04031

5 I -'"•095,528 05532 29 29.5 30 30.5 31 29.5 30 30.5 31 TEMPERATURE (oc) Figure 16. TPOD seatemperature profiles for an 8-hourinterval which includes the aircraftdata acquisition period.In all panels,the dottedprofile is for 25 min pastthe hour, and the dashedprofile is for 40 min past the hour.

creststhan in the bandsof ripplesin between,and on the 27th servedwarm water regions could propagatewith the phase cruisea sensorat 0.15-m depth in the SargassoSea indicateda speedof the internal wavesthey were associatedwith. 0.6øCtemperature variation, while anothersensor at 3-m depth The wind-drivenshear in the layervaries inversely with layer showed little variation. This is similar to the situation shown in depth. Shoalingwould increaseshear and thusentrainment at Figure 16. Subsurfacetemperature variations do not provethat the base,while deepeningwould reducethe wind-drivenflow there is a SST signature,but suchlarge variationsso closeto and perhapsbring shear below the critical level for overturn- the surfacestrongly suggest it. ing. If the Richardsonnumber goes above the critical value, Implicit in the fixed phaserelationship between the internal mixing is cut off. In Figure 16, the short pairs of horizontal waves and the shallow depth temperature variations is the lines at the 29.2øC abscissaposition indicate the predicted presumptionthat the temperaturedisturbance propagates with depth of the diurnal surfacelayer at the beginningand end of the phasespeed of the associatedinternal waves. This suggests each hour. The predictions,computed from the Fairall et al. that the internalwaves are organizingprocesses in the diurnal [1996a],warm layer model showa remarkableaccuracy when surface layer that affect its near surface temperature. The comparedwith the highestprofile duringeach hour. This sug- original Russian referenceswere not reviewed,but Fedorov geststhat the diurnal surfacelayer is in its normalstate except and Ginsburg[1988] offered no suggestionas to how the ob- when the troughof an internalwave passes by, at whichtime it WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,625

1.2 1.4 7 3:I FROM 2M SEACAT ,, n' 1.6 c• 3 ' . '• 30øCISOTHERM ' b 0 1.8 •E I '

øo 2

0 3:FROM TPODPROFILE • 2.2 E

I- 2.4

2.6 u.i 90 .

I I I I I I I I 29.4 29.6 29.8 30 30.2 30.4 30.6 30.8 100 2'•,_• TEMPERATURE (oc) AT 2 rn DEPTH 110 ! • I • Figure 17. Depth of the 30øCisotherm versus the tempera- ß d 120 ' tureat 2-m depth,developed from the TPOD profilesof Fig- 24øC ure 16. The solidcircles in Figure 17, in order of increasing 130 depth, correspondto profiles1, 2, 6, 8, 15, 17, 24, 22, 11, and 12 of Figure 16. 140

A 150 significantlydeepens, disrupting the normalprocesses within the layer.This imageis alsosupported by the 30øCisotherm in 3:160 .

Figure 20. i,u 170 ' The dashedcurves of Figure4 suggestthat the phasespeed of the 2- to 8-km featuresthat alignedso well in the bottom 180 , panelof Figure13 was in the 0.8 to 1.2m s-• range,a factor 190 of about1,6 timesthe currentspeed. This rate of translationis 15oC\ muchslower than the 3 m s- • phasespeed of thesemidiurnal tide. However,the generalsituation displayed in Figure3 is more complexthan a singlemode 1 internal wave of semidi- 210 200 ß urnalperiod. There are significantoscillations with periodsless 220 than an hour, and not all the isothermsrise and fall together. 10 8 6 4 For example,the 5 cm s-• currentspeed excursion at the TIME (UTC HOURS) on 28 NOV 92 positionindicated by the dottedline correspondsto significant Figure 19. The 30øCisotherms from applying(1) to the 3.75- local risesin the 15øCand 19øCisotherms, while the 24øCand min data (a) from the SEACAT at 2-m depthand (b) from 27øCisotherms are descendinglocally. This suggests the pres- TPOD data, with the 28øC,27øC, and 26øCisotherms from the enceof a higher-modewave. SEACAT dataof Figure15 and (d) the 24øC,21øC, 19øC, and Figure21 showsan analysisof the phasespeeds versus wave- 15øCisotherms from the TPOD data of Figure3. lengthfor the firstfive internalwave modes calculated from the N 2 profileobtained by averaging three conductivity-temperature-depth(CTD) profilestaken at R/V Vickers(2øS, 156.25øE) at 0700, 0800, and 1000 on November 28, 1992. The 0.8 to 1.2 m s-• rangefor wavelengthsof 2-8 km is enclosedby dashedlines. It is apparentthat a mode2 internalwave would i i i have the requisitephase speed, and mode 2 is consistentwith someof the out-of-phaseisotherm height variations seen in the 1- "'•';•.•.. / data. '-' ß-2"•.:• ' j -•' The phasespeed calculation for Figure 21 did not include -r 2 ' the current, but its effect was probablysmall. The current shownin Figure3 wasapproximately constant within the mixed layer.However, below --•40 or 50 m depth,its speed diminished rapidly,becoming less than 10 cm s-• by the bottomof the thermocline,and during that processthe current direction spiraledin a clockwisefashion. 29 29.5 30 30.5 31 If the phasespeeds are to match,the 2- to 8-km-wavelength TEMPERATURE(oc) SST features observed from the aircraft would have to be Figure 18. Profiles1, 2, 6, 8, 15, 17, 24, 22, 11, and 12 repro- causedby 2- to 8-km-wavelengthmode 2 internalwaves prop- ducedfrom Figure16 (correspondingto solidcircles in Figure agatingtoward 60ø, the samedirection as the internal tide. It 17, in orderof increasingdepth), with profile1 dotted,profile mightjust be consideredcoincidence that the short-wavelength 2 dashed,profile 6 solid,profile 8 dotted,etc. internalwaves propagated in the samedirection as the internal 12,626 WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

5O + the layer. The SST spatialpattern translationspeed is compa- TOTAL++++ +++++++++ rable to that of mode 2 internal wavesof similarwavelength. +++++ +++ +++ •'45 + The many sea surfacesensors employed in COARE on the z %+++ + + ++++ afternoonof November28, 1998,caught the upperocean in an n- 40 interestingstate. The surfacewas apparentlycooling at a mean O rate of 0.24øCh -1, under the influenceof an air-seaheat flux I-- 35 which shouldcause warming. Clearly, entrainmentof subsur- facewater was occurring, even though the localwind speed was E 30 - NORTH only1.6 m s-•. The extremestrains associated with internal wave modulation of the surfacelayer redistributewaters that 25 are initially laterally homogeneous,producing subsurface temperature patterns that propagate at wave phase velocities. 20 However,patterns in SST,which also appear to propagate,are 0500 0430 0400 0330 0300 seen as well. Some form of mixing must be invoked suchthat 1.2 the subsurfacepatterns penetrate to the sea surface. Can internal wave modulation of the mean entrainment rate cause the propagating patterns? We note that the vertical 1.6 • • • i13ooc• excursionsof the surfacelayer are of the order of 1 m only 1 m 1.8 belowthe surface.The local wind stress,while weak duringthis period, is distributed over a layer whose depth is strongly modulatedby the internal waves.Spatial variation in the sub- surfaceshear must result.The large near-surfacestrains should 7O correspondto lateral surfaceparticle displacementsthat are a significantfraction of the horizontalwavelength of the waves. E 80 The internal waves themselvesembody lateral motions that extend far below the diurnal surface layer. However, some

ILl 90 shear will be imparted acrossthe layer, and this can either reinforce or negate the wind driven shear. We conjecturethat as the afternoon progressesand near- 100 surfacestability decreases, internal-wave-related influences be- gin to modulate the near-surfaceentrainment rate. SST is, of 11o course,the integrated effect of all previousheating and en- o5)o 0430I 0400I 0330, I 0 • 00 trainment.In an initially homogenousocean with coolingrate TIME (UTC) on 28 NOV 92 modulatedby a singleinternal wave component,mixing will Figure 20. (top) The IMET currentat 5-m depth and (bot- producetemperature patterns that initially growwith time but tom) blowupsof the SEACAT data from Figure 19. eventuallyvanish after the internalwave has moved a complete wavelength.For a more complex, nonrepeatingpattern of modulation,a moving surfacetemperature signaturewill al- tide or they may have been of parasitic origin, but it seems waysbe present,although the apparentpropagation speed will implausiblethat their propagationdirection could be signifi- not alwaysreflect the phasespeed of the internal waves. cantlydifferent from 60ø and still maintainthe coherencedem- onstratedin the bottom panel of Figure 13. If these2- to 8-km featureswere actuallypropagating in somesignificantly differ- 2.5 ent direction, the apparent phase speed along the 60ø flight directionwould be correspondinglyhigher. The 60ø propagation direction is also suggestedby the wingtip-to-wingtipdata (Figure 11), whichindicated that the SST featureswere elon- gated in the directionperpendicular to 60ø .

8. Discussion and Conclusions The observationof an SST signalpropagating relative to the underlyingwater is unusual.We have presentedevidence of persistentSST spatial patternswhose temperature variations are not well correlated with either wind speed or the mean 0.5 squareslope of the sea surface.The SST patterns examined herepropagated at a speed(0.8-1.2 m s-•) thatwas too fast to be causedby the current(0.4-0.7 m s-q), andtoo slowto be I I I I I I explainedby the wind speed(2 m s-q) or the wind speed 0 1 2 3 4 5 6 7 8 relativeto thesurface (1.6 m s-I). We haveshown that internal WAVELENGTH (km) waves on the main thermocline can modulate the thickness of Figure 21. Phase speedversus wave length for the first five the diurnal surfacelayer, principallycausing it to deepenwith internal wave modes calculatedfrom measurementsmade by the passageof a trough,disrupting the normalprocesses within R/V Vickers(2øS, 156.25øE). WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST 12,627

We note that internal wave propagationconditions on No- program is the same relative standarddeviation we have as- vember 28 were nearly nondispersiveover horizontal wave- sumedin our assessmentof the SST pattern translationspeed. lengths4 km and greater(Figure 21). Also, the observedshift In 1997, ten flight lines of tOO- to 300-km length flown in in the SST pattern of 3.5 km is comparableto the scaleof the earlier years were repeated. The earlier ground tracks were smallest features observed, but much smaller than the scale of found to be so accurate, even with oscillations attributable to the most energetic SST signal component.Relative to the the aircraft-guidancesystem interaction, that none of the way wavelengthsof the SST pattern, only a modest amount of pointsneeded to be movedfor the repeat mission(R. N. Swift, signalpropagation has occurred. personalcommunication, 1998). The cooling rate model does not imply that the observed SST patternscorrelate perfectly with underlyinginternal wave displacements.Rather, SST representsthe integratedeffect of Acknowledgments.The authorsacknowledge the invaluablecon- previouscooling. Measurements that can establishthe direct tributionsof all the scientists,engineers, technicians and flight crews, relationshipbetween radiometric SST and the subsurfacemo- who made TOGA COARE possible.E.J.W. thanksPeggy LeMone of NCAR for discussions and access to the data from the forward- and tion field are required to identify the mechanismmore pre- side-lookingvideo cameras on the NCAR Electra, and Alexander cisely. Soloviev for discussions on his observations. He also thanks E. F. Bradley of the CSIRO, and Earl Gossard,Lev Ostrovsky,Andre Smir- nov, Len Fedor, and many other colleaguesat NOAA ETL for nu- Appendix merous discussions.D. E. Hines maintains the SRA, and he, D. W. On legst and 5 of Figure 1, NOAA WP-3D aircraft N43RF Shirk, and G. S. Mclntire installed the systemin the NOAA WP-3D aircraft for the TOGA COARE deployment.G. S. Mclntire and J. L. flew wingtip to wingtip with NCAR Electra aircraft 308D at Ward were in the Solomon Islands with the SRA. The authors thank 60-m height with about t00-m separationbetween their fuse- A. Plueddemann and M. Tomczak for the use of TPOD, Seacat, and lages.There was alsoa third aircraft, NOAA WP-3D N42RF, ADCP data from the IMET mooring,and A. Plueddemann,S. Ander- in front of them on thesetwo legsto form a tight triangle. All son,and the WHOI Upper Ocean ProcessesGroup for help with the three aircraft carried GPS receivers. There are intentional mooringdeployment, recovery, and subsequentdata processing.R.P. thanksthe officersand crew of the R/V Vickers.He was supportedby errors systcrnaticallyintroduced into the GPS SelectedAvail- National ScienceFoundation grant ATM-9110553. E.J.W. and D.E.H. a•ility (SA) orbitsand clocksto reducethe real-time absolute were supportedby the NASA Office of Earth Science.D.E.H. wasalso accuracyof the systemto about 50 m. But for aircraft flyingin supportedby the NOAA Climate Division. D.P.R. was supportedby NOAA and the Office of Naval Research. The work of S.P.B. was formation, the absolute errors would be identical and their supported by NSF grants ATM-9024436 and ATM-9110540 and relative positionswould be good to 2 to 5 m (W. B. Krabill, NOAA 56GP0154-01.R.A.W. was supportedby the National Science NASA GSFC, personalcommunication, 1997). Foundation, grant ATM95-25844. The GPS-corr½ctcdInertial NavigationEquipment (INE) positiondata for the NOAA aircraftindicated that N43RF was 80 m behind and 60 m to the left of N42RF on leg 1, and 60 m References behindand 50 rn to the left of it on leg 5. The relativepositions Apel, J. R., J. R. Holbrook, A. K. Liu, and J. J. Tsai, The Sulu Sea varied over a range of --•50 m along-trackand 20-30 m later- internalsoliton experiment, J. Phys.Oceanogr., 15, 1625-1651,1985. ally. These mean positionsand relative variationsare consis- Etkin, V. S., A. V. Kuzmin, and Yu. G. Trokhimovsky,IR and micro- tent with the images of N43RF recorded by a side-looking wave sea surface observationsunder calm weather conditions, in video camera on NCAR Electra 308D during these two legs. IGARSS'92, International Space Year: Space Remote Sensing, 92CH3041-1,pp. 260-262, IEEE, Piscataway,N.J., 1992. NASA Goddard SpaceFlight Center has had an Arctic Ice Ewing, G. C., Relation betweenband slicksat the surfaceand internal Mapping(AIM) programin operationsince 1991 to monitor waves in the sea, Science,3, 91-94, 1950. the elevationof the Greenlandice sheet [Krabillet al., 1995]. Fairall, C. W., E. F. Bradley, J. S. Godfrey, G. A. Wick, J. B. Edson, They map the elevationsover a swathwith a conicalscanning and G. S. Young, Cool skin and warm-layereffects on sea surface temperature,J. Geophys.Res., 101, 1295-1308, 1996a. laser from an aircraft flying 400 m above the terrain. The Fairall, C. W., E. F. Bradley, D. P. Rogers, J. B. Edson, and G. S. ground track is controlled by a guidancesystem that usesa Young, Bulk parameterizationof air-seafluxes for Tropical Ocean- singleGPS receiverand the SA signal[Wright and Swift, 1996] Global AtmosphereCoupled Ocean-Atmosphere Response Exper- to fly great circle ground tracksdefined by presetway points. iment, J. Geophys.Res., 101, 3747-3764, 1996b. The exact GPS orbits are obtained severalweeks post-flight Fedorov,K. N., and A. I. Ginsburg,Pripoverkhnostnyi sloi okeana,303 pp., Gidrometeoizdat,Moscow, 1988. (English translation, The and combinedin a differentialtechnique with the aircraft GPS Near-SurfaceLayer of the Ocean, translatedby M. Rosenberg,259 receiver data and simultaneouslyacquired ground receiver pp., VSP, Zeist, Netherlands,1992.) data to produceaircraft trajectorieswhose absolute accuracy is Garrett, C., and W. Munk, Internal waves in the ocean, Annu. Rev. t0 cm. This allows an assessmentof the real-time accuracyof Fluid Mech., 11, 339-369, 1979. Hagan, D. E., The profile of upwelling 11-•m radiancethrough the the aircraft guidancesystem. atmosphericboundary layer overlyingthe ocean,J. Geophys.Res., The projectguidelines were that the swathsfrom one year to 93, 5294-5302, 1988. anothermust have at leasta 50% overlapeverywhere to ensure Hagan, D. E., D. P. Rogers, C. A. Friehe, R. A. Weller, and E. J. that the height difference could be assessed.A wide swath Walsh, Aircraft observationsof sea surfacetemperature variability requires the laser to have a large off-nadir incidenceangle, in the tropicalPacific, J. Geophys.Res., 102, 15,733-15,747,1997. Jessup,A. T., and V. Hesany,Modulation of oceanskin temperature which degradesthe measurementsdue to uncertaintiesin air- by swell waves,J. Geophys.Res., 101, 6501-6511, 1996. craft roll attitude. Initially, useda 15ø incidence angle was used Krabill, W. B., R. H. Thomas, C. F. Martin, R. N. Swift, and E. B. to achieve a 200-m swath. Because the ground tracks were Frederick,Accuracy of airbornelaser altimetryover the Greenland found to be so accurate in earlier missions, in 1994 the inci- ice sheet, Int. J. Remote Sens., 16, 1211-1222, 1995. Kropfli, R. A., L. A. Ostrovsky,T. Stanton,E. A. Skirta, A. N. Keane, denceangle was reduced to t0 ø(140-m swathwidth). The 70-m and V. Irisov, Relationshipsbetween stronginternal wavesin the relativepositional standard deviation (half swathoverlap) for coastalzone and their radar and radiometricsignatures, J. Geophys. two transects flown at different times demonstrated in the AIM Res.,in press,1998. 12,628 WALSH ET AL.: INTERNAL WAVE COUPLING TO DIURNAL SURFACE LAYER AND SST

LaFond, E. C., and C. S. Cox, Internal waves, in The Sea, vol. 1, edited Webster,P. J., and R. Lukas,TOGA COARE: The Tropical Ocean- by M. N. Hill, pp. 731-763, Interscience,New York, 1962. Global AtmosphereCoupled Ocean-Atmosphere Response Exper- Langmuir, I., Surfacemotion of water inducedby wind, Science,87, iment, Bull. Am. Meteorol. Soc., 73, 1377-1416, 1992. 119-123, 1938. Weller, R., and S. Anderson,Surface meteorology and air-seafluxes in Liu, A. K., Y. S. Chang,M. K. Hsu, and N. K. Liang, Evolutionof a westernequatorial Pacific warm pool duringthe TOGA Coupled nonlinear internal waves in the East and South China Seas,J. Geo- Ocean-AtmosphereResponse Experiment, J. Clim., 9, 1959-1990, phys.Res., 103, 7995-8008, 1998. 1996. McLeish,W., On the mechanismsof wind-slickgeneration, Deep Sea Wright, C. W., and R. N. Swift, Applicationof new GPS aircraft Res., 15, 461-469, 1968. control/displaysystem to topographymapping of the Greenlandice Pinkel, R., M. Merrifield, J. Picaut, L. Washburn, S. Rutledge, cap,in Proceedingsof the Second International Airborne Remote Sens- M. McPhaden,and D. Siegal,Solitary internal waves in the western ing Conferenceand Exhibition:Technology, Measurement, and Anal- equatorialPacific Ocean, Geophys. Res. Lett., 24, 1603-1606, 1997. ysis,pp. 1-591-1-599,Environ. Res. Inst. of Mich., Ann Arbor, 1996. Price, J. F., R. A. Weller, and R. Pinkel, Diurnal cycling:observations Zatsepin,A. G., A. S. Kazmin, and K. N. Fedorov,Thermal andvisible and modelsof the upper oceanresponse to diurnalheating, cooling manifestationsof large internal waves on the ocean surface (in and wind mixing,J. Geophys.Res., 91, 8411-8427, 1986. Russian),Okeanologiya, 24, 586-592, 1984. (Englishtranslation, Ravier-Hay,P., and J. S. Godfrey,A model of diurnal changesin sea Oceanology,Engl. Transl.,24(4), 444-449, 1984.) surfacetemperature for the westernequatorial Pacific, TOGA Notes 11, pp. 5-8, Nova Southeast.Univ., Dania, Fla., 1993. M. Baumgartnerand R. A. Weller, Woods Hole Oceanographic Saunders,P.M., Aerial measurementof seasurface temperature in the Institution, Woods Hole, MA 02543. infrared,J. Geophys.Res., 72, 4109-4117, 1967. S. P. Burns,Department of MechanicalEngineering, University of Serra,Y. L., D. P. Rogers,D. E. Hagan, C. A. Friehe, R. L. Grossman, California, Irvine, CA 92717. R. A. Weller, and S. Anderson,The atmosphericboundary layer C. W. Fairall and E. J. Walsh (correspondingauthor), NOAA over the central and western equatorial Pacific Ocean observed EnvironmentalTechnology Laboratory, R/E/ET6, 325 Broadway, during COARE and CEPEX, J. Geophys.Res., 102, 23,217-23,237, Boulder,CO 80303-3328.(e-mail: [email protected]) 1997. D. E. Hagan, Jet Propulsion Laboratory, California Institute of Soloviev,A., and R. Lukas, Observationof large diurnalwarming Technology,4800 Oak Grove Drive, Pasadena,CA 91109-8099. eventsin the near-surfacelayer of the westernequatorial Pacific R. Pinkel and D. P. Rogers,Scripps Institution of Oceanography, warm pool, Deep SeaRes., Part I, 44, 1055-1076,1997. Universityof California,San Diego, La Jolla, CA 92093. Walsh, E. J., et al., Observationsof sea surfacemean squareslope under light wind during the Tropical Ocean-Global Atmosphere Coupled Ocean-AtmosphereResponse Experiment, J. Geophys. (ReceivedMay 29, 1997;revised March 10, 1998; Res., this issue. acceptedMarch 13, 1998.)