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Coupling of Internal Waves on the Main Thermocline to the Diurnal

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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 prop- agationof 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,
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