Brandt, Peter

Dengler, Marcus

Fischer, Tim

Visbeck, Martin

English

OceanRep

Tim Fischer, Marcus Dengler, Peter Brandt, Martin VisbeckUnderway acoustic surveyof internal wave shear and its use to estimatediapycnal mixing and transportsin the interior oceanBMBF-Verbundvorhaben   NordatlantikBMBF-Verbundvorhaben SOPRANRD 1   Ocean Circulation and Climate DynamicsDüsternbrooker Weg 20D-24105 Kielcontact: tfischer@geomar.de      www.geomar.depassive  scatterersdeducevelocity  fromDoppler-shifted  echoAcoustic imaging of internal wave shear  —  a proxy for turbulent mixingShear quantification from a moving ship  —  2 main issuesProcessing:  From acoustic pings to shear level to diapycnal diffusivityApplication at Tropical North Atlantic Oxygen Minimum Zone :Inferring diapycnal mixing and diapycnal oxygen transportReferencesspeed-dependent horizontal smoothing patchy distribution of acoustic scatterersminutedepth m  0 30 60 90 120 150 180 210 240 27050100150200250300350400450500log10 (S2)−7.5−7−6.5−6−5.5−5−4.5−4across-ship vertical shear(uncorrected)average and filter ADCP velocities across-ship corrected shear spectra:determine shear spectral level G relative to Garrett-Munk backgroundparametrize dissipation rate εfrom shear spectral level andinternal wave slopeε estimates allow estimation of diapycnal diffusivity10−10 10−9 10−810−1010−910−8   measured   parametrized      2 - parameter - fit  30oW   25oW   20oW   15oW    5oN   10oN   15oN m2/s  30oW   25oW   20oW   15oW    5oN   10oN   15oN m2/slongitudelatitudelog10       m2/s3  −30 −25 −20 −15246810121416−9.8−9.6−9.4−9.2−9−8.8−8.6longitudelatitudelog10 c mol*m3/kg2  −30 −25 −20 −1524681012141622.22.42.62.83longitudelatitudelog10  mol/m2/s  −30 −25 −20 −15246810121416−3−2.5−2−1.5longitudelatitudebathymetry and oxygen minimum concentration  −32 −30 −28 −26 −24 −22 −20 −18 −16 −1424681012141618m600050004000300020001000050 100 150 2000100200300400500600O2   μmol / kgdepth m26 27     kg / m3 10−10 10−9 10−8 10−7 10−6   m2 / s3depthtarget strength  200400600dB708090100110depthtarget strength gradient  200400600dB/m−2−1012depthalong−ship velocity  200400600m/s−0.2−0.100.10.2minutesdepthacross−ship velocity  50 100 150 200 250 300200400600m/s−0.2−0.100.10.2minutedepth m  50 100 150 200 25050100150200250300350400450500log10(S2)−8−7.5−7−6.5−6−5.5−5−4.5−4minutesdepth in m  0 20 40 60 80 100100200300400500m/s−0.2−0.100.10.2minutesdepth in m  0 20 40 60 80 100100200300400500m/s−0.2−0.100.10.20.005 0.01 0.02 0.0310−510−410−3G = 1.9G = 1vertical wavenumber in cpmS in  s−2 /cpm0.1 0.3 1 3 10 30 10000.050.10.150.20.250.30.35relative shear spectral level Gsampled fraction                                                       Polzin, K.,  J. Toole,  and R. Schmitt  ( 1995 ) :  Finescale parameterizations of turbulent dissi-                                                     pation, J. Phys. Oceanogr. 25, 306-328   -----  Gregg, M. C.,  T. B. Sanford,  and D. P. Winkel  ( 2003 ) : Reduced mixing from the breaking of internal waves in equatorial waters,  Nature 422, 513-515  -----  Banyte, Tanhua, Visbeck,Wallace, Karstensen, Krahmann, Schneider, Stramma  ( 2011 ) :  Diapycnal diffusivity at the upper boundary of the tropical NorthAtlantic oxygen minimum zone,  subm. to  J. Geophys. Res.  -----  Fischer, T. ( 2011 ) :  Diapycnal diffusivity and transport of matterin the open  ocean estimated from underway acoustic profiling,  PhD thesis, University of KielWhen moving, any observed periodic quantity will be reduced in variability by smoothing - as a function of shipspeed and distribution of involved horizontal wavelengths. Some degree of smoothing is unavoida-ble, because of needed noise reduction and because of the size of the acoustic footprint.slow fastremaining shear variability in percent -dependence on vertical wavelength and amount of averaging1 3 10 3000.010.020.030.040.050.0699.9 99averaging time in minutesvertical wavenumber in cpmat 0 knots1 3 10 3000.010.020.030.040.050.069995908070 50 30 10averaging time in minutesvertical wavenumber in cpmat 12 knotsConsequences:• Reduce averaging time to 1 minute• Obtain needed data precision by 2-D-filtering of   velocity data. Filter adapted to IW spectral shape.• Apply speed-dependent corrections to shear spectraAcoustic scatterers like plankton distribute inhomogeneously.Geometric acoustic beam spreading causes relatedspurious velocities.This bias affectsvelocity component indirection of movement.Across-shipvelocity componentis unaffected.Spurious internal wave shearin along-ship component when moving:Consequence:• Discard along-ship component unless on stationshear spectral level relative to Garrett-Munk background :distribution during 2 ship cruises< 1 kt> 9 ktacrossacrossalongalongsingle ping across-ship velocity componentafter 1-minuteaveraging andadapted 2-D filter:   ∆v = 0.5 cm/svelocity first-difference squaredvertical shear power spectrumG = ΦS/ΦS,GM = E/EGMFrom ε calculate diapycnal diffusivity using Osborn‘s relationK = 0.2 ε / N2                 Ansatz    ε = c 1a 2b       with  1 =  f/N    prop. to wave slope      and  2 = N2G    prop. to shear spectral levelε = 1/24 13/4 27/5       This parametrization for ε turns out to be a condensed version of the state-of-the-art parametrization [Polzin et al. 1995, Gregg et al. 2003] and is particularly appropriate for cruise work with short stations.−2.6 −2.4 −2.2 −2 −1.8 −1.6 −1.4−5−4.8−4.6−4.4−4.2−4−3.8−3.61 2 phase space coveragelog10 1log 10 2      Tropical Atlantic      midlatitudes          [Polzin et al 1995]Tropical Atlantic Oxygen Minimum Zonetypical / average profiles of oxygen, density and dissipation rateDiapycnal diffusivity Kestimated from microstructure measurements( during 3 cruises 2008-2010 )Topographic patterns in diapycnal mixing and diapycnal oxygen downfluxDiapycnal diffusivity Kestimated from underway acoustics( during 3 cruises 2008-2010 )55 independent K valuestime investment: 80 ship hours600 independent K values no extra ship time needed  CTDs are helpful for N2 input to the parametrization, but climatological N2 data would be sufficiently precise.MSS ADCP40μmol/kg50μmol/kg60     μmol/kg AfricaCabo VerdeSierra Leone Risefocus depth range10−610−510−410−610−510−4Dissipation rate εat locations of oxygenprofiles.Mean K = 1.2•10-5 m2/sIn agreement with simul-taneous Tracer Release Experiment[Banyte et al 2011]).  Oxygen gradient c at upper boundary of deep oxygen minimumDiapycnal oxygendownflux          ε  c   Mean  = 6 nmol/m2/saccounting for about30 % of total supply.10−83 • 10−9Internal wave shearTurbulent dissipation rateUnderway ADCPAcoustic Doppler Current Profilers (vessel mounted RDI Ocean Surve-yor) allow recording of finescale velocity fluctuations associated with internal wave shear. We use a 75 kHz broadband configuration at maximum pingrate for optimum results. Nonetheless, neighbouring frequencies and/or narrowband mode do also work.Microstructure profilesserve as „ground-truthing“ for mixing estimates from internal wave shear. Airfoil shear sensors on a tethered profiling probe (Sea & Sun Technology) sense microstruc-ture velocity fluctuations. These define , the dissipation rate of tur-bulent kinetic energy, as an indica-tor for mixing intensity. Instrument noise level is  = 7e-10 for single bins and = 1e-10 for 300m-depth-range-averages.  Mixing, oxygen gradient and flux - all three peak at Sierra Leone Rise and become weak above the adjacent abyssal plains to the North and to the Southeast. finescale shear and turbulence linkedAcknowledgments                                                                                              This study was supported by German Federal Ministry of Education                                                                                              and Research through the co-operative projects SOPRAN and NORTHATLANTIC. Large parts of data acquisition profited from participation in 3 cruises to the Tropical Atlantic that were part of theGerman Science Foundation‘s Sonderforschungsbereich 754 - „Climate Biogeochemistry Interactions in the Tropical Ocean“.Dr. G. Krahmann‘s valuable operating of the ADCP on cruise Meteor 83/1, the assistance of numerous grad students in takingmicrostructure profiles, and the friendly support of all crew members are highly appreciated.

Underway acoustic survey of internal wave shear; and its use to estimate diapycnal mixing and transports in the interior ocean

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