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About the contribution of the diapycnal heat flux to the heat budget of the mixed layer

About the contribution of the diapycnal heat flux to the heat budget of the mixed layer. Rebecca Hummels 1 , Marcus Dengler 1 , Bernard Bourles 2 1 GEOMAR Helmholtz Zentrum für Ozeanforschung , Kiel, Germany 2 LEGOS, IRD, CRHOB, Cotonou, Benin.

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About the contribution of the diapycnal heat flux to the heat budget of the mixed layer

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  1. AboutthecontributionofthediapycnalheatfluxtotheheatbudgetofthemixedlayerAboutthecontributionofthediapycnalheatfluxtotheheatbudgetofthemixedlayer Rebecca Hummels1, Marcus Dengler1, Bernard Bourles2 1GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany 2LEGOS, IRD, CRHOB, Cotonou, Benin TAV Meeting 2012, Kiel, Germany, 11.09.2012

  2. Motivation: SST variability in theAtlanticColdTongue (ACT) region • understandingof all contributingprocessesshapingseasonalcycleof SST isnecessary • interannualvariabilityofcoldtongue SSTs istiedtointerannualvariations in rainfallovertheadjacentcontinents

  3. Motivation: mixedlayerheatbalance • Contributionsto residual: • under-estimationofentrainment due tocoarsedrifterclimatology • under-estimationof latent heatflux due tobaddatacoveragefor relative humidity • Neglectionofdiapycnalheatflux out oftheML Foltz et. al 2003

  4. Motivation: mixedlayerheatbalance • Contributionsto residual: • under-estimationofentrainment due tocoarsedrifterclimatology • under-estimationof latent heatflux due tobaddatacoveragefor relative humidity • Neglectionofdiapycnalheatflux out oftheML • use a higherresolved, • morerecentdrifterclimatology Foltz et. al 2003

  5. Motivation: mixedlayerheatbalance • Contributionsto residual: • under-estimationofentrainment due tocoarsedrifterclimatology • under-estimationof latent heatflux due tobaddatacoveragefor relative humidity • Neglectionofdiapycnalheatflux out oftheML • use a higherresolved, • morerecentdrifterclimatology • use a longertimeseries • ofmeasurements Foltz et. al 2003

  6. Motivation: mixedlayerheatbalance • Contributionsto residual: • under-estimationofentrainment due tocoarsedrifterclimatology • under-estimationof latent heatflux due tobaddatacoveragefor relative humidity • Neglectionofdiapycnalheatflux out oftheML due toturbulence • use a higherresolved, • morerecentdrifterclimatology • use a longertimeseries • ofmeasurements Foltz et. al 2003 • use an extensive • observationalprogramtoestimatethisterm

  7. Data: Observationalprogram • Repetitive microstructuresectionswithinthecoldtongueregionformedby individual stationswithat least 3 profiles/station (8 cruisesresulted in > 1000 profiles) • Additional CTD stations • Shipboard ADCP measurements

  8. Data Treatment From MSS measurementstodiapycnalheatfluxes CTD sensors  T, C, p  Shear sensors  ? Dissipation rate of turbulent kinetic energy for isotropic turbulence is given by: Eddy diffusivities for mass can be estimated as: (Osborn, 1980) (Osborn and Cox, 1972)

  9. Diapycnalheatflux: Layer ofinterest MLD • Divergent profileofdiapycnalheatflux • heatloss due todiapycnalmixingischaracterizedbydiapycnalheatflux in thinlayerbelowthe ML •  thismeasureisincluded in the ML heatbudget

  10. Diapycnalheatflux out of ML: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast

  11. Diapycnalheatflux out of ML: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast

  12. Diapycnalheatflux out of ML: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast • in earlysummercomparedto September and November

  13. Mixed layerheatbudget 10°W, 0°N 3 phasesof ACT development: • Absence (January-April) • Development (May-August) • Maturephase (September- December)

  14. Mixed layerheatbudget 0°E, 0°N 23°W, 0°N 10°W, 0°N 10°W, 10°S

  15. Mixed layerheatbudget 23°W, 0°N Warming: atmosphericforcing, eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal), zonaland meridional heatadvection

  16. Mixed layerheatbudget 10°W, 0°N Warming: atmosphericforcing, eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal), zonal and meridional heatadvection

  17. Mixed layerheatbudget 0°E, 0°N Warming: atmosphericforcing (stronglyreduced), eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal) and meridional heatadvection

  18. Mixed layerheatbudget 10°W, 10°S Warming: eddyadvection, meridional heatadvection Cooling: atmosphericforcing, subsurfaceprocesses (entrainment, diapycnal) and zonal heatadvection

  19. Mixed layerheatbudget 0°E, 0°N 23°W, 0°N 10°W, 0°N 10°W, 10°S • Besidesat 23°W,0°N closed ML heatbudgetwithinuncertaintiesduringsampledperiods • Diapycnalheatfluxis an importantcoolingtermwithintheentireequatorial ACT regionwithinthedevelopmentphaseofthe ACT

  20. Background settingswithinthe ACT nSEC cSEC EUC • 4°S-2°N(equatorial ACT): • Flat MLDs • strong currents (EUC,cSEC,nSEC) • 10°S-4°S (southern ACT): • Deep MLDs • No strong currentbands • Elevated shearlevels ( • Moderate shearlevels • Enhanced dissipationratesbelow MLD • Background dissipationratesbelow MLD

  21. Parametrization Existingparametrizationschemesfortheequatorialregionarebased on a simple Ri (N²/S²) dependence: • PacanowskiandPhilander 1981 • Peters 1988 (2 different formulations) • KPP (Large et al 1994) • ZaronandMoum 2009 (2 different formulations) • Propose a simple dependencefittedtotheobservationaldataofthisstudy

  22. Parametrization 10°W, 0°N Parametrizations N²,S²  Ri  K 

  23. Parametrization MLD  Most existingparametrizationschemesclearyoverestimatetheheatlossofthemixedlayer due todiapycnalmixing  Seasonalparametrizedheatlossbased on independentdatasetwithnewfit isclosesttoobservations

  24. Parametrization 10°W, 0°N All individual termsofthemixedlayerheatbudgetat 10°W on theequatorareestimatedfromobservationsofthe PIRATA buoyandclimatologicalproducts

  25. Parametrization 23°W, 0°N • Large residual atthislocationremains • LargestdifferencestoFoltz et. al, 2003 are zonal advectionandeddyadvection

  26. Summary • New, extensive setof MSS observationsusedtoinfermagnitudeofdiapycnalheatlossesofthe ML in the ACT region; some regional andseasonalvariabilityresolved • The assessedvariabilityofthisterm was includedintothe ML heatbudgetat 4 characteristiclocationswithinthe ACT. The resultsclaimthediapycnalheatfluxthe dominant contributionforthecooling in theentireequatorial ACT regionand a negegiblecontributiontothecooling in the southern ACT • A newparametrizationisproposed, whichseemstoprovide plausible estimatesofthediapycnalheatlossofthe ML usingonlyobservationsofthePiratabuoy • The newparametrizationhastobefurthertested • Individual contributionstothe ML heatbudgetat 23°W needclarification

  27. Parametrization

  28. Parametrization Existing parametrizationschemesfortheequatorialregionarebased on a simple Ri (N²/S²) dependence: • PacanowskiandPhilander 1981 • Peters 1988 (2 different formulations) • KPP (Large et al 1994) • ZaronandMoum 2009 (2 different formulations) • Propose a simple dependencefittedtotheobservationaldataofthisstudy

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