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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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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
Motivation: SST variability in theAtlanticColdTongue (ACT) region • understandingof all contributingprocessesshapingseasonalcycleof SST isnecessary • interannualvariabilityofcoldtongue SSTs istiedtointerannualvariations in rainfallovertheadjacentcontinents
Motivation: mixedlayerheatbalance • Contributionsto residual: • under-estimationofentrainment due tocoarsedrifterclimatology • under-estimationof latent heatflux due tobaddatacoveragefor relative humidity • Neglectionofdiapycnalheatflux out oftheML Foltz et. al 2003
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
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
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
Data: Observationalprogram • Repetitive microstructuresectionswithinthecoldtongueregionformedby individual stationswithat least 3 profiles/station (8 cruisesresulted in > 1000 profiles) • Additional CTD stations • Shipboard ADCP measurements
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)
Diapycnalheatflux: Layer ofinterest MLD • Divergent profileofdiapycnalheatflux • heatloss due todiapycnalmixingischaracterizedbydiapycnalheatflux in thinlayerbelowthe ML • thismeasureisincluded in the ML heatbudget
Diapycnalheatflux out of ML: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast
Diapycnalheatflux out of ML: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast
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
Mixed layerheatbudget 10°W, 0°N 3 phasesof ACT development: • Absence (January-April) • Development (May-August) • Maturephase (September- December)
Mixed layerheatbudget 0°E, 0°N 23°W, 0°N 10°W, 0°N 10°W, 10°S
Mixed layerheatbudget 23°W, 0°N Warming: atmosphericforcing, eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal), zonaland meridional heatadvection
Mixed layerheatbudget 10°W, 0°N Warming: atmosphericforcing, eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal), zonal and meridional heatadvection
Mixed layerheatbudget 0°E, 0°N Warming: atmosphericforcing (stronglyreduced), eddyadvection Cooling: subsurfaceprocesses (entrainment, diapycnal) and meridional heatadvection
Mixed layerheatbudget 10°W, 10°S Warming: eddyadvection, meridional heatadvection Cooling: atmosphericforcing, subsurfaceprocesses (entrainment, diapycnal) and zonal heatadvection
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
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
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
Parametrization 10°W, 0°N Parametrizations N²,S² Ri K
Parametrization MLD Most existingparametrizationschemesclearyoverestimatetheheatlossofthemixedlayer due todiapycnalmixing Seasonalparametrizedheatlossbased on independentdatasetwithnewfit isclosesttoobservations
Parametrization 10°W, 0°N All individual termsofthemixedlayerheatbudgetat 10°W on theequatorareestimatedfromobservationsofthe PIRATA buoyandclimatologicalproducts
Parametrization 23°W, 0°N • Large residual atthislocationremains • LargestdifferencestoFoltz et. al, 2003 are zonal advectionandeddyadvection
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
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