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Mixed layer heat and freshwater budgets : Improvements during TACE

Mixed layer heat and freshwater budgets : Improvements during TACE. Rebecca Hummels 1 , Marcus Dengler 1 , Peter Brandt 1 , Michael Schlundt 1 1 GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany. Ocean Sciences Meeting 2014, Honolulu, Hawaii USA, 26.02.2014.

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Mixed layer heat and freshwater budgets : Improvements during TACE

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  1. Mixed layerheatandfreshwaterbudgets:Improvementsduring TACE Rebecca Hummels1, Marcus Dengler1, Peter Brandt1, Michael Schlundt1 1GEOMAR Helmholtz Zentrum für Ozeanforschung, Kiel, Germany Ocean Sciences Meeting 2014, Honolulu, Hawaii USA, 26.02.2014

  2. Motivation: Whylookat Mixed Layer (ML) heatbudgets in Tropics?    Annual-meanheatfluxthroughseasurfacecalculatedfromthe ECMWF 40-year reanalysis (Kallberg et al., 2005) Annual-meanSeaSurfaceTemperature (SST) from TMI satelliteobservations

  3. Motivation: SST variability in theAtlanticColdTongue (ACT) Whichprocessesdriveseasonal SST variability ? Interannualvariabilityof ACT SSTs istiedtointerannualvariations in rainfallovertheadjacentcontinents

  4. Motivation: Mixedlayerheatbudget individual contributions toheatbalance Sumandlocal storage • Contributionsto residual: • coarseresolutionofsurfacevelocityclimatology • baddatacoveragefor relative humidity • neglectionof diapycnal heatflux out oftheML Foltz et. al 2003

  5. Observationalprogram • repetitive microstructuresectionswithinthecoldtongueregion: 11 cruisesduring different seasons • individual stationswithat least 3 profiles (>2000 profiles) • shipboard ADCP measurements

  6. 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)

  7. Background settingswithinthe ACT nSEC cSEC EUC 3°S-1.5°N(equatorial ACT): • elevatedshearlevels(due to strong currents (EUC,cSEC,nSEC) • enhanceddissipationratesbelow MLD 10°S-4°S (southern ACT): • moderate shearlevels due tothe lack of strong currents • backgrounddissipationratesbelow MLD

  8. Diapycnalheatflux: Layer ofinterest MLD • Divergent profileofdiapycnalheatflux • heatloss due to diapycnal mixingischaracterizedby diapycnal heatflux in thinlayerbelowthe ML • thisvalueisincluded in the ML heatbudget

  9. Mixed layerheatbudget Evaluation atthe 4 PIRATA buoylocationswithinthe ACT 3 phasesof ACT development: • Absence (January-April) • Development (May-August) • Maturephase (September- December) 0°N, 10°W

  10. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss

  11. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux Warming: Cooling:

  12. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux Warming: Cooling: zonal and meridional heatadvection

  13. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux , eddyadvection Warming: Cooling: zonal and meridional heatadvection

  14. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux , eddyadvection Warming: Cooling: , entrainment zonal and meridional heatadvection

  15. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux Warming: Cooling: , entrainment zonal and meridional heatadvection , diapycnal

  16. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux , eddyadvection Warming: Cooling: , entrainment zonal and meridional heatadvection , diapycnal

  17. Mixed layerheatbudget 0°N, 23°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss netsurfaceheatflux , eddyadvection Warming: Cooling: , diapycnal , entrainment zonaland meridional heatadvection

  18. Mixed layerheatbudget 0°N, 10°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss Warming: netsurfaceheatflux, eddyadvection Cooling: zonal and meridional heatadvection, entrainment, diapycnal

  19. Mixed layerheatbudget 0°N, 0°E localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss Warming: netsurfaceheatflux (stronglyreduced), eddyadvection, meridional Cooling: zonal heatadvection, entrainment, diapycnal

  20. Mixed layerheatbudget 10°S, 10°W localstorage = netsurface - advection – eddyadvection - entrainment – diapycnal ML heatloss Warming: eddyadvectionand meridional heatadvection Cooling: netsurfaceheatflux, zonal heatadvection, entrainment, diapycnal

  21. Mixed layerheatbudget 0°N, 23°W 0°N, 10°W 0°N, 0°E 10°S, 10°W • closed ML heatbudgetwithinuncertaintiesduringsampledperiods • diapycnal heatfluxand zonal advectionarethetermsdominatingthecoolingwithintheequatorial ACT

  22. Freshwaterbudget 0°N, 23°W 0°N, 10°W Salinification: E-P>0, entrainment, meridional heatadvectionanddiapycnal saltflux Freshening: eddyadvectionandzonal heatadvection

  23. Freshwaterbudget 0°N, 23°W 0°N, 10°W Salinification: evaporation, entrainment, meridional heatadvectionand diapycnal saltflux Freshening: precipitation, eddyadvectionandzonal heatadvection • during ACT developmentmixedlayersalinityincreases • largestterms: entrainmentand diapycnal saltflux

  24. Summary and Outlook P • improvementofthe ML heatbudget a higherresolvedsurfacevelocityclimatology improvednetsurfaceheatfluxes (TropFlux) estimatesofthe diapycnal ML heatloss • closureofthebudgetswithintheincertaintieswithinthe ACT • identificationofmaincoolingtermsduring ACT development: diapycnal heatflux (partly zonal advection) in theentireequatorial ACT region • furtherrequiredimprovements (speciallyforinvestigationsofinterannualvariabilityof ML budgetcontributions): surfacevelocities resolutionof diapycnal ML heatloss

  25. Uncertainties 0°N, 23°W • Drifterand ARGO (usedhere) • OSCAR • Lumpkin et al., 2005 • choiceofsurfacevelocityproduct • seasonalvariabilityof diapycnal ML heatlossnot sufficientlyresolved

  26. Mixed layerheatbudget 0°N, 23°W 0°N, 10°W 0°N, 0°E 10°W, 10°S Improvements P • closed ML heatbudgetwithinuncertaintiesduringsampledperiods • diapycnal heatfluxand zonal advectionarethetermsdominatingthecoolingwithintheequatorial ACT

  27. Diapycnal ML heatloss: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast • in earlysummercomparedto September and November

  28. Diapycnal ML heatloss: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast

  29. Diapycnal ML heatloss: Seasonaland regional variability MLD • HeatlossoftheMLD due to turbulent mixingiselevated : • withintheequatorialregion • in the western equatorial ACT comparedtotheeast • in earlysummercomparedto September and November

  30. Uncertainties Comparisonof zonal and meridional velocityof different surfacevelocityproducts

  31. Parametrization

  32. 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

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

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

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

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