Mixing & TurbulenceMixing & TurbulenceSlide 3Slide 4Slide 5Stirring vs. MixingA Stirring Example?PowerPoint PresentationSlide 9Slide 10Slide 11Stirring vs. Eddy DiffusionSlide 13Slide 14BuoyancyBuoyancy & MixingExampleSlide 18Slide 19Purposeful Tracer ReleaseSlide 21Slide 22Stratified MixingAtlantic TemperatureAtlantic SalinityAtlantic OxygenAtlantic PhosphateConvectionSeasonal Temperature ChangeSlide 30Convection & the Conveyor BeltSlide 32Role of Sea IceSeasonal Sea IceMixing, Buoyancy, etc.Mixing & Turbulence•Mixing leads to a homogenization of water mass properties•Mixing occurs on all scales in ocean–molecular scales (10’s of m)–basin scales (1000’s of km)•Turbulence interactions cascade energy from big to small scalesMixing & Turbulence10 cm eddiesSmall-scale turbulenceShear-drivenMixing & Turbulence200 km eddiesMesoscaleGeostrophicMixing & Turbulence4 km eddiesSubmesoscale???Mixing & Turbulence•Stirring efficiently mixes property fields •All scales of motion contribute •Nearly all scales interact•Think of a kid’s playroom full of colored balls in neat piles (before the kids come)•Works if no restoring forces are presentStirring vs. MixingA single patch will be chaotically strained by flowReversibleA Stirring Example?•Advection of a blob of passive tracer•Seeded eddy model [Dyke & Robertson, 1985]Collection of Gaussian vortices that advect themselves & tracer distributionUsed in Abraham [1998] Nature, 391, 577QuickTime™ and a decompressorare needed to see this picture.Different Initial SeedsDifferent Squiggly PatternsRepeating many times …Averaging over space …… it looks like “diffusion”Stirring vs. Eddy DiffusionSnapshot Ensemble of Many SnapshotsStirring vs. Eddy Diffusion•Snapshots & synoptic surveys–Evidence of eddying motions, straining, etc.•Averaging smoothes the edges–The average result of stirring can be modeled as a diffusive process–Diffusion is now acting on larger (or eddy) scalesStirring vs. Mixing•If this experiment were repeated many times, the average patch will appear to be “diffused”•Concept of eddy diffusion•Eddy diffusion is >> molecular diffusionBuoyancy•Dense water sinks - light water floats–Density profile will increase with depth–Upward force due to ’s in  is called the buoyancy force•Buoyancy restricts vertical mixing of water massesBuoyancy & Mixing•Buoyancy is important to vertical mixing for two major processes–Asymmetric mixing in ocean interior–ConvectionExampleWater mass 1T=20 & S=36Water mass 2T=22 & S=351 >> 2Water mass 2 is buoyant relative to 112ExampleWater mass 1T=20 & S=36Water mass 2T=17 & S=351 ~ 2No net buoyancy12Buoyancy•Waters of same  mix easily, waters of different don’t (oil & vinegar)•Potential energy differences must be overcome by mechanical energy inputs•Mixing along isopycnal surfaces will be >>> than mixing across themPurposeful Tracer Release•SF6 release in Brazil Basin•Column inventory (mol m-2) •Top 14 mo•Bottom26 moPurposeful Tracer Release•SF6 release in Brazil Basin•Spatial average •Dashed - initial•Solid - 1 yr later•In one year, the SF6 plume has spread 400 m in vertical & 400 km in horizontalPurposeful Tracer Release•SF6 in color •Isopycnal surfaces in white lines•Top 14 mo•Bottom26 moStratified Mixing•Waters mix rapidly along isopycnals & slowly across them–Vertical eddy diffusivity ~10-5 - 10-3 m2/s–Horizontal eddy diffusivity ~102 - 104 m2/s•Gives rise to constancy in properties following isopycnalsAtlantic TemperatureeWOCE gallery – www.ewoce.orgAtlantic SalinityAtlantic OxygenAtlantic PhosphateConvection•Air-sea cooling & evaporation creates cool & saline surface waters •These waters are then denser than those just beneath them and they sink•Process is called convection•Annual & diurnal time scalesSeasonal TemperatureChange•OWS Papa•50N 145WSeasonal TemperatureChange•OWS Papa•50N 145WConvection & the Conveyor Belt•NADW production drives the conveyorConvection & the Conveyor BeltAABWNADWAAIWRole of Sea Ice•Formation of seasonal sea ice also is important•Sea ice salinities are ~2 to 5 psu•Reject brine water when formed –which is salty, cold and dense!!•Source of AABWSeasonal Sea IceMixing, Buoyancy, etc.•Turbulence drives mixing in the sea–Flow variations on many scales•Buoyancy is important–Drives convection –Asymmetric mixing within the interior•Active area of