Mixing & Turbulence

Mixing & Turbulence • Mixing leads to a homogenization of water mass properties • Mixing occurs on all scales in ocean • molecular scales (10’s of mm) • basin scales (1000’s of km) • Turbulence interactions cascade energy from big to small scales

Mixing & Turbulence 10 cm eddies Small-scale turbulence Shear-driven

Mixing & Turbulence 200 km eddies Mesoscale Geostrophic

Mixing & Turbulence 4 km eddies Submesoscale ???

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 present

Stirring vs. Mixing A single patch will be chaotically strained by flow Reversible

A Stirring Example? • Advection of a blob of passive tracer • Seeded eddy model [Dyke & Robertson, 1985] Collection of Gaussian vortices that advect themselves & tracer distribution Used in Abraham [1998] Nature, 391, 577

Different Initial Seeds Different Squiggly Patterns

Repeating many times …

Averaging over space … … it looks like “diffusion”

Stirring vs. Eddy Diffusion Snapshot Ensemble of Many Snapshots

Stirring 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) scales

Stirring 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 diffusion

Buoyancy • Dense water sinks - light water floats • Density profile will increase with depth • Upward force due to D’s in r is called the buoyancy force • Buoyancy restricts vertical mixing of water masses

Buoyancy & Mixing • Buoyancy is important to vertical mixing for two major processes • Asymmetric mixing in ocean interior • Convection

Example Water mass 1 T=20 & S=36 Water mass 2 T=22 & S=35 r1 >> r2 Water mass 2 is buoyant relative to 1 2 1

Example Water mass 1 T=20 & S=36 Water mass 2 T=17 & S=35 r1 ~ r2 No net buoyancy 1 2

Buoyancy • Waters of same r 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 them

Purposeful Tracer Release • SF6 release in Brazil Basin • Column inventory (mmol m-2) • Top 14 mo • Bottom 26 mo

Purposeful 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 horizontal

Purposeful Tracer Release • SF6 in color • Isopycnal surfaces in white lines • Top 14 mo • Bottom 26 mo

Stratified 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 isopycnals

Atlantic Temperature eWOCE gallery – www.ewoce.org

Atlantic Salinity

Atlantic Oxygen

Atlantic Phosphate

Convection • 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 scales

Seasonal TemperatureChange • OWS Papa • 50N 145W

Convection & the Conveyor Belt • NADW production drives the conveyor

Convection & the Conveyor Belt AAIW NADW AABW

Role 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 AABW

Seasonal Sea Ice

Mixing, 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 research

Mixing & Turbulence