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 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  ’s in  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  1 >>  2 Water mass 2 is buoyant relative to 1 1 2

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

Buoyancy 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 them

Purposeful Tracer Release SF 6 release in Brazil Basin Column inventory (  mol m -2 ) Top 14 mo Bottom26 mo

Purposeful Tracer Release SF 6 release in Brazil Basin Spatial average Dashed - initial Solid - 1 yr later In one year, the SF 6 plume has spread 400 m in vertical & 400 km in horizontal

Purposeful Tracer Release SF 6 in color Isopycnal surfaces in white lines Top 14 mo Bottom26 mo

Stratified Mixing Waters mix rapidly along isopycnals & slowly across them –Vertical eddy diffusivity ~ m 2 /s –Horizontal eddy diffusivity ~ m 2 /s Gives rise to constancy in properties following isopycnals

Atlantic Temperature eWOCE gallery –

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 Temperature Change OWS Papa 50N 145W

Seasonal Temperature Change OWS Papa 50N 145W

Convection & the Conveyor Belt NADW production drives the conveyor

Convection & the Conveyor Belt AABW NADW AAIW

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