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 before the kids come home...

• Works if no restoring forces are present

Stirring vs. Mixing

A single patch will be chaotically strained by flow

Reversible

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=36Water mass 2T=22 & S=35

1 >> 2

Water mass 2 is

buoyant relative to 1

1

2

Example

Water mass 1

T=20 & S=36Water 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

• SF6 release in

Brazil Basin

• Column inventory (mol 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 Temperatu

reChange

• OWS Papa

• 50N 145W

Seasonal Temperatu

reChange

• OWS Papa

• 50N 145W

Convection & the Conveyor Belt

• NADW production drives the conveyor

Convection & the Conveyor Belt

AABW

NADWAAIW

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