Transcript Importance of Photochemical Processes in the Sea

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
QuickTime™ and a
decompressor
are needed to see this picture.
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
1
T=20 & S=36
Water mass 2
T=17 & S=35
r1 ~ r2
No net buoyancy
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
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
AAIW
AABW
NADW
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