mixing & turbulence mixing leads to a homogenization of water mass properties mixing occurs on...
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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
• 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
• 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
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
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
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
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