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T. Torsvik
Turbulent motion enhances mixing and dispersion. This is achieved by a combination of stirring at large and intermediate length scale, and diffusion at the molecular level. Stirring extends the contact surface between fluid volumes, thereby increasing the area across which diffusive transfer can occur. As a result, regions of
intense turbulent motion tend to produce well mixed volumes of fluid. While stirring is usually considered to be a reversible process, mixing is irreversible. If for
instance a dye is inserted as a point source in a turbulent flow, the dye will eventually be mixed in the entire fluid volume and will not be accessible for extraction
except by some additional, artificial process that sorts the different fluid components.
Since the mean depth of the world ocean is 3795 m and the average flow velocity
usually exceeds 0.1 m/s, the characteristic Reynolds number for the ocean appears to
be about 4 × 10 8 , which is far greater than the critical Reynolds number of 10 4 . By
this estimate we would expect the world ocean to be dominated by large overturning
eddies maintaining a well mixed condition. However, density variation due to surface water heating and the distribution of salinity maintains a vertical stratification
in deep ocean and sea areas. This stratification is usually too strong to permit large
overturning eddies, so turbulent motion is usually found in patches with vertical
length scales of 1–100 m.
Due to the predominant effect of vertical stratification it is common to separate
between mixing that involves transfer of fluid across isopycnal surfaces, called diapycnal mixing, and mixing of water masses parallel to isopycnal surfaces, called
isopycnal mixing. Diapycnal mixing involves the mixing of water masses with different densities, which requires an overturning motion where dense water is lifted
above less dense water, hence involving a transfer from kinetic to potential energy.
The end result is a patch of water with intermediate density which will spread along
an isopycnal surface and contribute to weaken the vertical stratification. Isopycnal
mixing involves the redistribution of volumes of water with equal density, often
through the formation of mesoscale eddies with diameter of 50–300 km and a lifetime extending from a few days to months. Although the interior of the eddy is well
mixed, such mesoscale eddies may drift far from their origin, and a significant temperature difference can often be observed between the interior and exterior of such
eddies.
Turbulence is prominent in patches and localized regions in the ocean. Strong
turbulent motion is usually present in the upper boundary layer near the sea surface
and in the bottom boundary layer. In the upper boundary layer the wind stress generates waves which can induce turbulent motion, and the heat flux may create patches
of variable buoyancy. At the bottom boundary layer the frictional forces between the
bottom current and the sea floor are usually the origin of turbulent motion. Patches
of turbulent motion may also occur away from boundaries. Internal waves create
vertical gradients of current velocities, and breaking internal waves are well known
sources of diapycnal mixing. In regions where warm and salty water lies beneath
cold and less salty water, such as in Arctic regions, the difference in diffusion rate
of heat and salinity may lead to instabilities and create what is called double diffusive convection. The source of this instability is the fact that the diffusion of salt is
T. Torsvik
Turbulent motion enhances mixing and dispersion. This is achieved by a combination of stirring at large and intermediate length scale, and diffusion at the molecular level. Stirring extends the contact surface between fluid volumes, thereby increasing the area across which diffusive transfer can occur. As a result, regions of
intense turbulent motion tend to produce well mixed volumes of fluid. While stirring is usually considered to be a reversible process, mixing is irreversible. If for
instance a dye is inserted as a point source in a turbulent flow, the dye will eventually be mixed in the entire fluid volume and will not be accessible for extraction
except by some additional, artificial process that sorts the different fluid components.
Since the mean depth of the world ocean is 3795 m and the average flow velocity
usually exceeds 0.1 m/s, the characteristic Reynolds number for the ocean appears to
be about 4 × 10 8 , which is far greater than the critical Reynolds number of 10 4 . By
this estimate we would expect the world ocean to be dominated by large overturning
eddies maintaining a well mixed condition. However, density variation due to surface water heating and the distribution of salinity maintains a vertical stratification
in deep ocean and sea areas. This stratification is usually too strong to permit large
overturning eddies, so turbulent motion is usually found in patches with vertical
length scales of 1–100 m.
Due to the predominant effect of vertical stratification it is common to separate
between mixing that involves transfer of fluid across isopycnal surfaces, called diapycnal mixing, and mixing of water masses parallel to isopycnal surfaces, called
isopycnal mixing. Diapycnal mixing involves the mixing of water masses with different densities, which requires an overturning motion where dense water is lifted
above less dense water, hence involving a transfer from kinetic to potential energy.
The end result is a patch of water with intermediate density which will spread along
an isopycnal surface and contribute to weaken the vertical stratification. Isopycnal
mixing involves the redistribution of volumes of water with equal density, often
through the formation of mesoscale eddies with diameter of 50–300 km and a lifetime extending from a few days to months. Although the interior of the eddy is well
mixed, such mesoscale eddies may drift far from their origin, and a significant temperature difference can often be observed between the interior and exterior of such
eddies.
Turbulence is prominent in patches and localized regions in the ocean. Strong
turbulent motion is usually present in the upper boundary layer near the sea surface
and in the bottom boundary layer. In the upper boundary layer the wind stress generates waves which can induce turbulent motion, and the heat flux may create patches
of variable buoyancy. At the bottom boundary layer the frictional forces between the
bottom current and the sea floor are usually the origin of turbulent motion. Patches
of turbulent motion may also occur away from boundaries. Internal waves create
vertical gradients of current velocities, and breaking internal waves are well known
sources of diapycnal mixing. In regions where warm and salty water lies beneath
cold and less salty water, such as in Arctic regions, the difference in diffusion rate
of heat and salinity may lead to instabilities and create what is called double diffusive convection. The source of this instability is the fact that the diffusion of salt is