Chapter 3
NEAR-SURFACE TURBULENCE
Physical processes that determine the character and magnitude
of turbulence in the the upper ocean are surveyed and the reader
is exposed to the challenges of studying near-surface turbulence.
3. NEAR-SURFACE TURBULENCE
The main sources of turbulence in the near-surface layer of the ocean are
breaking surface waves, shear, and convection. Upper ocean turbulence
resulting from shear and convective instabilities may be substantially
influenced by the diurnal cycle of solar radiation and by precipitation events.
The shear that develops at the bottom of a shallow diurnal or rain-formed
mixed layer can greatly increase the turbulence generation (though on
relatively small scales) and thus the dissipation rate of turbulence. Below the
mixed layer (i.e., in the pycnocline), turbulence decays due to the stabilizing
effect of buoyancy forces. The dramatic effect of stratification is observed
under low wind speed conditions, when the turbulence regime depends
strongly on near-surface stratification, while the strong stratification is also
the result of reduced turbulent mixing.
The near-surface thermohaline stratification is eliminated quickly when
wave breaking starts. It is therefore reasonable to analyze the thermohaline
stratification effects separately, but we cannot do this for bubble-related
stratification. Bubbles from breaking waves alter density stratification. The
bubble volume fraction (void fraction) is high in spilling breakers; the
resulting increased stability affects near-surface dynamics. Conversely, the
bubble size distribution appears to depend on turbulence parameters. Beyond
the near-surface zone stirred by breaking waves, the average void fraction is
too small to influence the flow dynamics but may serve as a tracer for
detection of turbulence and coherent structures.
NEAR-SURFACE TURBULENCE
Physical processes that determine the character and magnitude
of turbulence in the the upper ocean are surveyed and the reader
is exposed to the challenges of studying near-surface turbulence.
3. NEAR-SURFACE TURBULENCE
The main sources of turbulence in the near-surface layer of the ocean are
breaking surface waves, shear, and convection. Upper ocean turbulence
resulting from shear and convective instabilities may be substantially
influenced by the diurnal cycle of solar radiation and by precipitation events.
The shear that develops at the bottom of a shallow diurnal or rain-formed
mixed layer can greatly increase the turbulence generation (though on
relatively small scales) and thus the dissipation rate of turbulence. Below the
mixed layer (i.e., in the pycnocline), turbulence decays due to the stabilizing
effect of buoyancy forces. The dramatic effect of stratification is observed
under low wind speed conditions, when the turbulence regime depends
strongly on near-surface stratification, while the strong stratification is also
the result of reduced turbulent mixing.
The near-surface thermohaline stratification is eliminated quickly when
wave breaking starts. It is therefore reasonable to analyze the thermohaline
stratification effects separately, but we cannot do this for bubble-related
stratification. Bubbles from breaking waves alter density stratification. The
bubble volume fraction (void fraction) is high in spilling breakers; the
resulting increased stability affects near-surface dynamics. Conversely, the
bubble size distribution appears to depend on turbulence parameters. Beyond
the near-surface zone stirred by breaking waves, the average void fraction is
too small to influence the flow dynamics but may serve as a tracer for
detection of turbulence and coherent structures.