THE NEAR-SURFACE LAYER OF THE OCEAN
The analysis of turbulence observations reveals different, often
contradictory, aspects of the role of surface waves. Since methodological
issues of turbulence measurements in the near-surface layer of the ocean still
greatly affect the study of wave-enhanced turbulence; one of the sections in
this chapter (Section 3.2) is devoted to the analysis of the challenges of
observing the near-surface turbulence.
In this chapter, we consider only local and essentially one-dimensional
models of near-surface turbulence; this is the so-called “small eddy” theory.
We consider the role of large eddies (Langmuir circulations, billows, ramplike structures) in Chapter 5 when discussing coherent structures in the nearsurface layer.
3.1 Free-Surface Turbulent Boundary Layer
An important feature of the upper ocean turbulent boundary layer is that
it develops near a free surface. In the near-surface layer affected by waves
and wave-breaking turbulence, the properties of the boundary layer may
differ substantially from the classic wall layer. In contrast to a rigid wall, the
tangential component of the velocity field at the free surface is not zero.
Velocity components in all directions disappear at the wall due to no-slip
conditions, while the free surface restricts motion in the normal direction
only.
3.1.1 Wave-following coordinate system
An important factor for interpreting near-surface data is the choice of
coordinate system. In a fixed coordinate system it is practically impossible to
study near-surface layers with thickness less than the maximum surface
wave height. In fact, any observational point between the wave trough and
crest will be alternately in water and in air.
The influence of surface waves on the near surface flow can be
provisionally divided into reversible (kinematic) and irreversible
deformations. The former are due to linear (irrotational) components of
surface waves, while the latter are caused by nonlinear (rotational)
components of surface waves and by turbulence. Examples of irrotational
waves are swell and long wind waves. Nonlinearity increases as wavelength
decreases. An extreme effect of wave nonlinearity is wave breaking.
A reasonable approach is to interpret the near-surface layer of the ocean
in a coordinate system linked to the ocean surface. Csanady (1984)
suggested that “...depth should be expressed in the coordinate system
connected with the surface produced by the nearly irrotational component of
144
The analysis of turbulence observations reveals different, often
contradictory, aspects of the role of surface waves. Since methodological
issues of turbulence measurements in the near-surface layer of the ocean still
greatly affect the study of wave-enhanced turbulence; one of the sections in
this chapter (Section 3.2) is devoted to the analysis of the challenges of
observing the near-surface turbulence.
In this chapter, we consider only local and essentially one-dimensional
models of near-surface turbulence; this is the so-called “small eddy” theory.
We consider the role of large eddies (Langmuir circulations, billows, ramplike structures) in Chapter 5 when discussing coherent structures in the nearsurface layer.
3.1 Free-Surface Turbulent Boundary Layer
An important feature of the upper ocean turbulent boundary layer is that
it develops near a free surface. In the near-surface layer affected by waves
and wave-breaking turbulence, the properties of the boundary layer may
differ substantially from the classic wall layer. In contrast to a rigid wall, the
tangential component of the velocity field at the free surface is not zero.
Velocity components in all directions disappear at the wall due to no-slip
conditions, while the free surface restricts motion in the normal direction
only.
3.1.1 Wave-following coordinate system
An important factor for interpreting near-surface data is the choice of
coordinate system. In a fixed coordinate system it is practically impossible to
study near-surface layers with thickness less than the maximum surface
wave height. In fact, any observational point between the wave trough and
crest will be alternately in water and in air.
The influence of surface waves on the near surface flow can be
provisionally divided into reversible (kinematic) and irreversible
deformations. The former are due to linear (irrotational) components of
surface waves, while the latter are caused by nonlinear (rotational)
components of surface waves and by turbulence. Examples of irrotational
waves are swell and long wind waves. Nonlinearity increases as wavelength
decreases. An extreme effect of wave nonlinearity is wave breaking.
A reasonable approach is to interpret the near-surface layer of the ocean
in a coordinate system linked to the ocean surface. Csanady (1984)
suggested that “...depth should be expressed in the coordinate system
connected with the surface produced by the nearly irrotational component of
144