Chapter 5. SPATIALLY-COHERENT STRUCTURES
turbulence, producing organized structures in the upper layer of the ocean
(possibly on scales different from the initial atmospheric scales). In Section
5.3, this process is described in the framework of a nonlinear diffusion
model. This process is also interesting because the energy accumulation on
large scales is accompanied by the development of narrow frontal zones
between large-scale structures. If a front becomes sharp enough, it may
interact with the wind stress via the mechanism of Stommel’s overturning
gate (Section 5.4).
Since the diurnal thermocline and rain-formed halocline are stably
stratified, they can develop internal wave motions. Under low wind speed
conditions the mixed layer depth reduces dramatically; as a result, the
diurnal thermocline or rain-formed halocline is found in the upper few
meters of the ocean. In this case, internal waves can be observed close to the
ocean surface (Section 5.5). In some situations, perhaps due to interaction
with shear or to resonant wave interactions, internal waves developing on the
shallow diurnal thermocline become strongly nonlinear and exhibit
billowing.
On horizontal scales l ~ h, the transition from two- to three-dimensional
boundary layer regimes occurs. Various types of organized structures like
penetrative convection, ramps, billows, and Langmuir cells are interpreted as
modes of the turbulent boundary layer instability. These spatially coherent
organized motions are discussed in Sections 5.6-5.8.
Ramp like-structures appear in near-surface horizontal temperature
records. The ramp-like structures have been found under both stable and
unstable stratification conditions. Their phenomenology and the theory of
this type of organized motion are discussed in Sections 5.6.
In Section 5.7, we try to distinguish between the myths and realities
regarding Langmuir circulations, one of the most controversial phenomena
in the history of oceanography. Convection is discussed in the last section of
this Chapter, Section 5.8. The importance of penetrative convection in the
dynamics of the surface mixed layer has been addressed with a nonlocal
parameterization scheme. How other types of organized structures contribute
to nonlocal transport, however, is not yet completely clear.
5.2 Self-Organization in Two-Dimensional Turbulence
On horizontal scales exceeding the thickness of the upper ocean
boundary layer, motions are quasi-two-dimensional due to stratification
and/or rotation effects. Three-dimensional flow at large Reynolds numbers is
chaotic and obeys statistical laws of isotropic turbulence, while a twodimensional flow is governed by the anisotropic turbulence law and has the
tendency to self-organization.
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turbulence, producing organized structures in the upper layer of the ocean
(possibly on scales different from the initial atmospheric scales). In Section
5.3, this process is described in the framework of a nonlinear diffusion
model. This process is also interesting because the energy accumulation on
large scales is accompanied by the development of narrow frontal zones
between large-scale structures. If a front becomes sharp enough, it may
interact with the wind stress via the mechanism of Stommel’s overturning
gate (Section 5.4).
Since the diurnal thermocline and rain-formed halocline are stably
stratified, they can develop internal wave motions. Under low wind speed
conditions the mixed layer depth reduces dramatically; as a result, the
diurnal thermocline or rain-formed halocline is found in the upper few
meters of the ocean. In this case, internal waves can be observed close to the
ocean surface (Section 5.5). In some situations, perhaps due to interaction
with shear or to resonant wave interactions, internal waves developing on the
shallow diurnal thermocline become strongly nonlinear and exhibit
billowing.
On horizontal scales l ~ h, the transition from two- to three-dimensional
boundary layer regimes occurs. Various types of organized structures like
penetrative convection, ramps, billows, and Langmuir cells are interpreted as
modes of the turbulent boundary layer instability. These spatially coherent
organized motions are discussed in Sections 5.6-5.8.
Ramp like-structures appear in near-surface horizontal temperature
records. The ramp-like structures have been found under both stable and
unstable stratification conditions. Their phenomenology and the theory of
this type of organized motion are discussed in Sections 5.6.
In Section 5.7, we try to distinguish between the myths and realities
regarding Langmuir circulations, one of the most controversial phenomena
in the history of oceanography. Convection is discussed in the last section of
this Chapter, Section 5.8. The importance of penetrative convection in the
dynamics of the surface mixed layer has been addressed with a nonlocal
parameterization scheme. How other types of organized structures contribute
to nonlocal transport, however, is not yet completely clear.
5.2 Self-Organization in Two-Dimensional Turbulence
On horizontal scales exceeding the thickness of the upper ocean
boundary layer, motions are quasi-two-dimensional due to stratification
and/or rotation effects. Three-dimensional flow at large Reynolds numbers is
chaotic and obeys statistical laws of isotropic turbulence, while a twodimensional flow is governed by the anisotropic turbulence law and has the
tendency to self-organization.
287