THE NEAR-SURFACE LAYER OF THE OCEAN
1) Logarithmic layer, where stratification is negligible (]= 0),
2) Free convection (] o f ), and
3) Marginal stability (] o f ).
Vertical dashed lines in Figure 1-17 indicate the logarithmic layer
regime. The regime of marginal stability is characterized by linear profiles of
nondimensional shear M
I and temperature gradient T
I , which is observed
starting for sufficiently large positive ] . The asymptotic regimes are
discussed in more detail in Section 3.4.
The Monin-Oboukhov similarity theory has provided an important
conceptual framework for understanding the dynamics of planetary
boundary layers. The constant stress layer assumption, which is the main
assumption of this theory, is however valid only within approximately 10%
of the total thickness of the planetary boundary layer. The Monin-Oboukhov
similarity theory was originally developed for the atmospheric boundary
layer. Its application to the upper ocean boundary layer has some specific
issues, which are discussed in Chapter 3
1.7.3 Surface mixed layer
The planetary boundary layers are subject to strong turbulence, and the
turbulent exchange coefficients are much higher within boundary layers than
outside of these regions. The surface mixed layer is a generic feature of the
upper ocean. Vertical profiles of temperature, salinity and other scalar
quantities show nearly constant values adjacent to the surface due to
continuous or episodic, but frequent, mixing. Wind-induced shear and waves
are important sources of turbulent mixing in the upper ocean. In addition,
thermal convection, in the form of loss of heat through long-wave radiation
flux and evaporative cooling cause turbulent mixing.
The model of an Ekman layer shows surface stress is carried away from
the boundary layer and toward the interior of the ocean. The Ekman
equations assume a dominant balance between the frictional force and the
Coriolis force while approximating the equations of motion. The velocity
vector then decays in amplitude by spiraling down away from the ocean's
surface toward the interior. The Monin-Oboukhov theory assumes that the
dominant balance is between the frictional force and buoyancy force. It helps
to explain how buoyancy fluxes due to diurnal warming, precipitation, or
horizontal advection suppress turbulence in the upper ocean. The Ekman and
Monin and Oboukhov theories represent a one-dimensional framework for
understanding planetary boundary layers, which may however be a
substantial oversimplification in certain cases (see Chapter 5).
The daily averaged depth of the mixed layer changes with season being
relatively shallow during the spring and summer and deeper during the fall
62
1) Logarithmic layer, where stratification is negligible (]= 0),
2) Free convection (] o f ), and
3) Marginal stability (] o f ).
Vertical dashed lines in Figure 1-17 indicate the logarithmic layer
regime. The regime of marginal stability is characterized by linear profiles of
nondimensional shear M
I and temperature gradient T
I , which is observed
starting for sufficiently large positive ] . The asymptotic regimes are
discussed in more detail in Section 3.4.
The Monin-Oboukhov similarity theory has provided an important
conceptual framework for understanding the dynamics of planetary
boundary layers. The constant stress layer assumption, which is the main
assumption of this theory, is however valid only within approximately 10%
of the total thickness of the planetary boundary layer. The Monin-Oboukhov
similarity theory was originally developed for the atmospheric boundary
layer. Its application to the upper ocean boundary layer has some specific
issues, which are discussed in Chapter 3
1.7.3 Surface mixed layer
The planetary boundary layers are subject to strong turbulence, and the
turbulent exchange coefficients are much higher within boundary layers than
outside of these regions. The surface mixed layer is a generic feature of the
upper ocean. Vertical profiles of temperature, salinity and other scalar
quantities show nearly constant values adjacent to the surface due to
continuous or episodic, but frequent, mixing. Wind-induced shear and waves
are important sources of turbulent mixing in the upper ocean. In addition,
thermal convection, in the form of loss of heat through long-wave radiation
flux and evaporative cooling cause turbulent mixing.
The model of an Ekman layer shows surface stress is carried away from
the boundary layer and toward the interior of the ocean. The Ekman
equations assume a dominant balance between the frictional force and the
Coriolis force while approximating the equations of motion. The velocity
vector then decays in amplitude by spiraling down away from the ocean's
surface toward the interior. The Monin-Oboukhov theory assumes that the
dominant balance is between the frictional force and buoyancy force. It helps
to explain how buoyancy fluxes due to diurnal warming, precipitation, or
horizontal advection suppress turbulence in the upper ocean. The Ekman and
Monin and Oboukhov theories represent a one-dimensional framework for
understanding planetary boundary layers, which may however be a
substantial oversimplification in certain cases (see Chapter 5).
The daily averaged depth of the mixed layer changes with season being
relatively shallow during the spring and summer and deeper during the fall
62