THE NEAR-SURFACE LAYER OF THE OCEAN
The size of convective parcels of cooler water increases with depth. If the
mixed layer is deep, as in the example form the Labrador Sea shown in
Figure 5-60, the horizontal size of the convective cells can exceed one
hundred meters. In this experiment the depth of the convective mixed layer
is equal to 350 m (Figure 5-60b). The horizontal profile of the vertical
velocity taken at 250 m (Figure 5-60c) reveals quasi-periodic structure.
Zhang et al. (2001) identified this structure as convection. The peak in the
spectrum calculated from the velocity record is at 0.007 Hz, which
corresponds to a 143 m wavelength for AUV speed 1 m s
-1 .
5.8.2 Penetrative convection
The unstable stratification of the mixed layer is usually bounded below
by a stratified pycnocline. One can imagine the mixed layer growing in
depth with thermals confined to the statically unstable depth range. In
reality, the largest thermals acquire enough kinetic energy as they fall
through the mixed layer that they can overshoot the base of the mixed layer,
working against the stratification. This is penetrative convection, which is a
type of organized motion. The penetrative convection produces a countergradient flux that is not properly accounted for if convective mixing is
modeled as merely a very strong vertical diffusion.
The cooling of the ocean from its surface is countered by the absorption
of solar radiation. The latter is a volume source for the upper meters of the
ocean. The thermals from the ocean surface, as they descend deeper into the
mixed layer, produce heat flux that is compensated by the volume absorption
of solar radiation.. This is another type of penetrative convection in the
5.8.3 Diurnal and seasonal cycle of convection
Below the wave-turbulent layer shear and convection are the main sources
of turbulent mixing (Section 3.1.4). Experimental studies conducted in the
atmospheric boundary layer show that for /
0.1
O
z L
the flow is primarily
driven by buoyant convection, where
3
0
/
/
O
T
p
L u
gQ
c
D
N
U
ª
º
¬
¼
is the
Oboukhov buoyancy length scale.
For 5 m s
-1 wind speed and net surface cooling Q 0 = 100 Wm
-2 , the
Oboukhov length scale in the upper ocean is L O ~ -15 m. This means that the
shear driven turbulent flow is confined within the upper approximately 1.5
m. In a 50 m deep mixed layer 97% of its depth will be driven by buoyant
convection during nighttime.
For much of the year, most of the ocean experiences a cycle of daytime
heating and night-time cooling which leads to a strong diurnal cycle in
390
upper ocean, which is considered in more detail in Section 4.5.1.
The size of convective parcels of cooler water increases with depth. If the
mixed layer is deep, as in the example form the Labrador Sea shown in
Figure 5-60, the horizontal size of the convective cells can exceed one
hundred meters. In this experiment the depth of the convective mixed layer
is equal to 350 m (Figure 5-60b). The horizontal profile of the vertical
velocity taken at 250 m (Figure 5-60c) reveals quasi-periodic structure.
Zhang et al. (2001) identified this structure as convection. The peak in the
spectrum calculated from the velocity record is at 0.007 Hz, which
corresponds to a 143 m wavelength for AUV speed 1 m s
-1 .
5.8.2 Penetrative convection
The unstable stratification of the mixed layer is usually bounded below
by a stratified pycnocline. One can imagine the mixed layer growing in
depth with thermals confined to the statically unstable depth range. In
reality, the largest thermals acquire enough kinetic energy as they fall
through the mixed layer that they can overshoot the base of the mixed layer,
working against the stratification. This is penetrative convection, which is a
type of organized motion. The penetrative convection produces a countergradient flux that is not properly accounted for if convective mixing is
modeled as merely a very strong vertical diffusion.
The cooling of the ocean from its surface is countered by the absorption
of solar radiation. The latter is a volume source for the upper meters of the
ocean. The thermals from the ocean surface, as they descend deeper into the
mixed layer, produce heat flux that is compensated by the volume absorption
of solar radiation.. This is another type of penetrative convection in the
5.8.3 Diurnal and seasonal cycle of convection
Below the wave-turbulent layer shear and convection are the main sources
of turbulent mixing (Section 3.1.4). Experimental studies conducted in the
atmospheric boundary layer show that for /
0.1
O
z L
the flow is primarily
driven by buoyant convection, where
3
0
/
/
O
T
p
L u
gQ
c
D
N
U
ª
º
¬
¼
is the
Oboukhov buoyancy length scale.
For 5 m s
-1 wind speed and net surface cooling Q 0 = 100 Wm
-2 , the
Oboukhov length scale in the upper ocean is L O ~ -15 m. This means that the
shear driven turbulent flow is confined within the upper approximately 1.5
m. In a 50 m deep mixed layer 97% of its depth will be driven by buoyant
convection during nighttime.
For much of the year, most of the ocean experiences a cycle of daytime
heating and night-time cooling which leads to a strong diurnal cycle in
390
upper ocean, which is considered in more detail in Section 4.5.1.