Chapter 5. SPATIALLY-COHERENT STRUCTURES
buoyant spreading of front. During several hours the ship crossed back and
forth across this front (Figure 5-27). The front appears to degrade
considerably during the time period; the temperature-salinity relationship
tends toward the equilibrium state, which is characterized by density ratio,
/
1
T
S
R
T
S
D
E
'
'
,
(5.24)
where
,
/
T
t
p S
T
D
U
w
w
and
.
/
S
t
p T
S
E
U
w w
are the expansion
coefficients of temperature and salinity in units kg m
-3 o C
-1 and kg m
-3 psu
-1 ,
and the temperature and salinity differences 'T and 'S are taken across the
frontal interface.
Equation (5.24) is shown by the straight diagonal line in Figure 5-29.
(Note that some contribution from spatial variation of the front is possible in
Figure 5-29.) According to the linear interpolation of the T-S tendency
(Figure 5-29), the compensated state for this frontal interface could be
achieved at
5
1 10
T T
D
' |
and
5
10
1
|
'S
S
E
, which corresponds to
030
.
0
|
'T
o C and
013
.
0
|
'S
psu.
After the interface achieves the compensated state ( R =1), it is no longer
affected by wind stress (see Section 5.4.4). The density ratio R is therefore
an important parameter characterizing the dynamics of sharp frontal
interfaces.
5.4.2 Statistics of sharp frontal interfaces in the western Pacific
warm pool
The presence of sharp frontal interfaces is associated with the
subduction process in the surface mixed layer, which is important in forming
the barrier layer in the warm pool area. The cases of sharp fronts described
in the previous section represent extreme situations. In many other cases, the
temperature, salinity, and density differences across fronts are relatively
small. Thus, these fronts cannot be seen visually on the ocean surface as in
the case shown in Figure 5-17, they may not be detected from the available
ADCP measurements because they are often shallower than 16 m (Figure
5-21 and Figure 5-22), and they cannot be seen in the bow turbulence
records as in the case shown Figure 5-24-Figure 5-26. These frontal
interfaces, however, can be detected in the bow density, salinity, and
temperature records due to huge local gradients within the interfaces.
Soloviev and Lukas (1997b) developed an algorithm for the automatic
detection of sharp frontal interfaces. The sharp frontal interface is detected in
a scalar property C when
)
/
(
/
dx
dC
std
n
dx
dC
!
. In order to detect the
sharpest frontal interfaces, Soloviev and Lukas (1997b) recommend to set n
= 4.
327
buoyant spreading of front. During several hours the ship crossed back and
forth across this front (Figure 5-27). The front appears to degrade
considerably during the time period; the temperature-salinity relationship
tends toward the equilibrium state, which is characterized by density ratio,
/
1
T
S
R
T
S
D
E
'
'
,
(5.24)
where
,
/
T
t
p S
T
D
U
w
w
and
.
/
S
t
p T
S
E
U
w w
are the expansion
coefficients of temperature and salinity in units kg m
-3 o C
-1 and kg m
-3 psu
-1 ,
and the temperature and salinity differences 'T and 'S are taken across the
frontal interface.
Equation (5.24) is shown by the straight diagonal line in Figure 5-29.
(Note that some contribution from spatial variation of the front is possible in
Figure 5-29.) According to the linear interpolation of the T-S tendency
(Figure 5-29), the compensated state for this frontal interface could be
achieved at
5
1 10
T T
D
' |
and
5
10
1
|
'S
S
E
, which corresponds to
030
.
0
|
'T
o C and
013
.
0
|
'S
psu.
After the interface achieves the compensated state ( R =1), it is no longer
affected by wind stress (see Section 5.4.4). The density ratio R is therefore
an important parameter characterizing the dynamics of sharp frontal
interfaces.
5.4.2 Statistics of sharp frontal interfaces in the western Pacific
warm pool
The presence of sharp frontal interfaces is associated with the
subduction process in the surface mixed layer, which is important in forming
the barrier layer in the warm pool area. The cases of sharp fronts described
in the previous section represent extreme situations. In many other cases, the
temperature, salinity, and density differences across fronts are relatively
small. Thus, these fronts cannot be seen visually on the ocean surface as in
the case shown in Figure 5-17, they may not be detected from the available
ADCP measurements because they are often shallower than 16 m (Figure
5-21 and Figure 5-22), and they cannot be seen in the bow turbulence
records as in the case shown Figure 5-24-Figure 5-26. These frontal
interfaces, however, can be detected in the bow density, salinity, and
temperature records due to huge local gradients within the interfaces.
Soloviev and Lukas (1997b) developed an algorithm for the automatic
detection of sharp frontal interfaces. The sharp frontal interface is detected in
a scalar property C when
)
/
(
/
dx
dC
std
n
dx
dC
!
. In order to detect the
sharpest frontal interfaces, Soloviev and Lukas (1997b) recommend to set n
= 4.
327