Chapter 5. SPATIALLY-COHERENT STRUCTURES
5.4.4 Interaction of sharp fronts with wind stress
High-resolution horizontal measurements in the open ocean thus reveal
frontal interfaces of width less than 100 m. These sharp frontal interfaces
have been observed in a wide range of wind speed conditions (0–15 m s
-1 ).
The frontal interfaces of less than ~100 m width reveal anisotropy with
respect to the wind stress direction (Figure 5-31).
Some of the frontal interfaces are narrow–sometimes only a few meters.
Detailed analysis of the record shown in Figure 5-26 reveals that the width
of the frontal interface is only about 1.5 m; the absolute values of the
horizontal temperature, salinity, and density gradients within this interface
reach 137.8
o
C km
-1 , 125.8 psu km
-1 , and 47.96 kg km
-1 respectively. In
particular, the density gradient within the interface is about 5x10
4 times
larger (!) than the mean horizontal density gradient (which is ~0.001 kg m
-3
km
-1 ). This interface looks pretty much like a discontinuity or a “wall” in the
upper ocean.
How can such discontinuities survive in the turbulent ocean? Soloviev
and Lukas (1997b) suggested that this happens because noncompensated
fronts narrower than 100 m could interact with the wind stress. When the
wind stress is directed toward lower density, gravitational instability may
trigger intensive vertical and cross-frontal mixing. In the process of mixing,
the interface may either reach the compensated state or entirely disappear; in
both cases, the interface is no longer prominent in the density field. When
the wind stress is directed toward higher density, vertical stratification
develops due to tilting, which inhibits mixing. As a result, the sharp interface
“freezes” and can drift down wind until the wind substantially changes either
its direction or speed with respect to the front. The lifetime of the sharp
frontal interfaces can therefore be linked to the synoptic time scale for
atmospheric processes (~5 days).
Soloviev et al. (2002) later identified this mechanism of the interaction
between a sharp front and wind stress as Stommel’s overturning gate (Figure
5-34). According to Stommel (1993):
“Future observational study of the three-dimensional structures within
the ‘mixed’ layer may show that direct driving of shear within the layer by
wind overwhelms the hypothetical density-driven exchanges of thermohaline
regulations (…) In the presence initially of a horizontal density gradient in
the direction of the displacement, one of two events will ensue. If the
displacement is toward larger mixed-layer density, then vertical stratification
develops. Vertical mixing eventually occurs. There is a horizontal exchange
of properties. On the other hand, if the wind-forced displacement of the
surface half of the mixed layer is toward smaller density below, rapid
gravitational instability will mix the two halves immediately, effectively
335
5.4.4 Interaction of sharp fronts with wind stress
High-resolution horizontal measurements in the open ocean thus reveal
frontal interfaces of width less than 100 m. These sharp frontal interfaces
have been observed in a wide range of wind speed conditions (0–15 m s
-1 ).
The frontal interfaces of less than ~100 m width reveal anisotropy with
respect to the wind stress direction (Figure 5-31).
Some of the frontal interfaces are narrow–sometimes only a few meters.
Detailed analysis of the record shown in Figure 5-26 reveals that the width
of the frontal interface is only about 1.5 m; the absolute values of the
horizontal temperature, salinity, and density gradients within this interface
reach 137.8
o
C km
-1 , 125.8 psu km
-1 , and 47.96 kg km
-1 respectively. In
particular, the density gradient within the interface is about 5x10
4 times
larger (!) than the mean horizontal density gradient (which is ~0.001 kg m
-3
km
-1 ). This interface looks pretty much like a discontinuity or a “wall” in the
upper ocean.
How can such discontinuities survive in the turbulent ocean? Soloviev
and Lukas (1997b) suggested that this happens because noncompensated
fronts narrower than 100 m could interact with the wind stress. When the
wind stress is directed toward lower density, gravitational instability may
trigger intensive vertical and cross-frontal mixing. In the process of mixing,
the interface may either reach the compensated state or entirely disappear; in
both cases, the interface is no longer prominent in the density field. When
the wind stress is directed toward higher density, vertical stratification
develops due to tilting, which inhibits mixing. As a result, the sharp interface
“freezes” and can drift down wind until the wind substantially changes either
its direction or speed with respect to the front. The lifetime of the sharp
frontal interfaces can therefore be linked to the synoptic time scale for
atmospheric processes (~5 days).
Soloviev et al. (2002) later identified this mechanism of the interaction
between a sharp front and wind stress as Stommel’s overturning gate (Figure
5-34). According to Stommel (1993):
“Future observational study of the three-dimensional structures within
the ‘mixed’ layer may show that direct driving of shear within the layer by
wind overwhelms the hypothetical density-driven exchanges of thermohaline
regulations (…) In the presence initially of a horizontal density gradient in
the direction of the displacement, one of two events will ensue. If the
displacement is toward larger mixed-layer density, then vertical stratification
develops. Vertical mixing eventually occurs. There is a horizontal exchange
of properties. On the other hand, if the wind-forced displacement of the
surface half of the mixed layer is toward smaller density below, rapid
gravitational instability will mix the two halves immediately, effectively
335