Chapter 2: SEA SURFACE MICROLAYER
can therefore serve as an indicator of the viscous sublayer depth. It is
remarkable that in dimensionless coordinates the thickness of the viscous
sublayer near the free surface is approximately half of what it would be near
a rigid wall. This is explained by the fact that only the vertical component of
turbulent fluctuation is effectively suppressed near the free surface; as a
result, turbulent eddies can penetrate closer to a free boundary than to a wall.
2.1.2 Thermal sublayer (cool skin)
The sea surface temperature may differ from the temperature of the
underlying mixed layer due to the presence of the aqueous thermal molecular
sublayer. This sublayer is also referred to as the cool skin of the ocean
(Saunders, 1967b).
Above the interface, there is a millimeter-thick atmospheric boundary
layer, where the vertical transport is also dominated by the molecular
diffusion. The main temperature difference across the air-sea interface is
observed in the atmospheric rather than oceanic molecular sublayer (Volkov
and Soloviev, 1986).
Figure 2-3 gives an example of the temperature profile in the upper 10
m of the ocean obtained with a free-rising profiler (Soloviev, 1992). For this
measurement, the profiler was equipped with a high-resolution temperature
probe (5 Pm diameter wire sensing element). The shunting of the micro-wire
probe by seawater is small due to the fact that its internal resistance was only
7 Ohms, while the area of the micro-wire surface for this thin wire is
extremely small (Azizyan et al., 1984).
The vertical temperature profile shown in Figure 2-3 was taken during
nighttime. The upper part of the profile reveals an abrupt temperature change
in the upper few millimeters due to the cool skin. This abrupt temperature
change near the surface is associated with the cool skin of the ocean. The
temperature difference across the cool skin in the example shown in Figure
2-3 is
0
b
T T T
'
§ -0.3
o
C, where 0
T is the sea surface temperature, and b
T
is the temperature of the bulk (diurnal mixed layer) water. The temperature
gradient below 2 meters represents the remnants of the diurnal thermocline
formed during the previous, daylight hours.
The temperature difference across the cool skin depends on the local
regime of air-sea interaction and thus varies in space and time. Historically,
much effort has been devoted to the cool skin parameterization. Saunders
(1967b) initially parameterized the averaged temperature difference across
the cool skin of the ocean T
' by ascribing a constant value to the
nondimensional coefficient,
0
/(Pr )
S
p
c u T
Q
O
U '
. Grassl (1976) found that
S
O varied with wind speed. The parameter S
O increased from 0 for calm
weather conditions to approximately 5 at moderate wind speeds.
71
can therefore serve as an indicator of the viscous sublayer depth. It is
remarkable that in dimensionless coordinates the thickness of the viscous
sublayer near the free surface is approximately half of what it would be near
a rigid wall. This is explained by the fact that only the vertical component of
turbulent fluctuation is effectively suppressed near the free surface; as a
result, turbulent eddies can penetrate closer to a free boundary than to a wall.
2.1.2 Thermal sublayer (cool skin)
The sea surface temperature may differ from the temperature of the
underlying mixed layer due to the presence of the aqueous thermal molecular
sublayer. This sublayer is also referred to as the cool skin of the ocean
(Saunders, 1967b).
Above the interface, there is a millimeter-thick atmospheric boundary
layer, where the vertical transport is also dominated by the molecular
diffusion. The main temperature difference across the air-sea interface is
observed in the atmospheric rather than oceanic molecular sublayer (Volkov
and Soloviev, 1986).
Figure 2-3 gives an example of the temperature profile in the upper 10
m of the ocean obtained with a free-rising profiler (Soloviev, 1992). For this
measurement, the profiler was equipped with a high-resolution temperature
probe (5 Pm diameter wire sensing element). The shunting of the micro-wire
probe by seawater is small due to the fact that its internal resistance was only
7 Ohms, while the area of the micro-wire surface for this thin wire is
extremely small (Azizyan et al., 1984).
The vertical temperature profile shown in Figure 2-3 was taken during
nighttime. The upper part of the profile reveals an abrupt temperature change
in the upper few millimeters due to the cool skin. This abrupt temperature
change near the surface is associated with the cool skin of the ocean. The
temperature difference across the cool skin in the example shown in Figure
2-3 is
0
b
T T T
'
§ -0.3
o
C, where 0
T is the sea surface temperature, and b
T
is the temperature of the bulk (diurnal mixed layer) water. The temperature
gradient below 2 meters represents the remnants of the diurnal thermocline
formed during the previous, daylight hours.
The temperature difference across the cool skin depends on the local
regime of air-sea interaction and thus varies in space and time. Historically,
much effort has been devoted to the cool skin parameterization. Saunders
(1967b) initially parameterized the averaged temperature difference across
the cool skin of the ocean T
' by ascribing a constant value to the
nondimensional coefficient,
0
/(Pr )
S
p
c u T
Q
O
U '
. Grassl (1976) found that
S
O varied with wind speed. The parameter S
O increased from 0 for calm
weather conditions to approximately 5 at moderate wind speeds.
71