THE NEAR-SURFACE LAYER OF THE OCEAN
structure of the upper turbulent boundary layer of the equatorial ocean
(Table 3-2). However, the Reynolds stress may interact with the horizontal
component of Earth’s rotation f y to exchange turbulent kinetic energy
between horizontal and vertical components (Garwood and Gallacher, 1985;
Garwood et al., 1985), resulting in the length scale
/
sin
G
y
L u
f
T
,
where T is the wind direction.
The effect of horizontal Coriolis acceleration on the turbulent eddies in
the equatorial turbulent boundary layer, however, appears to be relatively
small (Wang et al., 1996; Soloviev et al., 2001).
3.5.4 Boundary-layer horizontal pressure gradients
In the tropical ocean, the surface turbulent boundary layer has some
unique features because of its proximity to the equator. As mentioned above,
in Ekman’s solution for the drift of water in a rotating homogeneous ocean
when acted upon by a steady stress applied to the surface, the depth of the
spiral and the amplitude of the current increase without limit as the latitude
and vertical component of rotation approach zero. Stommel (1960) was first
to show that there is actually no singularity at the equator. However, to
remove the singularity, a zonal pressure gradient is required. At the equator,
such a pressure gradient cannot be balanced by the horizontal Coriolis
component but must be balanced by friction or inertial forces (Charney,
1960). The wind stress penetrates into the ocean through the surface mixed
layer, and the vertical turbulent viscosity provides the principle balance for
the zonal pressure gradient driving the Equatorial Undercurrent (McCreary,
1981).
Later, Lukas and Firing (1984) found evidence of geostrophic balance of
the Equatorial Undercurrent, which provides an alternative perspective
compared to the result of Charney (1960). The zonal pressure gradient and
the vertical turbulent viscosity were nevertheless still principle components
of the momentum balance at the equator.
It is known from hydraulics engineering that the longitudinal pressure
gradient can influence the structure of the turbulent boundary layer (White,
1986). Following Yaglom (1979), one can construct the so-called pressure
gradient length scale,
2 / /
p
L
u
p x
U w w . L p may be derived from the
momentum equations under the assumption that the horizontal pressure
gradient is approximately balanced by vertical mixing. Veronis (1960)
related a similar scale to the depth of the Equatorial Undercurrent.
Typical estimates of L p are given in Table 3-2. This horizontal pressure
gradient length scale exceeds all of the other mixed layer length scales. This
estimate suggests that the horizontal pressure gradient is not a major factor
in determining the vertical structure of the ocean mixed layer. It may
216
structure of the upper turbulent boundary layer of the equatorial ocean
(Table 3-2). However, the Reynolds stress may interact with the horizontal
component of Earth’s rotation f y to exchange turbulent kinetic energy
between horizontal and vertical components (Garwood and Gallacher, 1985;
Garwood et al., 1985), resulting in the length scale
/
sin
G
y
L u
f
T
,
where T is the wind direction.
The effect of horizontal Coriolis acceleration on the turbulent eddies in
the equatorial turbulent boundary layer, however, appears to be relatively
small (Wang et al., 1996; Soloviev et al., 2001).
3.5.4 Boundary-layer horizontal pressure gradients
In the tropical ocean, the surface turbulent boundary layer has some
unique features because of its proximity to the equator. As mentioned above,
in Ekman’s solution for the drift of water in a rotating homogeneous ocean
when acted upon by a steady stress applied to the surface, the depth of the
spiral and the amplitude of the current increase without limit as the latitude
and vertical component of rotation approach zero. Stommel (1960) was first
to show that there is actually no singularity at the equator. However, to
remove the singularity, a zonal pressure gradient is required. At the equator,
such a pressure gradient cannot be balanced by the horizontal Coriolis
component but must be balanced by friction or inertial forces (Charney,
1960). The wind stress penetrates into the ocean through the surface mixed
layer, and the vertical turbulent viscosity provides the principle balance for
the zonal pressure gradient driving the Equatorial Undercurrent (McCreary,
1981).
Later, Lukas and Firing (1984) found evidence of geostrophic balance of
the Equatorial Undercurrent, which provides an alternative perspective
compared to the result of Charney (1960). The zonal pressure gradient and
the vertical turbulent viscosity were nevertheless still principle components
of the momentum balance at the equator.
It is known from hydraulics engineering that the longitudinal pressure
gradient can influence the structure of the turbulent boundary layer (White,
1986). Following Yaglom (1979), one can construct the so-called pressure
gradient length scale,
2 / /
p
L
u
p x
U w w . L p may be derived from the
momentum equations under the assumption that the horizontal pressure
gradient is approximately balanced by vertical mixing. Veronis (1960)
related a similar scale to the depth of the Equatorial Undercurrent.
Typical estimates of L p are given in Table 3-2. This horizontal pressure
gradient length scale exceeds all of the other mixed layer length scales. This
estimate suggests that the horizontal pressure gradient is not a major factor
in determining the vertical structure of the ocean mixed layer. It may
216