THE NEAR-SURFACE LAYER OF THE OCEAN
where O is the wavelength, u
' is the vertical velocity jump,
0
'
/
g g U U
'
is the reduced gravity. For the large-amplitude internal wave observations of
Soloviev and Lukas (1996) (Figure 5-40),
4
0
/
5 10
U U
'
| u
. Typical
velocity of the diurnal jet is 0.2-0.3 m s
-1 . In the equatorial ocean, however,
there is substantial background shear. For a total velocity difference
~ 0.5 0.7
u
'
m s
-1 , formula (5.51) results in an estimate
160 310
O
m.
The V t -P contour plot in Figure 5-40d reveals wavelike disturbances of the
diurnal thermocline with a typical scale beginning from ~200 m, which is
consistent with the above estimate.
Observations from satellite images of strong diurnal warming events
near the California coast (Flament et al., 1994) indicated the formation of
coherent streaks, associated with the erosion and decay of the warming
layers during the night following strong diurnal warming. The horizontal
spacing of the streaks, observed at 1 km resolution, was ~4-8 km. This is
consistent with Figure 5-40 in which it is possible to trace a several
kilometer scale variation of the near-surface temperature of ~2
o C amplitude.
Internal waves of ~200 m wavelength cannot be resolved in the available
satellite images. The patterns between the two successive images separated
by 5 h 26 min (Flament et al. 1994, Figure 12b and c) look stationary, but
appear consistent with internal waves propagating at ~0.1 m s
-1 in the diurnal
thermocline.
The horizontal spacing of the streaks of 4-8 km observed by Flament et
al. (1994) is probably too large for the Kelvin-Helmholtz instability because
of the limitation on the maximum wavelength imposed by (5.51). The
internal wave–vortex resonance mechanism considered in Section 5.4.3
allows for perturbations with a wavelength of a few km to develop (but those
perturbations are not of Kelvin-Helmholtz type).
Internal waves in the near-surface stably stratified layers could modulate
the roughness of the ocean surface in the same way as the internal waves in
the main thermocline, making them visible in SAR images. Nonlinear
internal waves and billows in the near-surface layer of the ocean associated
with diurnal thermocline or rain-formed halocline may also modulate SST
producing specific spatial patterns seen in the infrared images of Walsh et al.
(1998).
5.6 Ramp-Like Structures
As emphasized in many studies (e.g., Thorpe, 1985; 1988; Csanady,
1984; Soloviev et al., 1988), the turbulent boundary layer at the ocean
surface has some similarity to the atmospheric turbulent boundary layer. The
atmospheric boundary layer exhibits spatially coherent organized motions in
the form of “ramps” (Antonia et al., 1979; Phong-Anant et al., 1980). Ramp354
where O is the wavelength, u
' is the vertical velocity jump,
0
'
/
g g U U
'
is the reduced gravity. For the large-amplitude internal wave observations of
Soloviev and Lukas (1996) (Figure 5-40),
4
0
/
5 10
U U
'
| u
. Typical
velocity of the diurnal jet is 0.2-0.3 m s
-1 . In the equatorial ocean, however,
there is substantial background shear. For a total velocity difference
~ 0.5 0.7
u
'
m s
-1 , formula (5.51) results in an estimate
160 310
O
m.
The V t -P contour plot in Figure 5-40d reveals wavelike disturbances of the
diurnal thermocline with a typical scale beginning from ~200 m, which is
consistent with the above estimate.
Observations from satellite images of strong diurnal warming events
near the California coast (Flament et al., 1994) indicated the formation of
coherent streaks, associated with the erosion and decay of the warming
layers during the night following strong diurnal warming. The horizontal
spacing of the streaks, observed at 1 km resolution, was ~4-8 km. This is
consistent with Figure 5-40 in which it is possible to trace a several
kilometer scale variation of the near-surface temperature of ~2
o C amplitude.
Internal waves of ~200 m wavelength cannot be resolved in the available
satellite images. The patterns between the two successive images separated
by 5 h 26 min (Flament et al. 1994, Figure 12b and c) look stationary, but
appear consistent with internal waves propagating at ~0.1 m s
-1 in the diurnal
thermocline.
The horizontal spacing of the streaks of 4-8 km observed by Flament et
al. (1994) is probably too large for the Kelvin-Helmholtz instability because
of the limitation on the maximum wavelength imposed by (5.51). The
internal wave–vortex resonance mechanism considered in Section 5.4.3
allows for perturbations with a wavelength of a few km to develop (but those
perturbations are not of Kelvin-Helmholtz type).
Internal waves in the near-surface stably stratified layers could modulate
the roughness of the ocean surface in the same way as the internal waves in
the main thermocline, making them visible in SAR images. Nonlinear
internal waves and billows in the near-surface layer of the ocean associated
with diurnal thermocline or rain-formed halocline may also modulate SST
producing specific spatial patterns seen in the infrared images of Walsh et al.
(1998).
5.6 Ramp-Like Structures
As emphasized in many studies (e.g., Thorpe, 1985; 1988; Csanady,
1984; Soloviev et al., 1988), the turbulent boundary layer at the ocean
surface has some similarity to the atmospheric turbulent boundary layer. The
atmospheric boundary layer exhibits spatially coherent organized motions in
the form of “ramps” (Antonia et al., 1979; Phong-Anant et al., 1980). Ramp354