198
6 Internal Waves
('" 80 m water depth), the waves had steepened showing shock on both the
front and back faces. Large amplitude solitons were developed on the front face
of the wave with approximately 40 m height. The velocities associated with
the internal tide reached 80 cm/s.
Lamb (1994) simulated the internal waves for the topography at the northern
side of George Bank, Canada. The water depth varied from 65 m, on the top
of the bank, to 260 m off the bank. The model successfully reproduced a
number of the observed features, including the large depression of pycnoclines
resulting in a hydraulic jump (shock wave) during off-bank flow, and two onbank propagating depressions every tidal period. Changing the density field
in a way which resulted in much stronger stratification induced internal wave
breaking by the end of off bank flow.
The main limitation of the present model techniques is the large temporal
variability in the background parameters of the ocean water, such as stratification and turbulent mixing, and lack of proper parameterization of the effects
of dissipation.
6.4.3 Surface Effects
While internal waves are usually detected by temperature or salinity fluctuations, there are other phenomena which have been linked to their occurrence.
These include slowing of a ship's passing, slicks of darker bands of muddy waters on the sea surface, sand waves formation, and affecting sound-scattering
layers in the ocean (Roberts, 1975). When the thermocline lies sufficiently close
to the surface, or when the amplitudes of internal waves are unusually large,
the internal breaker can appear at the sea surface. Historically, the Andaman
Sea was probably the region where such phenomena were first observed. According to LaFond (1966), breaking whitecaps emitted a low roar as they passed
a drifting ship on a calm sea. More evidence on the surface effects caused by
internal waves has been reported by Osborne and Burch (1980). Photographs
taken on board a survey vessel show a long band of breaking waves about 1.8
m high. Moreover, LANDSAT images have shown internal waves with crests as
long as 150 km, and wavelength as great as 15 km in the same area.
Burrage et al. (1996), using the ERS-l Synthetic Aperture Radar (SAR)
imagery detected a presence of internal waves on the Australian North West
Shelf (a description of the SAR sensor is given in Chap. 9). These images are
consistent with sea-truthing data obtained from a conventional current meter
and thermistor chain mooring. These instruments were sampled sufficiently
rapidly to resolve internal wave variability. Additional data were also obtained
from ships using a conductivity, temperature, and depth (CTD) probe to obtain
profiles of temperature, salinity and density at 2 m depth intervals.
Images containing thin, elongated curvilinear features, being almost certainly
signatures of bioslicks, were obtained under wind speeds of 2 m/s and less.
These sometimes appeared in association with broad or narrow rectilinear alternating light and dark bands which, based on the shelf and band geometry,
6 Internal Waves
('" 80 m water depth), the waves had steepened showing shock on both the
front and back faces. Large amplitude solitons were developed on the front face
of the wave with approximately 40 m height. The velocities associated with
the internal tide reached 80 cm/s.
Lamb (1994) simulated the internal waves for the topography at the northern
side of George Bank, Canada. The water depth varied from 65 m, on the top
of the bank, to 260 m off the bank. The model successfully reproduced a
number of the observed features, including the large depression of pycnoclines
resulting in a hydraulic jump (shock wave) during off-bank flow, and two onbank propagating depressions every tidal period. Changing the density field
in a way which resulted in much stronger stratification induced internal wave
breaking by the end of off bank flow.
The main limitation of the present model techniques is the large temporal
variability in the background parameters of the ocean water, such as stratification and turbulent mixing, and lack of proper parameterization of the effects
of dissipation.
6.4.3 Surface Effects
While internal waves are usually detected by temperature or salinity fluctuations, there are other phenomena which have been linked to their occurrence.
These include slowing of a ship's passing, slicks of darker bands of muddy waters on the sea surface, sand waves formation, and affecting sound-scattering
layers in the ocean (Roberts, 1975). When the thermocline lies sufficiently close
to the surface, or when the amplitudes of internal waves are unusually large,
the internal breaker can appear at the sea surface. Historically, the Andaman
Sea was probably the region where such phenomena were first observed. According to LaFond (1966), breaking whitecaps emitted a low roar as they passed
a drifting ship on a calm sea. More evidence on the surface effects caused by
internal waves has been reported by Osborne and Burch (1980). Photographs
taken on board a survey vessel show a long band of breaking waves about 1.8
m high. Moreover, LANDSAT images have shown internal waves with crests as
long as 150 km, and wavelength as great as 15 km in the same area.
Burrage et al. (1996), using the ERS-l Synthetic Aperture Radar (SAR)
imagery detected a presence of internal waves on the Australian North West
Shelf (a description of the SAR sensor is given in Chap. 9). These images are
consistent with sea-truthing data obtained from a conventional current meter
and thermistor chain mooring. These instruments were sampled sufficiently
rapidly to resolve internal wave variability. Additional data were also obtained
from ships using a conductivity, temperature, and depth (CTD) probe to obtain
profiles of temperature, salinity and density at 2 m depth intervals.
Images containing thin, elongated curvilinear features, being almost certainly
signatures of bioslicks, were obtained under wind speeds of 2 m/s and less.
These sometimes appeared in association with broad or narrow rectilinear alternating light and dark bands which, based on the shelf and band geometry,