6.2 Stability of Water Masses
185
2.5 minutes, but most have periods ranging from 5 to 20 minutes (Roberts,
1975). For example, temperature measurements made in Massachusetts Bay
(West Atlantic) indicate that the thermocline can heave up and down with
periods of 6 to 8 minutes with vertical displacements of 10 m, occurring at
17.2 m below the surface (Halpern, 1971). In the Andaman Sea, internal waves
with periods of about 20 minutes and amplitudes over 40 m were observed.
The existence of internal waves with a semi-diurnal period was observed during temperature/salinity measurements on the Australian North-West Shelf.
Vertical displacements of isopycnal surfaces were about 30 m, equal to nearly
half the water depth (Holloway, 1983).
The phase speed of short-period internal waves varies between 0.1 m/s and
1 mis, and their steepness is very small, equal to about 0.04. They are not
necessary of sinusoidal form; sometimes they are flattened on the crests when
the thermocline is shallow and peaked when the thermocline is deep.
Long internal waves with periods around 12 hours are known as semi-diurnal
internal tides or baroclinic tides. The amplitudes of internal tides are usually
2-10 m, while their length is about 3 x 10 4 m. A comprehensive review of
internal tide observations was published by Huthnance (1989).
There are a number of hypotheses on how internal waves are generated. There
are instances where some generating mechanisms are more likely to occur. For
example, it was shown that changes in the topography, e.g. slopes, sills, or
seamounts, may produce internal waves (Roberts, 1975; Holloway, 1983; Huthnance, 1989; Holloway et ai., 1997). Another possible mechanism for generation
of short-period internal waves is forcing provided by long-period waves. The relationship between short-period internal waves and the internal tides has been
observed east of the Straits of Gibraltar, the Bay of Biscay and on the California Coast (Roberts, 1975). It is also likely that winds, air-pressure fluctuations,
surface swell, as well as ships slowly moving through highly stratified water are
able to generate internal waves.
In the next section we will discuss the basic methods of quantitative description of internal waves in a simpler two-layer ocean and in an ocean with density
varying continuously with depth. However, we will start with a description of
the stability of water masses.
6.2 Stability of Water Masses
Understanding the stability of ocean water is the basic requirement needed to
understand the physics of internal waves. In general, a fluid element can be
in one of the three states: stable, unstable, or neutral stability. These three
states are demonstrated in Fig. 6.3 for a small ball displaced on a concave,
convex, and horizontal surface. In the stable state, when the ball is given a
small vertical displacement, there is a restoring force which acts on the ball to
return it to its original level. On the other hand, in the unstable state, when
185
2.5 minutes, but most have periods ranging from 5 to 20 minutes (Roberts,
1975). For example, temperature measurements made in Massachusetts Bay
(West Atlantic) indicate that the thermocline can heave up and down with
periods of 6 to 8 minutes with vertical displacements of 10 m, occurring at
17.2 m below the surface (Halpern, 1971). In the Andaman Sea, internal waves
with periods of about 20 minutes and amplitudes over 40 m were observed.
The existence of internal waves with a semi-diurnal period was observed during temperature/salinity measurements on the Australian North-West Shelf.
Vertical displacements of isopycnal surfaces were about 30 m, equal to nearly
half the water depth (Holloway, 1983).
The phase speed of short-period internal waves varies between 0.1 m/s and
1 mis, and their steepness is very small, equal to about 0.04. They are not
necessary of sinusoidal form; sometimes they are flattened on the crests when
the thermocline is shallow and peaked when the thermocline is deep.
Long internal waves with periods around 12 hours are known as semi-diurnal
internal tides or baroclinic tides. The amplitudes of internal tides are usually
2-10 m, while their length is about 3 x 10 4 m. A comprehensive review of
internal tide observations was published by Huthnance (1989).
There are a number of hypotheses on how internal waves are generated. There
are instances where some generating mechanisms are more likely to occur. For
example, it was shown that changes in the topography, e.g. slopes, sills, or
seamounts, may produce internal waves (Roberts, 1975; Holloway, 1983; Huthnance, 1989; Holloway et ai., 1997). Another possible mechanism for generation
of short-period internal waves is forcing provided by long-period waves. The relationship between short-period internal waves and the internal tides has been
observed east of the Straits of Gibraltar, the Bay of Biscay and on the California Coast (Roberts, 1975). It is also likely that winds, air-pressure fluctuations,
surface swell, as well as ships slowly moving through highly stratified water are
able to generate internal waves.
In the next section we will discuss the basic methods of quantitative description of internal waves in a simpler two-layer ocean and in an ocean with density
varying continuously with depth. However, we will start with a description of
the stability of water masses.
6.2 Stability of Water Masses
Understanding the stability of ocean water is the basic requirement needed to
understand the physics of internal waves. In general, a fluid element can be
in one of the three states: stable, unstable, or neutral stability. These three
states are demonstrated in Fig. 6.3 for a small ball displaced on a concave,
convex, and horizontal surface. In the stable state, when the ball is given a
small vertical displacement, there is a restoring force which acts on the ball to
return it to its original level. On the other hand, in the unstable state, when