6 Internal Waves
6.1 Introduction and Useful Definitions
The propagation of waves at the sea surface is strongly dependent on the density
difference between water and air, and on gravitational acceleration. However,
air density is small enough to be ignored in the theoretical analysis and practical
calculations of surface waves. In Chap. 1 we have shown that the density of
ocean water changes with depth. Therefore, it is quite likely that waves appear
along the density gradients within the ocean. Such waves are known as internal
waves.
While internal waves were known to exist in the ocean in the early 1900's,
it is only within the last three decades or so that technology has advanced to
a point where large numbers of observations are available. Most internal wave
measurement methods are based on recording the profiles of temperature and
salinity in the water column. The movement of points of equal temperature,
isotherms, or the movement of points of equal salinity, isopycnals, are the
manifestations of the passage of internal waves. Figure 6.1 (see colour plate
p.563) shows a time series of density on the Australian North- West Shelf (Burrage et al., 1996). In this figure, water density has been denoted in (Jt units as a
form of shorthand. The quantity (Jt is related to density Pw by the expression:
(Jt = Pw - 1000.
(6.1)
Thus, the density (Jt = 25 kg/m 3 corresponds to the density Pw equal to 1025
kg/m 3 .
The zone where the vertical density gradient is the greatest is known as a pycnocline. For completeness we define here the thermocline as a zone where
density variations are determined mostly by the temperature variations, and
halo cline where the density variations are controlled mostly by the salinity.
Figure 6.2 illustrates typical locations of the pycnocline and thermocline in the
tropical zone of the Atlantic Ocean (Miropolskiy and Monin, 1978). The thermocline is about 65 m below the surface, with the pycnocline slightly deeper,
at about 70 m. In the same figure, a vertical profile of the Brunt-Viiisiilii frequency, N (z) is also given. We will discuss this frequency later in this chapter.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999
6.1 Introduction and Useful Definitions
The propagation of waves at the sea surface is strongly dependent on the density
difference between water and air, and on gravitational acceleration. However,
air density is small enough to be ignored in the theoretical analysis and practical
calculations of surface waves. In Chap. 1 we have shown that the density of
ocean water changes with depth. Therefore, it is quite likely that waves appear
along the density gradients within the ocean. Such waves are known as internal
waves.
While internal waves were known to exist in the ocean in the early 1900's,
it is only within the last three decades or so that technology has advanced to
a point where large numbers of observations are available. Most internal wave
measurement methods are based on recording the profiles of temperature and
salinity in the water column. The movement of points of equal temperature,
isotherms, or the movement of points of equal salinity, isopycnals, are the
manifestations of the passage of internal waves. Figure 6.1 (see colour plate
p.563) shows a time series of density on the Australian North- West Shelf (Burrage et al., 1996). In this figure, water density has been denoted in (Jt units as a
form of shorthand. The quantity (Jt is related to density Pw by the expression:
(Jt = Pw - 1000.
(6.1)
Thus, the density (Jt = 25 kg/m 3 corresponds to the density Pw equal to 1025
kg/m 3 .
The zone where the vertical density gradient is the greatest is known as a pycnocline. For completeness we define here the thermocline as a zone where
density variations are determined mostly by the temperature variations, and
halo cline where the density variations are controlled mostly by the salinity.
Figure 6.2 illustrates typical locations of the pycnocline and thermocline in the
tropical zone of the Atlantic Ocean (Miropolskiy and Monin, 1978). The thermocline is about 65 m below the surface, with the pycnocline slightly deeper,
at about 70 m. In the same figure, a vertical profile of the Brunt-Viiisiilii frequency, N (z) is also given. We will discuss this frequency later in this chapter.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999