INTERNAL WAVES AND THEIR MEASUREMENT 
by E. C, LaFOND, Head, Marine Environment Studies Branch 
U.S. Navy Electronics Center 
San Diego, California 
INTRODUCTION 
Internal waves are undulating swells occur- 
ring between subsurface water layers of varying 
density, even though the density change be 
slight. By contrast, in a homogeneous fluid 
only surfaces waves are possible, and the ampli- 
tude of their motion decreases with depth. In 
the ocean, vertical density gradients are 
virtually always present. Thus internal waves 
are found in all oceans and probably most bays 
and lakes, and vary widely in amplitude, period, 
and depth. Although their amplitudes may exceed 
those of surface waves, internal waves are 
usually slower in speed. 
In a simple, two-layer density system, 
maximum amplitude exists at the boundary of the 
two layers and decreases linearly with distance 
above and below.t Ina multiple-layer or 
continuous-density gradient system, as in the 
sea, the wave motions become much more complex. 
Under these conditions multiple wave patterns, 
having different characteristics, have been ob- 
served at different levels. 
CAUSE OF INTERNAL WAVES 
The exact causes of specific internal 
waves have not yet been firmly established, but 
they are probably of varied origins. Experi- 
ments in the Norwegian fjords“ showed that a 
slow-moving sailboat initiated internal waves 
at the shallow-layer boundary of nearly fresh 
water and higher-density sea water. The inter- 
nal waves, produced by the keel of the slow- 
moving ship, reduced its speed and created the 
phenomenon known as "dead water". 
Since internal waves are commonly found at 
water mass boundaries or "fronts," they are 
probably produced through direct displacement 
of one water mass by another. The front is 
characterized by a group of relatively large 
internal waves followed by a change in the depth 
of the thermocline (fig. 1). Visual evidence 
of internal waves forming at water mass bound- 
aries has been shown in high-altitude photo- 
graphy3 of slick-type surface phenomena in 
Georgia Strait,* where such occurrences were 
attributed to large-scale discharge of water of 
*The channel between Vancouver Island and 
southwest British Columbia. 
varying density (fig. 2). Masses of fresh water 
created a tidal front, or zone of convergence 
and divergence, in which internal waves developed 
Internal waves, coincident with tidal periods 
whether semidiurnal or diurnal, were commonly 
observed; thus, it was concluded, tidal Hones 
must be instrumental in generating them. 29 
Certain internal waves may be created by 
two adjacent flows or by a flow impinging on a 
COB eT De oat shelf or other obstruction. Experi- 
ments~ that showed internal waves to occur when 
a tidal stream flowed against a coastal bank 
have been conducted. It was further proven that 
obstacles in the path of an advancing wave give 
rise to internal waves. The significant fact 
is that vertical oscillations in the thermal and 
density structure of sea, which are termed in- 
ternal waves, are apparently present in all 
oceans and at all depths. This indicates that 
their cause is likewise universal. 
MEASUREMENT EQUIPMENT AND TECHNIQUES 
Internal waves can be present only in water 
where a vertical density gradient exists. The 
vertical gradient may be caused either by tem- 
perature or salinity, or both. In fresh-water 
lakes, measurements of temperature alone are 
sufficient to establish the existence of density 
gradients. In the sea, measurements are com- 
monly of temperature since they are easily ac- 
complished, and the temperature and salinity 
gradients usually coincide. In addition, the 
salinity gradients are normally small. 
On one occasion, however, internal waves 
were directly measured by their vertical oscil- 
dations of density. A large, buoyant container 
was floated on a given density boundary and its 
depth recorded. A drum, filled with glass balls, 
was guided vertically by a cable. A recording 
manometer on the drum successfully furnished a 
{7-day record of the depth when it was weighted 
sufficiently to float on the maximum density 
gradient found in the southern Kattegat** in 
summer. 
Various instruments have been employed to 
measure vertical oscillation of temperature 
**An arm of the North Sea, between Sweden and 
Denmark. 
"Superior numbers refer to similarly numbered references at the end of this paper. 
