22. INTERNAL WAVES 

 PART I 



E. C. LaFond 



1. Introduction 



In a non-homogeneous element such as the sea, undulating swells, called 

 internal waves, form between subsurface water layers of varying density. By 

 contrast, in a homogeneous fluid only surface waves are possible, and the ampli- 

 tude of their motion decreases with depth. 



Internal waves exist in all oceans, 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 two layers, and decreases linearly with distance above and below 

 (Fjeldstad, 1933). In a multiple-layer or continuous-density gradient system, 

 as in the sea, the wave motions become much more complex. 



1900 



2000 



HOURS 



2100 



Fig. 1. Internal waves at a water-mass boundary, showing change in depth of thermocline 

 after group of internal waves. 



The exact causes of internal waves have not yet been firmly established, but 

 are probably of varied origins. Ekman (1904) beheved that a slow-moving ship 

 initiated internal waves at the shallow layer boundary of nearly fresh water 

 and higher-density sea- water. The internal 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 relatively large wave 

 followed by a few of decreasing size (Fig. 1). Visual evidence of internal waves 

 forming at water-mass boundaries is shown in pictures by Shand (1953), who 

 photographed, from high altitude, slick-type surface phenomena in Georgia 

 Strait, 1 and attributed such occurrences to large-scale discharge of water with 



1 The channel between Vancouver Island and southwest British Columbia. 



[MS received September, 1960] 



731 



