528 Internal Waves 



The disturbance between 31 January and 1 February, with great changes 

 in temperature and salinity, is of particular interest. The depth from which 

 these water masses originate, can be easily ascertained by means of the vertical 

 distribution of the temperature and salinity at the station — after elimination 

 of the regular semi-diurnal wave. Figure 217 shows the streamlines of this 

 disturbance which was caused by a powerful downward push of nearly 60 m 

 of the upper water masses. Its maximum is located in the region of the dis- 

 continuity layer and the disturbance decreases in intensity upward and down- 

 ward. This disturbance has also the characteristics of an internal wave. Its 

 shape reminds one of the disturbance which Sandstrom( 1908, p. 9) created by 

 his experiments in stratified water, when a gust of wind blew upon the water 

 surface and which is illustrated in Fig. 218. It cannot be doubted that at 



Fig. 218. Sandstrom's experiment creating an internal wave in stratified water by a gust 



of wind on the surface. 



anchor station 254 a similar process was observed by chance. Some external 

 disturbance can create such an "internal" wave. It travels along the boundary 

 surface with the velocity of internal waves. But observations at one station 

 alone are not sufficient to determine the direction of the wave. The period 

 was about 6-4 h. The length was therefore roughly 40 km, if c 2 = 172 cm/sec, 

 which makes it quite plausible that a gust of wind was the cause. 

 (d) Internal Waves When the Density is a Continuous Function of the Depth 



(a) Cellular waves and stability waves. Until now we have dealt with 

 internal waves at a discontinuity surface of density within a vast water mass. 

 These internal waves have their greatest amplitude at the boundary surface. 

 If there are several discontinuity layers in the vertical distribution of the 

 density, several internal waves can occur simultaneously. Therefore there 

 exists a greater variety of oscillations in such a water mass than in a homo- 

 geneous one. It is to be expected, that with an increasing number of dis- 

 continuity surfaces, the number of possible internal waves increases ac- 

 cordingly. This leads to the case of a continuous variation of the density 

 with depth and then we can expect an unlimited number of possible internal 

 waves. A water-mass with such continuous variation of the density with 

 depth behaves entirely different from homogenous liquids. 



Only the most elementary forms of such wave motions in a stable stratified 

 medium have been studied, as far as "short" waves are concerned. They 

 are the so-called cellulary waves, where the entire oscillating space is sub- 

 divided into "cells" of definite dimensions. In each cell the oscillation occurs 



