548 



Internal Waves 



reduced to U'h'— Uh = (see p. 524). The averaging thus eliminates all 

 currents associated with internal waves and leaves only the tidal current, 

 which belongs to the "external" wave. 

 (b) Recording Internal Waves and Interpretation of Such Records 



It has always been difficult to make extensive observations of internal 

 waves by repeated oceanographic series. O. Pettersson tried in 1909 to record 

 internal waves in not too deep waters by a suitable apparatus. He used for 

 his experiments at the oceanographic station of Borno (Gullmarfjord) a large 

 submarine float. By carefully balancing of the flout it was brought in equilibrium 

 with the discontinuity layer (layer of the maximum density gradient) and par- 

 ticipated in its vertical movement (Fig. 228, left). These vertical displacements 

 of the float, which correspond to the vertical displacements of the internal 

 waves, move along a guiding wire, which hangs from a submarine anchored 

 buoy (see Fig. 228, right). A pressure gauge is attached to the float and 



Fig.' 228. Left: float of the recording boundary gauge (Kullenberg and Pettersson). Right: 

 way of anchoring it to the bottom (Pettersson). 



registers vertical displacements. The technical difficulties of the installation 

 of the apparatus are considerable. Kullenberg was successful in making 

 a "recording boundary gauge" that could be used in the open ocean (see 

 H. Pettersson, 1938, p. 77). In 1932 extensive recordings of internal waves 

 were carried out simultaneously at several places in the central "Kattegat" 

 in the vicinity of the "Fladenbank" shoal (57°10'N., 11°45'E.) in order to 

 obtain a series of observations as long as possible. Only such observations 

 make it possible to ascertain if the waves are of the progressive or standing 

 type and also their direction, velocity and origin. Kullenberg (1935), and 

 Pettersson and Kullenberg (1933) subjected the observations which ex- 

 tended over 15 days to the harmonic analysis and discussed the results 

 thoroughly. Table 96 gives the data for two stations. For the M 2 tide, the 

 amplitude of the first station, which was located closer to the shoal, is con- 



