Internal Waves 555 



of the Coriolis force has to be considered and the period of the free internal 

 oscillation will approach the period of the inertia oscillation when the size 

 of the oscillating basin increases. Such conditions certainly occur in the 

 Baltic, where variations in the currents showed a period of the inertia oscil- 

 lation when the water masses are stratified (see Gustafson and Kullenberg, 

 1936; Kullenberg and Hela, 1936). These oscillations in the currents are 

 associated with internal waves of the entire stratified water-mass, and are 

 subjected to the period of the inertia oscillation. Because the oscillations 

 with the inertia period are in general caused by wind disturbances, they 

 persist for some time with a striking constancy and are dampened only slowly. 

 Approaching the coast the amplitude decreases and there they die off. 



O. Pettersson (1909) found very long periods of internal waves from once- 

 time-daily observations of the stratification of the water-masses of the Gulmar- 

 fjord at Borno. The main period of the internal vertical oscillations was 

 roughly 14 days, with amplitudes up to 25 m. Pettersson believed that these 

 oscillations were caused by the effect of the tide generating forces of the moon 

 (declination tide). But Wedderbljrn (1909, p. 602) could prove, by use of 

 equation (XVI. 43) for bays, that in this case one is dealing in all probability 

 with standing internal oscillations of the entire Skagerrak basin. If the en- 

 trance of the bay is assumed to be the line connecting Hanstholm (Denmark) 

 and Mandal (Norway) the width of the opening will be 50 km. With 

 / = 200 km, q-q' = 410 -3 , h = 100 or 200 m, h' = 20 m and with a cor- 

 rection for the width of the mouth (see equation (VI. 75), Table 22) one obtains 

 for the period T = 13-9 and 14 2 days respectively. This period derived by 

 theory is in good agreement with the observed one. But it is questionable, if the 

 effect of Coriolis force may be neglected in the case of such long periods. 



When the density varies continuously with depth and when the shape of 

 the bay is irregular, an infinite number of internal waves becomes probable. 

 But the waves of high order or of many nodes cannot be expected to last 

 very long because great velocity gradients generate friction which dissipates 

 their energy very fast. An exhaustive investigation on internal waves of the 

 Gulf of California, including this aspect was carried out by Sverdrup (1939, 

 1940, p. 170) and Munk (1941). The hydrographic survey of the Gulf of 

 California from 13 February to 19 March 1939 showed a wave-like pattern 

 in the dynamic topography of the sea surface and of the isobaric surfaces 

 (referred to the 1500 decibar surface). The phenomenon can be explained 

 only by the existence of standing internal waves of a period of roughly 7 days. 

 This wave is of the first order in a vertical direction, and the vertical dis- 

 placement vanishes at the bottom and at the surface. It is of the fourth order 

 in a horizontal direction with three nodal lines in the Gulf. A nodal line at 

 the entrance of the Gulf indicates that we have to do with a free oscillation. 

 Figure 231 shows the distribution of the water-masses and the position of the 

 various isobaric surfaces within the Gulf, as resulting from the oceanographic 



