Internal Waves 



527 



Figure 216 shows the variation of the temperature in several depths, as 

 observed in the serial observations. Neglecting a disturbance which occurred 

 at midnight, 31 January— 1 February one finds that in the vicinity of the 

 discontinuity layer there is a wave with a period of 12-3 h (semi-diurnal lunar 

 period), with a maximum amplitude in 100 m. In 125 m the wave is still 

 noticeable, but in the top layer and downward from 150 m, it is hardly de- 

 tectable. The variations in salinity are quite similar. The analysis of all the 

 observations reveals that at the boundary surface there was an internal tide 

 wave, with an amplitude of 3-6 m and a phase of 4 3 lunar h (referred to 

 the transit of the moon through the meridian at Greenwich). Current meas- 

 urements at 0, 25, and 50 m were taken simultaneously with the hydrographic 

 casts. The average velocity for the entire layer was 61 cm/sec with a phase 

 of 10-4 lunar h for currents flowing to the north. The difference in phase 

 between the vertical and horizontal motion is therefore 6 1 h, exactly a half 

 period. 



The amplitude of the velocity of the current has to be very small in the 

 lower layer because of its great depth; equation (XVI. 20) gives for Z = 36 m 

 and ti = 100 m, U' = 615 cm/sec in complete agreement with the observed 

 value. The influence of the internal tide wave on temperature and salinity 

 is limited to the immediate vicinity of the discontinuity layer of the density. 



Fig. 217. Streamlines of the lower layers at midnight from 31 January to 1 February at 



Anchor Station 254. 



Vertical displacements of water-masses above and below the discontinuity 

 layer must remain ineffective on the distribution of temperature and salinity 

 because of the nearly homogenous compositions of the layers. For this reason 

 no changes were observed in these properties at 50 and 150 m when the 

 internal wave passed. 



