J. Crease 135 
peak. This is more obvious in the correlogram from which the spectrum was 
made. It is at these longer periods observable by B/T and hydrographic station 
work that most observations have been made, and there are many reports in the 
literature of the presence of definite periodicities. Defant [8] (1961) describes a 
number in which there appear to be semidiurnal and diurnal waves. Anomalies 
in long oceanographic sections across the sea are often attributed without any 
direct evidence or justification to the transient effect of the waves. If they are 
present, they will seriously affect the interpretation of the mean temperature 
field and, as a consequence, of the acoustic regime as well. The fault of most of 
these measurements is that they were of relatively short duration, perhaps two 
or three days, and thus contained only a small number of waves of the suspected 
period. Haurwitz (1954) has shown, in a statistical treatment of four of the open 
ocean cases reported in the literature, that only one (by the German research 
ship Meteor) definitely indicated a semidiurnal tidal component that could not 
equally well be attributed to chance variations. Eventhough they may be random, 
these variations of long period are likely to be of considerable importance, as 
they may have amplitudes approaching 100 m. 
There are some difficulties inaccounting for diurnal and semidiurnal internal 
waves. It is natural to look for their cause in the tide-raising force that creates 
the surface tides but this does not immediately seem possible-the tidal force is 
acting on the whole body of the fluid, and without some resonance mechanism 
nothing much is to be expected in the way of internal waves. It has been noted 
that typical velocities in the midrange of the spectrum are 100 cm/sec, much 
lower than the speed of the tide, and there wouldn't seem to be much hope of 
resonance. However, to look at the possibilities, consider again Eq. (1) and note 
that for waves whose frequency o~f the internal wave velocity is given by the 
same expression as before except for a factor (1-f?/o)*%i.e., 
rae rion ceil 
Rea [ p reer 
Thus the rotation of the earth implies that for o>f the velocity becomes large 
and the wavelength long. There is now the possibility of some degree of resonance. 
However, f varies with latitude and the diurnal tide will strictly achieve 
resonance only at about 30° latitude, and for semidiurnal tide the resonance lati- 
tude will be closer to the poles. It is interesting that the USHO [9] observations 
off Bermuda in 33N should show the possible presence of diurnal waves and no 
semidiurnal waves. Thus there is no explanation so far for the semidiurnal waves 
except perhaps in high latitudes. Regarding these, a most significant set of 
observations were made by Reid [10] (1956) off California (Fig. 8.5). These are 
the observations referred to earlier in connection with breaking. The semi- 
diurnal period is very pronounced and the important point is that this record 
was obtained just off the continental borderland in deep water and records made 
farther offshore revealed nothing like this regularity. What appears to be 
happening here is that while the surface and internal waves are effectively un- 
coupled when moving over a flat bottom, the presence of an abrupt change of depth 
at the continental slope enables a transfer of energy to take place from the sur- 
