550 Internal Waves 



such an internal wave with such an inertia period corresponds to the natural 

 period of free oscillation of the Kattegat if the effect of the rotation of the 

 earth is taken into consideration (see later). For internal waves in the Baltic 

 Sea see also Lisitzin (1953). 



By hourly bathythermograph records off the coast of California (40 nm 

 offshore) over 6 days of anchor station on the thermocline in depths of 

 100-350 ft, well-developed internal waves were measured and analysed 

 (Reid, 1956). Waves of semi-diurnal period with amplitudes greater than 

 30 ft were found at the nearshore station (40 miles offshore in 1080 fm), 

 but little evidence of these waves or any other periodic phenomena was 

 found at the stations farther offshore though there was considerable 

 vertical fluctuation of isotherms. It is suggested that the waves found origi- 

 nate near the coast in the action of the surface tides and that, since the length 

 of such waves, if free, is short, they either dissipate before proceeding far 

 offshore or, more likely, are so distorted by the varying density structure 

 and water velocity that they are no longer recognized. 

 (c) Free Internal Oscillations of Large Regions; Internal Inertia Waves 



When a certain thermo-haline distribution of an oceanic region is balanced 

 by existing currents, and when this system is disturbed by some external 

 causes it will try to return to its former state of equilibrium. This always 

 takes place in the way of periodical oscillations around its state of equilibrium. 

 These oscillations will have the period of the free oscillation of the system. 

 The amplitude will depend upon the magnitude of the original disturbance 

 and will gradually decrease because of the influence of friction. The essential 

 data for such a case can be determined from a simple model, like a two-layer 

 ocean. The boundary surface between the upper layer (density q', thickness //') 

 and the lower layer (density q, thickness h) will be in equilibrium with the 

 existent currents, according to the Marguless condition for boundary layers. 

 Following Defant's (1940) theory, the frequency of the free oscillation 

 of the boundary surface (a n = 2njT n = frequency of the free internal 

 waves) can be expressed in first approximation by the quite accurate relation 



c„ = 



v 



/-» • v, trn-g Q—Q 

 (2rosinr/)) 2 ' 



(XVI. 39) 



where / is the length of the area of the oscillation, and where the Coriolis 

 force is taken into consideration. If the earth would not rotate (co = 0), 

 the frequency would be reduced to the equation for standing internal waves 

 in a basin of length / at the boundary surface between two liquids of different 

 density (see Chapter XVI/lc, p. 523). Then the period of the free oscillation is 



r r = 



w 



1 (? + ? 



(XVI. 40) 



g(Q-Q')\h h'j 



Let us assume that for a non-rotating earth the period T r is great, which 

 is in general the case, considering that the dimensions of the oscillating 



