658 MUNK [chap. 18 



It may occur to the reader that the long-period oscillations in sea-level may- 

 be the surface manifestation of internal wave activity (see Cox and La Fond, 

 Chapter 22). The two phenomena cover about the same frequency range. 

 Furthermore the computed surface amplitudes associated with observed 

 internal waves are not negligible as compared with measured surface amplitudes. 

 Nevertheless any attempt to correlate simultaneous measurements has met 

 with failure (Munk, Snodgrass and Tucker, 1959, p. 318). Internal waves may 

 still be an important extended source of long-wave activity, but the two phenom- 

 ena are not convincingly correlated at any time and place. 



In some instances the sea-level responds like an inverted barometer to 

 fluctuations in atmospheric pressure (Donn, 1958; Van Dorn, 1960a; Gossard 

 and Munk, 1955). The pressure fluctuations may be the surface manifestation 

 of internal waves in the atmosphere ; or again they may be associated with line 

 squalls. Ewing and Press (1953) have attributed the world-wide tide-gauge 

 disturbance accompanying the eruption of Krakatoa to the atmospheric 

 pressure wave associated with this great event. But here again, as in the case of 

 internal waves, one finds that in the usual situation there is no obvious co- 

 herence between simultaneous records of sea-level and atmospheric pressure. 

 Again this does not exclude the possibility of atmospheric pressure as an 

 extended source in the sense that the observed waves are the cumulative result 

 of random pressure impulses over the entire ocean surface. Inasmuch as the 

 propagation velocities of long waves in the atmosphere and ocean are not well 

 matched (1 100 km h-i versus 600 km h-i), this would represent an ofi'-resonance 

 flux of energy from one medium to the other. 



F. Waves in Ports 



The occurrence of undesirable oscillation in ports is usually referred to as 

 "seiching". There can be no doubt that the resonance characteristics of the 

 port itself are the chief factor in determining these oscillations. In most of the 

 literature it is assumed that it is the direct action of wind and pressure on 

 the water surface inside the port that is responsible for exciting the resonances. 

 It seems more likely that excitation through the mouth is the predominant 

 factor. This certainly must be true in the case of harbour seiches accompanying 

 severe tsunamis. A review of this subject has been given by Wilson (1957). 

 From a practical point of view it would seem desirable to determine the spectra 

 outside the harbour along the lines of Fig. 4, and to choose depth and lateral 

 dimensions of the harbour so as to "detune" it from the shelf resonances. For 

 regions with sharp outside spectral peaks this procedure might reduce the 

 amplitude inside the harbour by an order of magnitude ; for regions with broad 

 spectral bands, the advantage to be gained is minor (Miles and Munk, 1961). 



6. Tsunamis 



Because of the great damage caused by severe tsunamis, there have been 

 many general accounts. We shall mention only a few highlights. One of the best 



