18. LONG OCEAN WAVES 



W. H. MUNK 



1. Introduction 



"Long waves" liere refers to i:)eriod.s between swell and tides, that is, from 

 about I minute to 12 hours. This covers ten octaves. The most conspicuous 

 thing about waves in this frequency range is their absence. The energy^ con- 

 tained in the swell and tides is each of the order lO^ cm 2 ; the energy contained 

 in the entire intermediary range of frequencies is of the order of 1 cm^. 



Once every year or two, a "tsunami"' (or "tidal wave", or "seismic sea wave") 

 produces energies of 10^ cm^ in this intermediary range, and the long-wave 

 activity may remain above background for a week. Tsunamis are associated 

 with submarine volcanic eruptions, earthquakes or landslides, whereas the 

 background activity is meteorologically induced. To a large extent the interest 

 in long waves derives from the occurrence of occasional tsunamis. "Seiching" 

 in harbours and bays may affect safe anchoring conditions, and is another 

 source of interest. "Surges" or "storm tides" induced by severe storms are 

 typically associated with periods in excess of those of the ordinary astronomic 

 tides, and accordingly they are considered as a separate topic (Chajiter 17). 

 Nevertheless, the long-wave background to be described here has many of the 

 aspects of micro-surges, and the subject cannot be so neatly separated. 



2. The Instruments 



Observation of the intermediary frequencies puts exacting demands on 

 instrumentation and analysis. This is the penalty for working in a sjjectral 

 valley. The first requirement is to reduce the waves and swell by means of a 

 suitable low-pass filter. The traditional way to accomplish this is the Kelvin 

 tide-gauge (Fig. 1). This is still the principal source of observational evidence 

 concerning long waves. An obvious improvement is to add a high-pass filter 

 to reject the tides as well, thus making the long-wave recorder a band-pass 

 filter peaked in the intermediary frequencies. 2 One simple way of accomplishing 

 this is shown in Fig. 1. Another obvious improvement is to replace the orifice 

 (which responds non-linearly) with a capillary, as shown. Nearly all shore- 

 based long-wave recorders are elaborations on this simple theme (Munk, 

 Iglesias and Folsom, 1948; Van Dorn, 1956 and 1960; Snodgrass, 1958). In 

 general, pneumatic filtering is better adapted to this frequency range of a few 

 cycles per hour than is electronic filtering. Temperature effects are a serious 



1 Here used in the sense of the "mean-square elevation". Actually the energy per unit 

 area is pg (mean-square elevation). (See Cartwright, Chapter 15.) 



2 A low-pass filter only (e.g. a tide-gauge) is a more satisfactory long-wave instrument 

 than a high-pass filter only (a wave recorder with a slow leak). This is because it is easier 

 to detect tiny high-frequency wiggles superposed on low frequencies than tiny low- 

 frequency undulations underlying high frequencies. The distinction must be physiological : 

 the eye responds to the rate of change of the ordinate rather than to the ordinate itself. 



[MS received October, 1960\ 647 



