Waves 



117 



When the wind exceeds a certain critical 

 velocity (6 to 7.5 meters/sec according to 

 Munk, \947a, and Mandelbaum, 1956), its 

 influence on the ocean surface is markedly 

 greater than at low velocities. Such winds 

 are the chief cause of the main oceanic cur- 

 rents, but they also produce the sea or wind 

 waves that are responsible for many near- 

 shore processes of erosion and deposition 

 and for some of the depth zonation of shore- 

 living organisms. In the area off' southern 

 California several sets of waves are usually 

 present, one or more of which consist of 

 short-period (5 to 10 seconds) waves from 

 strong local winds such as those between 

 Point Arguello and San Nicolas Island. The 

 strong northeasterly Santa Anas also pro- 

 duce waves, but these reach only a short 

 distance seaward. Diurnal land breezes 

 are so weak and short in duration that they 

 rarely produce even small local waves; their 

 main effect is that of partly counteracting 

 the westerly winds and smoothing the sea 

 during early morning hours. The highest 

 waves of the region ordinarily occur in the 

 area between Point Arguello and San Nico- 

 las Island; these are commonly 1 to 2 meters 

 high, but by the time they have traveled to 

 the lee side of Santa Catalina Island they are 

 only one-third as high (Fig. 103). Storms, 

 such as the one in September 1939, create 

 larger waves of up to 6 meters which are 

 capable of considerable erosion of shores and 

 damage to shore structures (McEwen, 1935). 

 Waves 8 meters high produced by a storm 

 of November 14 and 15, 1958, capsized sev- 

 eral fishing boats as well as damaging shores. 



Particularly well developed during the 

 summer are the long-period (13 to 20 sec- 

 onds) waves originated by storms in the 

 Southern Hemisphere, but now far beyond 

 the influence of the causative winds and 

 therefore known as swell, in contrast to sea. 

 Occasional large waves of this origin wash 

 high on beaches during summer, spreading 

 consternation, sunbathers, and blankets. As 

 shown by Sverdrup (1947) and Munk 

 (1947Z)), the period and length of the waves 

 are a function of the distance from the storm 

 center; thus, the period and approach direc- 

 tion of swells serve as criteria for determining 



the paths foUowed by storms far at sea. 

 Even more useful for storm tracking are 

 longer period (up to 30 seconds) and faster 

 forerunners of swell (Munk, 1947c) which 

 have such a low amphtude that they cannot 

 be detected except by wave meters having 

 suitable frequency filters. 



More common are swells that enter the 

 region from several diff'erent small storm 

 centers. Nearly always present are the waves 

 from the North Pacific, from one or more 

 centers west of California, and from local 

 winds. These different sets of waves spread 

 throughout the continental borderland, and 



WAVE HEIGHT-FEET 



12 3 4 5 



Figure 103. Height frequency of waves obtained by esti- 

 mating from shipboard the heights of 75 to 100 successive 

 waves at points near the middles of various basins having 

 different degrees of exposure. The top three diagrams 

 were measured on June 17, 1957, and the next two on 

 June 18, both days having light winds. Bottom measure- 

 ment was on June 19 after the wind had increased to 

 about 40 knots. Wide vertical lines indicate mean heights 

 of waves in each diagram. Mean periods of waves were 

 5 to 7 seconds. 



