High-wind velocities are not uncommon north of 35°N but decrease significantly in frequen- 

 cy in southern California waters. The southern region of the area has steady, predominate 

 trade winds averaging 5 to 10 m/sec, blowing from the northeast in latitudes 5° to 25°N and 

 from the southeast 10°S to 5°N. Occasional high-velocity winds associated with local activi- 

 ty or tropical cyclones are observed along the coasts of Central America and Mexico. Other 

 areas, particularly the northern Baja and southern Cahfornia coast at latitudes 20° to 35°N, 

 have very low-velocity winds (only rarely do they increase above 17 m/sec). 



Wind-Generated Ocean Waves 



Ocean waves are considered here as part of the meteorological data because nearly 

 all near-surface waves are wind-generated and are directly related to wind velocity and dura- 

 tion. An exception would be the extremely rare tidal waves (tsunamis) generated by seismic 

 disturbances or volcanic eruptions. Quantitative values for wave height (the vertical distance 

 between a wave crest and the preceding trough) and period (length of time between succes- 

 sive crests) have been established as a function of wind speed, fetch (distance which the 

 wind has blown over water), duration of the wind, and decay distance (distance the waves 

 have progressed from the area of generation). Although ocean waves are generated as a 

 direct result of wind action, other variables such as currents, bottom topography, and local 

 winds affect their magnitude and direction. 



Wave data are generally observed as either "sea" (waves generated by local winds) or 

 "swell" (waves which travel beyond their source regions). The observed wave height is 

 usually termed the "significant height" and is defined as the average value of the highest one 

 third of all waves observed in a given wave train. "Extreme waves" are quite rare and occur 

 when two or more very high waves come together in constructive phase. Extreme wave 

 heights are discussed in the Extreme Meteorological Data Section and are defined by Thorn 

 (Ref. 37): 



Extreme Wave Height = 1.8 X Significant Wave Height. 



Figure 35 gives an empirically-derived curve of sustained wind speed versus significant wave 

 height for increasing and decreasing wind regimes. The figure indicates that for decreasing 

 winds the significant wave height will generally be higher because of the longer period of 

 time that the wind has blown. The right side of Fig. 35 thus demonstrates a "fully arisen" 

 sea (the total energy possible from a wind has been translated to wave energy). This figure 

 can be used to derive approximate values of significant wave heights from the wind data 

 given in the previous section. 



Wave period is highly important as an engineering consideration because it controls 

 the depths reached by given fracfions of the wave motion (amplitude and velocity). Figure 

 36 shows the attenuafion of wave mofion with depth given the period of the wave. For 

 example, the mofion of a wave with a 6-second period is 50 percent attenuated at about 4.5 

 meters of depth and becomes insignificant (4 percent of surface wave energy remaining) at 

 26 meters, while a 12-second wave is 50 percent attenuated at 23 meters and becomes insig- 

 nificant at II meters. (Note: Wave-induced drag forces scale with the square of the wave 

 motion.) 



24 



