Ocean swells are generated by storms blowing at sea, far from California. As the storm-generated waves move away from the storm area, the longer waves move faster, and thus separate from the shorter waves. This process of waves separating from each other according the the wave length is called dispersion.. As a result of dispersion, waves arriving in California which were generated by a single storm are mostly of one wave period, wavelength, and wave height.
A swell arriving in California may encounter another swell, generated by a different storm, arriving from a different direction,with a different period and wavelength. As these two (or more) swells interact, the phenomenon of wave interference occurs. Thus as two wave crests interact, a higher wave will result. Similarly, as two troughs interact, a deeper trough is generated. Also a wave crest and wave trough can interact to cancel each other, or interact destructively.
As swells appoach shallower water, they slow down and bend to become more nearly parallel to the shoreline. This is called refraction. It can cause wave energy to be focused on some areas, such as points of land, and cause wave energy to be lessened in other areas. Also, as waves refract and bend around small islands or shallow areas, they may interact on the side away from the swell direction, and may cause large waves to develop there.
The combination of these factors, dispersion, wave interference, and refraction, lead to complex patterns in which wave height in California can be quite variable from area to area. However, by measuring the direction, period and height of a swell or swells on offshore buoys or platforms, and by using information about the shape of the seafloor, scientists can predict wave height in Southern and Central California. Such work is being performed by the Coastal Data Information Program..