Waves provide nearly all the energy that drives physical processes along the California coast, and the occurrence of high waves relative to tidal extremes produces coastal flooding as well as beach and sea cliff erosion. Surface gravity waves are generated by wind blowing over the ocean and are characterized by their height, period, and direction of propagation. These wave parameters are jointly characterized by the wave "frequency-directional spectrum," which is the distribution of wave energy (proportional to height squared) as a function of wave frequency (the inverse of period) and direction. The wave spectrum of deep-water ocean waves depends in turn on the strength of the generating wind field (wind speed), the size of the area the wind is blowing over (fetch), and how long sustained wind speeds persist (duration). Big storms with persistent strong winds that blow over large ocean areas for several days generate high waves with long periods (i.e., low frequencies). Analysis of NOAA buoy wave spectra over the eastern North Pacific indicates that wave energy has been increasing since about 1980, with a general northward shift in storm track [Bromirski et al., 2005].
Changes in the wave climate across the North Pacific can be characterized by the distribution and trends in significant wave height (Hs, the average of the highest 1/3 of the waves). Because storm impacts on coastal processes depend in part on peak wave period, Tp, as well as Hs, a more complete descriptor of storm strength is wave power, PW, which incorporates both Hs and Tp, and provides an integral of storm event wave intensity. For example, changing hurricane intensity in the western North Atlantic has been associated with PW variability [Bromirski and Kosin, 2008]. Tp depends on both the duration and area (fetch) that persistent winds blow over, as well as wind speed, and is an important factor in the magnitude of coastal impacts.
Longer Tp waves produce higher wave runup (the vertical height waves travel up beach slopes), which increases flooding potential [Bromirski et al., 2012]. Larger, more intense extra-tropical cyclones (ETCs) generate longer period waves. Because coastal inundation depends on Hs and Tp, as well as timing relative to high tide, the distribution and trends of extreme PW events are important considerations in adapting to and mitigating impacts from changing storm characteristics and patterns as a result of global warming.
Wave power during extreme events, identified by Hs exceeding the 90th percentile level using wave model data since 1948, shows upward trends along the Central California coast (Figure, dots signify statistically significant regions). If this trend continues, coastal impacts will be exacerbated if projected sea level rise occurs.