Surface gravity waves play a major role in the exchange of momentum, heat, energy, and gases between the ocean and the atmosphere. Strong winds blowing over long fetches give rise to long-period waves, known as swell, that can propagate great distances from their source; hence, the surface wave field in a given region results from the combined response to both local and remote wind forcing. Surface winds off the California coast are marked by strong seasonality and regional scale variability associated with the coastal orography. As a consequence, a particular aspect of the surface wave variability in this region is the influence of these regional-scale high wind events that occur during spring and summer. These alongshore “expansion fan” winds have average speeds of ∼10 m/s and are the dominant forcing for waves off central/northern California, leading to relatively short period waves (8-10 s) that come predominantly from the north–northwest.

Waves are also modulated by ocean currents via wave–current interactions, which lead to variations in their direction, frequency, and amplitude. The surface current field in the California Current system (CCS) region is mostly dominated by balanced (rotational) motions in late winter/spring, while divergence is stronger in late summer/fall. Here, we propose a theoretical framework based on ray theory to assess the effects of current divergence and vorticity in the diffusion of wave action density. We show that the potential (divergent) component of the flow has no contribution to the diffusion of wave action.

In a separate study, we analyze a large ensemble of numerical experiments using the wave model WAVEWATCH III forced with idealized currents to investigate the role of divergent and rotational flows in modifying wave properties, including direction, period, directional spreading, and significant wave height (Hs). Finally, the results obtained using idealized currents are used to interpret the response of surface waves to realistic currents by running an additional set of simulations using the llc4320 MITgcm output in the CCS region.