UC San Diego

Coastal and maritime communities rely on wave models for adaptation and management of coastal hazards, maritime safety, and infrastructure planning. Accurate representation of ocean waves and their subsequent impact on air-sea fluxes is also important for numerical weather and climate prediction models. Ocean currents can have significant impact on accurate prediction of wave conditions as they modify wave direction, frequency, and amplitude. Resolving these effects is complicated in coastal regions, where ocean currents vary spatially and temporally across many scales. Thus, co-located measurements of ocean wave and current conditions are essential for better understanding and predicting surface wave conditions in nearshore environments. This thesis explores the variability of surface waves due to wave-current-tide interactions in coastal wave climates. Observations from a global network of over 200 moored wave buoys show that 87% of wave height records have significant inertial, diurnal, or semidiurnal variability. Diurnal variability in wave records is observed at high frequencies, related to wind forcing, and varies with latitude and distance from the coastline. Inertial variability has a strong seasonal cycle and is observed predominately in the Pacific Northwest. Finally, we find the strongest semidiurnal modulations in channels, on the East Coast, and in regions known for strong barotropic and baroclinic tides. These regional and seasonal patterns demonstrate how surface wave climate variability is influenced by distinct oceanographic phenomena. Ocean currents impact wave fields through both local effects from currents and water depth changes and non-local refractive effects from transverse gradients in currents and bathymetry. We demonstrate how the different mechanisms for wave-current interaction can lead to similar periodic modulations to ocean wave heights. Several examples of these modulations, influenced by tides and inertial currents, are presented. To characterize the exact influence of wave-tide interaction on the surface wave field measured by these buoys, we consider a case study at Fernandina Beach, FL where wave heights are modulated up to 25% over a tidal cycle. Observations show that wave heights increase when traveling with tidal currents and decrease when opposing them, with the most significant effects occurring in the long-period swell band. We use both analytical and numerical solutions to model these interactions, employing principles of geometrical optics and conservation of wave action. We consider the tide as a one-dimensional progressive shallow-water wave and find that the numerical model accurately reproduces the observed tidal modulation to wave amplitude and phase. Our model shows that the environmental conditions, wave conditions, and the relative speeds between the tide and surface waves determine the magnitude and direction of these modulations.