Tidal currents on the central Oregon shelf: Models, data, and assimilation

Abstract

Measurements of tidal currents on the central Oregon shelf are available from several sources, including recent high frequency (HF) coastal radar and Acoustic Doppler Profiler (ADP) deployments, and historical current moorings. In this paper we use a generalized inverse (GI) approach to compare these data to, and then assimilate them into, numerical models for the barotropic tides. Harmonic analysis of the data in short time windows using a modified admittance approach reveals that tidal currents on the Oregon shelf are highly variable in time, and can contain significant baroclinic components. Data from the winter months, when waters on the shelf are only weakly stratified, are found to be most nearly barotropic and thus most reasonable for assimilation into the shallow water equations model. The various data sources are used in several different combinations for assimilation and validation. Forcing the prior forward model with normal flow open boundary conditions obtained from a regional barotropic inverse model results in semidiurnal barotropic currents that are consistent (within estimated error limits) with all available data. In contrast, diurnal currents on the shelf are very sensitive to details of the model configuration, and are significantly improved by data assimilation. Very similar solutions result from assimilation of either the HF radar or ADP data sets. The high sensitivity of the diurnal band currents can be understood dynamically in terms of trapped shelf waves. A short (∼85 km long) section of shelf off the central Oregon coast is wide enough to allow first-mode barotropic shelf waves at the subinertial diurnal frequencies. This results in locally resonant large amplitude diurnal tidal currents that are very sensitive to details in the local forcing, and hence quite variable in time.