Methodology for a regional tidal model evaluation, with application to central California

Observations from disparate observational assets, including tide gauges, moorings, and high-frequency (HF) radars, were used to depict the tidal variability, and to evaluate model tidal simulations, for a region off central California, including the Monterey Bay. For this study, the hydrodynamic model was forced only with tides derived from a large-scale model for the northeast Pacific Ocean. Homogeneous density, and initially horizontally uniform density stratification, cases were considered. The model successfully reproduced tidal sea-surface height variations within the model domain, as determined by comparisons with sea level or bottom pressure measured at six locations. To achieve tidal currents with realistic amplitudes, as determined from HF radar and moored measurements, it was found that barotropic velocity, as well as sea level, from the large-scale regional tidal model must be included in specifying the open-boundary condition. However, even with such forcing, the model with homogeneous density field under-predicted the semidiurnal and diurnal barotropic currents as estimated from depth-averaged currents measured at 11 locations. In the diurnal frequency band, the observed surface and nearshore depth-averaged currents are likely influenced by meteorological forcing, which was not included in the model.The HF radar-measured surface tidal currents, both semidiurnal and diurnal, are consistent from year to year and between the winter season and the entire year. Semidiurnal surface tidal currents derived from year-long HF radar measurements do not resemble either the modeled or measured barotropic current fields. Rather, they exhibit amplitudes and small-scale spatial variability indicative of the presence of internal tides, thus indicating that model-derived barotropic tidal currents cannot be validated over large spatial extents using long time series of HF radar-derived surface currents. With initially horizontally uniform vertical density stratification, the model produced surface currents with spatial variability and amplitude range comparable to what was derived from HF radar surface current measurements, but the point by point comparisons are not impressive for this region of complex topography. Likewise, the subsurface current comparisons, performed at four deepwater locations, show considerable model-data differences. Possible reasons for these disparities include the effects of atmospheric forcing, spatially and temporally varying stratification, remotely generated coastally trapped waves, and remotely generated internal tides.