Modeling the internal tide in combination with wind-driven circulation on the Oregon shelf

Wind-driven alongshore currents and summer upwelling of cold waters are the dominant patterns of coastal circulation on the Oregon shelf, varying on scales of a few days to several months. This subinertial circulation can be modulated and affected by high-frequency barotropic and internal tides, particularly at the M₂ and K₁ frequencies. Using a 1-km horizontal resolution model based on the Regional Ocean Modeling System (ROMS), wind- and tidally-driven flows are studied in combination. The study period is May-August 2002, when data from the GLOBEC field program are available for model verification. Realistic time- and space-varying atmospheric forcing is provided by COAMPS and NCEP. Open boundary conditions are a combination of a solution from a larger scale, 3-km resolution ROMS model (run without tidal forcing) and barotropic tides from a data-assimilating shallow-water model. Modeled subtidal and tidal variability on the shelf are in good agreement with time-series of mooring velocity observations. The solutions reveal "hot spots" of the M₂ internal tide generation over the slope. These hot spots are generally found over regions of supercritical bathymetry, where the bottom slope is steeper than the slope of internal wave characteristics. Topographic energy conversion in these hot spots is well balanced by baroclinic energy flux divergence. M₂ baroclinic energy flux propagation on the shelf (across the 200-m isobath) is affected by the slope characteristic over the slope (supercritical or subcritical). Despite temporal and spatial variability in the baroclinic tidal energy fluxes on the slope and shelf, the integrated energy flux onto the shelf does not vary much in time. Experiments with bathymetry resolution show that the area-integrated generation of the internal tide energy on the slope is more affected than the integrated baroclinic tidal energy flux from the slope to the shelf. The model results show that not only the M₂, but also the K₁ tidal constituent can contribute to the surface current variability. In particular, the K₁ barotropic tide is intensified over the broadest portion of the Oregon shelf, consistent with the high-frequency radar observations of surface velocities and predictions of resonant zones from the analysis of shallow water equations. Our model reveals the non-trivial three-dimensional structure of the K₁ tide in this area.