Effects of small-scale, high-frequency ocean variability on surface material transport in the coastal ocean

Wind-driven coastal currents and summer upwelling of cold waters are the dominant patterns of variability along the Oregon coast, occurring on the temporal scales of several days to months. This variability is modulated, and potentially affected, by high-frequency processes associated with three-dimensional, baroclinic tidal flows (internal tides). To study the effect of internal tides on surface ocean material transport, the 1-km resolution model has been developed, based on the Regional Ocean Modeling System (ROMS), forced by realistic winds and up to 8 dominant tidal constituents. Lagrangian particle trajectories have been computed and analyzed using model current flow fields. Cases driven by wind only (WO), wind and the M2 semidiurnal tide in combination (M2), and wind and 8 tidal constituents (TW) are compared. Dispersion is computed for these cases using ensembles of particles. Based on these analyses, the surface tidal motions contribute to the dispersion of particles on the surface. The relative dispersion 1 day after release, averaged over continental shelf and slope, can be 70% greater in case TW than in case WO. In particular, dispersion is large over a wider part of the Oregon shelf where the diurnal K1 tidal component is known to be resonant. Lagrangian statistics have also been computed using 40-hour low-pass filtered surface TW fields (case TW-lp). Despite our expectation that this case would be close to WO, we find that the cross-shore average relative dispersion can be 40% greater in case TW-lp, which suggest the importance of the nonlinear effect of the surface horizontal tidal motions on subtidal circulation, e.g., via the Reynolds stress divergence term.