Wave–current interaction in an oceanic circulation model with a vortex-force formalism: Application to the surf zone

A vortex-force formalism for the interaction of surface gravity waves and currents is implemented in a three-dimensional (3D), terrain-following, hydrostatic, oceanic circulation model (Regional Oceanic Modeling System: ROMS; Shchepetkin and McWilliams, 2005). Eulerian wave-averaged current equations for mass, momentum, and tracers are included in ROMS based on an asymptotic theory by McWilliams et al. (2004) plus non-conservative wave effects due to wave breaking, associated surface roller waves, bottom streaming, and wave-enhanced vertical mixing and bottom drag especially for coastal and nearshore applications. The currents are coupled with a spectrum-peak WKB wave-refraction model that includes the effect of currents on waves, or, alternatively, a spectrum-resolving wave model (e.g., SWAN) is used. The coupled system is applied to the nearshore surf zone during the DUCK94 field measurement campaign. Model results are compared to the observations and effects of parameter choices are investigated with emphasis on simulating and interpreting the vertical profiles for alongshore and cross-shore currents. The model is further compared to another ROMS-based 3D coupled model by Warner et al. (2008) with depth-dependent radiation stresses on a plane beach. In both tests the present model manifests an onshore surface flow and compensating offshore near-bed undertow near the shoreline and around the breaking point. In contrast, the radiation-stress prescription yields significantly weaker vertical shear. The currents’ cross-shore and vertical structure is significantly shaped by the wave effects of near-surface breaker acceleration, vertical component of vortex force, and wave-enhanced pressure force and bottom drag.