Implementation of additional spectral wave field exchanges in a three-dimensional wave–current coupled WAVEWATCH-III (version 6.07) and CROCO (version 1.2) configuration: assessment of their implications for macro-tidal coastal hydrodynamics

Abstract. An advanced coupling between a three-dimensional ocean circulation model (CROCO) and a spectral wave model (WAVEWATCH-III) is presented to better represent the interactions of macro-tidal currents with winds and waves. In the previous implementation of the coupled interface between these two models, some of the wave-induced terms in the ocean dynamic equations were computed from their monochromatic approximations (e.g. Stokes drift, Bernoulli head, near-bottom wave orbital velocity, wave-to-ocean energy flux). In the present study, the exchanges of these fields computed from the spectral wave model are implemented and evaluated. A set of numerical experiments for a coastal configuration of the macro-tidal circulation off the Bay of Somme (France) is designed. The impact of the spectral versus monochromatic computation of wave-induced terms has a notable effect on the macro-tidal hydrodynamics, particularly in scenarios involving storm waves and opposing winds to tidal flows. This effect manifests as a reduction in the wave-induced deceleration of the vertical profile of tidal currents. The new implementation provides current magnitudes closer to measurements than those predicted using monochromatic formulations, particularly at the free surface. The spectral-surface Stokes drift and the near-bottom wave orbital velocity are found to be the spectral fields with the most impact, respectively increasing advection towards the free surface and shifting the profile close to the seabed. In the particular case of the Bay of Somme, the approximation of these spectral terms with their monochromatic counterparts ultimately results in an underestimation of ocean surface currents. Our model developments thus provide a better description of the competing effects of tides, winds, and waves on the circulation off macro-tidal bays, with implications for the study of air–sea interactions and sediment transport processes.