On the modulation of wave-current interactions by horizontal mixing and spatial resolution

The mechanics of rip currents are complex, involving interactions between waves, currents, water levels and bathymetry that pose particular challenges for numerical models. Horizontal turbulent diffusion in a rip system is difficult to measure using dye dilution or surfzone drifters, as shown by the range of published values for the horizontal diffusion coeffcient. Here, we study the effects of horizontal mixing on wave-current interactions by testing several diffusivity estimates in a fully coupled 3D wave-current model run at two different spatial resolution. Published results using very low diffusion have found near the shore the wave rays converge towards the rip channel because of refraction by the currents. We show that this process depends on the alongshore gradient of the rip current and that this gradient is modulated by both horizontal mixing and spatial resolution. We find that without the feedback of currents on waves, the flow is more sensitive to horizontal mixing with large alterations especially offshore and generally lower velocities. These modifications ascribed to mixing are similar to those induced by the feedback mechanism. When a large mixing coeffcient is used, we observe that: i) the behavior of the rip system is similar for both coupling modes (i.e. with and without the feedback of currents on waves) and for each resolution, ii) the evolution of the flow is more stable over time. Lastly, we show that the horizontal mixing strongly decreases the intensity of the 3D rip velocity, but not its vertical shear that is strongly dependent on the vertical mixing scheme and on the forcing terms.