The behavior of the near ocean surface under the combined action of waves and currents in shallow water

Drift current near the sea surface has an important effect on momentum, heat, and mass transfers across the air-sea interface, and consequently, influences physical, chemical, and biological processes within the upper ocean and the atmospheric surface layer. These currents also govern dispersion of oil slicks, chemical pollutants and other man-made discharges near the sea surface. Accurate estimates of sea surface drift currents are critical to forecasting slick transport and fate. The majority of existing drift models employ a drift factor and deflection angle, based on local wind speed, to estimate the sea surface drift vector. In addition, the models are restricted to deep water regions. The effects of wind induced shear and wave induced transport are lumped together in these formulations. In the present study the conservation of momentum, water mass, and turbulent energy equations are solved using an implicit finite difference method to predict the vertical distribution of current, turbulent energy, and eddy viscosity at one point. The model includes coupling between the wave and shear induced currents and is valid for both deep and shallow water. Input energy from the atmosphere to the turbulent energy and the current fields are represented through free surface boundary conditions. The numerical model showed excellent agreement to analytic solutions of the wind forced shear flow problems. The model was applied to predict surface drift currents for various wind speeds and gave results in general agreement with field observations and other numerical/theoretical studies. In deep water the model predicted drift factor and deflection angle decreased with increasing wind speed. In shallow water the model predicted drift factor and deflection angle were shown to be strongly affected by the wave-current interaction. The drift factor increased with increasing wind speed and the deflection angle decreased with increasing wind speed. The model was applied to three intentional oil spills which occurred in the Norwegian shelf in 1991. Comparisons are made between the modeled and observed trajectories of the oil spills. The model reproduces the observed trajectories with reasonable accuracy