Determinig the Effects of Stokes Drift on the Movement of Oil in the Gulf of Mexico

The impact of Stokes drift, a wave-driven mechanism of mass transport, is investigated for surface oil movement in the Gulf of Mexico. Stokes drift was neglected in trajectory forecasts in the Gulf during the Deepwater Horizon oil spill. The key considerations used in Gulf of Mexico trajectory forecasts were surface currents and wind drift. This study presents a physical argument for the importance of Stokes drift and questions the significance of wind drift over an oil slick. Furthermore, the magnitude and direction of the wind drift (2–5% of the wind speed and 20° to the right in the Northern Hemisphere) is very similar to that of the Stokes drift. For this reason, the differences between Stokes drift and the wind drift are examined using a vector comparison. The directional components of Stokes drift and the wind drift are found to be very similar although the magnitudes of the wind drift are found to be larger. When swell not associated with the local wind is present, however, the two drifts have significantly different directional components. Horizontal surface trajectories are computed for different atmospheric and oceanic conditions. Trajectory results are compared to satellite-derived oil locations using a center of mass comparison method. Analysis of trajectory forecasts and observed oil locations suggests that Stokes drift might play an important role in the movement of oil at the surface and that the magnitude of the wind drift may not be as large as most models presume. Key