Abstract
Symmetry considerations indicate that in steady-state conditions, and in the absence of a sea current, a horizontal interface must move in the direction of the surface geostrophic wind. With currents present, the interface velocity is the vector mean of the surface geostrophic wind and the current velocity, weighted respectively by the fluid masses per unit surface area which are affected by the viscous diffusion of vorticity upward and downward from the interface. In turbulent flow with neutral stratification, the ratio of these masses equals the square root of the density ratio. In transient conditions, the surface drift velocity has components in the direction of the isallobaric gradient and down a steeping surface slope. Wave action is responsible mainly for the spreading and the breakup of surface films.
