Abstract

Stokes drift associated with surface waves induces mass transport, which interacts with ocean currents to modify dynamics from situations without waves. The effect of Stokes drift can be parameterized through “Stokes forcing” terms derived from wave–current interaction theory, and an increasing number of regional and global ocean models incorporate these terms to represent wave effects on ocean currents. While the mixed layer scale currents’ response to Stokes drift has been actively studied, the dynamical understanding of large-scale responses, which is relevant to resolved currents in regional and global ocean models, remains limited. Here, we provide novel dynamical insights into the role of Stokes forcings with a view toward applications in coastal ocean modeling. A theoretical analysis of the linearized wave-averaged equation provides the following view: The inhomogeneity of the Stokes transport and corresponding local inertial adjustment induce horizontal convergence and divergence in the surface layer, which force the geostrophic flow through effective Ekman pumping. This interpretation is illustrated with idealized numerical simulations. When localized Stokes drift is imposed, the Lagrangian transport field exhibits a dipole circulation pattern on either side of the Stokes forcing. In the presence of a bottom slope, the transient wave forcing produces an irreversible change in the geostrophic current field through the generation of topographic Rossby waves. A simulation with realistic topography and wave forcing was also conducted. Forced only by Stokes transport, Lagrangian transport of O(1) m² s^-1 was generated and remained active for several days after a cyclone event due to the process described above. In the Pacific coastal region near Japan, it was estimated that the forcing through the Stokes transport can modify the wind stress forcing typically by 1-12 %.