Non-negligible impact of Stokes drift and wave-driven Eulerian currents on simulated surface particle dispersal in the Mediterranean Sea

Abstract. Numerical simulations of marine surface particle dispersal are a crucial tool for addressing many outstanding issues in physical oceanography with societal relevance, such as marine plastic pollution. However, the quality of these Lagrangian simulations depends on the ability of the underlying numerical model to represent the prevailing ocean circulation features. Here, we investigate how simulated marine surface particle dispersal changes, if the – often omitted or only approximated – impact of wind-generated surface waves on the upper ocean circulation is considered. We use velocity fields from a high-resolution coupled ocean-wave model simulation and a complementary stand-alone ocean model simulation for the Mediterranean Sea to answer the following questions: 1) How does the explicit representation of waves impact the simulated surface particle dispersal, and what is the relative impact of Stokes drift and wave-driven Eulerian currents? 2) How accurately can the wave impact be approximated by the commonly applied approach to advect particles with non wave-driven Eulerian currents and Stokes drift from stand-alone ocean and wave models? We find that the representation of surface waves tends to increase the simulated mean Lagrangian surface drift speed in winter through a dominant impact of Stokes drift, and to decrease the mean Lagrangian surface drift speed in summer through a dominant impact by wave-driven Eulerian currents. Furthermore, simulations that approximate the surface wave impact by including the Stokes drift (but ignoring the wave-driven Eulerian currents) do not necessarily yield a better estimate of the surface particle dispersal patterns with explicit representation of the wave impact than simulations that do not include any surface wave impact. Our results imply that – whenever possible – velocity fields from a coupled ocean-wave model should be used for surface particle dispersal simulations.