Lagrangian dispersion in realistic surface mixed layers

In the upper ocean, the surface mixed layer is rich in submesoscale flows characterized by large vertical velocities and significant vertical transport. In addition, the vertical flux is also modulated by a variety of smaller-scale features, with dynamics approaching three-dimensional turbulence. Surface gravity waves significantly influence the submesoscale regime, particularly through the formation of Langmuir circulations, which are a direct outcome of wave-current interactions. However, current models often parameterize these effects, leaving their precise impact on vertical transport unclear. This study addresses this gap by investigating the roles of wave-modulated submesoscale structures, parameterized turbulent mixing, and Langmuir circulations on Lagrangian particle movement, utilizing high-resolution ($\Delta x \lessapprox$ $100$ m) realistic ocean simulations able to resolve this smaller-scale dynamics. Our high resolution ($\Delta x = 30$ m) simulations reveal that Langmuir circulations dominate the vertical transport with their strong vertical velocities. This wave-induced vertical fluxes significantly affect Lagrangian particle movement, increasing their vertical displacement and Lagrangian relative horizontal diffusivity. These effects occur alongside downwelling from submesoscale features, suggesting that Langmuir circulations are integral in transporting biological and ecological materials vertically and horizontally in the ocean, while Stokes drift, another product of the wave-current interactions, have a lesser role in the particle stirring in this open-ocean simulation. This study also suggests that sub-grid-scale parameterization via diffusion may be limited when trying to reproduce the effects of ephemeral and heterogeneous small scale flows included in high-resolution Eulerian flows.