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.

Submesoscale eddies (those smaller than 50~km) are ubiquitous throughout the ocean, as revealed by satellite infrared images. Diagnosing their impact on ocean energetics from observations remains a challenge. This study analyzes a turbulent field of submesoscale eddies using airborne observations of surface currents and sea surface temperature, with high spatial resolution, collected during the S-MODE experiment in October 2022. Assuming surface current divergence and temperature are homogeneous down to 30 m depth, we show that more than 80% of the upward vertical heat fluxes, reaching ~227 W~m^{-2}, is explained by the smallest resolved eddies, with a size smaller than 15 km. This result emphasizes the contribution of small-scale eddies, poorly represented in numerical models, to the ocean heat budget and, therefore, to the climate system

Realistic simulations of Central California reveal interactions between shoaling internal tidal bores and submesoscale currents on the inner-shelf (30-60 m depth). These interactions comprise collisions between internal tidal upwelling, ‘forward’ bores (FBs) with submesoscale currents (SMCs) in the form of surface layer density fronts or filaments with downwelling secondary circulation. Along-shore oriented FBs collide with both cross-shore (perpendicular interaction) or along-shore (parallel interaction) oriented SMCs. In perpendicular interaction, FBs colliding into cross-shore oriented SMCs refract around the offshore tip of the downwelling front or filament. SMCs generally survive perpendicular interaction, despite partial disruption of downwelling secondary circulation by FBs. An example of parallel interaction demonstrates (1) blocking of FB propagation by elevated mixing and dense filament formation on the inner-shelf and (2) the subsequent destruction of the dense filament coincident with a decrease in vertical mixing and FB propagation underneath it. For both perpendicular and parallel interaction, FB propagation is modulated by a varying medium introduced by SMC density and current structure. The computational evidence of these interactions corroborates recent observations of interactions between small-scale, nearshore currents in the real ocean. This study motivates further exploration of interactions between fronts, filaments, internal tidal bores, and vortices in the nearshore.