Past studies separately demonstrate that vertical boundary layer turbulence can either sharpen or weaken submesoscale fronts in the surface mixed layer.  These studies invoke competing interpretations that separately focus on the impact of either vertical momentum mixing or vertical buoyancy mixing, where the former can favor sharpening (frontogenesis) by generation of an ageostrophic secondary circulation, while the latter can weaken the front (frontolysis) via diffusion or shear dispersion.  No study comprehensively demonstrates vertical mixing induced frontogenesis and frontolysis in a common framework.  Here, we develop a unified paradigm for this problem with idealized simulations that explore how a front initially in geostrophic balance responds to a fixed vertical mixing profile.  We evolve 2D fronts with the hydrostatic, primitive equations over a range of Ekman (Ek = 10^{-4} - 10^{-1}) and Rossby numbers (Ro = 0.25 - 2), where Ek quantifies the magnitude of vertical mixing and Ro quantifies the initial frontal strength.  We observe vertical momentum mixing induced, nonlinear frontogenesis at large Ro and small Ek and inhibition of frontogenesis via vertical buoyancy diffusion at small Ro and large Ek .  Symmetric instability can dominate frontogenesis at very small Ek; however, the fixed mixing limits interpretation of this regime.  Simulations that suppress vertical buoyancy mixing are remarkably frontogenetic, even at large Ek, explicitly demonstrating that buoyancy mixing is frontolytic.  We identify a controlling parameter (Ro^2 / Ek) that quantifies the competition between cross-front buoyancy advection and vertical diffusion.  This parameter approximately maps the transition from frontolysis to frontogenesis across simulations with active buoyancy and momentum mixing.

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.

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.