Three-dimensional Lagrangian transport in turbulence-resolving simulations of a submesoscale front

Three-dimensional transport at an upper-ocean front is examined by tracking an array of tracer particles in a large-eddy simulation (LES) model. The high-resolution LES has grid spacing of 2 m (horizontal) and 0.5-1.4 m (vertical). The front, which goes through symmetric instability followed by baroclinic instability, spawns both coherent structures and fine-scale turbulence. We find that a low-pass filter is able to separate the coherent structures in physical space from turbulence. The coherent structures consist of thin O(100) m vortex filaments and submesoscale eddies with vertical velocity as large as 5 mm/s, and the turbulence is spatially organized by these coherent structures. The vortex filaments and eddies provide direct pathways for vertical transport. The filament on the heavier side downwells the local (heavier) fluid to the bottom, spiraling around the eddy, while the filament at the lighter side upwells the local (lighter) fluid particles to the surface. The fluid particles near the bottom also upwell through the core of the eddy. Subsequently, the downwelled and upwelled particles slowly adjust to a smooth stable profile. The statistics of particle dispersion are also studied using single-particle, two-particle and multi-particle measures. Two-particle dispersion reveals a t³ regime as the particles are drawn into the coherent structures. Multi-particle dispersion statistics using tetrads of particles shows straining of the volumes into needle-like objects. The mixing properties experienced by the fluid particles, which are transported vertically across the mixed layer and laterally across the front, are examined and contrasted between upwelling and downwelling motions.