  The three-dimensional transport pathways, the time scales of vertical
transport, and the dispersion characteristics of submesoscale currents at an
upper-ocean front are investigated using material points (tracer particles)
that advect with the local fluid velocity. Coherent submesoscale vortex
filaments and eddies which dominate submesoscale (0.1 - 10 km) dynamics are
found to play a crucial role which is quantified here. These coherent
structures are generated and sustained through nonlinear evolution of
baroclinic instability. The collective motion of particles helps identify
common features of transport at the front. It is found that the particles in
the central region organize into inclined lobes, each associated with an eddy,
and the filaments associated with the heavy- and light-edges of the front
transfer edge particles to the lobes. This flux of new particles into the lobe
causes local particles to adjust, which leads to slumping of the front. The
particle motion in the vertical shows multiple time scales -- a fast time scale
with O(10) m vertical displacement within an hour and a slower near-inertial
time scale, comparable to the intrinsic time scale of the growing instability.
The fast time scale motions typically occur in the vortex filaments. The
overall slumping process is slower than what one might anticipate from the
large magnitude of vertical velocity in the filaments and requires a sustained
correlation over time between the lateral and the vertical motion. By tracking
clouds of particles, we show that their centers of mass downwell/upwell over
1-2 inertial time periods, after which an adjustment follows with a
sub-inertial time scale. The dispersion characteristics of the submesoscale
turbulent currents using single- and pair-particle statistics have been
investigated. The shape change in clusters of four particles reveals
deformation into thin, needle-like structures.
