  Ocean submesoscales, flows with characteristic size around 10m--10km, are
transitional between the larger, rotationally-constrained mesoscale and
three-dimensional turbulence. In this paper we present simulations of a
submesoscale ocean filament. The strong vertical transport associated with
submesoscale fronts and filaments ventilates the deeper water and provides a
route for the observed forward cascade of energy in the ocean. In our case, the
filament is strongly sheared in both vertical and cross-filament directions and
is unstable. Instability indeed dominates the early behaviour with a fast
extraction of kinetic energy from the vertically sheared thermal wind. The
filament is expected to be subject to instabilities, including symmetric and
Kelvin-Helmholtz instabilities. An instability emerges with a circulation that
does not match either, though symmetric modes become apparent later. The action
of the instability is sufficiently rapid that the filament does not respond in
a geostrophically balanced sense. Instead, it later exhibits vertically sheared
near-inertial oscillations with higher amplitude as the initial minimum
Richardson number decreases. Horizontal gradients strengthen only briefly as
the fronts restratify. These unstable filaments can be generated by strong
mixing events at pre-existing stable structures. The oscillations must also be
considered when designing initial conditions for numerical experiments of
submesoscale flows, where the initial state is intended to be balanced.
