The submesoscale (0.1-10km) ocean is dominated by highly anisotropic features. Fronts - step changes in density - remain balanced by the Coriolis acceleration of a down-front jet. These are created by the advection of larger-scale density structures by a strain flow, such as the flow at the edges of the vortices generated by western boundary currents.

Horizontal gradients in velocities and tracers become larger as they are drawn out by the strain flow, eventually leading to the onset of a new dynamical regime, where geostrophic and hydrostatic balances no longer describe the flow well. The strong vertical velocities that result provide a route for transport of tracers and momentum across the surface mixed layer and into deeper water, facilitating the observed forward cascade of energy in the ocean.

Here we study the evolution of a submesoscale filament, a pair of opposing fronts, and report on an energy pathway that begins with instability. The filament is unstable to symmetric instability, an exclusively submesoscale fluid instability that extracts kinetic energy from the vertically sheared jet at a front. The action of the instability on the filament removes velocity quickly, putting it out of geostrophic balance and kicking off a near-inertial oscillation within the filament. The oscillation is initially frontogenic at the surface, but this frontogenesis is not arrested by turbulence, the oscillation simply continues.

These unstable features have been observed in the ocean after strong wind mixing events, but the results here also serve as a caution for computational studies of filaments at this scale. Vertical mixing will generate surface-frontogenic secondary circulations in a dense filament; but instabilities will also contribute and the result may not generalise to all ocean features.