Routes to Turbulence Triggered by Inertial Oscillations at Ocean Fronts

Ocean fronts are often associated with enhanced levels of turbulent kinetic energy and dissipation, but the sources for this small-scale energy are not fully understood. Here, we will describe recent efforts to understand the dynamical coupling between fronts and wind-driven inertial oscillations using linear stability theory and numerical simulations. Two important results have emerged from this work. First, the addition of near-inertial shear can make the front more susceptible to symmetric instability (SI). This instability, catalyzed by the inertial oscillations, draws energy from the thermal wind flow and eventually becomes unstable to a secondary shear instability which transfers energy to small-scale turbulence. Second, a new type of parametric subharmonic instability (PSI) emerges when the inertial oscillations are sufficiently strong and the potential vorticity at the front is positive. Unlike SI, PSI primarily derives its energy from the inertial oscillations. Eventually, PSI also becomes unstable to a secondary shear instability, leading to enhanced small-scale turbulence and dissipation. These mechanisms represent pathways for down-scale energy transfer whereby energy in relatively larger-scales associated with inertial oscillations and the frontal structure can be transferred to small-scale turbulence in the frontal zone.