Turbulent patch in stably stratified flow

In the stably stratied thermocline of the open ocean, turbulence is one of the main phenomenons through which diapycnal transport and mixing of energy, mass, heat, nutrients and chemicals are accomplished. Oceanic turbulence drives vertical transport and mixing against the stabilizing eect of the ambient stratication through a variety of highly episodic processes occurring in mid-water. These intermittent signatures are the so-called stratied turbulent patch; a localized volume of turbulent activity.  A three-dimensional patch of turbulence is formed by an oscillating grid in both fresh water and an index-of-refraction matched stably stratied solution. Turbulence was varied by applying external force on the oscillating grid resulting in change of its frequency motion. Optical measurements including synchronized particle image velocimetry and planar laser induced fluorescence have been performed to capture the life cycle of the patch from its initial growth until it reaches critical size followed by its eventual collapse. The simultaneous capture of density and velocity allows for an assessment of both the buoyancy and turbulent  fluxes. The measurements of the growth, the quasi-steady regime and the collapse of the patch, indicated that the growth of the patch stops at a characteristic time, Nt = 4 for all the measured frequencies. The size of the patch was determined using several indicators, such as buoyancy, buoyancy variance, turbulent kinetic energy and enstrophy. All the indicators provide a similar behavior and self-similar patch size, which is directly proportional to the forcing frequency. The buoyancy flux terms were small, which was consistent with the observation that the patch thickness is approximately constant in the saturation regime. These observations can assist in better parameterizations of oceanic turbulent transport and mixing.