Direct Observations of Submesoscale Modulation of Ocean Surface Boundary Layer Turbulence

There is substantial theoretical and numerical evidence of tight coupling between the small-scale ocean surface boundary layer (OSBL) turbulence and submesoscale coherent structures. Turbulent mixing of momentum produces ageostrophic secondary circulation that fluctuates with the OSBL turbulence levels and alters the submesoscale frontal structure (as described in the transient turbulent thermal wind [T³W] framework). At the same time, a feedback due to the modulation of OSBL turbulence by frontal shear and stratification must also exist. Observations of these relationships remain challenging due to the wide scope of relevant scales and essentially non-equilibrium nature of such interactions.

Neutrally buoyant Lagrangian Floats equipped with high-resolution Acoustic Doppler Current Profilers (ADCPs) present a new way to conduct sustained multi-scale observations of multiscale boundary layer dynamics. While the floats accurately follow the large energy-containing boundary layer overturns, on-board ADCPs resolve both the driving shear structure and the centimeter-scale turbulent velocity fluctuations. High fidelity pulse-coherent Doppler processing allows visualization of evolution and structure of turbulent patches in response to changing forcing.

In a recent deployment in the Mediterranean Sea, a pair of floats documented modulation of small-scale turbulence associated with a subducting submesoscale front. The float observations present a Lagrangian view of a fluid parcel subducting from the fully turbulent mixed layer into the stratified transition layer underneath. The observational record shows pronounced suppression of OSBL turbulence underneath the dense side of the tilted submesoscale front where the actual subduction occurs. This localized shutdown of mixed-layer turbulence may allow thickening of the underlying transition layer, thus creating favorable conditions for vertical exchange between the mixed layer and the upper pycnocline.