Mechanisms Driving Enhanced Particle Export During a Submesoscale Re-stratification Event: Insights from Theory and Large Eddy Simulations

We investigate the mechanisms through which 1-10 km submesoscales and vertical mixing combine to influence the export of sinking particles from the mixed layer. Small-scale turbulence in the ocean mixed layer can act to re-suspend sinking particles, thereby decreasing the export rate. Submesoscales drive strong vertical circulation which can rapidly flush particles out of the mixed layer, while submesocale re-stratification suppresses small-scale turbulence and mixing.

We use large-eddy simulations to study the influence of small-scale turbulence and submesoscales on the rate and patterns of export of dense material from the upper ocean. Small-scale turbulence is forced by cooling the ocean surface, generating turbulent convection, while a horizontal density gradient allows submesoscales to develop. Sinking particles are modeled using passive tracers that move vertically relative to the fluid velocity with a constant 'slip' velocity. Several tracers are considered with sinking rates ranging from 10-100 m/day, covering a range of realistic values. The included figure shows the concentration of particles with a sinking speed of 100 meters / day (left) and vertical velocity (right) from the LES with active small-scale turbulence and a fully developed cyclonic submesoscale eddy.

We find that submesoscales can enhance particle export via two distinct mechanisms. First, strong subduction along submesoscale fronts transports particles into the thermocline where they settle gravitationally. Second, re-stratification induced by the submesoscales suppresses small-scale turbulence in the mixed layer. This inhibits the re-suspension of sinking particles into the mixed layer and increases the export flux. Finally, submesoscales leave behind an imprint in the particle concentration well below the mixed layer base.

To explain these results, we will introduce a simple and general theory which shows that the export rate can be significantly enhanced when the vertical mixing rate falls below a critical threshold which depends on various parameters including the mixed layer depth and the gravitational settling velocity. When applied to the LES, the theory accurately captures the increase in export flux. Broader implications of the theory will also be discussed.