Comparing the life cycles of a turbulent front and dense filament in the oceanic surface boundary layer

Submesoscale processes, typically shaped by intricate interactions between frontal dynamics and turbulence, have significant impacts on the transport of momentum, heat and biogeochemical tracers in the ocean. This study employs large-eddy simulations to investigate submesoscale frontogenesis and arrest in the ocean surface boundary layer. We compare a single-sided front with a dense filament, which can be viewed as a two-sided front. Both cases exhibit a similar life cycle, including frontogenesis driven by secondary circulation, frontal arrest due to the growth of instability and turbulence, and eventual frontal decay. One major difference is that the filament remains stationary throughout its life cycle, while the front propagates towards the denser side. Another distinction lies in the relative contributions of horizontal and vertical turbulent fluxes. In the filament case, horizontal (cross-front) turbulent flux dominates and effectively counteracts the frontogenetic tendency induced by secondary circulation, leading to frontal arrest. In contrast, both vertical and horizontal turbulent fluxes are crucial for the arrest of the single-sided front. Horizontal shear production is the primary source of turbulence in the filament, associated with the emergence of horizontal coherent eddies and consistent with the characteristics of horizontal shear instability. For the front, the development of horizontal eddies is less pronounced, and vertical shear production plays a more important role. This study reveals the similarities and differences between the dynamics of submesoscale fronts and filaments, as well as the role of turbulence in their evolution, providing insights for improved representation of these processes in ocean models.