Overlapping boundary layers in coastal oceans

Oceanic turbulence encompasses a variety of fluid dynamical processes, which control the turbulent mixing and determine the fate of nutrients in marine environment. Most of previous studies on boundary-layer flows were focused on either the upper mixed layer or the bottom boundary layer in deep ocean, or the Langmuir supercells in shallow-water regions where the boundary layer flow extends from the surface to the bottom floor. Here, we delve into the turbulent flow in the intermediate-depth coastal ocean using a fine-scale large eddy simulation method. The model consists of two distinct boundary layer regions driven by combined effects of a constant wind stress and a mean current, i.e. a surface boundary layer and a bottom boundary layer, assuming no heat flux exchange between the ocean surface and the overlying atmosphere. The flow is modelled by solving the grid-filtered Craik-Leibovich equations. The rotational effect, which has been neglected in previous numerical studies of Langmuir supercells, is included to better represent the real ocean flow. Simulations under different oceanic conditions are carried out to characterize the turbulent entrainment in the coastal oceanic boundary layer. The oceanic flow evolves through three stages: (1) a rapid deepening, (2) a slow entrainment, and (3) a prompt merger. Before the merger, the interior stratification suppresses the vertical mixing and inhibits the deepening of the two boundary layers. Once the stratification disappears, the two layers coalesce that leads to drastic enhancement of momentum transfer and turbulence intensity over the entire water column as compared to the scenario before the merger.