  Boundary layer turbulence in coastal regions differs from that in deep ocean
because of bottom interactions. In this paper, we focus on the merging of
surface and bottom boundary layers in a finite-depth coastal ocean by
numerically solving the wave-averaged equations using a large eddy simulation
method. The ocean fluid is driven by combined effects of wind stress, surface
wave, and a steady current in the presence of stable vertical stratification.
The resulting flow consists of two overlapping boundary layers, i.e. surface
and bottom boundary layers, separated by an interior stratification. The
overlapping boundary layers evolve through three phases, i.e. a rapid
deepening, an oscillatory equilibrium and a prompt merger, separated by two
transitions. Before the merger, internal waves are observed in the stratified
layer, and they are excited mainly by Langmuir turbulence in the surface
boundary layer. These waves induce a clear modulation on the bottom-generated
turbulence, facilitating the interaction between the surface and bottom
boundary layers. After the merger, the Langmuir circulations originally
confined to the surface layer are found to grow in size and extend down to the
sea bottom (even though the surface waves do not feel the bottom), reminiscent
of the well-organized Langmuir supercells. These full-depth Langmuir
circulations promote the vertical mixing and enhance the bottom shear, leading
to a significant enhancement of turbulence levels in the vertical column.
