Overlapping boundary layers in coastal oceans

TL;DR

This study investigates the merging process of surface and bottom boundary layers in coastal oceans through numerical simulations, revealing how internal waves and Langmuir turbulence influence turbulence and mixing.

Contribution

It introduces a detailed numerical analysis of boundary layer merging in coastal waters, highlighting the role of internal waves and Langmuir circulations in turbulence enhancement.

Findings

Boundary layers undergo three phases: deepening, oscillatory equilibrium, and merger.

Internal waves are excited by Langmuir turbulence, modulating bottom turbulence.

Post-merger, Langmuir circulations extend throughout the water column, increasing turbulence.

Abstract

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.