Overlapping Boundary Layers in Coastal Oceans
- 1 April 2022
- journal article
- research article
- Published by American Meteorological Society in Journal of Physical Oceanography
- Vol. 52 (4), 627-646
- https://doi.org/10.1175/jpo-d-21-0067.1
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. 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.Keywords
Funding Information
- Advanced Research Projects Agency (DE-AR0000920)
This publication has 62 references indexed in Scilit:
- Dynamics of Winds and Currents Coupled to Surface WavesAnnual Review of Fluid Mechanics, 2010
- Large eddy simulation of a stratified boundary layer under an oscillatory currentJournal of Fluid Mechanics, 2009
- Stratification Effects in a Bottom Ekman LayerJournal of Physical Oceanography, 2008
- Internal gravity waves generated by a turbulent bottom Ekman layerJournal of Fluid Mechanics, 2007
- Structure of the Entrainment Zone Capping the Convective Atmospheric Boundary LayerJournal of the Atmospheric Sciences, 1998
- An ocean large‐eddy simulation of Langmuir circulations and convection in the surface mixed layerJournal of Geophysical Research: Oceans, 1995
- Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterizationReviews of Geophysics, 1994
- THE FORM AND DYNAMICS OF LANGMUIR CIRCULATIONSAnnual Review of Fluid Mechanics, 1983
- Internal Waves in the OceanAnnual Review of Fluid Mechanics, 1979
- On the penetration of a turbulent layer into stratified fluidJournal of Fluid Mechanics, 1969