scholarly journals Nongeostrophic Baroclinic Instability over Sloping Bathymetry: Buoyant Flow Regime

2020 ◽  
Vol 50 (7) ◽  
pp. 1937-1956
Author(s):  
Lixin Qu ◽  
Robert Hetland
Keyword(s):  
Growth Rate ◽  
Flow Regime ◽  
Boundary Layers ◽  
Baroclinic Instability ◽  
Deep Ocean ◽  
Ocean Bottom ◽  
Coastal Zones ◽  
Buoyant Flow ◽  
Bottom Boundary ◽  
Bottom Boundary Layers

AbstractBaroclinic instabilities are important processes that enhance mixing and dispersion in the ocean. The presence of sloping bathymetry and the nongeostrophic effect influence the formation and evolution of baroclinic instabilities in oceanic bottom boundary layers and in coastal waters. This study explores two nongeostrophic baroclinic instability theories adapted to the scenario with sloping bathymetry and investigates the mechanism of the instability suppression (reduction in growth rate) in the buoyant flow regime. Both the two-layer and continuously stratified models reveal that the suppression is related to a new parameter, slope-relative Burger number Sr ≡ (M2/f2)(α + αp), where M2 is the horizontal buoyancy gradient, α is the bathymetry slope, and αp is the isopycnal slope. In the layer model, the instability growth rate linearly decreases with increasing Sr {the bulk form Sr = [U0/(H0f)](α + αp)}. In the continuously stratified model, the instability suppression intensifies with increasing Sr when the regime shifts from quasigeostrophic to nongeostrophic. The adapted theories are intrinsically applicable to deep ocean bottom boundary layers and could be conditionally applied to coastal buoyancy-driven flow. The slope-relative Burger number is related to the Richardson number by Sr = δrRi−1, where the slope-relative parameter is δr = (α + αp)/αp. Since energetic fronts in coastal zones are often characterized by low Ri, that implies potentially higher values of Sr, which is why baroclinic instabilities may be suppressed in the energetic regions where they would otherwise be expected to be ubiquitous according to the quasigeostrophic theory.

scholarly journals Turning Ocean Mixing Upside Down

2016 ◽  
Vol 46 (7) ◽  
pp. 2239-2261 ◽  
Author(s):  
Raffaele Ferrari ◽  
Ali Mashayek ◽  
Trevor J. McDougall ◽  
Maxim Nikurashin ◽  
Jean-Michael Campin
Keyword(s):  
Boundary Layers ◽  
Numerical Models ◽  
Meridional Overturning Circulation ◽  
Small Scale ◽  
Ocean Bottom ◽  
Overturning Circulation ◽  
Bottom Boundary ◽  
Bottom Boundary Layers ◽  
Meridional Overturning ◽  
Stratified Ocean

AbstractIt is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However, the observational evidence that the strong turbulent fluxes generated by small-scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal waters. Using a combination of theoretical ideas and numerical models, it is argued that abyssal waters upwell along weakly stratified boundary layers, where small-scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom. The abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters, driven by small-scale mixing in the stratified interior above the bottom boundary layers, and a slightly larger net upwelling, driven by the decay of small-scale mixing in the boundary layers. The crucial importance of upwelling along boundary layers in closing the abyssal overturning circulation is the main finding of this work.

Diffusive boundary layers over varying topography

2015 ◽  
Vol 769 ◽  
pp. 635-653 ◽  
Author(s):  
R. W. Dell ◽  
L. J. Pratt
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Deep Ocean ◽  
Ocean Modeling ◽  
Bottom Slope ◽  
Regional Ocean Modeling System ◽  
Bottom Boundary ◽  
Roms Model ◽  
Bottom Boundary Layers ◽  
Diffusive Boundary

Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom boundary layers to date have concentrated on constant bottom slopes. We present a study of how diffusive boundary layers interact with various idealized topography, such as changes in bottom slope, slopes with corrugations and isolated sills. We use linear theory and numerical simulations in the regional ocean modeling system (ROMS) model to show changes in bottom slope can cause convergences and divergences within the boundary layer, in turn causing fluid exchanges that reach far into the overlying fluid and alter stratification far from the bottom. We also identify several different regimes of boundary-layer behaviour for topography with oceanographically relevant size and shape, including reversing flows and overflows, and we develop a simple theory that predicts the regime boundaries, including what topographies will generate overflows. As observations also suggest there may be overflows in deep canyons where the flow passes over isolated bumps and sills, this parameter range may be particularly significant for understanding the role of boundary layers in the deep ocean.

scholarly journals Overlapping boundary layers in coastal oceans

2022 ◽  
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Deep Ocean ◽  
Finite Depth ◽  
Simulation Method ◽  
Surface Boundary ◽  
Vertical Column ◽  
Bottom Boundary ◽  
Bottom Boundary Layers ◽  
Langmuir Circulations

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.

Bottom Boundary Layers

2021 ◽  
Author(s):  
Stephen M. Henderson ◽  
Jeffrey R. Nielson
Keyword(s):  
Boundary Layers ◽  
Bottom Boundary ◽  
Bottom Boundary Layers

scholarly journals NUMERICAL MODELING OF A TURBULENT BOTTOM BOUNDARY LAYER UNDER SOLITARY WAVES ON A SMOOTH SURFACE

2018 ◽  
pp. 26
Author(s):  
Ahmad Sana ◽  
Hitoshi Tanaka
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Solitary Waves ◽  
Bottom Boundary Layer ◽  
Stream Velocity ◽  
Bottom Boundary ◽  
Sediment Movement ◽  
Bottom Boundary Layers ◽  
Turbulent Models

A number of studies on bottom boundary layers under sinusoidal and cnoidal waves were carried out in the past owing to the role of bottom shear stress on coastal sediment movement. In recent years, the bottom boundary layers under long waves have attracted considerable attention due to the occurrence of huge tsunamis and corresponding sediment movement. In the present study two-equation turbulent models proposed by Menter(1994) have been applied to a bottom boundary layer under solitary waves. A comparison has been made for cross-stream velocity profile and other turbulence properties in x-direction.

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