Tracer Transport within Abyssal Mixing Layers

AbstractMixing layers near sloped topography in the abyss are thought to play a critical role in the global overturning circulation. Yet the behavior of passive tracers within sloping boundary layer systems has received little attention, despite the extensive use of tracer observations to understand abyssal circulation. Here, we investigate the behavior of a passive tracer released near a sloping boundary within a flow governed by one-dimensional boundary layer theory. The spreading rate of the tracer across isopycnals is influenced by factors such as the bottom-intensification of mixing, the dipole of upwelling (in the boundary layer) and downwelling (in the outer mixing layer), and along-isopycnal diffusion. For isolated near-boundary tracer releases, the bulk diffusivity, proportional to the rate of increase of the variance of the tracer distribution in buoyancy space, is much less than what would be expected from averaging the diapycnal diffusivity over the tracer patch. This stems from the presence of the bottom boundary that prevents tracer diffusion through it. Furthermore, when along-isopycnal diffusion is weak, the boundary tends to drive the tracer up the slope toward less dense fluid on average due to asymmetries between boundary layer and interior flows. With strong along-isopycnal diffusion this upslope movement is reduced, while at the same time the average diapycnal spreading rate is increased due to a reduced influence of the bottom boundary. These results have implications for what can be learned about the characteristics of mixing near sloping boundaries from past and future tracer-release experiments.

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On the visual growth of a turbulent mixing layer

Journal of Fluid Mechanics ◽

10.1017/s0022112080002212 ◽

1980 ◽

Vol 96 (3) ◽

pp. 447-460 ◽

Cited By ~ 24

Author(s):

J. Jimenez

Keyword(s):

Turbulent Mixing ◽

Mixing Layer ◽

Vortex Sheet ◽

Spreading Rate ◽

Layer Growth ◽

Mixing Layers ◽

Turbulent Mixing Layer

Two models are discussed to account for the motion of the concentration interface in turbulent mixing layers. In the first one the interface is treated as a vortex sheet and its roll-up is studied. It is argued that this situation represents only the first stages of layer growth and another model is studied in detail in which a row of vortex cores entrains an essentially passive concentration interface with no vorticity. Both models give values of the spreading rate in approximate agreement with observations, and their relation is discussed.

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Observational evidence of diapycnal upwelling in a bottom enhanced mixing environment

10.5194/egusphere-egu2020-11565 ◽

2020 ◽

Author(s):

Marcus Dengler ◽

Martin Visbeck ◽

Toste Tanhua ◽

Jan Lüdke ◽

Madelaine Freund

Keyword(s):

Boundary Layer ◽

Continental Slope ◽

Center Of Mass ◽

Mixing Layer ◽

Observational Evidence ◽

Bottom Boundary Layer ◽

Turbulent Dissipation ◽

Bottom Boundary ◽

Order Of Magnitude ◽

Upward Displacement

In the framework of the Peruvian Oxygen minimum zone System Tracer Release Experiment (POSTRE) about 70 kg of trifluoromethyl sulfur pentafluoride (SF5CF3) was injected into the bottom boundary layer of the upper Peruvian continental slope at 250m depth in October 2015. Three different injection sites, at 10&#176;45&#8217;S, 12&#176;20&#8217;S and 14&#176;S were selected. At the tracer release sites and due to tide-topography interaction, mixing above the upper continental slope of Peru was intensified. Turbulent dissipation rates increase by about an order of magnitude in lower 50 to 100m above the bottom. During previous tracer release experiments, where tracer was injected into the stratified mixing layer above the bottom boundary layer, a change of the center of mass toward higher densities resulted. Newer theories suggest that this diapycnal downwelling is balanced by a diapycnal upwelling within the bottom boundary layer. Indeed, during the tracer survey it was found that the density of tracer&#8217;s center of mass had decreased by 0.13&#160;kg&#160;m-3. This corresponds to an upward displacement of 70-100m. Using microsctructure shear data from 8 cruises, we obtain a diapycnal velocity of about 0.5&#160;m&#160;day-1 within the bottom boundary layer. This suggests that on average, the tracer was trapped within the bottom boundary layer for a period between 1.5 and 3 month. Overall, our tracer study provides the first observational evidence of diapycnal upwelling occurring within the bottom boundary layer of a bottom enhanced mixing environment and supports recent ideas of a vigorous global overturning circulation.

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Control of the abyssal ocean overturning circulation by mixing-driven bottom boundary layers

10.1575/1912/27424 ◽

2021 ◽

Author(s):

◽

Henri F. Drake

Keyword(s):

Boundary Layer ◽

Boundary Layers ◽

Mixing Layers ◽

Overturning Circulation ◽

Bottom Boundary ◽

Brazil Basin ◽

Basin Scale ◽

Non Linear ◽

The Mean ◽

Abyssal Mixing

An emerging paradigm posits that the abyssal overturning circulation is driven by bottom-enhanced mixing, which results in vigorous upwelling in the bottom boundary layer (BBL) along the sloping seafloor and downwelling in the stratified mixing layer (SML) above; their residual is the overturning circulation. This boundary-controlled circulation fundamentally alters abyssal tracer distributions, with implications for global climate. Chapter 1 describes how a basin-scale overturning circulation arises from the coupling between the ocean interior and mixing-driven boundary layers over rough topography, such as the sloping flanks of mid-ocean ridges. BBL upwelling is well predicted by boundary layer theory, whereas the compensation by SML downwelling is weakened by the upward increase of the basin-wide stratification, which supports a finite net overturning. These simulated watermass transformations are comparable to best-estimate diagnostics but are sustained by a crude parameterization of boundary layer restratification processes. In Chapter 2, I run a realistic simulation of a fracture zone canyon in the Brazil Basin to decipher the non-linear dynamics of abyssal mixing layers and their interactions with rough topography. Using a hierarchy of progressively idealized simulations, I identify three physical processes that set the stratification of abyssal mixing layers (in addition to the weak buoyancy-driven cross-slope circulation): submesoscale baroclinic eddies on the ridge flanks, enhanced up-canyon flow due to inhibition of the cross-canyon thermal wind, and homogenization of canyon troughs below the level of blocking sills. Combined, these processes maintain a sufficiently large near-boundary stratification for mixing to drive globally significant BBL upwelling. In Chapter 3, simulated Tracer Release Experiments illustrate how passive tracers are mixed, stirred, and advected in abyssal mixing layers. Exact diagnostics reveal that while a tracer’s diapycnal motion is directly proportional to the mean divergence of mixing rates, its diapycnal spreading depends on both the mean mixing rate and an additional non-linear stretching term. These simulations suggest that the theorized boundary-layer control on the abyssal circulation is falsifiable: downwelling in the SML has already been confirmed by the Brazil Basin Tracer Release Experiment, while an upcoming experiment in the Rockall Trough will confirm or deny the existence of upwelling in the BBL.

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Diapycnal Transport near a Sloping Bottom Boundary

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0066.1 ◽

2020 ◽

Vol 50 (11) ◽

pp. 3253-3266

Author(s):

R. M. Holmes ◽

Trevor J. McDougall

Keyword(s):

Boundary Layer ◽

Mixing Layer ◽

Buoyancy Flux ◽

Bottom Boundary ◽

One Dimensional ◽

Dimensional Boundary ◽

Velocity Changes ◽

Stratified Ocean ◽

The One ◽

Sloping Bottom

AbstractThe diapycnal motion in the stratified ocean near a sloping bottom boundary is studied using analytical solutions from one-dimensional boundary layer theory. Bottom-intensification of the diapycnal mixing intensity ensures that in the stratified mixing layer (SML), where isopycnals are relatively flat, the diapycnal motion is downward toward denser fluid. In contrast, convergence of the diffusive buoyancy flux near the seafloor drives diapycnal upwelling in what we define as the bottom boundary layer (BBL). Much of the one-dimensional BBL is characterized by a stratification only slightly reduced from that in the SML because the maximum in the buoyancy flux at the top of the BBL, where the diapycnal velocity changes sign, must occur in well-stratified fluid. The diapycnal upwelling in the BBL is determined by variations not only in the magnitude of the buoyancy gradient but also in the curvature of isopycnals. The net diapycnal upwelling is concentrated in the bottom half of the BBL where the magnitude of the buoyancy gradient changes most rapidly. The curvature effect drives upwelling near the seafloor that only makes a significant contribution to the net upwelling for steep slopes. The structure of the diapycnal velocity in this stratified BBL differs from the case of a turbulent well-mixed BBL that has been assumed in some recent theoretical studies on bottom-intensified mixing. This work therefore extends recent theories in a way that should be more applicable to abyssal ocean observations where well-mixed BBLs are not common.

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Linear Instability Analysis of Confined Compressible Boundary/Mixing Layers Flow

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.275-277.522 ◽

2013 ◽

Vol 275-277 ◽

pp. 522-526

Author(s):

Zhi Yong Liu ◽

Xiang Jiang Yuan

Keyword(s):

Boundary Layer ◽

Mixing Layer ◽

Three Dimensional ◽

Linear Instability ◽

Mixing Layers ◽

Linear Stability Theory ◽

Slow Flow ◽

Boundary Layer Flows ◽

Boundary Mixing ◽

Linear Instability Analysis

A compressible supersonic confluent flow composed of boundary layers and mixing layers are studied by linear stability theory. The flow is confined in a two-dimensional adiabatic channel. A slower flow lying in the center mixes with faster boundary layer flows on both sides and two mixing layers are evolved near the centerline. Different unstable modes were discovered and the first mode was found to be most unstable. Three-dimensional disturbances were investigated and comparison of instability features was made with unconfined boundary layer flows. The investigation of different slow flow widths was also made and a smaller spacing between the boundary layer and mixing layer was found to suppress the growth of disturbance.

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