Observational evidence of diapycnal upwelling in a bottom enhanced mixing environment

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

<p>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°45’S, 12°20’S and 14°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’s center of mass had decreased by 0.13 kg m<sup>-3</sup>. 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 m day<sup>-1</sup> 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.</p>

scholarly journals Dissipation of Turbulent Kinetic Energy in the Oscillating Bottom Boundary Layer of a Large Shallow Lake

2020 ◽  
Vol 37 (3) ◽  
pp. 517-531 ◽  
Author(s):  
Aidin Jabbari ◽  
Leon Boegman ◽  
Reza Valipour ◽  
Danielle Wain ◽  
Damien Bouffard
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Bottom Boundary Layer ◽  
Inertial Subrange ◽  
Order Structure ◽  
Inertial Method ◽  
Bottom Boundary ◽  
Law Of The Wall ◽  
Order Of Magnitude

AbstractMixing rates and biogeochemical fluxes are commonly estimated from the rate of dissipation of turbulent kinetic energy ε as measured with a single instrument and processing method. However, differences in measurements of ε between instruments/methods often vary by one order of magnitude. In an effort to identify error in computing ε, we have applied four common methods to data from the bottom boundary layer of Lake Erie. We applied the second-order structure function method (SFM) to velocity measurements from an acoustic Doppler current profiler, using both canonical and anisotropy-adjusted Kolmogorov constants, and compared the results with those computed from the law of the wall, Batchelor fitting to temperature gradient microstructure, and inertial subrange fitting to acoustic Doppler velocimeter data. The ε from anisotropy-adjusted constants in SFM increased by a factor of 6 or more at 0.2 m above the bed and showed a better agreement with microstructure and inertial method estimations. The maximum difference between SFM ε, computed using adjusted and canonical constants, and microstructure values was 25% and 50%, respectively. This difference was 30% and 55%, respectively, for those from inertial subrange fitting at times of high-intensity turbulence (Reynolds number at 1 m above the bed of more than 2 × 104). Comparison of the SFM ε to those from law of the wall was often poor, with errors as large as one order of magnitude. From the considerable improvement in ε estimates near the bed, anisotropy-adjusted Kolmogorov constants should be applied to compute dissipation in geophysical boundary layers.

scholarly journals Study of the effects of Ekman dynamics in the bottom boundary layer on the Black Sea continental slope

2020 ◽  
Vol 20 (1) ◽  
pp. 1-16
Author(s):  
A. G. Zatsepin ◽  
V. V. Kremenetskiy ◽  
O. I. Podymov ◽  
A. G. Ostrovskii
Keyword(s):  
Boundary Layer ◽  
Continental Slope ◽  
Black Sea ◽  
Bottom Boundary Layer ◽  
The Black Sea ◽  
Bottom Boundary

On the Upwelling of Downwelling Currents

2008 ◽  
Vol 38 (11) ◽  
pp. 2482-2500 ◽  
Author(s):  
Ricardo P. Matano ◽  
Elbio D. Palma
Keyword(s):  
Boundary Layer ◽  
Continental Slope ◽  
Lateral Diffusion ◽  
Circulation Pattern ◽  
Volume Transport ◽  
Bottom Boundary Layer ◽  
Bottom Boundary ◽  
Proposed Model ◽  
Slope Currents ◽  
Current Flows

Abstract The term “downwelling currents” refers to currents with a downslope mass flux in the bottom boundary layer. Examples are the Malvinas and Southland Currents in the Southern Hemisphere and the Oyashio in the Northern Hemisphere. Although many of these currents generate the same type of highly productive ecosystems that is associated with upwelling regimes, the mechanism that may drive such upwelling remains unclear. In this article, it is postulated that the interaction between a downwelling current and the continental slope generates shelfbreak upwelling. The proposed mechanism is relatively simple. As a downwelling current flows along the continental slope, bottom friction and lateral diffusion spread it onto the neighboring shelf, thus generating along-shelf pressure gradients and a cross-shelf circulation pattern that leads to shelfbreak upwelling. At difference with previous studies of shelfbreak dynamics (e.g., Gawarkiewicz and Chapman, Chapman and Lentz, and Pickart), the shelfbreak upwelling in the proposed model is not controlled by the downslope buoyancy flux associated with the presence of a shelf current but by the along-shelf pressure gradient associated with the presence of a slope current. As these experiments demonstrate, shelfbreak upwelling will occur in flat-bottomed domains or even in the absence of a bottom boundary layer. The shelfbreak upwelling, moreover, is not evidence of the separation of the bottom boundary layer but of the downstream divergence of the slope currents, and its magnitude is proportional to the volume transport of that current. To prove this hypothesis, the results of a series of process-oriented numerical experiments are presented.

Mass transport under sea waves propagating over a rippled bed

1996 ◽  
Vol 314 ◽  
pp. 247-265 ◽  
Author(s):  
G. Vittori ◽  
P. Blondeaux
Keyword(s):  
Boundary Layer ◽  
Mass Transport ◽  
Outer Edge ◽  
Bottom Boundary Layer ◽  
Sea Waves ◽  
Bottom Boundary ◽  
Order Of Magnitude ◽  
Wave Slope ◽  
Rippled Bed ◽  
Sea Wave

Mass transport under a progressive sea wave propagating over a rippled bed is investigated. Wave amplitudes a* of the same order of magnitude as that of the boundary layer thickness δ* and of the ripple wavelength l* are considered. All the above quantities are assumed to be much smaller than the wavelength L* of the sea wave and much larger than the amplitude 2ε* of the ripples. The analysis is carried out up to the second order in the wave slope a*/L* and in the parameter ε*/δ* which is a measure of ripple steepness. Because of these assumptions, the slow damping of wave amplitude in the direction of wave propagation is taken into account. Attention is focused on the bottom boundary layer where an order (ε*/δ*)2 correction of the steady velocity components described by Longuet-Higgins (1953) is found. This correction persists at the outer edge of the bottom boundary layer and affects the solution in the entire water column.

Sign in / Sign up

Export Citation Format

Share Document