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

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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A numerical investigation into the bottom boundary layer flow and vertical structure of internal waves on a continental slope

Continental Shelf Research ◽

10.1016/s0278-4343(97)00067-8 ◽

1998 ◽

Vol 18 (1) ◽

pp. 31-65 ◽

Cited By ~ 15

Author(s):

Peter E. Holloway ◽

Belinda Barnes

Keyword(s):

Boundary Layer ◽

Numerical Investigation ◽

Continental Slope ◽

Internal Waves ◽

Boundary Layer Flow ◽

Vertical Structure ◽

Bottom Boundary Layer ◽

Bottom Boundary ◽

Layer Flow

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A Self-Contained Acoustic Scintillation Instrument for Path-Averaged Measurements of Flow and Turbulence with Application to Hydrothermal Vent and Bottom Boundary Layer Dynamics

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech1799.1 ◽

2005 ◽

Vol 22 (10) ◽

pp. 1602-1617 ◽

Cited By ~ 8

Author(s):

D. Di Iorio ◽

D. Lemon ◽

R. Chave

Keyword(s):

Boundary Layer ◽

Hydrothermal Vent ◽

Acoustic Scattering ◽

Temperature Fluctuations ◽

Energy Dissipation Rate ◽

Bottom Boundary Layer ◽

The Black Sea ◽

Small Scale ◽

Bottom Boundary

Abstract A self-contained acoustical scintillation instrument is described that has been used to measure flow and turbulence characteristics in two diverse oceanographic settings. This instrument is a battery-operated and internally logging acoustic propagation system that is ideally suited to monitor long-term flow and small-scale effective refractive index fluctuations. When the temperature variability dominates the acoustic scattering, as is the case of a hydrothermal vent plume, then a measure of the vertical buoyancy-driven flow, together with the root-mean-square temperature fluctuations, can be obtained. Results for vent structure Hulk of the Main Endeavour vent field of the Juan de Fuca Ridge show that the long-term (71 days) temperature fluctuations, together with the vertical flow, can be used to estimate heat flux density. Measurements also show oscillations in the log-amplitude variance that result from plume advection by the ambient tidal currents and demonstrate the need for a long time series measurement. When the turbulent velocity dominates the acoustic scattering, as is the case in some energetic bottom boundary layer flows, then the turbulent kinetic energy dissipation rate is derived, assuming isotropic and homogeneous models. The methodology and results are summarized from an application to the Bosporus Canyon of the Black Sea, to monitor the flow and turbulence associated with Mediterranean seawater inflow.

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On the Upwelling of Downwelling Currents

Journal of Physical Oceanography ◽

10.1175/2008jpo3783.1 ◽

2008 ◽

Vol 38 (11) ◽

pp. 2482-2500 ◽

Cited By ~ 63

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.

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