scholarly journals Moored Flux and Dissipation Estimates from the Northern Deepwater Gulf of Mexico

Fluids ◽  
2021 ◽  
Vol 6 (7) ◽  
pp. 237
Author(s):  
Kurt L. Polzin ◽  
Binbin Wang ◽  
Zhankun Wang ◽  
Fred Thwaites ◽  
Albert J. Williams
Keyword(s):  
Gulf Of Mexico ◽  
Velocity Vector ◽  
Layer Structure ◽  
Ekman Layer ◽  
Shear Instability ◽  
Turbulent Shear ◽  
Wave Band ◽  
Mixing Processes ◽  
Boundary Mixing ◽  
Turbulence Flux

Results from a pilot program to assess boundary mixing processes along the northern continental slope of the Gulf of Mexico are presented. We report a novel attempt to utilize a turbulence flux sensor on a conventional mooring. These data document many of the features expected of a stratified Ekman layer: a buoyancy anomaly over a height less than that of the unstratified Ekman layer and an enhanced turning of the velocity vector with depth. Turbulent stress estimates have an appropriate magnitude and are aligned with the near-bottom velocity vector. However, the Ekman layer is time dependent on inertial-diurnal time scales. Cross slope momentum and temperature fluxes have significant contributions from this frequency band. Collocated turbulent kinetic energy dissipation and temperature variance dissipation estimates imply a dissipation ratio of 0.14 that is not sensibly different from canonical values for shear instability (0.2). This mixing signature is associated with production in the internal wave band rather than frequencies associated with turbulent shear production. Our results reveal that the expectation of a quasi-stationary response to quasi-stationary forcing in the guise of eddy variability is naive and a boundary layer structure that does not support recent theoretical assumptions concerning one-dimensional models of boundary mixing.

scholarly journals Physical Transport Processes that Affect the Distribution of Oil in the Gulf of Mexico: Observations and Modeling

Oceanography ◽  
2021 ◽  
Vol 34 (1) ◽  
pp. 58-75
Author(s):  
Michel Boufadel ◽  
◽  
Annalisa Bracco ◽  
Eric Chassignet ◽  
Shuyi Chen ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Physical Environment ◽  
Large Scale ◽  
Transport Processes ◽  
Small Scale ◽  
Surface Mixed Layer ◽  
Physical Processes ◽  
Mixing Processes ◽  
Near Surface ◽  
Wide Range

Physical transport processes such as the circulation and mixing of waters largely determine the spatial distribution of materials in the ocean. They also establish the physical environment within which biogeochemical and other processes transform materials, including naturally occurring nutrients and human-made contaminants that may sustain or harm the region’s living resources. Thus, understanding and modeling the transport and distribution of materials provides a crucial substrate for determining the effects of biological, geological, and chemical processes. The wide range of scales in which these physical processes operate includes microscale droplets and bubbles; small-scale turbulence in buoyant plumes and the near-surface “mixed” layer; submesoscale fronts, convergent and divergent flows, and small eddies; larger mesoscale quasi-geostrophic eddies; and the overall large-scale circulation of the Gulf of Mexico and its interaction with the Atlantic Ocean and the Caribbean Sea; along with air-sea interaction on longer timescales. The circulation and mixing processes that operate near the Gulf of Mexico coasts, where most human activities occur, are strongly affected by wind- and river-induced currents and are further modified by the area’s complex topography. Gulf of Mexico physical processes are also characterized by strong linkages between coastal/shelf and deeper offshore waters that determine connectivity to the basin’s interior. This physical connectivity influences the transport of materials among different coastal areas within the Gulf of Mexico and can extend to adjacent basins. Major advances enabled by the Gulf of Mexico Research Initiative in the observation, understanding, and modeling of all of these aspects of the Gulf’s physical environment are summarized in this article, and key priorities for future work are also identified.

Frontal Instability and Energy Dissipation in a Submesoscale Upwelling Filament

2020 ◽  
Vol 50 (7) ◽  
pp. 2017-2035 ◽  
Author(s):  
Jen-Ping Peng ◽  
Peter Holtermann ◽  
Lars Umlauf
Keyword(s):  
Energy Dissipation ◽  
Lower Layer ◽  
Layer Structure ◽  
Quantitative Agreement ◽  
Shear Instability ◽  
Surface Velocity ◽  
Near Surface ◽  
Light Side ◽  
Frontal Instability ◽  
Symmetric Instability

AbstractBased on high-resolution turbulence microstructure and near-surface velocity data, frontal instability and its relation to turbulence are investigated inside a transient upwelling filament in the Benguela upwelling system (southeast Atlantic). The focus of our study is a sharp submesoscale front located at the edge of the filament, characterized by persistent downfront winds, a strong frontal jet, and vigorous turbulence. Our analysis reveals three distinct frontal stability regimes. (i) On the light side of the front, a 30–40-m-deep turbulent surface layer with low potential vorticity (PV) was identified. This low-PV region exhibited a well-defined two-layer structure with a convective (Ekman-forced) upper layer and a stably stratified lower layer, where turbulence was driven by forced symmetric instability (FSI). Dissipation rates in this region scaled with the Ekman buoyancy flux, in excellent quantitative agreement with recent numerical simulations of FSI. (ii) Inside the cyclonic flank of the frontal jet, near the maximum of the cross-front density gradient, the cyclonic vorticity was sufficiently strong to suppress FSI. Turbulence in this region was driven by marginal shear instability. (iii) Inside the anticyclonic flank of the frontal jet, conditions for mixed inertial/symmetric instability were satisfied. Our data provide direct evidence for the relevance of FSI, inertial instability, and marginal shear instability for overall kinetic energy dissipation in submesoscale fronts and filaments.

A three-dimensional law of the wall for turbulent shear flows

1975 ◽  
Vol 70 (1) ◽  
pp. 149-160 ◽  
Author(s):  
B. Van Den Berg
Keyword(s):  
Experimental Data ◽  
Pressure Gradient ◽  
Velocity Vector ◽  
Shear Flows ◽  
Three Dimensional ◽  
Turbulent Shear ◽  
Inertial Forces ◽  
Flow Angle ◽  
Law Of The Wall ◽  
Good Agreement

An extended law of the wall is derived for three-dimensional flows. It describes the variation of the magnitude and direction of velocity close to the wall. The effects of both the pressure gradient and the inertial forces have been taken into account. The derived wall law is valid only when the deviations from the simple law of the wall are not large. The most important feature of a three-dimensional wall law is the prediction of the rotation of the velocity vector near the wall. Comparison of the flow angle variations predicted by the present wall law with the few available experimental data shows good agreement.

scholarly journals Surface Relative Dispersion in the Southwestern Gulf of Mexico

2017 ◽  
Vol 47 (2) ◽  
pp. 387-403 ◽  
Author(s):  
Luis Zavala Sansón ◽  
Paula Pérez-Brunius ◽  
Julio Sheinbaum
Keyword(s):  
Gulf Of Mexico ◽  
Power Law ◽  
Linear Growth ◽  
Ekman Layer ◽  
Relative Dispersion ◽  
Loop Current ◽  
Robust Estimates ◽  
Surface Dispersion ◽  
Oceanic Currents ◽  
Particle Statistics

AbstractSurface dispersion properties in the southwestern Gulf of Mexico are studied by using a set of 441 drifters released during a 7-yr period and tracked for 2 months on average. The drifters have a drogue below the surface Ekman layer, so they approximately follow oceanic currents. This study follows two different approaches: First, two-particle (or pair) statistics are calculated [relative dispersion and finite-scale Lyapunov exponents (FSLEs)]. Relative dispersion estimates are consistent with theoretical dispersion regimes of two-dimensional turbulence: an exponential growth during the first 3 days, a Richardson-like regime between 3 and 20 days (in which relative dispersion grows as a power law in time), and standard dispersion (linear growth) for longer times. The FSLEs yield a power-law regime for scales between 10 and 150 km but do not detect an exponential regime for short separations (less than 10 km). Robust estimates of diffusivities based on both relative dispersion and FSLEs are provided. Second, two different dispersion scenarios are revealed by drifter trajectories and altimetric data and supported by two-particle statistics: (i) a south-to-north advection of drifters, predominantly along the western shelf of the region, and (ii) a retention of drifters during several weeks at the Bay of Campeche, the southernmost part of the Gulf of Mexico. Dominant processes that control the dispersion are the arrival of anticyclonic Loop Current eddies to the western shelf and their interaction with the semipermanent cyclonic structure in the Bay of Campeche.

scholarly journals On the equatorial Ekman layer

2016 ◽  
Vol 803 ◽  
pp. 395-435 ◽  
Author(s):  
Florence Marcotte ◽  
Emmanuel Dormy ◽  
Andrew Soward
Keyword(s):  
Shear Layer ◽  
Layer Structure ◽  
Ekman Layer ◽  
Far Field ◽  
Integral Boundary ◽  
Electrically Conducting ◽  
Incompressible Viscous Flow ◽  
Inner Sphere ◽  
Non Local ◽  
Layer Problem

The steady incompressible viscous flow in the wide gap between spheres rotating rapidly about a common axis at slightly different rates (small Rossby number) has a long and celebrated history. The problem is relevant to the dynamics of geophysical and planetary core flows, for which, in the case of electrically conducting fluids, the possible operation of a dynamo is of considerable interest. A comprehensive asymptotic study, in the small Ekman number limit $E\ll 1$, was undertaken by Stewartson (J. Fluid Mech., vol. 26, 1966, pp. 131–144). The mainstream flow, exterior to the $E^{1/2}$ Ekman layers on the inner/outer boundaries and the shear layer on the inner sphere tangent cylinder $\mathscr{C}$, is geostrophic. Stewartson identified a complicated nested layer structure on $\mathscr{C}$, which comprises relatively thick quasigeostrophic $E^{2/7}$- (inside $\mathscr{C}$) and $E^{1/4}$- (outside $\mathscr{C}$) layers. They embed a thinner ageostrophic $E^{1/3}$ shear layer (on $\mathscr{C}$), which merges with the inner sphere Ekman layer to form the $E^{2/5}$-equatorial Ekman layer of axial length $E^{1/5}$. Under appropriate scaling, this $E^{2/5}$-layer problem may be formulated, correct to leading order, independent of $E$. Then the Ekman boundary layer and ageostrophic shear layer become features of the far-field (as identified by the large value of the scaled axial coordinate $z$) solution. We present a numerical solution of the previously unsolved equatorial Ekman layer problem using a non-local integral boundary condition at finite $z$ to account for the far-field behaviour. Adopting $z^{-1}$ as a small parameter we extend Stewartson’s similarity solution for the ageostrophic shear layer to higher orders. This far-field solution agrees well with that obtained from our numerical model.

Time-dependent, non-monotonic mixing in stratified turbulent shear flows: implications for oceanographic estimates of buoyancy flux

2013 ◽  
Vol 736 ◽  
pp. 570-593 ◽  
Author(s):  
A. Mashayek ◽  
C. P. Caulfield ◽  
W. R. Peltier
Keyword(s):  
Reynolds Number ◽  
Dissipation Rate ◽  
Isotropic Turbulence ◽  
Buoyancy Flux ◽  
Shear Instability ◽  
Turbulent Shear ◽  
Mixing Efficiency ◽  
Small Scale ◽  
Stratified Turbulence ◽  
Flow Evolution

AbstractWe employ direct numerical simulation to investigate the efficiency of diapycnal mixing by shear-induced turbulence in stably stratified free shear layers for flows with bulk Richardson numbers in the range $0. 12\leq R{i}_{0} \leq 0. 2$ and Reynolds number $Re= 6000$. We show that mixing efficiency depends non-monotonically upon $R{i}_{0} $, peaking in the range 0.14–0.16, which coincides closely with the range in which both the buoyancy flux and the dissipation rate are maximum. By detailed analyses of the energetics of flow evolution and the underlying dynamics, we show that the existence of high mixing efficiency in the range $0. 14\lt R{i}_{0} \lt 0. 16$ is due to the emergence of a large number of small-scale instabilities which do not exist at lower Richardson numbers and are stabilized at high Richardson numbers. As discussed in Mashayek & Peltier (J. Fluid Mech., vol. 725, 2013, pp. 216–261), the existence of such a well-populated ‘zoo’ of secondary instabilities at intermediate Richardson numbers and the subsequent high mixing efficiency is realized only if the Reynolds number is higher than a critical value which is generally higher than that achievable in laboratory settings, as well as that which was achieved in the majority of previous numerical studies of shear-induced stratified turbulence. We furthermore show that the primary assumptions upon which the widely employed Osborn (J. Phys. Oceanogr. vol. 10, 1980, pp. 83–89) formula is based, as well as its counterparts and derivatives, which relate buoyancy flux to dissipation rate through a (constant) flux coefficient ($\Gamma $), fail at higher Richardson numbers provided that the Reynolds number is sufficiently high. Specifically, we show that the assumptions of fully developed, stationary, and isotropic turbulence all break down at high Richardson numbers. We show that the breakdown of these assumptions occurs most prominently at Richardson numbers above that corresponding to the maximum mixing efficiency, a fact that highlights the importance of the non-monotonicity of the dependence of mixing efficiency upon Richardson number, which we establish to be characteristic of stratified shear-induced turbulence. At high $R{i}_{0} $, the lifecycle of the turbulence is composed of a rapidly growing phase followed by a phase of rapid decay. Throughout the lifecycle, there is considerable exchange of energy between the small-scale turbulence and larger coherent structures which survive the various stages of flow evolution. Since shear instability is one of the most prominent mechanisms for turbulent dissipation of energy at scales below hundreds of metres and at various depths of the ocean, our results have important implications for the inference of turbulent diffusivities on the basis of microstructure measurements in the oceanic environment.

scholarly journals Generation of Bulk Shear Spikes in Shallow Stratified Tidal Seas

2009 ◽  
Vol 39 (4) ◽  
pp. 969-985 ◽  
Author(s):  
Hans Burchard ◽  
Tom P. Rippeth
Keyword(s):  
Wind Stress ◽  
Numerical Study ◽  
Layer Structure ◽  
Lower Boundary ◽  
Momentum Equation ◽  
Interfacial Stress ◽  
Shear Instability ◽  
Marginal Stability ◽  
Sea Bed ◽  
Wind Events

Abstract Recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability, suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing. The bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal, inertial, and synoptic time scales that has yet to be explained. In this paper observations from the seasonally stratified northern North Sea are presented for a time when the water column has a distinct two-layer structure. Bulk shear estimates, based on ADCP measurements, show a bulk shear vector that rotates in a clockwise direction at the local inertial period, with episodes of bulk shear spikes that have an approximately twice daily period, and occur in bursts that last for several days. To explain this observation, a simple two-layer model based on layer averaging of the one-dimensional momentum equation is developed, forced at the surface by wind stress and damped by (tidally dominated) sea bed friction. The two layers are then linked through an interfacial stress term. The model reproduces the observations, showing that the bulk shear spikes are a result of the alignment of the wind stress, tidal bed stress, and (clockwise rotating) bulk shear vectors. Velocity microstructure measurements are then used to confirm enhanced levels of mixing during a period of bulk shear spikes. A numerical study demonstrates the sensitivity of the spike generation mechanism to the local tidal conditions and the phasing and duration of wind events.

Two-Step Deglaciation: 14C-Dated High-Resolution δ18O Records from the Tropical Atlantic Ocean

1985 ◽  
Vol 23 (2) ◽  
pp. 258-271 ◽  
Author(s):  
W. H. Berger ◽  
J. S. Killingley ◽  
C. V. Metzler ◽  
E. Vincent
Keyword(s):  
High Resolution ◽  
Gulf Of Mexico ◽  
Oxygen Isotopes ◽  
Younger Dryas ◽  
Equatorial Pacific ◽  
Tropical Atlantic ◽  
The West ◽  
Mixing Processes ◽  
Sea Floor ◽  
Scale Shift

Eight box cores from the tropical Atlantic were studied in detail with regard to foraminiferal oxygen isotopes, radiocarbon, and Globorotalia menardii abundance. A standard Atlantic oxygen-isotope signal was reconstructed for the last 20,000 yr. It is quite similar to the west-equatorial Pacific signal published previously. Deglaciation is seen to occur in two steps which are separated by a pause. Onset of deglaciation is after 15,000 yr B.P. The pause is centered between 11,000 and 12,000 yr B.P., but may be correlative with the Younger Dryas (10,500 yr B.P.) if allowance is made for a scale shift due to mixing processes on the sea floor. Step 2 is centered near 10,000 yr B.P. and is followed by a brief excursion toward light oxygen values. This excursion (the M event) may correlate with the Gulf of Mexico meltwater spike.

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