Lateral entrainment in baroclinic currents II

The distribution, thickness, and mean grain size of surficial sediments on Sable Island Bank, Middle Bank, and Banquereau, Scotian Shelf, are used to re-evaluate interpretations made by others on mechanisms controlling long-term stability and net transport pathways of surface sand. A Holocene sand-ridge complex, the Sable Island Catena, extends 300 km across Sable Island Bank and Banquereau. This sand-ridge complex, which is up to 50 m thick, formed as a result of Holocene sediment transport on the outer banks and controls the modern-day distribution of bedforms and sediment size. A clockwise circulation of sand centred around Sable Island, which had been proposed as the mechanism maintaining the island, does not take place. Sand is transported from southwest to northeast across Sable Island Bank towards the Sable Island Catena; thereafter, sand is dispersed eastward. "Spillover" of sand from the banks to The Gully is not significant. Sand is trapped in depths less than 100 m. The processes proposed to have formed the Sable Island Catena are barotropic, storm-driven currents and associated sand transport and "fair-weather" dispersion of sand by strong, tidal flows or baroclinic currents, coupled with an abundant supply of sand-size sediment.

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The Nonlinear Dynamics of Time-Dependent Subcritical Baroclinic Currents

Journal of Physical Oceanography ◽

10.1175/jpo3034.1 ◽

2007 ◽

Vol 37 (4) ◽

pp. 1001-1021 ◽

Cited By ~ 11

Author(s):

G. R. Flierl ◽

J. Pedlosky

Keyword(s):

Nonlinear Dynamics ◽

Small Degree ◽

Mean Value ◽

Time Dependent ◽

Unstable Wave ◽

Amplitude Dependence ◽

Strongly Nonlinear ◽

System A ◽

Turbulent Characteristics ◽

Baroclinic Currents

Abstract The nonlinear dynamics of baroclinically unstable waves in a time-dependent zonal shear flow is considered in the framework of the two-layer Phillips model on the beta plane. In most cases considered in this study the amplitude of the shear is well below the critical value of the steady shear version of the model. Nevertheless, the time-dependent problem in which the shear oscillates periodically is unstable, and the unstable waves grow to substantial amplitudes, in some cases with strongly nonlinear and turbulent characteristics. For very small values of the shear amplitude in the presence of dissipation an analytical, asymptotic theory predicts a self-sustained wave whose amplitude undergoes a nonlinear oscillation whose period is amplitude dependent. There is a sensitive amplitude dependence of the wave on the frequency of the oscillating shear when the shear amplitude is small. This behavior is also found in a truncated model of the dynamics, and that model is used to examine larger shear amplitudes. When there is a mean value of the shear in addition to the oscillating component, but such that the total shear is still subcritical, the resulting nonlinear states exhibit a rectified horizontal buoyancy flux with a nonzero time average as a result of the instability of the oscillating shear. For higher, still subcritical, values of the shear, a symmetry breaking is detected in which a second cross-stream mode is generated through an instability of the unstable wave although this second mode would by itself be stable on the basic time-dependent current. For shear values that are substantially subcritical but of order of the critical shear, calculations with a full quasigeostrophic numerical model reveal a turbulent flow generated by the instability. If the beta effect is disregarded, the inviscid, linear problem is formally stable. However, calculations show that a small degree of nonlinearity is enough to destabilize the flow, leading to large amplitude vacillations and turbulence. When the most unstable wave is not the longest wave in the system, a cascade up scale to longer waves is observed. Indeed, this classically subcritical flow shows most of the qualitative character of a strongly supercritical flow. This result supports previous suggestions of the important role of background time dependence in maintaining the atmospheric and oceanic synoptic eddy field.

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Near-Inertial Waves in Strongly Baroclinic Currents

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0132.1 ◽

2013 ◽

Vol 43 (4) ◽

pp. 706-725 ◽

Cited By ~ 62

Author(s):

Daniel B. Whitt ◽

Leif N. Thomas

Keyword(s):

Gulf Stream ◽

Velocity Vector ◽

Numerical Solutions ◽

Wave Theory ◽

Two Dimensional ◽

Inertial Waves ◽

Geostrophic Current ◽

Minimum Frequency ◽

Critical Layers ◽

Baroclinic Currents

Abstract An analysis and physical interpretation of near-inertial waves (NIWs) propagating perpendicular to a steady, two-dimensional, strongly baroclinic, geostrophic current are presented. The analysis is appropriate for geostrophic currents with order-one Richardson numbers such as those associated with fronts experiencing strong, wintertime atmospheric forcing. This work highlights the underlying physics behind the properties of the NIWs using parcel arguments and the principles of conservation of density and absolute momentum. Baroclinicity introduces lateral gradients in density and vertical gradients in absolute momentum that significantly modify the dispersion and polarization relations and propagation of NIWs relative to classical internal wave theory. In particular, oscillations at the minimum frequency are not horizontal but, instead, are slanted along isopycnals. Furthermore, the polarization of the horizontal velocity is not necessarily circular at the minimum frequency and the spiraling of the wave’s velocity vector with time and depth can be in the opposite direction from that predicted by classical theory. Ray tracing and numerical solutions illustrate the trapping and amplification of NIWs in regions of strong baroclinicity where the wave frequency is lower than the effective Coriolis frequency. The largest amplification is found at slantwise critical layers that align with the tilted isopycnals of the current. Such slantwise critical layers are seen in wintertime observations of the Gulf Stream and, consistent with the theory, coincide with regions of intensified ageostrophic shear characterized by a banded structure that is spatially coherent along isopycnals.

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Instability of Meridional Baroclinic Currents*

Journal of Physical Oceanography ◽

10.1175/1520-0485(2002)032<1075:iombc>2.0.co;2 ◽

2002 ◽

Vol 32 (3) ◽

pp. 1075-1093 ◽

Cited By ~ 19

Author(s):

Alison Walker ◽

Joseph Pedlosky

Keyword(s):

Baroclinic Currents

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The turbidite-contourite-tidalite-baroclinite-hybridite problem: orthodoxy vs. empirical evidence behind the “Bouma Sequence”

Journal of Palaeogeography ◽

10.1186/s42501-021-00085-1 ◽

2021 ◽

Vol 10 (1) ◽

Author(s):

G. Shanmugam

Keyword(s):

Deep Water ◽

General Term ◽

Bottom Current ◽

Turbidity Currents ◽

Facies Model ◽

Bottom Currents ◽

Gravity Flows ◽

Slope Flow ◽

Baroclinic Currents ◽

Facies Models

AbstractThe underpinning problems of deep-water facies still remain unresolved. (1) The Tb, Tc, and Td divisions of the turbidite facies model, with traction structures, are an integral part of the “Bouma Sequence” (Ta, Tb, Tc, Td, Te). However, deposits of thermohaline contour currents, wind-driven bottom currents, deep-marine tidal currents, and baroclinic currents (internal waves and tides) also develop discrete rippled units, mimicking Tc. (2) The application of “cut-out” logic of sequences, which was originally introduced for the “Bouma Sequence”, with sharp basal contacts and sandy divisions containing well-developed traction structures, to muddy contorts with gradational basal contacts and an absence of well-developed traction structures is incongruent. (3) The presence of five internal divisions and hiatus in the muddy contoured facies model is in dispute. (4) Intersection of along slope contour currents with down slope sediment-gravity flows, triggering hybrid flows, also develops traction structures. (5) The comparison of genuine hybrid flows with down slope flow transformation of gravity flows is inconsistent with etymology of the term “hybrid”. (6) A reexamination of the Annot Sandstone at the Peira Cava type locality in SE France fails to validate either the orthodoxy of five internal divisions of the “Bouma Sequence” or their origin by turbidity currents. For example, the “Ta” division is composed of amalgamated units with inverse grading and floating mudstone clasts, suggesting a mass-transport deposit (MTD). The “Tb” and “Tc” divisions are composed of double mud layers and sigmoidal cross bedding, respectively, which suggest a tidalite origin. (7) Although it was reasonable to introduce a simplistic “Bouma Sequence” in 1962, at a time of limited knowledge on deep-water processes, it is obsolete now in 2021 to apply this model to the rock record amid a wealth of new knowledge. (8) The disconnect between 12 observed, but questionable, modern turbidity currents and over 10,000 interpreted ancient turbidites defies the doctrine of uniformitarianism. This disconnect is attributed to routine application of genetic facies models, without a pragmatic interpretation of empirical data. (9) A suggested solution to these problems is to interpret traction structures in the sedimentary record pragmatically on the basis of empirical field and experimental evidence, without any built-in bias using facies models, such as the “Bouma Sequence”. (10) Until reliable criteria are developed to distinguish traction structures of each type of bottom currents based on uniformitarianism, a general term “BCRS” (i.e., bottom-current reworked sands) is appropriate for deposits of all four kinds of bottom currents.

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Climatology of internal tides at a shelf-break location on the Australian North West Shelf

Marine and Freshwater Research ◽

10.1071/mf9880001 ◽

1988 ◽

Vol 39 (1) ◽

pp. 1 ◽

Cited By ~ 12

Author(s):

PE Hollaway

Keyword(s):

Internal Waves ◽

Temporal Variability ◽

Internal Tide ◽

Shelf Break ◽

Internal Tides ◽

North West ◽

Vertical Displacements ◽

Density Interfaces ◽

Baroclinic Currents

The concept of defining an internal tide climate is used as a means of providing an assessment of the amplitude of semi-diurnal vertical displacements of density interfaces and of horizontal baroclinic currents at a particular location. The analysis uses current meter and thermistor chain observations from North Rankin, a location just seaward of the shelf break on the Australian North West Shelf, spanning a period of 28 months. Contributions from both principal lunar (M2) and principal solar (S2) period internal waves are considered. The final climatological averages (monthly values) show the baroclinic currents to be comparable to or stronger than the semi-diurnal barotropic currents at the same location for the majority of the year (October through to May). The temporal variability closely follows the variability in the stratification with very weak baroclinic motion during the winter months (June to September).

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