Late Cenozoic evolution of Sackville Spur: a sediment drift on the Newfoundland continental slope

1990 ◽  
Vol 27 (6) ◽  
pp. 863-878 ◽  
Author(s):  
L. Kennard ◽  
C. Schafer ◽  
L. Carter
Keyword(s):  
Continental Slope ◽  
Large Scale ◽  
Lower Percentage ◽  
Western Boundary ◽  
Sediment Reworking ◽  
Grand Banks ◽  
The North ◽  
Sediment Drift ◽  
Labrador Current ◽  
Erosional Event

The Sackville Spur is a sediment drift feature that forms a northeastward extension of the Grand Banks continental slope between the 900 and 2500 m isobaths near latitude 48°N. At present, the Labrador Current (LC) and the Western Boundary Undercurrent (WBUC) appear to be the two major hydrodynamic forces controlling sedimentation patterns on the flanks of the spur. Near the upper part of the spur's north flank, a deep offshore component of the LC appears to be selectively winnowing silt and clay-size particles, leaving a lag deposit composed of about 43% sand-size material. The base of the north flank (≈2500 m) is in a zone in which sediments can be reworked by the fast-flowing core of the WUBC. Here surficial sediments are characterized by a relatively high percentage of fine (2–3[Formula: see text]) sand and by a lower percentage of silt compared with sediments observed near the spur crest.Reflection seismic data suggest that current-influenced deposition, associated predominantly with bottom-sediment reworking by the deeper offshore component of the LC, has been active over the uppermost part of the spur since Late Miocene to Early Pliocene time. The initiation of deep LC flow at this time is marked by a distinctive angular unconformity near the base of the spur drift deposit. Following this erosional event, deposition caused rapid progradation of the spur to the northeast. The latest phase of the spur's evolution is characterized by (i) intermittent erosion with concomitant large-scale submarine sliding; (ii) smaller scale mass-flow deposition; and (iii) a distinctive southeastward shift of its depocentre toward the Flemish Pass.

The Role of Bottom Vortex Stretching on the Path of the North Atlantic Western Boundary Current and on the Northern Recirculation Gyre

2007 ◽  
Vol 37 (8) ◽  
pp. 2053-2080 ◽  
Author(s):  
Rong Zhang ◽  
Geoffrey K. Vallis
Keyword(s):  
Continental Slope ◽  
North Atlantic ◽  
Western Boundary Current ◽  
Diagnostic Model ◽  
Western Boundary ◽  
Boundary Current ◽  
Grand Banks ◽  
The North ◽  
The North Atlantic ◽  
Recirculation Gyre

Abstract The mechanisms affecting the path of the depth-integrated North Atlantic western boundary current and the formation of the northern recirculation gyre are investigated using a hierarchy of models, namely, a robust diagnostic model, a prognostic model using a global 1° ocean general circulation model coupled to a two-dimensional atmospheric energy balance model with a hydrological cycle, a simple numerical barotropic model, and an analytic model. The results herein suggest that the path of this boundary current and the formation of the northern recirculation gyre are sensitive to both the magnitude of lateral viscosity and the strength of the deep western boundary current (DWBC). In particular, it is shown that bottom vortex stretching induced by a downslope DWBC near the south of the Grand Banks leads to the formation of a cyclonic northern recirculation gyre and keeps the path of the depth-integrated western boundary current downstream of Cape Hatteras separated from the North American coast. Both south of the Grand Banks and at the crossover region of the DWBC and Gulf Stream, the downslope DWBC induces strong bottom downwelling over the steep continental slope, and the magnitude of the bottom downwelling is locally stronger than surface Ekman pumping velocity, providing strong positive vorticity through bottom vortex-stretching effects. The bottom vortex-stretching effect is also present in an extensive area in the North Atlantic, and the contribution to the North Atlantic subpolar and subtropical gyres is on the same order as the local surface wind stress curl. Analytic solutions show that the bottom vortex stretching is important near the western boundary only when the continental slope is wider than the Munk frictional layer scale.

Impact of melting of the Laurentide Ice Sheet on sediments from the upper continental slope off southeastern Canada: evidence from Sm–Nd isotopesThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

2008 ◽  
Vol 45 (11) ◽  
pp. 1243-1252 ◽  
Author(s):  
R. K. Stevenson ◽  
X. W. Meng ◽  
C. Hillaire-Marcel
Keyword(s):  
Continental Slope ◽  
North American ◽  
Ice Sheet ◽  
Isotopic Signature ◽  
Grand Banks ◽  
The North ◽  
Climate Stability ◽  
Labrador Current ◽  
Younger Dryas Event ◽  
Glacial Tills

We present new Sm–Nd isotope data for sediments from a core located on the continental slope of the St. Pierre Bank of Canada’s east coast. The Nd analyses indicate that the sediments were derived from two principal sources: the North American Shield that yields an average early Proterozoic isotopic signature and a younger Proterozoic signature attributed to Appalachian crustal sources. The Appalachian-sourced sediments predominated during the last glacial maximum (LGM) and were associated with low sedimentation rates (<30 cm/ka), with the exception of a strong North American Shield signature present in a detrital carbonate layer that corresponds to Heinrich Layer 1 (H1). The dominance of the Appalachian signature decreased subsequent to H1. The Appalachian signatures closely follow the distribution of sediments interpreted as locally derived glacial tills, while the North American Shield signatures follow the distribution of hemipelagic mud that was likely deposited by the Labrador Current. The Nd data are consistent with the persistence of the Wisconsinan Ice Sheet coverage of Newfoundland and the Grand Banks after the LGM, although the coverage began to wane prior to 12.5 ka as evidenced by the increasing influence of the Labrador Current. However, an increase in the Appalachian isotope signature at the close of the Younger Dryas event likely indicates the final melting of the ice sheet covering the Grand Banks and the Avalon Peninsula, and the initiation of the Labrador Current’s modern circulation pathway.

scholarly journals A Model Study of the Spectral Structure of Boundary-Driven Rossby Waves and Related Altimetric Implications

2005 ◽  
Vol 35 (2) ◽  
pp. 218-231 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
North Pacific Ocean ◽  
Altimeter Data ◽  
Western Boundary ◽  
Spectral Structure ◽  
The North ◽  
Model Study ◽  
General Aspects ◽  
Coastal Kelvin Waves

Abstract A two-layer primitive equation box model of the North Pacific Ocean is used to highlight and analyze some general aspects of the linear large-scale boundary-driven oceanic variability that are detectable through altimeter observations. The model is forced by a white-noise wind, and a spectral analysis of the zonal and meridional, barotropic and baroclinic velocity components is carried out. Several dynamical features are identified in terms of boundary-driven Rossby waves, and their spatial structure and frequency dependence are examined theoretically and discussed in connection with recent studies based on altimeter data. In particular, the following aspects of the variability are analyzed: 1) beta-refracted baroclinic Rossby waves, which are found to be generated along the eastern boundary of the ocean by the passage of coastal Kelvin waves originating from the equatorial waveguide, and 2) westward-intensified barotropic Rossby waves, which originate from the western boundary of the ocean after reflection of longer waves generated in midocean. In the discussion the stress is put on dynamical aspects not yet fully understood and on the possibility that altimetry can provide further insight into their functioning.

Observations on depositional environments and benthos of the continental slope and rise, east of Newfoundland

1979 ◽  
Vol 16 (4) ◽  
pp. 831-846 ◽  
Author(s):  
Lionel Carter ◽  
Charles T. Schafer ◽  
M. A. Rashid
Keyword(s):  
Continental Slope ◽  
High Speed ◽  
Planktonic Foraminifera ◽  
Depositional Environments ◽  
Depth Interval ◽  
Western Boundary ◽  
Muddy Sand ◽  
Foraminiferal Assemblage ◽  
Labrador Current ◽  
Upper Slope

Sedimentologic, biologic, and morphologic criteria permit recognition of four depositional environments on the continental slope and rise, east of Newfoundland. The 'upper slope' (300–700 m) has a hummocky substrate with a mantle of terrigenous, gravelly muddy sand which is a mixture of ice-rafted detritus and sediment reworked from underlying glacial drift deposits. Reworking presumably took place during the last major lowering of sea level and it is continuing today under the influence of the Labrador Current and other oceanographic and biologically-related forces. The featureless bottom of the 'middle slope' (700–2000 m) is the principal depositional site of Recent mud. Fines, reworked from shelf and upper slope sediments, settle out together with fines transported to the area by the southeast-flowing Western Boundary Under-current (WBU). Compared to the upper slope this deeper environment receives less ice-rafted clasts, supports a richer macrofauna, and has a higher total species diversity of foraminifera. The 'lower slope' (2000–2500 m) is characterized by higher amounts of gravel and sand mixed with the mud, increasing numbers of current bedforms, and a more diverse foraminiferal assemblage, all of which correlate with the increasing power of the WBU with depth. The gravel was ice rafted probably at the end of the late Wisconsin to early Holocene and its presence on the seabed reflects the power of the WBU to inhibit deposition of Recent mud. The 'rise' (2500 to > 3000 m) is heralded by a subtle break in slope at about 2500 m. A high speed core of the undercurrent is situated in this area as indicated by the coarseness of the sediments (gravelly muddy sand) and the observed current bedforms. A marked increase in the numbers of benthonic and planktonic foraminifera is related primarily to the winnowing capacity of WBU. Numerous arenaceous deep sea forms first occur between 2500 and 3000 m and appear to reflect the reduced salinity, low temperature, high dissolved oxygen characteristics of the watermass that is associated with this depth interval.

scholarly journals Stochastic Forcing of Ocean Variability by the North Atlantic Oscillation

2009 ◽  
Vol 39 (1) ◽  
pp. 162-184 ◽  
Author(s):  
Kettyah C. Chhak ◽  
Andrew M. Moore ◽  
Ralph F. Milliff
Keyword(s):  
North Atlantic Oscillation ◽  
Ocean Circulation ◽  
Rossby Wave ◽  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Stochastic Forcing ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

scholarly journals A Climatology of Strong Large-Scale Ocean Evaporation Events. Part I: Identification, Global Distribution, and Associated Climate Conditions

2018 ◽  
Vol 31 (18) ◽  
pp. 7287-7312 ◽  
Author(s):  
Franziska Aemisegger ◽  
Lukas Papritz
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Past History ◽  
Spatiotemporal Variability ◽  
Western Boundary ◽  
Near Surface ◽  
Cold Air ◽  
The North ◽  
South Indian ◽  
The North Atlantic

This paper presents an object-based, global climatology (1979–2014) of strong large-scale ocean evaporation (SLOE) and its associated climatic properties. SLOE is diagnosed using an “atmospheric moisture uptake efficiency” criterion related to the ratio of surface evaporation and integrated water vapor content in the near-surface atmosphere. The chosen Eulerian identification procedure focuses on events that strongly contribute to the available near-surface atmospheric humidity. SLOE is particularly frequent along the warm ocean western boundary currents, downstream of large continental areas, and at the sea ice edge in polar regions with frequent cold-air outbreaks. Furthermore, wind-driven SLOE occurs in regions with topographically enforced winds. On a global annual average, SLOE occurs only 6% of the time but explains 22% of total ocean evaporation. An analysis of the past history and fate of air parcels involved in cold season SLOE in the North Atlantic and south Indian Oceans shows that cold-air advection is the main mechanism that induces these events. Extratropical cyclones thereby play an important role in setting the necessary equatorward synoptic flow. Consequently, the interannual variability of SLOE associated with the North Atlantic Oscillation and the southern annular mode reveals a very high sensitivity of SLOE with respect to the location of the storm tracks. This study highlights the strong link between transient synoptic events and the spatiotemporal variability in ocean evaporation patterns, which cannot be deduced from thermodynamic steady-state and climate mean state considerations alone.

scholarly journals Generation of Internal Waves by Eddies Impinging on the Western Boundary of the North Atlantic

2016 ◽  
Vol 46 (4) ◽  
pp. 1067-1079 ◽  
Author(s):  
L. Clément ◽  
E. Frajka-Williams ◽  
K. L. Sheen ◽  
J. A. Brearley ◽  
A. C. Naveira Garabato
Keyword(s):  
Internal Waves ◽  
North Atlantic ◽  
High Frequency ◽  
Large Scale ◽  
Acoustic Doppler Current Profiler ◽  
Relative Vorticity ◽  
Western Boundary ◽  
The North ◽  
Current Profiler ◽  
The North Atlantic

AbstractDespite the major role played by mesoscale eddies in redistributing the energy of the large-scale circulation, our understanding of their dissipation is still incomplete. This study investigates the generation of internal waves by decaying eddies in the North Atlantic western boundary. The eddy presence and decay are measured from the altimetric surface relative vorticity associated with an array of full-depth current meters extending ~100 km offshore at 26.5°N. In addition, internal waves are analyzed over a topographic rise from 2-yr high-frequency measurements of an acoustic Doppler current profiler (ADCP), which is located 13 km offshore in 600-m deep water. Despite an apparent polarity independence of the eddy decay observed from altimetric data, the flow in the deepest 100 m is enhanced for anticyclones (25.2 cm s−1) compared with cyclones (−4.7 cm s−1). Accordingly, the internal wave field is sensitive to this polarity-dependent deep velocity. This is apparent from the eddy-modulated enhanced dissipation rate, which is obtained from a finescale parameterization and exceeds 10−9 W kg−1 for near-bottom flows greater than 8 cm s−1. The present study underlines the importance of oceanic western boundaries for removing the energy of low-mode westward-propagating eddies to higher-mode internal waves.

scholarly journals Detecting flow features in scarce trajectory data using networks derived from symbolic itineraries: an application to surface drifters in the North Atlantic

2020 ◽  
Vol 27 (4) ◽  
pp. 501-518 ◽  
Author(s):  
David Wichmann ◽  
Christian Kehl ◽  
Henk A. Dijkstra ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Trajectory Data ◽  
Surface Transport ◽  
The North ◽  
The North Atlantic ◽  
Surface Drifters ◽  
Double Gyre

Abstract. The basin-wide surface transport of tracers such as heat, nutrients and plastic in the North Atlantic Ocean is organized into large-scale flow structures such as the Western Boundary Current and the Subtropical and Subpolar gyres. Being able to identify these features from drifter data is important for studying tracer dispersal but also for detecting changes in the large-scale surface flow due to climate change. We propose a new and conceptually simple method to detect groups of trajectories with similar dynamical behaviour from drifter data using network theory and normalized cut spectral clustering. Our network is constructed from conditional bin-drifter probability distributions and naturally handles drifter trajectories with data gaps and different lifetimes. The eigenvalue problem of the respective Laplacian can be replaced by a singular value decomposition of a related sparse data matrix. The construction of this matrix scales with O(NM+Nτ), where N is the number of particles, M the number of bins and τ the number of time steps. The concept behind our network construction is rooted in a particle's symbolic itinerary derived from its trajectory and a state space partition, which we incorporate in its most basic form by replacing a particle's itinerary by a probability distribution over symbols. We represent these distributions as the links of a bipartite graph, connecting particles and symbols. We apply our method to the periodically driven double-gyre flow and successfully identify well-known features. Exploiting the duality between particles and symbols defined by the bipartite graph, we demonstrate how a direct low-dimensional coarse definition of the clustering problem can still lead to relatively accurate results for the most dominant structures and resolve features down to scales much below the coarse graining scale. Our method also performs well in detecting structures with incomplete trajectory data, which we demonstrate for the double-gyre flow by randomly removing data points. We finally apply our method to a set of ocean drifter trajectories and present the first network-based clustering of the North Atlantic surface transport based on surface drifters, successfully detecting well-known regions such as the Subpolar and Subtropical gyres, the Western Boundary Current region and the Caribbean Sea.

scholarly journals The Thermally Driven Ocean Circulation with Realistic Bathymetry

2018 ◽  
Vol 48 (3) ◽  
pp. 647-665 ◽  
Author(s):  
Ada Gjermundsen ◽  
Joseph H. LaCasce ◽  
Liv Denstad
Keyword(s):  
Temperature Gradient ◽  
Surface Temperature ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
The North ◽  
Surface Gradient ◽  
Thermally Driven ◽  
Surface Temperature Gradient

AbstractThe global circulation driven solely by relaxation to an idealized surface temperature profile and to interior mixing is examined. Forcing by winds and evaporation/precipitation is excluded. The resulting circulation resembles the observed in many ways, and the overturning is of similar magnitude. The overturning is driven by large-scale upwelling in the interior (which is relatively large, because of the use of a constant mixing coefficient). The compensating downwelling occurs in the northern North Atlantic and in the Ross and Weddell Seas, with an additional, smaller contribution from the northern North Pacific. The latter is weaker because the Bering Strait limits the northward extent of the flow. The downwelling occurs in frictional layers near the boundaries and depends on the lateral shear in the horizontal flow. The shear, in turn, is linked to the imposed surface temperature gradient via thermal wind, and as such, the downwelling can be reduced or eliminated in selected regions by removing the surface gradient. Doing so in the northern North Atlantic causes the (thermally driven) Antarctic Circumpolar Current to intensify, increasing the sinking along Antarctica. Eliminating the surface gradient in the Southern Ocean increases the sinking in the North Atlantic and Pacific. As there is upwelling also in the western boundary currents, the flow must increase even more to accomplish the necessary downwelling. The implications of the results are then considered, particularly with respect to Arctic intensification of global warming, which will reduce the surface temperature gradient.

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