27 research outputs found
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A numerical study of the Magellan Plume
In this modeling study we investigate the dynamical mechanisms controlling the spreading of the Magellan Plume, which is a low-salinity tongue that extends along the Patagonian Shelf. Our results indicate that the overall characteristics of the plume (width, depth, spreading rate, etc.) are primarily influenced by tidal forcing, which manifests through tidal mixing and tidal residual currents. Tidal forcing produces a homogenization of the plume's waters and an offshore displacement of its salinity front. The interaction between tidal and wind-forcing reinforces the downstream and upstream buoyancy transports of the plume. The influence of the Malvinas Current on the Magellan Plume is more dominant north of 50°S, where it increases the along-shelf velocities and generates intrusions of saltier waters from the outer shelf, thus causing a reduction of the downstream buoyancy transport. Our experiments also indicate that the northern limit of the Magellan Plume is set by a high salinity discharge from the San Matias Gulf. Sensitivity experiments show that increments of the wind stress cause a decrease of the downstream buoyancy transport and an increase of the upstream buoyancy transport. Variations of the magnitude of the discharge produce substantial modifications in the downstream penetration of the plume and buoyancy transport. The Magellan discharge generates a northeastward current in the middle shelf, a recirculation gyre south of the inlet and a region of weak currents father north
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The Upstream Spreading of Bottom-Trapped Plumes
It is well known that numerical simulations of freshwater discharges produce plumes that spread in the
direction opposite to that of the propagation of coastally trapped waves (the upstream direction). The lack of
a theory explaining these motions in unforced environments deemed the numerical results suspect. Thus, it
became a common practice in numerical studies to add a downstream mean flow to arrest the development of
the upstream perturbation. This approach is generally unjustified, and it remains a matter of interest to
determine if the upstream displacement produced by models is a geophysical phenomenon or a consequence
of erroneous assumptions in the model setup. In this article, the results of highly idealized numerical experiments
are used to investigate these matters. It is shown that this phenomenon is associated with the
geostrophic adjustment of the discharge and that upstream motion is endemic to the baroclinic structure of
bottom-trapped plumes. It is also shown that downstream displacements are generated by the cross-shelf
barotropic pressure gradient generated by the propagation of coastally trapped waves. Sensitivity experiments
indicate that the speed of upstream propagation and the density structure of the plume are affected by
bottom friction, the slope of the bottom, and the magnitude of the density anomaly. Bottom friction in
particular slows down the progression of the plume and changes its density structure, producing a more
homogeneous downstream region and a more stratified upstream region
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The Spindown of Bottom-Trapped Plumes
This note considers the decay of a bottom-trapped freshwater plume after the causative freshwater inflow
has ceased. It is shown that shortly after the low-density inflow stops, the barotropic pressure field that it
created radiates away and the ocean circulation becomes controlled by baroclinic pressure gradients generated
by the remnants of the inflow. This produces a reversal of the circulation in the region downstream of the
inflow, after which the entire plume starts to move in the upstream direction. The decay of the plume is
henceforth controlled by upstream oceanic flow and dilution through cross-isopycnal mixing
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On the Upwelling of Downwelling Currents
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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The Impact of Boundary Conditions on the Upstream Spreading of Bottom-Trapped Plumes
This study shows that simulations of bottom-trapped plumes in periodic or closed domains generate a spurious cyclonic current that arrests the natural tendency of the plume to move upstream. Furthermore, it also shows that attempts to obstruct the upstream spreading lead to a bias of the fundamental characteristics of the plume.Keywords: Sloping bottom, Implementation, Continental shelf, Buoyant coastal discharge, General circulation modelKeywords: Sloping bottom, Implementation, Continental shelf, Buoyant coastal discharge, General circulation mode
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A numerical study of the Southwestern Atlantic Sheld circulation : barotropic response to tidal and wind forcing
This article analyzes the barotropic circulation in the Southwestern Atlantic Shelf using
a three-dimensional numerical model forced with winds and tides. South of 40°S, the shelf
circulation is dominated by the propagation of the semidiurnal tides. In this region the
diurnal tides are generally weak, except at the shelf edge where they resonate with northward
propagating, continental shelf waves. North of 40°S, the tidal circulation is relatively weak,
and the circulation is mainly driven by the winds. The wind-driven annual mean circulation
is characterized by a broad northeastward flow south of approximately 40°S and is
characterized by a southwestward flow farther north. The intense mixing associated with the
Patagonian tides enhances the bottom friction that balances the energy input from the wind
stress forcing. In contrast with previous results our simulation shows a detrainment of the
northward volume transport with latitude due to an offshore flow along the edge of the
Patagonian shelf break. The largest seasonal variations of the shelf circulation are observed
in the region between 45°S and 25°S where, during the fall, there is a development of a
clockwise gyre and a northeastward flow north of 40°S. The gyre weakens toward the
winter, and the northeastward flow reverses directions
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A numerical study of the Southwestern Atlantic Shelf circulation : stratified ocean response to local and offshore forcing
This article discusses the results of a suite of numerical simulations of the oceanic
circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its
mean circulation and seasonal variability and to determine the dynamical mechanisms
controlling them. Our experiments indicate that south of 40°S the mean circulation is
dominated by a general northeastward flow in the southern portion of the shelf, which is
controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the
offshore influence of the Malvinas Current farther north. The region from 40°S to
33°S presents the highest seasonal variability, with intrusions of cold sub-Antarctic waters
and the northward expansion of mixtures of the RĂo de la Plata waters in late fall and a
slower retraction of the plume during spring-summer. Wind stress variability seems to be
the primarily forcing mechanism for the plume dynamics. These model results are in
reasonable agreement with observations and previous model results. The present solutions
also reveal important additional features of the shelf response. The along-shelf circulation,
for example, is largely driven by the western boundary currents in the middle and outer
shelf, with induced transports that are 3 times larger than in experiments forced by
winds and tides. The analysis also indicates that the upstream influence of the Malvinas
Current is felt well beyond its retroflection point in the form of a northward middle-shelf
current and that the interaction of the Brazil Current with the Brazilian shelf topography
is primarily responsible for inducing steady shelf break upwelling
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Shelfbreak upwelling induced by alongshore currents: analytical and numerical results
Alongshore flow in the direction of propagation of coastal trapped waves can result in upwelling at the shelfbreak. The intensity of this upwelling can be comparable in magnitude to wind-driven coastal upwelling, with its associated ecological features. Recent numerical experiments by Matano & Palma indicate that this upwelling results from convergence of Ekman transport at the shelfbreak. The mechanism for this phenomenon can be understood in terms of steady solutions to the shallow water equations in the presence of Coriolis force and bottom drag. Matano & Palma interpreted their numerical results in terms of the arrested topographic wave, but did not present direct comparisons. Here we present a family of analytical solutions to the equations of the arrested topographic wave that shows striking quantitative agreement with earlier numerical resultsThis is the publisher's version of record. The original submission is copyrighted by Cambridge University Press and can be found here: http://www.tos.org/Keywords: shallow water flows, ocean processes, topographic effectsKeywords: shallow water flows, ocean processes, topographic effect
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Shelfbreak upwelling induced by alongshore currents: analytical and numerical results
Alongshore flow in the direction of propagation of coastal trapped waves can result in upwelling at the shelfbreak. The intensity of this upwelling can be comparable in magnitude to wind-driven coastal upwelling, with its associated ecological features. Recent numerical experiments by Matano & Palma indicate that this upwelling results from convergence of Ekman transport at the shelfbreak. The mechanism for this phenomenon can be understood in terms of steady solutions to the shallow water equations in the presence of Coriolis force and bottom drag. Matano & Palma interpreted their numerical results in terms of the arrested topographic wave, but did not present direct comparisons. Here we present a family of analytical solutions to the equations of the arrested topographic wave that shows striking quantitative agreement with earlier numerical results.Keywords: Topographic effects, Ocean processes, Shallow water flow
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A comparison of the circulation patterns over the Southwestern Atlantic Shelf driven by different wind stress climatologies
We compare the oceanic circulation patterns over the
Southwestern Atlantic Shelf (SWAS) forced by nine
different wind stress climatologies. The largest differences
are observed in experiments forced with the Hellerman
and Rosenstein [1983, hereafter HR83] and Trenberth
et al. [1990, hereafter TR90] winds. HR83 shows a
general northeastward flow near the shelf break. The
TR90 results shows a bifurcating path south of ~40°S
and a poleward flow north of 35°S. The most robust
circulation patterns are a broad northward flow and the
generation of coastal re-circulation cells in southern
Patagonia and the development of a southward jet in the
inner portion of the South Brazil Bight