Buoyant coastal currents

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Buoyant coastal currents

Ocean circulation derives its energy at the sea surface from two sources that define two circulation types: 1 wind-driven circulation forced by wind stress on the sea surface, inducing a momentum exchange, and 2 thermohaline circulation driven by the variations in water density imposed at the sea surface by exchange of ocean heat and water with the atmosphereinducing a buoyancy exchange. These two circulation types are not fully independent, since the sea-air buoyancy and momentum exchange are dependent on wind speed.

The wind-driven circulation is the more vigorous of the two and is configured as gyres that dominate an ocean region. The wind-driven circulation is strongest in the surface layer. The thermohaline circulation is more sluggish, with a typical speed of 1 cm 0. Wind stress induces a circulation pattern that is similar for each ocean. The depth penetration of the wind-driven currents depends on the intensity of ocean stratification: in those regions of strong stratification, such as the tropics, the surface currents extend to a depth of less than 1, metres about 3, feetand within the low-stratification polar regions the wind-driven circulation reaches all the way to the seafloor.

Near the thermal equator, where the warmest surface water is found, there occurs the eastward-flowing Equatorial Counter Current. At the geographic Equator a jetlike current is found just below the sea surface, flowing toward the east counter to the surface current. This is called the Equatorial Undercurrent. It attains speeds of more than 1 metre per second at a depth of nearly metres. It is driven by higher sea level in the western margins of the tropical ocean, producing a pressure gradient, which in the absence of a horizontal Coriolis force drives a west-to-east current along the Equator.

The wind field reverses the flow within the surface layer, inducing the South Equatorial Current. Equatorial circulation undergoes variations following the irregular periods of roughly three to eight years of the Southern Oscillation i.

Weakening of the east-to-west wind during a phase of the Southern Oscillation allows warm water in the western margin to slip back to the east by increasing the flow of the Equatorial Counter Current. Surface water temperatures and sea level decrease in the west and increase in the east. In the tropical Indian Ocean the strong seasonal winds of the monsoons induce a similarly strong seasonal circulation pattern. The subtropical gyres are anticyclonic circulation features.

The centre of the subtropical gyre is shifted to the west. This westward intensification of ocean currents was explained by the American meteorologist and oceanographer Henry M.

Stommel as resulting from the fact that the horizontal Coriolis force increases with latitude. This causes the poleward-flowing western boundary current to be a jetlike current that attains speeds of 2 to 4 metres 6.

This current transports the excess heat of the low latitudes to higher latitudes. The flow within the equatorward-flowing interior and eastern boundary of the subtropical gyres is quite different. It is more of a slow drift of cooler water that rarely exceeds 10 cm about 4 inches per second.

Associated with these currents is coastal upwelling that results from offshore Ekman transport. It carries about 30 million cubic metres 1 billion cubic feet of ocean water per second through the Straits of Florida and roughly 80 million cubic metres 2.

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Responding to the large-scale wind field over the North Atlantic, the Gulf Stream separates from the continental margin at Cape Hatteras. After separation it forms waves or meanders that eventually generate many eddies of warm and cold water. The warm eddies, composed of thermocline water normally found south of the Gulf Stream, are injected into the waters of the continental slope off the coast of the northeastern United States.

They drift to the southwest at rates of approximately 5 to 8 cm about 2 to 3 inches per second, and after a year they rejoin the Gulf Stream north of Cape Hatteras. Cold eddies of slope water are injected into the region south of the Gulf Stream and drift to the southwest. After roughly two years they reenter the Gulf Stream just north of the Antilles islands.

The path that they follow defines a clockwise-flowing recirculation gyre seaward of the Gulf Stream. Among the other western boundary currents, the Kuroshio of the North Pacific is perhaps the most like the Gulf Stream, having a similar transport and array of eddies. The Agulhas Current has a transport close to that of the Gulf Stream. It remains in contact with the margin of Africa around the southern rim of the continent.Buoyant coastal currents transport freshwater, heat, nutrients, sediments, pollutants, and biological organisms along many continental shelves and have important impacts on ecosystems, fisheries, and coastal circulation.

On the global scale, buoyant coastal currents are primarily responsible for the redistribution of freshwater. Considerable work is needed to improve our understanding of the pathways and mechanisms by which buoyant coastal currents adjust to coastline and topographic changes and the dynamics by which they move away from the coast. I have focused on several aspects of boundary current dynamics, and recently investigated the effects of coastline changes and the interaction of multiple currents of different densities.

The current moved across the mouth of the canyon for values of the ratio larger than unity. The significant result of this study, carried out in collaboration with Dave Sutherland MIT-WHOI Graduate Studentis that, although bottom friction is important in setting the position of the buoyant front, the separation process is driven by the inertia of the flow.

These results support the observational driven hypothesis that the path of the East Greenland Current is strongly influenced by the shelf bathymetry. Similarly, when a buoyant coastal current crosses a gap in the coastline, the fraction of buoyant water that enters the gap has been found, in the laboratory, to depend on the ratio of the channel width to the Rossby radius of deformation.

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This is true both for surface-trapped and slope-controlled currents. However, the details are different for the two kinds of currents.

A slope-controlled current can cross the gap less easily than a surface-trapped current, with a consequent increase of the buoyant flux into the gap. This results of this project were found in collaboration with Michelangelo Mariani, Michela De Dominicis, and Igino Angelini Undergraduate Guest Students and will be reported in two manuscripts in preparation. Boundary current dynamics become more complicated when multiple buoyant sources having different densities are present.

One relevant example of this scenario is the Gulf of Maine, where multiple rivers combine to form the western Maine coastal current. Even the simpler problem of how two buoyant currents align vertically and horizontally after they adjust to be in geostrophic balance, is still poorly understood. A combination of laboratory experiments and analytical calculations in collaboration with Jim Lerczak Oregon State U.

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Together with Domenico Mussardo and Yackar Mauzole Undergraduate Guest Studentswe compared these instabilities with single frontal instabilities previously observed by Griffiths et al.

In collaboration with Mike Spall WHOI I have been investigating the downwelling concentrated in strong currents subject to buoyancy loss near lateral boundaries.

Theoretical understanding of what controls the downwelling near boundaries its magnitude, length scales is weak and relies on parameterizations of poorly known turbulent mixing processes. There is a lack of clear understanding on whether the downwelling occurs over the spatial scale of the boundary current or within a narrower boundary layer.

I conducted laboratory experiments to resolve the turbulent mixing due to convective plumes and identify where downwelling takes place in order to determine whether the details of these small-scale turbulent processes need to be resolved explicitly in order to represent their influence on the larger-scale circulation. Cenedese C. Understanding the dynamics of the interaction between two river plumes.

buoyant coastal currents

Griffiths, R. Ageostrophic instability of ocean currents.After you enable Flash, refresh this page and the presentation should play.

Get the plugin now. Toggle navigation. Help Preferences Sign up Log in. To view this presentation, you'll need to allow Flash. Click to allow Flash After you enable Flash, refresh this page and the presentation should play. View by Category Toggle navigation. Products Sold on our sister site CrystalGraphics. Title: Circulation, structure and dynamics of a buoyant coastal current'. American Meteorological Society. Albuquerque New Mexico.

January Cold and Salty. Tags: albuquerque buoyant circulation coastal current dynamics structure. Latest Highest Rated. Title: Circulation, structure and dynamics of a buoyant coastal current' 1 Circulation, structure and dynamics of a buoyant coastal current. Salinity color and Temperature contour 5. Sectionally averaged fluctuating along-shore currents are geostrophically balanced.

In the near shore, weak stratification prevents bottom-stress from penetrating the entire water column. Surface currents are constrained by the coastline only within an internal Rossby radius from the coast. Whether your application is business, how-to, education, medicine, school, church, sales, marketing, online training or just for fun, PowerShow. And, best of all, most of its cool features are free and easy to use.

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All for free. Most of the presentations and slideshows on PowerShow.Chapman, D. The frontal trapping mechanism is remarkably robust, in fact so robust that the presence of a shelf break has little effect on the final location of the front.

Bottom stress is necessary for the frontal trapping mechanism, but the trapping isobath is relatively insensitive to the magnitude of the bottom friction coefficient. The near-surface part of the front is sometimes unstable, but it can be stabilized either by ambient stratification or by a weak background current in the direction of the buoyant inflow.

On some broad continental shelves, the accumulated influence of freshwater discharges dominates the large-scale circulation, establishing a fairly sharp change in water properties at or near the shelf break, typically separating relatively fresh and cold shelf waters from saltier and warmer slope waters. Such a shelfbreak front and its associated baroclinic currents are dominant features of the coastal circulation, strongly influencing both the alongshelf and cross-shelf transports of mass, momentum, nutrients, sediments, etc.

Coastal Currents

The best known and most studied example is the coastal circulation along the east coast of North America from the northern Labrador shelf to Cape Hatteras [e. Oxygen isotope measurements show that much of the continental shelf water found in the Middle Atlantic Bight MAB from Georges Bank to Cape Hatteras has a northern origin, at least as far north as the northern Labrador shelf e.

The details of the transit, however, are largely uncertain. There is considerable loss of shelf water along the way, but precisely where and by what processes remains unclear. Nevertheless, a persistent shelfbreak front is present along much of the Labrador shelf and through the MAB. It is particularly intriguing that the front remains close to the shelf break despite the fact that the shelf break is located at about m depth along the Labrador shelf, whereas it occurs at about m in the MAB.

The shelfbreak front in the MAB has been studied for many years, long enough to produce a climatology Linder and Gawarkiewiczand it is known to be quite variable. Recent observations at high resolution in both space and time have revealed even more complexity than previously anticipated Pickart et al. Small-scale 5—10 km hydrographic features and narrow baroclinic jets appear within the frontal region and then change or disappear within a few days.

External forcing e.

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Yet, the shelfbreak front is robust and resilient on long timescales; that is, it is virtually always present in some form, suggesting that some basic dynamics must be at work to establish and maintain the front despite such complications. If so, idealized process modeling should be capable of revealing these underlying dynamics.

There have been two basic approaches to modeling the steady or long-timescale behavior of a shelfbreak front. One approach considers the gradual filling of a two-dimensional cross-shelf and vertical frictionless shelf with freshened water from a local source e.

As the front separating fresher and ambient waters reaches the shelf edge, potential vorticity conservation restricts the continued seaward motion, thereby establishing the front at the shelf break.

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However, these models neither address the alongshelf formation and maintenance of the front nor account for the remote origin of the shelf water northern source for the MAB because the models are two-dimensional and the fresher water is supplied locally. Furthermore, the behavior depends critically on potential vorticity conservation, which is unlikely over the shelf where bottom friction typically generates a boundary layer that occupies an appreciable portion of the water column.

The second approach considers the three-dimensional evolution of an alongshelf current and the critical role of the bottom boundary layer.

Circulation, structure and dynamics of a buoyant coastal current' - PowerPoint PPT Presentation

Wright showed that offshore Ekman transport in the bottom boundary layer, caused by an imposed alongshelf current, can carry an existing surface-to-bottom front to the edge of a step shelf. Continued Ekman transport drains the alongshelf flow from the shelf, causing the foot or base of the front to move gradually downward along the face of the step until it reaches an equilibrium position where it carries all of the alongshelf flow.

Gawarkiewicz and Chapman examined the adjustment of a barotropic flow in an initially uniformly stratified coastal ocean and suggested that the action of the bottom boundary layer could itself form a front at the shelf break, where convergence within the bottom boundary layer causes flow to leave the bottom, moving upward along isopycnals.

This, in some respects, is a combination of the Wright and Gawarkiewicz and Chapman models.

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Eventually the depth becomes great enough that the alongshelf current and hence Ekman transport changes sign at the bottom. This creates a strong convergence in the bottom boundary layer that causes flow to leave the bottom and move upward along the front, as in the Gawarkiewicz and Chapman model.Thanks for helping us catch any problems with articles on DeepDyve.

We'll do our best to fix them. Check all that apply - Please note that only the first page is available if you have not selected a reading option after clicking "Read Article". Include any more information that will help us locate the issue and fix it faster for you. The bottom boundary layer exerts a powerful control over buoyant coastal currents that contact the bottom, providing a mechanism for trapping density fronts along isobaths.

Recent observations suggest that this mechanism may play a role in shelfbreak front dynamics. Here previous studies are extended to investigate frontal trapping by the bottom boundary layer in deeper water typical of shelf breaks and in the presence of ambient stratification.

A primitive-equation numerical model is used to study a buoyant current traveling along a vertical wall as it encounters shallow bottom topography typical of a continental shelf. At the initial point of contact, a surface-to-bottom front forms with an associated surface-intensified, geostrophic current. In the absence of bottom friction, the current shoals and continues along the shelf close to the coast. In the presence of bottom friction, buoyancy advection in the bottom boundary layer moves the front offshore across isobaths until it reaches a depth where the cross-isobath transport in the boundary layer nearly vanishes.

The frontal trapping mechanism is remarkably robust, in fact so robust that the presence of a shelf break has little effect on the final location of the front. Bottom stress is necessary for the frontal trapping mechanism, but the trapping isobath is relatively insensitive to the magnitude of the bottom friction coefficient.

The near-surface part of the front is sometimes unstable, but it can be stabilized either by ambient stratification or by a weak background current in the direction of the buoyant inflow. Enjoy affordable access to over 18 million articles from more than 15, peer-reviewed journals.

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Please enable Javascript on your browser to continue. Read Article. Download PDF. Share Full Text for Free beta. Web of Science. Let us know here. System error.Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer.

In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. The modelled river plume typically consisted of an offshore bulge and a coastal current.

The Kelvin number varied between 0. During the southwest monsoon the plume fringe twisted towards the south, while during the northeast monsoon it twisted towards north according to the reversal of monsoonal winds. The fresh water transport with respect to coastal currents varied in accordance with seasonal river discharge such that the value peaked in the wet season and dropped in the dry season.

The categorization of plume influenced area and realization of the direction of plume transport can be used for interpreting the dynamically and potentially active zones in the shelf off Kochi. The fresh water influx and the wave action make the coastal ocean a very dynamic and enriched ecosystem. The outward extension of buoyant plume depends on a large number of factors such as volume of outflow, inlet width, tidal action, wind, currents, local bathymetry, Coriolis acceleration etc.

Yankovsky and Chapman 2 suggested that the dynamics within the bulge are primarily cyclostrophic in nature, i.

The discharge into the coastal region can produce a strong buoyancy-driven coastal current that drags the low saline water to long distances. Since the fresh water influx has a strong dependence on seasonal rainfall, it drastically affects spatial extension and vertical gradients of the plume. This also has several effects on coastal zone properties such as reduction of salinity, increase in stratification and distribution of parameters like dissolved matters, pollutants, nutrients etc.

Schettini et al. In riverine plumes, fresh water is directly injected into the shelf region due to heavy river discharge, while in estuarine plumes the fresh water gets mixed with saline water within the estuary and as a result a modified buoyant plume protrudes out into the shelf. The disturbance in the plume frontal region may initiate cross frontal mixing through isopycnal layers resulting in the upward movement of nutrient-rich subsurface layers and leading to enhanced productivity of surface waters.

buoyant coastal currents

Jay et al. The studies by Nash and Moum 11 revealed that river plumes are one of the sources behind the creation of large-amplitude internal waves in the coastal ocean.

buoyant coastal currents

The direction of plume propagation is another major factor in establishing the plume behaviour and its environmental impact.

The offshore behaviour of the plume can be predicted by calculating Kelvin number as the ratio of source width to Rossby radius of deformation The plume continuously transports across the shelf and the dynamics associated with it eventually result in the dilution of plume with the ambient saline water 1314Coastal currents are coherent water masses in motion that are found in the region between the coastline and the edge of the continental shelf.

Coastal currents are important because the coastal zone is the place where most nutrients, pollutants, and sediments are introduced into the ocean, and where most larvae are generated and dispersed. Coastal currents are responsible for the transport and dispersal of these biological, chemical, and geological tracers in the water. For example, predictions of the advection and diffusion of spilled oil is dependent upon knowledge of the currents near the coast.

Coastal currents often are considered as being made up of two components, alongshore, or parallel to the coast, and cross-shore, or perpendicular to the coast. Away from the influence of inlets or river mouths, cross-shore flows are typically weaker than alongshore flows, yet the larger gradients in crossshore properties make the cross-shore flows better at redistributing those Skip to main content Skip to table of contents.

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Coastal Currents. How to cite. Introduction Coastal currents are coherent water masses in motion that are found in the region between the coastline and the edge of the continental shelf. This is a preview of subscription content, log in to check access. Bowden, K. Physical Oceanography of Coastal Waters.

Chichester: Ellis Horwood Limited. Google Scholar. Garvine, R. Estuarine plumes and fronts in shelf waters: a layer model. Journal of Physical Oceanography17 : — Komar, P. D, Beach Processes and Sedimentation2nd edn. Korso, P. The coastal jet: observations of surface currents over the Oregon continental shelf from HF Radar.

Oceanography10 2 : 53— Mofjeld, H. Tidal currents.


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