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

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

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

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

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

The influence of the Plata River discharge on the western South Atlantic shelf

The influence of the Plata, the second largest river in South America, extends along a coastal strip of 1300 km. Historical hydrographic and wind data and numerical simulations are combined to determine the seasonal and interannual variability of the Plata plume and its relationship to the magnitude of the river discharge and the intensity and direction of the wind stress. Our results indicate that the seasonal variability of the river plume is controlled by the alongshore component of the wind stress. During El Niño the effects of the wind and precipitation anomalies tend to compensate each other, preventing anomalous northeastward plume extensions associated to large outflow events. Numerical experiments confirm this finding and indicate that during El Niño the discharge from the Plata River spreads offshore

The salinity signature of the cross-shelf exchanges in the Southwestern Atlantic Ocean: Satellite observations

Satellite‐derived sea surface salinity (SSS) data from Aquarius and SMOS are used to study the shelf‐open ocean exchanges in the western South Atlantic near 35°S. Away from the tropics, these exchanges cause the largest SSS variability throughout the South Atlantic. The data reveal a well‐defined seasonal pattern of SSS during the analyzed period and of the location of the export of low‐salinity shelf waters. In spring and summer, low‐salinity waters over the shelf expand offshore and are transferred to the open ocean primarily southeast of the river mouth (from 36°S to 37°30′S). In contrast, in fall and winter, low‐salinity waters extend along a coastal plume and the export path to the open ocean distributes along the offshore edge of the plume. The strong seasonal SSS pattern is modulated by the seasonality of the along‐shelf component of the wind stress over the shelf. However, the combined analysis of SSS, satellite‐derived sea surface elevation and surface velocity data suggest that the precise location of the export of shelf waters depends on offshore circulation patterns, such as the location of the Brazil Malvinas Confluence and mesoscale eddies and meanders of the Brazil Current. The satellite data indicate that in summer, mixtures of low‐salinity shelf waters are swiftly driven toward the ocean interior along the axis of the Brazil/Malvinas Confluence. In winter, episodic wind reversals force the low‐salinity coastal plume offshore where they mix with tropical waters within the Brazil Current and create a warmer variety of low‐salinity waters in the open ocean

Respuesta barotrópica de los golfos norpatagónicos argentinos forzados por mareas y vientos Barotropic response of north Patagonian gulfs in Argentina to tidal and wind forcing

En este trabajo se presentan los resultados de un modelo hidrodinámico barotrópico de alta resolución no-lineal tridimensional para examinar la respuesta de los golfos norpatagónicos argentinos, San Matías (GSM), Nuevo (GN) y San José (GSJ), ante forzante de marea y patrones idealizados de viento superficial. La estructura de la circulación inducida por el viento predominante (Oeste) en los tres golfos consiste en dos giros independientes: uno anticiclónico en el Oeste con una corriente costera más intensa que fluye hacia el sur y un retorno más débil por el centro de los golfos y otro giro ciclónico en el Este, que establece la conexión con la Plataforma en el caso del GSM. El GN y GSJ tienen la comunicación con el exterior muy restringida. La inclusión de efectos disipativos producidos por la marea reduce la intensidad de los giros, pero la estructura espacial de la circulación permanece. Si se incrementa la magnitud del viento se intensifican principalmente los giros anticiclónicos y el transporte de intercambio en la boca del GSM y se profundiza la capa superficial de Ekman. En los experimentos forzados con vientos meridionales la circulación media se divide en dos giros cuyo sentido de circulación depende de la dirección del viento: un giro Norte ciclónico (anticiclónico) y un giro Sur anticiclónico (ciclónico) para vientos del Sur (Norte). Cuando los golfos son forzados simultáneamente por marea y vientos, las corrientes residuales de marea dominan la circulación media y contribuyen a un mayor aislamiento dinámico de los mismos con la plataforma exterior.This work presents the results of a high resolution three dimensional non-linear barotropic hydrodynamic model used to examine the response of three north Patagonian gulfs in Argentina (San Matías, SMG; Nuevo, NG; and San José, SJG) forced by tides and idealized patterns of surface winds. The structure of the circulation induced by the dominant winds (Westerly) in all three gulfs consists of two independent gyres: one anticyclonic gyre in the west with an intense southward coastal flow and weaker return flow in the middle of the gulfs and one cyclonic gyre in the east that, in the SMG's case, connects with the shelf. The other two gulfs (NG and SJG) have much more restricted connections with the exterior. The inclusion of tidal dissipation effects reduces the intensity of the gyres, but the general spatial pattern of the circulation remains. Increasing the wind magnitude mainly intensifies the anticyclonic gyre and the cross-shelf transport through the SMG mouth and deepens the surface Ekman layer. If the gulfs are forced with meridional winds, the circulation splits into two gyres whose sense of circulation depends on the wind direction: a northern cyclonic (anticyclonic) gyre and a southern anticyclonic (cyclonic) gyre for Southerly (Northerly) winds. When the gulfs are forced simultaneously by tides and winds, the residual tidal currents dominates the general circulation and contributes to a greater dynamical isolation of the gulfs from the exterior shelf

On the implementation of passive open boundary conditions for a general circulation model : the barotropic mode

The objective of this study is to evaluate the performance of several open boundary conditions applied to the Princeton ocean model. The focus is on passive open boundary conditions applied to the external mode, i.e., conditions that are applied when the mean flow at the open boundary is unknown and the values of the variables must be assumed or extrapolated from the interior solution. Three types of open boundary conditions (OBCs) are tested: (1) radiation conditions, (2) characteristic methods, (3) relaxation schemes. Numerical experiments are conducted in a zonal channel that include different forms of bottom topography. The experiments are designed to emphasize flow conditions dominated by direct wind forcing or wave radiation. Three sets of experiments are discussed: (1) forcing by a uniform, alongshelf wind stress; (2) the barotropic adjustment of an initial perturbation in the sea surface elevation; and (3) forcing by a traveling storm. The results of these experiments are compared with analytical solutions and the results of experiments using cyclic boundary conditions, or expanded domains. According to our results all radiation schemes of Orlanski's type perform poorly in either flows with strong nonlinear components or when the model response is dominated by the propagation of dispersive wave packets. The characteristic method recommended by Roed and Cooper [1987] provided reasonable responses in most cases, but we found problems in handling dispersive waves or variable wind forcing at the open boundaries. Although there were limitations in all the OBCs that we tested the best overall performances were for the conditions proposed by Flather [1976], combined with a local solution approach proposed by Roed and Smesdtad [1984], and the flow relaxation scheme developed by Martinsen and Engedahi [1987].Copyrighted by American Geophysical Union