Investigation of persistent coherent structures along the Southern Brazilian Shelf

Dentre as diferentes influências que a hidrodinâmica da plataforma continental sul brasileira sofre, uma de destaque é a pluma de água proveniente da Lagoa dos Patos, gerando no seu entorno uma região com elevada complexidade na dinâmica do escoamento. Além disto, ventos sazonais, sinóticos e variáveis, bem como as condições das correntes costeiras levam a padrões alternantes de escoamentos. Deste modo, foram investigadas ocorrências de estruturas coerentes persistentes neste ambiente. Para tal fim, foi utilizada uma metodologia numérica capaz de descrever as principais características fluidodinâmicas durante um ano climatológico. O código aberto de elementos finitos Telemac-Mascaret foi aplicado no domínio tridimensional, juntamente com o modelo k-épsilon para a descrição dos efeitos turbulentos. Estruturas persistentes de elevada turbulência foram identificadas na área de estudo, as quais ocorrem como perturbações de curvatura definidas por altas taxas de produção e dissipação turbulenta. Como resultado, há um aumento local na mistura entre porções de fluido, caracterizado por um fluxo ascendente. Este fenômeno pode ser atribuído as irregularidades na topografia do leito, associado as tensões de cisalhamento provocadas pela ação do vento. Os resultados obtidos por modelagem numérica contribuíram para aumentar a informação disponível sobre as estruturas de circulação nesta região.The freshwater-influenced region of the Patos Lagoon discharge to the Southern Brazilian Shelf (SBS) is a region of complex fluid dynamics. Seasoning and synoptic variable winds and coastal current conditions create an alternating flow pattern. The aim of this paper was to investigate the occurrence of persistent coherent structures in this environment. A numerical approach was chosen to describe the main hydrodynamic features of the region during a climatological year. The open-source finite-element code TelemacMascaret was applied to the three-dimensional domain. In addition the two-equation k-epsilon model described the turbulence mechanisms. The presence of persistent high-turbulent structures was identified within the study area. These occur as a strong curvilinear disturbance characterized by higher turbulent production and dissipation rates, which increase local mixture. As a result, upward circulation flow was observed, which may be due to irregularities in bottom topography associated with wind-induced stress forces. These results increase the information about the circulation structures of the study area by means of numerical modelling analysis

O impacto do Prata sobre o oceano

Os habitantes da região costeira que se estende da província de Buenos Aires, na Argentina, a Rio Grande, no sul do Brasil, sabem que a água do mar é quase 10°C mais quente em janeiro que em julho. Um observador mais atento notaria também mudanças de cor na sua superfície em diferentes épocas do ano. Para explicar tais observações, especula-se sobre a existência de freqüentes incursões de água fria oriundas da corrente das Malvinas. Neste artigo são apresentadas evidências que sugerem que as alterações mais importantes ao longo de uma faixa costeira de aproximadamente 1.000 km se devem à influência variável das águas que o rio da Prata derrama no oceano Atlântico

On the dynamics of mangueira bay-patos lagoon (Brazil)

Mangueira Bay is a semi-enclosed system that is very important to the communities living around the city of Rio Grande. It is influenced by multiple and conflicting human impacts. This embayment is connected to Patos Lagoon Estuary through a narrow inlet and its dynamics seem to be related to the dynamics of the estuarine area. This study aims to investigate the mechanisms that control the dynamics of Mangueira Bay under the predominant forcing conditions and evaluate the influence of baroclinic estuarine circulation on the dynamics of the area. The TELEMAC-2D model was used to carry out two- and three-dimensional simulations of Patos Lagoon Estuary. The modeling results indicate that the dynamics of Mangueira Bay are directly related to the dynamics of the main access channel, which are controlled by a combination of remote and local wind effects. A time lag occurs between the excursion of salt in Mangueira Bay and the excursion of salt in the main channel, with salinity penetrating in the embayment when winds from the SW become weaker. Marked horizontal stratification is present throughout the embayment, with salinity gradients changing direction as a function of ebb and flood currents. The area where the embayment connects to the main channel presents higher salinity variability and vertical stratification, while the inner limits are less affected by the dynamics of the channel

Straining and advection contributions to the mixing process of the patos lagoon coastal plume, Brazil

The Southern Brazilian Shelf is a region influenced by freshwater, and the evolution of stratification can present important ecological consequences in this area. The aim of this paper was to investigate the importance of straining and advection processes that affect the stratification and destratification of the water column along the Southern Brazilian inner shelf, a region that is influenced by the Patos Lagoon coastal plume. The study was carried out through 3‐D numerical modeling experiments and the results were analyzed using the potential energy anomaly equation and wavelet analysis. Results showed that the potential energy anomaly showed strong variability over a time scale of several days and followed the wind pattern over the study region, and was accompanied by the monthly modulation of river discharge and remote effects associated with variability in oceanic circulation. However, the most important events in synoptic time scales occurred in periods shorter than 20 days and were coincident with the passage of meteorological systems over the study region. Straining and advection were the most important mechanisms for the evolution of stratification in the adjacent coastal region. Nonlinearities and dispersion terms were as important as modulation effects, mainly during periods of high fluvial discharge. Close to the Patos Lagoon mouth, vertical advection explained most of the stratification evolution, due to the morphological characteristics in this region. In the frontal region and far field of the plume, the following two regions must be considered: the northeast part, which is characterized by the convergence of the coastal currents and ebb flows associated with the freshwater discharge that promote the domination of the cross‐shore straining and advection, and the southwest part, which is controlled by the coastal currents that result in the domination by alongshore straining and advection and cross‐shore advection terms. Close to the mouth of the Patos Lagoon, the occurrence of downward velocities generated downward displacement of the isopycnals, which decreases the potential energy anomalies, and vice versa. Near the frontal region, the anomalies were dependent on the intensity of the fluvial discharge. During moderate to high discharge events, the northeastward currents intensified mixing along shore, which decreased the potential energy anomalies. In the same way, the southwestward currents intensified the spreading of freshwater and increased the stratification and the potential energy anomalies

Dynamics of the patos lagoon coastal plume and its contribution to the deposition pattern of the southern brazilian inner shelf

The southern Brazilian shelf (SBS) is a region influenced by fresh water. The initial deposition of suspended sediments carried by this fresh water presents important ecological consequences for the area. The aim of this study was to investigate the contribution of the Patos Lagoon coastal plume to the deposition pattern observed along the inner continental shelf by providing estimates of estuarine‐shelf suspended sediment exchange. The study was carried out through three‐dimensional numeric modeling experiments on coupled hydrodynamic and morphodynamic processes. Results were analyzed using the dynamic method proposed by Garvine (1995) to study persistent plumes. The results were also analyzed using wavelet analysis. Results showed that the Patos Lagoon coastal plume behaves as a hypopycnal plume covering the first meters of the water column. The Patos Lagoon presents a dominant ebb condition with a mean discharge of 2088 m3 s−1 and an exportation rate of suspended matter of approximately 1.3 × 107 t yr−1. In the adjacent coastal region deposition prevails in sheltered regions up to 10 m deep, with enrichment in silt reaching up to 10% in the deceleration region of the Patos Lagoon coastal plume. The dynamic balance indicates a behavior typical of small to average‐sized plumes, with a well‐developed displacement along the coast that is mainly controlled by the alongshore wind component. The Coriolis force and bed shear stress present significant contributions during periods of moderate to high freshwater discharge when large‐scale plumes are formed. Transversally to the coast the force balance is associated with the Coriolis effect and wind influence, with a less important contribution from bed shear stress. The inner continental shelf adjacent to the Patos Lagoon entrance is dominantly influenced by plume occurrence. In this region northeasterly winds induce the formation of southwestward currents near the coast. These currents deflect around jetties and generate recirculation zones to the south. The formation of a cyclonic eddy occurs in this area and creates convergence zones that intensify the vertical velocities promoting the deposition of suspended sediments

Investigation of persistent coherent structures along the Southern Brazilian Shelf

Abstract The freshwater-influenced region of the Patos Lagoon discharge to the Southern Brazilian Shelf (SBS) is a region of complex fluid dynamics. Seasoning and synoptic variable winds and coastal current conditions create an alternating flow pattern. The aim of this paper was to investigate the occurrence of persistent coherent structures in this environment. A numerical approach was chosen to describe the main hydrodynamic features of the region during a climatological year. The open-source finite-element code Telemac-Mascaret was applied to the three-dimensional domain. In addition the two-equation k-epsilon model described the turbulence mechanisms. The presence of persistent high-turbulent structures was identified within the study area. These occur as a strong curvilinear disturbance characterized by higher turbulent production and dissipation rates, which increase local mixture. As a result, upward circulation flow was observed, which may be due to irregularities in bottom topography associated with wind-induced stress forces. These results increase the information about the circulation structures of the study area by means of numerical modelling analysis

Variability of the Spreading of the Patos Lagoon Plume Using Numerical Drifters

The Patos Lagoon coastal plume is a small-scale outflow that is strongly controlled by meteorological tides. However, the riverine discharge of the lagoon is subject to high decadal variability. Hence, the discharge amount alters the scale of this coastal plume and its effects over the inner shelf environment. This study uses hydrodynamic simulations and a Lagrangian model to estimate the spreading of the plume under two different discharge conditions.Through scale parameters, we characterized the contrasts of the plume structure between high discharge and low discharge conditions. During a strong discharge regime, the width and thickness of the plume are enhanced, and the inertial processes increase against the frictional effect of the wind. The consequences of these differences include higher values of alongshore and cross-shore spreading of the drifters for the strong discharge regime. These findings indicate that under similar wind conditions, different amounts of riverine discharges alter the extent to which the material delivered by the plume can spread over the inner continental shelf