Ilson Carlos Almeida da Silveira

A corrente do Brasil ao largo da costa leste brasileira

Existe limitada e esparsa informacao na literatura sobre a Corrente do Brasil, a corrente de contorno que fecha o Giro Subtropical do Atlântico Sul em sua borda oeste. Neste artigo, compilamos as informacoes atualmente disponiveis na literatura para fornecer uma visao dos aspectos cinematicos e dinâmicos desta corrente, enquanto flui ao largo do Leste Brasileiro. Discutimos sua composicao em termos de massas de agua e de sua extensao vertical em diversas latitudes. Tambem apresentamos levantamento atualizado sobre aspectos cinematicos desta corrente, tanto em termos de velocidades observadas e geostroficas como de transportes de volume. Informacoes sobre o meandramento e formacao de vortices na Corrente do Brasil, ao largo da costa Sudeste, e seu provavel papel na penetracao da Agua Central do Atlântico Sul. na quebra da plataforma continental, igualmente sao abordados.

Shelf break upwelling driven by Brazil Current Cyclonic Meanders

Analysis of hydrographic data and results of numerical modeling show the occurrence of upwelling associated with the cyclonic meanders of the Brazil Current, in the region known as the Southeast Brazil Bight or Santos Bight. The temperature and salinity distributions collected during summer and winter confirm the idea that meander-induced shelf break upwelling plays an important role in the pumping of South Atlantic Central Water from the slope region onto the continental shelf.

Post-LGM sedimentation on the outer shelf–upper slope of the northernmost part of the São Paulo Bight, southeastern Brazil

Abstract Sedimentary and isotope data as well as 14 C AMS dating on 15 box cores were used to identify the main post-Last Glacial Maximum (LGM) processes in the outer shelf and upper slope of the northernmost part of the Sao Paulo Bight, southeastern Brazil. Results show slight but significant variations in calcium carbonate, total sulphur and nitrogen contents as well as carbon and oxygen isotope ratios. Sedimentation rates, varying from 5 to 184 mm kyr −1 are controlled by the shelf and upper slope morphology, the Brazil Current meander dynamics, and the Coastal Water offshore motion.

Modelo paramétrico analítico para a estrutura de velocidade do sistema corrente do Brasil

Velocity-based parametric models are often used in the oceanographic literature either in theoretical studies that involves linear stability properties or as boundary-initial conditions for regional implementations of numerical models. However, most of these models were developed for the Gulf Stream (Niiler & Robinson, 1967; Luther & Bane, 1985; Robinson et al., 1988; Arango et al., 1992; Xue & Mellor, 1993; Gangopadhyay et al., 1997). Because the Brazil Current (BC)- Intermediate Western Boundary Current (IWBC) System is unique in terms of its vertical structure as it flows off SE Brazil, an original parametric model has been developed and is presented in this paper. The model parameterizes both the surface-core structure of the BC as well as the intermediate level-core of the IWBC, keeping their basic features such as the asymmetric configuration due to their interaction with the topography. In the BC case, the model allows for the current shape to be deformed if it is close to the shelf break or to acquire a hyperbolic-like shape when it is positioned far away from the continental margin. In the IWBC parameterization, the main characteristic of the model is to reproduce a core placed at 800-900 m depth, adjacent to the intermediate portion of the continental slope. Additionally, under the assumption that the currents are in geostrophic balance, a sectional model of the density structure was obtained from the velocity model. This density model was constructed based on the integration of the thermal wind equation. Finally, the parametric model has been successfully tested, it reproduced with great accuracy the structure of three velocity sections obtained from observations within the boundaries of the so-called Campos Bight: 21oS, 22oS e 23oS.

Methods for estimating the velocities of the Brazil Current in the pre-salt reservoir area off southeast Brazil (23 ∘ S–26 ∘ S)

The Brazil Current (BC) is likely the least observed and investigated subtropical western boundary current in the world. This study proposes a simple and systematic methodology to estimate quasi-synoptic cross-sectional speeds of the BC within the Santos Basin (23∘ S–26∘ S) based on the dynamic method using several combinations of data: Conductivity, temperature, and depth (CTD), temperature profiles, CTD and vessel-mounted Acoustic Doppler Current Profiler (VMADCP), and temperature profiles and VMADCP. All of the geostrophic estimates agree well with lowered Acoustic Doppler Current Profiler (LADCP) velocity observations and yield volume transports of -5.56 ±1.31 and 2.50 ±1.01 Sv for the BC and the Intermediate Western Boundary Current (IWBC), respectively. The LADCP data revealed that the BC flows southwestward and is ∼100 km wide, 500 m deep, and has a volume transport of approximately -5.75 ±1.53 Sv and a maximum speed of 0.59 m s−1. Underneath the BC, the IWBC flows northeastward and has a vertical extent of approximately 1,300 m, a width of ∼60 km, a maximum velocity of ∼0.22 m s−1, and a volume transport of 4.11 ± 2.01 Sv. Our analysis indicates that in the absence of the observed velocities, the isopycnal (σ0) of 26.82 kg m−3 (∼500 dbar) is an adequate level of no motion for use in geostrophic calculations. Additionally, a simple linear relationship between the temperature and the specific volume anomaly can be used for a reliable first estimate of the BC-IWBC system in temperature-only transects.

Can a minimalist model of wind forced baroclinic Rossby waves produce reasonable results

The linear theory predicts that Rossby waves are the large scale mechanism of adjustment to perturbations of the geophysical fluid. Satellite measurements of sea level anomaly (SLA) provided sturdy evidence of the existence of these waves. Recent studies suggest that the variability in the altimeter records is mostly due to mesoscale nonlinear eddies and challenges the original interpretation of westward propagating features as Rossby waves. The objective of this work is to test whether a classic linear dynamic model is a reasonable explanation for the observed SLA. A linear-reduced gravity non-dispersive Rossby wave model is used to estimate the SLA forced by direct and remote wind stress. Correlations between model results and observations are up to 0.88. The best agreement is in the tropical region of all ocean basins. These correlations decrease towards insignificance in mid-latitudes. The relative contributions of eastern boundary (remote) forcing and local wind forcing in the generation of Rossby waves are also estimated and suggest that the main wave forming mechanism is the remote forcing. Results suggest that linear long baroclinic Rossby wave dynamics explain a significant part of the SLA annual variability at least in the tropical oceans.

Eddy Formation in 2½-Layer, Quasigeostrophic Jets

Abstract The formation of nonlinear eddies in unstable 2½-layer, quasigeostrophic jets is investigated using a piecewise constant potential vorticity, “contour dynamical” model. Both infinite and semi-infinite jet dynamics are explored, considering a potential vorticity configuration with one front in each dynamically active layer. Unlike previous studies, the infinite basic jets have a double baroclinic mode structure, which allows the lower-layer transport to range from westward to eastward (with the upper-layer transport considered eastward in all cases). A variety of eddy-shedding events are obtained, and dipole vortices can form even with eastward lower-layer flows. For semi-infinite jets connecting to coastal currents along the boundary, coastal and retroflection eddies can be shed when baroclinic instability mechanisms are included. However, it seems that the unstable waves must either propagate westward or slowly in the eastward direction to allow this process to occur.

CARACTERIZAÇÃO DA OCEANOGRAFIA FÍSICA DO TALUDE CONTINENTAL E REGIÃO OCEÂNICA DA BACIA DE CAMPOS

RESUMO A circulacao oceânica, sobre o talude continental e o Plato de Sao Paulo, na Bacia de Campos e dominada pelo chamado “Sistema Corrente do Brasil”. Este e formado pela Corrente do Brasil (CB), pela Corrente de Contorno Intermediaria (CCI) e pelos meandros e vortices associados a ambos escoamentos. A coluna d’agua deste sistema de correntes e formada por um empilhamento de massas de agua, cujas profundidades e interfaces variam devido ao ajustamento baroclinico destes fluxos. Analise multiparametrica fisico-hidroquimica de massas de agua, de inedita aplicacao na margem continental brasileira, foi conduzida para identificar a composicao tanto da CB quanto da CCI. Encontrou-se que a CB transporta Agua Tropical e Agua Central do Atlântico Sul para sul-sudoeste, enquanto a CCI transporta essencialmente Agua Intermediaria Antartica e Agua Circumpolar Superior para norte-nordeste. A CB tem uma tipica espessura de 450-500 m, enquanto a CCI se estende desde a base da CB ate mais de 1.150 m de profundidade. O transporte de volume caracteristico da CB e de 6 Sv e o da CCI, de 4 Sv. O nucleo de velocidades da CB, cujos valores instantâneos podem atingir 1 m.s−1, esta localizado sempre em superficie. Ja o nucleo da CCI se encontra entre 800-900 m e aparentemente esta associado ao minimo de salinidade de toda coluna d’agua. Analise de 11 meses de series correntograficas sobre o poco Marlim revelam que CB e CCI formam um jato baroclinico medio, sem componente barotropica relevante. Esta caracteristica e crucial para explicar a natureza dinâmica dos amplos e recorrentes meandros e vortices formados na Bacia de Campos: tratam-se de meandros instaveis com propagacao horizontal virtualmente nula. Ou seja, as duas correntes eventualmente se acoplam durante a formacao do meandro. O meandro passa entao a crescer em amplitude em direcao ao oceano aberto, drenando energia potencial da CB e CCI e a transformando em cinetica na estrutura vortical. O perfil vertical caracteristico destes meandros decai exponencialmente com a profundidade, o que indica que a CB deve apresentar uma variabilidade subinercial bastante mais rica que a da CCI. Tambem e mostrado pela analise que os meandros, que sao cristas ou cavados de ondas de vorticidade baroclinicas, nao apresentam periodos dominantes. Seus comprimentos de onda, no entanto, sao tipicamente na faixa de 200-350 km.

CORRENTES E MASSAS DE ÁGUA NA PLATAFORMA CONTINENTAL

RESUMO A analise de dados coletados em varios projetos, principalmente HABITATS e DEPROAS, juntamente com a consulta a trabalhos anteriores, possibilitou a descricao das caracteristicas hidrotermodinâmicas da Plataforma Continental da Bacia de Campos (PCBC), incluindo suas variabilidades espaco-temporais e principais forcantes. Duas regioes com caracteristicas oceanograficas distintas foram identificadas na PCBC: a Parte Norte (PN-PCBC), localizada ao norte do Cabo de Sao Tome, e a Parte Sul (PS-PCBC), compreendida entre o Cabo de Sao Tome e o Cabo Frio. A principal diferenca oceanografica entre essas duas regioes e o grau de intrusao em direcao a costa das duas massas de agua oceânicas transportadas pela Corrente do Brasil (CB), Agua Tropical (AT) e Agua Central do Atlântico Sul (ACAS). O volume de ACAS na PS-PCBC e relativamente grande, tanto no verao quanto no inverno, pois essa massa de agua ocupa extensas areas das camadas de fundo e profunda, principalmente no extremo sul, proximo a Cabo Frio. Na PN-PCBC o volume relativo de ACAS e menor do que ao sul, havendo em geral predominio da AT. A ressurgencia costeira que ocorre na PS-PCBC e intermitente e controlada pelos ventos sazonais e sinoticos. Nos periodos de ressurgencia costeira, a ACAS apresenta volumes ainda maiores na PS-PCBC, principalmente em torno de Cabo Frio. Em qualquer epoca do ano, as correntes mais frequentes na PCBC tem direcao SO, alinhadas com as isobatas e deixando a costa a direita do movimento, desde a quebra da plataforma continental ate profundidades de 20-30 m, em resposta a tensao de cisalhamento dos ventos que sopram de E-NE, predominantes na regiao, e ao fluxo medio da CB. Essa circulacao para SO esta associada, em geral, a correntes convergentes a costa na camada de fundo, contribuindo para as intrusoes de ACAS em direcao a costa. Inversoes de direcao das correntes, para NE e deixando a costa a esquerda, estao associadas, principalmente, a tensoes de cisalhamento dos ventos de S-SO que ocorrem durante passagens de frentes frias. Essas frentes sao mais frequentes e intensas no inverno e na primavera. As correntes quase paralelas as isobatas apresentam pequeno cisalhamento vertical, sendo essencialmente barotropicas. Na direcao normal as isobatas, o primeiro modo baroclinico deve ser considerado para explicar a maior parte da variância subinercial das correntes. As correntes de mare sao menores do que as correntes subinerciais na direcao quase paralela a costa.

Is the Vertical Variability of the Ocean in Santos Bight, Brazil, Dominated by the Western Boundary Current Meanders?

The Brazil Current (CB) flows southwestward as vertically stratified and organized western boundary jet in the Brazilian shelf-break region ranging from 20–40° S, where the CB’s mass transport grows vertically. This geographical band show intense mesoescale activity due to passageway of eddies and meanders, superimposed over oceanic large-scale recirculation features which influence the oceanic circulation in the Santos Bight Pre-salt cluster area 300 km offshore. Based on 2-year observed data series of an oceanographic mooring array at Lula Field, and based on repeated hydrographic data (seawater temperature, salinity and N2 profiles) we used statistical and dynamical orthogonal modes in order to approach the local vertical current profile variability. We verified that it is 85 % explained by EOF-1. This variability is essentially of 1st baroclinic mode. Great part of it occupies the first 400–600 m water depth, with no predominant direction. We also found remarkable water column seasonal stratification. Albeit of relative weaker mean flows (0.1–0.2 m s−1), the study area is eddy dominated which are geostrophically adjusted to the 1st baroclinic mode. Furthermore, we observed that the significant directional variability over the Sao Paulo Plateau occurs far away from the mean current jets that flow parallel to the continental shelf-break geometry.Copyright