Dissertations / Theses on the topic 'Western boundary current instability'

This dissertation examines the nature of jet-boundary interactions and the role of barotropic instabilities in the Brazil Current system while still attached to the western boundary. The motivation was due to the frequent observation of bipolar features associated to the Brazil Current flow south of Cape Frio (RJ) and off Santos Bight (22º-28ºS). Such observations were mainly obtained from sea surface temperature images, and also from few \"in situ\" hydrographic and direct velocity measurements data. Therefore, our main focus is on the formation of bipolar features associated with the flow, and the main hypothesis is that barotropic instability is responsible for the generation of such dipoles along the western boundary current jet, while it has to deal with topographic variations along its path poleward. We address the system dynamics from a semi-theoretical perspective, and also through the application of numerical modeling on a process study approach. So it includes semi-theoretical studies of jet-lateral boundary interactions in idealized configurations relevant to the Brazil Current system off Cape Frio, considering the quasi-geostrophic theory as an appropriate approximation of the system we want to investigate. Additionally, numerical modeling is used through the construction of idealized scenarios where we simplify the physics in order to isolate the dynamical process of interest. All the dynamical analyzes were initialized from synoptic hydrographic data set which comprised the Brazil Current system off Cape Frio region. We validated the quasi-geostrophic theory we assumed and obtained the kinematics characteristics of the jet. In the dynamical analyses, we started the investigation from the simplest framework applied here, where we evaluate a piecewise constant potential vorticity field in a quasi-geostrophic contour dynamics model. It dealt with a meridionally-oriented jet flowing southward along a straight western boundary. Next, in a second model, we added more complexity in the system, idealizing western boundary coastline scenarios considering a quasi-continuous potential vorticity field in a quasi-geostrophic numerical model. Finally, in a third model, we constructed a more complex scenario for the Brazil Current jet that incorporated real topography and stratification of the water column in a primitive equation numerical model. Among our main finds, we verified that purely barotropic instability is able to generate vortex dipoles. Moreover, variations in the western boundary can indeed trigger perturbations in the jet and dipoles form. Therefore, sites with abrupt change in bathymetry and coastline orientation are preferred to the formation of the BC vortical dipoles. Vortex streets and instability trains can also develop downstream of such locations as consequence of perturbing a potentially batropically unstable jet. Hence, the horizontal shear is key to the generation of vortex dipoles. A weak shear does not allow these features to form, instead, frontal eddies are possibly generated, with the predominance of anticyclones. Although the three different model strategies applied here differ in dynamical configurations and approximations, they still kept nearly the same regime for the Brazil Current vortex-dipole formation. Topographic variations such as those associated with the change of coastline orientation near Cape Frio (23ºS) account for those vortex dipoles and streets be dominantly observed within the Santos Bight.
A presente tese examina a natureza das interações entre jato e contorno e o papel de instabilidades barotrópicas no sistema Corrente do Brasil (CB) quando este ainda se encontra fluindo junto à margem continental oeste. A motivação se deu através da frequente observação de feições bipolares associadas ao escoamento da CB ao sul de Cabo Frio (RJ) e ao largo da Bacia de Santos (entre 22º e 28ºS). Tais observações se devem principalmente a imagens termais de temperatura da superfície do mar, a algumas raras observações \"in situ\" através de dados hidrográficos e medições diretas de velocidade. Assim, o principal foco é na formação de feições bipolares associadas ao fluxo médio, sendo a principal hipótese a de que instabilidade barotrópica é responsável pela formação destes dipolos ao longo da corrente quando esta tem que lidar com variações da topografia. Este estudo tenta abordar os problemas em uma perspectiva semi-teórica, e também através de modelagem numérica em uma abordagem de estudos de processo. Assim, inclui estudos semi-teóricos em configurações idealizadas relevantes para o sistema Corrente do Brasil ao largo de Cabo Frio, considerando a teoria quase-geostrófica como a aproximação apropriada para a dinâmica do sistema que queremos avaliar. Adicionalmente, a modelagem numérica é usada através da construção de cenários idealizados onde simplificamos a física a fim de isolar os processos que queremos investigar. Toda a análise dinâmica partiu de um conjunto de dados que compreendeu o sistema Corrente do Brasil ao largo de Cabo Frio. Validamos a teoria quase-geostrófica que estamos considerando e obtivemos as características cinemáticas do jato. Nas análises dinâmicas, começamos a investigação do problema partindo do cenário mais simples utilizado aqui, onde consideramos um campo de vorticidade potencial discretizado em camadas horizontais em um modelo quase-geostrófico de dinâmica de contornos. O modelo incorporou a presença de uma linha de costa retilínea orientada meridionalmente no contorno oeste. Posteriormente, incluímos mais complexidade no sistema, idealizando diferentes cenários de linha de costa e considerando um campo de vorticidade potencial quase-contínuo em um modelo numérico quase-geostrófico. Finalmente, construímos um cenário ainda mais complexo para a Corrente do Brasil, o qual incorporou a topografia real da região e a estratificação da coluna de água em um modelo numérico de equações primitivas. Dentre as principais conclusões, pudemos comprovar que instabilidade barotrópica pode promover a formação de dipolos. Além disso, variações no contorno podem consequentemente ser gatilhos para gerar perturbações no jato e dipolos se formam. Assim, locais de mudança abrupta de batimetria e orientação de linha de costa são preferidos para a formação de dipolos vorticais. Rua de vórtices e trens de instabilidade também podem se desenvolver à juzante de tais locais como consequência de se perturbar um jato potencialmente barotropicamente instável. Consequentemente, um cisalhamento horizontal é chave para a geração de dipolos vorticais. Um cisalhamento relativamente fraco não permite a formação de tais feições, e em vez disso, vórtices frontais são possivelmente gerados, com a predominância de anticiclones. Apesar de as três diferentes estratégias aplicadas aqui diferirem em termos de configurações dinâmicas e aproximações, estas ainda mantiveram aproximadamente o mesmo regime para a formação de dipolos. Variações na topografia tais como aquelas associadas com mudanças de orientação de costa próximo a Cabo Frio (23ºS) contam com o fato de dipolos vorticais e rua de vórtices serem frequentemente observados dentro da Bacia de Santos.

O sistema de correntes de contorno oeste que flui ao largo da costa leste brasileira entre 15°S e 22°S, é composto pela Corrente do Brasil (CB) fluindo para sul, a Sub-Corrente Norte do Brasil (SNB) fluindo para norte e a Corrente de Contorno Oeste Profunda (CCP) também fluindo para sul. Vigorosos meandros e vórtices são observados à jusante dos Bancos de Abrolhos (BA) e Royal Charlotte (BRC) e da Cadeia Vitória-Trindade. O objetivo central desta dissertação é o estudo da estabilidade deste sistema de correntes, aqui denomidado Sistema CB-SNB-CCP, utilizando o modelo oceânico da Universidade de Princeton (POM) em um cenário idealizado. Buscamos responder o quão instável é este sistema e quais seriam as características das ondas instáveis geradas a fim de contribuir para o conhecimento acerca da rica dinâmica de meso-escala observada nesta região. Objetivamos também, elucidar o papel dos BA e BRC nesta dinâmica. Embasados na alta baroclinicidade do sistema de correntes de contorno ao largo do sudeste brasileiro, optamos por representar o escoamento CB-SNB-CCP através de um modelo paramétrico do campo de massa, calibrado com os dados hidrográficos oriundos dos Cruzeiros Abrolhos [Silveira et al., 2006]. Mantendo o caráter idealizado do estudo, também empregamos topografia de fundo analítica, onde representamos o talude da região por uma função tangente hiporbólica. O BA e o BRC foram aproximados através de funções gaussianas devidademente ajustadas aos contornos da isóbata de 80 m extraídas do conjunto ETOPO 2. Para identificar os mecanismos de crescimento das possíveis ondas instáveis, calculou-se o balanço de energia das simulações realizadas de acordo com o método de [Xue & Bane, 1997]. Os resultados de três experimentos numéricos realizados sugerem que o sistema CB-SNB-CCP é instável. Ciclones quase-estacionários do lado costeiro da CB surgem como o principal modo de variabilidade desta corrente. De acordo com a análise do balanço energético, o crescimento dessas feições resulta primariamente de instabilidade baroclínica do escoamento. A escala horizontal típica das ondas e vórtices instáveis modelados é dada pelo raio de deformação interno, como esperado pela teoria de instabilidade baroclínica de escoamentos realisticamente estratificados. Em particular, os resultados do experimento com o BA e o BRC idealizados comprovam que estes funcionam como gatilhos para o desenvolvimento de ondas instáveis, favorecendo amplamente o crescimento das estruturas verticais. Os trens de onda instáveis quase-estacionários aqui obtidos sugerem que provavelmente o meandramento da CB observado em latitudes que se estendem até 28°S pode ser parte de um único sistema que se origina na região dos BA e BRC.
The western boundary currents system that flows off the eastern brazilian coast between 15°S e 22°S is composed by the southward-flowing Brazil Current (BC), the northward-flowing North Brazil Under Current (NBUC) and the Deep Western Boundary Current (DWBC) that flows south. Vigorous meanders and eddies are observed downstream of the Abrolhos (AB) and Royal Charlotte Banks (RCB) and the Vitória-Trindade Ridge. The main goal of this dissertation is to study the stability of this currents system, hereby named BC-NBUC-DWBC system, using the Princeton University Ocean Model (POM) in an idealized scenerio. We seek to answer how unstable is this system and what are the characteristics of the unstable waves in order to contribute to the understanding of the rich mesoescale dynamics observed in this region. We also aim to elucidate the role of the AB and of the RCB on this dynamics. Based on the high degree of baroclinicity of the western boundary currents system off the southeastern brazilian coast, we opted to represent the BC-NBUC-DWBC system through a parametric model of the mass field, calibrated with hydrographic data from the Abrolhos Cruises [Silveira et al., 2006]. Maintaining the idealized character of the study, we have also employed an analytical bottom topography, in which the region\'s continental slope is approximated by hyperbolic tangent function. The AB and RCB were approximated by gaussian functions properly adjusted to the 80 m isobath extracted from the ETOPO 2 database. To identify the growth mecanism of the unstable waves, the energy budget of the simulations was calculated according to [Xue & Bane, 1997]. The results from the three experiments conducted here suggest that the BC-NBUC-DWBC system is indeed unstable. Quasi-stationary cyclones in the coastal side of BC arise as the main mode of variability of this current. According to the energy budget analisys, the growth of these features results primarily from baroclinic instability of BC-NBUC-DWBC flow. The typical horizontal scale of the modeled unstable waves and eddies is given by the internal radius of deformation, as expected by baroclinic instability theory of realistically stratified flows. The quasi-stationary unstable wave trains modeled in the present study also suggest that the meandering of the BC observed down to 28°S are probably part of a single system that originates at the AB and RCB region.

This thesis presents a numerical exploration of the dynamics governing rotating flow driven by a surface stress in the " sliced cylinder " model of Pedlosky & Greenspan (1967) and Beardsley (1969), and its close relative, the " sliced cone " model introduced by Griffiths & Veronis (1997). The sliced cylinder model simulates the barotropic wind-driven circulation in a circular basin with vertical sidewalls, using a depth gradient to mimic the effects of a gradient in Coriolis parameter. In the sliced cone the vertical sidewalls are replaced by an azimuthally uniform slope around the perimeter of the basin to simulate a continental slope. Since these models can be implemented in the laboratory, their dynamics can be explored by a complementary interplay of analysis and numerical and laboratory experiments. ¶ In this thesis a derivation is presented of a generalised quasigeostrophic formulation which is valid for linear and moderately nonlinear barotropic flows over large-amplitude topography on an f-plane, yet retains the simplicity and conservation properties of the standard quasigeostrophic vorticity equation (which is valid only for small depth variations). This formulation is implemented in a numerical model based on a code developed by Page (1982) and Becker & Page (1990). ¶ The accuracy of the formulation and its implementation are confirmed by detailed comparisons with the laboratory sliced cylinder and sliced cone results of Griffiths (Griffiths & Kiss, 1999) and Griffiths & Veronis (1997), respectively. The numerical model is then used to provide insight into the dynamics responsible for the observed laboratory flows. In the linear limit the numerical model reveals shortcomings in the sliced cone analysis by Griffiths & Veronis (1998) in the region where the slope and interior join, and shows that the potential vorticity is dissipated in an extended region at the bottom of the slope rather than a localised region at the east as suggested by Griffiths & Veronis (1997, 1998). Welander's thermal analogy (Welander, 1968) is used to explain the linear circulation pattern, and demonstrates that the broadly distributed potential vorticity dissipation is due to the closure of geostrophic contours in this geometry. ¶ The numerical results also provide insight into features of the flow at finite Rossby number. It is demonstrated that separation of the western boundary current in the sliced cylinder is closely associated with a " crisis " due to excessive potential vorticity dissipation in the viscous sublayer, rather than insufficient dissipation in the outer western boundary current as suggested by Holland & Lin (1975) and Pedlosky (1987). The stability boundaries in both models are refined using the numerical results, clarifying in particular the way in which the western boundary current instability in the sliced cone disappears at large Rossby and/or Ekman number. A flow regime is also revealed in the sliced cylinder in which the boundary current separates without reversed flow, consistent with the potential vorticity " crisis " mechanism. In addition the location of the stability boundary is determined as a function of the aspect ratio of the sliced cylinder, which demonstrates that the flow is stabilised in narrow basins such as those used by Beardsley (1969, 1972, 1973) and Becker & Page (1990) relative to the much wider basin used by Griffiths & Kiss (1999). ¶ Laboratory studies of the sliced cone by Griffiths & Veronis (1997) showed that the flow became unstable only under anticyclonic forcing. It is shown in this thesis that the contrast between flow under cyclonic and anticyclonic forcing is due to the combined effects of the relative vorticity and topography in determining the shape of the potential vorticity contours. The vorticity at the bottom of the sidewall smooths out the potential vorticity contours under cyclonic forcing, but distorts them into highly contorted shapes under anticyclonic forcing. In addition, the flow is dominated by inertial boundary layers under cyclonic forcing and by standing Rossby waves under anticyclonic forcing due to the differing flow direction relative to the direction of Rossby wave phase propagation. The changes to the potential vorticity structure under strong cyclonic forcing reduce the potential vorticity changes experienced by fluid columns, and the flow approaches a steady free inertial circulation. In contrast, the complexity of the flow structure under anticyclonic forcing results in strong potential vorticity changes and also leads to barotropic instability under strong forcing. ¶ The numerical results indicate that the instabilities in both models arise through supercritical Hopf bifurcations. The two types of instability observed by Griffiths & Veronis (1997) in the sliced cone are shown to be related to the western boundary current instability and " interior instability " identified by Meacham & Berloff (1997). The western boundary current instability is trapped at the western side of the interior because its northward phase speed exceeds that of the fastest interior Rossby wave with the same meridional wavenumber, as discussed by Ierley & Young (1991). ¶ Numerical experiments with different lateral boundary conditions are also undertaken. These show that the flow in the sliced cylinder is dramatically altered when the free-slip boundary condition is used instead of the no-slip condition, as expected from the work of Blandford (1971). There is no separated jet, because the flow cannot experience a potential vorticity " crisis " with this boundary condition, so the western boundary current overshoots and enters the interior from the east. In contrast, the flow in the sliced cone is identical whether no-slip, free-slip or super-slip boundary conditions are applied to the horizontal flow at the top of the sloping sidewall, except in the immediate vicinity of this region. This insensitivity results from the extremely strong topographic steering near the edge of the basin due to the vanishing depth, which demands a balance between wind forcing and Ekman pumping on the upper slope, regardless of the lateral boundary condition. The sensitivity to the lateral boundary condition is related to the importance of lateral friction in the global vorticity balance. The integrated vorticity must vanish under the no-slip condition, so in the sliced cylinder the overall vorticity budget is dominated by lateral viscosity and Ekman friction is negligible. Under the free-slip condition the Ekman friction assumes a dominant role in the dissipation, leading to a dramatic change in the flow structure. In contrast, the much larger depth variation in the sliced cone leads to a global vorticity balance in which Ekman friction is always dominant, regardless of the boundary condition.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009.
Includes bibliographical references (p. 257-264).
This thesis examines the nature of eddy-mean flow interactions in western boundary current jets and recirculation gyre dynamics from both theoretical and observational perspectives. It includes theoretical studies of eddy-mean flow interactions in idealized configurations relevant to western boundary current jet systems, namely (i) a study of the mechanism by which eddies generated from a localized forcing drive mean recirculation gyres through the process of nonlinear rectification; and (ii) a study of the role of eddies in the downstream evolution of a baroclinic jet subject to mixed instabilities. It also includes an observational analysis to characterize eddy-mean flow interactions in the Kuroshio Extension using data from the downstream location of maximum eddy kinetic energy in the jet. New insights are presented into a rectification mechanism by which eddies drive the recirculation gyres observed in western boundary current systems. Via this mechanism, eddies drive the recirculations by an up-gradient eddy potential vorticity flux inside a localized region of eddy activity. The effectiveness of the process depends on the properties of the energy radiation from the region, which in turn depends on the population of waves excited. In the zonally-evolving western boundary current jet, eddies also act to stabilize the unstable jet through down-gradient potential vorticity fluxes. In this configuration, the role of eddies depends critically on their downstream location relative to where the unstable time-mean jet first becomes stabilized by the eddy activity. The zonal advection of eddy activity from upstream of this location is fundamental to the mechanism permitting the eddies to drive the mean flows.
(cont.) Observational results are presented that provide the first clear evidence of a northern recirculation gyre in the Kuroshio Extension, as well as support for the hypothesis that the recirculations are, at least partially, eddy-driven. Support for the idealized studies' relevance to the oceanic regime is provided both by indications that various model simplifications are appropriate to the observed system, as well as by demonstrated consistencies between model predictions and observational results in the downstream development of time-mean and eddy properties.
by Stephanie N. Waterman.
Ph.D.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), February 2001.
Includes bibliographical references (p. 116-118).

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2001.
Includes bibliographical references (p. 116-118).
I study the exchange between a boundary current and flanking horizontal recirculations in a 'sliced-cylinder' rotating tank laboratory experiment. Two flow configurations are investigated: a single recirculation and a double, figure-8, recirculation. The latter case involves a hyperbolic point, while the former does not. I investigate the stirring and mixing under both steady and unsteady forcing. I quantify the mixing in each case using effective diffusivity, Keff, and a corollary effective length, Leff, as derived by Nakamura (1995, 1996). This approach involves diagnosing the geometric complexity of a tracer field. Geometric complexity is indicative of advective stirring. Because stirring creates high gradients, flows with high advective stirring also have high diffusion, and stronger overall mixing. I calculate effective length from images of dye in the tank and find much higher values of Leff in the unsteady hyperbolic cases than in the other cases. Slight unsteadiness in flows involving hyperbolic points gives rise to a chaotic advection mechanism known as 'lobe dynamics'. These lobes carry fluid in and out of the recirculations, acting as extremely effective stirring mechanisms. I demonstrate the existence of these exchange lobes in the unsteady hyperbolic (figure-8) flow. The velocity field in the tank is calculated utilizing particle image velocimetry (PIV) techniques and a time series U(t) demonstrates the (forced) unsteadiness in the flow. Images of dye in the tank show exchange lobes forming at this same forcing period, and carrying fluid in and out of the recirculation. Based on the results of these experiments, I am able to confirm that, at least in this controlled environment, basic geometry has a profound effect on the mixing effectiveness of a recirculation. I demonstrate radically increased stirring and mixing in the unsteady hyperbolic flow as compared to steady flows and flows without hyperbolic points. Recirculations are ubiquitous in the world ocean; they occur on a variety of scales, in many different configurations, and at all depths. Some of these configurations involve hyperbolic points, while others do not. Chaotic advection via lobe exchange may be an important component of the mixing at multiple locations in the ocean where hyperbolic recirculation geometries exist.
by Heather E. Deese.
S.M.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
O Atlântico tropical compreendido entre 20ºN e 20ºS apresenta-se hoje como chave para o entendimento das mudanças climáticas na Terra. Duas regiões despertam particular interesse: A banda equatorial do Atlântico onde o sistema de correntes interage com forçantes térmicos superficiais e ressurgência de Ekman, como a área sudoeste do Atlântico tropical (05°S-25°S / 20°W-47°W), onde parte da Corrente Sul Equatorial (CSE) penetra pela borda leste e contribui com muitas das correntes de fronteira oeste ao longo da plataforma continental brasileira. Entretanto, a variabilidade da dinâmica nestas regiões, que se mostra importante por sua contribuição sobre o clima da região nordeste do Brasil, apresenta-se pouco estudada. O presente trabalho investigou estes importantes sistemas do Atlântico tropical utilizando o ROMS (Regional Ocean Model System). A primeira área de estudo compreendida entre 20°S-20°N e 42°W 15°E, com resolução horizontal de 1/6º e 30 camadas sigma (que acompanham o terreno). Variações sazonais do transporte zonal, estrutura das correntes e distribuição da TSM (Março e Agosto) obtidos numericamente foram avaliados e comparados com dados: de literatura, experimentais do PIRATA e observados por satélite. Os resultados desta simulação mostram que o modelo é capaz de reproduzir os principais aspectos da Subcorrente Equatorial (SE), Contra-corrente Norte Equatorial (CNE), Corrente do Golfo (CG) e os ramos central e norte dos sistemas de Corrente Sul Equatorial (cCSE/nCSE), em diferentes seções ao longo do equador. A comparação entre a estrutura térmica nos primeiros 500 m simulada e do Programa PIRATA mostra uma Camada de Mistura (MLD) bem reproduzida, particularmente, a ressurgência que induz uma MLD mais rasa verificada nas boias mais à leste durante o inverno austral até o final da primavera austral. A evolução sazonal do sistema Piscina Quente do Atlântico Sul (SAWP) Língua Fria (Cold Tongue) foi bem representado, que é importante para futuras previsões de variabilidade climática sobre as fronteiras continentais da parte sudoeste do Atlântico tropical. Do lado sudoeste do Atlântico tropical (05°S-25°S / 20°W-47°W), O ROMS (Regional Ocean Model System) foi utilizado pela primeira vez nesta área para simular a circulação oceânica utilizando uma malha de resolução horizontal de 1/12º com 40 camadas sigma, que acompanham o terreno, para resolução vertical. Para avaliação preliminar da configuração do ROMS adotada foram analisadas as distribuições superficiais e verticais de temperatura, além de calculadas as evoluções sazonais da camada bem misturada e dos balanços, atmosféricos e oceânicos, envolvendo a troca de calor dentro da camada bem misturada. A ordem de grandeza das componentes oceânicas (principalmente a difusão vertical e a advecção horizontal) é da mesma ordem de grandeza dos forçantes atmosféricos e quase sempre opostos entre si, com alguma diferença de fase e transporte dentro das camadas mais superficiais. Resultados de variabilidade interanual foram comparados com os primeiros dois anos de perfis de temperatura observados advindos dos três fundeios do programa PIRATA-SWE (Projeto PIRATA, Extensão Sudoeste). A estrutura térmica simulada nas camadas mais superficiais do oceano está em concordância com os resultados obtidos in situ. Resultados de simulação apontam para uma larga e relativamente fraca CSE, composta por uma sequência de núcleos não bem definidos e próximos a superfície. O transporte que flui para oeste da CSE nos primeiros 400 m de profundidade ao longo da seção que atravessa as boias PIRATA-SWE, calculado para simulação do ROMS entre 2005-2007, apresenta um volume médio transportado de 14,9 Sv, com um máximo observado em JFM (15,7 Sv) e um mínimo durante MJJ (13.8 Sv). Os resultados de simulação indicam que em 2005-2007 o transporte para oeste da CSE foi modulado pela variabilidade da componente zonal do vento. Três seções zonais, posicionadas do continente até a posição da boia PIRATA, confirmam transporte mais intenso da Sub-corrente do Norte do Brasil (SNB), fluindo para norte, e uma diminuição no transporte da Corrente do Brasil (CB),que flui para sul, durante maio de 2006 e maio de 2007, quando a bifurcação do ramo sul da CSE alcança sua posição mais ao sul. Por outro lado, o máximo escoamento da CB foi registrado durante janeiro de 2006, janeiro de 2007 e março de 2007, com um mínimo da SNB fluindo para norte em dezembro de 2005 e outubro/dezembro 2006, correspondendo ao período em que a bifurcação do ramo sul da CSE alcança sua posição mais ao norte (OND). A Elevação da Superfície do Mar (ESM) e a Energia Cinética turbulenta (ECT) superficial calculada a partir das simulações e dos produtos AVISO Rio05 apontam na superfície para os mais altos níveis de energia de meso-escala ao longo do ramo central da CSE e da SNB/CB. Resultados de modelagem ecológica usando o modelo NPZD acoplado com o ROMS confirmam esta região como uma área oligotrófica. Resultados do modelo ecológico são comparados com SeaWifs dataset e a dinâmica e a produção primária são localmente discutidos. Estes resultados preliminares disponibilizam mais informações diante da complexidade da região de divergência da SCE e encoraja-nos a conduzir estudos mais detalhados a respeito da dinâmica e do transporte de massa nessa região utilizando o ROMS. Este trabalho também apresenta a necessidade de continuação, ampliação e extensão vertical para o sistema de observação PIRATA-SWE, especialmente com medidas de salinidade em mais níveis de profundidade, além da instalação de medidores de correntes

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 165-174).
The variability in the DWBC, its connection to the forcing in the northern North Atlantic and interaction with the Gulf Stream were explored from a combination of remote sensing and in-situ measurements in the western North Atlantic. Using satellite altimetry and Sea Surface Temperature (SST) we found evidence of the relation between changes in the Gulf Stream path and the variability in the temperature and velocity fields in the Slope Water. This relation was such that southward shifts of the main axis of the Gulf Stream were preceded by cold temperature anomalies and intensification of the southwestward flow. The analysis of 5.5 years of moored CTD and horizontal velocity data in the DWBC at 69 0W recorded during the period 2002-2008, showed that the variability along the DWBC is linked to changes in the dense water formation regions. The evolution of potential vorticity (PV) at the mooring site, characterized by a transition from deep to upper Labrador Sea Water (LSW), was similar to that observed in the Labrador Sea 6 to 9 years earlier, and imply spreading rates for the LSW that varied over time from 1.5 to 2.5cm/s. The time dependence of the spreading rates was in good agreement with changes in the strength of the DWBC at the mooring site. The evolution of the DWBC transport was explored in more detail from a 5- element moored array, also at 69'W. The results, for the period of 2004-2008, were consistent with the single mooring analysis. The variability measured from the array showed that upper, intermediate and deep water mass layers expand and contract at each other's expense, leading to alternating positive and negative PV anomalies at the upper-LSW, deep-LSW and Overflow Water (OW). Larger DWBC transports were associated with enhanced presence of recently ventilated upper-LSW and OW, rather than deep-LSW. The relative contribution of the different water masses to the observed circulation was investigated by inverting individual PV anomalies isolated from the observations. We found that changes in the depth-integrated circulation were mostly driven by changes in the OW.
by Beatriz Peña-Molino.
Ph.D.

A contourite drift complex on the J-Anomaly Ridge (JAR) and Southeast Newfoundland Ridge (SENR), offshore eastern Canada, records an extensive archive of North Atlantic circulatory and sedimentary dynamics formed under the influence of the Deep Western Boundary Current (DWBC). Seismic-reflection profiles constrained by drill sites from IODP Expedition 342 are used to map the spatial and temporal distribution of contourite sedimentation and to evaluate the Cenozoic history of the DWBC within a preexisting climatic framework. This study indicates three phases of sedimentation termed here Pre-Contourite-Drift Phase (~115-50 Ma), Active-Contourite-Drift Phase (~50-2.6 Ma), and Post-Contourite-Drift Phase (~2.6-0 Ma). Bottom current controlled sedimentation began at the boundary between Pre-Contourite-Drift Phase and Active-Contourite-Drift Phase (~50 Ma), and correlates to a long-term global cooling trend that initiated at the end of the Early Eocene Climatic Optimum. Within the Active-Contourite-Drift Phase at ~30 Ma depocenters shifted deeper and current energy and focus is interpreted to have increased in association with global oceanographic change at the Eocene-Oligocene transition. The beginning of Post-Contourite-Drift Phase sedimentation (~2.6 Ma) marks a shift in bottom current path towards shallower water depths, and corresponds with the onset of Northern Hemisphere ice sheets. These events of circulatory reorganization correlate with other North Atlantic seismic stratigraphic studies, suggesting that these events occurred throughout the North Atlantic. An improved understanding of long-term (>1000000 yr) dynamics of North Atlantic circulation in response to significant reorganization of Cenozoic climate provides important context towards refining models and prediction of oceanic response to contemporary climate change.
Master of Science

O sistema de correntes composto pela Corrente do Brasil (CB) e Corrente de Contorno Intermediária (CCI), 20°S e 28°S, são talvez um dos menos estudados sistemas de correntes de contorno subtropical de todo oceano. Dentro desta região, a CB desenvolve vigorosos meandros e vórtices. Uma combinação de simulações numéricas e estudos observacionais representam importantes ferramentas para investigar este fenômeno. Medidas diretas de velocidade são raras e usualmente restritas a curtos intervalos de tempo, para que padrões de longo termo sejam descritos. Similarmente, dados hidrográficos quase sinóticos e dados de ADCP são esparsos. Por outro lado, dados de XBT armazenados pela Marinha do Brasil são abundantes. Investigaremos aqui como que perfis de temperatura (T) amostrados por XBT podem complementar a climatologia regional e serem utilizados para gerar campos de inicialização e assimilação para modelos numéricos. Um método para a construção de perfis sintéticos de salinidade (S) é proposto considerando o complexo padrão da curva TS da região, o formato do perfil da temperatura in situ e a salinidade superficial. Mais especificamente, salinidade superficial e o perfil adimensional da temperatura in situ são usados para a reconstrução da salinidade nos primeiros 200 m da coluna de água, para perfis com profundidade local superior a 1899 m. O perfil não dimensional da salinidade climatológica é usado para as reconstruções no intervalo de 900-1300 m. Este perfil é então redimensionalizado usando valores estimados pela relação TS climatológica em seus extremos. A reconstrução do perfil de salinidade nas demais porções da coluna de água se dá pelo ajuste linear de um polinômio S(T), para cada intervalo de 25 m. Também é necessária a extrapolação dos perfis verticais de temperatura a partir da profundidade máxima de 1800 m (máxima profundidade das sondas de XBT) até o fundo. Propusemos aqui a utilização de um perfil climatológico de T não dimensional, para então fazer a redimensionalização e extensão da estrutura vertical de temperatura até a profundidade local. Cálculos geostróficos utilizando dados provenientes de cruzeiros hidrográficos quase sinóticos são empregados para testar a metodologia. Uma comparação entre o método proposto e as velocidades geostróficas calculadas por outras metodologias é conduzida. Esta comparação inclui velocidades geostróficas calculadas mediante salinidade constante (S=35), uma adaptação da teoria baseada em ajustes polinomiais para a curva TS, e mediante um polinômio ajustado diretamente entre anomalia do volume específico e temperatura. O método proposto neste trabalho foi o que melhor reproduziu, em termos médios, a estrutura vertical de correntes e o respectivo transporte de volume associado, dentre todos aqueles testados. Em particular, o transporte de volume da CB foi praticamente idêntico ao valor obtido com os campos in situ.
The Brazil Current (BC) and the Intermediate Western Boundary Current (IWBC) between 20°S and 28°S are perhaps two of the less studied subtropical boundary currents of the world ocean. Within this region, the BC develops vigorous meanders and rings. A combination of numerical simulations and observational studies are important tools to unravel these phenomena. Direct current measurements are rare and usually too short to depict the mean, long term circulation patterns. Similarly, quasi-synoptic hydrographic and ADCP data in the region are sparse. On the other hand, XBT data archived by the Brazilian Navy are abundant. Here we investigate how XBT temperature (T) profiles may improve the study region climatology and be used to generate initialization fields for numerical models. A method for constructing synthetic salinity (S) profiles is proposed considering the complex shape of the TS curve of the area, the shape of the in situ temperature profile and surface salinity. More specifically, surface salinity and nondimensionalized in situ temperature profiles are used for reconstructing salinity in the upper 200 m of the water column for locations with local depth greater than 1899 m. The mean nondimensionalized climatological salinity profile is used for the depth interval between 900 and 1300 m. The nondimensionalized profile is then redimensionalized using values estimated by the climatological TS relationship in its extrema. The salinity vertical profile of the remaining portions of the water column by linear fitting S to T each 25 m interval. It is also necessary to vertically extrapolate the temperature profiles from 1800 m (maximum depth reached by the XBT probes) to the ocean bottom. We propose using the nondimensionalized climatological T profile and the values of the temperature sampled at 1800 m as well as the bottom climatological temperature to redimensionalize it and extend the temperature vertical structure to the local depth. Geostrophic calculations using data from recent quasi-synoptic hydrographic surveys in the area are employed to test the methodology. A comparison between the currently proposed method and the geostrophic velocities calculated by other techniques is conducted. This comparison also includes geostrophic velocity estimates considering: constant salinity (S=35), an adaptation of classical techniques based solely on the TS curve shape, and by direct relating specific volume anomaly and temperature. The method proposed in this work is the one which best reproduces the current vertical structure and current volume transport in mean terms among all tested. In particular, the BC volume transport is virtually identical to the values obtained with the in situ fields.

This study investigates the upper ocean circulation along the west Australian coast, based on recent observations (WOCE ICM6, 1994/96) and numerical output from the 1/6 degree Parallel Ocean Program model (POP11B 1993/97). Particularly, we identify the source regions of the Leeuwin Current, quantify its mean and seasonal variability in terms of volume, heat and salt transports, and examine its heat balance (cooling mechanism). This also leads to further understanding of the regional circulation associated with the Leeuwin Undercurrent, the Eastern Gyral Current and the southeast Indian Subtropical Gyre. The tropical and subtropical sources of the Leeuwin Current are understood from an online numerical particle tracking. Some of the new findings are the Tropical Indian Ocean source of the Leeuwin Current (in addition to the Indonesian Throughflow/Pacific); the Eastern Gyral Current as a recirculation of the South Equatorial Current; the subtropical source of the Leeuwin Current fed by relatively narrow subsurface-intensified eastward jets in the Subtropical Gyre, which are also a major source for the Subtropical Water (salinity maximum) as observed in the Leeuwin Undercurrent along the ICM6 section at 22 degrees S. The ICM6 current meter array reveals a rich vertical current structure near North West Cape (22 degrees S). The coastal part of the Leeuwin Current has dominant synoptic variability and occasionally contains large spikes in its transport time series arising from the passage of tropical cyclones. On the mean, it is weaker and shallower compared to further downstream, and it only transports Tropical Water, of a variable content. The Leeuwin Undercurrent carries Subtropical Water, South Indian Central Water and Antarctic Intermediate Water equatorward between 150/250 to 500/750 m. There is a poleward flow just below the undercurrent which advects a mixed Intermediate Water, partially associated with outflows from the Red Sea and Persian Gulf. Narrow bottom-intensified currents are also observed. The 5-year mean model Leeuwin Current is a year-round poleward flow between 22 degrees S and 34 degrees S. It progressively deepens, from 150 to 300 m depth. Latitudinal variations in its volume transport are a response to lateral inflows/outflows. It has double the transport at 34 degrees S (-2.2 Sv) compared to at 22 degrees S (-1.2 Sv). These model estimates, however, may underestimate the transport of the Leeuwin Current by 50%. Along its path, the current becomes cooler (6 degrees C), saltier (0.6 psu) and denser (2 kg m -3). At seasonal scales, a stronger poleward flow in May-June advects the warmest and freshest waters along the west Australian coast. This advection is apparently spun up by the arrival of a poleward Kelvin wave in April, and reinforced by a minimum in the equatorward wind stress during July. In the model heat balance, the Leeuwin Current is significantly cooled by the eddy heat flux divergence (4 degrees C out of 6 degrees C), associated with mechanisms operating at submonthly time scales. However, exactly which mechanisms it is not yet clear. Air-sea fluxes only account for ~30% of the cooling and seasonal rectification is negligible. The eddy heat divergence, originating over a narrow region along the outer edge of the Leeuwin Current, is responsible for a considerable warming of a vast area of the adjacent ocean interior, which is then associated with strong heat losses to the atmosphere. The model westward eddy heat flux estimates are considerably larger than those associated with long lived warm core eddies detaching from the Leeuwin Current and moving offshore. This suggests that these mesoscale features are not the main mechanism responsible for the cooling of the Leeuwin Current. We suspect instead that short lived warm core eddies might play an important role.

Os meandros da Corrente do Brasil ao largo do sudeste brasileiro s~ao singulares em termos de sua composic¸ ~ao termohalina. Estes v´ ortices, principalmente na regi~ao de Cabo Frio e Cabo de S~ao Tom´e, constituem um sistema integrado a ressurg^encia costeira e a pr´opria Corrente do Brasil, caracterizando um sistema din^amico que propomos denominar Corrente do Brasil-v ´ ortice-ressurg^encia. Apresentamos uma metodologia de construc¸ ~ao de modelos param´etricos das feic¸ ~oes oceanogr´aficas (ou Modelos de Feic¸ ~ao) que comp~oem o sistema. Estes modelos s~ao capazes de construir tridimensionalmente feic¸ ~oes oceanogr´aficas individuais a partir de dados sin´oticos de superf´?cie e fundo para elaborac¸ ~ao de campos iniciais de modelos num´ericos, com o objetivo de compreender a interac¸ ~ao do sistema din^amico Corrente do Brasil-v´ ortice-ressurg^encia. Atrav´es da simulac¸ ~ao num´erica dos campos iniciais baseados nos Modelos de Feic¸ ~ao investigamos o processo que conduz `a assimetria termohalina dos v´ ortices da Corrente do Brasil, acima descrita. Esta foi devida a aproximac¸ ~ao desses v´ ortices junto ao talude continental e ao ajustamento barocl´?nico das correntes `as menores profundidades. Verificamos, ainda, que a penetrac¸a~o da A´ gua Central do Atla^ntico Sul pela camada de Ekman de fundo n~ao tem efeito significativo na estrutura dos v´ ortices. Realizamos experimentosnum´ericos adicionais com o objetivo de investigar quais os tipos de instabilidade geof´?sica estariam envolvidas no fen^omeno do crescimento dos meandros e v´ ortices da Corrente do Brasil. Analisando os resultados atrav´es de c´alculos de convers~ao de energia pudemos concluir que o sistema Corrente do Brasil-v´ ortice-ressurg^encia apresenta instabilidade mista ( barotr ´opica e barocl´?nica). Contudo, verificamos quantitativamente que a instabilidade barocl´?nica foi dominante. O sistema, no entanto, mostrou-se sens´?vel `a ac¸ ~ao da tens~ao de cisalhamento do vento de nordeste na regi~ao e consequentemente `a ressurg^encia costeira. Interpretamos tal sensibilidade como fator respons´avel pela relev^ancia da instabilidade barotr ´opica no processo de crescimento dos meandros da Corrente do Brasil. Por fim, verificamos que um meandro cicl ^onico frontal, quase-estacion´ario, em condic¸ ~oes especiais de crescimento por mecanismo de instabilidade, pode auxiliar no estabelecimento da ressurg^encia costeira. O v´ ortice do Cabo de S~ao Tom´e simulado apresenta velocidade de fase muito baixa e cresce em direc¸ ~ao a oceano aberto, de forma praticamente perpendicular `a quebra de plataforma. Logo, o crescimento do vo´ rtice de Cabo de Sa~o Tome´ faz com que o mesmo advecte A´ gua Costeira para seu interior provocando, por continuidade, afloramento da A´ gua central do Atla^ntico Sul junto `a costa. Assim, podemos dizer que h´a, de fato, uma interac¸ ~ao entre o sistema costeiro e a atividade de meso-escala da Corrente do Brasil.
The Brazil Current meanders off the Brazilian southeast coast are unique in terms of their thermohaline structure. These eddies, especially off the Cape Frio and S~ao Tom´e region constitute an integrated system with the Brazil Current and the coastal upwelling, characterizing a dynamic system which we propose to nominate as Brazil Current-eddy-upwelling system. We present a methodology of parametric model construction of the oceanographic features (or Feature Models) which compose the inferred system . These models are capable of three-dimensionally building independent the relevant features from synoptic surface and bottom data in order to compose the initial fields for numerical models with the objective of understanding the dynamics of the interaction of Brazil Current-eddy-upwelling system. By means of numerical simulation usingthe initial fields based on the Feature Models, we investigated the process which conducts to the thermohaline asymmetry of the eddies in the Brazil Current as mentioned above. This is due to the baroclinic approximation of the currents to shallower depths. The penetration of the Central Water in the South Atlantic by the bottom Ekman layer does not have a significant effect on the eddy structure. Additional numerical experiments were carried out with the objective of unders tanding which of the geophysical instabilities were involved in the growth of the Brazil Current meanders and eddies. The results analyzed through calculation of energy conversion allowed us to conclude that the Brazil Current-eddie-upwelling system presents mixed instability (barotropic and baroclinic). However, the baroclinic instability is dominant and is mostly associated to the vertical shear of the oceanic currents. The system, however,was sensitive to the action of the northeast wind shear and consequently to the coastal upwelling. Such sensibility was interpreted as the main responsible for the relevance of barotropic instability in the growth process of meanders in the Brazil Current. Lastly, we verified that a quasi-stationary frontal cyclonic meander, in special growth conditions by instability mechanisms, mayhelp in establishing coastal upwelling. The simulated S~ao Tom´e Cape Eddy presented a very low phase velocity and grew towards the open sea, almost perpendicularly to the continental shelf break. Therefore, the growth of the S~ao Tom´e Cape Eddy advects Coastal Water into its center and, therefore, lead to upwelling of South Atlantic Central Water near the coast. We can therefore say that there are in fact interaction mechanisms between the coastal system and the meso-scale activity of the Brazil Current.

Variabilidades de longo período têm sido detectadas no padrão de ventos em larga escala no hemisfério sul, com base em dados observacionais, reanálises e modelos. Estudos recentes têm sugerido algumas respostas oceânicas a mudanças no padrão atmosférico, desde a migração da Frente Subtropical, ao aumento do Vazamento das Agulhas e à intensificação dos giros subtropicais (GS). Apesar da importância do Atlântico Sul (AS) na distribuição global de calor, ainda não está claro a variabilidade de sua circulação e suas respostas às mudanças no campo do vento em larga escala. O presente estudo tem como objetivo investigar, usando resultados de uma simulação com o modelo numérico HYCOM (Hybrid Coordinate Ocean Model), a variabilidade do transporte de volume da Corrente do Brasil (CB) em sua região central (∼30ºS) frente ao campo de vento da Reanálise I do NCEP/NCAR (National Centers for Environmental Prediction/National Centers for Atmospheric Research). Os produtos do NCEP/NCAR apresentam intensificação e deslocamento em direção ao polo do cinturão de ventos de oeste no hemisfério sul (HS), o que ocasiona alteração do rotacional da tensão de cisalhamento do vento, ao longo de toda a bacia do AS, e a migração para sul das linhas de rotacional nulo e máximo em latitudes subtropicais. Mesmo com a intensificação do rotacional médio sobre o AS, a série computada do fluxo de retorno do transporte integrado de Sverdrup em 30ºS, através dos dados da reanálise, demonstram uma diminuição entre os anos de 1960 e 2010. Este comportamento é coerente com a tendência à redução do rotacional mediado zonalmente ao longo da mesma latitude. O transporte da CB computado com resultados do HYCOM apresenta uma tendência próxima à encontrada para o transporte de Sverdrup integrado, de cerca de 0,1 Sv por década, e estas apresentam correlação máxima de 0,6 com defasagem de 2 anos. Ao longo de 30°S, a leste da CB modelada, é encontrado um fluxo para norte identificado como o retorno de uma célula de recirculação, com transporte médio de 4,25±2,87Sv. Este fluxo também apresenta tendência à redução, a qual poderia estar relacionada com uma migração para sul de toda a estrutura da recirculação, acompanhando o deslocamento da confluência Brasil-Malvinas e da Frente Subtropical. O transporte residual na borda oeste (CB+Recirculação) apresenta uma intensificação do fluxo para sul, a qual poderia indicar um fortalecimento do GS, o que estaria em concordância com a intensificação e maior abrangência da Alta Subtropical observada no campo de Reanálise I. No entanto não foi encontrada correlação significativa entre o campo de vento e a variabilidade do transporte residual ao longo da fronteira oeste.
Long term variability has been detected in the South Hemisphere large-scale wind pattern, using models, reanalysis and observations. Recent studies have suggested an oceanic response to changes in the atmospheric pattern, such as the Subtropical Front migration, increase in the Agulhas leakage and intensification of the subtropical gyre (SG). Despite the importance of the South Atlantic in the global heat distribution, the variability of its circulation and response to changes in the large-scale wind field remains unclear. The current study aims to investigate, from a numerical simulation using HYCOM (Hybrid Coordinate Ocean Model), the variability of the Brazilian Current (BC) volume transport at its central region (∼30ºS) according to the wind field from Reanalysis I - NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The NCEP/NCAR results show an intensification and a poleward shift of the Southern Hemisphere westerly winds, leading changes in the wind curl along the SA, and a southern migration of the zero and maximum curl lines in subtropical latitudes. Even with the intensification of the mean wind curl over the SA, the time series of integrated Sverdrup return flow transport at 30ºS show a decrease between 1960 and 2010. This pattern is in agreement with the reduction trend in the zonal mean curl along the same latitude. The computed BC transport by HYCOM results present a similar trend to the integrated Sverdup transport (approximately 0.1 Sv per decade), with a 0.6 maximum correlation with a 2 year delay. Along the 30ºS, close to the western boundary, a north flow was identified as the return of a recirculation cell, with a 4,25±2,87 Sv mean transport. This flow also presents a reduction trend, which could be related to southern migration of the entire recirculation structure, following the Brazil-Malvinas confluence and Subtropical Front displacement. The residual transport at the western boundary (BC+Recirculation) shows an increase in the south flow that could indicates an increase of the SG, which is in agreement with the intensification and expansion of the Subtropical Ridge observed in the Reanalysis I. However, no significant correlation between the wind field and residual transport variability along the western boundary was found.

A Corrente de Contorno Oeste Profunda (CCP) é um dos principais componentes da Célula de Revolvimento Meridional do Atlântico e, portanto, é determinante para a manutenção do clima global. Ela é gerada nas regiões subpolares do Atlântico Norte e transporta as águas formadas neste sítio para o Hemisfério Sul. A sua trajetória bem como a sua variabilidade têm sido estudada ao longo dos tempos, mas informações acerva desta corrente no Atlântico Sul ainda são escassas. Estudos recentes mostram que ao sul da latitude de 8ºS a CCP se quebra em vórtices anticiclônicos e se propagam para sul. Reunimos um amplo conjunto de dados observacionais de cruzeiros oceanográficos e os utilizamos para observar estas feições ao largo da costa leste brasileira. Utilizamos o cálculo geostrófico referenciado em 4000 dbar a partir dos dados hidrográficos destes. Os resultados do cálculo geostrófico confirmam os achados na literatura, identificando estruturas vorticais com diâmetros entre 162 e 220 m e velocidades azimutais máximas ultrapassando 25 cm s-1. Um fundeio correntográfico instalado nas imediações da latitude 11ºS pela Universidade de Kiel no inicio do século foi utilizado para avaliar a regularidade e sazonalidade na formação dos anticiclones. Foi conferido que a formação de vórtices possui uma modulação sazonal, ocorrendo predominantemente em períodos que se observa uma maior intensidade da CCP, ou seja, de abril a setembro. Os resultados de simulação global do modelo HYCOM nos permitiram analisar a região de estudo de forma mais ampla, tanto espacial como temporalmente. A análise dos resultados de modelo sugere que a ocorrência de vórtices anticiclônicos da CCP entre 11ºS e 20ºS é bem regular e se propaga com velocidade velocidade de translação média de 5 cm s-1, porém é severamente interrompido ao se deparar com a complexa topografia ao sul de 17ºS. Entre os obstáculos topográficos, podemos destacar a Cadeia de Abrolhos (principalmente), o Monte Submarino Hot Spur e a Cadeia Vitoria Trindade, sendo que este último deflete para leste o escoamento associado provavelmente a uma CCP restabelecida. Embora os anticiclones da CCP tenham sido descritos há mais de uma década, acreditamos ser este o trabalho que primeiro mostra o destino e a destruição dessas feições ao largo da margem continental leste brasileira.
The Deep Western Boundary Current (DWBC) is one of the main components of the Atlantic Meridional Overturning Circulation and is therefor determinant in the maintainance of the global climate. This current has its origins in the subpolar region of the North Atlantic and transports the water mass formed at deep levels to the Southern Hemisphere. Its path as well as its variability have been studied in the last twenty years but information about the DWBC in the South Atlantic are still sparse. Recent investigations showed that the DWBC breaks into large anticyclonic eddies at around 8ºS. In this wrok, we gathered an ample observation data set of oceanographic cruises and used them to describe the vortical rings off the Eastern Brazilian coast. We emplyed the classic dynamics method referenced to 4,000 dbar to infer geostrophic velocity patterns from the data set. We indentify vortical structures of typically 162-220 km of diameter and azimuthal velocities higher than 25 cm s-1. A currentmeter mooring deployed by the University of Kiel in the begining of the century was used to investigate regularity and sazonality of the eddy formation and their passing of the 11ºS paralell. We found by inspecting the 5-year time series that there are about 3,7 events/year and a seasonal modulation dictated by the strength of the DWBC. More rings are shed during the April-September season when the DWBC is more intense and transports more. We also analyzed the output of a assimilative global simulation by the Hybrid Ocean Circulation Model (HYCOM) Consortium after validating it with our observational data set. Our results indicate that the anticyclones propagate at a characteristic speed of about 5cm s-1 until interacts with complex seamount topography at 17ºS. The topography is formed by th Abrolhos Ridge (17ºS), the Seamount Hot Sput (18ºS) and the Vitória-Trindade Ridge (20ºS). Moreover, it seems that the vast majority of the vortical features are destroted on the northernmost obstacle - the Abrolhos Ridge. Only a few survive and are destructed south. It seems from the analysis that a recoalesced DWBC existis nearby 20ºS, where it veers east and flows toward the interior of the Atlantic Ocean basin. We should emphasize that despite the DWBC anticyclones original description is more than a decade old, this is the first work to describe the fate of these large rings off the Eastern Brazil Continental Margin.

Denomina-se Sistema Corrente do Brasil (CB) ao sistema de correntes de contorno oeste formados pela CB, fluindo para sul-sudoeste, e Corrente de Contorno Intermediária (CCI), fluindo para norte-nordeste, associado aos meandros e vórtices. O sistema bordeja a margem continental brasileira sudeste ao longo de 1200-1800 m. A investigação da dinâmica das ondas baroclínicas de vorticidade superpostas ao Sistema Corrente do Brasil é conduzida através exclusivamente de análise de dados hidrográficos de dois cruzeiros de meso-escala (verão e inverno de 1993) do projeto Circulação Oceânica da Região Oeste do Atlântico Sul - COROAS, a componente brasileira do World Ocean Circulation Experiment - WOCE. Estes cruzeiros foram realizados na porção central do Embaiamento de São Paulo (\'23,5 GRAUS\' - \'27 GRAUS\'S). Objetivando-se a aplicação do Método Dinâmico, estimativa quantitativa de um Nível de Referência (NR) é conduzida comparando-se duas metodologias diferentes e independentes. A primeira utiliza o fato de que a CB transporta Água Tropical (AT) e Água Central do Atlântico Sul (ACAS) e de que a CCI transporta dominantemente Água Intermediária Antártica (AIA). Aplica-se então um Critério Termodinâmico, no qual a interface que separa ACAS e AIA é estimada para os dois cruzeiros via aplicação dos Teoremas de Shtokman. A outra metodologia consiste na obtenção de padrões verticais de velocidade baroclínica absoluta a partir de simulações numéricas com a versão seccional do Princenton Ocean Model - POM inicializados com 14 campos termohalinos interpolados a partir dos dados - é o Critério Dinâmico. Os dois critérios apresentam resultado médio que difere apenas cerca de 7 dbar um do outro. Assim, adota-se valor de 480 dbar como NR para os cálculos geostróficos como nível isobárico que demarca a interface média entre CB e CCI. Mapas de função de corrente geostrófica são gerados através de mapeamento objetivo. Tais mapas apresentam evidências inequívocas de que os ciclones e anticiclones, observados nos experimentos de verão e inverno de 1993, são estruturas de vórtices associadas a meandros do Sistema CB. Este aparecem como estruturas vorticais postadas em lados diametralmente opostos ao eixo da CB: ciclones no lado costeiro, e anticiclones no lado oceânico. A variação vertical das estruturas dos meandros e a existência de uma única inversão de sentido permite comprovar e concluir que são ondas baroclínicas de primeiro modo, corroborando especulações presentes na literatura. A construção de um modelo quase-geostrófico de duas camadas para um oceano não-viscoso no plano \'beta\' e de fundo plano permite a realização de análise de vorticidade potencial. Tal análise permite concluir que o campo de vorticidade básico devido à CB suplanta o planetário e que estas ondas baroclínicas são ondas que devem sua existência ao cisalhamento vertical e horizontal da corrente. A variação frontal em vorticidade potencial baroclínica do Sistema CB é de \'1,7x10 POT.-5 s POT. -1\'. A dominãncia do termo da vorticidade de estiramento, que responde por 60% da variação de vorticidade na frente, permite estabelecer que as ondas capturadas na malha hidrográfica são ondas longas dentro da classe de meso-escala. O princípio de conservação de vorticidade potencial é invocado numa análise que envolve a superposição dos campos de vorticidade potencial e função de corrente para a primeira camada. Esta análise apresenta tanto evidências robustas de propagação das ondas baroclínicas quanto indícios de crescimento do meandro ciclônico, ou seja, de processo de instabilidade. Face aos achados das análises anteriores, teoria linear e método das perturbações são utilizados para obter uma relação de dispersão para as ondas baroclínicas da CB. Esta relação é dependente da velocidade da CB e do gradiente de vorticidade potencial através da corrente
The Brazil Current System is formed by two western boundary currents that flow along the Southeast Brazil continental margin from surface to 1200-1800 m deep. These two currents are the southward-flowing Brazil Current (BC) and the northward-flowing Intermediate Western Boundary Current (IWBC). The investigation of the dynamics of the barociinic vorticity waves superimposed to the BC system is conducted in this work solely from hydrographic data anaiysis and manipulation from two oceanographic meso-scale surveys that were part of the COROAS Project, the Brazilian arm of the Worid Ocean Circulation Experiment. These two cruises sampled the central portion of the São Paulo Bight (23,5°- 27°S) in the summer and winter seasons of 1993. As it was intended to appiy the classical Dynamic Method to the data, a quantitative estimate of the reference levei (RL) is conducted through the comparison of two different and independent methodoiogies. The first uses the previous knowiedge that the BC transports Tropical Water and South Atlantic Central Water (SACW) as well as that the IWBC transports mainly Antarctic Intermediate Water (AAIW). This method, referred here as the Thermodynamic Criterion applies the Shtokman theorems to estimate the interface depth between SACW and AAIW. The second methodology, designated here as the Dynamic Criterion, consists of modeiing absolute baroclinic velocities for all 14 hydrographic transects using the sectional version of the Princeton Ocean Model. The results of the two methods differ oniy in 7 dbar and a RL of 480 dbar is adopted as to represent the average interface isobaric level between BC and IWBC in the following geostrophic calculations. Horizontal distributions of geostrophic stream function are generated using objective mapping. These distributions present unequivocal evidences of cyclonic and anticyclonic structures in both Summer and Winter 1993 cruises. These features are associated to meanders of the BC System. They are depicted in opposing sides of the current core. The cyclone is seen in the BC coastal side as the anticyclone is placed in its oceanic side. The vertical variation of these pattems with a single flow direction inversion confirms that these vortical features are part of a first baroclinic mode wave, which corroborates previous speculations found in the literature. In order to pursue a potential vorticity analysis, a data-derived two-layer quasi-geostrophic model is built assuming an invicid flat-bottomed ocean in the 3-plane. This analysis allowed to conclude that the basic vorticity field associated with both horizontal and vertical shear of the boundary currents are responsible for the baroclinic wave existence. The planetary vorticity gradient is one order of magnitude lower. The frontal variation in baroclinic potential vorticity is 1,7 x i0 s1. The dominance of the stretching vorticity, which accounts for 60% of the gradient variation, leads to classify these oscillatory motions as long meso-scale waves. The potential vorticity conservation principle is invoked on an analysis that consists of superimposing the first layer quasi-gesotrophic potential vorticity and geostrophic stream funtion maps. This analysis revealed that the baroclinic waves are propagating as well as evidences of meander growth, an indication of a possible geophysical instability mechanism, are seen. Given the findings of the previous analyses, linear theory and the perturbation method are used to derive a dispersion relation for the BC System first mode baroclinic waves. The wave frequency is function of the BC velocity as well as the potential vorticity cross-stream gradient.

A região oceânica adjacente ao Embaiamento de Tubarão (ET) e ao Banco de Abrolhos (BAb) talvez seja uma das menos investigadas da margem continental brasileira. Estudos acerca da circulação nesta porção da costa atualmente restringem-se a trabalhos realizados na década de 80 e 90. Recentemente, estudos focando a atividade de mesoescala e a variabilidade sazonal da circulação ao largo da costa leste brasileira, tem relatado padrões de escoamento complexos e despertado o interesse da comunidade científica na compreensão da dinâmica local. Buscando compreender o padrão de escoamento na região e dispondo de dados sinóticos dos anos de 2004 e 2005 provenientes do Projeto Abrolhos, este trabalho se propôs a estudar as principais feições encontradas na área em questão através de modelagem numérica. Com este intuito, o campo inicial foi construído através de dados hidrográficos sinóticos, interpolados objetivamente com campos climatológicos mensais do World Ocean Atlas 01. Para tal, os perfis dos dados sinóticos e climatológicos foram adimensionalizados e redimensionalizados, com imagens termais de satélite e campo sintético de salinidade. Após a construção do campo inicial, foi realizada simulação numérica com o Regional Ocean Modeling System - ROMS, para os cenários de inverno (2004) e verão (2005). No cenário de inverno foi identificado um anticiclone dentro do Embaiamento de Tubarão, aqui denominado Vórtice de Tubarão, entretanto o Vórtice de Vitória (VV) e o Vórtice de Abrolhos (VA) só foram encontrados no cenário de verão, estando o o VV associado a um anticiclone simétrico ao eixo da Corrente do Brasil. Os resultados refutam a hipótese de que os anticiclones ao largo do Banco de Abrolhos apresentariam a mesma estrutura dinâmica dos ciclones presentes no Embaiamento de Tubarão. Além disso, pode-se também, atestar a natureza não perene do VV.
The region adjacent to the Tubarão Embayment and Abrolhos Bank is maybe one of the less investigated of the Brazillian continental margin. Studies about the cirulation in this area are currently restricted to studies carried out during the 80\'s and 90\'s. Recently, studies focusing the mesoscale activity and the seazonal variability of the circulation off the Brazillian eastern coast have reported complex flow patterns and raised attention of the scientific community to the lack of knowledge of the local dynamics. In order to comprehend the flow pattern in the region, having synoptic data for 2004 and 2005 from the Abrolhos Project, this work aims to study the main features identified in the study area through numerical modelling. With this intent, the initial field was built through hydrographic synoptic data objectively interpolated with monthly climatological fields of the World Ocean Atlas 01. For that, the profiles of the synoptic and climatological data were non-dimensionalyzed and re-dimensionalyzed with thermal satellite images and synthetic salinity field. After the initial field was built, a numerical simulation with the Regional Ocean Modeling System - ROMS was carried out, for the winter (2004) and summer (2005) scenarios. In the winter scenario an anticyclone, here named Tubarão Eddy, was identified inside the Tubarão Embayment, while the Vitória Eddy (VE) and the Abrolhos Eddy (AE) were only identified in the summer scenario, being the VE associated with an anticyclone symetric to the Brazil Current axis. The results deny the hypothesis that the anticyclones offshore the Abrolhos Bank present the same dynamical structure as the cyclones present in the Tubarão Embayment. Besides, we state the non-permanent nature of the VE.

A circulação sazonal da Plataforma Continental Leste Brasileira (PCLB) entre 10oS e 16oS é investigada com base em um inédito conjunto de dados in situ e em modelagem hidrodinâmica. Os dados observados possibilitaram a compreensão da dinâmica da circulação em partes específicas da região de estudo e a sua interação com os diversos mecanismos forçantes, ilustrando a forte influência da circulação atmosférica e oceânica de meso/larga escalas, de processos transientes e da topografia local, na circulação regional, apresentando cenários distintos entre as estações do ano. Os resultados da modelagem hidrodinâmica não só complementaram os estudos observacionais, como também permitiram uma maior compreensão da variabilidade sazonal e espaço-temporal da circulação na PCLB, assim como a sua interação com os diversos mecanismos forçantes, para uma região mais ampla (10oS-16oS). A PCLB é fortemente afetada pela sazonalidade de larga escala do regime dos ventos alísios e da latitude da Zona de Convergência Intertropical (ZCIT). Seguindo esta dinâmica, durante as estações de primavera/verão, as correntes na Plataforma Continental (PC) ao sul de 12oS ficam sujeitas a ventos preferenciais de E-NE e, no oceano, a bifurcação da Corrente Sul Equatorial (bSEC) atinge a sua posição mais ao norte (~13oS em novembro). Nas regiões da quebra da PC e do talude continental, as correntes são também influenciadas pelas Correntes de Contorno Oeste (CCO). No domínio norte (10oS), a circulação na PC interna é para sul, seguindo o campo de ventos preferencial, enquanto a circulação na PC média e na quebra da PC é mais influenciada pelas correntes na região do talude continental, que são claramente dominadas pela dinâmica da Corrente Norte do Brasil/Sub-Corrente Norte do Brasil (CNB/SCNB). No domínio central (14oS), embora a circulação seja de certa forma similar à observada para o domínio norte, a circulação no talude continental apresenta uma maior variabilidade como resposta à presença dos fluxos contrários da Corrente do Brasil (CB) e da SCNB. A circulação no domínio sul (16oS) possui uma clara divisão ao longo da PC/talude continental, apresentando um intenso e organizado fluxo direcionado para sul na PC interna e média, como resposta à forçante remota do vento, e uma circulação polarizada na direção perpendicular à costa nas regiões da quebra da PC/talude continental, sugerindo uma associação com a CCO e com a topografia local do Banco Royal Charlotte. Durante as estações de outono/inverno, seguindo o regime sazonal dos ventos alísios e da latitude da ZCIT, a PC ao norte de ~20oS fica sujeita a ventos preferenciais de E-SE e as correntes sofrem uma reversão do fluxo médio, e a bCSE atinge seu limite sul (~17oS em julho). A circulação na PC interna e média no domínio norte durante estas estações, apresenta um fraco fluxo para norte com uma alta variabilidade. Enquanto na PC interna esta variabilidade está relacionada à entrada mais frequente de frentes frias, na PC média está relacionada com a circulação na região do talude continental. A circulação nesta região possui uma alta correlação com aquela na região da quebra da PC e ambas apresentam um fluxo médio para norte mais intenso e menos variável, que pode estar relacionado com o fluxo da CNB/SCNB, que ocupa toda a região do talude continental durante estas estações do ano. O fluxo médio para norte no domínio central, apresenta uma fraca intensidade e alta variabilidade nas regiões da PC interna e média e na quebra da PC, que pode estar relacionada com o aumento da frequência de sistemas frontais, ao fato desta região ser influenciada pela bifurcação do fluxo médio em superfície e, para a região da quebra da PC, à variabilidade da SCNB, que apresenta maiores episódios em que ocupa esta região. A CNB/SCNB exerce uma clara influência na circulação da região do talude continental. A circulação no domínio sul é principalmente gerada pelo vento remoto nas regiões da PC interna e média, enquanto nas regiões da quebra da PC e talude continental apresenta uma significativa influência da CCO.
The seasonal circulation of the Eastern Brazilian Shelf (EBS) between 10oS and 16oS is investigated based on original in situ data sets and hydrodynamic modeling. The observational data provided an understanding of the circulation dynamics within specific parts of the the study region and its interaction with the various forcing mechanisms, illustrating the strong influence of the large/meso scales atmospheric and oceanic circulation, transient processes and local topography on the regional circulation, presenting very distinct scenarios between seasons. The hydrodynamic modeling results not only complemented the observational studies but also allowed for a broader understanding of the seasonality and time-space variability of the EBS circulation, as well as its interaction with the various forcing mechanisms, in a wider region (10oS-16oS). The EBS is highly affected by large-scale seasonality in the trade wind regime and the latitude of the Intertropical Convergence Zone (ITCZ). Following this dynamics, during the spring/summer seasons, the currents within the continental shelf south of 12oS are influenced by preferential E-NE winds and, in the ocean, the bifurcation of the South Equatorial Current (bSEC) reaches its northernmost position (~13oS in November). At the shelf break and slope region, the currents are also influenced by the Western Boundary Currents (WBC). At the northern domain (10oS) the inner shelf circulation is to the south following the preferential winds, while the circulation at the mid shelf and shelf break is more influence by the currents at the slope, which are clearly dominated by the North Brazil Current/North Brazil Undercurrent (NBC/NBUC) dynamics. At the central region (14oS), although the circulation is somehow similar to that observed for the northern domain, the slope circulation presents a greater variability in response to the presence of the opposing Brazil Current (BC) and NBUC flow. At the southern domain (16oS) the circulation has a clear division along the continental shelf/slope regions, presenting an intense and organized southward flow at the inner and mid shelves, as a response to the remote wind forcing, and a polarized cross-shelf circulation at the shelf break/slope region, suggesting an association with the WBC and the local topography of the Royal Charlotte Bank. During the autumn/winter seasons, following the seasonal regime of the trade winds and the latitude of the ITCZ, the continental shelf north of ~20oS are influenced by preferential E-SE winds and the currents experience a reversal of the mean flow, and the bSEC reaches its southernmost position (~17oS in July). The circulation during these seasons at the northern domain inner and mid shelves presents a weak northward flow with a strong variability. While at the inner shelf this variability is related to the more frequent cold front passages at the mid shelf it is related to the slope region circulation. The circulation at this region has a high correlation with the circulation at the shelf break and both present a less variable and more intense northward net flow which could be related to the NBC/NBUC flow, that occupies the entire slope region during these seasons. The northward net flow at the central domain presents a weak intensity and high variability at the inner/mid shelves and shelf break regions, which could be related to the increase in the cold fronts frequency, to the fact that this region is under the influence of the bifurcation of the surface net flow and, for the shelf break region, to the NBUC variability which presents more episodes that occupies this region. At the slope region the NBC/NBUC exerts a clear influence in the circulation. The circulation at the southern domain is mainly driven by the remote wind forcing at the inner and mid shelves, while at the shelf break/slope regions presents a significant WBC influence.

O presente estudo compreende a análise de distribuição dos foraminíferos bentônicos vivos no talude continental da Bacia de Campos e Platô de São Paulo (entre 400 e 3000 m de profundidade), buscando entender os processos condicionantes dessa distribuição. Dados sedimentológicos, geoquímicos e microfaunísticos permitiram identificar três grupos na área de estudo. O grupo I inclui amostras do talude superior, médio e inferior (400-1300 m de profundidade), e é caracterizado por valores maiores de densidade de foraminíferos bentônicos, carbono orgânico total, concentração de fitopigmentos, biomassa de bactérias, menores valores de sortable silt e de conteúdo de carbonato de cálcio, e pela presença de espécies como Globocassidulina subglobosa, Reophax scorpiurus, Reophax subfusiformis, Reophax spiculotestus e Epistominella exigua. O grupo II, constituído de amostras do talude inferior e Platô de São Paulo (1900-3000 m de profundidade), é caracterizado por menores densidades de foraminíferos bentônicos, carbono orgânico total, concentração de fitopigmentos, biomassa de bactérias, maiores valores de sortable silt e de conteúdo de carbonato de cálcio, e pela presença de espécies como Saccorhiza ramosa, Rhizammina algaeformis, Karrerulina sp2. e Hyperammina rugosa. O grupo III (1900-3000 m de profundidade) diferencia-se do grupo II pela presença da Glomospira gordialis, Pyrgoella irregularis e Reophax helenae. Constatou-se que os processos hidrossedimentares (p.e. ação da Corrente do Brasil e Corrente de Contorno Intermediária junto ao fundo), o fluxo vertical de matéria orgânica particulada e concentração de fitopigmentos no sedimento são fatores controladores das condições tróficas no ambiente e estão relacionados às feições de mesoescala (meandros e vórtices de Cabo Frio, Cabo de São Tomé e Vitória), determinando, assim, variações na microfauna de foraminíferos bentônicos vivos na Bacia de Campos.
The present study comprises the analysis of the distribution of living benthic foraminifera on the continental slope of Campos Basin and Plateau of São Paulo (400-3000 m water depth) in order to understand the environmental processes determining this distribution. Sedimentological, geochemical and microfaunal data indicated the existence of three groups in the study area. Group I includes samples from the upper and middle slope (400-1300 m water depth) and is characterized by high values of benthic foraminifera density, total organic carbon, phytopigment concentration, biomass of bacteria, lower values of sortable silt and calcium carbonate content, and the presence of species such as Globocassidulina subglobosa, Reophax scorpiurus, Reophax subfusiformis, Reophax spiculotestus and Epistominella exigua. Group II, consisting of samples of the lower slope and Plateau of São Paulo (1900-3000 m water depth), is characterized by lower densities of benthic foraminifera, total organic carbon, phytopigment concentration, biomass of bacteria, higher values of sortable silt and calcium carbonate content, and the presence of species such as Saccorhiza ramosa, Rhizammina algaeformis, Karrerulina sp2. and Hyperammina rugosa. Group III (1900-3000 m water depth) differs from group II due to the presence of Glomospira gordialis, Pyrgoella irregularis and Reophax helenae. Hydro-sedimentary processes (e.g. action of the Brazil Current and Intermediate Western Boundary Current), the particulate organic matter flux and phytopigment concentration in the sediment are factors controlling the trophic conditions in the environment, and are related to features of mesoscale (meanders and Cabo Frio, Cabo de São Tomé and Vitória eddies), thereby determining changes in living benthic foraminifera in Campos Basin.

碩士
國立臺灣大學
大氣科學研究所
98
ABSTRACT By simulating the famouse Western Boundary Current (WBC), this thesis first integrate Chebyshev collocation method with Ghost Cell Immersed BoundaryMethod (GCIBM) into a powerful coupled model. WBC intensification is caused by wave energy accumulation at the western boundary of the ocean basins, causing a narrow region which has higher meridional velocity than anywhere else. Complex boundary geometry may also strongly affect the WBC which is difficult to handle in the numerical model. In order to solve this problem and increase flexibility, we use GCIBM to implicitly provide the boundary conditions for the numerical model. GCIBM is an advanced numerical method, based on the use of artificial forcing to represent the effect of the realistic boundaries on the flow. To accurately resolve the wave dynamic, we employee the Chebyshev Collocation method with exponential solution convergence rate. Combing GCIBM with Chebyshev Collocation method, we present high accuracy and flexibility barotropic model of different complexity to simulate the WBC intensification in the β plane ocean. The numerical results show that both Chebyshev Collocation method and central difference with GCIBM can provide a reasonable WBC. The width of the WBC agrees well with the theoretical one. It seems that during the transient state Rossby waves play an important role in the generation of WBC, causing the asymmetric property in the nearly symmetric ocean, while the nonlinear dynamics would redistribute the wave energy of the ocean. Besides single domain modeling, we use domain decomposition to integrate Chebyshev collocation method with GCIBM. The resluts sugesst that the arrangments of grids at the embedding regions are essential to the coupled model; Chebyshev collocation method and GCIBM would work efficeintly through domain decomposition with the embedding grids matching exactly between the subdomains. However, as long as the embedding grids don’t match each other; interpolation schemes are used in the two-way embedding algorithms, which would cause the mass flux inconsistency between the subdomains due to the difference grid resolutions. This numerical inconsistency would lead to error accumulation at the embedding regions, restricting the effective prediction time steps in the coupled models. Our research suggest that size ratio between GCIBM and Chebyshev collocation method subdomains should be larger than onesixth for the couplde model to run effectively; furthermore, we notice that both mass flux tuning and solution cnovergence rate matching between the subdomains could enlonggate the effective prediction time steps in the coupled models.

碩士
國立臺灣大學
海洋研究所
86
Analysising the bouy data of WOCE/KSVP, the RyuKyu arc showsits effect to the path of Kuroshio. A branch of Kuroshio turns toeastward and moves as wav es. Developing a barotropic numerical model to examine the influence of the r idge across the Western boundary current. According to model results, the rid ge leads toTaylor column effect, blocks the northward flow and forces it totur n to eastward. Analysising the balance of vorticity, in the southern region o f the ridge, some non-linear performance likerecirculation or Rossby wave can be found, and it has almost thesame dynamic structure of the Northwestern regi on of the generalwind-driven ocean circulation. The length of the ridge, its location and the width of the channel between boundary and ridgedetermine the pattern of this region.

In non-rotating fluids, boundary-layer separation occurs when the nearly inviscid flow just outside a viscous boundary-layer experiences an appreciable deceleration due to a region of adverse pressure gradient. The fluid ceases to flow along the boundary due to a flow recirculation region close to the boundary. The flow is then said to be "detached." In recent decades, attention has shifted to the study of boundary-layer separation in a rotating reference frame due to its significance in Geophysical Fluid Dynamics (GFD). Since the Earth is a rotating sphere, the so-called β-plane approximation f = f0 + βy is often used to account for the inherent meridional variation of the Coriolis parameter, f, while still solving the governing equations on a plane. Numerical simulations of currents on the β-plane have been useful in understanding ocean currents such as the Gulf Stream, the Brazil Current, and the Antarctic Circumpolar Current to name a few. In this thesis, we first consider the problem of prograde flow past a cylindrical obstacle on the β-plane. The problem is governed by the barotropic vorticity equation and is solved using a numerical method that is a combination of a finite difference method and a spectral method. A modified form of the β-plane approximation is proposed to avoid computational difficulties. Results are given and discussed for flow past a circular cylinder at selected Reynolds numbers (Re) and non-dimensional β-parameters (β^). Results are then given and discussed for flow past an elliptic cylinder of a fixed aspect ratio (r = 0.2) and at two angles of inclination (90°, 15°) at selected Re and β^. In general, it is found that the β-effect acts to suppress boundary-layer separation and to allow Rossby waves to form in the exterior flow field. In the asymmetrical case of an inclined elliptic cylinder, the β-effect was found to constrain the region of vortex shedding to a small region near the trailing edge of the cylinder. The shed vortices were found to propagate around the trailing edge instead of in the expected downstream direction, as observed in the non-rotating case. The second problem considered in this thesis is the separation of western boundary currents from a curved coastline. This problem is also governed by the barotropic vorticity equation, and it is solved on an idealized model domain suitable for investigating the effects that boundary curvature has on the tendency of a boundary current to separate. The numerical method employed is a two-dimensional Chebyshev spectral collocation method and yields high order accuracy that helps to better resolve the boundary-layer dynamics in comparison to low-order methods. Results are given for a selection of boundary curvatures, non-dimensional β-parameters (β^), Reynolds numbers (Re), and Munk Numbers (Mu). In general, it is found than an increase in β^ will act to suppress boundary-layer separation. However, a sufficiently sharp obstacle can overcome the β-effect and force the boundary current to separate regardless of the value of β^. It is also found that in the inertial limit (small Mu, large Re) the flow region to the east of the primary boundary current is dominated by strong wave interactions and large eddies which form as a result of shear instabilities. In an interesting case of the inertial limit, strong waves were found to interact with the separation region, causing it to expand and propagate to the east as a large eddy. This idealized the mechanism by which western boundary currents such as the Gulf Stream generate eddies in the world's oceans.