Intermediate Western Boundary Current Research Articles

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.

Moored Observations of the Currents and Transports of the Maluku Sea

Abstract The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin. Significance Statement The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages.

The Brazil Current quasi-stationary unstable meanders at 22°S–23°S

The most conspicuous oceanographic features off Southeast Brazil are the quasi-stationary eddies formed off Capes São Tomé (CSTE) and Frio (CFE) associated with the Brazil Current (BC). We use SST and SSH products to infer eddy growth rates. These two independent data sets reveal an average growth rate of 0.06day-1, and propagation speeds are virtually null. CFE tends to be more unstable and frequent than CSTE (3.5 vs. 2.3 events per year). CSTE can propagate toward the north or south while CFE only propagates southward. We perform potential vorticity (PV) inversion calculation using hydrographic data to understand how the meander growth occurs in the baroclinic jet formed by the BC and the Intermediate Western Boundary Current (IWBC). PV anomalies in one layer enhanced the anomalies on the other. This result offered in situ evidence of baroclinic conversion occurring during the eddy event captured in the observations. We then build a theoretical two-layer model calibrated with the observations seeking to explain why the eddies grow in place, and no accompanying unstable anticyclones are formed. The experiments lead us to conclude that the eddies’ quasi-stationarity is due to the almost pure baroclinic nature of the BC-IWBC jet. We also find that the western boundary inhibits the formation of the anticyclones. As observed in satellite imagery, the meanders develop as isolated disturbances that may grow simultaneously or not.

First description of archaeal communities in carbonate-rich seafloor and subseafloor sediments from the Southwestern Atlantic slope

Deep-sea sediments comprise one of the largest habitats on Earth. The archaeal groups contribute to a large fraction of the deep-sea benthic biomass, playing a key role in biogeochemical cycles. However, their diversity in deep-sea benthic ecosystems remains poorly understood, mostly because only recently novel taxa have been proposed, thus remodeling the phylogenetic tree. Despite the dificulty in obtaining cultivated representatives, the metabolic capabilities of archaea have lately been described through metagenomic data, indicating that archaeal taxa are highly versatile. Here, we aimed to reveal the diversity of archaeal communities in surface (0 to 15 cm depth) and subsurface (200 cm depth) sediments from a carbonate-rich region in the Southwestern Atlantic upper slope. We performed 16S rRNA gene sequencing, and found that the archaeal composition in surface sediments was mainly dominated by ammonia-oxidizing archaea within Nitrososphaeria class. The distribution of Nitrososphaeria ASVs (amplicon sequence variants) indicates the presence of several species or ecotypes. Contrastingly, the subsurface sediment was dominated by uncultivated anaerobic and poorly known archaea, including representatives of all supergroups (Asgard, TACK, DPANN and Euryarchaeota). These archaea have been described as having potentially diverse metabolic capabilities, including autotrophic and heterotrophic pathways, such as acetogenesis, methylotrophy, and degradation of labile and recalcitrant organic compounds. This indicates an important role in the remineralization of organic matter in the SW (Southwest) Atlantic slope. They are likely enriched due to the transport and mixing of sediments by the IWBC (Intermediate Western Boundary Current) along the continental slope. However, further studies are needed to reveal the geochemical and oceanographic drivers of the archaeal distribution. This study provides the first description of the archaeal communities in carbonate-rich sediments in the SW Atlantic slope, and adds new biodiversity insights to this geological feature, which is considered a vulnerable marine ecosystem, thus helping for future conservation strategies.

Sedimentation in the adjacencies of a southwestern Atlantic giant carbonate ridge

Although carbonate mounds have been investigated for 100 years, few studies focus on the giant variety. The Alpha Crucis Carbonate Ridge (ACCR), a ~17 x 12-km ring-shaped ridge formed by hundreds of mounded structures, located between the 300 and 800-m isobaths and reaching a maximum height of 340 meters above the adjacent seafloor, is the first giant carbonate mounded feature described for the SW Atlantic margin. This study provides the first multiproxy approach to investigate sediments covering the ACCR and its adjacencies. Most of the area is located under the Intermediate Western Boundary Current (IWBC) flow, which carries the nutrient-rich Antarctic Intermediate Water (AAIW). Radiocarbon aging shows pronounced differences for the shallow layers (MIS3 for the top of the mounds and late Holocene for the adjacencies). Grain size data indicate the prevalence of sandy fractions on top of the mounds and muddy sediments in the adjacent areas. Fe/Ca and Ti/Ca proxies allowed for identifying mainly biogenic sedimentation in the area. However, the input of allochthonous terrigenous sediment is necessary for mound buildup, and values of Fe and Ti collected on the top of the mounds are significant. End-Members distributions and metal concentrations also allowed for recognition of distinct sources of sediment. εNd and Ln(Fe/K) indicated two primary terrigenous sources, the Precambrian rocks of the Brazilian shield (Cabo Frio end-member) and the multiple lithologies drained by the Rio de la Plata basin. Redox condition proxies indicated that the area is submitted to oxic conditions, probably reflecting the action of the IWBC. This work provides the first insight into an integrated grain-size and geochemical characterization of the Alpha Crucis Carbonate Ridge (southwestern Atlantic margin).

A vertical-mode extrapolation scheme applied to the Brazil Current domain: quasi-synoptic scenarios

The Brazil Current (BC) is a western boundary current, with an average vertical extension of 500 m, which occupies the southwestern margin of Atlantic Ocean. Recently, the South Brazil Bight - SBB (23º S - 28º S) has been intensively explored by the oil and gas industries, which brought a high demand in operational oceanography and ocean state monitoring. In this study we present a new technique based on the vertical dynamic modes of oscillation, to extrapolate velocity profiles in the SBB, with potential applications to data acquired by Vessel Mounted Acoustic Doppler Current Profilers (VMADCP). Using four quasi-synoptic crosssectional velocity data, acquired by Lowered Acoustic Doppler Current Profilers (LADCP), we observed that the mean circulation of this region can be characterized as a 2-dynamical mode system. This mean circulation is mainly dominated at surface by the BC and at intermediate levels by the Intermediate Western Boundary Current (IWBC), which flows northward. Furthermore, observed mesoscale vortical features in the SBB, such as eddies and meanders, are shown to be primarily barotropic-mode dominated. In our extrapolation methodology, the use of an independent set of velocity time series obtained by four moorings has shown to be a valid way for obtaining the relative contribution of each dynamical mode in the velocity field in the SBB. Normalized mean squared errors below 10% and radar diagrams show that the presented methodology was able to reproduce the main dynamic features observed by LADCP transects. Also, with this technique, we extrapolated three VMADCP sections and the results agreed with previous observations.

Geomorphological imprint of opposing ocean bottom currents, a case study from the southeastern Brazilian Atlantic margin

The combination of oceanographic processes and a negligible fluvial terrestrial sediment contribution makes the Santos Basin margin (southwestern Brazil) a unique environment for studying thew morphological imprint of bottom currents on the seafloor morphology. We divide the seafloor into four domains: outer shelf, upper, middle, and lower slope, based on an analysis of sub-bottom Chirp profiles, multichannel seismic data, hydrographic data, numerical simulation outputs, and a seismic-derived bathymetry map. A clinoform at the outer shelf and upper slope, topped by erosional morphologies, developed below the SW-flowing Brazil Current (BC). At the bottom of the upper slope, a rugged and mounded surface around 600 mbsl coincides with the current flow reversal to the underlying NE-flowing Intermediate Western Boundary Current (IWBC). The smooth middle slope exhibits sheeted contourites interrupted by occasional channel-and-mound complexes.From the middle slope base, salt tectonics becomes a major player in setting the seafloor morphology. The slope-parallel Santos Channel develops above a subsurface diapir and the Cabo Frio Fault. The latter divides between the landward gap in subsurface Albian salt and the basinward rich diapir realm. Hence, the lower slope is reworked by the SW-flowing Deep Western Boundary Current (DWBC); however, salt tectonics dictates its morphology. Our study consists of a unique example for studying the morpho-sedimentary imprint of a counter-current flowing on the continental slope and also for exploring the interaction between bottom flows with a suite of endogenic processes, such as salt tectonics, fluid flows, and mass movements.

Can the Intermediate Western Boundary Current recirculation trigger the Vitória Eddy formation?

South of the Vitória-Trindade Ridge, a seamount chain off East Brazil, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the Vitória Eddy. It was recently found that the Intermediate Western Boundary Current (IWBC), which flows equatorward below the BC, cyclonically recirculate within Tubarão Bight. We present an analysis of AVISO observations that suggest that the Vitória Eddy formation is conditioned by the strength of the BC upstream of Tubarão Bight. A weak BC is prone to local meandering and eddy formation in the bight, while a strong BC suppresses eddy formation in the bight but triggers downstream meander growth. To study the effects of the IWBC recirculation on the BC meandering and the Vitória Eddy formation, we formulate a simple two-layer quasi-geostrophic model. In the model, the BC is represented by a meridional jet in the upper layer and the IWBC recirculation is a steady eddy in the lower layer. The lower-layer eddy effectively acts as a topographic bump, affecting the upper-layer jet via the stretching term \(\psi _{2}/{R_{d}^{2}}\), where ψ2 is the lower-layer streamfunction and Rd is the baroclinic deformation radius. Based on the AVISO sea-surface height data and previous observational studies, we define a stationary eddy and reference jet. We conduct a number of initial-value problem experiments varying the upper-layer jet speed. A weak upper-layer jet slowly meanders and develops a cyclone above the lower-layer eddy. As we increase the jet velocity, the meandering is faster and the cyclone is larger. But a too-strong jet has an opposite effect: the potential vorticity anomalies induced by the lower-layer eddy are quickly swept away, leading to explosive downstream meander growth; no cyclone is formed above the lower-layer eddy. In all cases, the initial meandering trigger is a linear process (the steering of the upper-layer jet by the lower-layer eddy). But even when the upper-layer jet is weak, nonlinearity quickly becomes important, dominating the dynamics after 10 days of simulation. The downstream meander growth is fully nonlinear. Our idealized QG model confirms that the IWBC recirculation can trigger the Vitória Eddy formation and elucidates the mechanisms involved in this process.

On the Steadiness and Instability of the Intermediate Western Boundary Current between 24° and 18°S

AbstractThe Intermediate Western Boundary Current (IWBC) transports Antarctic Intermediate Water across the Vitória–Trindade Ridge (VTR), a seamount chain at ~20°S off Brazil. Recent studies suggest that the IWBC develops a strong cyclonic recirculation in Tubarão Bight, upstream of the VTR, with weak time dependency. We herein use new quasi-synoptic observations, data from the Argo array, and a regional numerical model to describe the structure and variability of the IWBC and to investigate its dynamics. Both shipboard acoustic Doppler current profiler (ADCP) data and trajectories of Argo floats confirm the existence of the IWBC recirculation, which is also captured by our Regional Oceanic Modeling System (ROMS) simulation. An “intermediate-layer” quasigeostrophic (QG) model indicates that the ROMS time-mean flow is a good proxy for the IWBC steady state, as revealed by largely parallel isolines of streamfunctionand potential vorticity; ascatter diagram also shows that the IWBC is potentially unstable. Further analysis of the ROMS simulation reveals that remotely generated, westward-propagating nonlinear eddies are the main source of variability in the region. These eddies enter the domain through the Tubarão Bight eastern edge and strongly interact with the IWBC. As they are advected downstream and negotiate the local topography, the eddies grow explosively through horizontal shear production.

The Alpha Crucis Carbonate Ridge (ACCR): Discovery of a giant ring-shaped carbonate complex on the SW Atlantic margin

Recently acquired bathymetric and high-resolution seismic data from the upper slope of Santos Basin, southern Brazilian margin, reveal a major geomorphological feature in the SW Atlantic that is interpreted as a carbonate ridge - the Alpha Crucis Carbonate Ridge (ACCR). The ACCR is the first megastructure of this type described on the SW Atlantic margin. The ~17 × 11-km-wide ring-shaped ACCR features tens of >100-m-high steep-sided carbonate mounds protruding from the surrounding seabed and flanked by elongated depressions. Comet-like marks downstream of the mound structures indicate that the area is presently influenced by the northward flow of the Intermediate Western Boundary Current (IWBC), a branch of the Subtropical Gyre that transports Antarctic Intermediate Water. Abundant carbonate sands and gravels cover the mounds and are overlain by a biologically significant community of living and dead ramified corals and associated invertebrates. The IWBC acts as a hydrodynamic factor that is responsible for both shaping the bottom and transporting coral larvae. We contend that the ACCR was formed by upward fluid flow along active sub-surface faults and fractures that formed by lateral extension generated by the ascending movement of salt diapirs at depth. The ACCR provides an important modern and accessible analogue for a seabed carbonate build-up related to sub-surface hydrocarbon systems.

Circulação Oceânica a 1000 Metros na Região da Cadeia Vitória - Trindade

A circulacao oceânica de subsuperficie desempenha um papel crucial nos transportes oceânicos de calor e massa e consequentemente no sistema climatico terrestre. Algumas regioes oceânicas ate os dias atuais ainda apresentam poucos estudos que busquem caracterizar aspectos basicos relativos a dinâmica oceânica existente em niveis intermediarios e profundos, como e o caso de parte significativa da regiao oceânica adjacente a costa leste brasileira. Este trabalho tem como objetivo a caracterizacao da circulacao oceânica a 1000 metros e a estrutura termohalina na regiao da Cadeia Vitoria – Trindade (CVT) a partir de perfiladores lagrangeanos Argo. Os resultados indicam que a profundidade de 1000 metros e uma regiao de transicao na coluna d’agua entre a circulacao intermediaria e a profunda, sobretudo sob o ponto de vista da analise da trajetoria. Ao norte da cadeia, o fluxo e dominado por uma corrente de norte para sul, atribuido a Corrente de Contorno Profunda (CCP). Enquanto no sul da cadeia, um fluxo zonal de leste para oeste domina a regiao oceânica e a Corrente de Contorno Intermediaria (CCI) aparece junto ao talude indo para norte, enquanto um fluxo contrario para sul ocorre mais afastado da costa. Na analise termohalina da regiao, as caracteristicas termohalinas da Agua Intermediaria Antartica (AIA) dominam o nivel de 1000 metros, o que sugere que ao norte da cadeia a CCP influencie no transporte da AIA para sul.

Environmental controls on the distribution of living (stained) benthic foraminifera on the continental slope in the Campos Basin area (SW Atlantic)

Abstract Living (stained) benthic foraminifera from deep-sea stations in the Campos Basin, southeastern Brazilian continental margin, were investigated to understand their distribution patterns and ecology, as well as the oceanographic processes that control foraminiferal distribution. Sediments were collected from 1050 m to 1950 m of water depth during the austral winter of 2003, below the Intermediate Western Boundary Current (IWBC) and the Deep Water Boundary Current (DWBC). Based on statistical analysis, vertical flux of particulate organic matter and the grain size of sediment seem to be the main factors controlling the spatial distribution of benthic foraminifera. The middle slope (1050 m deep) is characterized by relatively high foraminiferal density and a predominance of phytodetritus-feeding foraminifera such as Epistominella exigua and Globocassidulina subglobosa. The occurrence of these species seems to reflect the Brazil Current System (BCS). The above-mentioned currents are associated with the relatively high vertical flux of particulate organic matter and the prevalence of sandy sediments, respectively. The lower slope (between 1350 and 1950 m of water depth) is marked by low foraminiferal density and assemblages composed of Bolivina spp. and Brizalina spp., with low particulate organic matter flux values, muddy sediments, and more refractory organic matter. The distribution of this group seems to be related to episodic fluxes of food particles to the seafloor, which are influenced by the BCS at the surface and are deposited under low deep current activity (DWBC).

Inorganic and organic geochemical fingerprinting of sediment sources and ocean circulation on a complex continental margin (São Paulo Bight, Brazil)

Abstract. In this study, we use inorganic (metal) and organic (bulk and molecular) markers in sediment samples of the south-eastern Brazilian margin to investigate the response of geochemical fingerprints to the complex hydrodynamic processes present in the area. Results indicate the potential of export of terrigenous siliciclastic and organic constituents to the upper slope, even in an area with limited fluvial supply.Metal contents and especially the ln(Ti ∕ Al) and ln(Fe ∕ K) ratios make it possible to recognise the extension of shelf sediments toward the upper slope. Potassium, here expressed as ln(K ∕ Sc) and ln(K ∕ Al) ratios used as proxies of illite–kaolinite variations, proved to be an important parameter, especially because it allowed us to decipher the imprint of the northward flow of the Intermediate Western Boundary Current (IWBC) in comparison to the southward flows of the Brazil Current (BC) and Deep Western Boundary Current (DWBC). Using organic matter analyses, we were able to evaluate the extent of terrestrial contributions to the outer shelf and slope, even without the presence of significant fluvial input. In addition, molecular markers signify a slight increase in the input of C4-derived plants to the slope sediments, transported from distant areas by the main alongshore boundary currents, indicating that the terrestrial fraction of the organic matter deposited on the slope has a distinct origin when compared to shelf sediments.

The recirculation of the intermediate western boundary current at the Tubarão Bight – Brazil

Lagrangian floats and current meter measurements from two moored arrays are analyzed, in combination with altimetry data, in order to investigate the recirculation of Antarctic Intermediate Waters (AAIW), and of the Intermediate Western Boundary Current (IWBC) at the Tubarão Bight, in the vicinity of the Vitória-Trindade Ridge (VTR), Brazil. Results from a high-resolution numerical simulation provide a complementary view of the flow at intermediate and surface levels. The data depict a topographically-induced cyclonic recirculation at intermediate levels, and five Argo floats are successively trapped inside the bight for two-and-a-half years, performing a total of 10 closed clockwise gyres during this period of time. In situ measurements at the western side of the bight show an intense alongshore flow at intermediate levels, with averaged velocities at 800m of ~30cm/s, and peak velocities exceeding 50cm/s, magnitudes comparable to the Brazil Current (BC) flow at surface levels. The recirculation extends from at least 1000m deep up to 370m, reaching sometimes depths as shallow as 150m, but is mostly uncoupled from the surface flow during the one-and-a-half year long current meter record. Three different flow patterns are observed, and simulated, at surface levels inside the bight during the time the recirculation is well established at intermediate levels: a shallow cyclonic circulation, somewhat akin to the Vitória Eddy; a recurrent anticyclonic flow that encompasses the entire bight; and a southwestward-oriented circulation, associated with the BC being reorganized in a coherent flow after negotiating its way through the VTR channels. A significant portion (about 50% according to the model) of the inflow of intermediate waters recirculates, enhancing the flow of the IWBC within the bight, and increasing the age of AAIW that will eventually cross the VTR on its way to lower latitudes. Although the data are not conclusive about a preferential pathway of the intermediate flow across the VTR, the simulation indicates that up to 70% will exit through the main channel located between the Besnard Bank and the Vitória Seamount, while 30% will follow eastward to the south of the VTR, in a region of intense mesoscale activity, and then cross the ridge farther east.

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 (σ 0) 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.

A NUMERICAL STUDY OF THE BRAZILIAN SHELF/SLOPE REGION SOUTH OF 13S USING THE OCEAN MODEL ROMS

The oceanic features in the eastern and southeastern brazilian shelf/slope south
 of 13S is investigated using ROMS (Regional Ocean Model System). The model
 integration was 9 years and it was forced with: i) 6-hourly synoptic atmospheric
 data from NCEP; ii) initial and boundary conditions from OCCAM (Ocean Climate
 Circulation Advanced Modelling) monthly mean climatology and iii) tidal
 forcing from TPXO 7.1 global data set. The model results were compared with
 observations, which consisted in thermodynamic MDL (Mixed Layer Depth) climatology,
 satellite data, measurements from tide gauges along the shelf and currents
 measurements values from literature. The simulated currents represented well the
 BC (Brazil Current)-IWBC (IntermediateWestern Boundary Current) System. The
 BC - IWBC system at 22S cross-shelf section, where the mean alongshelf velocity
 represents our simulation capability of reproducing the western boundary currents,
 showed poleward BC and a opposing IWBC. At this section, the BC velocity core
 is in 50 m with 0.41 m.s−1 and the IWBC core around 800 m with 0.15 m.s−1.

Lagrangian observations in the Intermediate Western Boundary Current of the South Atlantic

Subsurface float measurements at 800m depth carried out from 1994 to 2003 in the Brazil Basin are used to characterise the equatorward Intermediate Western Boundary Current (IWBC) and its connections to the ocean interior. Transversally, the boundary flow is less than 100km wide, and most intense at 10–20km from the 800m isobath. Its average velocities range from ∼0.1ms−1 to 0.3ms−1 depending on latitude, with individual daily values as high as 0.7ms−1. The flow meridional extent exhibits 3 contrasted domains: (i) from 27°S to the Vitoria–Trindade Ridge at 20°30′S, the boundary flow intensifies northward along a relatively smooth topography. A counter current adjacent to it on its seaward side feeds it with intermediate water from the northern limb of the subtropical gyre. (ii) At latitudes 20–15°S characterised by a very irregular topography, the IWBC becomes weaker with even no real proof of its presence at 18–15°S. An intense mesoscale variability prevails there, which apparently takes over from the boundary flow to ensure the northward transport of water to 15°S, where the IWBC re-forms. (iii) North of this latitude, the boundary flow increases again to ∼10°S along smooth isobaths, then decreases when encountering a rougher topography and the zonal jets of the equatorial current system. A counter current present from ∼5°S to 14°S, partly fed from the boundary flow, contributes to its drainage. The IWBC shows two main input locations, at 27–23°S and 15–12°S in the southern parts of the two latitudinal domains of smooth topography where the northward current increases. Output locations coincide with major capes in the continental slope geometry, at 20°S and 18°S (the southeastern and northeastern corners of the Abrolhos Bank), at 8°S near the Recife Plateau, and at 5°S near Cape São Roque.

A two-layer approximation to the Brazil Current–Intermediate Western Boundary Current System between 20°S and 28°S

It has been shown that the vertical structure of the Brazil Current (BC)–Intermediate Western Boundary Current (IWBC) System is dominated by the first baroclinic mode at 22°S–23°S. In this work, we employed the Miami Isopycnic Coordinate Ocean Model to investigate whether the rich mesoscale activity of this current system, between 20°S and 28°S, is reproduced by a two-layer approximation of its vertical structure. The model results showed cyclonic and anticyclonic meanders propagating southwestward along the current axis, resembling the dynamical pattern of Rossby waves superposed on a mean flow. Analysis of the upper layer zonal velocity component, using a space-time diagram, revealed a dominant wavelength of about 450 km and phase velocity of about 0.20 ms −1 southwestward. The results also showed that the eddy-like structures slowly grew in amplitude as they moved downstream. Despite the simplified design of the numerical experiments conducted here, these results compared favorably with observations and seem to indicate that weakly unstable long baroclinic waves are responsible for most of the variability observed in the BC–IWBC system.

Is the meander growth in the Brazil Current system off Southeast Brazil due to baroclinic instability?

Temporally-growing frontal meandering and occasional eddy-shedding is observed in the Brazil Current (BC) as it flows adjacent to the Brazilian Coast. No study of the dynamics of this phenomenon has been conducted to date in the region between 22 ° S and 25 ° S. Within this latitude range, the flow over the intermediate continental slope is marked by a current inversion at a depth that is associated with the Intermediate Western Boundary Current (IWBC). A time series analysis of 10-current-meter mooring data was used to describe a mean vertical profile for the BC-IWBC jet and a typical meander vertical structure. The latter was obtained by an empirical orthogonal function (EOF) analysis that showed a single mode explaining 82% of the total variance. This mode structure decayed sharply with depth, revealing that the meandering is much more vigorous within the BC domain than it is in the IWBC region. As the spectral analysis of the mode amplitude time series revealed no significant periods, we searched for dominant wavelengths. This search was done via a spatial EOF analysis on 51 thermal front patterns derived from digitized AVHRR images. Four modes were statistically significant at the 95% confidence level. Modes 3 and 4, which together explained 18% of the total variance, are associated with 266 and 338-km vorticity waves, respectively. With this new information derived from the data, the [Johns, W.E., 1988. One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras. Dyn. Atmos. Oceans 11, 323–350] one-dimensional quasi-geostrophic model was applied to the interpolated mean BC-IWBC jet. The results indicated that the BC system is indeed baroclinically unstable and that the wavelengths depicted in the thermal front analysis are associated with the most unstable waves produced by the model. Growth rates were about 0.06 (0.05) days −1for the 266-km (338-km) wave. Moreover, phase speeds for these waves were low compared to the surface BC velocity and may account for remarks in the literature about growing standing or stationary meanders off southeast Brazil. The theoretical vertical structure modes associated with these waves resembled very closely to the one obtained for the current-meter mooring EOF analysis. We interpret this agreement as a confirmation that baroclinic instability is an important mechanism in meander growth in the BC system.

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

Modelos paramétricos de velocidade de correntes são utilizados na literatura oceanográfica para estudos teóricos envolvendo propriedades de instabilidade, ou como condições de contorno e iniciais para implementações regionais de modelos numéricos. Estes, no entanto, foram desenvolvidos na sua maioria para a Corrente do Golfo (Niiler & Robinson, 1967; Luther & Bane, 1985; Robinson et al., 1988; Arango et al., 1992; Xue & Mellor, 1993; Gangopadhyay et al., 1997). O sistema formado pela Corrente do Brasil (CB) e pela Corrente de Contorno Intermediária (CCI) apresenta características dinâmicas singulares enquanto flui ao largo do SE brasileiro, quando comparada as demais correntes de contorno oeste do oceano mundial. Neste trabalho, apresentamos um modelo paramétrico analítico original. O modelo parametriza a estrutura tanto da corrente de superfície (a CB) como da corrente intermediária (a CCI), visando preservar suas características essenciais. No caso da CB, suas principais características são a presença de um núcleo em superfície e o comportamento ocasionalmente assimétrico longitudinalmente devido ao fenômeno do meandramento. Fato que a faz aproximar e afastar o escoamento superficial da quebra de plataforma e do talude superior. Já para a CCI, a principal característica é apresentar o núcleo em torno dos 800-900 m de profundidade, geralmente junto ao talude intermediário. Adicionalmente ao modelo seccional de velocidades, um outro modelo de estrutura de densidade é obtido, assumindo que o campo de velocidades é estacionário e geostrófico: tratam-se, portanto, de campos básicos. Este modelo de densidade é construído a partir da integração da relação do vento térmico. Por fim, a parametrização é confrontada com três seções de velocidades baroclínicas obtidas a partir de observações dentro dos limites da bacia de Campos: 21ºS, 22ºS e 23ºS.