Brazil Current System Research Articles

Development of a multigrid operational forecast system for the oceanic region off Rio de Janeiro State

We propose the development of a multigrid open-source operational forecast system, with the oceanic region off Rio de Janeiro state as our first case study. The system is composed by three nested ROMS domains, employing Mercator Ocean at the lateral open boundaries of the outermost grid, and GFS (Global Forecast System) at the surface open boundary of all grids. Two schemes were built for the operational forecast workflow, free (without Data Assimilation) and assimilated (with data assimilation). For evaluation, we performed hindcast simulations using Mercator and ERA5 as input data for different parts of the Southeastern Brazilian oceanic region, covering various regimes of the unique Brazil Current system. The modelled data was compared against velocity components from three oceanic buoys, SST from MUR (Ultra-high Resolution Sea Surface Temperature), and SSH from AVISO. The comparison against the buoys showed a good representation of the velocity field by ROMS, performing better than Mercator for most scenarios. ROMS also presented good performances for SST and SSH. For the three-nested forecast system, the velocity fields presented comparable magnitudes against Mercator, without the presence of jet-like instabilities, indicating a stable system. We also implemented the I4D-Var method in a 2-day analysis for the simulation of 7-day forecasts, finding an improvement in the modelled forecasts initialized from the assimilated analysis for both velocity components, with an overall reduction in the RMSE and bias, and an increase in correlation.

Evaluation of the relationship between biomass of living (stained) benthic foraminifera and particulate organic matter vertical flux in an oligotrophic region, Campos Basin, southeastern Brazilian continental margin

This study investigates the relationship between the vertical flux of particulate organic matter (POM) and the biomass of living benthic foraminifera in a region at the continental slope of the Campos Basin (between 400 and 3000 m). Vertical POM fluxes to the seafloor were modeled as a function of remote sensing chlorophyll-a concentration, sea surface temperature, and derived products such as primary production rates and estimates of dominant phytoplankton size. Measurements of biovolume predicted the biomass of benthic foraminifera. The mean calculated vertical POM fluxes (24.2 ± 17.3 mgCm−2.day−1) were consistent with observations from other oligotrophic environments and correlated positively with the total organic carbon (TOC) content in the sediments. The total biomass of benthic foraminifera, and the biomasses of 14 individual foraminifera species also showed significant correlations with the estimated vertical POM fluxes. The spatial distribution of these fluxes, sediment carbon, and foraminifera biomass in Campos Basin depicted the expected decrease with local depth but our data suggested that the foraminifera communities were influenced by mesoscale eddies associated with the Brazil Current System.

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).

Reviews on HYCOM and MyOcean with focus on Brazil Current System, between Cabo Frio and Cabo de São Tomé

Products of numerical models, such as HYCOM (Hybrid Coordinate Ocean Model) and MyOcean, are widely used in scientific circles. In the present study, these two databases were compared for a radial between Cabo Frio and Cabo de São Tomé, aiming their application for the Brazil Current System. Therefore, this study presents analysis for the percentage of kinetic energy in each longitudinal band of the BC, and also for it volume transport and for it limits depth of occurrence.

Variabilidade do sistema corrente do Brasil associada aos cenários de mudanças climáticas A1F1 e B1

The aim of this work is to investigate the behavior of volume transport and advective heat flux patterns which are related to the Brazil Current System (BCS) according to the climate change scenarios suggested by the Intergovernmental Panel on Climate Changes. The results deriving from the global model Climate Model version 2.1 (GFDL-CM2.1) for future scenarios of climate change A1FI and B1 and for the scenario that represents the climate in the Twentieth Century (20C3M) have been used in this work. Volume transport and advective heat flux integrated for the whole BCS as well as for each water mass which constitutes the system have been calculated applying these data. Estimates were made for three zonal sections (23ºS, 25ºS and 30ºS) and then were used in the Wavelet Analysis. The performed analysis showed the occurrence of interannual variability for the estimated properties in the region. It also showed that the BCS shows changes in both intensity and variability of these parameters when comparing A1FI and B1 to 20C3M. Results from Wavelet Analysis for the total volume transport and advective heat flux integrated per section show that the period between three and six years prevails in sections 23ºS and 25ºS. The variability in 30ºS however, is significantly higher than eight years. It could also be observed the change of the variability of the total volume transport and heat flux temporal series towards higher frequencies when they are submitted to A1FI and B1 climate changes scenarios.

Vertical structure, energetics, and dynamics of the Brazil Current System at 22°S–28°S

We use four current meter moorings and quasi‐synoptic hydrographic observations in conjunction with a one‐dimensional quasi‐geostrophic linear stability model to investigate downstream changes in the Brazil Current (BC) System between 22°S and 28°S. The data set depict the downstream thickening of the BC. Its vertical extension increases from 350 m at 22.7°S to 850 m at 27.9°S. Most of this deepening occurs between 25.5°S and 27.9°S and is linked to the bifurcation of the South Equatorial Current at intermediate depths (Santos bifurcation), which adds the Antarctic Intermediate Water flow to the BC. Geostrophic estimates suggest that the BC transport is increased by at least 4.3 Sv (∼70%) to the south of that bifurcation. Moreover, the Santos bifurcation is associated with a substantial increase in the barotropic component of the BC System. On average, the water column average kinetic energy (IKE) is 70% baroclinic to the north and 54% barotropic to the south of the bifurcation. Additionally, the BC shows conspicuous mesoscale activity off southeast Brazil. The water column average eddy kinetic energy accounts for 30–60% of the IKE. Instabilities of the mean flow may give rise to these mesoscale fluctuations. Indeed, the linear stability analysis suggests that the BC System is baroclinically unstable between 22°S and 28°S. In particular, the model predicts southwestward‐propagating fastest growing waves (∼190 km) from 25.5°S to 27.9°S and quasi‐standing most unstable modes (∼230 km) at 22.7°S. These modes have vertical structures roughly consistent with the observed eddy field.

Distribution of living planktonic foraminifera in relation to oceanic processes on the southeastern continental Brazilian margin (23°S–25°S and 40°W–44°W)

The vertical distribution (0 to 100m) of planktonic foraminifera was investigated based on 40 tow samples that were collected in eight stations, during the austral summer of 2002, in a geographically restricted area (23°S–25°S and 40°W–44°W) on the southeastern Brazilian continental margin. Species' abundances are low (less than 10 specimens/m3), which is typical of an oligotrophic area. The foraminifera assemblage is mainly composed of warm water species (Globigerinoides ruber white and pink forms), with a predominance of spinose and symbiont-bearing species. Temperature and inorganic nutrient enrichment of the surface are the main factors that control foraminiferal abundance and diversity; nevertheless salinity can also influence the ecological descriptors. The role of the deep chlorophyll maximum (DCM) in the distribution of foraminifera is not always clear, but the increase in the abundance of G. ruber (white and pink) seems to be related to a deeper DCM, and high salinities (S>36.5). The ecological habitat of these species is affected by the depth of the mixed layer, with a predominance of the white form in deeper layers. Increases in the foraminiferal diversity are related to the dynamics of the Brazil Current system, which displaces the area of high productivity in the euphotic zone off the coast. The abundances of Globigerina bulloides, Globigerina falconensis, Globigerinella calida and Globigerinella siphonifera follow the nutrient enrichment of the surface water mass, corroborating the usefulness of these species as paleoproductivity proxies in the study area. These data confirm the use of diversity measurements and assemblages composition for reconstructing past water column structures in subtropical oceans.

Results of the research cruises of FRV ‘Walther Herwig’ to South America. LXXV: trans-Atlantic oceanographic measurements during South America Expeditions 1966-1976 of the Institute of Sea Fisheries, Hamburg, Germany

Summary During the 1960s, with the new research platform FRV ‘Walther Herwig’ being available for long-distance exploratory fishery and ichthyology research, the Institute of Sea Fisheries started a series of South America Expeditions. During these expeditions large-scale oceanographic sections were performed which cross major oceanic surface current systems between 40°N and 40°S. By means of historic oceanographic data, as available through the World Data Centre A, Washington, the thermohaline situation along these sections is presented and described in relation to large-scale oceanic features like the North Atlantic Subpolar Gyre System, the northern Subtropical Gyre, the Atlantic Equatorial Current System, the southern Subtropical Gyre, the Brazil Current and the Malvinas Current System. Data obtained during the 1970s reveal large-scale oceanographic properties within the Subtropical Convergence belt at 40°S, between South America and South Africa. Based on these data, an analysis on the baroclinic current velocities of the upper 1000 m is presented. This analysis documents the deep reaching influence of the two boundary currents in this region, the Brazil Current system in the west, and the Agulhas/Benguela Current system in the east of the transect along 40°S. Also designed as large-scale ichthyology transects, the oceanographic observations performed during leg I of the Antarctic Expedition of the Federal Republic of Germany during 1975/1976 along 50°W and 40°W, reveal thermal properties across the Subtropical Convergence, in the sub-Antarctic surface waters and in Antarctic surface waters.

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.

Tropical instability vortices in the Atlantic Ocean

This paper examines the physical characteristics and dynamics of Atlantic tropical instability vortices based on observational data from 1997 to 2000 and a high‐resolution numerical model simulation. Most prominent during boreal summer, the vortices are characterized by 400 km spatial scales, westward translation at 0–40 cm s−1, anticyclonic circumferential velocities of 30–110 cm s−1, and SST anomalies that decrease from 2°C in midbasin to 0.5°C or less near the western boundary. In contrast, the sea level anomalies grow as the vortices progress westward, reaching maximum amplitudes of 14 cm near 40°W. The large circumferential velocities, leading to relative vorticities of −4 × 10−6 s−1 and Rossby numbers of 0.5, indicate that centrifugal effects may play an important role in the vortices' vorticity balance. We address the vortices' vertical structure and vorticity dynamics by examination of a high‐resolution numerical model. There is a reasonably good agreement between the model and observations. Simulated vortices are confined mostly to the mixed layer, which increases westward in depth from 30 to 100 m near the western boundary. In the eastern basin, potential vorticity within the vortices is not conserved but decreases because of wind stress forcing. In contrast, in the western basin, decreases in potential vorticity are due mostly to the vortices' acquisition of Southern Hemisphere water. We estimate an annualized cross‐equatorial transport of up to 1.2 Sv associated with the vortices, which is eventually contributed to the North Brazil Current system.

Flow and recirculation of Antarctic Intermediate Water across the Rio Grande Rise

The flow of the low‐salinity Antarctic Intermediate Water (AAIW) at 700–1150 m depth across the Rio Grande Rise and the lower Santos Plateau is studied under the auspices of the World Ocean Circulation Experiment (WOCE) in the context of the Deep Basin Experiment. Our data set consists of several hydrographic sections, a collection of 15 RAFOS float trajectories, and records from 14 moored current meters. The data were gathered during different intervals between 1990 and 1994. The inferred flow field strongly supports a basinwide anticyclonic recirculation cell in the subtropical South Atlantic underneath the wind‐driven gyre. Its center, which appears to be southeast of the Rio Grande Rise, separates the eastward advection of AAIW below the South Atlantic Current from the westward flowing, recirculating AAIW. The two near‐shelf limbs closing the circumference of AAIW flow are formed in the east by the deep Benguela Current, potentially modulated by salty inflow of Indian Ocean Intermediate Water, and in the west by the Brazil Current system. Further important circulation elements are the Brazil‐Falkland (Malvinas) Confluence Zone at 40°S and an unnamed divergence at 28°S close to the 1000 m isobath. The resulting broad southward flow of AAIW augments the share of modified, i.e., saltier, intermediate water in the source region of the South Atlantic Current, while the smaller northward flow marks the source of a narrow equatorward Western Intermediate Boundary Current, ultimately leaving the South Atlantic. This shelf‐trapped jet is clearly documented in hydrographic data from 19°S and in nearby current meter records. The jet contrasts a sluggish flow across this latitude east of 35°W. A continuous flow of AAIW from its subpolar region in the southwestern Argentine Basin all along the western slope toward the equator appears unlikely between 35°S and 25°S.

On the eddy field in the Agulhas Retroflection, with some global considerations

Moored instrument data are used to describe and summarize selected characteristics of the observed time‐dependent flow at low frequencies, that is, the eddy field, in the Agulhas Retroflection. In addition, the basic features of the eddy kinetic energy (KE) distributions in the Agulhas Current System (ACS) are compared with those from analogous strong current regimes in other oceans, concentrating on those from the Gulf Stream System (GSS). Similarities between the eddy field in the ACS and the GSS are found to include the vertical structure of KE as well as its distribution with frequency. This correspondence between the eddy fields in the ACS and GSS is probably associated with a consonant physical mechanism (mixed barotropic/baroclinic instability) for eddy generation in the presence of comparable upper and intermediate layer mean flow characteristics. It is also pointed out (for the first time to my knowledge) that the upper 1000 m or so (σθ ≤ 27.4) of the ACS and GSS can have a very similar distribution of transport in potential density classes. Relationships between the eddy field and the general circulation for the ACS and GSS with the Kuroshio Current System and the North Brazil Current System are described in less detail and scope. It seems probable that the characteristics of the global eddy field are strongly affected by the interbasin‐scale upper and intermediate layer replacement flows associated with the global thermohaline circulation.

Physical oceanography of the Amazon shelf

The Amazon shelf is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind stress from the northeasterly tradewinds and strong along-shelf flow associated with the North Brazil Current. Although the large volume of river discharge produces a pronounced salinity anomaly, the water motions on the shelf are dominated by the other forcing factors. Tidal velocities of up to 200 cm s −1 are generally oriented in the cross-shelf direction. Tide-induced mixing influences the position and structure of the bottom salinity front that separates the well-mixed nearshore region from the stratified plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and thus reduce the bottom stress. At subtidal frequencies, motion is primarily along-shelf toward the northwest, both in the plume and in the ambient, high-salinity water of the outer-shelf. The plume is generally 5–10 m thick, with a salinity of 20–30 psu. The along-shelf velocity within the plume varies as a function of the along-shelf wind stress. This variability results in large temporal variations in plume structure and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the result of a large-scale pressure gradient associated with the North Brazil Current system.