Brazil-Malvinas Confluence Region Research Articles

Satellite and in situ measurements of water vapour in the Brazil‐Malvinas Confluence region

AbstractThis decade‐long study from the INTERCONF programme addresses the data gap on ocean dynamics in the Southern Hemisphere. Focusing on the Brazil‐Malvinas Confluence (BMC), we investigated the effect of temperature differences between the warm Brazil Current (BC) and the cold Malvinas Current (MC) on water vapour in the marine atmospheric boundary layer (MABL). Our results show a clear distinction: warmer BMC waters have 32% more water vapour (2 kg·m−2 on average) compared to the MC. This highlights the direct link between ocean temperatures and atmospheric processes. Analysis of radiosonde data alongside satellite measurements showed better agreement over cooler waters with lower water vapour, leading to lower variability. This suggests less atmospheric turbulence and improved data compatibility, especially for satellite retrievals such as AIRS. Comparisons between reanalysis data (CFSR), satellite sounders (AIRS) and radiosondes (RS) showed consistent air temperature profiles, with average errors within the 10% threshold for satellite measurements. While capturing humidity variations remains a challenge, especially at high concentrations (indicated by higher mean squared error values), our study highlights the reliability of satellite data, particularly over the cold BMC region. This research highlights the importance of studying the interactions between ocean fronts and atmospheric phenomena for a complete picture of Southern Hemisphere ocean dynamics. It offers valuable insights for scientists across disciplines, providing a broad perspective on the results and their significance in different contexts.

Holocene palaeoceanographic history of the western South Atlantic

Although the circulation of the western South Atlantic tightly controls the meridional heat and salt transport, our knowledge about its long-term past changes is still fragmented and sparse. While many studies focused their attention on the last glacial and deglaciation periods, Holocene studies mostly lack of an integrated analysis from the subtropical and midlatitude regions. The analysis of selected regionally distributed palaeoceanographic records along the western South Atlantic between 27° and 39°S reveals that, although weaker in amplitude than the dramatic shifts of the last glacial cycle, the Brazil-Malvinas Confluence region showed substantial changes throughout the Holocene. The shift in the location of both southern and northern source waters in the Atlantic implied a strengthening of the Southern Atlantic Meridional Overturning Circulation since the Mid-Holocene. This, together with the progressive southward displacement of the Brazil-Malvinas Confluence confirms the progressive weakening of the Atlantic Meridional Overturning Circulation towards the preindustrial times.

Planktonic foraminifera assemblages from the Brazil–Malvinas Confluence: palaeoceanographic implications of sub-surface temperature reconstructions in the western South Atlantic

Planktonic foraminifera census data have been widely used to reconstruct changes in ocean ecosystems as well as ocean chemistry and circulation through calibration functions. Here we analyse new core-top census data from 22 sites in the western South Atlantic, improving the geographical coverage and spatial resolution of the environmental gradients from the region covered by the Brazil–Malvinas Confluence (˜38°S–53°W). We combine them with previous data to provide an up-to-date compilation of the western South Atlantic planktonic foraminifera–calibration data set. We study the relationship between the assemblages present in the core-top samples and the most dynamic environmental variables from the region to establish the environmental variable(s) more resolved by the assemblages. Therefore, we develop and assess a new calibration function using the data set and testing several statistical models at different water depths. Our results reveal that the distribution and abundance of the species in the Brazil–Malvinas Confluence region mainly reflect the influence of two environmental variables: the mixed layer temperature and mixed layer depth (57% of total variance). The most precise reconstructions were obtained when using sub-surface temperatures between 40 and 50 m water depth. The application of the calibration function to a Holocene sediment core at ˜37°S–53°W revealed mean annual sub-surface temperature reconstructions between 8°C and 11°C, confirming the northward displacement of the Brazil–Malvinas Confluence during the onset of the Holocene and suggesting a major influence of the Malvinas Current during the entire Holocene at the studied site.

The Storm Tracks Response to Changes in Atmospheric Greenhouse Gas Concentration at the South of Brazil and Southwest Atlantic Ocean

Here we investigate the ocean-atmosphere coupling and the contribution of the Sea Surface Temperature (SST) variations in: 1) Brazil-Malvinas Confluence (BMC) region, 2) Southwest Atlantic Ocean and 3) Southern Brazil. Numerical simulations of the ECHAM5/MPI-OM coupled ocean-atmosphere model were used to analyze the changes in the seasonal trajectory of the extratropical cyclones, in terms of intensification of physical mechanisms and implications for future scenarios. The numerical experiment for the future scenario considered an atmospheric CO2 concentration of approximately 770 ppm, which represents an increment of more than 350 ppm over the current values recorded by the Mauna Loa reference station. For this scenario, the results indicated a Storm Tracks (ST) displacement of 5° latitude toward south and changes of the meridional transport of sensible heat, close to 50°S. The increase in SST induces ST intensification and consequently an increase in the occurrence of extratropical cyclones. Overall, in the BMC region, we found a change in the pattern of cyclogenetic activity occurrence, with less frequent, but more intense events. On the Southern Brazilian region, the results of this study indicate increases in rainfall during summer months, whereas, a decrease in frequency and an increase in intensity were found for wintertime. We suggest that these changes could impact the climate dynamic of the Brazilian South coast, with a magnitude yet unknown.

The use of mesoscale eddies by juvenile loggerhead sea turtles (Caretta caretta) in the southwestern Atlantic.

Marine animals, such as turtles, seabirds and pelagic fishes, are observed to travel and congregate around eddies in the open ocean. Mesoscale eddies, large swirling ocean vortices with radius scales of approximately 50–100 km, provide environmental variability that can structure these populations. In this study, we investigate the use of mesoscale eddies by 24 individual juvenile loggerhead sea turtles (Caretta caretta) in the Brazil-Malvinas Confluence region. The influence of eddies on turtles is assessed by collocating the turtle trajectories to the tracks of mesoscale eddies identified in maps of sea level anomaly. Juvenile loggerhead sea turtles are significantly more likely to be located in the interiors of anticyclones in this region. The distribution of surface drifters in eddy interiors reveals no significant association with the interiors of cyclones or anticyclones, suggesting higher prevalence of turtles in anticyclones is a result of their behavior. In the southern portion of the Brazil-Malvinas Confluence region, turtle swimming speed is significantly slower in the interiors of anticyclones, when compared to the periphery, suggesting that these turtles are possibly feeding on prey items associated with anomalously low near-surface chlorophyll concentrations observed in those features.

Air-sea heat fluxes associated to mesoscale eddies in the Southwestern Atlantic Ocean and their dependence on different regional conditions

Heat fluxes between the ocean and the atmosphere largely represent the link between the two media. A possible mechanism of interaction is generated by mesoscale ocean eddies. In this work we evaluate if eddies in Southwestern Atlantic (SWA) Ocean may significantly affect flows between the ocean and the atmosphere. Atmospherics conditions associated with eddies were examined using data of sea surface temperature (SST), sensible (SHF) and latent heat flux (LHF) from NCEP–CFSR reanalysis. On average, we found that NCEP–CFSR reanalysis adequately reflects the variability expected from eddies in the SWA, considering the classical eddy-pumping theory: anticyclonic (cyclonic) eddies cause maximum positive (negative) anomalies with maximum mean anomalies of 0.5 °C (−0.5 °C) in SST, 6 W/m2 (−4 W/m2) in SHF and 12 W/m2 (−9 W/m2) in LHF. However, a regional dependence of heat fluxes associated to mesoscale cyclonic eddies was found: in the turbulent Brazil–Malvinas Confluence (BMC) region they are related with positive heat flux anomaly (ocean heat loss), while in the rest of the SWA they behave as expected (ocean heat gain). We argue that eddy-pumping do not cool enough the center of the cyclonic eddies in the BMC region simply because most of them trapped very warm waters when they originate in the subtropics. The article therefore concludes that in the SWA: (1) a robust link exists between the SST anomalies generated by eddies and the local anomalous heat flow between the ocean and the atmosphere; (2) in the BMC region cyclonic eddies are related with positive heat anomalies, contrary to what is expected.

Influência de vórtices na concentração de clorofila da confluência Brasil-Malvinas: Mecanismos inferidos por sensoriamento remoto

This study aims to (a) identify the spatial patterns in the surface phytoplankton abundance, here indexed as satellite chlorophyll-a concentration ([chla]), associated with mesoscale eddies of the Brazil-Malvinas Confluence (BMC) and (b) infer which are the physical mechanisms responsible for such patterns. In this work, the 57 largest eddies in the broad BMC region were selected from a global eddy dataset based on merged satellite altimetry data. Using those eddies, the main mechanisms which may eddies affect the [chla] were tested: (a) the eddy-pumping, (b) the eddy Ekman pumping and (c) the eddy advection of meridional [chla] gradients. Mean spatial chlorophyll and chlorophyll patterns associated with the selected eddies, as well as correlations between variables that describe the [chla], and its spatial anomalies (Δ[chla]), distribution and the eddies’ physical characteristics, were calculated. Positive and negative anomalies associated with cyclonic and anticyclonic eddies’ interiors, respectively, and significant inverse correlations between eddies’ amplitudes and the difference between Δ[chla] inside and outside the eddies, were found as evidence of eddy-pumping mechanism. The [chla] spatial pattern associated to the eddy advection of meridional gradients was evident in the eastern side of the features, as low (high) [chla] being advected from south (north) in anticyclonic (cyclonic) eddies. The magnitude of the difference between Δ[chla] in the east and west eddies’ hemispheres presented itself more dependent on the [chla] meridional gradient intensity than on the eddy rotational velocity. Furthermore, the non-linear parameter of the studied eddies suggest that BCM eddies are capable to transport the water parcels trapped inside them, helping in the propagation of the eddy-pumping signature.

A Lagrangian study of the Brazil-Malvinas confluence: Lagrangian coherent structures and several lyapunov exponents

This paper presents an analysis of specific Lagrangian patterns in an unsteady chaotic flow, applied to the Brazil-Malvinas confluence region. Satellite tracked ocean drifters and Ocean General Circulation Model (OGCM) data are used to calculate time dependant structures and patterns. In particular, the local flow properties, namely the dispersion along real and virtual drifters’ trajectories are investigated. The method’s key advantage is to condense critical Lagrangian information into a single scalar field also known as a Lyapunov exponent. Three fields, each corresponding to a particular method of extraction are calculated from the OGCM data. These fields can be thought as maps of sensitivity with respect to initial conditions, in a finite time interval. The maximum ridges in these fields are known to represent the underlying structure of chaotic flows which organise the transport through the regional oceanic dynamic features. When used in conjunction with real drifter data, such maps and tools can be of significant use in predicting the dispersion properties of a given area. This has important applications in Search and Rescue (SAR) and oil fate scenarios.

Sea surface temperature anomalies driven by oceanic local forcing in the Brazil-Malvinas Confluence

Sea surface temperature (SST) anomaly events in the Brazil-Malvinas Confluence (BMC) were investigated through wavelet analysis and numerical modeling. Wavelet analysis was applied to recognize the main spectral signals of SST anomaly events in the BMC and in the Drake Passage as a first attempt to link middle and high latitudes. The numerical modeling approach was used to clarify the local oceanic dynamics that drive these anomalies. Wavelet analysis pointed to the 8–12-year band as the most energetic band representing remote forcing between high to middle latitudes. Other frequencies observed in the BMC wavelet analysis indicate that part of its variability could also be forced by low-latitude events, such as El Nino. Numerical experiments carried out for the years of 1964 and 1992 (cold and warm El Nino-Southern Oscillation (ENSO) phases) revealed two distinct behaviors that produced negative and positive sea surface temperature anomalies on the BMC region. The first behavior is caused by northward cold flow, Rio de la Plata runoff, and upwelling processes. The second behavior is driven by a southward excursion of the Brazil Current (BC) front, alterations in Rio de la Plata discharge rates, and most likely by air-sea interactions. Both episodes are characterized by uncoupled behavior between the surface and deeper layers.

Modulation mechanisms of marine atmospheric boundary layer at the Brazil‐Malvinas Confluence region

AbstractThe influence of the Brazil‐Malvinas Confluence (BMC) region on the marine atmospheric boundary layer (MABL) is investigated through in situ data analysis of five different cruises (2004 to 2008) and numerical experiments with a regional atmospheric model. Two different groups of numerical experiments were performed in order to evaluate the relevance of static stability and hydrostatic balance physical mechanisms for the MABL instability. The first group used monthly climatological sea surface temperature (SST) as bottom boundary condition while the second used daily updated Advanced Microwave Scanning Radiometer–EOS SST data together with radiosondes and surface data assimilation. A reasonable agreement between numerical results and QuikSCAT wind data was observed through correlation coefficients and mean square error values. In terms of the horizontal structure of the MABL, stronger winds were found over the warm side of the BMC region as well as over the thermal front itself, which supports the coexistence of both modulation mechanisms. The analyzed patterns of surface atmospheric thermal advection showed a clear interaction between the synoptic and regional scales. The signature of the oceanic thermal front (almost meridionally oriented) on the air temperature at 2 m makes the temperature advection strongly determined by the zonal component of the wind. The analysis of momentum budget terms did not show a clear and reasonable explanation of the existence or predominance of the modulation mechanisms, and it also suggested the relevance of other effects, such as the idea based on unbalanced Coriolis force and turbulence/friction effects.

Brazil-Malvinas confluence: effects of environmental variability on phytoplankton community structure

This study investigates the relationships between the spring phytoplankton community and environmental factors in the Brazil-Malvinas confluence region. Phytoplankton community composition was determined by the high performance liquid chromatography/CHEMTAX approach, complemented with microscopic examination. Abiotic factors included temperature, salinity, dissolved inorganic macronutrients (ammonium, nitrite, nitrate, phosphate and silicate), water column stability and upper mixed layer depth (UMLD). These environmental variables were reasonably informative to explain the variability of the phytoplankton communities (44% of variation explained). Cluster and canonical correspondence analyses allowed discrimination of four zones (coastal, Sub-Antarctic, tropical and intermediate zones), also identifiable in the T–S diagrams and in the nutrient spatial distribution patterns. The presence of nutrient-rich Sub-Antarctic waters was a major oceanographic feature, associated with diatoms and dinoflagellates. However, in the Sub-Antarctic zone, biomass was particularly low, probably as a result of grazing pressure, as suggested by chemical and biological indicators. In contrast, in oligotrophic tropical waters, phytoplankton was mainly composed by small nanoflagellates and cyanobacteria. A large intermediate zone was also dominated by nanoflagellates, mainly Phaeocystis antarctica, probably favored by strong water column stability. The coastal zone exhibited fairly similar conditions to those in the intermediate zone, but with deeper UMLD, a favorable condition for diatom growth. These results emphasize the importance of the properties of water masses and also biological processes such as grazing in structuring phytoplankton communities in the region.

Variabilidade das anomalias de temperatura da superfície do mar no oceano atlântico sudoeste e sua relação com o fenomeno El Niño-Oscilação Sul

Este trabalho apresenta um estudo sobre a variabilidade dos campos de anomalia da temperatura da superfície do mar (ATSM), obtidos através imagens de satélites em regiões sob a influência dos diferentes regimes oceanográficos e/ou biofísicos no Oceano Atlântico Sudoeste (ATS). Para tanto, séries temporais de ATSM de 1985 a 2006 foram investigadas em nove sub-áreas no ATS, divididas nos domínios subtropical, intermediário e subantártico. Análise espectral aplicada às séries de ATSM revelou ciclos no período interanual entre 1,6 e 3,7 anos. Análise de correlação cruzada foi usada para estabelecer a relação entre os ciclos presentes na ATSM e os eventos do fenômeno El Niño Oscilação Sul (ENOS), representados aqui a partir do Índice Multivariado do ENOS (MEI). Os resultados evidenciam uma correlação estatisticamente significante, defasada no tempo, entre o ENOS e anomalias termais no ATS, porém sem uma associação direta entre os episódios quentes (frios) de El Niño (La Niña) e ATSMs positivas (negativas). Dentre as diversas áreas de estudo, a correlação é máxima sobre a região da Confluência Brasil-Malvinas (-0,78), com defasagem de tempo de 9 meses após a La Niña de maio de 1988 a maio de 1989.

Dynamics of the Brazil-Malvinas Confluence: Energy Conversions

In this work, we investigated the mesoscale dynamics of the Brazil-Malvinas Confluence (BMC) region. Particularly, we were interested in the role of geophysical instability in the formation and development of the mesoscale features commonly observed in this region.We dynamically analyzed the results of numerical simulations of the BMC region conducted with 'Hybrid Coordinate Ocean Model' (HYCOM). We quantified the effect of barotropic and baroclinic energy conversions in the modeled flow and showed the dominance of the latter in the region.

Brazil-Malvinas Confluence upper ocean temperature anomalies induced by an ENSO wind forcing

The Brazil-Malvinas Confluence (BMC) region constitutes one of the most important ocean features in the South Atlantic Ocean basin. Understanding the BMC upper ocean temperature variability could represent one of the key factors to understanding its connection to local and global ocean circulation. Some important local and remote ocean processes are related to the annual and interannual variability of its thermal/salt front position. This paper investigates the upper ocean variability in the BMC region under the influence of an El Niño/Southern Oscillation-type wind stress field using an oceanic general circulation model (Modular Ocean Model version 4.0). The results revealed the occurrence of positive anomalies, with values ranging from 0.5ºC to 1.0ºC (centered at 40.5ºS and 55.0ºW), near the BMC region associated with upper ocean velocity anomalies during the peak of the ocean kinetic energy phase. This mechanism was a response of a volume transport balance between the Brazil Current (BC) and the Malvinas Current (MC), the latter being accelerated by the Antarctic Circumpolar Current (ACC). The occurrence of positive anomalies was always associated with an enhanced BC meridional surface volume transport, together with a decrease of the ACC zonal volume transport, vertically integrated across the Drake Passage, and a correspondent decrease in the upper layer MC volume transport. It was possible to see negative upper layer temperature anomaly predominance just before the peak ocean kinetic energy month and positive anomaly predominance immediately following this month. The occurrence of upper layer temperature anomalies in the BMC region was not restricted to just the peak ocean kinetic energy month since they occurred throughout the perturbed experiment integration time.

Multiyear measurements of the oceanic and atmospheric boundary layers at the Brazil‐Malvinas confluence region

This study analyzes and discusses data taken from oceanic and atmospheric measurements performed simultaneously at the Brazil‐Malvinas Confluence (BMC) region in the southwestern Atlantic Ocean. This area is one of the most dynamical frontal regions of the world ocean. Data were collected during four research cruises in the region once a year in consecutive years between 2004 and 2007. Very few studies have addressed the importance of studying the air‐sea coupling at the BMC region. Lateral temperature gradients at the study region were as high as 0.3°C km−1 at the surface and subsurface. In the oceanic boundary layer, the vertical temperature gradient reached 0.08°C m−1 at 500 m depth. Our results show that the marine atmospheric boundary layer (MABL) at the BMC region is modulated by the strong sea surface temperature (SST) gradients present at the sea surface. The mean MABL structure is thicker over the warmside of the BMC where Brazil Current (BC) waters predominate. The opposite occurs over the coldside of the confluence where waters from the Malvinas (Falkland) Current (MC) are found. The warmside of the confluence presented systematically higher MABL top height compared to the coldside. This type of modulation at the synoptic scale is consistent to what happens in other frontal regions of the world ocean, where the MABL adjusts itself to modifications along the SST gradients. Over warm waters at the BMC region, the MABL static instability and turbulence were increased while winds at the lower portion of the MABL were strong. Over the coldside of the BC/MC front an opposite behavior is found: the MABL is thinner and more stable. Our results suggest that the sea‐level pressure (SLP) was also modulated locally, together with static stability vertical mixing mechanism, by the surface condition during all cruises. SST gradients at the BMC region modulate the synoptic atmospheric pressure gradient. Postfrontal and prefrontal conditions produce opposite thermal advections in the MABL that lead to different pressure intensification patterns across the confluence.

Multi-sensor satellite and in situ measurements of a warm core ocean eddy south of the Brazil–Malvinas Confluence region

A warm core eddy was detected south of the Brazil–Malvinas (Falkland) Confluence (BMC) region in satellite images of the southwestern Atlantic Ocean in late 2002. The structure was also sampled by in situ instruments deployed by a ship in 2 days of November 2002. An analysis of satellite data was performed to account for the lifecycle, dimension, surface temperature, surface chlorophyll concentration, surface height anomaly and displacement of the eddy since it was detached from the Brazil Current in September 2002. Satellite data were derived from several sources such as the AMSR-E, MODIS and radar altimeter. The structure lasted 64 days at south of the Brazil–Malvinas Confluence region later being re-assimilated by the Brazil Current return flow. In situ data collected during 2 days showed that the eddy was about 150 m deep, 5.5 °C warmer and 1 practical salinity unit saltier than adjacent waters. The salt anomaly associated to the eddy was estimated as 1.37 × 10 12 kg while its heat content was 4.24 × 10 18 J. These are typical estimates for eddies present at the BMC region. Sea surface temperature (SST), chlorophyll concentration, and sea surface height anomaly time series were analyzed for the eddy's center along its trajectory path throughout its lifetime. Mean translational velocities for the eddy during its lifetime were computed from visual interpretation and by using an adaptation of the Maximum Cross-Correlation (MCC) method on AMSR-E SST images. The overall deviation between the two methods was 26%. This suggests that the MCC method has a potential to be applied in monitoring programs to automatically account for the translational velocities of eddies in the BMC region.

Águas modais subtropicais do Atlântico Sul na base Levitus.

Mode waters are layers of vertically homogeneous water found over a large geographical area, occur within or near the top of the permanent pycnocline. The mode waters are characterized by the conservation of their properties of the formation area (temperature, salinity, potential vorticity). In South Atlantic Ocean, the subtropical mode waters are formed within the Brazil-Malvinas Confluence region, and keep their properties along the subtropical gyre. The objective of this work is to identify and to characterize subtropical mode waters from data of temperature and salinity of the Levitus database. The applied methodology consisted on the construction and analysis of zonal and meridional sections of potential temperature and salinity and volumetric T-S diagrams. Two mode waters were identified on the analysis, one with potential temperature (q) ~14-16°C and another one with q ~ 11-12°C. These results were verified with information of hydrographic data collected presented in literature and will be used for future comparison with numerical models simulation.

Estudo teórico da dinâmica da confluência Brasil-Malvinas

The study of the mesoscale dynamics of the confluence of two western boundary currents was conducted in this work via a quasi-geostrophic inviscid f-plane two-layer ocean theoretical approach. The simplified vertical structure was calculated through the employment of a dynamic calibration scheme based on the dynamical mode structure of the Brazil-Malvinas Confluence Region. The Brazil-Malvinas mode structure was computed from climatological hydrography of the area delimited by 35o-40oS and 50o-60oW. Two contour dynamics models were constructed: a linear and a nonlinear version. The basic flow configuration consisted of two converging western boundary currents that form a zonal eastward current in the upper layer. The lower layer flow was essentially divergent as a result of a westward zonal jet impinging on the western border. This vertical shear choice assured that the system was baroclinically unstable. The results of the model experiments showed that the presence of the confluence, the western meridional boundary and the barotropic mode in the dynamical structure of the basic flow favored long wave patterns. The three experiments conducted with the nonlinear model exhibited the development of both a reflection pattern and vortical dipoles. The dipoles pinched off from either the retroflection lobe (i.e., the primary crest of the wave train) or the primary trough when the baroclinically unstable current system was perturbed at the boundary vicinities. It was verified that the nonlinear model simulations followed the instability properties predicted by the linear model in terms of meander growth rates, phase speeds and most unstable wavelengths. This suggested that while the baroclinic instability mechanism was responsible for the temporal growth of the meanders, the nonlinear effects caused the dipole isolation and pinch off of the finite amplitude meanders. These dipoles could leave and propagate away from the current axis.

Chlorophyll variability and eddies in the Brazil–Malvinas Confluence region

Ocean-color data from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) have been used to investigate temporal and spatial variability of chlorophyll-a concentration in the Brazil–Malvinas Confluence (BMC) region (30–50°S and 30–70°W). Our analysis is based on 60 monthly averaged and 230 weekly averaged images (9×9 km2 resolution) that span from October 1997 to September 2002 in the Southwestern Atlantic Ocean. A nonlinear model was used to fit the annual harmonic of the chlorophyll concentration anomalies in the region. Analysis of the spatial variability of model parameters has shown the existence of a marked annual cycle at several locations in the study region. We also have observed shorter-period oscillating features on smaller spatial scale, associated with the BMC dynamics. These mesoscale features have periods of about 9–12 weeks and have a marked northward propagation with higher chlorophyll values relative to the surrounding waters. Further investigation of these mesoscale features with an advanced very high-resolution radiometer thermal infrared and TOPEX/POSEIDON (T/P) altimeter data has unveiled interesting eddy-like surface structures in the BMC region. These multiple eddies are clearly visible on both thermal and color imagery off southern Brazil. Furthermore, the SeaWiFS images have shown a decrease in chlorophyll-a concentration as these eddies propagate northward, due to mixing with chlorophyll-poor tropical waters carried southward by the Brazil Current. Several rings along the shelf region, which are probably generated by shear instabilities, also have been detected by the ocean-color images.

Variability of the South Atlantic upper ocean circulation: A data assimilation experiment with 5 years of TOPEX/POSEIDON altimeter observations

Dynamical interpolation of five years of TOPEX/POSEIDON altimeter data (October 1992-September 1997) is performed in the South Atlantic basin, through the use of a simple quasigeostrophic model and a simple nudging data assimilation technique. The resulting data set is analysed in order to evidence the role of mesoscale eddies on the interannual variability of the South Atlantic ocean circulations. The annual mean circulations and the eddy fields reveal some significant interannual variability in the Brazil-Malvinas confluence region and in the Aghulas Retroflection region. But this interannual variability spreads out also in large extensions of those previous regions as well as in other areas of the South Atlantic basin. In particular, significant variability shows up in the upper return branch of the subtropical gyre that is interpreted in relation with the drift of some Aghulas rings. The role of those latter rings which are crucial for the Indian-Atlantic interocean exchange processes, is further investigated in the paper, and the fate of five specific rings analysed in detail. The overall influence of topography on the circulation is clearly evidenced, especially through its interaction with eddies.