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[1] 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.

Mestrado em Meteorologia e Oceanografia Física Este trabalho, realizado no âmbito do Mestrado em Meteorologia e Oceanografia Física da Universidade de Aveiro, surge ligado à campanha realizada pelo IPIMAR (NeoMav), a bordo dos Navios de Investigação Noruega e Tellina em Julho de 2007. Nele se faz uma análise dos dados recolhidos pelo CTDF a bordo dos dois navios, caracterizando os campos de temperatura, salinidade, densidade e concentração de clorofila-a na área mais costeira, ao largo de Aveiro, para o período de 2 a 7 de Julho de 2007. A informação recolhida por estes navios foi complementada com: i) uma configuração numérica de circulação, realizada com o modelo ROMS-AGRIF, com uma resolução aproximada de 900m, forçado em condições realistas; ii) dados obtidos por satélite para temperatura de superfície (da série NOAA, sensor AVHRR) e concentração de clorofila-a (sensor MODIS, satélite Terra). São estudadas as respostas da plataforma média e plataforma interior aos forçamentos atmosféricos que consistiram na alternância entre um regime de ventos de Norte, seguido de relaxamento e nova intensificação do vento de Norte, no Verão, na zona em estudo. As características mais marcantes são: a ascensão de água fria, durante o afloramento; o aparecimento de uma contracorrente para Norte na plataforma interior, junto à costa, durante o relaxamento; o aparecimento de uma frente que separa as águas afloradas mais frias das águas oceânicas mais quentes; a assinatura das descargas do rio Douro na salinidade, com uma pluma de água menos salgada a deslocar-se para Sul, associada ao jacto e para o largo, associada ao transporte de Ekman induzido pelo vento. Os resultados obtidos através do modelo demonstraram ser bastante consistentes com as observações e com as Imagens de Satélite, pelo que este modelo numérico reproduz realisticamente os resultados. ABSTRACT: The present work, as a dissertation of the Master’s Degree Programme in Meteorology and Physical Oceanography at the University of Aveiro, is associated with a survey (NeoMav), conducted by the IPIMAR, with two Investigation boats, Noruega and Tellina, in July of 2007. An analysis of the data provided by the CTDF is made in order to characterize temperature, salinity, density and chlorophyll-a concentration, in the region near the cost of Aveiro, in the western Iberian region. The information of the both cruises was complemented with: a fine resolution model ROMS-AGRIF, with 900 m of resolution approximately, forced in realistic conditions; ii) data from satellite imagery of surface temperature (series NOAA, sensor AVHRR) and chlorophyll-a concentration (sensor MODIS, Satellite Terra). The responses of middle and inner shelves are studied due to the forcing of the atmospheric conditions, during the alternation between Southward winds and relaxation, in the summer, in the region of the study. The most striking features are: the upwelled water during the upwelling; the appearance of a Northward counter current during the relaxation; the appearance of a front, which separates cold upwelled waters from warmer oceanic water; the signature of less salinity lens from Douro river, with a plume going Southward, associated to the flow, and offshore, associated to the Ekman transport induced by the wind. There is plenty of agreement between the results from the numeric model, the observations and the satellite imagery.

O estudo da circulação oceânica é de grande importância para a compreensão do sistema climático terrestre. Apesar disso, o conhecimento sobre diversas regiões ainda é incipiente e, muitas vezes, baseado num pequeno montante de dados como, por exemplo, a região oceânica ao sul da Cadeia Submarina Vitória-Trindade (CSVT). Estudar a variabilidade nessa região é o objetivo central do projeto MOVAR (Monitoramento da Variabilidade do Transporte entre o Rio de Janeiro- RJ e a Ilha da Trindade- ES) que, para isso, realiza a coleta periódica de dados de batitermógrafos descartáveis (XBT) ao longo de uma seção ao largo da Bacia de Campos, chamada seção AX97. Como contribuição ao projeto, a seção foi explorado utilizando, além dos dados mensurados, os resultados de duas simulações numéricas realizas com o OCCAM (Ocean Circulation and Climate Advanced Modelling Project). Os resultados mostraram que o aumento da resolução espacial de 1/4o (OC4) para 1/12o (OC12) gerou melhoras significativas nos resultados modelados e, consequentemente, nas estimativas da variabilidade oceânica.O transporte de volume da CB foi calculado utilizando o método dinâmico considerando dois níveis de referência, 680 m e 400 m. O nível mais raso foi considerdo mais adequado para região e resultou no tranporte médio de -5,1 ± 0,9 Sv para os dados do OC12 e de -3,3 ± 2,1 Sv para os dados de XBT. Além dos valores médios, a variabilidade na região foi investigada usando dados do OC12 e foi constatado que a corrente apresenta mudanças de intensidade e posição resultando em ciclos com frequências próximas a anual e semi-anual. Quando a CB esta fraca, o fluxo de água pelo canal secundário (mais distante da costa) da CSVT é pequeno e a corrente apresenta um padrão meandrante ao sul da CSVT. Quando o transporte é maior, a bifurcação da CB é evidente sendo o fluxo intenso nos dois canais, a corrente de contorno se reorganiza junto ao talude logo depois de passar pela cadeia. Os cenários parecem se suceder e podem estar associados à propagação de vórtices; quando a corrente se intensifica ela se torna mais instável, o meandramento se inicia para dissipar energia e, neste processo, pode gerar um vórtice. The study of ocean circulation is very important for the undestanding of Earths climate. Besides this importance, many oceanic regions are poorly known due to undersampling, which is the case of the area south of Vitória-Trindade Ridge (CSVT). Understanding the variability in this region is the main goal of the MOVAR project (Variability monitoring between Rio de Janeiro-RJ and Trindade Island-ES). To accomplish that, a regular sampling using expandable bathythermograph (XBT) is made offshore of Campos Basin (between the shelf break around Cabo Frio and Trindade Island) along the section called AX97. As a contribution to this project, in this study the MOVAR section is explored using the results of two simulations made with OCCAM (Ocean Circulation and Climate Advanced Modelling Project). Results showed that increasing resolution from 1/4o (OC4) to 1/12o (OC12) produces improvements in the model outputs and thus in the overall results and interpretation. Brazil Current (CB) volume transports were calculated using the classic dynamic method, where two reference surfaces were tested, 400 m and 680 m. The shallower was considered more realistic for the study area with relative mean transport of -5,1 ± 0,9 Sv (1 Sv =106 m3 s-1) for the OC12 data and -3,3 ± 2,1 Sv for the XBT data. Furthermore, the variability was investigated using OC12 simulation. This study found out (reinforced) that CB position and intensity vary in time producing cycles near to annual and semi-annual periods. When the CB is weak, it unveils a meandering pattern south of the CSVT and the water fluxes through the second (offshore) channel on the ridge are weak. When the current is strong, the fluxes are intense through both channels. Both scenarios seem to succeed each other and may be associated with the generation of eddies. When the current gets stronger, it becomes more unstable and then starts meandering as a form to lose energy, sometimes, propagating an eddy.

International audience; Submesoscale activity over the Argentinian shelf is investigated by means of high resolution primitive equation numerical solutions. These reveal energetic turbulent activity (visually similar to the one occasionally seen in satellite images) at scales O(5 km) in fall and winter that is linked to mixed layer baroclinic instability. The air-sea heat flux responsible for (i) deepening the upper ocean boundary layer (at these seasons) and (ii) maintaining a cross-shelf background density gradient is the key environmental parameter controlling submesoscale activity. Implications of submesoscale turbulence are investigated. Its mixing efficiency estimated by computing a diffusivity coefficient is above 30 m(2) s(-1) away from the shallowest regions. Aggregation of surface buoyant material by submesoscale currents occurs within hours and is presumably important to the ecosystem.

[1] The Community Climate System Model version 3 is used to analyse changes in water mass subduction rates in the South Atlantic Ocean over the 21st century. The model results are first compared to observations over 1950–2000, and shown to be rather good. The subduction rates do not change significantly over the 21st century, but the densities at which water masses form become significantly lighter. The strong westerly winds in this region do not change much, which suggests small changes to the rate at which the Atlantic sector of the Southern Ocean takes up heat and carbon dioxide over the 21st century.

Sedimentological parameters of 206 box-core tops and 387 van Veen grab samples were analysed so as to understand the distribution, source and sedimentation rates in the shelf and upper slope of the southern Southeastern Brazilian margin. Most of the parameters (bulk organic matter constituents, δ13C, and eNd) can be grouped on northern and southern sectors, separated by a sharp boundary. This suggests that sediment sources and sedimentary processes are different in both regions. The southern sector of the study area is more influenced by cold waters coming from the southern portion of the South American shelf and the organic fraction revealed an important role played by the primary productivity in the sedimentation. Also, eNd values indicate that part of the inorganic fraction of the southern sector sediments are allochtonous, probably coming from younger rocks of the Andean chain probably reaching the shelf through the La Plata river runoff. In the northern part of the study area the sedimentation is controlled almost exclusively by the meandering of Brazil Current. Compositional and isotope bulk organic parameters showed a more complex mixing of terrigenous and pelagic fractions. Also, eNd values are associated with the precambrian rocks of the Brazilian shield. The whole area exhibits sedimentation rates, varying from 5 to 660 mm kyr−1.

The delivery of freshwater to the North Atlantic during Heinrich Stadial 1 (HS1) is thought to have fundamentally altered the operation of Atlantic meridional overturning circulation (AMOC). Although benthic foraminiferal carbon isotope records from the mid-depth Atlantic show a pronounced excursion to lower values during HS1, whether these shifts correspond to changes in water mass proportions, advection, or shifts in the carbon cycle remains unclear. Here we present new deglacial records of authigenic neodymium isotopes – a water mass tracer that is independent of the carbon cycle – from two cores in the mid-depth South Atlantic. We find no change in neodymium isotopic composition, and thus water mass proportions, between the Last Glacial Maximum (LGM) and HS1, despite large decreases in carbon isotope values at the onset of HS1 in the same cores. We suggest that the excursions of carbon isotopes to lower values were likely caused by the accumulation of respired organic matter due to slow overturning circulation, rather than to increased southern-sourced water, as typically assumed. The finding that there was little change in water mass provenance in the mid-depth South Atlantic between the LGM and HS1, despite decreased overturning, suggests that the rate of production of mid-depth southern-sourced water mass decreased in concert with decreased production of northern-sourced intermediate water at the onset of HS1. Consequently, we propose that even drastic changes in the strength of AMOC need not cause a significant change in South Atlantic mid-depth water mass proportions.

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