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Fixed N (nitrate, nitrite, and ammonium) is a limiting nutrient for photosynthesis in the surface ocean. The rates and relative importance of N cycling processes, however, are temporally and spatially complex, which hamper their direct measurement and quantification. The South Atlantic subtropical front separates the Atlantic Ocean and the subantarctic, an area which can elucidate information about water masses both entering and leaving the basin. Through the GEOTRACES programme, an oceanographic section across 40°S in the South Atlantic is used to investigate biogeochemical cycling of nitrogen and carbon in this region. Hydrographic data, in combination with the isotopic composition of nitrate (NO3-), particulate organic carbon and particulate nitrogen (δ15NNO3, δ18ONO3, δ13CPOC, δ15NPN), is used to provide integrative measurements for temporally and spatially variable processes of the marine N-cycle and C-cycle. A thorough examination of the stable isotope cycling of particulate and dissolved N across the subtropical front is used to quantify the supply of fixed N to the mixed layer. The relative importance of nitrate from the subsurface, N2 fixation, terrestrial input and atmospheric deposition in supplying production is determined. Typically, 30-50% of the export flux in the subtropical water masses is sourced from N2 fixers and up to 75% within the Brazil Current. This finding suggests that diazotrophs may be abundant in the South West Atlantic providing a source of new N to this region. To assess the basin scale N-cycling processes, the deep water masses were analysed to reveal the origin and history of NO3-. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilisation by algae. This enrichment in δ15NNO3 is not present in the subtropical North Atlantic or the return flow of the North Atlantic Deep Water (NADW), which decreases from ~5.9‰ in the newly formed intermediate waters to ~4.8‰ in the NADW at 40°S. The modification of isotopic signatures through the subtropical Atlantic can be calculated as an incorporation of 26-36 Tg N yr-1 of newly fixed N from an isotopic source of -1‰ (N2 fixation). The extent of N addition is higher than estimated rates of N loss within the Atlantic and surpasses the amount of N deficit supplied to the basin. Fixed N inputs and losses through the global ocean are investigated by the assessment of remineralised nitrate added to the ocean interior. A lower δ15N is observed in Atlantic remineralised nitrate in comparison to the Pacific. The relative importance of N2 fixation and pelagic denitrification within each ocean basin is quantified and through this approach, N2 fixation rates are estimated at 92-116 Tg N yr-1 in the Pacific and 24-32 Tg N yr-1 in the Indian Ocean. Combining Atlantic N2 fixation of ~32 Tg N yr-1 with Indo-Pacific, global N2 fixation rates can be estimated at 142-184 Tg N yr-1. The high inputs in the Pacific suggest that excess P is the dominant control on the success of N2 fixers. However, estimates of new N addition to the Atlantic indicate other mechanisms such as the recycling efficiency of P and supply of Fe to the surface ocean increase N2 fixation rates above this threshold. The organic matter supplied to sediments is principally derived from phytoplankton across the subtropical front. High organic content is associated with the productive Brazil-Malvinas Confluence region where a diverse supply of nutrients sustains elevated biomass. The Rio Plata outflow is characterised with high δ15NNO3 and δ15NPN, suggesting denitrification processes occur in the estuary. A low δ13C source associated with high Al concentrations is identified on the western slope, indicating a supply of terrestrial derived C to the deep ocean. The fractionation of C uptake by phytoplankton is assessed in subtropical and subantarctic waters. In the subantarctic, CO2[aq] and growth rates determine the extent of C isotope fractionation. In this region, low species diversity and a small range in cell size enable the fractionation from CO2[aq] and growth rate to be expressed in phytoplankton. In subtropical water masses a larger range of cell size is the principal determinant of C fractionation. Increased surface area to volume is the main mechanism for increasing C uptake, arguing against the use of δ13CPOC as a palaeoproxy. The low δ13CPOC and δ15NPN observed in the subtropics (from C fractionation and N2 fixation) contrast the heavier signatures in the subantarctic. These observations are propagated to the sediments, wherein organic matter shifts are determined by changes in the subtropical front over time. The results of this study have greatly improved knowledge of N and C cycling within the South Atlantic, providing new insight into the cycling of these two important elements in the surface and deep ocean, on a regional and global scale.

The green porcelain crab, Petrolisthes armatus, has recently invaded oyster reefs of the South Atlantic Bight at mean densities of up to several thousand individuals m-². Despite the crab’s tremendous densities and wide-spread occurrence, its population dynamics, the reasons for its success, and its ecological impacts have remained unknown. We used field monitoring in two estuaries of coastal Georgia to assess spatial and temporal patterns of distribution, demographics, reproduction, and effects on native crabs. We used field and mesocosm experiments with constructed oyster reef communities of varying native species richness and adult porcelain crab additions to assess why the invader is successful and how it impacts native species and communities. We found P. armatus distributed throughout the estuaries, primarily in the lower regions and low intertidal. Sex ratios were 1:1 throughout the year. During warmer months mean densities ranged from 1,000-11,000 crabs m-², 20-90% of mature females were gravid, and numerous recruits were present. Despite decreases in density of 64->99% in the winter, populations rebounded in the spring. Maximum mean densities were 37 times the highest densities ever recorded and population fecundity exceeded that of the native range by an order of magnitude, but correlations did not show significant negative effects of P. armatus on native crabs. Field experiments suggested that invasion was successful due to tremendous recruitment overwhelming biotic resistance by native species richness or predation. The crab only needed structure to invade, but the presence of adult conspecifics significantly enhanced recruitment (i.e., intraspecific “invasional meltdown”). We documented several impacts on native biota, including the (1) suppression of oyster growth, benthic algal biomass, native crab recruitment, and native goby densities and the (2) enhancement of bivalve recruitment, macroalgal cover, and survivorship of oyster drills. We did not, though, see an effect on native taxonomic richness. The large direct and indirect effects of P. armatus on growth, survivorship, and recruitment of virtually all of the most common native species on oyster reefs in the short-term (4-12 weeks) and at relatively low experimental densities (750-1500 crabs m-²) imply considerable long-term consequences for a major hard-substrate habitat of the South Atlantic Bight.

A high-resolution astronomic tidal model has been developed that includes detailed inshore regions of the Atlantic Intracoastal Waterway and associated estuaries along the South Atlantic Bight. The unique nature of the model’s development ensures that the tidal hydrodynamic interaction between the shelf and estuaries is fully described. Harmonic analysis of the model output results in a database of tidal information that extends from a semi-circular arc (radius ~750 km) enclosing the South Atlantic Bight from the North Carolina coast to the Florida Keys, onto the continental shelf and into the full estuarine system. The need for tidal boundary conditions (elevation and velocity) for driving inland waterway models has motivated the development of a software application to extract results from the tidal database which is the basis of this thesis. In this tidal toolbox, the astronomic tidal constituents can be resynthesized for any open water point in the domain over any interval of time in the past, present, or future. The application extracts model results interpolated to a user’s exact geographical points of interest, desired time interval, and tidal constituents. Comparison plots of the model results versus historical data are published on the website at 89 tidal gauging stations. All of the aforementioned features work within a zoom-able geospatial interface for enhanced user interaction. In order to make tidal elevation and velocity data available, a web service serves the data to users over the internet. The tidal database of 497,847 nodes and 927,165 elements has been preprocessed and indexed to enable timely access from a typical modern web server. The iii preprocessing and web services required are detailed in this thesis, as well as the reproducibility of the Tidal Toolbox for new domains.

The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S / The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S

Esta tese visa estudar o impacto da temperatura superfí-cie do mar (TSM) do Atlântico Sul na circulação atmos-férica regional, com ênfase na América do Sul. Os efeitos do Atlântico Sul na atmosfera são examinados através de dois conjuntos de simulações numéricas (ensemble), com o modelo atmosférico do NCAR (CCM3), forçados com a TSM observacional sobre dois domínios: o oceano global e o Atlântico Sul Subtropical, entre 20ºS e 60ºS. Uma metodologia de tratamento do ensemble é aplicada com o intuito de diminuir a variabilidade interna da atmosfera e ressaltar a resposta devido à forçante local de TSM. A comparação entre os experimentos numéricos através das análises de EOF e SVD mostrou que o Atlântico Sul exerce influência sobre a intensidade e posição geográfica da Zona de Convergência do Atlântico Sul (ZCAS) durante os meses de verão austral, associado a uma circulação ciclônica junto à costa sudeste brasileira, que advecta ar relativamente quente e úmido para a banda de precipitação oceânica. Este trabalho também mostra a importância conjunta da TSM do Atlântico Sul e Pacífico equatorial na modulação da intensidade e posição da ITCZ durante os meses de outono e inverno. Os resultados confirmam ainda o El Niño Oscilação Sul como um fator de impacto remoto na intensidade de chuvas na região sul do Brasil durante o inverno. / This work aims to study the impact of South Atlantic sea surface temperature (SST) on regional atmosfheric circulation. In order to achieve this purpose we used the atmospheric global circulation model Community Climate Model (CCM 3) from National Center for Atmosfheric Research (NCAR). the effects of South Atlantic on atmosphere are examined through two sets of numerical simulations (ensemble) generated with different SST configurations. The integrations were forced by SST from observational data over two domains: global ocean and South Atlantic from 20ºS to 60ºS. Looking for the reduction of the internal variability of the atmosphere and for increase of the response from the local SST forcing, a forcing, a special treatment was applied to the ensembles. The statistical methodology used in this study includes simple analysis such as climatology, variance and correlation as well as Empirical Orthogonal Functions and Singular Value Decomposition. the comparasion between the numerical experiments showed that the South Atlantic influences the interannual variability of precipitation over South America, especially with respect to the intensity and geographic position od South Atlantic Convergenge Zone (SACZ) during the austral summer season. The anomalous SST of the Subtropical South Atlantic induces the heat exchange between ocean and atmosphere, which togeter with a cyclonic circulation near the southern Brazilian coast, advects relatively moist and warm air to the oceanic part of SACZ. The increase of convection over this band inhibits the solar radiation in the surface, preventing the continuous increase of SST (negative feedback). This work also shows the importance of SST from the South Atlantic and Equatorial Pacific in modulating the intensity and position of ITCZ during the autumn and winter monhths. This study also confirms the El Niño - Southern Oscillation (ENSO) as a remote impact factor controling the intensity of rain over southern Brazil mainly during winter.

Esta tese visa estudar o impacto da temperatura superfí-cie do mar (TSM) do Atlântico Sul na circulação atmos-férica regional, com ênfase na América do Sul. Os efeitos do Atlântico Sul na atmosfera são examinados através de dois conjuntos de simulações numéricas (ensemble), com o modelo atmosférico do NCAR (CCM3), forçados com a TSM observacional sobre dois domínios: o oceano global e o Atlântico Sul Subtropical, entre 20ºS e 60ºS. Uma metodologia de tratamento do ensemble é aplicada com o intuito de diminuir a variabilidade interna da atmosfera e ressaltar a resposta devido à forçante local de TSM. A comparação entre os experimentos numéricos através das análises de EOF e SVD mostrou que o Atlântico Sul exerce influência sobre a intensidade e posição geográfica da Zona de Convergência do Atlântico Sul (ZCAS) durante os meses de verão austral, associado a uma circulação ciclônica junto à costa sudeste brasileira, que advecta ar relativamente quente e úmido para a banda de precipitação oceânica. Este trabalho também mostra a importância conjunta da TSM do Atlântico Sul e Pacífico equatorial na modulação da intensidade e posição da ITCZ durante os meses de outono e inverno. Os resultados confirmam ainda o El Niño Oscilação Sul como um fator de impacto remoto na intensidade de chuvas na região sul do Brasil durante o inverno. / This work aims to study the impact of South Atlantic sea surface temperature (SST) on regional atmosfheric circulation. In order to achieve this purpose we used the atmospheric global circulation model Community Climate Model (CCM 3) from National Center for Atmosfheric Research (NCAR). the effects of South Atlantic on atmosphere are examined through two sets of numerical simulations (ensemble) generated with different SST configurations. The integrations were forced by SST from observational data over two domains: global ocean and South Atlantic from 20ºS to 60ºS. Looking for the reduction of the internal variability of the atmosphere and for increase of the response from the local SST forcing, a forcing, a special treatment was applied to the ensembles. The statistical methodology used in this study includes simple analysis such as climatology, variance and correlation as well as Empirical Orthogonal Functions and Singular Value Decomposition. the comparasion between the numerical experiments showed that the South Atlantic influences the interannual variability of precipitation over South America, especially with respect to the intensity and geographic position od South Atlantic Convergenge Zone (SACZ) during the austral summer season. The anomalous SST of the Subtropical South Atlantic induces the heat exchange between ocean and atmosphere, which togeter with a cyclonic circulation near the southern Brazilian coast, advects relatively moist and warm air to the oceanic part of SACZ. The increase of convection over this band inhibits the solar radiation in the surface, preventing the continuous increase of SST (negative feedback). This work also shows the importance of SST from the South Atlantic and Equatorial Pacific in modulating the intensity and position of ITCZ during the autumn and winter monhths. This study also confirms the El Niño - Southern Oscillation (ENSO) as a remote impact factor controling the intensity of rain over southern Brazil mainly during winter.

The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S / The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S

D.Litt. et Phil. / Please refer to full text to view abstract