Дисертації з теми "Western tropical Atlantic"

Processes occurring during the early life of marine fishes encompassing the larval, settlement, and juvenile stages can have important impacts on recruitment and subsequent population dynamics. Yet these life stages remain poorly understood, especially in coral reef-associated species of commercial and recreational fisheries interest. Two years (2003-2004) of monthly sampling of 17 stations along a transect spanning the east-west axis of the Straits of Florida revealed consistent spatiotemporal patterns in larval abundance, growth, and mortality of several snapper and barracuda species. Much of the species-specific variability in these patterns tracked adult life history, and spatial (several snapper species) and temporal (Sphyraena barracuda) patterns in larval growth were related to larval food availability. While no patterns were identified in larval mortality rates, tethering experiments examining relative rates of predation on late-stage Lutjanus griseus larvae in surface waters of the lower Florida Keys revealed that relative predation rate and probability of predation in oceanic areas seaward of the reef was significantly greater than over reef or nearshore seagrass/hardbottom habitats. The combined effects of mortality during these early stages in concert with variability in early life traits caused selective mortality to be pervasive throughout the early life stages of snappers and barracudas. Patterns in selective mortality were investigated by tracking and repeatedly sampling several cohorts of larvae in 2007 and 2008, and for the first time in tropical reef fishes, linking young pelagic larvae with settlement-stage fish and juveniles. In agreement with the growth-mortality hypothesis, large size-at-hatch and fast larval growth conveyed a survival advantage in most species examined, but several switches in the direction of selection with ontogeny and over time occurred, and were contrary to this hypothesis. Consistent patterns of trait-mediated selective mortality lower trait variability in the surviving population, while inconsistencies in these patterns may contribute to the high degree of variability that characterizes these early life stages. Results presented in this dissertation help fill knowledge gaps critical to the understanding and modeling of dispersal and connectivity in several economically valuable snapper and barracuda species. In addition, the identification of life history traits important to the survival of individuals through the larval and into the juvenile stage, has implications for future management of these ecologically and economically valuable species.

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016.
Cataloged from PDF version of thesis. Page 156/Figure B-1 contains text that runs off the edge of the page margin.
Includes bibliographical references.
Paleoclimate archives place the short instrumental record of climate variability in a longer temporal context and allow better understanding of the rate, nature and extent by which anthropogenic warming will impact natural and human systems. The ocean is a key component of the climate system and records of past ocean variability are thus essential for characterizing natural variability and quantifying climate sensitivity to radiative forcing. Coral skeletons are high-resolution archives of tropical sea surface temperatures (SSTs), but inconsistencies call the accuracy of existing coral proxy records into question. In this thesis, I first quantify the errors associated with the traditional coral thermometer, Sr/Ca, by comparing in situ logged SST with Sr/Ca-derived SST in four corals on the same reef. I show that intercolony disparities in mean Sr/Ca, amplitude of variability, and trend are not due to differences in water temperature, but rather to "vital effects" that result in a ±2° C uncertainty on reconstructed SST. I then expand, refine, and test a new paleothermometer, Sr-U, across multiple coral species and through time. I show that Sr-U captures spatial SST variability with an uncertainty of ±0.6° C. When applied to two corals outside of the calibration, Sr-U accurately captures the mean SST and the 20th century trend in the Western Tropical Atlantic. Finally, I apply Sr-U to a coral from the Little Ice Age (LIA) to address uncertainties in the magnitude of western tropical Atlantic cooling during a 95-year period spanning 1465-1560. Results suggest the region was 1.1° C±0.6°C cooler than the 1958-1988 mean, but within error of early 20th century SST at this site. Critically, several periods of warmth, equivalent to the 1958-1988 mean, occurred during a solar minimum that is widely believed to have been a cool period of the LIA. My results indicate that Sr/Ca exaggerates the actual cooling by almost 3° C. My record demonstrates the value of Sr-U and highlights the need for continuous accurate SST records to better constrain the amplitude, drivers, and mechanisms of LIA tropical climate change.
by Alice Elizabeth Alpert.
Ph. D.

Paleotempestology, the study of past tropical cyclones (TCs) using geological proxy techniques, is a growing discipline that utilizes data from a broad range of sources. Most paleotempestological studies have been conducted using “established proxies”, such as grain-size analysis, loss-on-ignition, and micropaleontological indicators. More recently researchers have been applying more advanced geochemical analyses, such as X-ray fluorescence (XRF) core scanning and stable isotopic geochemistry to generate new paleotempestological records. This is presented as a four article-type thesis that investigates how changing climate conditions have impacted the frequency and paths of tropical cyclones in the western North Atlantic basin on different spatial and temporal scales. The first article (Chapter 2; Oliva et al., 2017, Prog Phys Geog) provides an in-depth and up-to-date literature review of the current state of paleotempestological studies in the western North Atlantic basin. The assumptions, strengths and limitations of paleotempestological studies are discussed. Moreover, this article discusses innovative venues for paleotempestological research that will lead to a better understanding of TC dynamics under future climate change scenarios. The second article (Chapter 3; Oliva et al., submitted, The Holocene) presents the development of the first database summarizing the most up-to-date paleotempestological proxy data available for TC reconstructions for the western North Atlantic basin. Subsets of this new database are then used to reconstruct TC variability in the western North Atlantic basin. Using our new developed subsets, we investigate a key hypothesis, the Bermuda High Hypothesis that has been proposed to have influenced TC paths over centennial to millennial timescales. Results show an oscillation in the distribution of TC landfalls along the North American coast, suggesting a centennial oscillation in the mean summer position of the high pressure system. We suggest that a more serious, millennial scale shift in the Bermuda High to a northeastern (NE position) may have occurred at ~3000 and ~1000 cal yr BP. The third article (Chapter 4; Oliva et al., under review, Marine Geology) presents a local multi-proxy reconstruction of TC activity during the past 800 years from Robinson Lake, Chezzetcook Inlet in Nova Scotia, Canada. Here, we are testing the more recent use of the XRF scanning approach to paleotempestology at a local scale. Two sediment cores were extracted from Robinson Lake that were dated by 210Pb and 14C, analyzed for organic matter content, benthic foraminifera and thecamoebians, sediment grain size, and a range of elements and elemental ratios determined by XRF core scanning. Results show two periods of low TC activity based on multiple proxies including XRF technology: one from ~1150 to 1475 CE (800 – 475 cal yr BP) and the other from 1670 CE (280 cal yr BP) to the present, with the intervening period from ~1475 to 1670 CE (475 – 280 cal yr BP) as a time of more frequent and possibly higher magnitude TC activity. The fourth article (Chapter 5. Oliva et al., in preparation, Canadian Journal of Earth Sciences) explores the potential use of stable oxygen isotopes in tree ring α-cellulose to reconstruct past local TC activity surrounding areas of known TC strikes. Cores of 12 Picea mariana trees were extracted adjacent to Robinson Lake, Chezzetcook Inlet, Nova Scotia in order to test more contemporary and historically documented records of TC activity in this region as per Chapter 4. TCs precipitate 18O-depleted rain, leaving a unique signature in the source water that trees use to form cellulose. Using an autoregressive integrated moving average (ARIMA AR-1) model to detrend the data, local and regional time series were reconstructed. Local reconstructions led to most (> 95%) hurricanes and all major hurricane (± 1 year) being recorded in the isotope record, whereas the regional reconstruction shows no major hurricane, only a few hurricanes (< 40%) and one signal with a higher error (> 1 year). This thesis contributes to advancing our knowledge in paleotempestology of the western North Atlantic basin by: 1) bringing an up-to-date current status on paleotempestology, 2) the development and ongoing use of a new paleotempestology database for the western North Atlantic basin publicly available, 3) a local scale study using new XRF core elemental technology and 4) the exploratory use of tree-ring α-cellulose oxygen isotopic analysis based on contemporary and historical documents at local sites.

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O Atlântico tropical compreendido entre 20ºN e 20ºS apresenta-se hoje como chave para o entendimento das mudanças climáticas na Terra. Duas regiões despertam particular interesse: A banda equatorial do Atlântico onde o sistema de correntes interage com forçantes térmicos superficiais e ressurgência de Ekman, como a área sudoeste do Atlântico tropical (05°S-25°S / 20°W-47°W), onde parte da Corrente Sul Equatorial (CSE) penetra pela borda leste e contribui com muitas das correntes de fronteira oeste ao longo da plataforma continental brasileira. Entretanto, a variabilidade da dinâmica nestas regiões, que se mostra importante por sua contribuição sobre o clima da região nordeste do Brasil, apresenta-se pouco estudada. O presente trabalho investigou estes importantes sistemas do Atlântico tropical utilizando o ROMS (Regional Ocean Model System). A primeira área de estudo compreendida entre 20°S-20°N e 42°W 15°E, com resolução horizontal de 1/6º e 30 camadas sigma (que acompanham o terreno). Variações sazonais do transporte zonal, estrutura das correntes e distribuição da TSM (Março e Agosto) obtidos numericamente foram avaliados e comparados com dados: de literatura, experimentais do PIRATA e observados por satélite. Os resultados desta simulação mostram que o modelo é capaz de reproduzir os principais aspectos da Subcorrente Equatorial (SE), Contra-corrente Norte Equatorial (CNE), Corrente do Golfo (CG) e os ramos central e norte dos sistemas de Corrente Sul Equatorial (cCSE/nCSE), em diferentes seções ao longo do equador. A comparação entre a estrutura térmica nos primeiros 500 m simulada e do Programa PIRATA mostra uma Camada de Mistura (MLD) bem reproduzida, particularmente, a ressurgência que induz uma MLD mais rasa verificada nas boias mais à leste durante o inverno austral até o final da primavera austral. A evolução sazonal do sistema Piscina Quente do Atlântico Sul (SAWP) Língua Fria (Cold Tongue) foi bem representado, que é importante para futuras previsões de variabilidade climática sobre as fronteiras continentais da parte sudoeste do Atlântico tropical. Do lado sudoeste do Atlântico tropical (05°S-25°S / 20°W-47°W), O ROMS (Regional Ocean Model System) foi utilizado pela primeira vez nesta área para simular a circulação oceânica utilizando uma malha de resolução horizontal de 1/12º com 40 camadas sigma, que acompanham o terreno, para resolução vertical. Para avaliação preliminar da configuração do ROMS adotada foram analisadas as distribuições superficiais e verticais de temperatura, além de calculadas as evoluções sazonais da camada bem misturada e dos balanços, atmosféricos e oceânicos, envolvendo a troca de calor dentro da camada bem misturada. A ordem de grandeza das componentes oceânicas (principalmente a difusão vertical e a advecção horizontal) é da mesma ordem de grandeza dos forçantes atmosféricos e quase sempre opostos entre si, com alguma diferença de fase e transporte dentro das camadas mais superficiais. Resultados de variabilidade interanual foram comparados com os primeiros dois anos de perfis de temperatura observados advindos dos três fundeios do programa PIRATA-SWE (Projeto PIRATA, Extensão Sudoeste). A estrutura térmica simulada nas camadas mais superficiais do oceano está em concordância com os resultados obtidos in situ. Resultados de simulação apontam para uma larga e relativamente fraca CSE, composta por uma sequência de núcleos não bem definidos e próximos a superfície. O transporte que flui para oeste da CSE nos primeiros 400 m de profundidade ao longo da seção que atravessa as boias PIRATA-SWE, calculado para simulação do ROMS entre 2005-2007, apresenta um volume médio transportado de 14,9 Sv, com um máximo observado em JFM (15,7 Sv) e um mínimo durante MJJ (13.8 Sv). Os resultados de simulação indicam que em 2005-2007 o transporte para oeste da CSE foi modulado pela variabilidade da componente zonal do vento. Três seções zonais, posicionadas do continente até a posição da boia PIRATA, confirmam transporte mais intenso da Sub-corrente do Norte do Brasil (SNB), fluindo para norte, e uma diminuição no transporte da Corrente do Brasil (CB),que flui para sul, durante maio de 2006 e maio de 2007, quando a bifurcação do ramo sul da CSE alcança sua posição mais ao sul. Por outro lado, o máximo escoamento da CB foi registrado durante janeiro de 2006, janeiro de 2007 e março de 2007, com um mínimo da SNB fluindo para norte em dezembro de 2005 e outubro/dezembro 2006, correspondendo ao período em que a bifurcação do ramo sul da CSE alcança sua posição mais ao norte (OND). A Elevação da Superfície do Mar (ESM) e a Energia Cinética turbulenta (ECT) superficial calculada a partir das simulações e dos produtos AVISO Rio05 apontam na superfície para os mais altos níveis de energia de meso-escala ao longo do ramo central da CSE e da SNB/CB. Resultados de modelagem ecológica usando o modelo NPZD acoplado com o ROMS confirmam esta região como uma área oligotrófica. Resultados do modelo ecológico são comparados com SeaWifs dataset e a dinâmica e a produção primária são localmente discutidos. Estes resultados preliminares disponibilizam mais informações diante da complexidade da região de divergência da SCE e encoraja-nos a conduzir estudos mais detalhados a respeito da dinâmica e do transporte de massa nessa região utilizando o ROMS. Este trabalho também apresenta a necessidade de continuação, ampliação e extensão vertical para o sistema de observação PIRATA-SWE, especialmente com medidas de salinidade em mais níveis de profundidade, além da instalação de medidores de correntes

Large river plumes and frontal zones are important physical features influencing plankton distribution in the marine environment. In the western tropical North Atlantic Ocean (WTNA) the Amazon River plume may extend over an area reaching 1.5 x 106 km2. The freshwater plume creates a low-density lens in the surface 25m and supplies silicon and phosphorus to the WTNA. These physical and chemical gradients create an ideal environment for large-scale blooms of diatom diazotroph associations (DDAs), a symbiotic relationship between nitrogen-fixing cyanobacteria and chain-forming diatoms. While the physical and chemical properties of the plume with regard to influences on phytoplankton have been reported, zooplankton distributions and the fate of enhanced primary production in the plume are largely unknown. I investigated mesozooplankton (>200 μm) composition and grazing in the Amazon River plume-influenced WTNA in spring (May-June 2010) and fall (Sept.-Oct. 2011). Changes in zooplankton distribution and grazing occurred over the sea surface salinity (SSS) gradient from low salinity and mesohaline plume waters to high salinity oceanic waters. Distinct communities were identified in each season along the salinity gradient with several taxa primarily constrained in the surface plume waters (e.g., Lucifer faxoni). The plume appears to function as an “extended estuary”, with a number of taxa (e.g., decapods, euphausiids, and fish larvae) utilizing the plume as a nursery habitat or dispersal mechanism for larval stages. Mesozooplankton grazing was elevated in plume waters compared to oceanic waters and was 2-3 times higher in the fall vs. spring. These patterns suggest a lag in the peak mesozooplankton abundance and grazing in response the observed spring DDA bloom, at least in low salinity plume waters. Comparison of micro- and mesozooplankton grazing along the SSS gradient supported a transition from an “export” food web in waters with SSS < 33 where mesozooplankton grazing dominated and potential for export via fecal pellet production is higher, to a “retention” food web at SSS above 33 where microzooplankton grazing was highest and recycling of nutrients in surface waters is predicted. Using molecular techniques to investigate feeding on DDAs and other N- fixers, I found that copepods consumed DDAs (Hemialus-Richelia and Rhizosolenia- Richelia, diatom-diazotroph respectively) as well as the colonial cyanobacterium Trichodesmium. Investigation of mesozooplankton grazing more broadly on other cyanobacteria with 16S rRNA sequencing revealed consumption of Synechococcus, Prochlorococcus, and the unicellular diazotroph UCYN-A Candidatus Atelocyanobacterium thalassa. Together, these results have important implications for our understanding of biogeochemical cycling in the WTNA, and other regions with abundant DDAs (e.g., the Mekong and Congo River plumes).

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Este trabalho apresenta uma análise de dados hidrográficos coletados em quatro campanhas oceanográficas do programa REVIZEE/SCORE-N. Estas campanhas foram realizadas durante os períodos de Março-Maio de 1995, Outubro-Novembro de 1997, Maio-Junho de 1999, e Julho-Agosto 2001, na área da Plataforma Continental do Amazonas e região oceânica adjacente. Esta área corresponde a Zona Econômica Exclusiva do Norte do Brasil (ZEE-N). As informações obtidas in situ foram complementadas com os resultados do Experimento ATL6 Projeto CLIPPER, realizado com a versão 8.1 do modelo numérico francês de circulação oceânica geral OPA. As análises dos dados hidrográficos permitiram descrever a variação sazonal e espacial das quatro massas de água encontradas na região: Água Costeira (AC), Água Tropical (AT), Água Central do Atlântico Sul (ACAS) e Água Intermediária da Antártica (AIA). Através das medidas observacionais e dos resultados numéricos foi possível avaliar o deslocamento da massa de água amazônica, que atingiu mais de 300 km de distância perpendicular à costa durante o período de máxima descarga do rio (Abril-Maio), mas que foi deslocada para Noroeste durante a época de baixas vazões do rio (Outubro-Novembro). Os resultados indicaram também que a ACAS sofre uma expansão de cerca de 70 m (49oW) e de 220 m (50oW) durante o período de transição das descargas fluviais, projetando-se sobre a plataforma. As análises permitiram identificar a descarga fluvial de águas doces, e o afluxo de águas sub-superficiais de máxima salinidade provenientes do Atlântico Sul, transportadas para a região pelo sistema Corrente Sul Equatorial (CSE)/Corrente Norte do Brasil (CNB)/Sub-corrente Norte do Brasil (SCNB), como os dois principais processos que contribuem para a formação das Camadas de Barreira (CB) na área de estudo. Durante Março-Maio, período correspondendo às altas descargas do rio Amazonas, foram observadas espessuras de CB da ordem de 50 m, governadas pelo estabelecimento de uma forte picnoclina induzida pela mudança brusca do gradiente de salinidade no interior da camada isotérmica. Na investigação dos principais mecanismos físicooceanográficos de conexão entre a ZEE-N e o Oceano Atlântico tropical, verificou-se a ocorrência sistemática de intrusão sub-superficial de águas do Atlântico Sul via SCNB, que após retroflexão (43ºW), alimenta as subcorrentes equatoriais (SCE e SCNE). As análises das estações localizadas mais ao Norte da região estudada (49ºW), e os resultados das simulações do Experimento ATL 6 mostraram a presença de águas provindas do Atlântico Norte na área de estudo, identificando a Sub-corrente de Fronteira Oeste (SCFO) (alimentada a 50ºW pela recirculação da CNE), como principal responsável pelo transporte das águas do Atlântico Norte para a ZEE-N

A newly proposed tropical cyclogenesis sequence that describes the transition of a tropical wave's critical layer to a tropical cyclone is used to examine two formation cases in the western North Pacific basin. Typhoon Nuri (2008), formed from a precursor easterly wave during the Tropical Cyclone Structure 2008 field experiment, and Typhoon Man-yi (2007), formed within an equatorial Rossby wave as it interacted with a monsoon trough. In each case, i) the critical layer of the parent wave protects a proto-vortex from an external hostile environment and allows it to strengthen until it becomes a self-sustained entity and ii) the intersection between the wave trough and critical latitude, within the Kelvin cat's eye, is the preferred location for tropical cyclogenesis. Numerical simulations suggest that the so-called "bottom-up" pathway to tropical cyclogenesis is favored within Typhoon Man-yi's critical layer. Additionally, Tropical Rainfall Measurement Mission (TRMM) composite analyses of 55 developing easterly waves indicate that as genesis approaches, i) convection is favored in the Kelvin cat's eye circulation, ii) the convective contribution to total rain rate becomes dominant, iii) the radius of maximum convection decreases, and iv) a convective-type heating profile is present. These findings support the "bottom-up" development model within easterly wave critical layers.

The first two chapters of this study examine faunal composition and latitudinal patterns in diversity of fishes in warm-temperate and tropical estuarine waters of the western Atlantic from La Plata River (35&\sp\circ&S), Argentina, to Chesapeake Bay (37&\sp\circ&N), USA (33 faunal reports). Species distributions generally corresponded with previously recognized marine zoogeographic regions, and cluster analyses also showed that faunal similarity among estuarine sampling sites was related to the type of sampling gear used and to the type of habitat that was sampled in each estuary. The total number of fish species reported is positively related to the size of the area sampled and number of individuals collected, although latitude (or temperature related phenomena) played a critical role in determining richness and equitability among species. The results of this study show that interaction between disturbance frequency or magnitude (e.g. temperature and salinity variations) and the rate at which dominance is achieved can result in a predictable pattern of fish species diversity in estuaries. In the last chapter the structure of estuarine fish assemblages in Patos Lagoon (32&\sp\circ&S), Brazil, and York River (37&\sp\circ&N), USA is described using historical bottom trawl data, to examine similarities between geographically isolated fish assemblages. Within broad limits structural assemblage patterns were correlated with temperature changes, although the intensity of seasonal changes differed between them. The lowest winter temperatures in the York River were correlated with a pronounced seasonal species emigration from the estuary, a pattern which was not observed in the more thermally moderate Patos Lagoon. Patos Lagoon was more diverse in terms of equitability and species richness than the York River at any particular point in time, but the seasonal pattern of use of the estuary (i.e. nursery and feeding grounds) was similar between localities, and did not differ from other temperate or tropical estuaries. Comparing fish assemblages using a nondimensional diversity index (i.e, H&\sp\prime&) masks interesting differences in species richness and relative abundance, and measurements of species richness and equitability (rarefaction and relative species rank techniques) independent of sample size should be used in order to determine richness and equitability differences among systems.

La Terre est un système complexe aux multiples interactions et le lieu de vie d’innombrables espèces vivantes. Dès lors, de nombreux travaux ont permis depuis des décennies, d’améliorer notre compréhension du climat de la Terre. Toutefois de nombreuses zones d’ombre restent aujourd’hui inexplorées. La région du Nord-Est (NE) du Brésil et de l’Atlantique tropical de l’Ouest est à ce jour relativement peu étudiée. Pourtant, l’hydroclimat singulier du NE Brésil et le rôle de l’Atlantique tropical de l’Ouest dans le transport interhémisphérique de chaleur font de cette région une zone importante d’un point de vue climatique. Parmi les traceurs couramment employés pour reconstruire les paléoenvironnements, les biomarqueurs lipidiques sont de précieux supports d’information. De nombreuses propriétés environnementales telles que la température de l’océan, la composition du couvert de végétation ou les conditions hydroclimatiques peuvent être reconstruites à partir de l’abondance, de la distribution ou de la composition isotopique de ces molécules. Ce projet de thèse vise à reconstruire les variations hydroclimatiques passées dans la région de l’extrême NE du Brésil, en relation avec la circulation de surface de l’Atlantique tropical de l’Ouest par la caractérisation du matériel organique d’une carotte de sédiments marins prélevée sur la marge brésilienne qui couvre les derniers 305 000 ans. L’analyse de la matière organique (MO) aux échelles totale et moléculaire a, dans un premier temps, permis de caractériser les sources de MO et notamment la composition des végétations actuelles dans la région d’étude. Ensuite, les températures océaniques de surface et de subsurface, qui ont respectivement été reconstruites à partir de la distribution des alcénones à longues chaînes et des alkyl tétraéthers de glycérols isopréniques, mettent en lumière des réchauffements d’ampleur en subsurface lors des trois dernières déglaciations. L’action conjointe du ralentissement de la circulation océanique et de l’intensification de l’Agulhas Leakage pendant ces périodes est proposée pour expliquer le réchauffement de l’Atlantique tropical de l’Ouest. Enfin, les variations passées du couvert de végétation au NE Brésil, déduites des propriétés des n-alcanes à longues chaînes, sont apparues étroitement liées aux changements hydroclimatiques. La température de surface de l’Atlantique tropical et l’intensité des vents des alizés semblent jouer un rôle majeur dans le contrôle des variations passées de végétation et de l’hydroclimat du Nord-Est du Brésil. En résumé, ce travail a notamment permis d’une part de mieux comprendre le rôle de l’Atlantique tropical en tant que réservoir de chaleur pendant les déglaciations et d’autre part de mieux contraindre les influences contrôlant les variations hydroclimatiques passées au Nord-Est du Brésil
The Earth is a complex system with many interactions and is also home to countless species. Consequently, extensive research has enhanced our comprehension of the Earth's climate over the past few decades. Nonetheless, numerous areas remain unexplored. The North-East (NE) region of Brazil and the western tropical Atlantic have received limited attention so far. The distinctive hydroclimate of northeastern Brazil and the contribution of the tropical western Atlantic to interhemispheric heat transfer make these regions climatologically important. Lipid biomarkers are among the frequently utilized proxys for palaeoenvironmental reconstructions, and they provide valuable insights. Numerous environmental factors, including ocean temperature, vegetation composition, and hydroclimate conditions, can be deduced from the abundance, distribution, and isotopic composition of these compounds. The objective of this thesis project is to reconstruct previous hydroclimatic fluctuations in the extreme northeastern area of Brazil, associated with surface circulation in the western tropical Atlantic. This task will be achieved through the characterisation of organic material obtained from a marine sediment core sampled from the Brazilian margin spanning the last 305, 000 years. The characterization of organic matter (OM) at both total and molecular level was employed to identify the sources of OM and, specifically, the composition of the modern vegetation within the study area. Moreover, the reconstruction of surface and subsurface ocean temperatures respectively from the distribution of long-chain alkenones and isoprenoid glycerol alkyl tetraethers highlighted significant subsurface warmings over the last three deglaciations. The cause of the warming of the tropical western Atlantic during these periods is suggested to be the combined effect of reduced ocean circulation and increased intensity of the Agulhas Leakage. Finally, the study has found a close relationship between hydroclimatic changes and past variations in vegetation cover in northeastern Brazil, as inferred from long-chain n-alkanes properties. This relationship is largely influenced by the upper ocean temperature of the tropical Atlantic and the intensity of South-East trade winds. The present study allowed us to better understand the role of the tropical Atlantic as a heat reservoir during deglaciations and to better constrain the influences controlling past hydroclimatic variations in northeastern Brazil

碩士
國立臺灣大學
大氣科學研究所
101
In this study, we target on the interannual relationship between the Western North Pacific Subtropical High (WNPSH) and the Tropical Atlantic sea surface temperature (SST) in the boreal summer. We find that the interannual variance of the WNPSH has become significantly larger after the early 1980s. The interannual correlation between the WNPSH index and global SST is also characterized by a shift on the late 1970s and early 1980s. SST is one of the important factors that affect the WNPSH. The effects of the SST fluctuation in the Northern Indian Ocean (NIO), the Western Pacific (WP), Marine Continent (MC) and the Equatorial Center Pacific (ECP) have been widely examined in recent years. But the effect of the Tropical Atlantic (TA) was hardly explored. The result of correlation analysis with the TA SST shows a quadrupole structure of streamfunction over Pacific and Indian Ocean. Using SVD analysis, we find the variability modes with the circulation patterns associated with the quadrupole structure of streamfunction and the corresponding SST pattern with warm SST in TA and NIO and cold SST in WNP. We think this mode is similar to the air-ocean interaction mode which found by Wang et al. (2013) via Principal Component Analysis. Numerical experiments using the ICTP AGCM/slab ocean model (Kucharski et al., 2013) confirm that the warm TA SST can cause the descending flow by Walker Circulation over the WNP, and also influence the NIO-WNP coupling system in the boreal summer, therefore enhance WNPSH directly and indirectly.