Dissertations / Theses on the topic 'Antarctic Ocean Climate'

The Southern Ocean plays a major role in the global overturning circulation, providing an important route for the return flow of deep water subducted in the North Atlantic. The World Ocean Circulation Experiment (WOCE) provided an unprecedented picture of the state of the world’s oceans and set new standards for high quality in-situ hydrographic data. This study combines the existing WOCE data set with new hydrographic sections, and output from global and regional ocean models to examine the mean state of the Southern Ocean circulation and the balance of fluxes around the Antarctic Circumpolar current. A historical data set in the region of Drake Passage is examined to study the large-scale water mass variability between 1926-2005. The water mass properties of the Lower Circumpolar Deep Water is constant within error bounds throughout the data set. A warming and freshening signal in the surface waters from 1997-2005 to the north of the Sub-Antarctic Front along SR01b is also presented. The major part of this work is based around an inverse study of the Southern Ocean that combines the WOCE data-set with contemporary sections, and other forcing fields to examine the balance of fluxes throughout the Southern Ocean. The study examines the effect of different parameterisations of the dianeutral mixing in the Southern Ocean, in light of the differing views of localised deep turbulent mixing from observations, and an adiabatic ocean interior from residual mean studies, The freshwater balance in the model is presented and its implications on the water mass formation and transformation of the upper and lower cells of the overturning circulation is discussed in detail.

Ocean warming near the Antarctic ice shelves has critical implications for future ice sheet mass loss and global sea level rise. A global climate model with an eddying ocean is used to quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized 2xCO(2) experiment. The results indicate that relatively large warm anomalies occur both in the upper 100 m and at depths above the shelf floor, which are controlled by different mechanisms. The near-surface ocean warming is primarily a response to enhanced onshore advective heat transport across the shelf break. The deep shelf warming is initiated by onshore intrusions of relatively warm Circumpolar Deep Water (CDW), in density classes that access the shelf, as well as the reduction of the vertical mixing of heat. CO2-induced shelf freshening influences both warming mechanisms. The shelf freshening slows vertical mixing by limiting gravitational instabilities and the upward diffusion of heat associated with CDW, resulting in the buildup of heat at depth. Meanwhile, freshening near the shelf break enhances the lateral density gradient of the Antarctic Slope Front (ASF) and disconnect isopycnals between the shelf and CDW, making cross-ASF heat exchange more difficult. However, at several locations along the ASF, the cross-ASF heat transport is less inhibited and heat can move onshore. Once onshore, lateral and vertical heat advection work to disperse the heat anomalies across the shelf region. Understanding the inhomogeneous Antarctic shelf warming will lead to better projections of future ice sheet mass loss.

The western Antarctic Peninsula (WAP) is a hotspot of climatic and oceanographic change, with a 6°C rise in winter atmospheric temperatures and >1°C warming of the surface ocean since the 1950s. These trends are having a profound impact on the physical environment at the WAP, with widespread glacial retreat, a 40% decline in sea ice coverage and intensification of deep water upwelling. The main objective of this study is to assess the response of phytoplankton productivity to these changes, and implications for the marine carbon and nitrogen cycles in the WAP coastal zone. An extensive suite of biogeochemical and physical oceanographic data was collected over five austral summer growing seasons in northern Marguerite Bay between 2004 and 2010. Concentrations and isotopic compositions ( 15N, 13C, 14C) of dissolved nitrate, dissolved inorganic carbon species, particulate nitrogen, organic carbon and chlorophyll a are used in the context of a substantial ancillary dataset to investigate nutrient supply, phytoplankton productivity and nutrient uptake, export flux and the fate of organic material, and the factors underpinning pronounced seasonal and interannual variability. High-resolution biogeochemical time-series data for surface and underlying seawater, sea ice brine, sediment trap material and coretop sediments allow detailed examination of carbon and nitrogen cycle processes under contrasting oceanographic conditions and the interaction between these marine processes and air-sea exchange of climate-relevant CO2. This study shows that the WAP marine environment is currently a summertime sink for atmospheric CO2 in most years due to high productivity and biological carbon uptake sufficient to offset the CO2 supply from circumpolar deep waters, which act as a persistent source of heat, nutrients and CO2 across the shelf. For the first time, CO2 sink/source behaviour is parameterised in terms of nitrate utilisation, by exploiting the relationship between CO2 and nitrate concentrations, and deriving the nitrate depletion at which surface ocean CO2 is undersaturated relative to atmosphere and carbon sink behaviour is achieved. This could have vast utility in examining CO2 sink/source dynamics over greater spatial and temporal scales than by direct CO2 measurements, of which availability is more limited. This study documents abrupt changes in phytoplankton productivity, nitrate utilisation and biological CO2 uptake during a period of rapid sea ice decline. In fact, nitrate utilisation, particulate organic matter production and biological CO2 uptake all decrease by at least 50 % between a sea ice-influenced, high productivity season and one of low sea ice and low productivity. The key driver of interannual variability in production and export of organic material is found to be upper ocean stratification and its regulation of light availability to phytoplankton. Productivity, CO2 uptake and export are maximal when stratification is sufficient to provide a stable well-lit surface environment for phytoplankton growth, but with some degree of mixing to promote export of suspended organic matter. Strong stratification causes intense initial production, but retention of suspended organic particles in the surface ocean induces a self-shading effect, and overall productivity, CO2 uptake and export fluxes are low. When stratification is weak, mixing of phytoplankton over a larger depth range exposes cells to a wider range of light levels and reduces photosynthetic efficiency, thus total productivity and CO2 uptake. A conceptual model is developed here, which attempts to describe the mechanism by which sea ice dynamics exert the principal control on stratification and therefore productivity and CO2 uptake at the WAP, with potential application to other regions of the Antarctic continental shelf. Although meteoric waters (glacial melt and precipitation) are more prevalent in surface waters throughout the study, sea ice meltwater variability is driven by large and rapid spring/early summer pulses, which stabilise the upper ocean and initiate phytoplankton growth. The timing and magnitude of these sea ice melt pulses then exert the key control on stratification and seasonal productivity. In a low sea ice year of this study, the sea ice trigger mechanism was absent and productivity was low. This strongly suggests that ongoing sea ice decline at the WAP and greater frequency of such low sea ice years is likely to drive a dramatic reduction in productivity and export, which would substantially reduce the capacity of the summertime CO2 sink in this region. Ongoing warming and ecosystem change are thus likely to have severe impacts on net CO2 sink/source behaviour at the WAP over the annual cycle, and the role of the Southern Ocean in regulating atmospheric CO2 and global climate. Finally, factors influencing the stable isotopic signature of particulate organic carbon ( 13CPOC), a common paleo-proxy, are assessed. 13CPOC is greatly influenced by seasonal shifts in diatom assemblages and isotopically heavy sea ice material, so cannot be used as a robust proxy for ambient CO2 in the coastal Southern Ocean.

The location of fronts has a direct influence on both the physical and biological processes in the Southern Ocean. However, until recently fronts have been poorly resolved by available data and climate models. In this thesis we utilise a combination of high resolution satellite data, model output and ARGO data to improve our basic understanding of fronts. A method is derived whereby fronts are identified as local maxima in sea surface height gradients. In this way fronts are defined locally as jets, rather than continuous-circumpolar water mass boundaries. A new climatology of Southern Ocean fronts is presented. This climatology reveals a new interpretation of the Subtropical Front. The currents associated with the Subtropical Front correspond to the western boundary current extensions from each basin, and we name these the Dynamical Subtropical Front. Previous studies have instead suggested that the Subtropical Front is a continuous feature across the Southern Ocean associated with the super gyre boundary. A comprehensive assessment of the relationship between front locations and wind stress is conducted. Firstly, the response of fronts to a southward shift in the westerly winds is tested using output from a 100 year climate change simulation on a high resolution coupled model. It is shown that there was no change in the location of fronts within the Antarctic Circumpolar Current as a result of a 1.3° southward shift in the westerly winds. Secondly, it is shown that the climatological position of the Subtropical Front is 5-10° north of the zero wind stress curl line, despite many studies assuming that the location of the Subtropical Front is determined by the zero wind stress curl. Finally, we show that the nutrient supply at ocean fronts is primarily due to horizontal advection and not upwelling. Nutrients from coastal regions are entrained into western boundary currents and advected into the Southern Ocean along the Dynamical Subtropical Front.

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Sea level rise (SLR) threatens coastal communities, infrastructure, and ecosystems. Worldwide, stakeholders critically depend on SLR projections with the associated uncertainty for risk assessments, decision-making and coastal planning. Recent research suggests that the Antarctic ice sheet mass loss during the 21st century may contribute up to an additional one meter of global SLR by year 2100. While uncertainty still exists, this value would double the ‘likely’ (> 66% probability) range of global SLR (0.52-0.98 m) by the year 2100, as found by Chapter 13 on Sea Level Change in the Fifth Assessment Report by the Intergovernmental Panel on Climate Change. Here, we present three studies that assess mechanisms relevant to 21st century local, regional, and global SLR. Appendix A examines the effect of large-scale oceanic and atmospheric circulation variability on extreme sea levels along the East Coast of North America. Appendices B and C analyze ocean warming on the Antarctic shelf and its implications for future ice shelf basal melt and Antarctic Ice Sheet mass loss. These studies will contribute to more accurate projections of local, regional, and global SLR. In Appendix A, we analyze long-term tide gauge records from the North American eastern seaboard and find an extreme SLR event during 2009-2010. Within this two-year period, coastal sea levels spiked between Montauk, New York and Southern Canada by up to 128 mm. This two-year spike is unprecedented in the tide gauge record and found to be a 1-in-850 year event. We show that a 30% reduction in strength of the Atlantic meridional overturning circulation (AMOC) and a strong negative North Atlantic Oscillation (NAO) index caused the extreme SLR event. Climate models project that the AMOC will weaken and NAO variability will remain high over the 21st century. Consequently, extreme SLR events on the Northeast Coast could become more frequent during the 21st century in response to climate change and SLR. In Appendix B, we use a fine-resolution global climate model (GFDL CM2.6) that resolves an eddying ocean. With this state-of-the-art coupled model, we quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized experiment of doubling of the atmospheric CO2 concentration. The results show that the CO2 forcing leads to the shelf region warming both in the upper 100 m ocean and at depths near the sea floor. These warming patterns are controlled by different mechanisms. In the upper 100 m, the heat anomalies are primarily controlled by increased heat transport into the shelf region associated with the warmer near-surface waters from lower latitudes. Below 100 m, the heat anomalies develop due to increased onshore intrusions of relatively warm Circumpolar Deep Water and reduced vertical mixing of heat in the water column. A complete heat budget analysis is performed for the Antarctic shelf region as well as for six subdomains and three depth ranges (0-100 m, 100-700 m, and 700-1000 m). The results show that certain regions of the Antarctic shelf are more susceptible to large CO2-forced warming. These findings have implications for future Antarctic Ice Sheet mass loss and SLR, and can provide more detailed and accurate ocean boundary conditions for dynamical ice sheet models. In Appendix C, we use CM2.6 to examine the connections among ocean freshening and the magnitude and location of ocean warming on the Antarctic shelf. We find that CO2 forcing freshens the Antarctic shelf seas via increases in local precipitation, sea ice loss, liquid runoff, and iceberg calving. The freshening induces three heat budget-relevant responses: (1) reduced vertical mixing; (2) strengthening of the Antarctic Slope Front (ASF); and (3) increased eddy kinetic energy (EKE) near the ASF. First, heat can accumulate at depth (100-1000 m) as freshening increases the vertical stratification on the shelf and reduces upward mixing of heat associated with diffusion and convective processes. Second, freshening near the shelf break strengthens the ASF by increasing the lateral density gradient and by steepening and deepening the associated isopycnals. This response limits cross-ASF onshore heat transport at many locations around Antarctica. Third, EKE increases near the ASF may contribute to shelf warming by increasing cross-ASF onshore eddy heat transport. These results demonstrate the importance of shelf freshening to the development of positive heat anomalies on the Antarctic shelf. The findings provide new insight to the location of future shelf warming and ice shelf basal melting as well as provide significant information for projecting regional and global SLR.

Pteropods are holoplanktonic gastropod molluscs found globally. Although species diversity is greater at lower latitudes, species abundance is greater at temperate and polar latitudes. Declines in pteropod populations have not only been correlated to declines of their major predators, but pteropods have also been used as bioindicators of global environmental changes such as ocean acidification. With high latitude abundances, pteropods provide significant sustenance for species such as the Atlantic salmon in the Atlantic Ocean and Pleuragramma antarcticum in the Southern Ocean. Because pteropods eat phytoplankton and other pteropods, factors that affect pteropod abundance influence many trophic levels. This dissertation explores ecological, physiological and trophodynamic relationships of pteropods when considering the influences of environmental factors observed to be altering the western Antarctic Peninsula's marine ecosystem. Over the last few decades very few studies have reported the distributions of pteropods along the western Antarctic Peninsula, in particular south of the Gerlache Strait. The ecological study provided the first detailed report of the pteropods Spongiobranchaea australis and Clione antarctica along the western Antarctic Peninsula south of the Gerlache Strait, and their local distribution was correlated to the region's major water masses and mesoscale water mass circulation. The physiological study of S. australis and C. antarctica yielded the first account of their metabolism, ratios of oxygen consumed to nitrogen excreted, proximate body composition, primary substrates oxidized, and enzymatic activities along the study's latitudinal gradient; the first report of S. australis' physiology anywhere around Antarctica. The final chapter utilized a comprehensive Ecopath with Ecosim model of the western Antarctic Peninsula's marine ecosystem. The model was used to explore the trophodynamic significance of pteropods within their polar marine ecosystem as well as changes in whole ecosystem trophodynamics by employing various climate change scenarios expected to alter the Peninsula's marine ecosystem over the next 40 years. The sum of these studies provides a foundation for exploring pteropods as bioindicators of environmental change along the western Antarctic Peninsula, a region currently experiencing considerable climate anomalies.

Chapter 1: Antarctic fishes are extremely cold adapted. Despite their inability to upregulate heat shock proteins, recent studies have demonstrated a capacity for heat response in these animals. A cDNA microarray study looked at the Notothenioid fish Trematomus bernacchii and revealed heat sensitivities for hundreds of genes, two of which code for members of the CCAAT/Enhancer-binding protein (C/EBP) family of transcription factors. These molecular switches are best known for their roles in apoptosis, inflammation and cell cycle arrest. This dissertation further elucidates the role of C/EBP-delta in the Antarctic fishes T. bernacchii and Pagothenia borchgrevinki. Chapter 2: C/EBP-delta is constitutively expressed in unstressed, field-acclimated (ca. -1.86°C) animals in a highly tissue-specific manner. White muscle tissue contains the highest C/EBP-delta concentration, which is further increased in response to sublethal heat stress at 2.0 or 4.0°C. This response is mostly acute and transitory, but a lesser upregulation was observed in fishes held for one month at 4.0°C. Chapter 3: The heat-induced nuclear translocation of C/EBP-delta--as determined by immunohistochemistry--appears to be time, tissue and species specific with spleen, heart and retinae being particularly responsive in certain situations. Chapter 4: Protein concentrations of proliferating cell nuclear antigen are tissue specific and variably heat responsive. Surprisingly, levels appear to be positively correlated with C/EBP-delta. Chapter 5: Flow cytometry revealed increasingly high temperatures reduce the proportion of G1 cells while increasing the abundance of apoptotic cells. Chapter 6: These findings are discussed in the context of global climate change and the cellular stress response.

Depuis les années 90, l’Antarctique de l’Ouest, dont le secteur d’Amundsen, affiche une importante perte de masse provenant principalement de l’accélération des glaciers côtiers en réponse à une fonte océanique plus conséquente sous les plateformes de glace. Ces plateformes sont généralement confinées est agissent comme un verrou pour l’écoulement. En subissant davantage de fonte basale, les plateformes deviennent fragiles et les glaciers en amont s’accélèrent, contribuant ainsi à augmenter le niveau des mers. L’avenir de l’Antarctique de l’Ouest est particulièrement préoccupant car sa configuration rend la calotte sujette à une instabilité marine. Par ailleurs, ces plateformes pourraient s’affaiblir sous l’effet d’une augmentation de la fonte de surface dans un climat plus chaud (hydrofracturation), rendant là aussi une instabilité possible. L’arrivée de ces instabilités pourrait être freinée ou compensée par l’évolution du bilan de masse de surface qui se compose majoritairement de précipitations neigeuses, sporadiquement augmenté par la pluie, et légèrement amoindri par la sublimation et le runoff. Cette thèse porte sur la modélisation de l’ensemble des processus atmosphériques et océaniques pouvant faire évoluer la contribution de l’Antarctique de l’Ouest au niveau des mers.Pour cela une projection océanique représentant les cavités sous-glaciaires a d’abord été réalisée avec le modèle NEMO. La circulation induite par la fonte océanique modifie la réponse de l’océan côtier à un futur changement de circulation atmosphérique, si bien qu’utiliser des modèles de climat ne représentant pas les cavités donne une indication faussée du réchauffement de l’océan autour de la calotte. Nous avons également mis en évidence une rétroaction positive entre la fonte sous-glaciaire et le retrait de la ligne d’échouage, entraînant une augmentation de la fonte jusqu’à 2.5 fois. Ces résultats indiquent la nécessité de coupler des modèles de calotte et d’océan pour établir des projections futures, même si les projections envisagées dans cette thèse restent relativement idéalisées.Pour établir des projections de bilan de masse de surface, il est nécessaire d’utiliser un modèle atmosphérique avec une représentation fine des processus polaires, notamment ceux liés au manteau neigeux. Ainsi nous avons utilisé le modèle atmosphérique régional MAR pour établir des projections dans le secteur d’Amundsen. Nous avons d’abord montré que MAR est approprié pour représenter le climat de surface observé en Antarctique de l’Ouest. Nous avons trouvé qu’aucun des modes climatiques (ASL, SAM, ENSO) n’expliquent plus de 50% de la variance de la fonte et du SMB en été à l’échelle interannuelle, et il est donc difficile d’utiliser des projections des modes climatiques comme indication de l’évolution du climat de surface.Forcé par le signal multi-modèle CMIP5 dans le scénario rcp85, MAR prévoit une augmentation du bilan de masse de surface de 30-40% d’ici 2100. Cette augmentation est équivalente à une baisse de 0.33 mm/an de niveau des mers, ce qui compenserait l’effet de la dynamique si celle-ci restait à son niveau actuel (0.26 mm/an). Ces projections indiquent également 5 à 15 fois plus de fonte de surface sur les plateformes du secteur Amundsen, mais la quasi-totalité de la fonte produite chaque année continue à regeler dans la couche de neige annuelle, et ne devrait donc contribuer de manière importante ni au bilan de masse de surface ni à l’hydrofracturation.Il ressort de ces travaux qu’un couplage océan/calotte dans les modèles de climat est primordial pour simuler le futur de l’Antarctique et de l’océan Austral. Une représentation fine des processus liés à la fonte de surface et au regel dans le névé est également essentielle car la possibilité d’hydrofracturation des plateformes dans un climat plus chaud relève d’un équilibre subtil entre l’augmentation de l’accumulation, de la température, et les rétroactions liées à l’albédo et à l’humidité
West Antarctica, and particularly the Amundsen sector, has shown since the 1990s a large increase of mass loss related to coastal glacier acceleration in response to an increase of oceanic melt underneath ice shelves. Ice shelves play a buttressing role for ice-stream and increased oceanic melt therefore lead to ice shelves thinning and glacier acceleration, which contributes to sea level rise. West Antarctica is of particular concern because its configuration is prone to marine ice-sheet instability. It has been suggested that ice shelves weaken under large surface melt in a warmer climate (hydrofracturing), possibly leading to another kind of instability. Instabilities could be slowed down or compensated by future Surface Mass Balance (SMB) that consists mainly of snowfall, sporadic rainfall, and is slightly reduced by sublimation and runoff. The main objective of this PhD work is to model the atmospheric and oceanic processes that will most likely affect the future West Antarctic contribution to sea level rise.First, oceanic projections have been developed using the NEMO ocean model. The ocean circulation induced by ice-shelf basal melting affects the ocean response to future changes in surface winds. Therefore, models that do not represent ice-shelf cavities produce wrong warming patterns around Antarctica. A positive feedback between oceanic melting and grounding-line retreat has been identified and can increase melt rates by a factor of 2.5. These results are strong incentive to couple ocean and ice sheet models, although the projections proposed here are relatively idealized.To run SMB and surface melting projections, an atmospheric model with a fine representation of polar processes, including those related to the snowpack, is needed. MAR is found to be an appropriate tool to simulate the present-day surface climate in the Amundsen region. We find that none of the large climate modes of variability (ASL, SAM, ENSO) explains more than 50% of surface melt and SMB summer variance at the interannual timescale. The use of climate mode variability projections to estimate the future surface climate of West Antarctica is therefore not trivial.Forced by the CMIP5 multi-model mean under the RCP8.5 scenario, MAR predicts an increase of SMB by 30-40% for the end of the 21st century. This increase corresponds to 0.33 mm yr-1 of sea level drop down, which is higher than the current West Antarctic contribution of ~0.26 mm yr-1 from ice dynamics. Surface melt is also projected to increase by a factor of 5 to 15 over the Amundsen ice shelves, but most of it is projected to refreeze in the annual snow layer, so future melting should not have a strong contribution to SMB or hydrofracturing.To conclude we show that coupled ocean and ice sheet climate models are essential to simulate the future of Antarctica and Southern Ocean. A fine representation of surface melt and refreezing processes within the snowpack is also crucial as possible hydrofracturing is threatening in a warmer climate and it comes within a delicate equilibrium between snowfall, air temperature, and feedback related to albedo and humidity

Marine sedimentation from the Adélie Land continental margin of East Antarctica provides unique high resolution records of Holocene environmental change. The subannually resolved sediment cores MD03-2601 (66°03.07’S, 138°33.43’E) and IODP-318-U1357B (66°24.7990′S, 140°25.5705′E) from the Dumont d’Urville Trough,Adélie Land, document atmospheric and oceanic processes impacting on biogenic sedimentation on the Adélie Land continental shelf during the Holocene. Resin embedded, continuous polished thin sections from each core were analysed for diatom content and sediment microfabric using scanning electron microscope backscattered electron imagery. The sediments contained repeating sequences of seasonal diatom-rich laminae which enabled multi-taper method time series analysis. Time series analysis shows that in the Hypsithermal there appears to have been an external (solar) control on interannual sedimentation as well as internal controls (e.g. the southern annular mode, SAM, and El Nino-southern Oscillation, ENSO); whilst in the Neoglacial internal climatic modes exerted a much stronger control. Quasi-biennial (2 – 3 year) peaks commonly occurred in analysis of both Hypsitherml and Neoglacial sequences. The distribution of resting spore-rich laminae in these sections suggests that a multidecadal (>50-years) variation between phasing of the SAM and ENSO systems may exert an important control on interannual environmental variability in the sections analysed. The distribution of diatom-derived biomarker proxies, namely C25 highly branched isoprenoid (HBI) alkenes, was compared to the diatom lamina-based record in core MD03-2601. At the Holocene scale, HBI diene and triene molecules have a positive association to sea ice associated diatom-rich laminae, with greater abundances of both HBI molecules and sea ice associated diatom laminae in the Neoglacial interval. However, at a sub-annual resolution there is no strong association between lamina type and HBI concentrations. This is attributed to a combination of: (i) the HBI alkenes recording a different signal to that of the diatom-rich laminae; (ii) interannual variation in HBI export that is greater than inter-seasonal variation, for which there is little modern data for comparison; (iii) possible diagenetic alteration of the HBI signal.

Neste trabalho a variabilidade climatica do oceano Austral e a ocorrencia da Onda Circumpolar Antartica (OCA) sao investigadas. Foram usados os dados de uma simulaçao do modelo numerico acoplado do Nacional Centre for Atmospheric Research/ Community System Model - NCAR CCSM de 150 anos, e um conjunto de dados climatologicos como base de comparaçao dos dados do modelo. estes foram obtidos da Re-analise do NCEP/NCAR (National Center for Envirommental Prediction) para o periodo de janeiro de 1948 a julho de 2002. Com o intuito de analisar o comportamento sazonal e anual foram analisadas as climatologias e utilizada a tecnica de analise harmonica das variaveis de temperatura da superficie do mar (TSM), pressao ao nivel do mar (PNM), componentes meridional (Vy) e zonal (Vx) do vento. Para analisar o comportamento interanual dessas variaveis sao utilizados diagramas Hovmoeller, espectros de potencia, alem de tecnicas estatisticas como Empirical Ortogonal Functions (EOF) e singular Value Decomposition (SVD).
In this study the climatic variability of the Southern ocean and the Antarctic Circumpolar Wave (ACW) are investigated. The National Center for Atmospheric Research/ Community System Model _ NCAR CCSM coupled model 150 years simulation data is compered with the climatology data from the Nacional Center for Envirommental Prediction - NCEP/NCAR Re-analysis, for a period from january/1948 until july/2002. Annual and seasonal climatology and harmonic analysis are used for the following variables: sea surface temperature (SST), sea level pressure (SLP), meridional and zonal wind. Hovmoeller diagrams, potencial spectra and statistics methods such as Empirical Ortogonal Functions (EOF) and Singular Value Decomposition (SVD) are used to analyze changes in interannual behavior of this variables.

Abstract This thesis examines the Mid-Pliocene climatic extreme ca. three million years ago (Ma) which was the latest longtime warm period. It is an important topic because the climate back then was warmer compared with the present. The bipolar regions are studied because they represent the largest areas that control the global climate. This study is based on clay mineral research that may significantly improve our knowledge of the Mid-Pliocene climate when combined with other palaeoenvironmental data. The paleoclimatological objectives of this study were: 1) to investigate how clay minerals reflect the Mid-Pliocene Global Warmth event, 2) to study ice sheet development at high latitudes, especially in East-Antarctica, and the history of ice rafting and sea ice, especially in the Arctic Ocean. This thesis deals with the clay mineral distribution and compositional analysis of the Pliocene-aged marine sediment sequences provided by the Ocean Drilling Program (ODP). The first studied site, Site 1165, is located at the continental rise of Prydz Bay, East Antarctica, and the second studied site, Site 911, is located at the Yermak Plateau, north of Svalbard, in the Arctic Ocean. The Pliocene smectite clay minerals at Site 1165 were mainly derived from Antarctic continental sources and transported to the site primarly by bottom currents related to warm events during the last 5 Ma. The evidence obtained in this study shows that the East Antarctic ice sheet may have been a dynamic ice sheet during the past 5 Ma, especially during the Mid-Pliocene. The results from the Mid-Pliocene possibly suggest a general warming trend. Based on the composition of the heavy minerals and clay minerals, at Site 911, the Pliocene smectite clay minerals were mainly transported within sea ice by the Siberian branch of the Transpolar Drift. The results indicate a warming trend at approximately 3 Ma after which they indicate a shift back to glacial conditions. Based on this study, the Mid-Pliocene Global Warmth can be observed in both the Arctic and Antarctic regions.

La premiere partie comporte l'etude des signatures climatiques des isotopes stables sur des forages du dome c (antarctique). L'echantillonnage est aleatoire. On elimine le bruit par interpolation lineaire et filtres passe-bas. On etudie la methode de la fenetre et les splines d'ajustement d'ordre 2. La seconde partie est l'etude du microrelief glaciaire a l'echelle decimetrique

Ce travail traite de la simulation numérique du climat des grandes calottes de glace, en particulier des calottes de l'Antarctique et du Groenland, toujours existantes, dans des conditions climatiques différentes, à l'aide de modèles de circulation générale de l'atmosphère (MCGA). Le MCGA à grille variable LMDz a été adapté aux spécificités du climat polaire et validé pour le climat actuel. L'approche d'une grille variable, qui permet d'utiliser le MCGA à haute résolution spatiale (autour de 100 km) sur la région d'intérêt à un coût numérique raisonnable, a été validée en analysant la dynamique atmosphérique au bord de la région ciblée à l'aide d'un schéma de suivi des cyclones individuels. Des simulations du climat du Dernier Maximum Glaciaire (DMG) ont été faites pour le Groenland et l'Antarctique et analysées en tenant compte des archives glaciaires disponibles. Une explication possible des différences entre les deux méthodes principales de reconstruction des paléotempératures - l'analyse des isotopes de l'eau et la mesure directe de la température de la glace dans le trou de forage - au centre du Groenland a pu être proposée. Cette explication est basée sur des changements de paramètres climatiques locaux. C'est la première fois que l'approche de grille variable a été utilisée dans un MCGA pour des simulations du climat polaire à l'échelle de quelques années. Les simulations paléoclimatiques faites avec LMDz sont à une résolution spatiale inégalée à ce jour. Finalement, le climat du DMG, simulé par plusieurs MCGA dans le cadre du projet international PMIP (Paleoclimate Modelling Intercomparison Programme), a été analysé, et des implications des résultats pour l'interprétation des enregistrements glaciaires ont été discutées.

Un modèle d'écoulement de la glace est développé en tenant compte du couplage vitesse-température. Ce modèle permet l'étude des réactions d'une calotte polaire aux variations du climat et est appliqués à la ligne de courant "Ridge B - Vostok - Glacier Byrd". (Antarctique de l'Est). Les processus à prendre en compte, les équations de base ainsi qu'une étude bibliographique sont présentés au chapitre (1). Au chapitre (II) dans le but d'interpréter les forages glaciaires, nous développons un outil dans lequel les vitesses d'écoulement sont déduites de la géométrie de la nappe de glace et de l'alimentation par précipitation. A partir du champ de vitesse ainsi calculé, le champ de température est obtenu en résolvant l'équation de la chaleur dépendant du temps avec des conditions aux limites qui tiennent compte de la fusion éventuelle à la base de la calotte de glace ainsi que de la température dans le socle rocheux. Cet outil est utilisé (chapitre III) pour calculer l'âge de la glace dans le carottage de Vostok. Il apparait que la principale incertitude sur la chronologie vient de la mauvaise connaissance de l'accumulation de glace en amont du forage. Le modèle de température permet également d'interpréter le profil de température mesuré à Vostok et d'en tirer des informations sur le flux géothermique et sur l'accumulation. Le modèle thermo-mécanique (chapitre IV) simule l'évolution de l'épaisseur de glace au cours du temps. Les vitesses d'écoulement sont intégrées numériquement et couplées aux température à chaque pas de temps. Un modèle d'ice-shelf est également développé et les déplacements de la ligne d'échouage sont calculés en fonction de la dynamique de la calotte glaciaire et de celle de l'ice-shelf. Des études de sensibilité sont effectuées pour les divers paramètres du modèle. Les variations d'altitude à Vostok pour le dernier cycle climatique sont obtenues et ne dépassent pas 100 m.

Le changement climatique et les bouleversements qu’il implique en Arctique ont véritablement renouvelé l’intérêt pour cet espace. Celui-ci soulève des enjeux de différentes échelles (aussi bien régionale que globale), mais aussi de différentes natures (économiques, politiques, sociales, environnementales), qui constituent autant de problématiques juridiques et interrogent la pertinence du droit applicable. Cependant, et c’est là l’enjeu juridique principal de ce sujet : le cadre juridique existant fait preuve d’une grande complexité, dont la cohérence et la pertinence vis-à-vis de la région ont été largement remises en cause. Mais à ce mal unique – la complexité – les auteurs ne semblent pas s’accorder sur les solutions à adopter. Ces divergences soulignent l’intérêt de déterminer si le cadre juridique applicable à l’océan Arctique peut être consi-déré comme « adéquat » dans le sens où il permettrait une gestion répondant aux critères d’un système juridique satisfaisant. Du point de vue de la méthode, l’étude demandait de mettre de l’ordre dans le désordre apparent. A cette fin, la systématisation, à travers la réalisation d’un tableau synoptique analysant l’ensemble des normes de droit international applicables à la région, a été nécessaire. En conclusion, en dépit de sa diversité, le cadre juridique peut néanmoins être considéré comme satisfai-sant du point de vue substantiel (complétude) et formel (cohérence). Au-delà de la simple cohérence, la construc-tion actuelle d’un droit de l’Arctique amène à identifier un processus d’ordonnancement au niveau régional, appa-rentant de plus en plus cet encadrement à un véritable « système juridique ». Cette régionalisation est juridiquement indispensable, mais il faut néanmoins admettre qu’elle ne permet pas d’assurer l’action mondiale qui demeure indispensable face au problème global du changement climatique. Si elle constitue une étape probablement néces-saire, elle représente surtout une étape supplémentaire à cette mise en oeuvre généralisée et renforce dès lors la fragmentation du droit international, et donc sa complexité
Climate change and the disruption it implies in the Arctic have really renewed the interest in this space. This raises issues of different scales (both regional and global), but also of different natures (economic, political, social, environmental), which constitute as many legal issues and question the relevance of the applicable law. However, and this is the main legal issue here: the existing legal framework is extremely complex, whose consistency and relevance concerning the region have been widely questioned. But to this unique problem - com-plexity - the authors do not seem to agree on the solutions to adopt. These differences underline the interest to determine whether the legal framework for the Arctic Ocean can be considered as "adequate" in the sense that it would enable a management that meets the criteria of a satisfying legal system. Regarding the method, the study demanded to put the apparent disorder in order. To this end, systematization was necessary; it was done through the creation of a synoptic table analyzing all the standards of international law applicable to the region,. In conclusion, despite its diversity, the legal framework can nevertheless be considered satisfactory from a substantive (completeness) and formal (coherence) point of view. Beyond simple coherence, the current cons-truction of an Arctic law leads to the identification of an scheduling process at the regional level, this framework resembling more and more a real "legal system". This regionalisation is legally essential. Nevertheless, it must be recognized that it does not ensure the worldwide action which remains essential in the face of the global problem that is climate change. If it constitutes a probably necessary step, it represents above all an additional step in this generalized implementation and therefore reinforces the fragmentation of international law, and its complexity

L'objectif de ce travail est de contribuer à la compréhension du cycle atmosphérique du soufre en Antarctique par l'utilisation d'un Modèle de Circulation Générale Atmosphérique (MCGA). Les versions "Antarctique" et "soufre" du MCGA LMD-ZT ("Laboratoire de Météorologie Dynamique- Zoom Traceurs") ont été fusionnées pour l'étude à haute résolution Antarctique du cycle du soufre dans les moyennes et hautes latitudes Sud. Une méthode de forçage "latéral" de la circulation atmosphérique antarctique par des analyses du Centre Européen de Prévision Météorologique à Moyen Terme (CEPMMT) a été spécifiquement développée et appliquée. Le modèle a d'abord été évalué. Il représente correctement le cycle saisonnier des espéces soufrées aux sites d'observation en Antarctique. Plusieurs défauts ont cependant été identifiés , discutés, et certains ont été étudiés par des expériences numériques spécifiques. Ceci nous a permis, dans la suite de ce travail, de décrire de manière critique le cycle du soufre en Antarctique à partir des résultats du modèle. Le modèle a été utilisé pour trois applications. La première a été une mise en oeuvre directe du modèle, tel qu'il a été évalué, dans le but d'estimer les distributions spatiales, le cycle saisonnier dans les régions centrales, et le bilan annuel des espèces soufrées en Antarctique. La deuxième se présente sous forme d'études de sensibilité à la formulation des émissions de sulfure de diméthyle (DMS) océanique, que la phase d'évaluation a révélé comme déterminante pour la modélisation du cycle du soufre. Pour la troisième, l'adjoint du modèle de transport (qui, en première approximation, permet de remonter le temps), complété par l'adjoint du module chimique spécifiquement développé pour ce travail, a été mis en oeuvre pour une recherche quantitative de l'origine géographlque et de l'âge des espèces soufrées en Antarctique.

The implications of ocean freshening from accelerating Antarctic land-ice loss are poorly understood, due to the scarcity of observations near the Antarctic coast, and the high spatial and temporal resolution required to resolve Antarctic continental shelf processes in ocean models. Here, a high-resolution global ocean--sea-ice model is used to investigate the response of Antarctic continental shelf circulation to increasing meltwater. Two freshwater perturbation experiments are conducted, using projected Antarctic ice-loss rates under RCP 4.5 and RCP 8.5 emissions scenarios at 2100.We find that surface freshening near the Antarctic coast increases stratification and reduces the formation of cold, dense waters on the Antarctic continental shelf that, in the current climate, drive abyssal ocean circulation and ventilation. In our simulations, the connection between the abyssal ocean and the cold Antarctic shelf collapses within 10 years following the application of projected 2100 meltwater forcing, as downwelling surface waters on the continental shelf are freshened by glacial runoff, leaving them too buoyant to sink to the abyssal ocean. Around Antarctica, coastal freshening increases lateral density gradients between the cool, fresh shelf and the warm, saline open ocean, strengthening frontal structures that separate the adjacent water-masses, and accelerating geostrophic currents that flow westward along the coast and along the continental shelf break. This process acts to homogenise shelf waters and increasingly isolate the cool continental shelf from the warmer open ocean, leading to a net cooling on the continental shelf. Acceleration of the circumpolar coastal current results in remote temperature feedbacks unique to these experiments; most notable being a strong cooling signal on the West Antarctic shelf, an historically warm region associated with high rates of ice shelf melt, generated by the advection of cold Weddell Sea shelf waters around the Antarctic Peninsula by the strengthening coastal current. However, shelf cooling is not a circumpolar response to freshening. Deep warm anomalies arise in the Ross Sea, Adelie Coast, and Prydz Bay continental shelf dense water source regions under enhanced meltwater forcing, as full depth convention collapses and surface cooling fluxes diminish. Warming is strongest in the Ross Sea, where the absence of strong frontal structures and zonal currents at the shelf break grants warm, open ocean waters uninhibited access to the shelf. On the Prydz Bay and Adelie Coast shelf, warming due to convective shutdown is counterbalanced by decreased shoreward advective heat transport, due to strengthening frontal structures at the shelf break. As such, we find that coastal freshening induces both warm and cool anomalies at different locations along the Antarctic continental shelf, suggesting that shelf freshening by meltwater can both accelerate ice shelf melt (a positive feedback) and inhibit melt (a negative feedback) depending on regional factors. These findings improve our understanding of the complex suite of climate effects associated with the melting of Antarctica's land-ice. Both the disruption of abyssal overturning and shifts in heat transport to ice shelves have implications for sea-level rise; as reduced deep ocean ventilation may increase oceanic heat content (effecting steric sea-level), and accelerating land-ice loss, in regions of warming, adds mass to the oceans (effecting eustatic sea-level).

The Southern Ocean circulation plays a central role in the dynamics of past and future global climate change. However, due to a scarcity of observations and the difficulty of accurately modelling the Southern Ocean, we lack a comprehensive understanding of how the circulation responds to change. The energetic eddy field directly impacts the response of the circulation, but the small scale of the eddies has generally been below the resolution of numerical ocean models. This thesis makes use of two high resolution idealised ocean models to investigate the role of eddies in modifying the response of the Southern Ocean circulation to changing wind stress and surface buoyancy forcing. The simulations demonstrate that the eddy field is significant in the dynamical response, but that the impact on different aspects of the circulation is complex and subtle. The Antarctic Circumpolar Current (ACC) transport increases only weakly in response to enhanced Southern Ocean wind stress, due to the enhanced eddy field, which efficiently eliminates momentum input. In contrast, the eddy field only partially compensates wind- driven increases in the upper overturning circulation, resulting in a moderate overturning increase in response to enhanced westerly winds. Southern Ocean heat uptake is also shown to be dependent on changes in the eddy field. Mid-depth warming occurs primarily due to a decrease in the upward eddy heat flux, associated with reduced isopycnal temperature gradients in a warmer climate. However, increased wind stress may reduce the mid-depth heat uptake; a transient cooling trend arises from the wind-driven enhancement of the vertical eddy heat flux. The uncoupled idealised models also permit an in-depth analysis of the response of the Southern Ocean overturning to changes in surface buoyancy forcing. A suite of buoyancy forcing perturbations show that mid-latitude heat and freshwater fluxes may be as significant as wind stress in altering the strength of the upper overturning circulation. Analysis of the transient model response to a range of surface buoyancy forcing perturbations also indicates that recent observations are consistent with a slowdown of the lower overturning cell. In summary, this thesis has refined our understanding of how the large scale Southern Ocean circulation responds to atmospheric change and the role of the eddy field in modifying that response. This work provides a basis to interpret the more complicated response of coupled and coarse resolution models.

Thesis (Ph. D.)--Florida State University, 2003.
Advisor: Dr. Doron Nof, Florida State University, College of Arts and Sciences, Dept. of Oceanography. Title and description from dissertation home page (viewed Oct. 6, 2003). Includes bibliographical references.

Early 1990s to late 2000s freshening (ΔS ≈ -0.001–0.002) and warming (Δθ ≈ 0.02°C–0.035°C) of bottom waters was detected in the southern Pacific Ocean, and Ross Sea source waters progressively freshened during the past four decades. This study investigates potential freshwater anomaly sources and quantifies their effect. Glacial melt water inputs to the GCT increased by 1.3 km^3 per decade (1976– 2007), more rapidly so after 2000 (6.8 km^3 per decade), freshening local Shelf Water by 0.0004 per decade. Lighter basal melt inputs to the LAT started in 1994 and also picked up after 2000 to 14.9 km^3 per decade, lowering the local Antarctic Surface Water salinity by -0.017 per decade. Upstream in the Amundsen Sea surface water freshened by -0.03 per decade (1994–2007) mostly (50%) from larger melt water inputs from the Pine Island (17.7 km^3 per decade) and Dotson (14.8 km^3 per decade) glaciers. Two decades of steady (1978-2000) strengthening of sea ice productivity (200 km^3 per decade) within the Ross Sea Polynya suddenly reversed to weakening (-98.6 km^3 per decade) and resulted in Shelf Water freshening (-0.02 per decade) thereafter. To fully account for the observed variability in Ross Sea waters, the progressive (1992- 2011) adjustment of the density field and induced advective contributions are estimated based on a simplified three-layer stratification. Eastern (western) inflow (outflow) of light surface (dense shelf) water increased by 28% (15%) to 1.11 Sv (1.01 Sv) by 2011; whereas a sluggish intermediate inflow (0.02 Sv) of Modified Circumpolar Deep Water turned into outflow after 2007, thus contributing 0.09 Sv by 2011 to the ventilation of deep waters farther offshore. The estimated evolution of overturning and advective salt fluxes in the Ross Sea yield overall freshening of water masses similar to those derived from observations. Volumetric mean salinities declined at -0.07 per decade for Antarctic Surface Water, -0.05 per decade for Modified Circumpolar Water, and -0.03 per decade for Shelf Water. Outflow intensification of Shelf Water mixtures is also consistent with bottom water property changes (freshening and warming) measured farther downstream in the southern Pacific Ocean.

Dissertação de Mestrado em Química apresentada à Faculdade de Ciências e Tecnologia
The Antarctica and Southern Ocean ecosystems have been suffering changes through the last 30 years, in particular raising of the ocean temperature. These changes occur in a non-circumpolar distribution, for which there are more affected zones than others and, consequently more species in particular regions. It is necessary to understand how these species deal with the environmental changes that are influencing its habitat and how these changes will alter the global marine ecosystems. As Antarctica is the habitat of numerous endemic species, such studies are urgently needed. Due to climate impacts, it is possible to observe evidence of change in the Antarctic food web, where Antarctic krill (Euphausia superba) has decreased its important role in some regions, allowing other zooplankton species to have that role. It is of extreme importance to study the lower trophic levels of this food web, so it is possible to verify the effects of climate change in the species that live in these waters that may help higher trophic levels. Fatty acid analysis allows to assess the response of species to environmental stressors and identify potential food sources, in a trophic ecology context. The zooplankton species of Euphausia superba, Euphausia triacantha, Themisto gaudichaudii, Thysanoessa spp were collected between December 2016 and January 2017, in three locations with distinct characteristics: Antarctic waters, Intermediate and Sub-Antarctic waters. After GC-MS analysis, it was possible to verify that the species E. triacantha, T. gaudichaudii and Thysanoessa spp. presented a better body condition than E. superba. Besides, it was also possible to observe that these three species all revealed better conditions in waters with a higher temperature (Sub-Antarctic waters) than in waters with lower temperatures, closer to the ones found in the Southern Ocean. However, E. superba exhibited a different fatty acid profile (characterized by the absence of highly unsaturated fatty acids in its constitution and dominance of saturated fatty acids) from what is reported in literature and to what it was found for the remaining species studied in this thesis. Further investigation is needed in order to understand how E. superba deals with climate changes for a better understanding of these results.
Os ecossistemas presentes na Antártida e no Oceano Antártico têm vindo a sofrer alterações ao longo dos últimos 30 anos, especialmente com o aumento de temperatura dos oceanos. Estas alterações não ocorrem de um forma homogénea, pelo que existem zonas mais afetadas do que outras, havendo, também por isso, espécies que são mais afetadas por estas alterações. Torna-se assim necessário perceber como as espécies lidam com estas alterações e como estas mudanças poderão alterar os sistemas marinhos globais. A Antártida é o habitat de numerosas espécies endémicas, tornando-se importante o estudo da sua cadeia trófica num contexto de alterações climáticas. Devido aos impactos ambientais, existe já evidência de uma alteração desta cadeia trófica, com o camarão da Antártida (Antarctic krill Euphausia superba) a diminuir o seu papel preponderante em algumas regiões, passando a outras espécies de zooplâncton esse papel. Assim, é de extrema importância o estudo na base desta cadeia trófica, de modo a analisar os possíveis efeitos de mudanças ambientais nas espécies que habitam na Antártida e com um papel chave para os níveis tróficos mais elevados. A análise do perfil de ácidos gordos permite verificar se as espécies estão em stress devido a mudanças ambientais, num contexto de ecologia trófica. No presente estudo, as espécies de zooplâncton Euphausia superba, Euphausia triacantha e Themisto gaudichaudii e o género Thysanoessa spp. foram recolhidas entre dezembro de 2016 e janeiro de 2017, em 3 locais com características diferentes: águas Antárticas, Intermédias e Sub-Antárticas. Após análise por GC-MS, foi possível verificar que as espécies E. triacanta e T. gaudichaudii e o género Thysanoessa spp. apresentavam uma melhor condição corporal do que E. superba. Além disso, também foi possível verificar que todas estas espécies se encontravam em melhores condições (maior abundância de ácidos gordos na sua composição e maior abundância de ácidos gordos essenciais) em águas com temperaturas mais elevadas (águas sub-Antárticas) do que em águas com temperaturas mais baixas, próximas das encontradas no Oceano Antártico. No entanto, E. superba mostrou um perfil de ácidos gordos distinto dos apresentados pelas outras espécies estudadas nesta tese (independentemente da sua localização) e do que é reportado na literatura (ausência de ácidos gordos altamente insaturados presentes na sua constituição e domínio de ácidos gordos saturados). Mais estudos para compreender as alterações que esta espécie vive nesta região são necessários para um melhor entendimento destes resultados.

The Antarctic Circumpolar Current (ACC) is well known for its multiple bands with large meridional property gradients in the upper waters, each associated with a deep-reaching current core. A revised nineteen-year time series (1992?2011) of altimeter data from the CNES/CLS AVISO is analyzed to identify and trace the spatial distribution of ACC fronts. Specific contours of sea surface height (SSH) are selected within narrow continuous bands of relative maxima SSH slope in the Southwest Atlantic Ocean sector, where they closely follow the distribution of ACC fronts derived from inspection of concurrent high-resolution profile data at hydrographic stations. When applied to the full circumpolar belt, the frontal distribution derived from these new altimeter-based indicators also agrees well with the traces of current jets and in-situ dynamic height fields calculated from concurrent Argo profile data. The temporal variability of ACC fronts is analyzed in relation to dominant modes of atmospheric forcing variability in the Southern Ocean. All three ACC fronts have experienced large seasonal to decadal variability throughout the satellite altimetry era. The general seasonal tendency for each of these jets, with respect to long-term mean positions, is to be located farther to the south during the austral summer and to north in the winter. Circumpolar-mean annual frontal locations show a consistent linear trend of southward migration. However, the estimated decadal variability of the frontal distributions is highly localized, and due to selective response mechanisms to atmospheric variability. A persistent poleward drift of ACC fronts is observed in the Indian sector consistent with increasing sea surface temperature trends. In contrast, a vacillation in the meridional location of ACC fronts is observed in the Pacific sector in association to minor sea surface cooling trends. Therefore, unlike in the Indian sector, the regional Pacific Ocean response is significantly sensitive to dominant atmospheric forcing indices. Mesoscale eddies derived from instabilities at strong current cores are successfully identified with specific SSH gradient criteria. The new estimates of rings population in the Southern Ocean are tightly linked to interannual to decadal atmospheric variability. Increased number of mesoscale eddies correlate with positive SAM forcing about two years earlier, or negative ENSO forcing two to three months earlier. These cross-correlations might explain a prominent peak in rings abundance estimated during 2000 and 2001, and the short-lived maximum that appeared in 2010. There are no persistent trends in the estimated sea surface slope across Drake Passage, and therefore neither in the transport of the ACC. High cross-correlation between the abundance of mesoscale eddies and atmospheric forcing suggests that the overall ACC system is in an eddy-saturated state. However, Drake Passage positive sea level slope anomalies were two-year lagged with negative SAM forcing and with positive ENSO events. These regional responses are characteristic of eastward-propagating signals from a buoyancy-dominated Pacific sector of the Southern Ocean.

The biological process of the carbon cycle in the Antarctic Ocean is controlled by the photosynthetic activity of the primary producers. The amount of fixed carbon does not only depend on the photosynthetic activity but also on the carbon losses due to respiration. Thus, the ratio photosynthesis to respiration (rP/R) is an important parameter to predict the effect of climate change on the Antarctic ecosystem. Indeed, the ongoing changes in climate change are influencing the dynamics of environmental conditions, which has tremendous effects on the phytoplankton community. Therefore, two ecologically relevant species from the Southern Ocean were here investigated: the diatom Chaetoceros sp. and the prymnesiophyte Phaeocystis antarctica, studying the changes in the rP/R under global climate change conditions. Three main parameters were examined i.e temperature, salinity and iron limitation. The P/R ratio was significantly affected by temperature, while salinity had only a secondary importance, although with species-specific differences. More specifically, the values were ranging from 12.3 to 7.5 for Chaetoceros sp. and from 12.4 to 2.5 for P. antarctica. The changes in this ratio were principally due to variations in respiration, rather than in photosynthesis. Chaetoceros sp. appears to be less flexible in the regulation of the extent of photoprotective mechanisms (non-photochemical quenching and alternative electrons), but its photoprotective level was generally higher than in P. antarctica. Regarding iron limitation, data were successfully collected only for Chaetoceros sp.. The P/R ratio, equal to 2.8, did not change under iron limitation, with iron limited cells showing a very efficient acclimation to the lowered assimilatory metabolism by decreasing their respiratory losses.

The ocean is the largest sink of anthropogenic carbon from the atmosphere and therefore the magnitude of ocean carbon uptake largely determines the airborne fraction of emissions and the ultimate severity of surface climate change. However, climate-feedbacks on ocean carbon uptake over the historical period and in the future are uncertain. In particular, much uncertainty in the ocean carbon response hinges on the influence of wind-driven changes in the Southern Ocean, which is the most significant region of anthropogenic carbon uptake. Here I show that the Southern Hemisphere westerly winds simulated by the Coupled Model Intercomparison Project Phase 3 (CMIP3) and CMIP5 climate models have significant biases in their pre-industrial and satellite era-climatologies, relative to observationally based estimates. I also show that the models project the westerlies to intensify and shift poleward under anthropogenic forcing over the 20th and 21st centuries, but that they significantly underestimate the trends over the satellite era. I then use a novel experimental design, wherein I isolate the influence of the models pre-industrial wind bias on simulations of ocean carbon uptake and climate. I do this by using the UVic Earth System Climate Model (ESCM) with an ensemble of members, each forced by the winds from an individual CMIP model. I show here that the climate model pre-industrial wind bias can significantly increase ocean carbon uptake in transient climate change simulations, reducing the airborne fraction and projected climate change. By contrast, the simulated wind-changes over the 20th and 21st centuries reduce ocean carbon uptake, largely through an increase in outgassing from the Southern Ocean. However, I show that this transient- wind effect is i) smaller than the pre-industrial bias effect and ii) does not occur when using a variable formulation for the Gent-McWilliams coefficient of eddy diffusivity in the coarse resolution model, under simulated or observed wind-changes. I then go on to demonstrate that the simulated transient wind-changes significantly reduce the Antarctic sea-ice area simulated by the UVic ESCM. I also test the influence of fresh water input to the Southern Ocean from dynamic Antarctic Ice Sheet mass loss, which is a forcing absent from the CMIP5 models. The magnitude of the fresh water effect is small and has little influence on the sea-ice area trends simulated by the CMIP5 models over the historical era. These results have significant implications for previous model-based studies of the ocean carbon cycle, as well as for the quantification of the wind-induced uncertainty in future climate projections by current Earth System Models.
Graduate
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University of Technology, Sydney. Faculty of Science.
The Antarctic marine ecosystem is unique and dynamic, changing seasonally and forming specialised niche habitats including open ocean, sea ice and meltwater environments. Phytoplankton are key species in the structure and function of the Antarctic ecosystem, instrumental in the regions biogeochemistry, fundamental to the food web and strong contributors to global primary production and carbon sequestration. Understanding the photosynthetic plasticity of Antarctic phytoplankton is essential to understanding the effects global change is likely to have on primary production in the region. Through a series of experiments, this thesis explores the processes of light acclimation, photoprotection and photoinhibition in Antarctic microalgae under different environmental stressors, comparing photophysiological responses of species known to inhabit the sea-ice, meltwater and pelagic regions of Antarctic waters. The photosynthetic properties of three Antarctic diatoms (Fragilariopsis cylindrus, Pseudo-nitzschia subcurvata and Chaetoceros sp.) to changes in salinity, temperature and light were compared. Large heterogeneities in the photoprotective capacity of the three species and several distinct physiological strategies in response to the rapid changes in the ambient environment were observed (Publication I). Similarly, photosynthesis and net primary productivity was species-specific with large differences between environmental conditions (Publication II). Fast induction kinetics and pulse amplitude modulated fluorometry were used to demonstrate high levels of flexibility in light acclimation capabilities of sea ice algae from the east Antarctic. Inhibitors and pigment analyses identified xanthophyll cycling as the critical mechanism for photoprotection and preferred means by which sea ice diatoms regulated energy flow to PS1 (Publication III). While immunoblot analyses of natural communities measured minimal D1 protein breakdown in algae exposed to irradiances up to 200 µmol photons m⁻² s⁻¹. These data showed that sea ice diatoms had low intrinsic susceptibility to PSII photoinactivation and strong irradiance-dependent induction of non-photochemical quenching that was independent of protein resynthesis (Publication IV). The remaining chapters investigated photoprotective strategies and photosynthetic plasticity of phytoplankton under nutrient limitation. Nitrogen depletion in F. cylindrus had a strong influence on non-photochemical quenching capacity and resulted in the impairment of photosynthetic electron transport resulting in the formation of Qʙ non-reducing PSII centres within the photosystem (Publication V). The influence of iron-limitation and high light stress on the growth and physiology of Southern Ocean phytoplankton revealed a community-based response of measurable changes in pigment ratios, photosynthetic capacity and community composition (Publication VI). Iron-limited phytoplankton altered the allocation of photosynthetically derived energy, increasing photoprotective pigment pools and down-regulating photochemistry, at the expense of photosynthetic plasticity.