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Previous studies found large biases between individual observational and model estimates of historical ocean anthropogenic carbon uptake. We show that the largest bias between the Coupled Model Intercomparison Project phase 5 (CMIP5) ensemble mean and between two observational estimates of ocean anthropogenic carbon is due to a difference in start date. After adjusting the CMIP5 and observational estimates to the 1791-1995 period, all three carbon uptake estimates agree to within 3Pg of C, about 4% of the total. The CMIP5 ensemble mean spatial bias compared to the observations is generally smaller than the observational error, apart from a negative bias in the Southern Ocean and a positive bias in the Southern Indian and Pacific Oceans compensating each other in the global mean. This dipole pattern is likely due to an equatorward and weak bias in the position of Southern Hemisphere westerlies and lack of mode and intermediate water ventilation.

Understanding the physics of multiphase plumes and their simulation through numerical modeling has been an important area of research in recent times in the area of environmental fluid mechanics. The two renowned numerical modeling types that are commonly used by researchers today to simulate multiphase plumes in nature are the mixed-fluid and the two-fluid integral models. In the present study, a detailed review was performed to study and analyze the two modeling approaches for the case of a double plume (upward moving inner plume with downward moving annular outer plume) with the objective of ascertaining which of these models represent the prototype physics in the integral plume model equations with a higher degree of completeness and accuracy. A graphical user interface was designed to facilitate running the models. By comparison to laboratory scale experimental data and through sensitivity analyses, a rigorous effort was made to determine the most appropriate choice of initial conditions needed at the start of the model computation and at the peeling locations and to obtain the most consistent values of the different model parameters that are necessary for calibration of the two models. Consequently, with these selected sets of initial conditions and model parameters, the models were run and their outputs compared against each other for three different case studies with ambient conditions typical of real environmental data. The dispersed phases considered were air bubbles in two cases and liquid CO2 droplets for the third case, with water as the continuous phase in all cases. The entrainment coefficient was found to be the most important parameter that affected the model results. In all the three case studies conducted, the mixed-fluid model was found to predict about 30% higher values for the peel heights and the DMPR (Depth of Maximum Plume Rise) than the two-fluid model.

Much research has been devoted to understanding the ocean carbon cycle because of its prominent role in controlling global climate. Coastal oceans remain a source of uncertainty in global ocean carbon budgets due to their individual characteristics and their high spatial and temporal variability. Recent attempts to establish general patterns suggest that temperate and high-latitude coastal oceans act as sinks for atmospheric carbon dioxide (CO2). In this thesis, carbon cycling in two Canadian coastal ocean regions is investigated, and the uptake of atmospheric CO2 is quantified. A combination of ship-board measurements and highly temporally resolved data from an autonomous mooring was used to quantify the seasonal to multi-annual variability in the inorganic carbon system in the Scotian Shelf region of the northwestern Atlantic for the first time. The Scotian Shelf, unlike other shelf seas at similar latitude, acts as a source of CO2 to the atmosphere, with fluxes varying over two orders of magnitude in space and time between 1999 and 2008. The first observations of the inorganic carbon system in the Amundsen Gulf region of the southern Beaufort Sea, covering the full annual cycle, are also presented. Air-sea CO2 fluxes are computed and a carbon budget is balanced. The Amundsen Gulf system acts as a moderate sink for atmospheric CO2; seasonal ice-cover limits winter CO2 uptake despite the continued undersaturation of the surface waters. Biological production precedes the ice break-up, and the growth of under-ice algae constitutes nearly 40% of the annual net community production. The Scotian Shelf may be described as an estuarine system with an outflow of surface water, and intrusion of carbon-rich subsurface water by a combination of wind-driven mixing, upwelling and convection, which fuels the CO2 release to the atmosphere. In contrast, Amundsen Gulf may be described as an anti-estuarine, or downwelling, system, with an inflow of surface waters and an outflow of subsurface waters. Wind-driven and convective mixing are inhibited by ice-cover and restrict the intrusion of carbon- and nutrient-rich waters from below, maintaining the CO2 uptake by the surface waters. / PhD Thesis

Thesis advisor: Hilary Palevsky / The ocean plays a key role in global carbon cycling, taking up CO2 from the atmosphere. A fraction of this CO2 is converted into organic carbon through primary production in the surface ocean and sequestered in the deep ocean through a process known as the biological pump. The ability of the biological pump to sequester carbon away from the atmosphere is influenced by the interaction between the annual cycle of ocean mixed layer depth (MLD), primary production, and ecosystem processes that influence export efficiency. Gravitational sinking of particulate organic carbon (POC) is the largest component of the biological pump and the aspect that is best represented in Earth System Models (ESMs). I use ESM data from CESM2, an ESM participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), to investigate how a high-emissions climate change scenario will impact POC flux globally and regionally over the 21st century. The model simulates a 4.4% decrease in global POC flux at the 100 m depth horizon, from 7.12 Pg C/yr in the short-term (2014-2034) to 6.81 Pg C/yr in the long-term (2079-2099), indicating that the biological pump will become less efficient overall at sequestering carbon. However, the extent of change varies across the globe, including the largest POC flux declines in the North Atlantic, where the maximum annual MLD is projected to shoal immensely. In the future, a multi-model comparison across ESMs will allow for further analysis on the variability of these changes to the biological pump. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Earth and Environmental Science.

Les rivières sont une source importante de constituants biogéochimiques pour les océans. Jusqu’à présent, les modèles océaniques globaux représentaient de manière inadéquate ou ignoraient simplement les apports continentaux de nutriments, de carbone, d’alcalinité provenant des rivières. En particulier, les perturbations anthropiques des apports fluviaux au cours du 20 ème siècle et leurs conséquences sur l’état physique et biogéochimique des océans - notamment la zone côtière - n’ont pas encore été analysées à l’aide d’un modèle global prenant en compte la circulation tridimensionnelle de l’océan. L’objectif principal de cette thèse était donc d’intégrer les apports biogéochimiques provenant des rivières dans un modèle océanique global afin d’améliorer la compréhension du cycle du carbone de l’océan côtier et son évolution au cours du 20 ème siècle. Dans un premier temps, mon travail a visé à l’amélioration des connaissances concernant le rôle des apports biogéochimiques fluviaux sur le cycle du carbone océanique à long-terme, en se focalisant sur la période préindustrielle. Pour cela, j’ai estimé les apports des rivières en utilisant des modèles permettant d’estimer l’érosion chimique et le transfert de matière organique desécosystèmes terrestres à l’océan. Ces apports fluviaux ont ensuite été ajoutés dans le modèle biogéochimique océanique HAMOCC et leurs impacts sur la production primaire océanique et les flux de CO2 entre l’atmosphère et l’océan ont été analysés. Les résultats nous ont permis de quantifier un dégazage de CO 2 préindustriel de 0.23 Pg C yr -1 pour l’océan global, principalement localisé à proximité de l’embouchure des rivières. Le modèle a également démontré l’existence d’un transfert inter-hémisphèrique de carbone, avec un plus grand apport des rivières à l’océan dans l’hémisphère nord, et un transfert de l’hémisphère nord à l’hémisphère sud où un dégazage net se produit. Une augmentation considérable de la production primaire océanique induite par les apports des rivières a également été prédite.La modélisation biogéochimique de l’océan côtier a ensuite été améliorée, en augmentant la vitesse de minéralisation de la matière organique dans les sédiments côtiers et en incluant la dégradation de la matière organique dissoute d’origine terrestre (tDOM) dans l’océan. Par ailleurs, notre analyse suggère un temps de résidence des eaux dans la zone côtière significativement plus courte (14-16 mois en moyenne) que celui estimé jusqu’à présent (>4 ans). Ce temps de courte résidence implique un transfert efficace de matière organiquede l’océan côtier à l’océan ouvert, un état autotrophe net de l’océan côtier, ainsi qu’un puit de CO 2 (0.06-0.08 Pg C yr -1) pour la période préindustrielle, contrairement aux hypothèses précédemment proposées dans la littérature.Dans le dernier chapitre, les perturbations océaniques induites par les changements de la concentration en CO 2 dans l’atmosphère, de la physique de l’océan et des apports biogéochimiques fluviaux au cours du 20 ème siècle ont été analysées. Les résultats indiquent que la réduction de production primaire nette (NPP) observée dans les océans tropicaux et subtropicaux, pourrait être entièrement compensée par une augmentation de la NPP dans l’océan austral et dans les systèmes côtiers de type «EBUS». Les simulations montrent aussi que l’augmentation des apports fluviaux provoque une augmentation de NPP océanique à l’échelle de l’océan côtier (+15 %) et à l’échelle globale (+ 4 %). En conclusion, cette thèse a permis de démontrer l’importance d’inclure la variabilité spatio-temporelle des apports fluviaux et des processus biogéochimiques de l’océan côtier dans la description du cycle du carbone océanique global. Les améliorations apportées au modèle océanique global HAMOCC permettront d’affiner les prédictions du rôle de l’océan dans le cycle du carbone au cours du 21 ème siècle. / River deliver vast amounts of terrestrially derived compounds to the ocean. These fluxes are of particular importance for the coastal ocean, which is recognized as a region of disproportionate contribution to global oceanic biological fluxes. Until now, the riverine carbon, nutrient and alkalinity inputs have been poorly represented or omitted in global ocean biogeochemistry models. In particular, there has yet to be a model that considers the pre-industrial riverine loads of biogeochemical compounds to the ocean, and terrestrial inputs of organic matter are greatly simplified in their composition and reactivities in the ocean. Furthermore, the coastal ocean and its contribution to the globalcarbon cycle have remained enigmatic, with little attention being paid to this area of high biological productivity in global model analysis of carbon fluxes. Lastly, 20 th century perturbations in riverine fluxes as well as of the physical and biogeochemical states of the coastal ocean have remained unexplored in a 3-dimensional model. Thus, the main goals of this thesis are to integrate an improved representation of riverine supplies in a global ocean model, as well as to improve the representation of the coastal ocean in the model, in order to solve open questions with respect its global contributions to carbon cycling.In this thesis, I first aimed to close gaps of knowledge in the long-term implications of pre-industrial riverine loads for the oceanic cycling of carbon in a novel framework. I estimated pre-industrial biogeochemical riverine loads and their spatial distributions derived from Earth System Model variables while using a hierarchy of state-of-the-art weathering and organic matter land-ocean export models. I incorporated these loads into the global ocean biogeochemical model HAMOCC and investigated the induced changes in oceanic biological production and in the air-sea carbon flux, both at the global scale and in a regional shelf analysis. Finally, I summarized the results by assessing the net land sink of atmospheric carbon prescribed by the terrestrial models, and comparing it to the long-term carbon outgassing determined in the ocean model. The study reveals a pre-industrial oceanic outgassing flux of 231 Tg C yr -1 ,which is found to a large degree in proximity to the river mouths. The model also indicates an interhemispheric transfer of carbon from dominant northern hemisphere riverine inputs to outgassing in the southern hemisphere. Furthermore, I observe substantial riverine-induced increases in biological productivity in the tropical West Atlantic (+166 %), the Bay of Bengal (+377 %) and in the East China Sea (+71 %), in comparison to a model simulation which does not consider the riverine inputs.In addition to considering supplies provided by riverine fluxes, the biogeochemical representation of the coastal ocean is improved in HAMOCC, by firstly increasing organic matter remineralization rates in the coastal sediment and by secondly explicitly representing the breakdown process of terrestrial dissolved organic matter (tDOM) in the ocean. In an analysis of the coastal fluxes, the model shows a much shorter residence time of coastal waters (14-16 months) than previously assumed, which leads to an efficient cross-shelf transport of organic matter and a net autotrophic state for both the pre-industrial timeframe and the present day. The coastal ocean is also revealed as a CO2 sink for the pre-industrial time period (0.06-0.08 Pg C yr -1 ) in contrary to to the suggested source in published literature. The sink is however not only caused by the autotrophic state of the coastal ocean, but it is likely also strongly influenced by the effects of biological alkalinity production, as well as both physical and biogeochemical characteristics of open ocean inflows.In the final chapter, 20 th century oceanic perturbations due to changes in atmospheric CO 2 concentrations and in the physical climate, and to increases in riverine nutrient supplies were investigated by using sequential model simulations. The model results show that the decrease in the net primary production (NPP) in the tropical and subtropical oceans due to temperature-induced stratification may be completely compensated by increases in the Southern Ocean and in Eastern Boundary Upwelling Systems (EBUS). The model also reveals that including increases in riverine supplies causes a global ocean NPP increase of +4 %, with the coastal ocean being a particularlystrongly affected region (+15 %).This thesis shows a strong necessity to represent spatio-temporal changes in riverine supplies and of the coastal ocean state in spatially explicit global models in order to assess changes of the global cycling of carbon in the ocean in the past and potentially in the future. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

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