Dissertations / Theses on the topic 'Deep ocean circulation'

Thesis (Ph. D.)--Woods Hole Oceanographic Institution and Massachusetts Institute of Technology, 1990.
Funding was provided by the Office of Naval Research and a Secretary of the Navy Graduate Fellowship in Oceanography. References : p. 117-121.

Les différents projets présentés dans cette thèse contribuent à la compréhension de plusieurs aspects clés de la circulation océanique. Le premier aspect que nous étudions porte sur les processus physiques à l'origine du mélange lié à la marée; deux processus ont été mis en évidence. Depuis la latitude critique vers l'équateur, la marée interne transfert son énergie à des ondes plus petite échelle via des instabilités triadiques résonnantes impliquant les ondes proche inertielles. Depuis la latitude critique vers le pôle, les ondes de marée interne continuent de transférer leur énergie à des ondes plus petite échelle, mais étonnamment ce transfert se fait entre la marée interne et des ondes évanescentes.Dans la deuxième étude, nous étudions l'effet d'un courant moyen sur la propagation et la dissipation des ondes de marée interne, générées à la topographie dans des simulations haute résolution. Dans ce cas, la dépendance en latitude de la dissipation de la marée interne est plus lisse et plus proche d'une constante. Ce changement de la dépendance en latitude peut être lié au décalage des fréquences des ondes de marée interne par effet Doppler, ce qui induit la génération d'ondes secondaires plus petite échelle.Dans la troisième étude, nous étudions l'effet d'une perturbation générée en amont sur la circulation dans le bassin amont dû à l'interaction entre la perturbation et un seuil hydrauliquement contrôlé. Les ondes de Kelvin et topographiques de Rossby, générées par une variation de l'afflux d'eau dans le bassin amont, perturbent l'écoulement au dessus du seuil et ainsi l'export d'eau. Cette perturbation est due à la réfraction des ondes sur le seuil à chaque passage, une fois qu'elles ont fait le tour du bassin amont
The various projects presented in this thesis contribute to our understanding of various key aspects of the oceanic circulation. The first aspect that we investigate is the physical processes responsible for this tidal mixing, and we identify two processes. Equatorward of the critical latitude, internal tides transfer their energy to smaller-scale waves via triadic resonant instabilities involving near-inertial waves. Poleward of the critical latitude, internal tides still transfer energy to smaller-scale waves, but surprisingly this transfer takes place between the internal tide and evanescent waves.In the second study, we investigate the effect of a mean current on the propagation and the dissipation of internal tides generated at the topography in high-resolution simulations. In that case, the latitudinal dependence of the tidal energy dissipation is found to be smoother and closer to a constant. This change in the latitudinal dependence can be linked to the Doppler shift of the frequency of the internal tides, which impacts the generation of smaller-scale secondary waves.In the third study, we study the effect of an upstream disturbance on the upstream circulation by interaction with a hydraulically controlled sill. The Kelvin and topographic Rossby waves, generated by a change in the upstream inflow, perturb the flow through the channel and hence the water export. This perturbation is due to the refraction of the waves at the sill at each passage, once they go around the upstream basin

The seasonal and interannual variation of the water properties and deep water renewal in the Strait of Georgia are examined. Temperature, salinity and dissolved oxygen data acquired over an intensively sampled one year period are presented to show the timing of renewal periods and the variation in properties across the Strait and with depth. The northward propagation of the renewal signal and of its variation across the Strait of Georgia are discussed. A volumetric analysis provides temperature and salinity averages of the Strait of Georgia waters and of renewal water during several stages of each renewal period. Estimates of the renewal volume are calculated using a heat and salt budget method. Data collected in the central part of the Strait over several decades are presented to show the interannual variation in water properties. Correlation coefficients relating the wintertime air temperatures to the deep water properties some time later in the Strait are given.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

La période froide du Dernier Maximum Glaciaire était caractérisée, en regard de notre climat moderne, par une couverture de glace de mer australe accrue, une circulation profonde Atlantique moins profonde et une plus faible concentration en CO2 dans l’atmosphère. Ces différences sont bien connues grâce aux observations indirectes mais difficiles à représenter dans les simulations issues des modèles de climat. En effet, ces modèles simulent fréquemment une concentration en CO2 atmosphérique trop élevée, une circulation océanique trop profonde dans l’Atlantique et une banquise présentant une distribution trop circulaire dans l’océan austral ainsi qu’une étendue hivernale et une amplitude saisonnière trop faibles. Ces désaccords modèle-données observés au Dernier Maximum Glaciaire remettent en cause la représentation numérique de certains processus climatiques essentiels. Plusieurs études soulignent le rôle majeur de la glace de mer australe sur la capacité de stockage de carbone de l’océan et la circulation océanique profonde. Je me suis donc focalisée sur cette région pour mieux com-prendre les processus associés à ce stockage. Grâce aux simulations réalisées avec le modèle système terre iLOVECLIM, j’ai pu démontrer que les incertitudes liées à la représentation des calottes polaires ont un impact limité sur les variables examinées ici. En revanche, d’autres choix de conditions aux limites (affectant le volume de l’océan, l’ajustement de l’alcalinité) peuvent entraîner des modifications importantes du contenu total en carbone de l’océan. Je montre également que l’utilisation d’une paramétrisation simple de la plongée des saumures résultant de la formation de glace de mer permet d’améliorer significativement la simulation de la glace de mer australe, de la circulation océanique profonde et de la concentration en CO2 atmosphérique. Un ensemble de simulations incluant l’impact de différentes paramétrisations océaniques est utilisé pour montrer que la circulation océanique très profonde simulée par notre modèle ne peut être attribuée à une glace de mer australe insuffisante. En revanche, les processus de convection dans l’océan austral semblent clefs pour améliorer à la fois la glace de mer australe, la circulation océanique profonde et la concentration en CO2 atmosphérique auDernier Maximum Glaciaire
Compared to the present-day climate, the cold period of the Last Glacial Maximum was characterized by an expanded sea-ice cover in the Southern Ocean, a shoaled Atlantic deep ocean circulation and a lower atmospheric CO2 concentration. These changes are well-documented by indirect observations but difficult to represent in simulations of climate models. Indeed, these models tend to simulate a too high atmospheric CO2 concentration, a too deep Atlantic deep ocean circulation, and a sea-ice cover with a too circular distribution in the Southern Ocean and a too small winter extent and seasonal amplitude. The model-data discrepancies observed at the Last Glacial Maximum call into question the model representation of some important climate processes. Several studies have underlined the crucial role of the Southern Ocean sea ice on ocean carbon storage capacity and deep circulation. I have therefore focussed on this region to improve our understanding of the processes associated with this storage. Thanks to simulations performed with the Earth System Model iLOVECLIM, I have demonstrated thatthe uncertainties related to ice sheet reconstructions have a limited impact on the variables examined in this study. In contrast, other choices of boundary conditions (influencing the ocean volume and alkalinity adjustment) can yield large changes of carbon sequestration in the ocean. I also show that a simple parameterization of the sinking of brines consequent to sea-ice formation significantly improves the simulated Southern Ocean sea ice, deep ocean circulation and atmospheric CO2 concentration. A set of simulations including the effects of diverse ocean parameterizations is used to show that the too deep ocean circulation simulated by our model cannot be attributed to an insufficient sea-ice cover, whereas convection processes in the Southern Ocean seem crucial to improve both the Southern Ocean sea ice, the deep ocean circulation and the atmospheric CO2 concentration at the Last Glacial Maximum

The work presented in this Thesis concerns the large-scale circulation of the Indian Ocean and follows three lines of investigation: (i) decadal variability of the subtropical gyre circulation; (ii) decadal variability of the deep meridional overturning circulation (MOC); and (iii) the influence of diapycnal diffusivity on quasi-steady MOC states. The decadal variability of the subtropical gyre transport over the ocean interior (away from boundary currents) is investigated using hydrographic data from 32°S. Estimates of the relative gyre transports are: 41 ± 5.1 Sv (1 Sv = 106 m3s-1) for 1987, 42 ± 7.0 Sv for 1995 and 58 ± 7.0 Sv for 2002. This represents a 40% increase from 1987 to 2002. The main areas of change in the geostrophic transports are just east of Madagascar Ridge and around Broken Plateau, which is consistent with differences observed in the isopycnal depths in these areas. Maps of contoured velocity suggest that most of the change happened between 1995 and 2002, which supports the transport estimates. The 1987 and 2002 hydrographic data are then combined with a regional model of the Indian Ocean to investigate the impact that changes in conditions near 32°S might have on the deep MOC. The model has lateral open boundaries at 35°S for the Southern Ocean and 122°E for the Indonesian Throughflow. The meridional velocity field dominates over density at the southern boundary (SB) in determining the basin-wide deep circulation on decadal timescales. The initial adjustment of the deep MOC to the first 5-6 years of model integration and shows a large sensitivity to the SB conditions. With ‘best’ estimates of the flow field near 32°S the model shows a 6 Sv and 16 Sv deep MOC for 1987 and 2002, respectively. There are also changes in the zonal structure of the deep circulation. The results suggest that the Indian Ocean exhibits decadal variability in the size and structure of the deep MOC. Furthermore, the apparent inconsistency between previous non-GCM and regional GCM studies may be a result of the lateral boundary conditions, rather than a conflict in the model dynamics. 200-year model integrations suggest that quasi-steady MOC states in the Indian Ocean are reached on century time scales. The size, structure and adjustment time of the quasi-steady deep MOC are controlled by the distribution of diapycnal diffusivity (Kd). The zonal mean distribution of Kd required to support the prescribed deep inflow at the model SB can be estimated using a one-dimensional (1-D) advective-diffusive balance in isopycnal layers. The 18 Sv overturning circulation put forward by Ferron and Marotzke [2003] (FM) collapses when their model configuration is integrated to quasi-steady state under a number of different Kd regimes. With a diagnosed Kd field only 70% of the FM circulation can be supported in quasi-steady state, and the Kd values are an order of magnitude larger than recent observations suggest. The results imply that one can get a good a priori estimate of the Kd-field required to support a quasi-steady model MOC by applying a 1-D advective-diffusive balance in isopycnal layers to the SB conditions. Overall, the research highlights the need to implement improved estimates of (nonuniform) Kd in ocean GCMs when investigating quasi-equilibrium model states.

The Southern Ocean (SO) is a critical component in the global ocean conveyor. As the only conduit linking the Atlantic, Indian and Pacific Oceans, as well as an important region of upwelling and water mass formation, it is thought to have played a key role in modulating Earth’s past climate. Changes in the circulation of SO deep and bottom waters over the last 1.5 million years are investigated using stable carbon isotope $δ^{13}C$ measurements made on the tests of the benthic foraminfer Cibicidoides ($δ^{13}C_{b}$), and the rare earth element concentrations and Neodymium isotope ($ɛ_{Nd}$) values of marine sediments and their authigenic ferromanganese coatings. Being a proxy for past seawater nutrient contents, $δ^{13}C_{b}$ provides important insights into both past ocean circulation and the potential storage of remineralised organic carbon within the deep ocean, while simultaneously providing information on the past ventilation state of the deep ocean interior. As seawater $ɛ_{Nd}$ remains unaffected by biological fractionation or air-sea exchange processes, reconstructions of past deep and bottom water $ɛ_{Nd}$ provides a tool with which to study past changes in the circulation and mixing of these water masses. A suite of previously published late Holocene (0-6 ka) and Last Glacial Maximum (LGM; 18-24 ka) $δ^{13}C_{b}$ data are used alongside newly acquired $δ^{13}C_{b}$ data from the Amundsen Sea in the eastern Pacific sector of the SO to investigate past changes in the pattern of circum-Antarctic seawater carbon isotope composition. The $δ^{13}C$ signature of deep and bottom waters was much more heterogenous during the LGM than the late Holocene, with negative $δ^{13}C$ excursions occurring within the Atlantic and Indian sectors of the SO below c. 2-3 km water depth. Some of this negative $δ^{13}C$ signal was advected through the SO to the Pacific sector, but this appears to have been restricted by bathymetric barriers within the SO. New $δ^{13}C_{b}$ data spanning the last 800 ka from the Amundsen Sea are presented and suggest differing modes of bottom water formation in the Atlantic vs Pacific sectors of the SO during glacial periods of the last 800 ka. An authigenic $ɛ_{Nd}$ record measured on sediments from a core located in the deep Indian Ocean is used to investigate the palaeocirculation history of modified Circumpolar Deep Water (mCDW) within the Indian Ocean during the last 1.5 million years. Shifts towards more radiogenic $ɛ_{Nd}$ values during glacial periods are interpreted as reflecting a decreased entrainment of deep waters sourced in the North Atlantic (Northern Component Water, NCW) within CDW, which led to a reduced advection of an unradiogenic $ɛ_{Nd}$ NCW signal to the core site. $ɛ_{Nd}$ and REE measurements made on sediments from two cores located on the Pacific-Antarctic Ridge in the western Pacific sector of the SO (to the north of the Ross Sea Embayment) are used to reconstruct the bottom water palaeocirculation in this region across the last 540 ka. The proportion and $ɛ_{Nd}$ signature of Ross Sea Bottom Water (RSBW) bathing these core sites has fluctuated throughout the last 540 ka. These fluctuations suggest the rate and location of bottom water formation within the Ross Sea, and the supply of terrigenous material with radiogenic $ɛ_{Nd}$ values with which to isotopically `labelled' RSBW, may have changed in the past.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988.
Includes bibliographical references (p. 142-146).
This thesis is a study of the effect of geothermal sources on the deep circulation, temperature and salinity fields. In Chapter 1 background material is given on the strength and distribution of geothermal heating. In Chapter 2 evidence for the influence of a hydrothermal system in the rift valley of the Mid-Atlantic Ridge on nearby property fields and a model of the flow around such a heat source are presented, with an analysis of a larger-scale effect. Results of an analytical model for a heat source on a #-plane in Chapter 3 show how the response far from the source can have a structure different from the forcing because of its dependence on two parameters: a Peclet number (the ratio of horizontal advection and vertical diffusion), and a Froude-number-like parameter (the ratio of long wave phase speed to background flow speed) which control the relative amount of damping and advection of different vertical scales. The solutions emphasize the different behavior of a dynamical field like temperature compared to tracers introduced at the source. These ideas are useful for interpreting more complicated solutions from a numerical model presented in the final chapter.
by Kevin G. Speer.
Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 160-163).
Physical oceanographers have known for several decades the total amount of abyssal mixing and upwelling required to balance the deep-water formation, but are still working to understand the mechanisms and locations-how and where it happens. From observational studies, we know that areas of rough topography are important and the hundreds of Grand-Canyon sized canyons that line mid-ocean ridges have particularly energetic mixing. To better understand the mechanisms by which rough topography translates into energetic currents and mixing, I studied diffusive boundary layers over varying topography using theoretical approaches and idealized numerical simulations using the ROMS model. In this dissertation, I show a variety of previously unidentified characteristics of diffusive boundary layers that are likely relevant for understanding the circulation of the abyssal ocean. These boundary layers share many important properties with observed flows in abyssal canyons, like increased kinetic energy near topographic sills and strong currents running from the abyssal plains up the slopes of the mid-ocean ridges toward their crests. They also have a previously unknown capacity to accelerate into overflows for a variety of oceanographically relevant shapes and sizes of topography. This acceleration happens without external forcing, meaning such overflows may be ubiquitous in the deep ocean. These boundary layers also can force exchange of large volumes of fluid between the relatively unstratified boundary layer and the stratified far-field fluid, altering the stratification far from the boundary. We see these effects in boundary layers in two- and three-dimensions, with and without rotation. In conclusion, these boundary layer processes, though previously neglected, may be a source of a dynamically important amount of abyssal upwelling, profoundly affecting predictions of the basin-scale circulation. This type of mechanism cannot be captured by the kind of mixing parameterizations used in current global climate models, based on a bottom roughness. Therefore, there is much work still to do to better understand how these boundary layers behave in more realistic contexts and how we might incorporate that understanding into climate models.
by Rebecca Walsh Dell.
Ph.D.

Recently observed ²³⁰Th concentrations in 2007 and 2009 documented very high ²³⁰Th values within the Atlantic layer in the Canada Basin of the Arctic Ocean. Similar levels of high ²³⁰Th had only been previously observed in the Alpha Ridge region, implying that the Alpha Ridge is the potential source of the high ²³⁰Th waters. As the Alpha Ridge is downstream in the classic cyclonic circulation, that circulation is believed to have changed. Motivated by this, a three-dimensional Arctic ²³⁰Th model is configured for the first time to study such change. To simulate the tracer, I coupled a scavenging model, which describes the exchange of ²³⁰Th (and ²³¹Pa) between the dissolved and particulate phases, to an offline NEMO model (the Nucleus for European Modelling of the Ocean) that provides the advection and mixing processes that redistribute the tracers within the ocean. As the scavenging rates of such tracer elements are strongly affected by oceanic particle concentrations, the scavenging rates are parameterized as a function of ice concentration, which, to a great extent, influences the biological processes in the water. Model output produced an increase of ²³⁰Th concentration in the south Canada Basin. Sensitivity experiments confirm such change is not caused by a change in the particle field but a change in the intermediate circulation from cyclonic to anticyclonic throughout the Amerasian Basin. This shift in circulation is the reason for a subsequent transport of high ²³⁰Th concentration from the Alpha Ridge to the south Canada Basin. The model circulation and density fields suggest that the change in the flow is caused by increased dense water flux into the Arctic Ocean, primarily through the Barents Sea route. This increase of dense water inflow alters the density distribution in the Arctic and results in a quick adjustment in the Atlantic layer (~1 year) through propagation of boundary trapped internal Kelvin waves.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

La circulation méridienne grande échelle des océans se maintient grâce à un équilibre subtil entre différents mécanismes. Dans le cas d’un forçage thermique atmosphérique, le point crucial qui apparaît est le mélange turbulent équatorial qui réchauffe les eaux profondes. L’objet de cette thèse est d’expliciter ce mélange. Il a été motivé par l’observation de fines couches homogènes se superposant de façon intermittente sur la verticale. Ces couches s’insèrent dans la dynamique de plus grande échelle des jets équatoriaux. Une exploration numérique a permis d’obtenir une représentation réaliste de la dynamique équatoriale constituée de jets extra-équatoriaux et de jets équatoriaux profonds. La combinaison de ces deux systèmes de jets forme des niches de faibles valeurs de vorticité potentielle, propices à l’instabilité inertielle et au mélange. Une étude numérique de très haute résolution. A recréé dans ce contexte des petites structures actives aux propriétés identiques à celles observées en Atlantique. Le phénomène s’avère irréversible et son développement amène l’écoulement de plus grande échelle à s’ajuster aux zones marginales des critères d’instabilité inertielle. Ces mélanges équatoriaux permettent d’étendre la distribution globale des coefficients de diffusivité turbulente jusqu’à l’équateur où le mélange dû au déferlement des ondes internes n’est pas actif. La résolution atteinte par les simulations numériques ne permet pas encore de conclure sur une cascade d’énergie vers les échelles du mélange mais invite à de nouvelles études vers les plus petites échelles
The large scale meridional overturning circulation in the ocean is maintained by a combination of several mechanisms. This dissertation focuses on turbulent equatorial mixing, one of the key mechanisms to balance heat forcing. The motivation for the research is observations of thin layers of well-mixed density and tracers distributed intermittently over the vertical that coincide with particular features of a larger scale pattem of equatorial jets. A large number of numerical experiments at moderately high resolution was performed in order te successfully replicate the observed equatorial dynamics, which involves both quasi-barotropic extra-equatorial jets and small vertical scale equatorial deep jets. The combination of the two systems of jets builds low-potential-vorticity niches, favouring the development of inertial instability and mixing. A very high resolution simulation was performed to study the fonnation of fine-scale layer structures with properties similar to those observed in the Atlantic. The creation of layers appears to be irreversible and the spatial distribution of the layers matches that of the marginal condition for inertial stability. The new appreciation gained for the importance of equatorial mixing leads one te revisit the question of the global distribution of diapycnal diffusivity coefficients, the estimation of which is often based on the breaking of internal waves, whose activity decreases with latitude. The resolution of the simulations performed for this study is not fine enough to produce a direct cascade of energy toward mixing scales but invites further investigation in that area

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Radioactive isotopes can be used in paleoceanography both for dating samples and as tracers of ocean processes. Here I use radiocarbon and uranium series isotopes to investigate the ocean's role in climate change over the last deglaciation. I present a new method for rapid radiocarbon analyses as a means of age-screening deep-sea corals for further study. Based on age survey results, I selected forty corals from the Drake Passage and thirteen from the Reykjanes Ridge off Iceland and dated them with uranium series isotopes. The uranium series dates give independent ages that allow radiocarbon to be used as a tracer of circulation and carbon cycle changes. The radiocarbon records generated from the Drake Passage corals show increased stratification in the Southern Ocean during the last glacial maximum (LGM) that disappeared during the start of the deglaciation as atmospheric CO2 began to rise during Heinrich Stadial 1 (HI). Considering these data and using a simple mass budget calculation, I show that the drop in atmospheric radiocarbon activity during H1 can be explained given direct carbon exchange between the radiocarbon-depleted deep ocean and atmosphere, e.g. through the Southern Ocean. The Drake Passage radiocarbon records also show evidence for decreased air-sea gas exchange in the Southern Ocean during the Antarctic Cold Reversal/Belling-Allered coincident with the hiatus in the deglacial CO2 rise. During this time period in the North Atlantic, radiocarbon reconstructions from deep-sea corals collected from off Iceland show a similar ventilation rate to that observed today and during the Holocene. To further investigate changes in North Atlantic ventilation over the last deglaciation, I used an inverse model to assess the consistency of sedimentary 2m1 Pa/ 230Th ratios from the Holocene, Hl, and the LGM with the modern circulation. Although sedimentary 231Pa/230Th has been used to infer changes in the strength of the meridional overturning circulation in the past, I find that published data are consistent with the modern circulation during the LGM and Hi. These findings highlight the importance of giving due regard to the uncertainties in the behavior and spatial distribution of paleoceanographic tracers.
by Andrea Burke.
Ph.D.

The Southern Ocean is vitally important in understanding climate change, both regionally and globally. This is due partly to physical factors such as the Antarctic Circumpolar Current (ACC), which facilitates the interchange of heat, nutrients and carbon dioxide between the world oceans. These physical factors also include the seasonal variation in sea-ice around the Antarctic continent and the strong Southern Hemisphere westerly winds, which are responsible for the divergence-driven upwelling of deep water to the surface, south of the Polar Front. The Upper branch of the Circumpolar Deep Water (UCDW) is warm, relative to Antarctic waters, and is high in dissolved inorganic carbon and nutrients, in particular iron. It contributes to carbon dioxide regulation, therefore, through temperature effects on solubility, the out-gassing of carbon dioxide and also via its effect on primary production and the biological pump. This project examines long-term trends in the meridional circulation of UCDW and in upper-ocean structure, primarily in the Australian region (110- 160E, 40-70S) of the Southern Ocean. This is achieved by the use of the Simple Ocean Data Assimilation (SODA) 2.0.2/2.0.4 reanalysis of ocean climate variability, which combines output from an ocean global circulation model with observational data, through a sequential data estimation scheme. Trends are produced for 1958-2007, in five-degree latitudinal bands that approximate the ACC frontal zones, for UCDW and mixed layer hydrodynamic variables and these are related to trends in Southern Hemisphere winds and global climate indices. In addition, similar trends are also produced for the shorter period 1997-2007, for which satellite data are available, and these trends are related to trends in chlorophyll-a, a proxy for phytoplankton biomass, and primary production. As well as these trends, interannual variability in chlorophyll-a in spring and summer, and its controls, are also studied.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Le traceur transitoire que constitue le tritium est devenu l'un des outils les plus prometteurs dens l'etude de la circulation oceanique generale et de la capacite de l'ocean a absorber le gaz carbonique anthropogene. L'utilisation simultanee du tritium et de son descendant par voie radioactive l'helium-3 ajoute une information supplementaire: la paire de traceurs tritium et helium-3 constitue une horloge dans l'etude des masses d'eau. En plus de son origine "tritiumgenique", l'helium-3 est emis au niveau des dorsales oceaniques et il constitue un traceur de choix dans l'etude de la circulation oceanique profonde. Toutes les mesures d'helium-3 et de tritium ont ete faites par spectrometrie de masse. Protocole analytique, limite de detection et reproductibilite de la methode sont reportes dans ce travail. Sont abordes au niveau de l'interpretation des resultats: -1) les donnees d'helium-3 a la campagne merou a (ete 1982) ont permis de localiser une source active et de mettre en evidence un courant de retour intermediaire (centre vers 1000 m de profondeur), prolongement du courant profond upwelle dans la partie meridionale du bassin. -2) l'utilisation des donnees de tritium concernant la mission phycemed 1 (avril 1981) a permis de decrire la grande variabilite spatio-temporelle des processus de convection ayant lieu dans le bassin nord; le temps de renouvellement des eaux profonbdes du golfe du lion est evalue a 11 +ou- 2 ans. Les circulations profondes et de subsurface au niveau des detroits de sardaigne et de gibraltar sont precisees. -3) l'etude simultanee, suivant des niveaux isopycnaux, des donnees tritium et helium-3 de la mission topogulf (ete 1983) a permis de localiser les zones ou les processus de convection sont actifs. Une approche theorique relative a l'utilisation simultanee du tritium et de l'helium-3 est abordee. Elle utilise un modele de melange distinguant, pour une masse d'eau consideree, le temps de transit du temps de ventilation. La validite de "age tritium-helium" est testee par comparaison avec les temps de transit et de ventilation determines par le modele

Cette thèse apporte de nouvelles perspectives sur la circulation profonde de l’Océan Austral et sur le cycle du CO2. Elle se base sur la signature géochimique et minéralogique de la fraction terrigène des sédiments récupérés dans le secteur Atlantique Sud (carotte MD07-3076Q) et Indien Sud (MD12-3396Q) de l’Océan Austral.Une étude préliminaire des propriétés géologiques des sources potentielles de sédiments pour l’Océan Austral, ainsi que l’étude des processus de transports impliqués de l’émission d’une particule à son dépôt sur les fonds marins a été cruciale à la réalisation d’une étude de provenance dans cette zone complexe. Ces informations préliminaires ont démontré la nécessité de travailler sur plusieurs fractions granulométriques dans l’Atlantique Sud pour pouvoir retracer efficacement les différentes masses d’eau. Une telle approche a permis : (1) l’identification de la provenance de différentes fractions granulométriques ; (2) l’assignation de chaque fraction granulométrique à une masse d’eau spécifique ; (3) la reconstruction de l’évolution passée des principales masses d’eau profondes ; (4) de reconstruire les changements paléoenvironnementaux en Amérique du Sud pendant l’Holocène.De manière générale, nos données supportent les conclusions dessinées à l’aide d’autres proxys avec un Océan Atlantique Sud dominé par les masses d’eaux de l’hémisphère sud (i.e. AntArctic Bottom Water, AABW, et Circumpolar Deep Water, CDW) pendant la dernière période glaciaire, et un Océan Atlantique Sud dominé par les masses d’eau de l’Atlantique Nord (North Atlantic Deep Water, NADW) à partir du Bølling Allerød (B/A).De plus, cette thèse a permis de faire la distinction entre les AABW et l’ACC (Courant Circumpolaire Antarctique, qui comprend la CDW) dans les deux secteurs étudiés de l’Océan Austral. Cela a rendu possible la quantification de la contribution relative à la sédimentation des principales masses d’eau dans chaque secteur (i.e. AABW, CDW, et NADW dans le secteur Atlantique Sud ; ACC et AABW dans le secteur Indien Sud). Grâce à cette distinction, il a été possible de fournir des évidences d’activités encore inconnues de l’AABW au court du temps, et d’étudier les interactions dynamiques entre les AABW et la CDW dans le secteur Atlantique Sud de l’Océan Austral. Cette étude démontre que les évènements d’Heinrich (HE) 1, 2, et 3 sont précédés par une modification de la circulation profonde de l’Océan Austral. En particulier, ces modifications sont associées à une augmentation significative de la contribution de l’AABW à la sédimentation. De ce fait, ce travail met en lumière l’idée qu’un changement de la circulation profonde dans l’Océan Austral peut jouer un rôle, ou même déclencher les HE. Nos données indiquent également que, même si aucun changement dans le CO2 atmosphérique n’est observé pendant l’HE 2, la turbulence était plus forte pendant l’HE 2 que pendant l’HE 1. Cela suggère que les échanges entre l’océan profond et l’atmosphère pourraient avoir été coupés en raison de la présence d’une barrière dynamique/physique résultant de conditions prévalant en raison de la faible obliquité pendant la dernière période glaciaire. A la toute fin de cette dernière période glaciaire, nos données indiquent un « pulse » de la vitesse et de l’extension vers le Nord de la AABW dans le secteur Atlantique de l’Océan Austral. Ce "pulse" est concomitant de la première augmentation de la ventilation enregistrée dans la même carotte de sédiment et se termine avec le début du stadial d’Heinrich (HS) 1, une période d’augmentation du CO2 atmosphérique. Ces observations suggèrent que cette rapide incursion de la AABW dans l’Océan Atlantique Sud serait responsable de la rupture de la barrière physique/dynamique séparant l’océan profond, riche en CO2, et la surface, permettant ainsi l’échange entre ces eaux riches en CO2 et l’atmosphère pendant l’HS 1, et donc l’augmentation du CO2 atmosphérique pendant la dernière déglaciation
This PhD thesis provides new insights into both the deep Southern Ocean circulation and CO2 cycle based on geochemical, radiogenic isotopes and clay mineralogical signatures of the terrigenous fractions transported by the main deep-water masses using sediment cores recovered in the South Atlantic sector (core MD07-3076Q, central South Atlantic) and in the South Indian sector (core MD12-3396Q) of the Southern Ocean.A careful preliminary study of the geological properties of the potential sediments sources to the Southern Ocean and of the source-to-sink transportation patterns of detrital particles was crucial to develop a well constrained provenance study in this rather complex area. These preliminary data demonstrated that it was absolutely necessary to work on distinct grain-size fractions in the South Atlantic Ocean in order to efficiently track different water masses. Such an approach successfully permits: (1) provenance identification of the various grain-size fractions (clay, cohesive silt and sortable silt); (2) an assignment of each grain-size fraction to a specific water-mass; (3) the reconstruction of the main deep water-mass pathways past changes, (4) to reconstruct paleoenvironmental changes over South America during the Holocene.Overall, our data support the conclusions drawn using other proxies: Strong southern deep-water masses (i.e. AntArctic Bottom Water, AABW, and Circumpolar Deep Water, CDW) and weak southern North Atlantic deep-water (NADW) prevailed during the last glacial period. The transition from this glacial state to the modern period likely happened during the Bølling Allerød (B/A) with: (1) a southward migration and a deepening of the NADW into the South Atlantic sector of the Southern Ocean, (2) a retreat to the South and a slowdown of the southern deep-water masses.In addition, this study successfully permits the distinction between the AABW and the ACC (that includes CDW) in both sectors of the Southern Ocean. This enabled the quantification of the relative contribution to sedimentation of all the main deep-water masses in each sector (i.e. AABW, CDW and NADW in the South Atlantic sector; ACC and AABW in the South Indian sector). Thanks to this distinction, it was possible to provide evidences of previously unknown AABW activities through time, and to study the dynamical interactions between the AABW and the LCDW in the South Atlantic sector of the Southern Ocean. We show that Heinrich Events (HE) 1, 2 and 3 are preceded by a modification of the Southern Ocean overturning circulation, and especially associated to a significant increase in AABW contribution to sediment deposition. Consequently, this work highlights that a change in the Southern Ocean overturning circulation can play a major role or even trigger the Heinrich Events. Our data also indicates that the turbulent mixing was stronger during HE 2 than during HE 1, even though HE 2 is muted in atmospheric CO2 records. This suggests that the exchanges between the deep ocean and the atmosphere might have been disabled due to dynamical/physical barrier resulting from background conditions involved by low obliquity during the last glacial period. At the very end of this glacial period, our data indicate a substantial “pulse” of AABW speed and northern extent in the South Atlantic sector of the Southern Ocean. This "pulse" is concomitant with the first increase of the ventilation recorded in the same sediment core and ends with the beginning of the HS 1, a period of rising atmospheric CO2. These observations suggest that this rapid incursion of AABW into the South Atlantic Ocean may be responsible of the breakdown of the physical/dynamical barrier between the deep CO2-rich ocean and the surface, enabling the exchange between these CO2-rich waters and the atmosphere during the HS 1, and thus, the rise of atmospheric CO2 during the last deglaciation

To explore whether the dispersion of sediments in the North Atlantic can be related to modern and past Atlantic Meridional Overturning Circulation (AMOC) flow speed, particle size distributions (weight%, Sortable Silt mean grain size) and grain-size separated (0–4, 4–10, 10–20, 20–30, 30–40 and 40–63 μm) Sm-Nd isotopes and trace element concentrations are measured on 12 cores along the flow-path of Western Boundary Undercurrent and in the central North Atlantic since the Last glacial Maximum (LGM). North Atlantic is a useful place to explore how size-specific sediment provenance is related to sedimentary inputs and deep-current advection because mantle-derived materials in Iceland is a unique sedimentary source compared to crustal-derived terranes in Europe, Greenland and North America. The four main processes transporting sediments from continents to the North Atlantic (bottom currents, turbidity currents, ice-rafting events, airborne inputs) can be well distinguished through the size-specific physical and geochemical records. When primarily advected by the bottom currents, Holocene sediments show that the finer-sized fractions (0–4, 4–10, 10–20 μm) were transported further, and the coarser size fraction (40–63 μm) matched local inputs. In the deep coretops (> 2700 m) proximal to southern Greenland, fine-slit size fraction (10–20 μm) instead of clay size fraction (0–4 μm) observed more Icelandic-material contribution. In the past, the 20–30, 30–40 and 40–63 μm particles in the shallower Iceland-proximal core (1249 m) reflect Icelandic composition variation due to the abrupt volcanic eruption around 13–9 ka; while in the deeper Iceland-proximal core (2303 m) they were sensitive to the changing bottom flow speed. Downstream in cores proximal to southern Greenland (> 2272 m) and eastern North America (3555 m), composition of the 20–63 μm sediments could be used as an indicator for the retreating of the Greenland and Laurentide Ice Sheets which affect the sediment accessibility of the covered geological terranes; while the 0–4, 4–10 and 10–20 μm particles were more sensitive towards the changing direction (northern-sourced or southern-sourced) and velocity of the bottom current. In the open North Atlantic, the composition of the 0–10 μm particles were less variable between the cold and warm climate intervals compared to the 10–63 μm particles, and the 30–40 and 40–63 μm size fractions were sensitive towards both ice-rafting events and bottom flow direction. During LGM, shallower and vigorous northern-sourced water (NSW) was observed overlaying the deeper southern-sourced water (SSW), with the boundary between 2133 to 2303 m in southern Iceland, and ~ 2272 m in southern Greenland. Reduced NSW occurred during Heinrich Stadial 1, until AMOC above ~ 3500 m recovered to vigorous modern-like version no later than ~ 13.5 ka. Sluggish overflow was observed in North Atlantic between 12.2–11.7 ka above ~ 3500 m. Reduction of Iceland-Scotland Overflow Water occurred around 9.7 ka, and started recovering to its modern vigorous no later than ~ 8.6 ka. These relative past AMOC strength variations (vigorous/sluggish) are firstly converted to actual bottom-current speed (in cm/s) using laminar advection model in this work: vertical settling velocity of particle having the most Icelandic contribution is calculated by Stokes’ Law, and the lateral deep-sea current speed is calculated when the vertical settling depth and the lateral advection distance of the particle traveled before settling are constrained. Primary modelling errors originating from temperature/salinity variations in past deep seawater, winnowing process in fine particles, basaltic-signature dilution by crustal input, and lateral advection pathways of Icelandic-material are further discussed, indicating relatively low modelling error (< ~ 10–20 %). The modelling results agree well with modern deep-sea current speed measurements and backtrack-trajectory eddy resolving model (Ocean model for the Earth Simulator, OFES18), indicating reasonable quantifications of past AMOC flow speeds.

Two-thirds of submarine volcanism in the Earth's ocean basins is manifested along mid-ocean ridges and the remaining one-third is revealed along intraoceanic arcs and seamounts. Hydrothermal systems and the circulation patterns associated with these volcanic settings remove heat from the solid Earth into the deep ocean. Hydrothermal circulation continues to remove and redistribute heat in the crust as it ages. The heat and mass fluxes added to the deep ocean influence mixing in the abyssal ocean thereby affecting global thermohaline circulation. In addition to removing heat, hydrothermal processes extract chemical components from the oceanic and carry it to the surface of the ocean floor, while also removing certain elements from seawater. The resulting geochemical cycling has ramifications on the localized mineral deposits and also the biota that utilize these chemical fluxes as nutrients. In this dissertation, I analyze observed conductive heat flow measurements in the Panama Basin and borehole thermal measurements in Brothers Volcano and use mathematical models to estimate advective heat and mass fluxes, and crustal permeability. In the first manuscript, I use a well-mixed aquifer model to explain the heat transport in a sediment pond in the inactive part of the Ecuador Fracture Zone. This model yields mass fluxes and permeabilities similar to estimates at young upper oceanic crust suggesting vigorous convection beneath the sediment layer. In the second manuscript, I analyze the conductive heat flow measurements made in oceanic between 1.5 and 5.7 Ma on the southern flank of the Costa Rica Rift. These data show a mean conductive heat deficit of 70%, and this deficit is explained by various hydrothermal advective transport mechanisms, including outcrop to outcrop circulation, transport through faults, and redistribution of heat by flow of hydrothermal fluids in the basement. In the third manuscript, I analyze the borehole temperature logs for two sites representative of recharge and discharge areas of hydrothermal systems in the Brothers Volcano. I develop upflow and downflow models for fluids in the borehole and formation resulting in estimated of flow rates and permeabilities. All three independent research works are connected by the common thread of utilizing relatively simple mathematical concepts to get new insights into hydrothermal processes in oceanic crust.
PHD

Cette étude concerne la mise en euvre de méthodes numériques d'optimisation de type contrôle optimal appliquées à un problème d'assimilation de données en océanographie. Il s'agit d'étudier la faisabilité de la méthode dans un problème de grande taille et dans un modèle turbulent, caractéristique des circulations océaniques aux latitudes moyennes. Le modèle de circulation utilisé est un modèle quasi-géostrophique stratifié en plusieurs couches. Les données à assimiler sont les mesures altimétriques satellitaires (hauteur de la surface libre de l'océan). Elles se trouvent uniquement sur la couche de surface. Le vecteur de contrôle est choisi comme étant la condition initiale du système dynamique sur toutes les couches. Sur le plan théorique, sont étudiés : - l'existence et l'unicité de la solution de l'équation linéarisée ; - l'existence et l'unicité de l'état adjoint ; - la convergence de la méthode de contrôle par des suites minimisantes. Nous étudions ensuite la qualité de l'identification des circulations en profondeur en connaissant les informations de surface. Nous abordons aussi les différentes stratégies d'assimilation en temps (séquentielle, incrémentale, ...). Une étude au second ordre de la fonctionnelle permet d'estimer l'erreur de l'identification et de quantifier la propagation des informations de surface en profondeur. Un test de la validation croisée généralisée sur notre problème pour déterminer le coefficient de régularisation est fait grâce à cette étude au second ordre.

Etude de la flore des diatomees de forages oceaniques et de coupes situees a terre: site 647 de l'odp pour l'oligocene; formation des moronites et coupes de cobatillas en espagne, coupes de saint-laurent la vernede et d'uzes dans le gard, coupes du "tripoli di contignaco" en italie et forages dsdp des sites 116, 334, 335 et 545 pour le miocene; site 646 de l'odp pour le plio-pleistocene. L'interpretation biostratigraphique permet de mettre en evidence des hiatus de la sedimentation ,lies a la circulation oceanique profonde et a la presence d'upwellings cotiers

Thesis (Ph.D.)--University of Hawaii at Manoa, 2008.
For weak Yanai wave amplitude, currents resembling the TEJs are obtained, but only within the beam. They are the mean Eulerian flow, which cancels the Stokes drift of the Yanai waves, yielding a zero-mean Lagrangian flow: the water parcels conserve their potential vorticity (PV) and are stationary over a wave cycle. With stronger amplitude, the Yanai waves become unstable, and lose their energy to small vertical scales where it is dissipated. The resulting vertical decay of the Yanai waves provides a source of PV, allowing water parcels to move meridionally within the beam. This process results in TEJs with a mean Lagrangian zonal flow extending to the west of the beam.
In the Pacific and Atlantic oceans, a complex equatorial current structure is found below the thermocline. The currents are zonal with typical speeds from 5 to 20 cm s-1 and extend as deep as 2500 m. The structure can be divided into two overlapping parts: the Tall Equatorial Jets (TEJs), with large vertical scale and alternating with latitude, and the Equatorial Deep Jets (EDJs), centered on the equator and alternating in the vertical with a wavelength of several hundred meters.
In the present study, using idealized numerical simulations and analytical solutions, we demonstrate that the TEJs could result from a rectification of a beam of monthly-periodic Yanai waves that is generated in the eastern part of the basin by instabilities of the swift equatorial surface currents.
This circulation poses a computational and a theoretical challenge. First, state-of-the-art high-resolution regional models and Ocean General Circulation Models (OGCMs) typically produce a rather weak, inaccurate and incomplete picture of the circulation. Second, the most promising existing theory, based on the rectification of intraseasonal Yanai waves, cannot account for the basin-wide presence of the TEJs.
Includes bibliographical references (leaves 148-157).
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158 leaves, bound 29 cm

A growing body of Nd isotope data derived from fish debris and Fe-Mn crusts suggests that the Pacific was characterized by deep-water mass formation in both the North and South Pacific during the Early Paleogene. However, the South Pacific source has not been identified to date. Here we present new fossil fish debris neodymium isotope data from the South Pacific and southern tropical Pacific Ocean Drilling Program and Deep Sea Drilling Project Sites 323, 463, 596, 865 and 869 (paleowater depths spanning 1500 to 5000m) to reconstruct the water mass composition over the time interval ~80 to ~24 Ma. The data indicate a relatively unradiogenic South Pacific water mass composition, and the composition of Nd increases with distance northward. The new tropical Pacific data are consistent with existing records from that region. Analyses of detrital sediment Nd isotopic composition, combined with the dissolved Nd composition recorded by fish debris, suggests that the South Pacific water mass convected in the Pacific sector of the Southern Ocean. We designate this water mass South Pacific Deep Water (SPDW). The Nd isotopic composition of SPDW is more radiogenic than initially hypothesized and the relatively small increase in isotopic composition (from ~-6 to ~-4) during the transit from the Southern Ocean to the tropical Pacific suggests a faster rate of overturning circulation during the greenhouse climate interval than previously thought.

The physical oceanography in the northwest Indian Ocean is largely controlled by the seasonal monsoon. The seasonal variability in circulation is complex. Many studies have investigated processes in the Persian (Arabian) Gulf and Arabian Sea, but little is understood about the Sea of Oman. This thesis incorporated observations from Argo floats, surface drifters and satellite imagery to study the deep and surface circulation in the northwest Indian Ocean. An assessment of four independent moorings located in the Sea of Oman and Arabian Sea, as well as a model skill comparison of the Simple Ocean Data Assimilation (SODA) model, contributed to understanding the dynamics in this region. Spatial patterns of surface current velocity produced from surface drifter data from 1995-2009 agreed with previously known surface currents. The Somali Current, East Arabian Current, Equatorial Current, Northeast/Southwest Monsoon Current, Great Whirl and Ras al Hadd Jet were all identified. During the Southwest Monsoon, flow direction was to the east and southeast in the Arabian Sea. The Somali Current flowed northeast along the Somali Coast extending to the East Arabian Current along the Oman coast. During the Northeast Monsoon, evaporation increased over the Arabian Sea, which resulted in a salinity gradient. This imbalance caused low-salinity surface water from the northeast Indian Ocean to flow into the northwest Indian Ocean as the Northeast Monsoon Current. Current direction reversed with the change of wind direction from the Southwest Monsoon to the Northwest Monsoon. Many characteristics seen at the surface were also identified in the subsurface as deep as 1500m. The comparison of moored observations to the Argo observations co-located in space and time showed reasonable agreement with the largest salinity difference of 0.23 and largest temperature difference of 0.78?C. The Murray Ridge mooring had a temperature correlation of 0.97 when compared to Argo observations. Argo observations were compared with SODA model numerical output from 1992-2001, and, after Argo, were assimilated from 2002-2009. With assimilation of Argo data into the SODA model, the temperature and salinity from the model numerical output improved, with most differences between model numerical output and Argo observations falling within one standard deviation.

Previous numerical and theoretical results based on constant diapycnal diffusivity suggested the abyssal meridional overturning circulation (MOC) should weaken as winds over the Southern Ocean intensify. We corroborate this result in a simple ocean model, but find it does not hold in more complex models. First, models with a variable eddy transfer coefficient and simple yet dynamic atmosphere and sea-ice models show an increase, albeit slightly, of the abyssal MOC under increasing winds. Second, the abyssal MOC significantly strengthens with winds when diapycnal diffusivity is parameterized to be energetically supported by the winds. This tests the emerging idea that a significant fraction of the wind energy input to the large-scale ocean circulation is removed by mesoscale eddies and may then be transferred to internal lee waves, and thence to bottom-enhanced diapycnal mixing. A scaling theory of the abyssal MOC is extended to incorporate this energy pathway, corroborating our numerical results.
Graduate
0415
gstanley@uvic.ca

This dissertation combines mineralogical data and petrographic and field observations with geochemical analysis (major, trace and isotope) to provide new insights into the hydrology and geochemistry of mid-ocean ridge hydrothermal systems. Two study areas were chosen to study two different aspects of hydrothermal circulation: high-temperature on-axis hydrothermal systems were studied using samples from the Hess Deep Rift (Cocos Plate, Equatorial Pacific) and low-temperature off-axis hydrothermal systems were studied at the Troodos Ophiolite in Cyprus. Significant findings include the documentation of a previously unknown warm fluid that pervades the lavas leaching Li from newly formed crust. This finding corroborates a model of broad hydrothermal discharge in the sheeted dikes. In the off-axis low-temperature regime, lateral flow of warm fluid is documented in the lavas, advecting heat from the oceanic lithosphere, with minor geochemical changes to the lavas. The sedimentary cover was found to influence alteration in two ways. The longer an area remains unsedimented allowing the free ingress and egress of seawater, the deeper the enrichment of alkali metals is observed. The maximum enrichment in alkali metals (K, Rb, Cs) however, is similar in both locations. The sedimentary cover can also modify the seawater before it becomes impermeable to fluid flow; early metaliferrous oxide sediments react with seawater, creating a fluid that mobilizes and fractionates the REEs and Y. The fractionation results in negative Ce anomalies, positive Eu anomalies, and negative Y anomalies. Basalts altered under these conditions also lack the ubiquitous Fe-oxides and Fe-oxyhydroxides that are commonly associated with alkali metal uptake. In situ trace element analysis of alteration minerals formed at low-temperature confirmed that secondary phyllosilicates are strongly enriched in alkali metals (K, Rb, Cs and Li), Ba is found in adularia and zeolites, Sr is hosted in carbonates, and no phases were found to be enriched in U. The concentrations of K2O, Rb, Cs (as well as B) are highest in celadonites, whereas Li concentrations are highest in smectites (saponite, Al-saponite, beidellite) and smectite-chlorite mixtures, and much higher than previously reported. Alkalis are also taken up into palagonite, with Li having the highest concentrations, over 1000 ppm in one analysis. Crystal chemical factors were found to be the dominant control on trace element uptake, and for the phyllosilicates no correlation was found between the temperature, age of the crust, texture of the phyllosilicates. In phyllosilicates the K, Rb and Cs are adsorbed as exchange cations, with enrichment (Cs > Rb > K) increasing with decreasing hydration energy, whereas the uptake of Li and B does not correlate with the hydration energy. Lithium concentrations also do not correlate with the Mg content, suggesting substitution of Li for Mg is not the only mechanism of Li uptake into phyllosilicates as has been suggested.
Graduate