Ocean circulation is a fundamental component of the Earth's climate system. The Atlantic thermohaline circulation, driven by deep convection at high latitudes, has a central role in regulating European climate through its transport and redistribution of heat. Either directly or indirectly, Mediterranean Water (MW) provides the high salinities found in the Nordic Seas which are required to precondition for deep convection. The precise mechanisms and pathways remain undetermined and yet are fundamental to understanding the effect that any change in the Mediterranean Outflow may have on circulation and ultimately on climate. This thesis investigates the transport of MW in the North Atlantic Ocean using hydrographic and model data. The primary objective is to quantify how much MW flows northwards and how much flows westwards from its source in the Gulf of Cadiz. Two boxes (Med Boxes) were constructed using hydrographic sections in the eastern North Atlantic enclosing the Strait of Gibraltar and Mediterranean Outflow. Inverse methods were used to determine the velocity field from which volume transports and salt fluxes were calculated. The main northward flow of MW across 41°N occurs east of 12°W, at depths of 500 to 1500 m, and transports 50 to 75% of the MW. An advective westward pathway across 20°W transports MW at similar depths into the ocean interior between 35°N and 40°N. Insignificant salt fluxes across the southern section (24°N) at MW depths confirm that no MW crosses the southern Med Box boundary. The net overturning circulation of the Med Box, with inflowing surface waters and outflowing intermediate waters, is attributed both to the exchange at the Strait of Gibraltar and also to water mass transformation associated with entrainment of North Atlantic Water into the Mediterranean Outflow. The magnitude of the circulation provides an estimate of this entrainment of 1.6 ± 0.6 Sv, within the Med Box boundaries. For comparison with these observational estimates, the model used is OCCAM, a level model from the Ocean Circulation and Climate Advance Modelling Project. The primary reason for choosing this model is that experiments have been undertaken with the Strait of Gibraltar both open and closed, enabling a new approach to the investigation of MW transport. MW is identified using the salinity difference between the two model experiments. Although the model circulation (in the open-Strait run) has a similar net overturning to the hydrography, different MW transport pathways are observed. A weak and variable northward pathway along the Iberian Peninsula and a strong southwestward flow from the Gulf of Cadiz result in a higher proportion of MW flowing westwards (61%) than northwards (39%). Seasonal variability is observed, with maximum westward flow in Autumn (70%) and minimum in Spring (50%). There is no evidence in the model that the northward boundary current provides a direct route for Mediterranean salt to reach the Nordic Seas.

<p>The present thesis deals with the ocean circulation from two viewpoints: <i>Pro primo</i>, the dependence of the global thermohaline ocean circulation (THC) on the parameterization of the small-scale vertical mixing processes in the interior of the ocean, and, <i>pro secundo</i>, the dynamics of the circulation in the Nordic Seas. The THC is found be crucially dependent on the parameterization of the small-scale vertical mixing, two types of which have been compared: The commonly used constant diffusivity and a, physically more plausible, stability-dependent parameterization. For constant diffusivity the circulation weakens when the equator-to-pole surface density difference is decreased, consonant with commonly held prejudices. However, for stability-dependent diffusivity the circulation is enhanced. This conclusion has been reached using two investigative techniques, viz. a scale analysis as well as a numerical zonally-averaged and equatorially symmetric THC model. However, if asymmetric flows are considered, the dynamics become more complex to interpret. It has, nevertheless, been concluded that when the degree of asymmetry of the surface-density distribution is taken to be fixed, the response of the circulation to changes of the surface-density distribution corresponds to that from the symmetric investigation.</p><p>The studies of the Nordic Seas are mainly based on satellite-altimetric data providing Sea-Level Anomalies (SLAs). These are utilized to estimate the seasonal cycle as well as the inter-annual variability of the depth-integrated flows. The seasonal cycle is examined using the winter-to-summer difference of the barotropic flow, with focus on the entire region as well as on two sections extending from a common point in the central Norwegian Sea to Svinøy on the Norwegian coast and to the Faroe Islands, respectively. The total barotropic transport is estimated to be around 10 Sv larger during winter than in summer, of which 8 Sv are associated with the barotropic re-circulation gyre in the interior of the Norwegian Sea, the remainder being linked to the Atlantic inflow across the Iceland-Scotland Ridge. The inter-annual variability of the circulation in the Nordic Seas is investigated on the basis of a theoretical analysis permitting independent calculation of the barotropic flow along closed isobaths using SLA data as well as wind data. The barotropic flow based on SLA data is found to co-vary with the flow estimated using wind data.</p>

The present thesis deals with the ocean circulation from two viewpoints: Pro primo, the dependence of the global thermohaline ocean circulation (THC) on the parameterization of the small-scale vertical mixing processes in the interior of the ocean, and, pro secundo, the dynamics of the circulation in the Nordic Seas. The THC is found be crucially dependent on the parameterization of the small-scale vertical mixing, two types of which have been compared: The commonly used constant diffusivity and a, physically more plausible, stability-dependent parameterization. For constant diffusivity the circulation weakens when the equator-to-pole surface density difference is decreased, consonant with commonly held prejudices. However, for stability-dependent diffusivity the circulation is enhanced. This conclusion has been reached using two investigative techniques, viz. a scale analysis as well as a numerical zonally-averaged and equatorially symmetric THC model. However, if asymmetric flows are considered, the dynamics become more complex to interpret. It has, nevertheless, been concluded that when the degree of asymmetry of the surface-density distribution is taken to be fixed, the response of the circulation to changes of the surface-density distribution corresponds to that from the symmetric investigation. The studies of the Nordic Seas are mainly based on satellite-altimetric data providing Sea-Level Anomalies (SLAs). These are utilized to estimate the seasonal cycle as well as the inter-annual variability of the depth-integrated flows. The seasonal cycle is examined using the winter-to-summer difference of the barotropic flow, with focus on the entire region as well as on two sections extending from a common point in the central Norwegian Sea to Svinøy on the Norwegian coast and to the Faroe Islands, respectively. The total barotropic transport is estimated to be around 10 Sv larger during winter than in summer, of which 8 Sv are associated with the barotropic re-circulation gyre in the interior of the Norwegian Sea, the remainder being linked to the Atlantic inflow across the Iceland-Scotland Ridge. The inter-annual variability of the circulation in the Nordic Seas is investigated on the basis of a theoretical analysis permitting independent calculation of the barotropic flow along closed isobaths using SLA data as well as wind data. The barotropic flow based on SLA data is found to co-vary with the flow estimated using wind data.

Made available in DSpace on 2015-03-26T13:50:02Z (GMT). No. of bitstreams: 1 texto completo.pdf: 3607155 bytes, checksum: 0ba931da234b886d1f1b534578664639 (MD5) Previous issue date: 2009-07-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The global thermohaline circulation (THC) is the transport of ocean water masses caused by differences in the sea water density due to variations in temperature and salinity. Studies have shown that increased precipitation in high latitudes of the Northern Hemisphere and the sea ice melting of the Arctic region generates a freshwater flow of in the North Atlantic which is able to shutoff the formation of deep water and hence desintensifies the THC. Considering the above, this study investigates the anomalous pattern of the oceanic and atmospheric circulation by introducing an increase of 1 Sverdrup (Sv) (1 Sv = 106m3s-1) of freshwater into the North Atlantic, based on simulations performed with the LOVECLIM model. The results show that the weakening of the THC leads to strong cooling in the North Atlantic region whereas the extratropical Southern Hemisphere warms. The weakening of the THC also changes the patterns of atmospheric circulation, inducing a reduction in the subtropical jet speed due to the smooth thermal gradient between the equator and the southern hemisphere polar region. Furthermore, the intertropical convergence zone moves southwards and changes the precipitation regime of the north and the northeastern part of Brazil. The reduction of the THC also leads to changes in the baroclinic instability in the middle and high latitudes of the Southern Hemisphere. / A Circulação Termohalina Global (CTG) consiste no transporte de massas d água oceânicas associado a diferenças na densidade da água do mar devido a variações de temperatura e salinidade. Estudos têm demonstrado que o aumento da precipitação em altas latitudes do Hemisfério Norte e o derretimento do gelo da região do Ártico podem gerar um fluxo de água doce no Oceano Atlântico Norte, capaz de interromper a formação de água profunda e, conseqüentemente, reduzir a CTG. Diante do exposto, o objetivo do trabalho é investigar o comportamento anômalo das circulações oceânica e atmosférica devido a um aumento de 1 Sverdrup (Sv) (1 Sv = 106m3s-1) no transporte de água doce no Atlântico Norte, com base em simulações realizadas com um modelo climático acoplado (LOVECLIM). Os resultados demonstram que o enfraquecimento da CTG provoca um forte resfriamento no Atlântico Norte enquanto que a região extratropical do Hemisfério Sul aquece. A inibição da CTG também muda os padrões da circulação atmosférica, se observa uma redução na corrente de jato subtropical devido o menor gradiente térmico entre o equador e a região polar austral. Além disso, a zona de convergência intertropical desloca-se para sul alterando o regime de precipitação das regiões norte e nordeste do Brasil. Por outro lado existe um enfraquecimento da instabilidade baroclínica nas latitudes médias e altas do Hemisfério Sul.

Due to its relevance for global climate, the realistic representation of the Atlantic meridional overturning circulation (AMOC) in ocean models is a key task.<br> In recent years, two paradigms have evolved around what are its driving mechanisms: diapycnal mixing and Southern Ocean winds. This work aims at clarifying what sets the strength of the Atlantic overturning components in an ocean general circulation model and discusses the role of spatially inhomogeneous mixing, numerical diffusion and winds. Furthermore, the relation of the AMOC with a key quantity, the meridional pressure difference is analyzed. <br><br> Due to the application of a very low diffusive tracer advection scheme, a realistic Atlantic overturning circulation can be obtained that is purely wind driven.<br> On top of the winddriven circulation, changes of density gradients are caused by increasing the parameterized eddy diffusion in the North Atlantic and Southern Ocean. The linear relation between the maximum of the Atlantic overturning and the meridional pressure difference found in previous studies is confirmed and it is shown to be due to one significant pressure gradient between the average pressure over high latitude deep water formation regions and a relatively uniform pressure between 30°N and 30°S, which can directly be related to a zonal flow through geostrophy. Under constant Southern Ocean windstress forcing, a South Atlantic outflow in the range of 6-16 Sv is obtained for a large variety of experiments. Overall, the circulation is winddriven but its strength not uniquely determined by the Southern Ocean windstress. <br><br> The scaling of the Atlantic overturning components is linear with the background vertical diffusivity, not confirming the 2/3 power law for one-hemisphere models without wind forcing. The pycnocline depth is constant in the coarse resolution model with large vertical grid extends. It suggests the ocean model operates like the Stommel box model with a linear relation of the pressure difference and fixed vertical scale for the volume transport. However, this seems only valid for vertical diffusivities smaller 0.4 cm²/s, when the dominant upwelling within the Atlantic occurs along the boundaries. For larger vertical diffusivities, a significant amount of interior upwelling occurs. It is further shown that any localized vertical mixing in the deep to bottom ocean cannot drive an Atlantic overturning. However, enhanced boundary mixing at thermocline depths is potentially important. <br><br> The numerical diffusion is shown to have a large impact on the representation of the Atlantic overturning in the model. While the horizontal numerical diffusion tends to destabilize the Atlantic overturning the verital numerical diffusion denotes an amplifying mechanism. / Wegen ihrer Bedeutung für das globale Klima ist die realistische Darstellung des Atlantischen meridionalen overturnings in Ozeanmodellen eine zentrale Aufgabe.<br> In den letzten Jahren haben sich zwei verschiedene Hypothesen darüber entwickelt, was diese Zirkulation antreibt: diapyknische Vermischung und Winde im südlichen Ozean.<br> Die vorliegende Arbeit zielt darauf aufzuklären, welche Rolle eine räumlich inhomogene Verteilung der Vermischung, die numerische Diffusion und Winde beim Bestimmen der Stärke des Atlantischen overturnings spielen. Ausserdem wird die Beziehung des Atlantischen overturnings zu meridionalen Druckgradienten untersucht. <br><br> Durch Anwenden eines sehr gering diffusiven Tracer-Advektionsschemas kann eine realistische Zirkulation erzeugt werden, die rein von den Winden im südlichen Ozean getrieben wird. Ausgehend von der windgetriebenen Zirkulation werden Änderungen der Dichtegradienten durch Verstärkung der parametrisierten Eddy Diffusion im Nordatlantik und südlichen Ozean hervorgerufen. Dadurch wird das Bild einer vom Wind bestimmten Zirkulation in der letztendlich Druckgradienten nicht ausschlaggebend sein würden, modifiziert. Das lineare Verhältnis zwischen dem Maximum des Atlantischen overturnings und dem meridionalen Druckgradienten wird bestätigt und erklärt. Diese Linearität ist auf einen signifikanten Druckgradienten zwischen den Tiefenwasserbildungsgebieten und einem zwischen 30°N and 30°S homogenen Druck zurückzuführen. Der Volumentransport bei 30°S variiert über eine Bandbreite von 10 Sv für verschiedene Experimente unter konstantem Wind über dem südlichen Ozean. Zusammenfassend ist die Zirkulation zwar windgetrieben aber ihre Stärke nicht allein vom Wind bestimmt. <br><br> Die Skalierung des Atlantischen overturnings ist linear mit vertikaler Vermischung, was die Skalierung mit einem Exponenten von 2/3 in ein-hemisphärischen Modellen ohne Wind-Antrieb nicht bestätigt. Die Tiefe der Pyknokline bleibt mit der groben vertikalen Auflösung konstant. Die Ergebnisse deuten darauf hin, dass das Ozeanmodell sich wie das Stommel-Box Modell verhält mit einer linearen Beziehung zum meridionalen Druckgradienten und einer festen vertikalen Skala für den Volumentransport. Das scheint jedoch nur für Diffusivitäten kleiner als 0.4 cm²/s zu gelten, wenn das Aufsteigen im Atlantischen Ozean bevorzugt an den Ozeanrändern statt findet. <br><br> In Bezug auf den Antrieb des Atlantischen overturnings wird gezeigt, dass vertikale Vermischung in der Nähe des Ozeanbodens keinen Einfluss hat. Verstärkte vertikale Vermischung an den Ozeanrändern in der Tiefe der Thermokline jedoch ist potentiell wichtig. <br><br> Die numerische Diffusion hat einen grossen Einfluss auf das Atlantische overturning im Modell. Während die horizontale numerische Diffusion das overturning eher zu destabilisieren tendiert, bewirkt die vertikale numerische Diffusion einen Verstärkungsmechanismus.

L'Eau Antarctique de Fond constitue la principale masse d'eau océanique par son volume, et nourrit la composante la plus profonde et la plus lente de la circulation océanique. Les processus qui régissent son cycle de vie sont donc clé pour la capacité de stockage de l'océan en carbone et chaleur aux échelles centennales à multi-millénaires. Cette thèse tente de caractériser et quantifier les principaux processus responsables de la destruction (synonyme d'allègement et de remontée) de l'Eau Antarctique de Fond dans l'océan abyssal. A partir d'une estimée issue d'observations de la structure thermohaline de l'océan mondial et de diagnostics fondés sur le budget de densité des eaux profondes, les rôles respectifs du chauffage géothermal, du mélange turbulent par déferlement d'ondes internes et de la géométrie des bassins sont évalués. Il est montré que la géométrie de l'océan gouverne la structure de la circulation de l'Eau Antarctique de Fond. La contribution du déferlement des ondes internes, bien que mal contrainte, est estimée insuffisante pour maintenir un rythme de destruction de l'Eau Antarctique de Fond comparable à celui de sa formation. Le chauffage géothermal a quant à lui un rôle important pour la remontée des eaux recouvrant une large surface du lit océanique. Les résultats suggèrent une réévaluation de l'importance du mélange au niveau des détroits et seuils profonds, mais aussi du rôle fondamental de la forme des bassins, pour l'allègement et le transport des eaux abyssales. / Antarctic Bottom Water is the most voluminous water mass of the World Ocean, and it feeds the deepest and slowest component of ocean circulation. The processes that govern its lifecycle are therefore key to the ocean's carbon and heat storage capacity on centennial to multi-millennial timescales. This thesis aims at characterizing and quantifying processes responsible for the destruction (synonymous of lightening and upwelling) of Antarctic Bottom Water in the abyssal ocean. Using an observational estimate of the global ocean thermohaline structure and diagnostics based on the density budget of deep waters, we explore the roles of basin geometry, geothermal heating and mixing by breaking internal waves for the abyssal circulation. We show that the shape of ocean basins largely controls the structure of abyssal upwelling. The contribution of mixing powered by breaking internal waves, though poorly constrained, is estimated to be insufficient to destroy Antarctic Bottom Water at a rate comparable to that of its formation. Geothermal heating plays an important role for the upwelling of waters covering large seafloor areas. The results suggest a reappraisal of the role of mixing in deep straits and sills, but also of the fundamental role of basin geometry, for the lightening and transport of abyssal waters.

Erkenntnisse aus paläoklimatologischen Studien, theoretischen Betrachtungen und Modellsimulationen deuten darauf hin, dass anthropogene Emissionen von Treibhausgasen und Aerosolen zu großskaligen, singulären Klimaereignissen führen könnten. Diese bezeichnen stark nichtlineare, abrupte Klimaänderungen, mit regionalen bis hin zu globalen Auswirkungen. Ziel dieser Arbeit ist die Entwicklung von Modellen zweier maßgeblicher Komponenten des Klimasystems, die singuläres Verhalten aufweisen könnten: die atlantische thermohaline Zirkulation (THC) und der indische Monsun. Diese Modelle sind so konzipiert, dass sie den Anforderungen der "Integrated Assessment"-Modellierung genügen, d.h., sie sind realistisch, recheneffizient, transparent und flexibel. <br /> <br /> Das THC-Modell ist ein einfaches, interhemisphärisches Boxmodell, das anhand von Daten kalibriert wird, die mit einem gekoppelten Klimamodell mittlerer Komplexität erzeugt wurden. Das Modell wird durch die globale Mitteltemperatur angetrieben, die mit Hilfe eines linearen Downscaling-Verfahrens in regionale Wärme- und Süßwasserflüsse übersetzt wird. Die Ergebnisse einer Vielzahl von zeitabhängigen Simulationen zeigen, dass das Modell in der Lage ist, maßgebliche Eigenschaften des Verhaltens komplexer Klimamodelle wiederzugeben, wie die Sensitivität bezüglich des Ausmaßes, der regionalen Verteilung und der Rate der Klimaänderung. <br /> <br /> Der indische Monsun wird anhand eines neuartigen eindimensionalen Boxmodells der tropischen Atmosphäre beschrieben. Dieses enthält Parmetrisierungen der Oberflächen- und Strahlungsflüsse, des hydrologischen Kreislaufs und derHydrologie der Landoberfläche. Trotz des hohen Idealisierungsgrades ist das Modell in der Lage, relevante Aspekte der beobachteten Monsundynamik, wie z.B. den Jahresgang des Niederschlags und das Eintritts- sowie Rückzugsdatum des Sommermonsuns, zufrieden stellend zu simulieren. Außerdem erfasst das Modell die Sensitivitätdes Monsuns bezüglich Änderungen der Treibhausgas- und Aerosolkonzentrationen, die aus komplexeren Modellen bekannt sind. <br /> <br /> Eine vereinfachte Version des Monsunmodells wird für die Untersuchung des qualitativen Systemverhaltens in Abhängigkeit von Änderungen der Randbedingungen eingesetzt. Das bemerkenswerteste Ergebnis ist das Auftreten einer Sattelknotenbifurkation des Sommermonsuns für kritische Werte der Albedo oder der Sonneneinstrahlung. Darüber hinaus weist das Modell zwei stabile Zustände auf: neben dem niederschlagsreichen Sommermonsun besteht ein Zustand, der sich durch einen schwachen hydrologischen Kreislauf auszeichnet. Das Beachtliche an diesen Ergebnissen ist, dass anthropogene Störungen der plantetaren Albedo, wie Schwefelemissionen und/oder Landnutzungsänderungen, zu einer Destabilisierung des indischen Monsuns führen könnten. <br /> <br /> Das THC-Boxmodell findet exemplarische Anwendung in einem "Integrated Assessment" von Klimaschutzstrategien. Basierend auf dem konzeptionellen und methodischen Gerüst des Leitplankenansatzes werden Emissionskorridore (d.h. zulässige Spannen an CO2-Emissionen) berechnet, die das Risiko eines THC-Zusammenbruchs begrenzen sowie sozioökonomische Randbedingungen berücksichtigen. Die Ergebnisse zeigen u.a. eine starke Abhängigkeit der Breite der Emissionskorridore von der Klima- und hydrologischen Sensitivität. Für kleine Werte einer oder beider Sensitivitäten liegt der obere Korridorrand bei weit höheren Emissionswerten als jene, die von plausiblen Emissionsszenarien für das 21. Jahrhundert erreicht werden. Für große Werte der Sensitivitäten hingegen, verlassen schon niedrige Emissionsszenarien den Korridor in den frühen Jahrzehnten des 21. Jahrhunderts. Dies impliziert eine Abkehr von den gegenwärtigen Emissionstrends innherhalb der kommenden Jahrzehnte, wenn das Risko eines THC Zusammenbruchs gering gehalten werden soll. <br /> <br /> Anhand einer Vielzahl von Anwendungen - von Sensitivitäts- über Bifurkationsanalysen hin zu integrierter Modellierung - zeigt diese Arbeit den Wert reduzierter Modelle auf. Die Ergebnisse und die daraus zu ziehenden Schlussfolgerungen liefern einen wertvollen Beitrag zu der wissenschaftlichen und politischen Diskussion bezüglich der Folgen des anthropogenen Klimawandels und der langfristigen Klimaschutzziele. / Concerns have been raised that anthropogenic climate change could lead to large-scale singular climate events, i.e., abrupt nonlinear climate changes with repercussions on regional to global scales. One central goal of this thesis is the development of models of two representative components of the climate system that could exhibit singular behavior: the Atlantic thermohaline circulation (THC) and the Indian monsoon. These models are conceived so as to fulfill the main requirements of integrated assessment modeling, i.e., reliability, computational efficiency, transparency and flexibility. <br /> <br /> The model of the THC is an interhemispheric four-box model calibrated against data generated with a coupled climate model of intermediate complexity. It is designed to be driven by global mean temperature change which is translated into regional fluxes of heat and freshwater through a linear down-scaling procedure. Results of a large number of transient climate change simulations indicate that the reduced-form THC model is able to emulate key features of the behavior of comprehensive climate models such as the sensitivity of the THC to the amount, regional distribution and rate of change in the heat and freshwater fluxes. <br /> <br /> The Indian monsoon is described by a novel one-dimensional box model of the tropical atmosphere. It includes representations of the radiative and surface fluxes, the hydrological cycle and surface hydrology. Despite its high degree of idealization, the model satisfactorily captures relevant aspects of the observed monsoon dynamics, such as the annual course of precipitation and the onset and withdrawal of the summer monsoon. Also, the model exhibits the sensitivity to changes in greenhouse gas and sulfate aerosol concentrations that are known from comprehensive models. <br /> <br /> A simplified version of the monsoon model is employed for the identification of changes in the qualitative system behavior against changes in boundary conditions. The most notable result is that under summer conditions a saddle-node bifurcation occurs at critical values of the planetary albedo or insolation. Furthermore, the system exhibits two stable equilibria: besides the wet summer monsoon, a stable state exists which is characterized by a weak hydrological cycle. These results are remarkable insofar, as they indicate that anthropogenic perturbations of the planetary albedo such as sulfur emissions and/or land-use changes could destabilize the Indian summer monsoon. <br /> <br /> The reduced-form THC model is employed in an exemplary integrated assessment application. Drawing on the conceptual and methodological framework of the tolerable windows approach, emissions corridors (i.e., admissible ranges of CO2- emissions) are derived that limit the risk of a THC collapse while considering expectations about the socio-economically acceptable pace of emissions reductions. Results indicate, for example, a large dependency of the width of the emissions corridor on climate and hydrological sensitivity: for low values of climate and/or hydrological sensitivity, the corridor boundaries are far from being transgressed by any plausible emissions scenario for the 21st century. In contrast, for high values of both quantities low non-intervention scenarios leave the corridor already in the early decades of the 21st century. This implies that if the risk of a THC collapse is to be kept low, business-as-usual paths would need to be abandoned within the next two decades. <br /> <br /> All in all, this thesis highlights the value of reduced-form modeling by presenting a number of applications of this class of models, ranging from sensitivity and bifurcation analysis to integrated assessment. The results achieved and conclusions drawn provide a useful contribution to the scientific and policy debate about the consequences of anthropogenic climate change and the long-term goals of climate protection. <br><br>--- <br> Anmerkung:<br> Die Autorin ist Trägerin des von der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Potsdam vergebenen Michelson-Preises für die beste Promotion des Jahres 2003/2004.

Two long-lived halogen radionuclides (36Cl, T1/2 = 301 kyr, and 129I, T1/2 = 15.7 Myr) have been studied by means of Accelerator Mass Spectrometry (AMS) at the Uppsala Tandem Laboratory. The 36Cl measurements in natural samples using a medium-sized tandem accelerator (~1 MeV/amu) have been considered. A gas-filled magnetic spectrometer (GFM) was proposed for the separation of 36Cl from its isobar, 36S. Semi-empirical Monte-Carlo ion optical calculations were conducted to define optimal conditions for separating 36Cl and 36S. A 180° GFM was constructed and installed at the dedicated AMS beam line. 129I has been measured in waters from the Arctic and North Atlantic Oceans. Most of the 129I currently present in the Earth's surface environment can be traced back to liquid and gaseous releases from the nuclear reprocessing facilities at Sellafield (UK) and La Hague (France). The anthropogenic 129I inventory in the central Arctic Ocean was found to increase proportionally to the integrated 129I releases from these reprocessing facilities. The interaction and origin of water masses in the region have been clearly distinguished with the help of 129I labeling. Predictions based on a compartment model calculation showed that the Atlantic Ocean and deep Arctic Ocean are the major sinks for the reprocessed 129I. The variability in 129I concentration measured in seawater along a transect from the Baltic Sea to the North Atlantic suggests strong enrichment in the Skagerrak–Kattegat basin. The 129I inventory in the Baltic and Bothnian Seas is equal to ~0.3% of the total liquid releases from the reprocessing facilities. A lake sediment core sampled in northeastern Ireland was analyzed for 129I to study the history of the Sellafield releases, in particular the nuclear accident of 1957. High 129I concentration was observed corresponding to 1990 and later, while no indication of the accident was found. The results of this thesis research clearly demonstrate the uniqueness and future potential of 129I as a tracer of processes in both marine and continental archives.

Generalized linear stability theory is used to calculate the optimal initial conditions that result in transient amplification of the thermohaline circulation (THC) in a zonally-averaged single basin ocean model. The eigenmodes of the tangent linear model verify that the system is asymptotically stable but the nonnormality of the system permits the growth of perturbations for a finite period through the interference of nonorthogonal eigenmodes. It is found that the maximum amplification of the THC anomalies occurs after 6 years with both the thermally driven and salinity driven components playing major roles in the amplification process. The transient amplification of THC anomalies is due to the constructive and destructive interference of a large number of eigenmodes and the evolution over time is determined by how the interference pattern evolves. It is found that five of the most highly nonnormal eigenmodes are critical to the initial cancellation of the salinity and temperature contributions to the THC while 11 oscillating modes with decay timescales ranging from 2 to 6 years are the major contributors at the time of maximum amplification. This analysis demonstrates that the different dynamics of salinity and temperature anomalies allows the dramatic growth of perturbations to the THC on relatively short (interannual to decadal) timescales. In addition the ideas of generalized stability theory are used to calculate the stochastic optimals which are the spatial patterns of stochastic forcing that are most efficient at generating variance growth in the THC. It is found that the optimal stochastic forcing occurs at high latitudes and induces low-frequency THC variability by exciting the salinity-dominated modes of the THC. The first stochastic optimal is found to have its largest projection on the same five highly nonnormal eigenmodes found to be critical to the structure of the optimal initial conditions. The model’s response to stochastic forcing is not controlled by the least damped eigenmodes of the tangent linear model but rather by the linear interference of these highly nonnormal eigenmodes. The process of pseudoresonance suggests that the nonnormal eigenmodes are excited and sustained by stochastically induced perturbations which in turn lead to maximum THC variance. Finally, it was shown that the addition of wind stress did not have a large impact on the nonnormal dynamics of the linearised system. Adding wind allowed the value of the vertical diffusivity to be reduced to achieve the same maximum linearised THC amplitude as was used in the case with no wind stress.

Generalized linear stability theory is used to calculate the optimal initial conditions that result in transient amplification of the thermohaline circulation (THC) in a zonally-averaged single basin ocean model. The eigenmodes of the tangent linear model verify that the system is asymptotically stable but the nonnormality of the system permits the growth of perturbations for a finite period through the interference of nonorthogonal eigenmodes. It is found that the maximum amplification of the THC anomalies occurs after 6 years with both the thermally driven and salinity driven components playing major roles in the amplification process. The transient amplification of THC anomalies is due to the constructive and destructive interference of a large number of eigenmodes and the evolution over time is determined by how the interference pattern evolves. It is found that five of the most highly nonnormal eigenmodes are critical to the initial cancellation of the salinity and temperature contributions to the THC while 11 oscillating modes with decay timescales ranging from 2 to 6 years are the major contributors at the time of maximum amplification. This analysis demonstrates that the different dynamics of salinity and temperature anomalies allows the dramatic growth of perturbations to the THC on relatively short (interannual to decadal) timescales. In addition the ideas of generalized stability theory are used to calculate the stochastic optimals which are the spatial patterns of stochastic forcing that are most efficient at generating variance growth in the THC. It is found that the optimal stochastic forcing occurs at high latitudes and induces low-frequency THC variability by exciting the salinity-dominated modes of the THC. The first stochastic optimal is found to have its largest projection on the same five highly nonnormal eigenmodes found to be critical to the structure of the optimal initial conditions. The model’s response to stochastic forcing is not controlled by the least damped eigenmodes of the tangent linear model but rather by the linear interference of these highly nonnormal eigenmodes. The process of pseudoresonance suggests that the nonnormal eigenmodes are excited and sustained by stochastically induced perturbations which in turn lead to maximum THC variance. Finally, it was shown that the addition of wind stress did not have a large impact on the nonnormal dynamics of the linearised system. Adding wind allowed the value of the vertical diffusivity to be reduced to achieve the same maximum linearised THC amplitude as was used in the case with no wind stress.