This study presents an analysis of observed Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5°N on submonthly to interannual time scales compared to variability characteristics produced by a selection of five high- and low-resolution, synoptically and climatologically forced OGCMs. The focus of the analysis is on the relative contributions of ocean mesoscale eddies and synoptic atmospheric forcing to the overall AMOC variability. Observations used in this study were collected within the framework of the joint U.K.-U.S. Rapid Climate Change (RAPID)-Meridional Overturning Circulation & Heat Flux Array (MOCHA) Program. The RAPID-MOCHA array has now been in place for nearly 6 years, of which 4 years of data (2004-2007) are analyzed in this study. At 26.5°N, the MOC strength measured by the RAPID-MOCHA array is 18.5 Sv. Overall, the models tend to produce a realistic, though slightly underestimated, MOC. With the exception of one of the high-resolution, synoptically forced models, standard deviations of model-produced MOC are lower than the observed standard deviation by 1.5 to 2 Sv. A comparison of the MOC spectra at 26.5°N shows that model variability is weaker than observed variability at periods longer than 100 days. Of the five models investigated in this study, two were selected for a more in-depth examination. One model is forced by a monthly climatology derived from 6-hourly NCEP/NCAR winds (OFES-CLIM), whereas the other is forced by NCEP/NCAR reanalysis daily winds and fluxes (OFES-NCEP). They are identically configured, presenting an opportunity to explain differences in their MOCs by their differences in forcing. Both of these models were produced by the OGCM for the Earth Simulator (OFES), operated by the Japan Agency for Marine-Earth Science & Technology (JAMSTEC). The effects of Ekman transport on the strength, variability, and meridional decorrelation scale are investigated for the OFES models. This study finds that AMOC variance due to Ekman forcing is distributed nearly evenly between the submonthly, intraseasonal, and seasonal period bands. When Ekman forcing is removed, the remaining variance is the result of geostrophic motions. In the intraseasonal period band this geostrophic AMOC variance is dominated by eddy activity, and variance in the submonthly period band is dominated by forced geostrophic motions such as Rossby and Kelvin waves. It is also found that MOC variability is coherent over a meridional distance of ~8° throughout the study region, and that this coherence scale is intrinsic to both Ekman and geostrophic motions. A Monte Carlo-style evaluation of the 27-year-long OFES-NCEP timeseries is used to investigate the ability of a four year MOC strength timeseries to represent the characteristics of lengthier timeseries. It is found that a randomly selected four year timeseries will fall within ~1 Sv of the true mean 95% of the time, but long term trends cannot be accurately calculated from a four year timeseries. Errors in the calculated trend are noticeably reduced for each additional year until the timeseries reaches ~11 years in length. For timeseries longer than 11-years, the trend's 95% confidence interval asymptotes to 2 Sv/decade.

The Meridional Overturning Circulation (MOC) plays an important part in the Earth's climate, but the mechanisms that determine MOC response to climate change remain unclear. In particular, the relative importance of the adiabatic and diabatic dynamics in MOC is still under debate. This study aims to explore the relationship between the air-sea density flux and isopycnal MOC, and examine the possibility of diagnosing the adiabatic component of MOC from the air-sea density flux. This is done here using the concept of the "push-pull" mode, which consists of the adiabatic push into the deep ocean in the Northern Hemisphere and pull out of the deep ocean in the Southern Hemisphere. The evolutions of the isopycnal MOC and the "push-pull" mode are qualitatively similar. The maximum streamfunctions of the "push-pull" modes and isopycnal MOC both decrease by 3-5 Sv during 100 years, and their decrease is very similar to each other in the deep layers. In particular, the slope of the downward linear trend in the maximum is about -5 Sv per 100 years in both the "push-pull" modes and isopycanl MOC at the equator. The decrease in actual isopycnal MOC is faster at heavier densities than that at lighter densities. The first EOF mode of eigenvectors of the "push-pull" mode explains less percentage of variance than in the case of the isopycnal MOC at the equator. The detection of the global changes in MOC from the surface fluxes alone is feasible, if the surface fluxes are measured with sufficient accuracy.

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

The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S / The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S

Dans le cadre des études relatives à la sûreté sismique des installations industrielles, on est amené à se préoccuper de la stabilité de structures libres (des équipements, des containers, des fûts. . . ) posées à même le sol. De nombreuses méthodes ont permis d’établir des critères de sûreté réputés conservatifs, sans qu’il soit besoin de représenter finement le comportement dynamique de l’objet. Dans le cadre de cette thèse, on a cherché à analyser la capacité de prédiction par des modèles numériques du mouvement de corps rigides libres soumis à des séismes impliquant impacts et glissements. Pour cela, on s’est appuyé sur deux campagnes expérimentales qui ont été menées au laboratoire EMSI du CEA/Saclay sur des blocs parallélépipédiques en acier, élancés et disposant de 4 appuis non ponctuels usinés avec des tolérances standards. Dans un premier temps, des essais de lâcher (bloc immobile en appuis sur deux pieds, puis lâché sans source d’excitation extérieure), souvent analysés dans la littérature comme un mouvement plan, ont fait apparaître un mouvement 3D reproductible dans les premiers instants consécutifs au lâcher. L’analyse fine de ce mouvement a permis, d’une part, de conclure qu’il était dû à des défauts de géométrie des pieds et, d’autre part, d’élaborer un modèle numérique représentatif incluant ces défauts. Dans un second temps, il a été question d’étudier l’aptitude du modèle numérique à représenter le comportement dynamique au cours du temps d’un bloc rigide élancé non idéal soumis à des excitations sismiques. Les blocs ont été soumis à 100 réalisations d’un processus stationnaire (essais de variabilité) puis 100 fois à la même accélération (essais de répétabilité). D’un point de vue statistique, et malgré les incertitudes expérimentales, ce travail a permis d’exhiber une bonne adéquation entre les résultats des modèles numériques et les résultats expérimentaux. En outre, il a permis de quantifier la durée au-delà de laquelle une prédiction du comportement ne peut plus être considérée comme pertinente. Pour finir, on s’est attaché à appliquer des outils classiques de fiabilité au problème de bloc rigide soumis à des séismes, ainsi que la méthode récente des Subset Simulations. / In the field of nuclear safety, the stability of free standing structures like containers, barrels or electronical devices is considered to be an important matter. Until now, the literature written on the subject presents some stability criteria known to be conservative without needing to represent in detail the object behavior. This thesis attempts to analyse the capacity numerical models have to predict the behavior of blocks submitted to seismic acceleration, with impacts and friction. To this effect, two experimental campaigns were carried out in the EMSI laboratory (CEA Saclay, France) on slender massive prismatic steel blocks, with 4 machined (i.e. non-ideal) feet. First of all, release tests (the block is in an unsteady position on 2 feet, kept still with a wire, then the wire is cut and the block is released without any ground motion) are usually analysed as a plane motion. Our experiments have shown a reproducible out-of-plane (3D) motion during the first seconds of the release. A detailed analysis highlighted the fact that this 3D motion is induced by geometrical defects on the block feet, and allowed us to build an accurate numerical model of this behavior. The ability of this numerical model to match the dynamic behavior of a non-ideal rigid slender block has been questioned. In a second campaign, 4 blocks were subjected on the one hand to 100 realisations of a stationnary process, and on the other hand 100 times to the same excitation. This accounts for an analysis of the variability of two 100-samples of results obtained under two different input variability levels. From a statistical point of view, despite experimental uncertainties this article demonstrates a good agreement between numerical and experimental results. Finally, some classical tools of reliabily were applied to the rocking block problem, as well as a newer method called Subset Simulation.

京都大学 / 0048 / 新制・課程博士 / 博士(農学) / 甲第21466号 / 農博第2309号 / 新制||農||1064(附属図書館) / 学位論文||H31||N5161(農学部図書室) / 京都大学大学院農学研究科地域環境科学専攻 / (主査)教授 清水 浩, 准教授 中嶋 洋, 教授 飯田 訓久 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

In structural mechanics there are several occasions where a linearized formulation of the original nonlinear problem reduces considerably the computational effort for the response analysis. In a broader sense, a linearized formulation can be viewed as a first-order expansion of the dynamic equilibrium of the system about a `static¿ configuration; yet caution should be exercised when identifying the `correct¿ static configuration. This paper uses as a case study the rocking response of a rigid block stepping on viscoelastic supports, whose non-linear dynamics is the subject of the companion paper, and elaborates on the challenge of identifying the most appropriate static configuration around which a first-order expansion will produce the most dependable results in each regime of motion. For the regime when the heel of the block separates, a revised set of linearized equations is presented, which is an improvement to the unconservative equations published previously in the literature. The associated eigenvalues demonstrate that the characteristics of the foundation do not affect the rocking motion of the block once the heel separates.

In this thesis, I explore the climate system's response to symmetric abrupt and transient CO₂ forcing across a range of concentrations, from ⅛ ⨉ to 8⨉CO₂, relative to pre-industrial levels. I use two CMIP6 class models: the CESM Large Ensemble (CESM-LE) model configuration and the NASA Goddard Institute for Space Studies Model E2.1-G (GISS-E2.1-G). I use a hierarchy of (1) fully coupled atmosphere-ocean-sea-ice-land, (2) slab ocean, and (3) prescribed sea surface temperature simulations to analyze and support the findings. First, I find an asymmetric response in global mean surface air temperature (𝚫𝜯_s) and effective climate sensitivity (EffCS) between colder and warmer experiments. The 𝚫𝜯_s response at 8⨉CO₂ is more than a third larger than the corresponding cooling at ⅛⨉CO₂. I attribute this assymetry primarily due to the non-logarithmic CO₂ forcing, not to changes in the radiative feedbacks. Second, I identify a non-monotonic response of EffCS in the warmer scenarios, with a minimum occurring at 4⨉CO₂ (3⨉CO₂) in CESM-LE (GISS-E2.1-G). This minimum in the warming simulations is associated with a non-monotonicity in the radiative feedback. Similar non-monotonic responses in Northern Hemisphere sea-ice, precipitation, the latitude of zero precipitation-minus-evaporation, and the strength of the Hadley cell are also identified. Comparing the climate response over the same CO₂ range between fully coupled and slab-ocean versions of the same models, I demonstrate that the climate system’s non-monotonic response is linked to changes in ocean dynamics, associated with a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Third, to establish the significance of North Atlantic cooling in driving the non-monotonic changes in the radiative feedback, I conducted additional atmosphere-only (AMIP) simulations using the same models but with prescribed sea surface temperatures (SSTs) restricted to different regions. Through these simulations, I uncovered that the minimum EffCS value, characterized by notably negative radiative feedbacks, primarily originates from relative cooling of the sea surface temperature (SST) in the tropical and subtropical North Atlantic. This cooling of SSTs contributes to an increase in low-level cloud content in the eastern region of the North Atlantic, subsequently leading to a pronounced negative (stabilizing) feedback response. Furthermore, I investigated the state dependence of the effective radiative forcing (ERF) from 1/16 ⨉ to 16⨉CO₂. I found that ERF increases with CO₂ concentration due to the increase in Instantaneous Radiative Forcing (IRF). Specifically, the IRF increases at higher CO₂ values primarily due to stronger stratospheric cooling induced by CO₂ forcing. On the other hand, the radiative adjustments counteract the IRF increase, causing the ERF to rise at a slower pace compared to the corresponding increase in IRF induced by higher CO₂ concentrations. Lastly, I studied the winter storm tracks in the Southern Hemisphere, focusing on experiments up to 8⨉CO₂. Through this analysis, I identified a non-linear response in the low latitude storm tracks. It is projected that the storm tracks will experience an intensification by the end of the century. However, my findings reveal that this intensification does not scale linearly with CO₂ forcing. In fact, the storm tracks shift poleward, including a reduction of the storm tracks at low-mid latitudes and intensification at mid-high latitudes.

The waters of the Indian Ocean southeast of Africa are a crucial junction for surface and deep ocean processes that serve as vital controls on Earth’s climate system. At the surface, the Agulhas Current, its retroflection, and Agulhas Leakage transfer water from the Indian and South Atlantic. The addition of this heat and salt to the Atlantic Basin helps drive the Meridional Overturning Circulation and the formation of deep water in the North Atlantic Ocean. On the timescales of centuries, the Meridional Overturning Circulation ultimately returns this water back to the Indian Ocean in the form of North Atlantic Deep Water. Proxy reconstructions show that the vigor of ocean overturning is immensely important to the global climate system, driving changes in atmospheric CO₂ concentrations and temperature and precipitation patterns across the planet. I use x-ray fluorescence core scanning, sediment provenance techniques, and core images from International Ocean Discovery Program Site U1474, located in the Natal Valley of the southwest Indian Ocean, to investigate past changes in the Agulhas Current and North Atlantic Deep Water. 40K/40Ar provenance ages measured on the clay fraction of sediment from Site U1474 indicate that, despite its great distance from the core site, the Zambezi is the most important factor influencing the deposition of terrigenous sediment in the Natal Valley. We present these results in a quantitative way, reinforcing the conclusions of previous studies. However, a comparison to newly available proxy records influenced by current speed and hydroclimate suggests that the strength of the Agulhas does not have a major influence on terrigenous sediment sources, at least at the headwaters of the Agulhas Current. Instead, I suggest that low-latitude hydrologic processes driven by zonal and meridional temperature gradients in conjunction with sea level are responsible for sediment source variability. In core photos, I found evidence for deep water variability in the Natal Valley in the form of millimeter-to-centimeter scale layers of olive-green sediment. To an overwhelming extent, these layers are formed during glacial periods, especially at their termination. I complement observations at Site U1474 with published proxy data for oxygen concentrations and measurements of total organic carbon percent in the Natal Valley and by extending our search for these green layers to core sites around the world. With these data, it is possible to confidently connect these layers to organic carbon concentrations in the sediment, reduction-oxidation processes in sediments following burial, and the local concentration of dissolved oxygen in the deep water. There are comparable fluctuations in the abundances of green layers in core sites in the path of North Atlantic Deep Water during glacial cycles, where more frequent and more intense green layer formation is driven by higher bottom water oxygen concentrations. Peaks in the abundance of green layers approximately 250 ka and 900 thousand years ago coincide with global scale excursions toward isotopically light benthic carbon isotopes. Connecting the green layers to the release of isotopically light organic carbon from sediments leads me to propose that long-observed fluctuations in the carbon cycle may be attributable to deep ocean oxygenation.

21% av de växthusgaser som släpps ut i Sverige kommer från byggsektorn. Motsvarande siffra för avfall är 40% (Boverket, 2023). Ett fokus måste därför ligga på att använda hållbara alternativ. KL-trä är förnyelsebart samt minskar genom sin höga prefabriceringsgrad avfall på arbetsplatsen (Ramboll, u.å). Trä har många fördelar, men det finns fortfarande en skepsis i byggbranschen när det kommer till större byggnader. Därför är det viktigt att studera och bli bättre på att hantera de svagheter som byggmaterialet har så att tröskeln för att använda det sänks och vi i byggbranschen kan jobba mot ett mer hållbart byggande. Denna studie fokuserar på problematiken stjälpning. En lätt stomme påverkas mer av horisontalkrafter än vad en tyngre gör. Kontroll av statisk jämvikt görs antingen för hand eller med datorhjälp. Finita Elementmetoden, FEM, är en kraftfull metod för att förutsäga hur byggnaden kommer bete sig. Ett problem med FEM-modeller är att de snabbt blir komplexa och svårtolkade. Stelkroppsmetoden är ett sätt att kontrollera statisk jämvikt genom en handberäkning. Byggnaden ses som en sammanhängande stel kropp som roterar kring läsidans nedre kant. Metoden är väldigt förenklad. I verkligheten kan man förvänta sig att byggnaden roterar kring en linje längre in och att det finns en eftergivlighet i väggarna som håller emot. Flytt av denna linje i markplan benämns i rapporten som momentpunkt. Hur mycket av mothållande vägg i lovart som beräknas stabilisera benämns som lastbredd. Syftet med studien är att skapa en bättre förståelse för FEM-modellen samt hur stelkroppsmetoden kan varieras för att skapa en mer verklighetstrogen bild. Detta för att i framtida byggnationer göra det enklare för konstruktören att välja KL-trä som byggmaterial. För att uppnå detta varieras momentpunktens placering och lastbredd av mothållande långsida i en handberäkning med stelkroppsmetoden som grund. Detta resultat jämförs mot en FEM-modell för att sedan se vid vilket värde på momentpunkt och lastbredd som beräkningarna korrelerar. Studien är både explanativ och explorativ och utförs som en fallstudie av jämförande karaktär. Referensobjektet är en sex våningar hög kontorsbyggnad under uppförande i Östersund. Stommen består av ytterväggar i KL-trä samt ett pelar-balksystem av limträ i byggnadens mitt. Resultatet från handberäkningen presenteras i en tabell där det är intressant att undersöka vid vilka kritiska punkter som statisk jämvikt ej infaller. För en momentpunkt som ej är indragen krävs det för denna byggnad en lastbredd på 5 av 29 meter. Flyttas momentpunkten in 4 m så krävs en lastbredd på 13 m. Enligt en icke modifierad stelkroppsmetod erhålls ett mothållande moment på 30 MNm vilket kan jämföras med det stjälpande momentet på 13 MNm. Det resulterande momentet uppgår då till 17 MNm. FEM-modellen korrelerar med handberäkningen med det resulterande momentet på 6,6 MNm då momentpunkten väljs till 4 m. Motsvarande lastbredd blir i handberäkningen 24 m. Momentpunkten 3 m ger en korrelation vid 10,1 MNm. Det ger en motsvarande lastbredd på 27 m. Vid en momentpunkt under 2 m ger FEM konsekvent högre resulterande moment. Någon rotationspunkt kunde ej finnas för FEM-modellen, med följden att den lastbredd och momentpunkt som FEM-modellen motsvarar i en handberäkning förblir obestämd. / 21% of the greenhouse gases emitted in Sweden originate from the building sector. The corresponding figure for waste is 40% (Boverket, 2023). Therefore, the focus must be on using sustainable alternatives. Cross Laminated Timber (CLT) is renewable, and through its high degree of prefabrication, reduces waste at the construction site (Ramboll, u.å). Wood has many advantages, but there is still skepticism in the building sector when it comes to larger buildings. Therefore, it is important to study the building materials weaknesses so that the threshold for using it is lowered. This study focuses on the challenge of overturning. A lightweight structure is more affected by horizontal forces than a heavier one. Checking of static equilibrium is done either by hand or with computer assistance. The Finite Element Method, FEM, is a powerful method used by computers for predicting how a building will behave. A problem with FEM models is that they quickly become complex and difficult to interpret. The rigid body method is a way of checking static equilibrium through calculations done by hand. The building is seen as a coherent rigid body that rotates around the lower edge of the leeward side. This method is very simplified. In reality, one would expect the building to rotate around a line further in, and the stabilizing walls not to be completely rigid. The placement of this line in relation to the origin line is referred to as the rotation line. Load width is defined as the width of the stabilizing wall in the windward direction that is included in the calculation. The study aims to create a better understanding of the FEM model and how the rigid body method can be varied to generate a more realistic model. This is to facilitate CLT as a building material in future constructions. To achieve this, the position of the rotation line and the load width are varied in a calculation done by hand using the rigid body method. This result is compared against a FEM model to analyze where the rotation line and load width correlate in the different models. The study is both explanatory and exploratory and is carried out as a case study of a comparative nature. The reference object is a six-story office building in Östersund. The structure consists of outer walls in CLT and a post and beam system of glulam in the center of the building. The result from the calculation done by hand is shown in a table. Critical values occur where static equilibrium no longer can be verified. For the origin line, a loading width of 5 out of 29 meters are required for this building. If the rotation line is placed 4 m inward, a load width of 13 m is required. According to an unmodified rigid body method, a resisting moment of 30 MNm is obtained, which can be compared with the overturning moment of 13 MNm. The resulting moment totals 17 MNm. The FEM model correlates with the hand calculation with the resulting moment of 6.6 MNm when the rotation line is placed 4 m inward from the origin line. The corresponding load width is 24 m in the calculation done by hand. The rotation line at 3 m gives a correlation at 10.1 MNm. This gives a corresponding load width of 27 m. For rotation lines below 2 m, FEM consistently gives higher resulting moments. The rotation line specific to the FEM model could not be found, with the consequence that the load width and rotation line that the FEM model corresponds to in a calculation done by hand remains undetermined.