http://hdl.handle.net/1956/923 Search the Channel search http://bora.uib.no/simple-search http://hdl.handle.net/1956/2393 Title: On the Nordic Seas’ role in the Atlantic Meridional Overturning Circulation <br/> <br/>Authors: Iovino, Doroteaciro <br/> <br/>Abstract: In the present climate, the North Atlantic thermohaline circulation (THC) plays a fundamental role in the global transport of heat at high latitudes. The response of the North Atlantic Ocean-Nordic Seas THC to surface forcing and basin geometries in an idealized one-hemisphere basin is analyzed to better understand the processes that are fundamental to the modeled circulation. Focusing on the dynamics of the Nordic Seas, analytical and numerical modeling highlight the relevance of a sill (Greenland-Scotland Ridge) in setting the properties of the water masses formed in and exported from a marginal sea. Finally, the influence of the convective activity in the Greenland Sea for the overflow, and thus the overturning, is assessed using hydrographic data (from 1950 to present), a regional ocean model, and a unique tracer release experiment. Greenland Sea Gyre water is estimated to contribute less than 1 Sv, and there is no evidence for causality between changes in the Greenland Sea and the overflow. http://hdl.handle.net/1956/2357 Title: Crude oil components with affinity for gas hydrates in petroleum production <br/> <br/>Authors: Borgund, Anna Elisabet <br/> <br/>Abstract: Some crude oils are believed to contain natural inhibiting components that can prevent hydrate plugging of oil pipelines in petroleum production. A method for classification of the oils that form hydrate plugs, as opposed to those that are not problematical, can change the hydrate inhibiting strategies for oil companies, and result in both economical savings and environmental improvements. Furthermore, an identification of natural hydrate plug inhibiting components can eventually give rise to development of more economical and environmentally friendly inhibitors that can be added to crude oil in pipelines. This thesis addresses the issues of chemical characterisation of crude oil with respect to identification of natural plug inhibiting components. Natural plug inhibiting components are probably hydrate surface active, e.g. an acid fraction has previously been shown to be able to convert a plugging oil into a non-plugging oil when added in a low concentration. In this work, methods for extraction of surface active compounds in crude oil have been established; two methods for acid extraction have been tested (liquid-liquid and ion exchange), and in addition components with affinity for freon hydrate and ice surfaces have been extracted. The extracts have been characterised with chromatographic and spectroscopic methods, e.g. an HPLC method for separation of extracts into four groups have been developed; non-polar compounds, saturated carboxylic acids, phenolic compounds and polyfunctional compounds. The results show that the ion exchange is more effective than the liquidliquid method for extraction of acids from crude oil. Freon hydrates are found to extract a specific fraction with polar compounds from crude oil, while ice does not seem to be a good surface for extraction of components. Compounds which absorb to hydrate surfaces are found to be dominated by saturated carboxylic acids, and to contain lower amounts of phenols and polyfunctional compounds. In general, crude oil extracts primarily contain components of intermediate molecular weights. This means that neither high-molecular compounds such as asphaltenes, nor simple low-molecular petroleum acids and bases are present to a large extent in these extracts. FTIR analysis can to some degree differentiate between freon hydrate extracts from plugging and non-plugging crude oils. GC-MS is not suited for the extracts used in this thesis due to limitations regarding molecular weights and low volatility. LC-MS analysis with Ion Trap MS has been tested at the University of Newcastle, but was not optimal for our samples. The freon hydrate extracts have been analysed for compounds similar to a biosurfactant structure, but no such compounds could be found. No structural identification of the natural inhibiting components have been obtained. However, new methods for extraction of components with affinity for freon hydrate and ice surfaces have been developed and acid extraction methods from the literature have been tested. In addition, a set of analytical methods that can characterise the fractions have been established. One reason for the difficulty of identifying the natural inhibiting components can be that they are present in very low amounts, but it may also be that the distribution of compounds in the fractions is as important as the presence of specific molecules. http://hdl.handle.net/1956/2025 Title: Static and dynamic studies of foam and foam-oil interactions <br/> <br/>Authors: Vikingstad, Anne Kari http://hdl.handle.net/1956/1307 Title: Aspects on interactions between mid- to high lattitude atmospheric circulation and some surface processes <br/> <br/>Authors: Byrkjedal, Øyvind <br/> <br/>Abstract: The Arctic is a hot topic in Climate Research. A large number of signs of a warming Arctic Climate have been identified the latest years. This is of major concern in light of the increasing atmospheric content of greenhouse gases. The climate research community projects future warming of the climate in the high latitudes as a response to increased amounts of anthropogenic release of greenhouse gases since the pre-industrial era. The overall objectives of this work has been to study the mid- and high latitude climate and climate variability, and to evaluate how well some climate processes that contribute to determine the Arctic climate and variability are represented and simulated in climate models. A new data set of storm tracks trajectories and statistics over the Northern Hemisphere for the period 1948-2002 has been developed. The variability of the cyclones extending to the Nordic Seas is studied in particular, and it is found that both the number of storms and their intensity exhibits a strong decadal and interannual variability. The ocean volume transports into and out of the Nordic Seas shows a relatively close relation to the wintertime cyclone intensity and cyclone count. To have confidence in future projections of climate, it is necessary to evaluate how the model behaves in a climate regime different from modern day. To do this two model simulations of the last glacial maximum (LGM) was performed. The reconstructions of sea surface temperatures in the Nordic Seas in LGM differ from perennial sea ice cover to having open ocean during the summer. The large scale atmospheric circulation patterns of the two different climate reconstructions are studied. It is found that the perennial sea ice cover produces a circulation pattern which may be too zonal to support the existence of the large north Eurasian ice sheets. In the case with seasonally open ocean the air masses carries larger amounts of heat and moisture towards the ice sheets and represents a larger degree of meridional circulation. The current general circulation models, including several of those used by the IPCC, show considerable disagreement in simulating present day high latitude climate. This is of major concern and reduces the confidence in future model projections of high latitude climate. To investigate how turbulent vertical exchange processes in the Arctic boundary layer is represented by the climate models a simulation with high vertical resolution in the lower part of the atmosphere is performed. This reveals that the coarse vertical resolution commonly employed in the climate models are unable to reproduce important exchange processes in the Arctic boundary layer. In the case of our model this results in a warm bias over the Arctic Ocean. By increasing the vertical resolution we achieve a better representation of vertical turbulent exchange processes with the result of reproducing more realistic surface fluxes and surface air temperatures. <br/> <br/>Description: This thesis is based on the following papers: Paper I Sorteberg A., Kvamstø N. G. and Byrkjedal Ø. (2005). Wintertime Nordic Seas Cyclone Variability and its Impact on Oceanic Volume Transports Into the Nordic Seas. In: The Nordic Seas, An Integrated Perspective. H. Drange, T. Dokken, T. Furevik, R. Gerdes and W. Berger. Washington DC, American Geophysical Union. 158: 137-156 Paper II Byrkjedal Ø., Kvamstø N. G., Meland M. and Jansen E. (2006). Sensitivity of Last Glacial Maximum Climate to sea-ice condition in the Nordic Seas. Climate Dynamics 26(5): 473-487 Paper III Byrkjedal Ø., Esau I. and Kvamstø N. G. (2006). Sensitivity of simulated wintertime Arctic atmosphere to vertical resolution in the ARPEGE/IFS model. Submitted to Climate Dynamics http://hdl.handle.net/1956/804 Title: The sensitivity of the present day Atlantic meriodinal overturning circulation to freshwater forcing <br/> <br/>Authors: Otterå, Odd Helge; Drange, Helge; Bentsen, Mats; Kvamstø, N. G.; Jiang, Dabang <br/> <br/>Abstract: Mounting evidence indicates that the Atlantic Meridional Overturning Circulation (AMOC) was strongly reduced during cold climate episodes in the past, possible due to freshwater influx from glacial melting. It is also expected that the freshwater input to high northern latitudes will increase as human-induced global warming continues, with potential impacts on the AMOC. Here we present results from a 150 years sensitivity experiment with the Bergen Climate Model (BCM) for the present-day climate, but with enhanced runoff from the Arctic region throughout the integration. The AMOC drops by 30% over the first 50 years, followed by a gradual recovery. The simulated response indicates that the present-day AMOC might be robust to the isolated effect of enhanced, high-latitude freshwater forcing on a centennial time scale, and that the western tropical North Atlantic may provide key information about the long-term variability, and by that monitoring, of the AMOC. http://hdl.handle.net/1956/624 Title: Evaluation of ocean carbon cycle models with data-based metrics <br/> <br/>Authors: Matsumoto, K.; Sarmiento, J. L.; Key, R. M.; Aumont, O.; Bullister, J. L.; Caldeira, Ken; Campin, J. M.; Doney, S. C.; Drange, Helge; Dutay, J. C.; Follows, M.; Gao, Yongqi; Gnanadesikan, A.; Gruber, N.; Ishida, A.; Joos, F.; Lindsay, K.; Maier-Reimer, E.; Marshall, J. C.; Matear, R. J.; Monfray, P.; Mouchet, A.; Najjar, R.; Plattner, G. K.; Schlitzer, R.; Slater, R.; Swathi, P. S.; Totterdell, I. J.; Weirig, M. F.; Yamanaka, Y.; Yool, A.; Orr, J. C. <br/> <br/>Abstract: New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new databased metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model. http://hdl.handle.net/1956/435 Title: Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from denstity overturns <br/> <br/>Authors: Fer, I.; Skogseth, R.; Haugan, Peter Mosby <br/> <br/>Abstract: Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity-temperature-depth profiles and short-term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1-dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, e, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn’s model assuming a constant mixing efficiency. Survey-averaged profiles suggest enhanced mixing near the bottom with values of Kz and e, when averaged within the overflow, equal to 10 x 10-4 m2 s-1 and 3 x 10-8 W kg-1, respectively. Kz is found to decrease with increasing buoyancy frequency as N-1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2p/3600 s-1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream. http://hdl.handle.net/1956/430 Title: Ice shelf water overflow and bottom water formation in the southern Weddell Sea <br/> <br/>Authors: Foldvik, A.; Gammelsrød, T.; Østerhus, Svein; Fahrbach, E.; Rohardt, G.; Schröder, Michael; Nicholls, Keith W.; Padman, L.; Woodgate, R. A. <br/> <br/>Abstract: [1] Cold shelf waters flowing out of the Filchner Depression in the southern Weddell Sea make a significant contribution to the production of Weddell Sea Bottom Water (WSBW), a precursor to Antarctic Bottom Water (AABW). We use all available current meter records from the region to calculate the flux of cold water (<−1.9°C) over the sill at the northern end of the Filchner Depression (1.6 ± 0.5 Sv), and to determine its fate. The estimated fluxes and mixing rates imply a rate of WSBW formation (referenced to −0.8°C) of 4.3 ± 1.4 Sv. We identify three pathways for the cold shelf waters to enter the deep Weddell Sea circulation. One path involves flow constrained to follow the shelf break. The other two paths are down the continental slope, resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36°W and 37°W. Mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m. Probably aided by thermobaricity, the plume water at this depth can flow at a speed approaching 1 m s−1, implying that the flow is occasionally supercritical. We postulate that such supercriticality acts to limit mixing between the plume and its environment. The transition from supercritical to slower, more uniform flow is associated with very efficient mixing, probably as a result of hydraulic jumps.