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1
Butler, Adam. "Statistical modelling of synthetic oceanographic extremes." Thesis, Lancaster University, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.430015.
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Rojas, Mendoza Jorge Enrique. "Modelling and essential control of an oceanographic monitoring remotely operated underwater vehicle." Master's thesis, Pontificia Universidad Católica del Perú, 2017. http://tesis.pucp.edu.pe/repositorio/handle/123456789/9516.
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Ocean pollution and contamination of the water are serious problems because of its rapid
increase and spread, having a negative effect on people, animals and the environment.
Due to this, new technologies to monitor and measure environmental parameters are
being developed. Remotely Operated Underwater Vehicles (ROVs) have become a
commonly used robotic platform in oceanographic monitoring and analysis. The ROVBWSTI,
designed by Fraunhofer IOSB-AST Institute, is an underwater modular vehicle
capable of fulfilling numerous tasks, especially in the area of environmental sensoring.
The motion of the ROV is commanded through a joypad controller, and functional
requirements of autonomy are not implemented yet.
Motivated by this fact, this master thesis focuses on the modelling of the dynamics of
the remotely operated vehicle, considering its motion, existing ocean currents, effects
of gravitation and buoyancy. Moreover, the concrete effect of the thrusters on the
ROV is analysed and identified. Furthermore, the detailed identification of the dynamic
and hydrodynamic parameters required in the model is considered, based on empirical
estimations, computational methods and experimental tests. The obtained approach is
simulated and optimized, using real motion trials as a reference. After the successful
modelling, the design of an essential control system that includes set-point regulation
and waypoint tracking is performed and simulated.
As a result, it obtains an accurate dynamic model of the remotely operated vehicle that
was successfully simulated and compared with real motion tests. On the other hand,
the proposed control system applied to the model adequately achieves its purpose of
regulation and way point tracking that allows the autonomy of the vehicle.Tesis
3
Jenter, Harry Leonard. "Modelling bottom stress in depth-averaged flows." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/58501.
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Thesis (Ph. D.)--Joint Program in Oceanographic Engineering (Massachusetts Institute of Technology, Dept. of Civil Engineering; and the Woods Hole Oceanographic Institution), 1989.Includes bibliographical references (leaves 140-145).
The relationship between depth-averaged velocity and bottom stress for wind-driven flow in unstratified coastal waters is examined here. The adequacy of traditional linear and quadratic drag laws is addressed by comparison with a 2 1/2-D model. A 2 1/2-D model is one in which a simplified 1-D depth-resolving model (DRM) is used to provide an estimate of the relationship between the flow and bottom stress at each grid point of a depth-averaged model (DAM). Bottom stress information is passed from the DRM to the DAM in the form of drag tensor with two components: one which scales the flow and one which rotates it. This eliminates the problem of traditional drag laws requiring the flow and bottom stress to be collinear. In addition, the drag tensor field is updated periodically so that the relationship between the velocity and bottom stress can be time-dependent. However, simplifications in the 2 1/2-D model that render it computationally efficient also impose restrictions on the time-scale of resolvable processes. Basically, they must be much longer than the vertical diffusion time scale. Four progressively more complicated scenarios are investigated. The important factors governing the importance of bottom friction in each are found to be 1) non-dimensional surface Ekman depth ... is the surface shear velocity, f is the Coriolis parameter and h is the water depth 2) the non-dimensional bottom roughness, zo/h where zo is the roughness length and 3) the angle between the wind stress and the shoreline. Each has significant influence on the drag law. The drag tensor magnitude, r, and the drag tensor angle, 0 are functions of all three, while a drag tensor which scales with the square of the depth-averaged velocity has a magnitude, Cd, that only depends on zo/h. The choice of drag law is found to significantly affect the response of a domain. Spin up times and phase relationships vary between models. In general, the 2 1/2-D model responds more quickly than either a constant r or constant Cd model. Steady-state responses are also affected. The two most significant results are that failure to account for 0 in the drag law sometimes leads to substantial errors in estimating the sea surface height and to extremely poor resolution of cross-shore bottom stress. The latter implies that cross-shore near-bottom transport is essentially neglected by traditional DAMs.
by Harry Leonard Jenter, II.
Ph.D.
4
Sandery, Paul Anthony, and paul sandery@flinders edu au. "Seasonal Variability of Water Mass Properties in Bass Strait: Three-dimensional oceanographic modelling studies." Flinders University. Chemistry, Physics and Earth Sciences, 2007. http://catalogue.flinders.edu.au./local/adt/public/adt-SFU20070831.093503.
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The climatology of the seasonal cycle of water mass variation and transformation in Bass Strait, south-eastern Australia, is studied using a high resolution three-dimensional sigma-coordinate hydrodynamic model coupled with data from observations and previous studies.
Model forcing consists of the principal tidal constituents from the Australian National Tidal Centre and long-term monthly mean atmospheric forcing fields from NCEP reanalysis. The initial density field is established using temperature and salinity means and annual and semi-annual harmonics from the CARS2000 hydrographic atlas. This is also used to prescribe incoming water mass properties at model open-sea boundaries with seasonal variation. Far-field forcing is included with open-sea boundary parameterisation of residual sea-level representing both the South Australian Current and the East Australian Current. Lagrangian and Eulerian tracer methods are used to derive transport timescales, such as age, residence times and flushing times. These are used to examine and summarise model predictions and as a diagnostic tool in sensitivity studies.
Currents, sea-level and water mass properties in the model compare favourably with previous studies and observations, despite limitations in the model and in the data used for comparison. The seasonal cycle, in model results, is characterised by formation of a shallow (< 20 m) saltier surface-layer in late spring to summer and subsequent downward mixing and erosion of the salinity field in autumn to winter with water mass from the west. This leaves behind water mass with positive age and salinity anomalies in areas of low flushing. In late winter-early spring most parts of this water mass leave the Strait interior. These areas are thought to be related to the source water of the Bass Strait Cascade. The residual circulation in all model experiments is shown to be related to seasonal-mean sea-level anomalies, arising from both barotropic and baroclinic adjustment, both in and surrounding the Strait.
5
Shannon, Lynne Jane. "Modelling the oceanographic transport of young Cape anchovy Engraulis capensis by advective processes off South Africa." Master's thesis, University of Cape Town, 1995. http://hdl.handle.net/11427/21784.
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Bibliography: pages 107-121.A Monte-Carlo type model has been developed to investigate the importance of passive transport by currents above the thermocline for anchovy recruitment off South Africa. Simulation studies indicate that mean year-class strength of Cape anchovy is relatively robust to altered advective processes off South Africa. This occurs despite the fact that changed flow alters the likelihood of offshore advection and hence losses of anchovy from the system. Two different approaches have been taken to address the effects of altered advection, and the applicability of each is discussed. One approach involves altering westward advection in proportion to the mean current field (derived from Acoustic Doppler Current Profiler measurements), and the other, altering westward and northward advection by the addition of fixed offshore current velocities. The proportional approach did not affect year-class strength significantly, whereas the other approach, which incorporated large changes in the flow field, yielded statistically significant differences in predicted year-class strengths between advection scenarios. Reduced flow in the latter approach led to a mean year-class strength 2.7 times stronger than a proposed base flow scenario (which incorporated westward and northward drift in addition to the ADCP currents), whereas enhanced flow resulted in a mean year-class strength of similar magnitude to that of the base flow scenario. Changed flow may alter the geographic distribution of eggs and larvae, which might in turn influence recruitment of young-of-the-year anchovy to the South African purse-seine fishery. The north-flowing shelf-edge jet current plays an important role in transporting anchovy eggs and larvae from spawning grounds in the south to nursery areas frn1her north along the west coast of South Africa. Enhanced model advection westward and norward from the spawning grounds in the south serves to transport anchovy into the region of the jet current. However, advection into unproductive waters offshore is also enhanced and prevents good recruitment under these flow conditions. On the other hand, reduced westward and northward advection in the model, shown through wind records to characterise El-Nino years in coastal areas of South Africa, serves to retain anchovy reproductive products and often transports young anchovy into coastal areas, preventing offshore loss. Therefore the advection model suggests that good year-class strengths (in terms of numbers) are likely to be supported in years when westward and northward advection are reduced. A further reduction in westward advection may be less favourable by causing advective losses offshore along the south coast of South Africa. This may be viewed in terms of an "optimal environmental window" hypothesis, where reduced westward advection is favourable for anchovy survival off South Africa, but further reduction of westward advection as well as enhanced westward advection appear unfavourable. It is concluded that although passive transport, of anchovy in South African waters is relatively robust, it may account for a substantial proportion of recruitment variability.
6
Botella, Juan 1967. "Mesoscale variability and mean flow interaction near the Gulf Stream as seen by satellite altimetry and numerical modelling." Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/39410.
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Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2001.Includes bibliographical references (p. 115-120).
The interaction between the eddy field and the mean flow near the Gulf Stream is studied here using satellite altimeter measurements and an eddy resolving numerical model. The eddy vorticity flux in the quasigeostrophic framework is obtained from the stream function standard deviation and spatial correlation function assuming the correlation function is homogeneous. An analytical expression is found for the stream function correlation using the altimetric and numerical data. Cases when the correlation function is anisotropic are compared to the isotropic case previously studied by Hogg (1993), who found that the eddy vorticity flux drives two counter rotating gyres on either side of the stream. The anisotropy can be important in the eddy vorticity flux, even when its departure from the isotropic case is small. Meridional or zonal anisotropies can drive recirculation gyres similar in strength and position to the ones driven by the isotropic case. The results when including anisotropy in the diagonal direction suggest that the homogenoeus assumption may not be valid.
by Juan Botella.
S.M.
7
Scarfe, Bradley Edward. "Oceanographic Considerations for the Management and Protection of Surfing Breaks." The University of Waikato, 2008. http://hdl.handle.net/10289/2668.
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Although the physical characteristics of surfing breaks are well described in the literature, there is little specific research on surfing and coastal management. Such research is required because coastal engineering has had significant impacts to surfing breaks, both positive and negative. Strategic planning and environmental impact assessment methods, a central tenet of integrated coastal zone management (ICZM), are recommended by this thesis to maximise surfing amenities. The research reported here identifies key oceanographic considerations required for ICZM around surfing breaks including: surfing wave parameters; surfing break components; relationship between surfer skill, surfing manoeuvre type and wave parameters; wind effects on waves; currents; geomorphic surfing break categorisation; beach-state and morphology; and offshore wave transformations. Key coastal activities that can have impacts to surfing breaks are identified. Environmental data types to consider during coastal studies around surfing breaks are presented and geographic information systems (GIS) are used to manage and interpret such information. To monitor surfing breaks, a shallow water multibeam echo sounding system was utilised and a RTK GPS water level correction and hydrographic GIS methodology developed. Including surfing in coastal management requires coastal engineering solutions that incorporate surfing. As an example, the efficacy of the artificial surfing reef (ASR) at Mount Maunganui, New Zealand, was evaluated. GIS, multibeam echo soundings, oceanographic measurements, photography, and wave modelling were all applied to monitor sea floor morphology around the reef. Results showed that the beach-state has more cellular circulation since the reef was installed, and a groin effect on the offshore bar was caused by the structure within the monitoring period, trapping sediment updrift and eroding sediment downdrift. No identifiable shoreline salient was observed. Landward of the reef, a scour hole ~3 times the surface area of the reef has formed. The current literature on ASRs has primarily focused on reef shape and its role in creating surfing waves. However, this study suggests that impacts to the offshore bar, beach-state, scour hole and surf zone hydrodynamics should all be included in future surfing reef designs. More real world reef studies, including ongoing monitoring of existing surfing reefs are required to validate theoretical concepts in the published literature.
8
Ponte, Rui Vasques de Melo. "Observations and modelling of deep equatorial currents in the central Pacific." Thesis, Massachusetts Institute of Technology, 1988. http://hdl.handle.net/1721.1/58499.
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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 (leaves 178-180).
Analysis of vertical profiles of absolute horizontal velocity collected in January 1981, February 1982 and April 1982 in the central equatorial Pacific as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program, revealed two significant narrow band spectral peaks in the zonal velocity records, centered at vertical wavelengths of 560 and 350 stretched meters (sm). Both signals were present in all three cruises, but the 350 sm peak showed a more steady character in amplitude and a higher signal-to-noise ratio. In addition, its vertical scales corresponded to the scales of the conspicuous alternating flows generically called the equatorial deep jets in the past (the same terminology will be used here). Meridional velocity and vertical displacement spectra did not show any such energetic features. Energy in the 560 sm band roughly doubled between January 1981 and April 1982. Time lagged coherence results suggested upward phase propagation at time scales of about 4 years. East-west phase lines computed from zonally lagged coherences, tilted downward towards the west, implying westward phase propagation. Estimates of zonal wavelength (on the order of 10000 km) and period based on these coherence calculations, and the observed energy meridional structure at this vertical wavenumber band, seem consistent, within experimental errors, with the presence of a first meridional mode long Rossby wave packet, weakly modulated in the zonal direction. The equatorial deep jets, identified with the peak centered at 350 sm, are best defined as a finite narrow band process in vertical wavenumber (311-400 sm), accounting for only 20% of the total variance present in the broad band energetic background. At the jets wavenumber band, latitudinal energy scaling compared well with Kelvin wave theoretical values and a general tilt of phase lines downward towards the east yielded estimates of 10000-16000 km for the zonal wavelengths.
by Rui Vasques de Melo Ponte.
Ph.D.
9
Wang, Caixia. "Diagnosis of physical and biological controls on phytoplankton distribution in the Gulf of Maine-Georges Bank region." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/59508.
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Thesis (M.S.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1999.Includes bibliographical references (leaves 92-100).
The linkage between physics and biology is studied by applying a one-dimensional model and a two-dimensional model to the Sargasso Sea and the Gulf of Maine- Georges Bank region, respectively. The first model investigates the annual cycles of production and the response of the annual cycles to external forcing. The computed seasonal cycles compare reasonably well with the data. The spring bloom occurs after the winter mixing weakens and before the establishment of the summer stratification. Sensitivity experiments are also carried out, which basically provide information of how the internal bio-chemical parameters affect the biological system. The second model investigates the effect of the circulation field on the distribution of phytoplankton, and the relative importance of physical circulation and biological sources by using a data assimilation approach. The model results reveal seasonal and geographic variations of phytoplankton concentration, which compare well with data. The results verify that the seasonal cycles of phytoplankton are controlled by both the biological source and the physical advection, which themselves are functions of space and time. The biological source and the physical advection basically counterbalance each other. Advection controls the tendency of the phytoplankton concentration more often in the coastal region of the western Gulf of Maine than on Georges Bank, due to the small magnitude of the biological source in the former region, although the advection flux divergences have greater magnitudes on Georges Bank than in the coastal region of the western Gulf of Maine. It is also suggested by the model results that the two separated populations in the coastal region of the western Gulf of Maine and on Georges Bank are self-sustaining.
by Caixia Wang.
M.S.
10
Dale, Andrew W. "The oceanography and modelling of the Pontevedra Ria (NW Spain)." Thesis, University of Plymouth, 2003. http://hdl.handle.net/10026.1/1966.
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A multidisciplinary study the oceanography of the Pontevedra Ria (NW Spain), including hydrography, biogeochemistiy and biogeochemical modelling, has been performed. The hydrographical variability of the Pontevedra Ria was dependent on freshwater inputs and upwelling of nutrient-rich East North Atlantic Central Water from the shelf. Intrusions of the Poleward Current were also detected during winter. A stratified box model approach predicted that upwelling water fluxes into the ria of 2-4x10³ m³ s-1, of which >30% rises to the surface waters inside the ria. Freshwater residence time varied from ~4-9 d in the central ria and 1-4 d in the internal ria. Nutrients concentrations showed a strong fluvial and oceanic signal, with a clear zone of near-bed aerobic remineralisation in the internal ria. Nutrient flux experiments showed that high nutrient fluxes, particularly ammonium (3.5 mg N mˉ² hˉ¹), coincided with period of high water influx to the ria. These were related to stirring of quasi-benthic phytodetrital fluff. Denitrification was a major fate for particulate organic nitrogen in the sediment, averaging 2.5 mg N m ˉ² h ˉ¹ for the spring and dry season. A non-steady state nutrient budget revealed that the central and internal zones of the Pontevedra Ria display different biogeochemical characteristics. Net community production (NCP) based on phosphate uptake was spatially and temporally variable, with rates of 9.6 and 20.2 mg C m ˉ² h ˉ¹ in central and internal rias in spring, respectively, and 30.3 and 29.0 mg C m ˉ² h ˉ¹ in the dry season. Previously unquantified benthic nutrient inputs were important, and up to 25% of NCP was due to the sediment nutrient flux in the dry season. Denitrification calculated with the nutrient budget equalled 1.82 and 5.66 mg N m ˉ² h ˉ¹ in the dry season in the central and internal ria, respectively, and was equal to 27 and 42% of dry season NCP. The robustness of the box model was questioned, and found to be an unsuitable modelling approach for the Rias Bajas. This had clear implications for predicting NCP and net nutrient budgets to the coastal zone. Salinity and temperature were simulated with the commercially-available simulation shell, ECoS, to within the analytical error of the observed data. Inorganic nutrient concentrations and benthic effluxes were qualitatively and quantitatively agreeable with observed data. Phytoplankton growth in ECoS was limited by up to 30% by phosphorus rather than nitrogen, as previously believed. The annual evolution of NCP was successfully reproduced by examining the chlorophyll-normalised rate of organic carbon production. Mean NCP In the spring and dry season was 46.5 and 147 mg c m ˉ² h ˉ¹, which agreed well with the literature. There were clear discrepancies between box model and ECoS-derived nutrient export to the Atlantic ocean. The definitions of constituent uptake and remineralisation processes between the two approaches were examined In the context of biogeochemical modelling and environmental management of the Rias Bajas.
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Cossa, Obadias J. "Modelling the oceanic circulation in the Delagoa Bight." Doctoral thesis, University of Cape Town, 2018. http://hdl.handle.net/11427/27911.
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The ROMS is used to investigate the oceanic circulation in the Delagoa Bight region, near the southwestern end of the Mozambique Channel. The model is initially configured at a horizontal resolution of 1/10° (9.9km) over the domain 30.1-43°E,22.13-30.8° S. Subsequent configurations nested an inner grid of resolution 1/30° (3.3km) over the region 32.43-36.43°E,24.09-27.71°S Several sensitivity experiments were performed with and without the inner grid or with or without tidal forcing. Using only the outer coarse resolution grid, the first experiment (DELAGI) does not include tidal forcing whereas tides are included in the second (DELAG-II) experiment. DELAG-III and DELAG-IV both use the inner higher resolution grid but exclude and include tidal forcing respectively. The model was evaluated against observations, namely the WOA, Pathfinder SST and AVISO SSH. The results showed that ROMS adequately resolves the oceanic features in the region, namely the pathways of the anticyclonic eddies from the northern Mozambique Channel and from Madagascar, and the instances when the DBLE is present or absent. The model is also able to reproduce the main water masses and their sources in the region. Water masses found in the centre of the Bight enter through the northeastern sector, either by intrusion of pulses or instabilities of the southwards flowing current. When the DBLE is well established, upwelling is likely to contribute to the water masses in the lower layers. The transport of water towards the Bight from the east was found to be less than that from the north. The model also succeeds in representing the thermocline structure of the DBLE but it fails to capture the local salinity maximum. When tidal forcing is included, the speed of the flow close to the coast increases. The model also revealed the influence of the Inhambane Cyclone on the Delagoa Bight as well as on the region to its south. This cyclone, which is generated in the flow near Inhambane, is similar to Natal Pulses which occur in the Agulhas Current. An eddy detecting and tracking system was used with both the model outputs and VISO SSH to determine the statistics of the DBLE, namely its dimensions, amplitudes and life-times. A maximum radius of 59.52 km, life span of 126 days and an amplitude of 27.27 cm were found. It was also demonstrated that this feature is generated northeast of the Bight. When the inner grid was included in the simulations, anticyclonic features were generated within the Bight with a maximum diameter of 85.4 km and life span of 12 day. These anticyclonic features dominate the circulation when the DBLE is absent (less than 30 percent of the total period of the simulation). The possibility of several cyclonic cores existing simultaneously in the Bight was also demonstrated. Two cores were found with life spans of more than 12 days. When the number of cores is greater than two, they tend to be short lived. Analysis of energy conversion rates showed that the generation of both the DBLE and the Inhambane cyclones is mainly by barotropic instabilities, although in both regions of their generation, weak baroclinic instabilities were also found.
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Manyilizu, Majuto Clement. "Numerical modelling of the coastal ocean off Tanzania." Master's thesis, University of Cape Town, 2009. http://hdl.handle.net/11427/17968.
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Includes bibliographical references (pages 71-89).In this model study of the coastal ocean off Tanzania, the Regional Ocean Modelling System (ROMS) was employed to model the coastal ocean off Tanzania over the domain of 5°N-15°S and 38-55°E. It was integrated for ten years with monthly mean Comprehensive Ocean and Atmosphere Data Sets (COADS) winds and heat fluxes. Initial and lateral boundary conditions were derived from the World Ocean Atlas. The model was used to simulate the annual cycle, and the sea surface temperature (SST) output compared with the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) sea surface temperature (SST) measurements for the same region. Although broadly comparable, the model SST was generally warmer than that of TMI data. The high SSTs in the Tanzanian coastal waters (greater than 28°C) occur from December to May while SSTs of less than 28°C occur during the rest of the year. The East African Coastal Current (EACC) experiences its lowest spatial and temporal average speeds (about 0.4ms- 1) in February and its maximum speed (1.7 ms⁻¹) in July. Speeds of greater than 1 ms⁻¹ occur during both transition seasons north of 6°S. The meridional wind stresses appear to be positively correlated with the EACC(r>0.6) in all locations and they are statistically significant (p<0.05). The annual cycle of the model flow in the southern Tanzanian waters seems to be positively correlated with the flow to the north of Madagascar (r=0.57 and p=O.O5). The flow in these regions changes in phase with each other from October to April and June to July with minimum speeds in November. For the other months, the flow in these regions is out of phase with each other. The model currents off southern Tanzania attain their maximum speeds in August when the South West monsoon is fully developed while the flow north of Madagascar attains its maximum speed in September when the South West monsoon fades. However, the flow in the southern Tanzanian waters is more affected by the reversal of winds over the tropical western Indian Ocean (r=0.69, p=0.01) than that north of Madagascar (r=0.51, p=0.09). This difference results in a larger annual speed range in the flow off southern Tanzania (about 0.4 ms⁻¹ ) than that to the north of Madagascar (about 0.3ms⁻¹). The ROMS model realistically simulates the annual cycle of the sea surface temperature and heat flux, the East African Coastal Current and the annual cycle of the flow entering the coastal ocean off the southern part of Tanzania. However, studies which integrate the large scale domain and regional coupled ocean-atmosphere interactions are needed to better understand of the East African climate and ocean variability. Such model results combined with suitable remote sensing and in situ observations will help improve understanding of the circulation and properties of the coastal ocean off Tanzania.
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Carrie, A. L. S. "Wave refraction modelling and longshore sediment transport." Thesis, University of East Anglia, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372553.
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Carson, Nuala. "Numerical modelling of landfast sea ice." Thesis, University of Liverpool, 2014. http://livrepository.liverpool.ac.uk/18773/.
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Landfast sea ice is a recurring seasonal feature along many coastlines in the polar regions. It is characterised by a lack of horizontal motion, for at least 20 days, and its attachment to the coast or seabed. It can form as a result of restrictive geometry, such as channels or embayments, or through the grounding of thick ice ridges which add lateral stability to the ice cover. Due to its stationary and persistent nature, landfast ice fundamentally modifies the exchange of heat and momentum between the atmosphere and ocean, compared with more mobile pack ice. The current generation of sea ice models is not capable of reproducing certain aspects of landfast ice formation and breakup. In this work two landfast ice parameterisations were developed, which describe the formation and breakup of landfast ice through the grounding of thick ice ridges. The parameterisations assume the sub-grid scale distribution of ice draft and ocean depth, the two parameters important in determining the occurrence of grounded ridges. The sub-grid scale distribution of grounded ice is firstly defined by assuming that ice draft and ocean depth are independent. This parameterisation allowed ice of any thickness to occur and ground at any depth. Advancing from this the sub-grid scale distribution of the grounded ice was restricted in an effort to make it more realistic. Based on Arctic ice scour observations ice was prevented from grounding in regions where the draft thickness was much larger than the ocean depth. Both parameterisations were incorporated into a commonly used sea ice model, the Los Alamos Sea Ice Model (CICE), to which a multi-category ocean depth distribution from high resolution global bathymetry data (ETOPO1) was included. The parameterisations were tested in global standalone format (i.e. no active ocean) with realistic atmospheric forcing. Both parameterisations were found to improve the spatial distribution and the seasonal cycle of landfast ice compared to the control (i.e. no landfast ice parameterisation) in the Arctic and Antarctic. However, the grounded ridges produced by the parameterisations were very stable, and tended to become multiyear leading to the production of multiyear landfast ice, which was particularly widespread in the Antarctic. It was found that tides have a significant impact on both grounded and landfast ice. In some polar locations tides were found to increase the occurrence of landfast ice, by increasing the production of thick ridges which were able to ground. Conversely, in some regions, tides were found to decrease the occurrence of landfast ice, as strong tidal and residual currents increased the mobility of the grounded ridges and landfast ice. This thesis finishes by considering whether a sea ice model could be used to further our understanding of the physical landfast ice system. Analytically derived characteristic numbers, which describe the ability of landfast ice to form, were found to fully describe the formation of landfast ice within the sea ice model CICE during idealised 1D scenarios. For these scenarios the key parameters controlling ice motion were found to be the external forcing component, the width of the ice cover, the internal ice strength, and the thickness of the ice. However, an exact characteristic variable able to describe the occurrence of landfast ice in an idealised 2D scenario could not be found analytically, nor could it be inferred numerically, and this remains an area for further research. This thesis examines different methods of modelling landfast sea ice and provides the sea ice modelling community with a means to parametrise landfast ice formation as a result of grounded ridges without having to work at very fine resolution, as this is computationally inefficient.
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Julião, Heloise Pavanato. "Abundância e distribuiçãoda baleia jubarte (Megaptera novaeangliae) na costa do Brasil." reponame:Repositório Institucional da FURG, 2013. http://repositorio.furg.br/handle/1/4023.
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Dissertação(mestrado) - Universidade Federal do Rio Grande, Programa de Pós–Graduação em Oceanografia Biológica, Instituto de Oceanografia, 2013.Submitted by Cristiane Gomides (cristiane_gomides@hotmail.com) on 2013-10-09T18:43:46Z No. of bitstreams: 1 Heloise.pdf: 1525937 bytes, checksum: 44441e69ced9544eaba26ec6b8f8e2d9 (MD5)
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População é a unidade fundamental da conservação e sua forma mais simples de monitoramento envolve a amostragem temporal regular para a determinação do status populacional. Uma das populações de baleia jubarte do Hemisfério Sul utiliza a costa do Brasil entre maio e dezembro para se reprodução e criação dos filhotes. Esta população, denominada “estoque reprodutivo A” pela Comissão Internacional da Baleia, tem mostrado sinais de recuperação após um marcado declínio devido a caça e um longo período de moratória. Esta população se concentra principalmente no Banco dos Abrolhos (BA), onde águas calmas e quentes parecem constituir um hábitat ideal. Este estudo teve o objetivo de estimar o tamanho da população de jubartes para o ano de 2011, bem como predizer a distribuição de grupos na costa brasileira. O método de amostragem de distâncias foi implementado, e modelos hierárquicos Bayesianos foram propostos para estimar a abundância. Modelos auto-regressivos condicionais foram aplicados para predizer a densidade em células de 0.5° de latitude e longitude. O tamanho da população foi estimado em 10,160 baleias (Cr.I.95%=6,607-17,692). As maiores densidades foram encontradas entre o Banco dos Abrolhos e a Baía de Todos os Santos (BA). Os resultados sugerem que o aumento populacional acarreta a expansão da população para além do Banco dos Abrolhos.
Population is the fundamental unit of conservation and its simplest monitoring tool involves regular sampling over time for population assessing status. One of the Southern Hemisphere humpback whale populations winters at the Brazilian coast typically from May to December where breeding and calving occur. This population, labeled as “breeding stock A” by International Whaling Commission, has shown signs of recovery after the long period of whaling. The goal of this study was to estimate the population size of humpback whales up to 2011, and predict group distribution along the Brazilian coast. Distance sampling methods were implemented and hierarchical Bayesian models were proposed to estimate abundance. Conditional auto-regressive models were used to predict the density in a lattice of 0.5° of latitude and longitude. Population size was estimated at 10,160 whales (Cr.I.95%=6,607-17,692). Highest densities were predicted to occur between Abrolhos Bank and Todos os Santos Bay (BA). The results suggest that the population increase leads to a population expansion beyond Abrolhos Bank.
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Jones, Ian David. "Isopycnic modelling of the North Atlantic heat budget." Thesis, University of Liverpool, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318284.
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Veitch, Jennifer Anne. "Equilibrium dynamics of the Benguela system : a numerical modelling approach." Doctoral thesis, University of Cape Town, 2009. http://hdl.handle.net/11427/12153.
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Includes abstract.Includes bibliographical references (p. 237-255).
The Regional Ocean Modelling System (ROMS) is used to systematically investigate equilibrium conditions and seasonal variations of the Benguela system, including both the large-scale flow regime as well as the coastal upswelling regime. A shelf-edge poleward flow exists in the northern Benguela region and is driven primarily by the wind-stress curl via the Sverdup relation. As such, it is strongly seasonal and is most intense during spring and summer when the wind-stress curl is most negative. The poleward flow deepens as it moves southward and between 25-27° much of it veers offshore due to the nature of of the wind-stress curl. In the mean state, the Benguela Current is characterized by two streams: the more inshore stream is topographically controlled and follows the run of the shelf-edge. The offshore stream is driven by nonlinear reactions of passing Aghulas rings and eddies and does not have a striking seasonal signal. The model simulates all seven of the major upswelling cells within its domain.
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Queiroz, Eurico Tiago Justino. "Modelling Benguela niños using the regional oceanic modeling system (ROMS)." Master's thesis, University of Cape Town, 2007. http://hdl.handle.net/11427/6499.
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Includes bibliographical references (leaves 132-141).Pierre Florenchie
This study is framed by three questions: firstly, could the Regional Oceanic Modelling System (ROMS) reproduce the seasonal cycle of the equatorial Atlantic? Secondly, what is the nature of the link between remote forcing in the western equatorial Atlantic and Benguela Niños/Niñas? Thirdly, what is the impact of these events on the equatorial Atlantic Ocean SST and circulation patterns? The results obtained suggest that the model is very sensitive to different wind stress forcing, particularly in respect of the impact on the mixed layer characteristics. As a result the equatorial upwelling is overestimated in both temporal and spatial scales.
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Wadley, Martin Robert. "Modelling the bottom water circulation in the Vema Channel." Thesis, University of East Anglia, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.482778.
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Su, Lin 1966. "Modelling study of nutrients cycles in the North Atlantic Atlantic Ocean." Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40002.
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We have coupled a 3-dimensional physical planetary geostrophic ocean general circulation model to different biological models to investigate the interaction between physical and biological processes. A 1-dimensional particle cycle model with two particle size classes has been developed and coupled to the physical model as well.The ocean model (Zhang et al., 1992) is based on the planetary geostrophic equations in spherical coordinates. The model equations include the full prognostic temperature and salinity equations. The momentum equations are diagnostic and include geostrophic balance, and a linear friction term in order to provide a western boundary current. The wind stress is applied at the top level of the model. The temperature and salinity distributions used in the surface boundary restoring condition are taken from climatological data. The model domain consists of a flat-bottomed box of 60$ sp circ$ longitude extending between 5$ sp circ$N and 65$ sp circ$N. The horizontal resolution is 2.3$ sp circ$ in both latitude and longitude with 14 levels in the vertical.
The physical model is first coupled to a biological model where new production is given by a restoring condition of surface nitrate towards its observed concentration. The coupled model is used to examine Martin et al.'s (1987) hypothesis that lateral transport and decomposition of slow or non-sinking organic matter can cause a non-local balance between the remineralization rate and the overlying new production rate in open ocean regions. The role of the Gulf Stream in nutrient transport is examined. The model results agree well with the North Atlantic nutrient transport calculated from observed nutrients and hydrographic data. The model results suggest that the thermohaline overturning circulation and the Gulf Stream horizontal recirculation play an important role in the North Atlantic nutrient distribution.
The physical model is then tested in the seasonal mode, and coupled with a biological model which is based on nitrate limiting the rate of new production. The model simulated seasonal oxygen cycle agrees well with the results of observational studies and 1-dimensional model simulations. The oxygen utilization rate below the euphotic zone provides a useful estimate of new production.
A 1-dimensional time dependent particle cycling model with two particle size classes based on Clegg and Whitfield (1990) is then developed. The simulated total organic carbon concentration and large particle flux are consistent with observations and other 1-dimensional model simulations. The downward transport of organic carbon is mainly accomplished by the fast sinking large particles, which comprise a small fraction of the total particulate mass. The steady state version of the particle model is also coupled with the 3-dimensional physical model. The magnitudes of simulated organic carbon flux and total organic matter concentration are comparable with observations.
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Herrington, Sian Joscelyn. "The modelling of mixotrophy in the oligotrophic Atlantic." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/359061/.
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In the oligotrophic Atlantic Ocean small algae are the dominant fixers of inorganic carbon. In situ experiments have shown that a large proportion of these algae are mixotrophs - eating bacteria (bacterivory) as well as obtaining energy from sunlight. Bacterivory performed by algae has implications for our understanding of the role of ultraplankton (<5 μm) in biogeochemical cycling. The motivation of this thesis is to explore how mixotrophy may be modelled in the subtropical Atlantic using a data driven approach. An ecosystem model incorporating ultraplankton mixotrophy was developed, constructed and parameterised using in situ data, initially through network analysis and later using a μ-Genetic Algorithm technique. The model highlights the key role of mixotrophy in the cycling of nutrients, in a region where fast nutrient turnover is important for the functioning of the ecosystem. In addition, the model reveals that bacterivory is the predominant route of nutrient acquisition for these mixotrophs and suggests that mixotrophy in this low nutrient region is an adaptive rather than a survival mechanism. This thesis also addresses wider questions related to model structure and assumptions. The need for an explicit dissolved organic phosphate variable in an ecosystem model for the oligotrophic Atlantic is questioned through in situ radio-nucleotide bioassay techniques. Additionally, ultraplankton spatial variability is statistically assessed and used to demonstrate that a zero-dimensional model is not necessarily applicable to an entire region, despite the ultraplankton community within that region being statistically similar according to multivariate analyses. Furthermore a comparison of in situ to remotely sensed data shows that ocean colour is limited in its ability to detect ultraphytoplankton, making the use of such data to calibrate and assess future models difficult. This thesis therefore not only contributes to our ability to model the oligotrophic Atlantic but more broadly to our understanding of the role of mixotrophs within it.
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Yoo, D. "Mathematical modelling of wave-current interacted flow in shallow waters." Thesis, University of Manchester, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376157.
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Van, Wellen Eur Ing Erik. "Modelling of swash zone sediment transport on coarse grained beaches." Thesis, University of Plymouth, 1999. http://hdl.handle.net/10026.1/1761.
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A review is presented which assesses the importance of the swash zone as a potential contributor to the longshore and cross-shore transport on steep coarse-grained beaches. Based on this review it is apparent that the swash zone on such beaches forms an important contributor to both the longshore and the cross-shore transport. The review also identifies that the swash zone is neglected in all but a few sediment transport models. In addition, a lack of available shingle beach field data against which to validate existing and new transport models is also reported. Two surf zone integrated equations are presented with the aim of producing simple and physics based formulae relating the total longshore transport (TLT) to the main parameters such as wave height at breaking. In addition, a surf and swash zone inclusive transport formula is developed based on an existing numerical model for the calculation of shingle transport. These formulae, together with existing TLT formulae are evaluated against existing, synthesised and new field data collected during this study. A mathematical model (STRAND) is developed which quantifies sediment transport in the swash zone. The model combines recent advances in the understanding of swash zone dynamics with physics-based predictive transport equations and is computationally efficient. Sensitivity analyses on the model confirm the high potential for transport in the swash zone, both cross-shore and longshore. The STRAND model gives good results when tested against existing data and new field data from shingle beaches at Shoreham-by-Sea and Lancing. Although originally developed for shingle beaches, the model is also validated using data from sand beaches, thus encompassing a wider variety of sediment sizes than many models have used for tests in the past. The swash zone on steep beaches is found to be responsible for as much as two thirds of the volumetric longshore transport. The model also indicated high and rapidly fluctuating cross-shore transport rates, thus contradicting existing transport distribution models. Therefore, sediment transport in the swash zone on steep beaches can no longer be ignored as an important contributor to the overall longshore and cross-shore transport budgets.
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Gan, Jianping 1962. "Upper ocean modelling in Baie des Chaleurs." Thesis, McGill University, 1995. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=28752.
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An eddy-resolving upper ocean model is developed to study the dynamics and thermo-dynamics of Baie des Chaleurs (BdC, 47.5-48.5N, 65.5-66.5W), Gulf of St. Lawrence (GSL), Canada. The model has primitive equation dynamics with two active layers embedded with a Kraus-Niiler type mixed layer model at the top.Forced by observed wind, atmospheric heat fluxes, river runoff and appropriate remote forcing (in particular, the Gaspe Current, GC), the model demonstrates that the mean cyclonic general circulation pattern in the bay is a consequence of the intrusion of the GC. In the mixed layer, atmospheric heat fluxes and horizontal thermal advection play a key role in the thermal balance at the eastern part of the bay. The local mixed layer fluctuations are controlled by wind and GC induced divergence. The entrainment (and its corresponding heat flux) is important at the western part of the bay and changes the mean mixed layer depth on a time scale of more than a week. Varying GC intensified the flow variations induced by the wind in the bay and improved simulation results as compared with observations.
Sensitivity runs are conducted to study the effects of external forcing, important physical processes and the internal physical parameterisation on the model results and to compare these with the main model run. Experiments show that nonlinearity is very important in determining the circulation pattern in the bay. Changing external thermal forcing also modifies dynamical processes in the BdC. The fluctuations in the near surface temperature are mainly due to latent and sensible heat changes. The parameter study indicates that, the model is not overly sensitive to changes in most of the parameters, but suggests that sensitivity of the mixed layer physical parameters depends on the dynamical and thermodynamic system applied.
Hydrographic and current meter data are used first to study the variability of both the dynamics and thermodynamics in the BdC and its relation to the separation/intrusion of the unsteady GC. A numerical model is then applied to gain insight into the problem. The time scales of interest range from tidal to seasonal.
The results show that the kinetic energy in the BdC is dominated by the semi-diurnal tide (M$ sb2$) and periods of 5-10 days for high and low frequency bands, respectively. Most of the energy in the low frequency band is found to be induced by wind-related forcing.
Both observations and model results indicate that seasonal variations in the BdC are strongly related to the characteristics of separation/intrusion of the GC, which is mainly controlled by its transport magnitude as well as phase, duration and strength of its acceleration (or deceleration). The separation occurs when (adverse) vorticity having an opposite sign from that existing upstream is generated near the separation area. Although the separation can be generated in a decelerating GC, it can also occur in an accelerating GC when the GC is strong enough to advect upstream vorticity necessary to form a recirculation and the related adverse vorticity downstream. Nonlinearity is critical to the separation. Nevertheless, separation can be generated in a linear current with strong deceleration. The GC intrudes either along the coastline (attachment) into the bay by a non-separated GC or following the separation of the GC (reattachment). Effects of various physical processes on the separation/intrusion and variability of eddies in the BdC are examined.
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DeTracey, Brendan. "Modelling interannual sea ice variability in the Gulf of St. Lawrence." Thesis, McGill University, 1993. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=68167.
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An uncoupled, modified Hibler ice model has been applied to the Gulf of St. Lawrence for three different winters of varying severity, in order to examine interannual sea ice variability. The simulation was initialized with observed November sea surface temperatures, and forced by weekly geostrophic winds, monthly averaged meteorological data and model geostrophic surface currents.Results showed a general correlation with observations, reproducing differences in the sea ice cover between the years chosen. Neglecting oceanic effects caused excessive ice formation in the northwest Gulf and produced discrepancies between the observed and modelled ice edge.
Sensitivity studies revealed a high sensitivity to variations in both the forcing fields and the model free parameters. Further modelling studies must include a coupled ocean component, and force the ice component with weekly meteorological data to improve the accuracy of the prediction.
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Castaneda, Julian Jose. "Modelling and measuring (by H.F. radar) dispersion in the coastal zone." Thesis, University of Southampton, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241137.
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Gamoyo, Majambo J. "Modelling dispersal and connectivity of broadcast spawning corals in the Western Indian Ocean." Doctoral thesis, University of Cape Town, 2018. http://hdl.handle.net/11427/29633.
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Coral reef degradation is happening at an alarming rate all over the world due to multiple stressors with elevated sea surface temperature being the root cause. Using the Regional Ocean Modelling System and an individual-based model for the western Indian Ocean, this thesis explored the general circulation patterns (both large and mesoscale) important to dispersal and connectivity of broadcast corals while identifying regions that act as a source of larvae and those that receive larvae. Because habitat destruction and fragmentation through severe bleaching and mortality threaten coral reef health, projected thermal stress from Global Climate Models was explored to quantify future bleaching scenarios that might impact the reproductive timing and larval dispersal. Evaluation of the ROMS configuration for the western Indian Ocean shows that the basin-scale circulation patterns of the region are appropriately captured with the mean volume transports consistent with those derived from observation. Using the eddy detection algorithm, a description of the Southern Gyre as a key aspect of the Somali Current system was identified. The Southern Gyre is associated with barotropic instabilities associated with the northward flowing Somali Current. Rossby waves arriving at the East African coast and the strength of the monsoon winds are also responsible for the evolution and intensification of the gyre. The aggregated trajectories from the Lagrangian model highlight the dominant dispersal pathways and barriers to dispersal following release. The general circulation plays an important role in the dispersal of reef larvae over the study region. At a short pelagic larval duration, most of the released larvae settle back to or near natal reefs, but as the pelagic duration increases, the number of isolated reefs and islands decreases. Even with increased pelagic duration, some reefs (e.g., Agalega and Tromelin) are completely isolated. The mean dispersal distance from release to settlement varied across the region with larvae released along the East African coast dispersed an average of 405 km before settling while those in the Seychelles archipelago dispersed about 101 km. Different blocks of clusters were observed with 16 clusters observed when the pelagic duration is shorter (5 days), compared to seven clusters when the pelagic duration is longer (60 days). The warming trends and bleaching thermal stress shows that among the 636 reef pixels in the study region, about 56% showed positive sea surface temperature trends during the study period (1985- 2016). The frequency of bleaching level thermal stress has also increased over the same period, a tendency that climate models project to continue. Even under optimistic scenarios (such as the Representative Concentration Pathway RCP 4.5), most coral reefs are projected to experience severe bleaching and possible mortality by the 2050s. Low to moderate thermal stress are projected over reefs along the East African coast and near the northwest tip of Madagascar and thus these regions may act as potential climate refugia while increasing the potential of reefs to cope with climate change.
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Hopkins, Joanne E. "Statistical modelling and variability of the subtropical front, New Zealand." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/63759/.
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Ocean fronts are narrow zones of intense dynamic activity that play an important role in global ocean-atmosphere interactions. Of particular significance is the circumglobal frontal system of the Southern Ocean where intermediate water masses are formed, heat, salt, nutrients and momentum are redistributed and carbon dioxide is absorbed. The northern limit of this frontal band is marked by the Subtropical Front, where subtropical gyre water convergences with colder subantarctic water. Owing to their highly variable nature, both in space and time, ocean fronts are notoriously difficult features to adequately sample using traditional in-situ techniques. We therefore propose a new and innovative statistical modelling approach to detecting and monitoring ocean fronts from AVHRR SST images. Weighted local likelihood is used to provide a nonparametric description of spatial variations in the position and strength of individual fronts within an image. Although we use the new algorithm on AVHRR data it is suitable for other satellite data or model output. The algorithm is used to study the spatial and temporal variability of a localized section of the Subtropical Front past New Zealand, known locally as the Southland Front. Twenty-one years (January 1985 to December 2005) of estimates of the front’s position, temperature and strength are examined using cross correlation and wavelet analysis to investigate the role that remote atmospheric and oceanic forcing relating to the El Nino-Southern Oscillation may play in interannual frontal variability. Cold (warm) anomalies are observed at the Southland Front three to four months after peak El Nino (La Nina) events. The gradient of the front changes one to two seasons in advance of extreme ENSO events suggesting that it may be used as a precursor to changes in the Southern Oscillation. There are strong seasonal dependencies to the correlation between ENSO indices and frontal characteristics. In addition, the frequency and phase relationships are inconsistent indicating that no one physical mechanism or mode of climate variability is responsible for the teleconnection.
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Jones, John Eric. "Numerical modelling of tides, surges, residual circulation and salinity in shelf seas." Thesis, University of Liverpool, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240598.
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Marcinko, Charlotte L. J. "Modelling and observational studies of dinoflagellate bioluminescence within the Northeast Atlantic." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/384575/.
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Backeberg, Bjorn Christoph. "Modelling the mesoscale variability in the greater Agulhas Current system using hybrid coordinate Ocean model." Doctoral thesis, University of Cape Town, 2009. http://hdl.handle.net/11427/6458.
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The ocean circulation dynamics in the greater Agulhas Current system are dominated by mesoscale variability, which is highly non-linear, and therefore difficult to measure and simulate accurately. Moreover, the shedding of Agulhas rings from the retroflection south of Africa, which is the dominant mechanism by which warm and saline water flows from the Indian into the Atlantic Ocean, is thought to be a crucial component of the thermohaline circulation. With the goal of providing an accurate simulation of the greater Agulhas Current system, and in particular its mesoscale variability, a high resolution Hybrid Coordinate Ocean Model is set up in a nested configuration. In two 11 year simulation experiments, the effect of a higher order momentum advection scheme on the simulated ocean dynamics is tested and evaluated against available satellite observations and in-situ measurements. Quantitative analyses and model validation methods are developed to objectively evaluate the simulation experiments. The resultant skewness analyses and spatial variograms are objective measures for assessing the model simulation and additionally provide new insights on the mesoscale dynamics of the greater Agulhas Current system. A 4th order momentum advection scheme is shown to significantly improve the simulation of the region, in particular the dynamics of the southern Agulhas Current and the retroflection are greatly improved. From the analyses of the two model simulations in conjunction with satellite observations and in-situ measurements, it is found that the Indo-Atlantic inter-ocean exchange, and the shedding of Agulhas rings from the retroflection, is sensitive to the strength of the Agulhas Current, which in turn is influenced by the flow dynamics in the Mozambique Channel and south of Madagascar. Mesoscale eddies drifting from these source regions to the Agulhas Current play an important role, and the connection between the Agulhas Current and the respective source regions provides a link to large-scale variability in the Indian Ocean, which in turn is related to interannual modes of variability such as the Indian Ocean Dipole and El NiÑo Southern Oscillation.
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Meyiwa, Sbongile. "Numerical modelling of Tropical Cyclone Dineo and its rainfall impacts over north-eastern South Africa." Master's thesis, Faculty of Science, 2019. http://hdl.handle.net/11427/31174.
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Widespread flooding over parts of Mozambique, Zimbabwe, Malawi, Botswana as well as north-eastern South Africa was experienced in February 2017. The flooding was associated with Tropical Cyclone Dineo that was generated in the Mozambique Channel on 12 February 2017 and made landfall over the south-central coast of Mozambique on 15 February. This study investigates the atmospheric circulation and potential mechanisms responsible for the heavy rainfall that occurred during the passage of ex-Tropical Cyclone Dineo inland from the Mozambican coast with focus on the rainfall patterns over north-eastern South Africa. Output from the Weather Research and Forecasting (WRF) model, the Climate Forecast System Reanalyses version 2 (CFSv2) atmospheric reanalysis, satellite derived rainfall and wind data, and station rainfall data are used for this purpose. Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, WRF model rainfall and rainfall station data indicated that many parts of north-eastern South Africa experienced large amounts of rainfall during the final stages of Dineo (16-17 February 2017) while Mozambique experienced heavy rainfall soon after the cyclone made landfall. An inland trough ahead of Dineo led to substantial rainfall at this time over Malawi and Botswana. Furthermore, analysis of the station data revealed that in north-eastern South Africa some stations recorded about 80 % of their total monthly rainfall from this event. The WRF model run indicated low level monsoonal north-easterly moisture fluxes feeding into Dineo in the Mozambique Channel. Subsequent convergence over south-eastern Africa between this flow and the south-easterly cyclonic flux associated with Dineo led to substantial rainfall over Mozambique, Zimbabwe, Botswana and north-eastern South Africa. Although the 2016/17 tropical cyclone season recorded below average numbers of storms, it is suggested that the conditions prior to the storm iv formation were favourable for the track of Tropical Cyclone Dineo and landfall on the southcentral Mozambican coast.
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Viljoen, Anél. "Investigation of the nearshore, episodic poleward current in the southern Benguela : a numerical modelling approach." Master's thesis, University of Cape Town, 2006. http://hdl.handle.net/11427/6448.
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Includes bibliographical references.Fisheries are of great economic importance on the South African West Coast (the Southern Benguela). The St Helena Bay region is a key nursery habitat for these fisheries because of its retention, upwelling and stratified water column characteristics. However, these characteristics also result in other outcomes such as hypoxia and harmful algal blooms (HAB's) which impact on the habitat suitability character of the system. A nearshore, episodic poleward current has been observed in this region, and it is believed that this current plays an important role in the incidence of HABs as well as hypoxia events. The drivers and dynamics of this nearshore, episodic poleward current have not been clearly understood, nor thoroughly investigated, due to the complexity of the scales and processes. However, the importance of this current in transporting harmful algae from the north into St Helena Bay and its role in habitat hypoxia has emphasized the need to understand its dynamics.
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Dupont, Frederic. "Comparison of numerical methods for modelling ocean circulation in basins with irregular coasts." Thesis, McGill University, 2001. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=37886.
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Comprehension of global oceanic currents and, ultimately, of climate variability requires the use of computer modelling. Although much effort has been spent on the accuracy of traditional finite difference (FD) models used in ocean modelling, there are still concerns, especially since these models have a crude representation of the geometry of oceanic basins. Such a crude representation may influence the accuracy of modelling boundary currents, or unrealisticly represent the impinging of eddies or the propagation of Kelvin waves along the coastline. This motivated the use of alternative modelling techniques applied on completely irregular geometries such as finite element (FE) and spectral element (SE) methods. In this thesis, we want to investigate the accuracy and cost-effectiveness of these three numerical methods in irregular domains and to understand to which extent the unstructured grid FE and SE methods constitute an improvement over the more traditional FD methods. To accomplish this, we limit ourselves to modelling the shallow water equations in presence of irregular coastlines with no bottom topography.In the first part of the thesis, we compare the performances of FD methods on Cartesian grids with FE and SE methods in various geometries for linear and non-linear applications. We argue that the SE method is to a certain extent superior to FD methods. In a second part, we study the influence of step-like walls on vorticity budgets for wind-driven shallow water FD models. We show that vorticity budgets can be very sensitive to the FD formulation. This has certain implications for using vorticity budgets as a diagnostic tool in FD models. In the final part, we use a SE shallow water model for investigating the "inertial runaway problem" in irregular domains for the single-gyre Munk problem. Ideally, one would like the statistical equilibrium observed at large Reynolds number to be insensitive to model choices that are not well founded, e.g., the precise value of the viscous coefficient, and choice of dynamic boundary condition. Simple models of geophysical flows are indeed very sensitive to these choices. For example, flows typically converge to unrealisticly strong circulations, particularly under free-slip boundary conditions, even at rather modest Reynolds numbers. This is referred to as the "inertial runaway problem". We show that the addition of irregular coastlines to the canonical problem helps to slow considerably the circulation, but does not prevent runway.
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Phelps, Jack. "Modelling hydrodynamic transport and larval dispersal in North-East Atlantic Shelf seas." Thesis, University of Liverpool, 2015. http://livrepository.liverpool.ac.uk/2026939/.
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This thesis presents a series of numerical modelling studies into hydrodynamic transport and larval dispersal. The initial investigation seeks to evaluate retention timescales in Liverpool Bay. The flushing time and residence time are equal to 136 days and 103 days respectively, however small concentrations of seawater are retained over several years due to vigorous tidal mixing. The age distribution is shown to be highly variable and dependent upon tracer input duration, however salinity can be used to estimate the mean age, which is not directly observable in practice. Chapters 3, 4 and 5 all focus upon the dispersal of meroplanktonic larvae and aim to determine how larval behaviour affects their transport. Vertical migration is a significant influence upon larval dispersal within each case study, although the effect of this behaviour is conditional upon local hydrodynamic conditions. For example diel vertical migration promotes dispersal in the western Irish Sea, however the identical swimming pattern facilitates local retention in the eastern Irish Sea. The ecological implications of these findings are discussed. This thesis concludes with an investigation into the impact of large CO2 leakages on the marine carbonate system at potential carbon sequestration sites in the North Sea. Perturbations to seawater pH are found to vary according to the rate, duration and location of CO2 input. The northern North Sea is particularly vulnerable to large perturbations ( > 1 pH units) during the summer months, as the strong seasonal thermocline suppresses CO2 outgasing.
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Watts-Rodrigues, Pedro Paulo Gomes. "Modelling nitrous oxide production in two contrasting British estuaries : the Forth and the Tyne." Thesis, University of Newcastle Upon Tyne, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.275513.
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Lucas, Marc A. "On steady and variable buoyancy forcing in the Atlantic : an idealised modelling study." Thesis, University of Southampton, 2005. https://eprints.soton.ac.uk/18666/.
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This study examines the response of the thermohaline circulation in the north Atlantic to steady and variable buoyancy forcing. The model used is a version of the MOMA model (Webb, 1996), updated to include a free surface and Gent & McWilliams mixing. The model’s resolution is coarse, 4 x4 degrees with 15 levels in the vertical. In a first instance, the model’s response to 14 different fixed thermal profiles is investigated, by systematically keeping the equator temperature fixed and then the northernmost temperature fixed. The results show that the models response differs for these two sets of experiments as one setup favours stratification while the other favours convection. In a second instance, the restoring field is made to oscillate over 17 different periods, ranging from 6 months to 32,000 years. The model's meridional overturning circulation (MOC) exhibits a very strong response on all timescales greater than 15 years, up to and including the longest forcing timescales examined. The peak-to-peak values of the MOC oscillations reach up to 125% of the steady-state maximum MOC and exhibit resonance-like behaviour, with a maximum at centennial to millennial forcing periods (depending on the vertical diffusivity). This resonance-like behaviour stems from the existence of two adjustment time scales, one of which is set by the vertical diffusion and another, which is set by the basin width. Finally, the study is extended to a double hemisphere basin. Again, the model's MOC exhibits a very strong response on all timescales in both hemispheres, up to and including the longest forcing timescales examined for either set of experiments with the amplitude of the oscillations reaching up to 140% of the steady-state maximum MOC and exhibiting resonance-like behaviour, with a maximum at centennial to millennial forcing periods. This resonance like behaviour is identical to what has been observed in a single hemisphere and occurs for the same reasons. What is novel is that when the forcing in the southern subordinate hemisphere lags that of the northern by half a period, the amplitude of the response is substantially greater for large forcing periods (millennial and above), particularly in the subordinate (southern) hemisphere. This happens because the basin has in effect two sources of deep water. This leads to colder bottom waters and thus greater stratification, which in turn stabilises the water column and thus reduces the value of the minimum overturning. The considerable deviation from the quasi-equilibrium response at all timescales above 15 years for both the single hemisphere and the double hemisphere experiments is surprising and suggests a potentially important role of the ocean circulation for climate even at Milankovich timescales.
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Levasseur, Anne. "Observations and modelling of the variability of the Solent-Southampton Water estuarine system." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/63761/.
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Understanding the effect of physical forcing on estuarine functioning is of major importance to determine the rate of exchange of water, sediments, pollutants and nutrients between the continent and the ocean. The combination of numerical models and discrete datasets is used to describe and investigate processes of natural variability in the partially-mixed, non-turbid, macrotidal Solent-Southampton Water estuarine system (UK). The estuarine circulation and the response of wind forcing is examined using a three dimensional, free surface, finite volume and finite element grid model. Results from short-term (three months) simulations have been compared against data (ADCP measurements, tidal elevations and salinity distributions) collected in spring 2001 in Southampton Water and the Solent. The model reproduces the unique tidal curve of Southampton Water and the partially-mixed conditions prevailing in the upper estuary. The contribution of the local wind forcing (wind intensity · 12 m s−1) to changes in water level is estimated to be up to 6 cm in Southampton Water in the model. The modelled salinity stratification varies over a semi-diurnal cycle with the highest stratification occuring at mid-ebb. Wind forcing is more efficient in altering stratification at ebb than flood. The temporal and spatial variability of light attenuation is also investigated. Turbidity is demonstrated to be the major contribution to light attenuation using a time-series of discrete data collected in 2001, 2002 and 2003. A typical seasonal cycle of the coefficient of light attenuation is revealed, with a minimum in May-June and a maximum occuring in September-October. A second dataset of continuous measurements (10-minute interval) demonstrates the spring-neap modulation of the turbidity. The mouth of Southampton Water is more exposed to tidal mixing and therefore more turbid than the mid-estuary. A five-compartment zero-dimensional pelagic ecosystem model including a sediment compartment has been developed to assess the impact of the variability of the light attenuation on the timing and the magnitude of the spring phytoplankton bloom. Using high resolution irradiance forcing and a constant coefficient of attenuation k set to the minimum May-June value, simulations compare well with discrete data of chlorophyll a, and less successfully with zooplankton and Dissolved Inorganic Nitrogen. A sensitivity analysis indicates that interranual variability in the phytoplankton spring bloom originates in order of importance from 1) parameterization of k 2) the variation of the seasonal cycle of surface irradiance 3) the intrinsic dynamics determined by the combination of fixed parameters of the ecosystem model.
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Wood, Christopher Charles. "Modelling macro-nutrient release and fate resulting from sediment resuspension in shelf seas." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/359077/.
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Armitage, John J. "Modelling the controls on melt generation during continental extension and breakup." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/66263/.
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Rifting is the process that leads to the formation of oceans. Rifting is the break up of continents, leading to the formation of new oceanic floor between the two continental plates. Although the concept of continental rifting is accepted within the scientific community, it is still debated what controls the volume and composition of igneous material generated at these constructive plate boundaries. Here I present the results of dynamic modelling of rifted margins. I have explored the consequences of margin and mantle structure on the melt generated during continental extension and breakup. The central aim is to understand how melting affects the rifting of continents, especially in the North Atlantic. In order to understand the enigmatic melt production observed around the North Atlantic various tools are developed for interpreting the model output. These are predictions of primary major element composition of the melt, rare-earth element composition of the melt, predictions of the crystallised mid-oceanic ridge basalt composition and the seismic velocity of the lower crust. The thickness of the lithosphere has a very large impact on the subsequent rifting style. Extension of a 125 km thick thermally and rheologically defined lithosphere that has no prior thinning produces little melt during breakup. The Southeast Greenland margin rifted above a pre-thinned lithosphere and at initial fast half spreading rates. Further- more, to generate the thickness, chemistry and seismic velocities observed off this margin, rifting was coincident with the arrival of a 50 km thick, 200 ◦C thermal anomaly. This thermal anomaly is not a plume, rather an exhaustible thermal layer that has drained along the sub-lithospheric topography from a distal plume. The melts generated are high in MgO, and depleted in TiO. They are depleted in rare-earth elements. This would lead to high seismic velocities within the underplate being, as observed off Southeast Greenland.
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Roberts, Zoe Louise. "The application of adaptive mesh modelling techniques to the study of open ocean deep convection." Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/65672/.
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The rapid cooling of the waters at high latitudes creates an unstable strati cation which in turn leads to localised overturning (sinking) of the water column. This process is called open ocean deep convection (OODC). The process of OODC occurs in stages. Initially, individual convective elements known as plumes form and cold, dense water descends from the surface. Over time these plumes build up to produce a well-mixed `chimney' of cold dense uid. This chimney then slumps and sinks, and restratication (the return to a stable state throughout the water column) occurs. It is widely accepted that OODC plays a main role in driving the thermohaline circulation (THC) and hence has a potentially major role in climate. However, the mechanisms of OODC itself are not fully understood, and there is much debate surrounding how it contributes to THC. One di- culty is that OODC tends to occur sporadically in only a few isolated regions around the globe, making direct observations dicult. As a result, theoretical and numerical investigations have become key to the development of our understanding of OODC. The scale on which OODC occurs presents a further issue, with traditional numerical representations (parameterisations) of OODC in global circulation models (GCMs) omitting convective detail due to resolution. Due to the scales on which OODC occurs, it has been dicult to numerically investigate the nature of OODC in the small scale at the same time as resolving basin scale circulation. With the advent of nite element methods and adaptive meshing techniques, it is now possible to study OODC in regional models without the need to parameterise. One such model, the Imperial College Ocean Model (ICOM) is employed in this thesis for these purposes. ICOM is a 3D nite element, non-hydrostatic model with an adaptive, unstructured mesh and non-uniform resolution, allowing modelling of i the gyre circulation and resolution of OODC simultaneously. As the use of an adaptive, unstructured mesh model is novel in investigating Greenland Sea open ocean deep convection, it is of interest to assess the accuracy of the ICOM model, and the amount of numerical diusion present. The classical uid dynamics problem of parallel plate convection provides a simple test problem for this purpose. A series of tests investigating the linear stability of various temperature gradients were performed in order to diagnose the amount of numerical diusivity associated with hexahedral, tetrahedral and adaptive meshes within ICOM, and ICOM was further compared with a leading GCM (MITgcm). The use of the linear instability problem was found to be a useful case against which to test numerical models in an attempt to diagnose implicit diusivity and viscosity. A series of experiments were conducted in order to identify any prevailing dierences between model convection in xed and adaptive mesh congurations, under varying durations of applied cooling, and using varying extents of horizontal cooling. The adaptive mesh proved to be highly suitable for studying the convective problem, it was less computationally expensive and free from the numerical instability observed on the xed mesh. The sensitivity of model convection to the introduction of stratication was investigated. Uniform cooling was applied across the surface of a domain initialised with a weak stratication over the surface 1500m and a more strongly stratied region below, and the development of a convective layer was observed within the initial upper layer. Convection was constrained to the upper layer of stratication, and some penetrative convection was identied in the early stages of the model run.
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Hemmings, John Christopher Paul. "Quantitative modelling of spatial variability in the north Atlantic spring phytoplankton bloom." Thesis, University of Southampton, 1999. https://eprints.soton.ac.uk/42095/.
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The effects of variability in the physical environment on the development of the spring phytoplankton bloom are investigated using a physically forced model of the annual plankton cycle in the ocean mixed layer. The model is optimised to fit survey data from the eastern North Atlantic, collected over a 1500 x 1500 km area between 39N and 54N, from April-June 1991, establishing the feasibility of using spatially distributed point-in-time data in model calibration. Measurements made below the seasonal pycnocline show the existence of an empirical relationship between preformed nitrate and salinity in this area, allowing salinity-based estimates of pre-bloom mixed layer nitrate concentration to be made. These estimates provide important additional constraints for the model. The observed spatio-temporal patterns, at scales between 36 km and 1500 km, in nutrients, chlorophyll and measures of bloom progression derived from these data with reference to pre-bloom nitrate are discussed, together with the corresponding patterns in seasonal stratification. During the spring bloom, when biogeochemical concentrations vary rapidly in response to the developing stratification, absence of data defining its history limits the value of comparison between point-in-time observations and model results. Predictions of variation in stratification at the seasonal time-scale from general circulation models (GCMs) can be used in place of observational data to force ecosystem models. However, the degree to which observations are used to constrain the model solutions should allow for both model error in stratification and misrepresentation of the seasonal development of stratification by the observations. The latter occurs due to sampling error associated with short-term fluctuations. It can be corrected for if a suitable contemporary sea surface temperature data set is available to define the variation of mixed layer temperature at the seasonal time-scale. It is shown that the accuracy of the GCM predictions can be improved by the application of meteorology specific to the year of observation. It is also shown that the sensitivity of the ecosystem model predictions to error in the physical forcing can be reduced by matching model and observations by a stratification measure, rather than by time, when comparing fields. The survey data show an important contribution to the stratification arising from the 'tilting' action of vertical shear on pre-existing horizontal buoyancy gradients in the winter¬ time mixed layer. This effect was severely underestimated by the GCM. The discrepancy can be accounted for by the absence of density fronts and mesoscale dynamics in the model. Ecosystem model results suggest that spatial variance in Zooplankton grazing, due to the effect of differences in the depth and duration of winter-time mixing on the over-wintering success of plankton populations, is one of the major factors controlling the spatial and temporal characteristics of the phytoplankton bloom.
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Weaver, Andrew John. "Numerical and analytical modelling of oceanic/atmospheric processes." Thesis, University of British Columbia, 1987. http://hdl.handle.net/2429/27560.
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Two problems in oceanic/atmospheric modelling are examined in this thesis. In the first problem the release of fresh water from a midlatitude estuary to the continental shelf is modelled numerically as a Rossby adjustment problem using a primitive equation model. As the initial salinity front is relaxed, a first baroclinic mode Kelvin wave propagates into the estuary, while along the continental shelf, the disturbance travels in the direction of coastally trapped waves but with a relatively slow propagation speed. When a submarine canyon extends offshore from the estuary, the joint effect of baroclinicity and bottom relief provides forcing for barotropic flow. The disturbance now propagates along the shelf at the first coastally trapped wave mode phase speed, and the shelf circulation is significantly more energetic and barotropic than in the case without the canyon. For both the experiments with and without a canyon an anticyclonic circulation is formed off the mouth of the estuary, generated by the surface outflow and deeper inflow over changing bottom topography. As the deeper inflow encounters shallower depth, the column of fluid is vertically compressed, thereby spinning up anticyclonically due to the conservation of potential vorticity. This feature is in qualitative agreement with the Tully eddy observed off Juan de Fuca Strait. A study of the reverse estuary (where the estuarine water is denser than the oceanic water) shows that this configuration has more potential energy available for conversion to kinetic energy than the normal estuary. Bass Strait may be considered as a possible reverse estuary source for the generation of coastally trapped waves. Model solutions are compared with field observations in the Bass Strait region and with the results of the Australian Coastal Experiment. The effects of a wider shelf and a wider estuary are examined by two more experiments. For the wider shelf, the resulting baroclinic flow is similar to that of the other runs, although the barotropic flow is weaker. The wide estuary model proves to be the most dynamic of all, with the intensified anticyclonic circulation now extending well into the estuary.
In the second problem the effect of the horizontal structure of midlatitude oceanic heating on the stationary atmospheric response is examined by means of a continuously stratified model and a simple two level model, both in the quasigeostrophic β-plane approximation.
Solutions are obtained for three non-periodic zonal heating structures (line source, segmented cosine, and segmented sine). Little difference is observed between the solutions for these two different models (continuously stratified and two level). There are two cases which emerge in obtaining analytic solutions. In case 1, for large meridional wavenumbers, there exists a large local response and a constant downstream response. In case 2, for small meridional wavenumbers, the far field response is now sinusoidal. A critical wavenumber separating these two cases is obtained. The effect of oceanic heating on the atmosphere over the Kuroshio region is examined in an attempt to explain the large correlations observed between winter Kuroshio oceanic heat flux anomalies, and the winter atmospheric surface pressure and 500 & 700 mb geopotential heights, both upstream and downstream of the heating region. In both models, the response is consistent with the observed
correlations. When western North Pacific heating and eastern North Pacific cooling are introduced into the models, a large low pressure response is observed over the central North Pacific. This feature is in excellent agreement with the observed correlations. A time dependent, periodic, two level model (with and without surface friction) is also introduced
in order to study the transient atmospheric response to oceanic heating. The height at which the thermodynamic equation is applied is found to be crucial in determining the response of this model. When the heating is entered into the model near to the surface, unstable modes are prevalent sooner than they would be when the heat forcing is applied at a higher level. As in the steady state models, two cases dependent on the meridional wavenumber ɭ emerge in the analysis. For small scale meridional heating structures (large ɭ), the response consists of an upper level high and a lower level low which propagate eastward with time. For large scale meridional heating structures (small ɭ) the response essentially consists of a wavenumber 3-4 perturbation superimposed on the solution for large ɭ.Science, Faculty of
Mathematics, Department of
Graduate
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Braby, Laura. "A study of Mesoscale Eddies, the Agulhas current and the evolution of its meanders using satellite observations and numerical modelling experiments." Doctoral thesis, Faculty of Science, 2019. http://hdl.handle.net/11427/31213.
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The Agulhas Current is the strongest western boundary current in the Southern Hemisphere and plays an important role in the exchange of heat and salt between the Indian and South Atlantic Ocean basins, thereby affecting global climate. The variability in the northern Agulhas Current is influenced by both cyclonic and anticyclonic mesoscale eddies, originating from the Mozambique Channel and south of Madagascar (known as source region eddies) and which propagate toward the offshore edge of the Agulhas Current. Using a combination of an eddy-tracking data set with in-situ surface drifter observations and altimetry-derived geostrophic currents, it is shown that source region eddies dissipate upon approaching the Agulhas Current. Their entrainment into the Agulhas Current affects its mean velocity and offshore position through a transfer of momentum, with anti-cyclonic eddies consistently increasing the Agulhas Current’s velocity by 0.16 ± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 ± 0.16 m.s-1 and shifting the current up to 144 ± 85 km offshore. These velocity anomalies propagate downstream at rates of 44 km.d-1 (anti-cyclonic eddies) and 23 km.d-1 (cyclonic eddies). Whilst existing numerical models are successfully able to capture many aspects of the Agulhas Current, many models are unable to accurately represent the observed eddy dissipation and interaction processes, affecting our understanding of mesoscale variability within in the current. In this study, we compare two simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy tracking algorithm assess the local effect of the changed wind stress on source region eddies and their interaction with the northern Agulhas Current. There is an overall reduction in eddy kinetic energy (EKE) of 33% over the Agulhas Current domain. Changes in eddy pathways, properties and energy conversion terms, resulting from the change in forcing from absolute to relative winds (the wind speed relative to the current speed) have resulted in significantly different mesoscale eddies in the regional HYCOM. The effects of the change in wind forcing on the variability within the Agulhas Current were examined and the differences between the two simulations were found to be very small. Finally, the evolution of meanders in the Agulhas Current, including the properties and dissipation of smaller meanders as well as mesoscale Natal Pulses type meanders, were assessed using both HYCOM experiments and compared to satellite observations. The representation of smaller meanders (under 50km in size) improved with the changed in wind forcing. However, larger Agulhas Current meanders (greater than or equal to 50km) which previously occurred too frequently in the regional HYCOM, are now too infrequent in the regional HYCOM, with an average of 1.1 meanders occurring each year. A decrease in the frequency of larger meanders was observed from the location offshore of Port Edward (30.22° E, 31.05° S) to the region of the ACT array (27.48° E, 33.35° S), in the satellite data as well as both model experiments, indicating that some of the meanders have dissipated and that both regional HYCOM models are able to resolve this.
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Arfeuille, Gilles. "Modelling the interannual variability of the Arctic sea ice cover." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=21505.
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A thermodynamic-dynamic sea ice model based on the granular material rheology of Tremblay and Mysak is used to study the interannual variability of the Arctic sea ice cover during the 41-year period 1958--1998. The sea ice model is coupled to both a mixed layer ocean model and a one-layer thermodynamic atmospheric model. The model is first run with monthly climatology for most of the thermodynamic and dynamic forcing components to obtain a stable periodic seasonal cycle. For the 41-year run, the monthly wind stress forcing is derived from analyzed sea level pressures from the National Centers for Environmental Prediction (NCEP Reanalysis) data. The atmospheric thermodynamic forcings are based on monthly climatology.In this thesis we explore the high-latitude sea ice circulation and thickness changes due to year-to-year variations in the wind field. We focus our study on the interannual variability of the sea ice. volume in the Arctic Basin, and the subsequent changes in the export of sea ice from the Arctic Basin into the northern North Atlantic via Fram Strait. (Abstract shortened by UMI.)
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Burger, Jessica. "Drivers of short-term variability in phytoplankton production in an embayment of the southern Benguela upwelling system: an observational and modelling study." Master's thesis, Faculty of Science, 2018. http://hdl.handle.net/11427/30063.
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In the southern Benguela upwelling system (SBUS), the wind-driven supply of nutrient-rich water from depth sustains elevated levels of primary productivity. St Helena Bay (SHB), a coastal embayment in the SBUS positioned north of an upwelling centre, is an area of water mass retention. In addition to supporting 40-50% of total SBUS productivity, SHB often experiences harmful algal blooms (HABs) and hypoxic conditions that are difficult to predict given the high sub-seasonal variability that characterises this region. To better understand this variability, net primary production (NPP), nitrate and ammonium uptake, and phytoplankton community composition were measured for ten days during the upwelling season at an anchor station in SHB. A period of active upwelling (days 1-5) was followed by one of relaxation (day 6-10), together constituting an “upwelling cycle”. During upwelling, the mixed layer was deeper than the euphotic zone and phytoplankton were light-limited, evidenced by high ambient nutrient concentrations and relatively low rates of NPP and nitrate uptake. During relaxation, water column stratification increased, restricting phytoplankton production to a shallow, well-lit surface layer in which nitrate was exhausted after three days. The subsequent decline in NPP and nitrate uptake rates confirms that nutrient availability succeeded light as the ultimate control on productivity during the relaxation phase. Of the three phytoplankton size classes investigated (0.7-2.7 µm, 2.7-10 µm, >10 µm), the 2.7-10 µm fraction contributed most to the measured increases in biomass and nutrient uptake rates. This was unexpected given that large (>10 µm) diatoms typically dominate in upwelling systems; however, the 2.7-10 µm size fraction achieved a faster growth rate and sustained it for longer than the other size classes. The success of this size fraction may be partly due to a capacity for luxury nitrate uptake, evidenced by a low biomass C:N ratio and a nitrate uptake rate that was decoupled from NPP. Throughout the experiment, the phytoplankton community comprised mainly Chaetoceros spp. and Skeletonema costatum. These diatoms occupy a large size range (2-80 µm), although it is likely that they mainly occurred in the 2.7-10 µm size class during the experiment. They also produce resting spores that may provide a selective advantage during seeding in highly variable upwelling systems, increasing their chances of proliferating when conditions become favourable. Once the water column stratified, the phytoplankton community diversified, with dinoflagellates and the large diatom, Coscinodiscus gigas (200-500 µm), becoming more abundant. The contribution of C. gigas to biomass and productivity was not fully accounted for in the measurements because collected seawater was screened (200 µm mesh) prior to incubation. However, a simple N₃P₃ ecological model parameterized with the observations suggests that their contribution would have been minimal. The hydrographic data indicate that another upwelling cycle commenced by day 10 of the experiment. This likely prevented the further proliferation of dinoflagellates, some of which are HAB species, that may have succeeded the small diatoms given a longer period of quiescence. One implication of this is that understanding the rapid cycling between light and nutrient limitation, as induced by an actively upwelling versus stratified water column, may advance our capacity to predict the occurrence of HABs in SHB.
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Andersson, Elinor. "Starttillståndets inverkan på hydrologisk prognososäkerhet i HYPE-modellen." Thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-296384.
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SMHI:s hydrologiska prognos- och varningstjänst använder sig av meteorologiska ensembleprognoser som indata i hydrologiska modeller. De hydrologiskaensembleprognoserna tar därmed hänsyn till framtida osäkerhet i temperatur och nederbördoch används som underlag vid utfärdandet av risker och varningar för höga flöden. För närvarande beaktas dock inte osäkerheten i modellens starttillstånd, vilket består av de tillståndsvariabler i modellen som beskriver bland annat markvattenhalt och snötäcke. I dennastudie undersöktes hur starttillståndet i den hydrologiska modellen HYPE inverkar på prognoser i syfte att kvantifiera osäkerheten och på sikt möjliggöra säkrare prognoser.Studien hade tre mål: 1) Ta fram ett förslag på hur starttillståndet kan varieras för att ge en god uppskattning av prognososäkerheten relaterat till det hydrologiska starttillståndet. 2) Undersöka sambandet mellan starttillståndens spridning och det hydrologiska prognosfelet. 3) Analysera hur årstider, avrinningsområdens area, sjöprocent, skogsprocent och höjd över havet inverkar på prognososäkerheten. En central hypotes var att mindre skillnad mellan starttillståndets vattenföring och den observerade vattenföringen vid prognosstart resulterar i mer träffsäkra prognoser. Studien begränsades av att starttillstånden endast genererades med hjälp av störningar i drivdata.Indata till HYPE-modellen var femton temperatur- och nederbördsserier som manipulerats i syfte att skapa en ensemble av olika starttillstånd. Denna ensemble användes sedan för att göra vattenföringsprognoser med observerad temperatur och nederbörd som drivdata. Studien omfattade 76 avrinningsområden från hela Sverige med data för perioden 1999-2008. Prognoser utfördes varje dygn och ensemblespridningen utvärderades 2, 4 och 10 dygn in i prognosen. Samma utvärderingar utfördes även på autoregressiva prognoser, vilket innebär att modellerad rättas utefter observerad vattenföring.Resultaten indikerade ett samband mellan ensemblespridning och prognosfel, vilket innebär att spridning kan användas som ett mått på starttillståndets osäkerhet. Prognosfelet korrelerade positivt med skogsprocent och negativt med avrinningsområdenas area, sjöprocent och höjd över havet. Samma samband uppvisades mellan dessa områdesvariableroch spridning. Spridningen var störst på vintern och våren då normalisering skett med medelvattenföring över tio år, och under vår och sommar då normalisering skett med medelvattenföring per månad. Hypotesen att mindre skillnad mellan starttillståndets vattenföring och den observerade vattenföringen vid prognosstart resulterar i mer träffsäkraprognoser bekräftades av resultaten. Implementering av en ensemble av olika starttillstånd i operationella prognoser vid SMHIs hydrologiska prognos- och varningstjänst föreslås i syfte att kvantifiera osäkerheten och därigenom utöka bedömningsunderlaget vid utfärdande av risker och varningar.The Hydrological Forecast and Warning Service of The Swedish Meteorological and Hydrological Institute (SMHI) use meteorological ensemble forecasts as input in hydrological models. The hydrological ensemble forecasts take the uncertainty of future temperature and precipitation into account and serve as the basis of issued risks and warnings of high flows. Currently not considered is the uncertainty of the initial state, which consists of state variables in the model describing for instance soil water content and snow pack. This study assessed the impact of the initial state on forecasts in the hydrological model HYPE aiming to quantify the uncertainty and eventually enable more accurate forecasts.There were three aims of this study : 1) Evaluate a suggestion about how the initial state can be varied to give a good estimation of forecast uncertainty related to the hydrological initial state. 2) Examine the relationship between the spread of initial states and the hydrological forecast error. 3) Analyze the impact of seasons, catchment area, lake percentage, forest percentage and elevation on forecast uncertainty. A central hypothesis was that a smaller difference between the discharge of the initial state and the observed discharge results in more accurate forecasts. A restriction of the study was that the initial states only could be generated by disturbances of forcing data in before the forecast.Input data to the HYPE model were fifteen temperature and precipitation series, manipulated to generate an ensemble of different initial states. This ensemble was then used to make discharge forecasts with observed temperature and precipitation as forcing data. The study was performed on 76 catchments all over Sweden with data from the time period 1999-2008. Forecasts were made every day and the ensemble spread was evaluated 2, 4 and 10 days into the forecast. Autoregressive forecasts where the modelled discharge is corrected after the observed discharge were executed and evaluated as well. The results indicated a relationship between ensemble spread and forecast error, which implies that the spread can be used as a measure of the uncertainty of the initial state. The forecast error and ensemble spread correlated positively to forest percentage and negatively to catchment area, lake percentage and elevation. The same trend was detected between spread and catchment characteristics. The spread was biggest in winter and spring when normalization was made with mean discharge for the ten-year period and in spring and summer when normalization was done with mean discharge per month. The hypothesis that a smaller difference between the discharge of the initial state and the observed discharge results in more accurate forecasts was confirmed by the results. An implementation of an ensemble of different initial states in operational forecasts at SMHI’s Hydrological Forecast and Warning Service is suggested in order to further quantify the uncertainty of hydrological forecasts, and thereby improve the basis of judgment when issuing risks and warnings.
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Kelly-Gerreyn, Boris Adrian. "Modelling sedimentary biogeochemical processes in a high nitrate, UK estuary (the Gt. Ouse) with emphasis on the nitrogen cycle." Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.273834.
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Marin-Moreno, Hector. "Numerical modelling of overpressure generation in deep basins and response of Arctic gas hydrate to ocean warming." Thesis, University of Southampton, 2014. https://eprints.soton.ac.uk/364170/.
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This thesis is split into the two scientific topics studied; overpressure development in deep basins and present-day and future gas hydrate dissociation in the Arctic. Locating and quantifying overpressure is essential to understand basin evolution and hydrocarbon migration in deep basins and thickly sedimented continental margins. The first part of this thesis develops two new methods, including an inverse model, to impose seismic and geological constraints on models of overpressure generated by the disequilibrium compaction and aquathermal expansion mechanisms. The results provide greater understanding of a low velocity zone (LVZ), inferred from wide-angle seismic data, in the centre of the Eastern Black Sea Basin (EBSB). The application of both methods in the study area indicate that the LVZ located within the Maikop formation, at ~3500-6500 m depth below the seabed (mbsf), is linked to overpressure generated, mainly, by disequilibrium compaction.
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Tremblay, Louis-Bruno. "Modelling sea ice as a granular material, with applications to climate variability." Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=34472.
Full textAbstract:
A new dynamic service model based on granular material rheology is presented. The service model is coupled to both a mixed-layer ocean model and a 1-layer thermodynamic atmospheric model which allows for an ice-albedo feedback. Land is represented by a 6-meter thick layer with a constant base temperature. A 10-year integration including both thermodynamic and dynamic effects and incorporating prescribed climatological wind stress and ocean current data was performed in order for the model to reach a stable periodic seasonal cycle. The commonly observed lead complexes, along which sliding and opening of adjacent ice floes occur in the Arctic sea-ice cover, are well reproduced in this simulation. In particular, shear lines extending from the western Canadian Archipelago toward the central Arctic, often observed in winter satellite images, are present. The ice edge is well positioned both in winter and summer using this thermodynamically coupled ocean-ice-atmosphere model. The results also yield a sea-ice circulation and thickness distribution over the Arctic which are in good agreement with observations. The model also produces an increase in ice formation associated with the dilatation of the ice medium along sliding lines. In this model, incident energy absorbed by the ocean melts ice laterally and warms the mixed layer, causing a smaller ice retreat in the summer. This cures a problem common to many existing thermodynamic-dynamic sea-ice models.The origin and space-time evolution of Beaufort Sea ice anomalies are studied using data and the sea-ice model described above. In particular, the influence of river runoff, atmospheric temperature and wind anomalies in creating anomalous sea ice condition in the Beaufort Sea is studied. The sea-ice model is then used to track the position of an ice anomaly as it is transported by the Beaufort Gyre and the Transpolar Drift Stream out of the Arctic Basin.
It can be inferred from driftwood data collected in the Canadian Arctic Archipelago that very different sea-ice drift patterns were present in the Arctic Ocean during the Holocene. In this study, the sea-ice model described above is used to examine the different modes of Arctic sea-ice circulation during this period, and also to infer characteristics of century-to-millennial scale changes in Arctic atmospheric circulation. (Abstract shortened by UMI.)