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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Investigation into decadal scale variability in the North Atlantic Ocean

Brown, S. C. 2002 (has links)
The subsurface density of the North Atlantic displays considerable variability on decadal timescales. In this thesis the variability is examined from two directions. First I show direct observations of oceanic Rossby waves, using historical subsurface temperature data in the North Atlantic. Previously, such waves have only been directly observed using satellite data. Subsurface temperature data are used to investigate westward propagating Rossby waves across the North Atlantic at 30.5°N to 34.5°N, during 1970-74 and 1993-97. The latter period is when high resolution satellite altimetry data are available for comparison. The comparison was very favourable, with observed phase speeds east of the mid-Atlantic ridge, 3.6 cm s-1 in the altimeter data and 3.4 cm s-1 for the corresponding period in the hydrographic data. The phase speed of Rossby waves (west of the mid-Atlantic ridge) was observed to be 6.1 cm s-1 in 1970-74, compared with 4.3 cm s-1 in 1993-00. Interannual variability of the phase speed was also observed in the altimeter data, this variability was greater in the west than the east. Data from a coupled climate model (HadCM3) are used to assess the ability of an assimilation scheme, based on temperature-salinity preservation, to represent salinity given temperature data. The assimilation scheme was amended to account for meridional frontal movements. The adjustment improves the pre-assimilated salinity in comparison to the true-salinity in all areas south of 55°N, in the North Atlantic. Finally initial condition experiments are performed to determine the predictability of sea surface temperature (SST) and heat flux. The temperature and salinity fields resulting from the assimilation scheme are used as initial conditions in a twin experiment of HadCM3. When the temperature and salinity of the initial ocean state are perfectly known, the SST anomaly forecast skill is up to 5 years. When the assimilation scheme is used to provide these initial conditions for a forecast, the skill of predicting the SST anomaly is almost 2 years. There was no skill in predicting the fluxes in HadCM3 on a seasonal or annual timescale. However the average winter flux anomaly, when averaged over 5 years was found to be predictable.

Kelvin Waves in a shoaling Shallow sea

Jesuthan, Vasthiampillai Joseph 2009 (has links)
No description available.

A study of dense water formation over Rockall Bank

O'Neill, Clare Katherine 2008 (has links)
Rockall Bank is a large undersea bank situated in the north-east Atlantic. The region features steep bathymetry with the depth of Rockall Trough reaching 3000 m, rising to the surface at Rockall rock itself. Winter convective mixing in this area is strong and can reach 600 m or more. As this is deeper than a significant proportion of the bank, the water column above the bank becomes cooler than in the surrounding area, and a "cold water patch" forms. This water has been observed moving off the slope as a dense water cascade, a process that is important for shelf-ocean exchange and ventilation and which is also biologically important. This research contributes to knowledge of the dense water formation over Rockall Bank by analysing remotely-sensed data as well as numerical model results. Within this study: i) satellite sea surface temperature data were obtained for a ten year time series and the presence and properties of the surface cold water patch were investigated, and ii) the POLCOMS numerical model was adapted to simulate the Rockall Bank under different climatological conditions. It was demonstrated that a surface cold patch could be seen regularly in the satellite SST data, though more often there is a cold front associated with the Bank with no distinct cold patch. The duration of the cold patch and the duration of the front are inversely linked, and the cold patch is seen for longer in winters with a greater winter temperature difference across the Bank. The model successfully simulated the formation of the dense water in three test years, and the results show that the Bank greatly amplifies the effect of changing meteorological conditions. Dense water formation is very sensitive to changes in the winter air temperature, with a 2°C temperature decrease leading to a three-fold increase in the density difference between the water over the Bank and the surrounding area. The model results show a limited amount of cascading during the cooler years, but no cascading at all in the warmer year. Such large changes in the dense water formation caused by a relatively small change in air temperature suggests that climate change will have a significant impact on this system.

Phytoplankton as indicators for eutrophication in Europe's regional seas

McQuatters-Gollop, Abigail 2008 (has links)
Eutrophication of marine and coastal waters is a growing concern throughout Europe's regional seas and an historical problem in the Black Sea and regions of the North-East Atlantic, particularly the North Sea. As the base of the marine pelagic food web, phytoplankton are sensitive indicators of environmental change and therefore may be used as indicators of eutrophication for the monitoring, management and mitigation of the effects of nutrient loading on coastal and marine ecosystems. However, due to the interactive effects of climate and eutrophication, it can be difficult to separate the climatedriven response of phytoplankton from changes induced by excess nutrients. Therefore, the aim of this work is to separate these two signals in order to explore eutrophication effects. Without historical knowledge of 'pristine' or unimpacted ecosystem states it is difficult to identify and assess the severity and magnitude of change. Even w~re spatially and temporally comprehensive ecological datasets are available, equivalent nutrient timeseries are rare and a method of linking phytoplankton dynamics to eutrophication is required. Because open sea ecosystems are less impacted by anthropogenic nutrients than those near shore, offshore regions may be used as reference areas in comparison with coastal systems to investigate the effects of nutrient loading. Changes observed solely in coastal systems are mo$1likely a result of local processes (such as eutrophication) while those observed in both open sea and coastal areas are probably a response to large-scale drivers (such as climate). Therefore the comparison of coastal and open sea data may reveal different (or similar) patterns ofchange in phytoplankton indicators. Throughout most of the North-East Atlantic climate appears to override the effects of nutrients on phytoplankton dynamics, although the two drivers have been found to have synergistic effects resulting in increasing chlorophyll levels in the coastal North Sea. Additionally, the 1980s North-East Atlantic regime shift is clearly visible in coastal and open sea chlorophyll concentrations and diatom and' dinoflagellate abundances, demonstrating the sensitivity of phytoplankton as indicators at both the biomass and functional group scales. In the Black Sea, an observed decrease in chlorophyll appears to be at least partially a result of changes in climate and is not solely attributable to the 'recovery' of the Black Sea ecosystem. Black Sea chlorophyll has also undergone a possible recent (2002) regime shift, although its significance is difficult to determine due to the short time-series of chlorophyll data available. . The successful use of phytoplankton as indicators of eutrophication in these two disparate sea regions at two different ecological scales suggests that the method of comparing coastal and open sea phytoplankton data could be applied to other European seas as a means of distinguishing betWeen the effects of climate and eutrophication.

Large-scale study of Calanus in the North Atlantic Ocean : macroecological patterns and potential impacts of climate change

Helaouet, Pierre 2009 (has links)
Marine ecosystems show natural fluctuation throughout a large range of spatial and temporal scales. Despite the large amount of study devoted to the North Atlantic Ocean, drivers of those fluctuations remain unclear. By changing global climate, polluting, introducing exotic species, expanding and intensifying land uses and overharvesting biological resources, human activities have degraded the global ecosystem and drastically accelerated species extinction rates. Consequences of this human forcing become apparent in the progressive degradation of ecosystem that are used by humans (Schroter et al. , 2005), climate change- induced shifts in species distributions toward the poles (Parmesan et al. , 1999) and higher elevations (Wilson et al. , 2005), and in rapidly changing phenology (Edwards & Richardson, 2004). Data collected by the Continuous Plankton Recorder (CPR) constitutes, by both their temporal and biogeographical extends, one of the most useful datasets to investigate further major marine management issues as the distinction between anthropogenic, climatically forced and natural ecosystems fluctuations. The present work is a contribution to environmental change biology focused on copepods Calanus species as key structural species characteristic of the North Atlantic Ocean and adjacent seas. The purpose is to (1) identify environmental factors leading to the large-scale distribution patterns of Calanus that occurred in the North Atlantic Ocean, and (2) to propose and investigate new methods to assess both fundamental and realised niches of a dominant species in these basins. Most current approaches using Hutchinson concept of ecological niches to model species distribution belong to correlative or mechanistic models. A correlative approach has been developed to assess statistical relationships between the observed spatial distributions of two congeneric species and a set of environmental variables characteristic of the studied area. The method is designed to show the seasonal dynamics of environmental restriction driving observed distributions. Both Calanus finmarchicus and C. helgolandicus environmental preferences and optimum have been defined for 11 environmental parameters. A principal component analysis (PCA) has been used (1) to quantify the importance on the spatial distribution of each environmental parameter and (2) to identify the ecological niche. A numerical analysis based on Multiple Response Permutation Procedures (MRPP) was utilised to assess the breath of each niche and to compare them. The egg production rate of Calanus finmarchicus has been defined to investigate the link between physiology, macroecological patterns and ecological niches. It typically assesses the fundamental niche as in opposition to the correlative approach, the model based on a fundamental biological process is more focused on the potential response of C. finmarchicus to environmental conditions. The simplicity of the method which used only Sea Surface Temperature (SST) allows us to use IPCC scenarios and predict a shift in distribution over the 21st century.

Wide angle reflection studies at sea

Walker, Christopher David Terence 1978 (has links)
A self contained, free floating, recoverable sonobuoy system is described, for use in marine reflection and refraction surveys, together with a mathematical derivation of the wave amplitudes to be expected in such investigations. A detailed examination of the reflection and refraction results from several areas in the North Atlantic, obtained in the summers of 1973 and 1974 during the course of two Durham University Geophysical Surveys, is presented, in conjunction with simple processing procedures designed to extract information concerning the physical composition of the sea floor sediments, on the basis of the theory developed in the text.

Oceanographic studies of the Aegean Sea

Sultan, S. A.-R. 1981 (has links)
During October/November 1978, oceanographic observations were made on the 7/78 Cruise of the R.R.S. "Shackleton" in the N.W. Aegean Sea. The study area is located within 22°30' to 24°30'E of longitude and from 39°15' to 40°25'N of latitude. Data were collected using STD probe, NIO bottles, expendable bathythermographs and surface continuous records of salinity and temperature using a thermosalinograph. The circulation and water masses in the N.W. Aegean Sea are studied in terms of vertical sections of the horizontal distribution of water properties, individual station curves T/S, S/D and T/D, geostrophic speeds and dynamic topography. Isopycnic analyses using relative salinity are made for subsurface water layers. Two main features are observed in the surface waters; these are: (a) A mixed layer of characteristics; depths 15-55m, densities 27.4-28.6 gm/cm, salinities 37.4-38.7°/oo, and temperatures 15.5-17.2°C. (b) A temperature inversion layer of characteristics; depths 35-90m, densities 27.7-28.8 gm/cm, and temperatures 16.3-18.0°C. In the water usually deeper than 75m, three water masses are recognized: (1) Intermediate water characterized by maximum salinity 38.95°/oo, temperatures 16.30-15°C and densities 28.5-29.0 gm/cm. It is present at depths between 75 and 200m. (2) Deep water present at core depth about 350m and is characterized by salinity maximum 38.92°/oo, temperature 14.00°C and density 29.2 gm/cm. (3) Bottom water present deeper than 400m and is characterized by a well-mixed layer of salinity 38.87°/oo, temperature 13.39°C and density 29.32 gm/cm. The horizontal circulation is dominated by a central cyclonic gyre, an intrustion of high salinity water at the east and southeast, and coastal currents associated with freshwater discharge from nearby rivers.

Distribution of certain trace elements in sea water and sediments in relation to local sources, Swansea Bay and adjacent parts of the Bristol Channel

Vivian, C. M. G. 1976 (has links)
No description available.

A finite element solution to the shallow water equations incorporating a moving boundary

Wahab, A. K. 1995 (has links)
This thesis presents a finite element procedure for the solution of the Shallow-Water Equations dealing with tide generated flows in estuaries and coastal areas incorporating a dynamic land-water interface. The flow algorithm employed is an implementation of the explicit Two-Step Taylor-Galerkin finite element method. The 2-dimensional, depth averaged flow domain is discretised into linear triangular 3-noded elements. A background mesh storing the bathymetric information of the domain is generated using a procedure based on the Delaunay Triangulation technique but with the nodes optimally positioned to take into account the contours of the bed. The moving boundary component is separate from the flow computation. They are coupled at the end of each time-step. Nodes on the land-water interface are moved according to the tidal level and bed slope. Elements and nodes are deleted or created along the boundary so as to ensure good quality triangular elements at all stages of the tide. To limit computational time in the moving boundary algorithm, only the affected areas are meshed. The frequency of movement is monitored so that an optimum balance between accurate presentation of the land-water boundary and fast computational needs is achieved. The presence of structures in the intertidal zone is also dealt with.

Some aspects of sedimentary bodies in parts of the Bristol Channel

Turner, S. R. 1976 (has links)
No description available.

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