Browsing by Author "Roy, Claude"

Now showing 1 - 7 of 7
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    Analysis of variable scatterometer wind fields in the Benguela upwelling region
    (2001) Johnson, Ashley Stratton; Shillington, Frank; Nelson, G; Roy, Claude
    The dissertation seeked to understand the effectiveness of satellite based data collection of wind fields along the west coast of southern Africa, pertaining to particular synoptic atmospheric systems. A comparison between data from two automatic coastal weather stations and measurements obtained by the NASA scatterometer (NSCAT) for the period 1 December 1996 to 31 May 1997 yielded a correlation coefficient of 70%.
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    Comparative analysis of the seasonal sea surface temperature & wind stress in the four major eastern boundary current systems
    (2006) Lamont, Tarron; Shillington, Frank; Roy, Claude
    The coastal upwelling regions along northwest Africa (the Canary system), southwest Africa (the Benguela system), North America (the California system), and South America (the Peru-Chile system) were studied and compared on a seasonal timescale. A 17 -year NOAA Pathfinder Sea Surface Temperature (SST) dataset with a spatial resolution of 9km was used to describe the large-scale temporal and spatial variability of upwelling within the four regions. An upwelling index (OSST) was derived in order to add to the patterns of upwelling variability described from the sea surface temperature. The upwelling index was also used to describe the similarities and differences between the four upwelling regions. A 10-year ERS wind stress dataset with a spatial resolution of 10 x 10 was used to derive the offshore Ekman Transport in each of the regions. The offshore Ekman Transport was used to supplement the description of SST's in each system. Principal Component Analysis was used to investigate the variance structure of the anomalies of the sea surface temperature in each of the four regions. The results of the principal component analysis are interpreted in terms of the underlying physical dynamics.
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    A description of the seasonality and its variability in a numerical ocean model of the Southern Benguela Region
    (2004) Bergman, Selwyn; Shillington, Frank; Roy, Claude
    The Southern Benguela has, for various reasons, been a focal point for research particularly in recent years. One of the most notable reasons for this interest can he found in the presence of economically viable fish species and their relationship to the process of upwelling. A numerical model of the entire Southern Benguela Region has previously been set up in order to assist furlher studies in the region. This dissertation presents the use of the model in an investigation on the seasonality and the interannual variability in temperature, salinity and general circulation in the region. Hovmuller Plots of the climatology and the associated anomalies were calculated in order to determine the characteristics of the seasonal cycle. It also became possible to deduce a vertical structure of upwelling.
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    Descriptive analysis of a virtual transect through the Benguela Upwelling System
    (2003) Chang, Nicolette; Shillington, Frank; Roy, Claude
    With the limitations of in situ observations of the ocean, numerical modeling of the ocean can easily provide clues to the structures and processes that lie therein. This project is an example of the examination of numerical model output. In this case, the area of interest is an area of ocean off Namaqualand which displays a reaction to local wind forcing. A transect was performed off the coast using output from ROMS (Regional Ocean Modeling System), forced under realistic winds obtained from weekly averaged ERS data. Temporal and spatial analysis yield information on the climatological characteristics of this region.
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    Modelling the effects of spatio-temporal spawning variability on the transport and retention success of sardine (Sardinops sagax) eggs and larvae around South Africa
    (2018) McGrath, Alice; Moloney, Coleen L; Van der Lingen, Carl D; Hermes, Juliet C; Roy, Claude
    The transport and retention of sardine (Sardinops sagax) eggs on the west and south coasts of South Africa were investigated using a Lagrangrian, particle tracking, individual-based model (IBM), coupled with two 3-D hydrodynamic Regional Ocean Modeling System (ROMS) model configurations of different generations. The same IBM is coupled with PLUME, an older ROMS model configuration used in a previous sardine IBM for the southern Benguela, as well as BENGSAFE, a more recent, better resolved ROMS model configuration. Results from the two runs were compared to see if and how the underlying ocean model affects modelled retention and transport. An updated IBM, extended spatially and temporally to better represent sardine spawning and recruitment, was then coupled with the BENGSAFE model configuration, and assessed. The effects of spawning area, and month, year and depth of particle release, were investigated for each coupled 3D-IBM simulation using linear models. The portion of particles transported from the south coast to the west coast was a focus throughout, as this is of great importance to the management of the sardine fishery. In all simulations, three main systems were identified depending on where particles were released and recruited. Two of these systems were retention-based: particles released on the west coast and retained in the west coast recruitment area (WC-WC), and particles released on the south coast and retained in the south coast recruitment area (SC-SC). The other system is transport-based and represents particles released on the south coast and transported by ocean dynamics to recruit in the west coast recruitment area. Results were similar for the west coast when either hydro- dynamic model configuration was used; depth of release proved to be important in this system with its effect varying depending where on the west coast particles were released. Differences occurred on the south coast, with more transport to the west coast and more offshore loss in the higher resolved BENGSAFE IBM than the PLUME IBM. The difference was attributed to the positioning of the PLUME model's boundary in close proximity to the Agulhas Current, and the conditions at this boundary not properly resolving the current. The initial BENGSAFE IBM was spatially restricted to match the PLUME IBM, and therefore south coast spawning and recruitment was not fully represented. Therefore, the BENGSAFE IBM was spatially extended on the south coast and run for a longer time period. Similar results were obtained in the extended BENGSAFE IBM and the initial BENGSAFE IBM. However south coast retention increased and the average percentage of particles transported to the west coast decreased slightly. Spawning area proved to be an important determinant of retention and transport success in all recruitment systems, and the further east a particle was spawned the less likely it was to be transported to the west coast, and the more likely it was to be retained on the south coast. Transport to the west coast was most successful in late winter; a significant monthly effect on transport success was present. Correlations between standardised anomalies for stock assessment model estimated recruitment and IBM modelled retention/transport for the west and south coast were not significant, highlighting that retention and transport are not the only determinants of recruitment success. The extended BENGSAFE IBM can act as a base level to which extra layers of biological complexities can be added, such as horizontal and vertical egg distribution matching observations, incorporating gonadosomatic index (GSI) values, diurnal vertical migration and buoyancy.
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    Specific applications of satelite remote sensing to the Benguela ecosystem
    (2005) Weeks, Scarla Jeanne; Shillington, Frank; Barlow, RG; Roy, Claude
    The objective of this thesis was to utilise high resolution satellite data, from the NOAA AVHRR and OrbView-2 SeaWiFS sensors, to investigate the upper layer dynamics of the Benguela ecosystem in more detailed space and time scales than previously undertaken. The standard for SeaWiFS bio-optical algorithms and processing parameters are not ideal for the highly productive Benguela waters. Hence, a detailed investigation was undertaken, and the processing parameters modified for more optical application to Benguela waters. Examination of the individual radiances used in the bio-optical algorithm revealed that constituents, other than chlorophyll, were at times contributing significantly to the in-water light field. The approach adopted in this thesis was that the application of satellite data to Benguela waters should be considered either as qualitative, for event scale phenomena, or quantitative, for the longterm study.
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    The variability of Lagrangian transport in the southern Benguela Current upwelling system
    (2020) Ragoasha, Moagabo Natalie; Reason, Christopher; Herbette, Steven; Veitch, Jenny; Cambon, Gildas; Roy, Claude
    This study analyses the physical mechanisms that impact Lagrangian pathways and transport in the southern Benguela upwelling system (SBUS),an environment in which currents are key components of many important ecological processes, including the dispersal of marine larvae. Physical advection by currents is an important mechanism for egg and larvae transport success in the SBUS since the spawning areas and recruitment areas are separated by a long distance. High-resolution numerical model simulations of the SBUS coupled with particle tracking experiments are used to investigate Lagrangian pathways between the Cape Peninsula (34◦S) and St Helena Bay(32◦S) and how they are linked to the oceanic circulation. Transport success, given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released, is used quantify the alongshore connectivity between the two regions. We have identified and quantified the following physical drivers: (i) Benguela Jet, (ii) offshore Ekman transport,(iii) inner shelf poleward current, (iv) mesoscale eddies to be responsible for the spatial and temporal variability of the alongshore connectivity. The Benguela Jet was found to be the dominant driver of the connectivity at both seasonal and interannual timescales. Moreover, the presence of anti-cyclonic eddies near the shelf-edge negatively impact transport success by advecting particles into the open ocean. The opposite occurs with shelf-edge eddies as they transport particles onshore onto the shelf and the Benguela Jet contributing to positive transport success anomalies. These findings will provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs.