Browsing by Author "Shillington, Frank"
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- ItemOpen AccessAgulhas current variability determined from space : a multi-sensor approach(2011) Rouault, Marjolaine; Shillington, FrankSatellite remote sensing datasets including more than 6 years of high frequency Sea Surface Temperature (SST) imagery as well as surface current observations derived from 18 years of merged-altimetry and over 2 years of Advanced Synthetic Aperture Radar (ASAR) observations are combined to study the variability of the Agulhas Current. The newly available rangedirected surface currents velocities from ASAR, which rely on the careful analysis of the measured Doppler shift, show strong promise for monitoring the meso to sub-mesoscale features of the surface circulation. While the accuracy of ASAR surface current velocities suffers from occasional bias due to our current inability to systematically account for the wind-induced contribution to the Doppler shift signal, the ASAR surface current velocities are able to consistently highlight regions of strong current and shear. The synaptic nature and relatively high resolution of ASAR acquisitions make the ASAR derived current velocities a good complement to altimetry for the study of sub-mesoscale processes and western boundary current dynamics. Time-averaged range-directed surface currents derived from ASAR provide an improved map of the mean Agulhas Current flow, clearly showing the location of the Agulhas Current core over the 1000 m isobath and identifying the region at the shelf edge of the north-eastern Agulhas Bank as one of the most variable within the Agulhas Current. To determine the variability of the Agulhas Current, an algorithm to track the position of the current is developed and applied to the longer merged-altimetry and SST records. Limitations associated with altimetry near the coast favour the use of the SST dataset to track the position of the Agulhas Current in its northern region. In the southern Agulhas, where the current lies further from the coast, altimetry is suited to monitoring the position of the Agulhas Current. The front detection analysis conducted on the SST dataset in the northern Agulhas reveals the complex nature of Natal Pulses. The downstream passage of the Natal Pulses is associated with the generation of secondary offshore meanders at the inshore edge of the current. Perturbations formed during the passage of Natal Pulses evolve rapidly to either dissipate, re-merge with the initial Natal Pulse or in some rare occasion, detach from the Agulhas Current.
- ItemOpen AccessAnalysis of variable scatterometer wind fields in the Benguela upwelling region(2001) Johnson, Ashley Stratton; Shillington, Frank; Nelson, G; Roy, ClaudeThe 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%.
- ItemOpen AccessAspects of sea level variability in the southwest Indian Ocean and the east coast of Africa - (latitude 0-35°S and from the coast to 60°E)(2013) Amollo, Joseph Odhiambo; Shillington, FrankAnalysis of tide gauge sea level observations of varying durations in the southwest Indian Ocean and the East coast of Africa (Lamu, Mombasa, Zanzibar, Durban, Port La Rue and Port Louis) show variability which are related to global, regional time scales, local weather and climatic changes, oceanographic and hydrological forcing that manifest in both short and long time scales. The investigations on the tide gauge sea level observations are conducted through the separation of the total sea level measurements into the contributing components (tides and residuals) using a Matlab in built software (t-tide). Short time scale sea level variability in the southwest Indian Ocean is due to the effects of tides which exhibit tidal range variations with latitude and shelf width, storm surges resulting from tropical cyclones passage especially in the mid-latitude region, atmospheric pressure fluctuations over the surface of the sea and local wind fields. Sea surface temperature variations during summer and winter result in differential heating of the ocean surface and contribute to the observed sea level variability at seasonal time scale especially in the region 25°S and southwards where the temperature differences are large. The equatorial region is characterized by a near constant sea surface temperature that sustains thermal expansion of the upper layer of the ocean water throughout the year. Monsoon periods show significant and variable wind speeds that impact on sea level variability in the southwest Indian Ocean and the East coast of Africa and are greatest during the summer monsoon (from June to August). On longer time scales (Interannual and decadal), sea level variations in this region is mostly influenced by the El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). During the 1997/98 El Nino event, the sea levels are significantly higher than normal at the coast and the islands. During the 2000/2001 La Nina, the sea levels are significantly lower than normal at the coasts in the southwest Indian Ocean. Indian Ocean Dipole effects are significant in the southwest Indian Ocean during the period 2006 through to 2008 and are more enhanced in 2007. The annual highest sea levels in this region are influenced by the year to year changes in weather pattern and the perigean cycle of the tides on a 4.4 year period but their secular trends are not statistically significant.
- ItemOpen AccessThe bio-optical detection of harmful algal blooms(2005) Bernard, Stewart; Probyn, Trevor; Shillington, FrankAn analytical framework for the simulation and quantitative interpretation of ocean colour data is presented, providing an inverse reflectance algorithm designed for the detection of harmful algal blooms. The adopted framework focuses on establishing quantitative relationships between optically important algal intracellular properties and inherent optical properties (IOPs), such as the absorption and backscattering coefficients, and the resultant effects on remote-sensing reflectance. A principal aim of the study is to establish the determinant variables of the IOPs associated with natural algal assemblages, and provide a means of simulating these IOPs. Algal size is an important determinant of optical properties, and the study demonstrates algal IOP simulation, using equivalent particle size distributions that can be simply parameterised with regard to effective cell diameter. Statistical analyses of causal variability are also conducted on absorption data from a variety of natural algal assemblages, revealing the relative importance of cell size, intracellular Chi a concentration, and accessory pigment complement. An improved understanding of algal angular scattering is regarded as key to the analytical modelling of ocean colour, and the use of two-layered spherical models for the simulation of algal scattering properties is investigated. Preliminary validation of the combined use of the equivalent size and two-layered models indicates that they are capable of adequately simulating the remote-sensing reflectance properties of high biomass bloom waters.
- ItemOpen AccessBio-optical investigation of phytoplankton production in the southern Benguela ecosystem(2011) Lamont, Tarron; Barlow, RG; Shillington, FrankThis study aims to evaluate various existing models for the estimation of PP in the southern Benguela ecosystem, and to investigate the photophysiological responses of phytoplankton to changes in environmental conditions.
- ItemOpen AccessCharacteristics of near surface circulation patterns in the Benguela as derived from the ADCP(Acoustic Doppler Current Profiler).(2002) Iita, Aina T; Shillington, Frank; Rae, Christopher DuncombeThis study forms part of one of the Benguela Environment Fisheries Interaction and Training (BENEFIT) program projects, whose main objective is to implement effective and quality ADCP data collection in the Benguela region. The thesis has two main components, firstly it undertook to investigate and assess the data quality, processing methods and software of ADCP data that are used to collect and process the ADCP data available in the Benguela region. An inter-comparison was made between the two different formats for data collection, i.e. raw and RDI proprietary "TRANSECT"processed data (which is more readily available in South Africa), to evaluate their respective accuracy in depicting current flow. The raw data were validated (edited and calibrated) using the CODAS software package before the current vectors could be drawn. TRANSECT -processed data could not be validated therefore vectors were drawn from the un-validated data. Data used here was collected during a monthly monitoring survey on 06-08 August 2001 on board one of the South African research vessel Algoa. The comparison results showed that TRANSECT -processed data includes unedited errors, noise and biases, which are already averaged into the data by the program and that the raw data presented a more realistic current flow. Secondly, the study undertook to apply the ADCP data collected to describe the state of oceanography of the Benguela region during the survey periods. Data from monitoring survey in 06-08 August 2001 was used to describe the southern Benguela while the second data set from a cruise conducted off Namibia in October 2000 was used to describe the oceanography of the central and northern Benguela. Most of the structures observed were in support of the literature and confirming previous studies of the region. In the southern region, the equator ward shelf jet off Cape Peninsula was observed to be forced offshore and entrained in an anticyclonic-like feature, which appeared to be part of a wann Agulhas filament. In northern Nambia, a strong poleward movement of the wann Angolan water was measured.
- ItemOpen AccessThe chemical and bio-optical characterisation of gelbstoff in southern African waters : a preliminary analysis(1996) Bernard, Stewart; Shillington, FrankThis study will attempt to begin the bio-optical characterisation of gelbstoff in southern African waters. Gelbstoff is a collective term, in itself perhaps an indication of a poorly understood phenomenon, given to a complex group of macromolecular organic compounds. It is the common bio-optical properties of these compounds that cause such an association, specifically the exponential decrease of absorption with increase in wavelength, resulting in typical absorption spectra decaying exponentially from a maximum in the ultra-violet. It is the accurate measurement or inferral of these spectra that is the primary aim of any bio-optical investigation of gelbstoff.
- ItemOpen AccessComparative analysis of the seasonal sea surface temperature & wind stress in the four major eastern boundary current systems(2006) Lamont, Tarron; Shillington, Frank; Roy, ClaudeThe 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.
- ItemOpen AccessA description of the seasonality and its variability in a numerical ocean model of the Southern Benguela Region(2004) Bergman, Selwyn; Shillington, Frank; Roy, ClaudeThe 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.
- ItemOpen AccessDescriptive analysis of a virtual transect through the Benguela Upwelling System(2003) Chang, Nicolette; Shillington, Frank; Roy, ClaudeWith 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.
- ItemOpen AccessA descriptive analysis of the genesis and translation of a dipole vortex from the Agulhas retroflection region(2000) Whittle, Christo Peter; Shillington, Frank; Rae, Christopher DuncombeAn anomalous leakage of Agulhas Current water into the south-east Atlantic Ocean, exhibiting a mushroom-like shape, was observed during routine observations of A VHRR satellite imagery in early December 1996. The development of this anomaly was followed on the sea surface temperature (SST) imagery and it was tentatively identified as a consequence of filament interaction between the Agulhas retroflection and an occluding Agulhas ring. This interpretation prompted a cruise onboard the FR.S Africana with the objective of conducting a hydrographic survey of the Agulhas ring and the associated filament near Cape Town. A descriptive analysis, gleaned from A VHRR satellite imagery and in situ data, of the hydrographic characteristics of a vortex dipole, surveyed during this cruise, is presented in this thesis. An analysis of water mass properties and geostrophic flow patterns determined that an Agulhas ring and a cyclonic eddy, containing Benguela Current water in its core, constituted a dipole vortex in the south-eastern Atlantic Ocean. During the period of the hydrographic survey, the secondary vortex exhibited an anticlockwise rotation of 8. 6°/day around the Agulhas ring A warm filament, originating from the western Agulhas Bank, was entrained between the two counter-rotating eddies, thus resulting in the mixing of Agulhas Bank water into the South Atlantic Ocean. Satellite altimetry and A VHRR imagery were used to "backtrack" the vortex dipole to its origin at the Agulhas retroflectiOn. By combining interpretations from the altimetry and A VHRR imagery, it was possible to describe the complex interactions the dipole displayed with the retroflection and the Agulhas Bank as it franslated in a north-westerly direction. The mushroom configuration, identified earlier on SST imagery, betrayed the presence of an adjacent pair of circulatory features of opposing spm. As the dipole translated northward, it interacted with the Agulhas Bank and the cyclone was strained, becoming a filament as it was forced between the Agulhas ring and the Agulhas Bank. West of Cape Town the dipole was re-established when the cyclone redeveloped, changing the orientation of the dipole so that a filament was drawn directly from the Agulhas Bank.
- ItemOpen AccessDistinguishing cyanobacteria from algae using bio-optical remote sensing(2014) Matthews, Mark William; Shillington, Frank; Bernard, StewartThis study advances the use of remote sensing for eutrophication and cyanobacterial bloom detection in inland and near-coastal waters. The hypothesis that prokaryotic cyanobacteria can be systematically differentiated from algae (or eukaryotic species) on the basis of their distinctive bio-optical features is tested using a novel in situ bio-optical dataset and remotely sensed data from the Medium Resolution Imaging Spectrometer (MERIS). The in situ dataset was collected between 2010 and 201 2 from three optically-diverse South African inland waters. An empirical algorithm, called the maximum peak-height (MPH) algorithm, was developed for operational determinations of trophic status (chlorophyll-α), cyanobacterial blooms and surface scum from MERIS. The algorithm uses top-of-atmosphere data to avoid the large uncertainties associated with atmospherically corrected water leaving reflectance data in optically-complex and turbid waters. The detailed analysis of the variability of the optical properties of the three diverse reservoirs provides new knowledge of the inherent optical properties of South African inland waters which have previously not been described. The study also provides the first detailed investigation of the effects of intracellular gas vacuoles on the optics of phytoplankton using a two-layered sphere model. The results demonstrate how gas vacuoles impart distinctive bio-optical features to cyanobacteria and cause backscattering to be enhanced. An advanced inversion algorithm is developed for detecting phytoplankton assemblage type and size from water leaving reflectance data. The algorithm, based on a direct solution of the equation of radiative transfer using Ecolight-S radiative transfer model, successfully distinguishes between phytoplankton assemblages dominated by small-celled cyanobacteria and those dominated by large-celled dinoflagellate species. It also provides reliable estimates of phytoplankton biomass (chl-α), and the absorption coeficients of phytoplankton and combined non- phytoplankton particulate and dissolved matter. Finally, the application of the MPH algorithm to a time series of MERIS data from 2002 to 2012 in South Africa's 55 largest reservoirs is likely to be the most comprehensive assessment of eutrophication and cyanobacteria occurrence from earth observation data yet performed. The results confirm that widespread cyanobacterial blooms and eutrophication remain issues of critical concern for water quality in South Africa.
- ItemOpen AccessDrivers of coastal sea level variability along the east and south of South Africa(2019) Nhantumbo, Bernardino João; Shillington, Frank; Backeberg, Björn; Nilsen, Jan Even; Reason, ChristopherSea level rise and variability is of great concern in the coastal areas where a significant part of the global population is settled. Therefore, understanding regional and local long-term sea level variability as well as its trend is critical. On the other hand, quantifying how the sea level has varied on different timescales and why, is critical for understanding sea level changes, and crucial for improving future global, regional, and local projections. In this study, monthly mean sea level records of seven individual tide gauges, from the east and south coast of South Africa were used to analyse the embedded timescales of variability. These timescales were separated through the Empirical Mode Decomposition (EMD) method. This is the first time that the EMD method has been applied to southern African tide gauge records. The sensitivity of the EMD method when dealing with data gaps was tested on artificially created gaps in monthly mean synthetic altimetry sea level records, representing the seven individual tide gauges under consideration. The missing values were filled by linear interpolation, average value and linear trend value. The results suggested that whichever gap filling method is applied, the separated EMD timescales will display a distorted temporal structure of the continuous time series. As a consequence, monthly mean tide gauge sea level records were optimised by filling the gaps as best as possible using satellite altimetry data and the adjacent tide gauge records where possible, and then the oscillatory timescales of variability were separated using the EMD method with the intent to determine their physical drivers. However, identifying a single driver for each separated timescale is challenging due to our limited knowledge of how sea level is linked to the various forcing mechanisms. Therefore, the timescales of sea level variability extracted using the EMD were grouped into sub-annual and interannual timescales, and their relationship to possible driving mechanisms was investigated. The sub-annual timescale indicates how sea level responds to the mesoscale and synoptic weather systems in the annual cycle, including seasonal and annual large-scale wind and atmospheric pressure pattern changes. The interannual timescale indicates an association with the climate indices including El Niño-Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode through large-scale sea surface temperature patterns and large-scale pressure and wind patterns. In addition, the results have suggested that the studied coastal sea level has an association with the Agulhas Current at both sub-annual and interannual timescale through absolute dynamic topography variations at the Agulhas Current core locations. However, due to limitations in Agulhas Current data, the study was limited to East London and Port Elizabeth and the results suggested that the Agulhas Current contribution is responsible for over 62% of the monthly sea level variability at East London. However, the results were not sufficiently consistent to suggest a firm conclusion at Port Elizabeth.
- ItemOpen AccessEquilibrium dynamics of the Benguela system : a numerical modelling approach(2009) Veitch, Jennifer Anne; Shillington, Frank; Penven, PierrickThe 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.
- ItemOpen AccessEstimating the spatial and temporal variability of primary production from a combination of in situ and remote sensing data a southern Benguela case study(2013) Williamson, Robert I; Field, John G; Shillington, Frank; Jarre, Astrid; Potgieter, AnetThe aim of this thesis is to produce fine resolution estimates of primary production in three-dimensional space at the temporal scale that these events develop. It is hypothesized that complex relationships among time sequences of physical and biological processes that influence primary production can be automatically discovered from archives of data. This study uses an archive of in situ ship-board data containing subsurface temperature and phytoplankton distribution profiles. Each profile is associated in time and space with satellite remotely-sensed wind, sea surface temperature and surface chlorophyll a data. The bottom depth, season and location of each profile are also recorded. The archive of depth profiles is simplified by mapping each profile onto one of twelve representative profile clusters obtained using the k-means clustering algorithm so that each cluster contains a set of similar profiles and their corresponding data. Relationships between remotely sensed surface features and chlorophyll a profiles are first obtained from a static Bayesian network using same day data. This is then taken further by analysing time-series of satellite data to predict likely temperature and chlorophyll a profiles for each pixel of a 4 km resolution satellite image.
- ItemOpen AccessIdentification and characterisation of submesoscale activity over the continental shelf in the Bay of Biscay(2014) Mawren, Daneeja; Guillaume, Charria; Veitch, Jennifer Anne; Shillington, FrankFrontal instabilities commonly detected in regions of freshwater influence (ROFIs), are considered as fundamental processes in the generation of submesoscale features (a few kilometres to tens of kilometres spatial scale and temporal variability of the order of a day). Consequently, the present study (part of DYMETER project, IFREMER) aims at exploring the development mechanisms of these structures over the continental shelf in the Bay of Biscay. The project is principally based on the analysis of remotely sensed images (high resolution ocean colour MODIS 800m) of Chlorophyll-α concentration, Sea Surface Temperature and Suspended Particulate Matter in view of identifying and characterizing submesoscale activity developing in the vicinity of the Loire River. A 10-year dataset (2003-2013) was explored and satellite ocean colour images revealed a spatial variability in the surface chlorophyll-α distribution. In this context, several events on daily, seasonal and interannual scales were selected and compared but the difficulty lies in tracking the continuous evolution of submesoscale structures in time and space due to the presence of clouds obscuring remote optical sensors. Hydrodynamic parameters like wind forcing and river runoffs were also studied to examine their impact on the submesoscale dynamics. Singularity exponent analysis was performed on the chlorophyll-α images to highlight distinct frontal structures which revealed in turn, a multitude of submesoscale fronts and filaments widespread in the upper ocean. The probability density function (PDF-skewness performed on singularity exponents) was used to characterize the submesoscale structures. The analyses carried out showed that during winter, frontal structures gain in intensity due to a high river outflow regime and dominant wind influence. As such, the PDF curve is skewed-right (strong frontal gradient) in winter and skewed-left (weak frontal gradient) in summer. Wavenumber spectrum analysis was also used to characterize submesoscale structures. Slope values ranging between -0.2 to -0.4 were noted but results obtained did not display significant differences in frontal spatial scales with time. Further investigations (beyond the scope of this thesis) will need to be undertaken to evaluate interactions between hydrodynamics and biogeochemistry (using satellite observations and coupled physical-biogeochemical models) for targeted events in the river plumes.
- ItemOpen AccessInvestigation of the nearshore, episodic poleward current in the southern Benguela : a numerical modelling approach(2006) Viljoen, Anél; Shillington, Frank; Brundrit, GeoffFisheries 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.
- ItemOpen AccessLatent heat flux and South African summer rainfall variability(1997) Courtney, Shaun; Shillington, FrankThe main purpose of this study was to determine if a relationship exists between oceanic latent heat flux and the summer rainfall of South Africa. Such a link would be useful for a better understanding and the prediction of monthly variability in the summer rainfall for South Africa. In order to investigate possible relationships between oceanic latent heat flux and summer rainfall in South Africa, point to point and point to field statistical correlations were made between gridded monthly COADS derived bulk oceanic heat fluxes and area averaged rainfall for the period 1950 to 1988. Correlations with the oceanic latent heat flux were not significant when the summer area averaged rainfall was divided into early and late summer seasons. This is due to the fact that different oceanic latent heat flux regions correlate with a different rainfall region each month. The results of monthly latent heat flux and summer rainfall demonstrated that there exists a statistical link between oceanic latent heat flux and summer rainfall and that this link could prove useful in the prediction of summer rainfall patterns. These results of the correlation between monthly latent heat flux and summer rainfall show that summer rainfall can be grouped into an all-area index that can be used as a proxy for the entire summer rainfall region. Results of these correlations further show that there are three major mechanisms that are at work over the six month summer rainfall period. These mechanisms show a link between the oceanic latent heat flux and summer rainfall variability. This study has shown that various oceanographic areas in the surrounding oceans correlate at different lags with South African summer rainfall. These correlations can be useful as precursors in predicting wetter or dryer rainfall events. Areas identified by correlation of oceanic latent heat flux regions as important as precursors for summer rainfall prediction are similar to those areas other researchers have identified in studies using OLR, SST and upper level winds (Jury 1995). This study adds weight to the already existing knowledge of these precursors of rainfall predictability.
- ItemOpen AccessModeling the effects of environmental and ecological processes on the transport, mortality, growth and distribution of early stages of Cape Anchovy (Engraulis Encrasicolus) in the Benguela system(2003) Parada, Carolina E; Mullen, Christian; Shillington, Frank; Field, John G; Fréon, PierreAn individual based modelling approach was used to study environmental factors and processes influencing the early life history of anchovy in the southern Benguela region and on the Agulhas Bank. The intention was to then establish the link between these factors and processes and the recruitment success of anchovy in the southern Benguela region. Specific factors that were explored were transport from the spawning grounds to the nursery area, advection, temperature- dependent growth and mortality, vertical migration behaviour, retention in the nursery area, as well as the possibility of a second nursery area on the Eastern Agulhas Bank. Eight individual based models were coupled to the output of a 3-D hydrodynamic model to study dispersion processes. Particles representing eggs and larvae were released (spawned) over the Agulhas Bank, and their movements were tracked during their transport to the west coast (the recruitment area). Although the eggs and larvae were initially considered to be neutrally-buoyant, passive particles, increasing levels of complexity were progressively incorporated by adding processes such as particle buoyancy, temperature-dependency of growth and mortality of eggs and larvae, vertical behaviour and retention. A series of experiments was run by setting the parameters representing the factors being investigated by the model, and estimating a primary, quantifiable response variable representing the dynamics of the system (e.g. particles successfully reaching the nursery area). An analysis of variance was employed to assess the significance and the sensitivity of each model to changes in the parameter values. A pattern-oriented analysis was then systematically applied to assess the validity of the results of the model. The results indicate that five processes and their interactions strongly influence the transport of spawning products arriving at and subsequently being retained in the nursery area: (1) the location of the spawning, (2) the buoyancy of the eggs, (3) transport by the jet current and its inter-annual variability; (4) the 3-D structure of the current in the nursery area on the west coast, (5) The swimming abilities of the pre-recruits in both the vertical and the horizontal planes. Two factors appear to be of major importance in effecting mortality during the period leading up to recruitment, namely temperature and offshore losses through advective processes. A conceptual model of the life history of anchovy in southern Benguela is presented and discussed in the light of the hypothesis proposed by Hutchings et al. (1998). A number of modifications to this hypothesis are proposed, specifically an extension of the spawning area, and the addition of several key biological processes. A new element that is proposed in this thesis is the possibility of the Eastern Agulhas Bank as an additional nursery and spawning area.
- ItemOpen AccessModelling Benguela niños using the regional oceanic modeling system (ROMS)(2007) Queiroz, Eurico Tiago Justino; Shillington, Frank; Monteiro, PedroThis 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.