Bibliography: leaves 119-133. / Regional wind-stress variability over the Benguela Upwelling System is described using 16 months (01 August 1999 29 November 2000) of satellite derived QuikSCA T wind data. The QuikSCA T data are compared to the climatologies presented by Kamstra (1985) and Bakun and Nelson (1991), as well as the long-term climatology (1968-1996) of the surface vector wind speed field off the coast of southern Africa, as derived from the 2.5° resolution NCEPINCAR reanalysis dataset. Broad scale similaritie"s are found between the QuikSCA T and the long-term NCEPINCAR climatology (1968-1996) data sets. This allows one to have confidence in using this scatterometer data to investigate details of spatial and temporal variability over the Benguela System. During summer, wind-stress maxima are found at approximately 17, 29 and 34°S. These maxima strengthen in late summer. The seasonal northward migration of the South Atlantic Anticyclone becomes apparent in late autumn, when the strongest wind-stress occurs north of 28°S. A significant wind-stress minimum is observed to develop slightly north of Cape Columbine (33°S) during autumn. To the north (10-23.5°S) the Benguela is characterised by relatively strong south-easterly wind-stress during winter. To the south (24-35°S) the Benguela is characterised by relatively weak westerly to south-westerly wind-stress during winter. A southward migration of southeasterly wind-stress is observed during early spring. By November the entire Benguela Upwelling System is once again characterised by southerly to south-easterly wind stress. Wind-stress variability is investigated using both a type of artificial neural network, known as the Kohonen Self Organising Map (SOM), as well as a wavelet analysis. Two independent SOM studies are conducted. The first study produced a 6x4 SOM output array, which is used to examine seasonal variability as well as the temporal evolution of two synoptic-scale wind events. For the second study both a SOM and a wavelet analysis are applied to an extracted data set to find that the system can be divided into six discrete wind regimes, 10-15°S; 15.5-18.5°S; 19-23.5°S; 24-28.5°S; 29-32.5°S; and 33-35°S. The wavelet power spectra for these wind cells span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10-23.5°S, the majority of the power occurs during winter, with a 6-16 day periodicity. Further south, 24-35°S, the majority of the power occurs in the summer. Here a bi-modal distribution occurs, with peaks of 6-16 and 35-40 days. Lastly a case study sequence of the spatial distribution of wind-stress, windstress curl and SST, at a location off the west coast of southern Africa (25-300S and 12-17°E), is discussed in relation to an intense, upwelling favourable, wind event that occurred from 11-20 February 2000.

Includes bibliographical references. / The South Atlantic Ocean acts as a key region of ocean teleconnections and water mass transformation, directly transporting waters from the Southern, Pacific and Indian Oceans towards the North Atlantic and feeding the upper branch of the Atlantic Meridional Overturning Circulation. To date, this region remains under sampled over long time periods. Models form useful platforms for the wholistic study of such regions where long term datasets are not available. However, it is important to first gauge to what extent the model deviates from available observations. In a detailed analysis, this study evaluates an eddy resolving 1/12° HYbrid Coordinate Ocean Model simulation in the South Atlantic and Agulhas regions with reference to available observations and literature. The outcomes of this research contribute to improving the configurations of future ocean models by identifying the limitations of the model analysed. In addition, analysis of the 50 year (1960-2009) simulation reveals that, in this model simulation, mesoscale activity and sea surface temperature trends in the Cape Basin are not related to Agulhas Current transport changes. These findings highlights the need for long term observational databases and further research into climate related trends and interannual variability of the ocean, in particular around Southern Africa.

Investigating the variability of the Agulhas leakage, the volume transport of water from the Indian Ocean to the South Atlantic Ocean, is highly relevant due to its potential contribution to the Atlantic Meridional Overturning Circulation (AMOC) and the global circulation of heat, salt and freshwater, hence to the global climate. As observations of the leakage are scattered in time and space, ocean model outputs are unavoidable to describe its mechanisms of variability. However, the Agulhas leakage is part of a system that still represents a challenge to modellers, because of its degree of non-linearity. As a consequence, this thesis reports the study of the mechanisms of variability of the Agulhas leakage in six ocean model outputs of varying resolution, vertical discretization and boundary conditions. Inter-comparing several model outputs allow one to identify which characteristics of variability are robust. It also provides valuable information on the model speci_cs required to correctly represent the mechanisms of variability of the Agulhas leakage and hence its contribution to climate variability. The Agulhas leakage is estimated at the Good Hope line, a section through the centre of the Cape Basin, using a Eulerian threshold integration method based on the thermo- haline properties of water masses along the line. The threshold method also takes into account the thermohaline biases of each model. Other methods have been tested but the former was selected because it yields an Agulhas Leakage that represents 60% of the actual magnitude as determined by a passive tracer method (only available in one simulation) and captures nearly all of the temporal variability.

In March 1997 a two-weeks field survey was conducted in Langebaan Lagoon and Saldanha Bay. The aim of this survey was to farther our understanding of the processes driving the mixing and the exchange at the Langebaan Lagoon-Saldanha Bay interface. The parameters measured included currents, water-levels, temperature, salinity, density and wind. The nature of the flow at the Langebaan Lagoon inlets was ascertained by combining statistical analysis of the measurements to a theoretical understanding of the system hydrodynamics. The flow in the vicinity of the straight was predominantly driven by the tide. It was found that during high tidal range periods, there existed an asymmetry between the ebb and the flood flows at both of the lagoon's inlets. When tidal forcing was strong, water particles released at the lagoon inlets during the ebb were subject to long drifts. The outflow from the east inlet appeared to take the form of a turbulent jet. At the west inlet strong frictional interactions between the flow and land boundaries occurred, causing the flow to rapidly expand and lose momentum and therefore impeding the formation of a jet. It was established that, generally, buoyancy forcing on the Langebaan Lagoon outflow would be small and that water issuing from the lagoon during the ebb would remained attached to the sea-bed as it propagated into Saldanha Bay. However, when Saldanha Bay was strongly stratified, the east inlet ebb jet would lift off from the bottom as it reached the 8m depth contour. The large drifts resulting from the sink-like nature of the inflow and the jet-like nature of the outflow induced a very rapid and strong exchange between Langebaan Lagoon and Saldanha Bay. The propagation of the lagoon effluent also contributed extensively to vertically stir the water-column in Big Bay. As the tidal range weakened, the regions of influence of the ebb and the flood overlapped to a greater extent and the exchange between the lagoon and the bay decreased significantly. The asymmetry between the ebb and the flood flows at the Langebaan Lagoon inlets generated a Lagrangian residual circulation, with the east inlet constituting the entrance for Saldanha Bay water, while the west inlet would be the exit route for Langebaan Lagoon water. Southerly winds, contributed to the overall residual circulation by driving water out of the Lagoon. Bibliography: 126-133 pages.

Word processed copy. / Includes bibliographical references. / Harmful algal blooms (HABs), typically attributed to dinoflagellate species, occur along the west coast of South Africa, particularly during the latter part of the upwelling season. As part of the Benguela Current Large Marine Ecosystem (BCLME) programme, a buoy has been developed locally for monitoring the development and occurrence of HABs. The mooring is situated three and a half kilometres offshore from Lambert's Bay, downstream from the Cape Columbine upwelling cell, on the west coast of South Africa, and collects high frequency time series data, available in real time. The instrument package on the buoy provides bio-optical data analogous to remote sensing reflectance, temperature and current profile data, and fluorescence. The locally developed mooring and observing system is the only real-time, bio-optical mooring in southern Africa. Its development, and configuratoin of the instruments, data collection and telecommunications systems, are fully documented in this thesis.

The Marginal Ice Zone (MIZ) in the Southern Ocean is a dynamic area shown to vary seasonally in width and extent northward. Remote sensing is the only tool available to give a large-scale picture of sea-ice conditions but it is important to note that only surface properties are visible in remote sensing products. Little is known about the hydrographical properties of the Atlantic Sector of the Southern Ocean in the austral winter as it is difficult to obtain shipboard observations. Using observations from southern elephant seals, it is possible to analyse properties such as salinity and temperature within the MIZ. These properties may then be related to observations of sea ice concentration obtained by remote sensing to qualitatively describe their relationships. This work provides an example of how data from selected seal observations and satellite data can be used operationally to define the hydrography of the MIZ, focusing on the years 2005 to 2009. Results show that the properties of the underlying water in the MIZ appear to be fairly constant in the region of 15°W to 45°E for the months of July and August. A warmer pool of water is evident in the east of this region. Based on what is known about the physics of the region and what is seen spatially regarding changes in temperature and salinity, the warm pool may be brought about by eddies along the South West Indian Ridge. Results also show that surface concentrations of sea ice may have an effect on the underlying properties of the water. A more detailed description of the characteristics and features in the MIZ may further help to understand key processes in the region. Changes in subsurface temperature and salinity may influence the formation of ice. This may in turn influence large to mesoscale processes in both the ocean and atmosphere. Understanding how these physical properties change and what may cause them to change can help further understanding of larger scale processes.

Bibliography: pages 152-161. / The Agulhas Current flows along the eastern coast of southern Africa as one of the largest western boundary currents in the world's ocean. On overshooting the southern tip of Africa at approximately 20°E, a retroflection loop is formed that causes the current to double back on itself and to form the easterly flowing Agulhas Return Current. No focussed investigation to establish the hydrography of this important component of global ocean circulation has to date been carried out.

Includes bibliographical references. / This study exploits the remotely sensed sun-induced fluorescence signal to infer phytoplankton assemblage dynamics in the southern Benguela via in situ bio-optical moorings, and through the use of satellite sensors.

Includes bibliographical references (leaves 189-200). / Weather stations across the northern KwaZulu-Natal coastline recorded over 100 mm of rainfall over the 11112 February 2005, with Cape St. Lucia and Richards Bay measuring 111 mm and 96.8 mm, respectively. This heavy rainfall was associated with a mesoscale convective system (MCS) that initiated through small convective storms beginning early in the afternoon on 11 February 2005 and eventually decayed in the early morning hours on the 12th. The high-lying topography of the eastern escarpment and high diurnal surface heating possibly provided the trigger for the event. It was also identified that a combination of synoptic features in and around South Africa contributed to the evolution of the system. This particular MCS is investigated with a non-hydrostatic numerical model (MM5) to help determine which processes were important in its initiation and development, as well as what factors contributed to the associated heavy rainfall. The model is also used to conduct sensitivity tests to determine the role that local features, such as the regional topography and sea surface temperature, played in the evolution of the system. Through the various MM5 simulations, it was evident that the eastern escarpment played a key role in triggering the convective event, while it also had an influence on the low level winds that advected moisture into the region. A sea surface temperature sensitivity simulation highlighted the important role that the Agulhas Current plays in supplying moisture to fuel extreme precipitation events in South Africa. The significance of resolving large-scale features in the mid-latitudes in numerical simulations of weather events in South Africa was identified when excluding these features from the simulation. Through these simulations it was identified that the development of the MCS and the heavy nocturnal precipitation was due to a combination of the continuous moisture supply into the region, a conditionally unstable atmosphere, and uplift due to low level convergence and the local topography.

Bibliography : leaves 81-85. / Although upwelling was found to be more or less perennial along most of the coast south of 16 °S, seasonal variations were observed for both the SST and the upwelling favorable wind conditions. Inter-annual variability is common, and with these datasets it was possible to highlight periods of anomalous conditions. Results indicated that both the seasonal and inter-annual variability between the northern and southern parts of the Benguela system is quite different, with stronger seasonality observed in the southern Benguela.