The western boundary current rings shed from the Agulhas retroflection may be responsible for a considerable transfer of heat, salt and energy from the South Indian into the South Atlantic Ocean. Few hydrographic measurements have been collected from Agulhas rings in the South Atlantic Ocean and their characteristics and influence on the waters of the Cape Basin through which they pass are thus little known. The temperature, salinity, and nutrient data presented in the thesis were collected from three Agulhas rings on a number of recent hydrographic cruises in the South Atlantic Ocean. Temperature profiles, conductivity-temperature-depth measurements, nutrient data, GEOSAT altimeter data, and NOAA-11 satellite imagery were used to investigate one of the rings in May 1989. It had previously been postulated that the rings could have an important effect on the Benguela upwelling system and this thesis demonstrates the interaction of the ring with a filament from the upwelling system. An adverse influence of this interaction on the anchovy larval population is postulated, and cited as a possible cause of the very poor anchovy yearclass of 1989. The other two rings were encountered during winter (August 1990 and June 1992), closer to the retroflection, and only hydrographic observations were possible. One of the rings showed a very deep isothermal surface layer and evidence of a deep pycnostad at its centre. The deep stad is shown to be likely due to vortex stretching and possible sources for the water in the stad are suggested. Comparative hydrographic characteristics, water mass structure, velocity fields, and the potential for contribution to interbasin transfer of the three rings are presented and discussed in the thesis.

Includes bibliographical references. / A combination of numerous factors, including geographic position, regional orography and local sea surface temperatures, ensures that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events.

Word processed copy. / Includes bibliographical references (leaves 167-181). / In this thesis the variability of the South Atlantic climate is investigated. In order to account for both large and regional scales, two ocean models are applied. An ocean general circulation model, forced with the National Centre for Environmental Prediction / National Centre for Atmospheric Research reanalysis is used to investigate the South Atlantic leading modes and also investigate the South Atlantic response to El Niño-Southern Oscillation induced climate variability. A regional ocean model is used to investigate variability in the South East Atlantic., especially in the Angola Benguela Frontal Zone.

Ocean colour remote sensing is a valuable tool for deriving information about key biogeochemical variables over inland, coastal and ocean waters at scales unachievable via in situ techniques. However, broader use of ocean colour data is still limited by the need for users to choose among a seemingly complicated range of available satellite products and to understand the limitations and constraints of these products across a wide range of water types. This issue could benefit from the capability to seamlessly apply and blend watertype appropriate algorithms into a single output product that provides optimal retrievals over a wide range of water types. The assessment of the fuzzy membership of satellite remote sensing reflectance (Rᵣₛ) to pre-defined regional optical water types (OWTs) provides a framework for application and blending of OWT-appropriate algorithms on a per-pixel basis. This study presents the first characterization of the OWTs in the coastal waters of South Africa. The OWTs are determined through stepwise fuzzy c-means clustering of a systematically expanding and modified database constructed from in situ, synthetic and regionally extracted Medium Resolution Imaging Spectrometer (MERIS) Rᵣₛ. A database division allows separate and more detailed clustering of phytoplankton-dominated Rᵣₛ and backscattering-dominated Rᵣₛ into six and five classes respectively. Chlorophyll α (Chl α) algorithms are assigned per OWT based on lowest error and uncertainty. The blended Chl α product consists of weighted retrievals from five different algorithms, including two 4th order polynomial exponential algorithms utilizing the blue-green spectral region, two red-NIR band ratio algorithms, and a neural network. The algorithm blending procedure retrieves satellite-derived Chl α concentration ([Chl α]) with lower RMS error and uncertainty compared to individual algorithms and provides improved capability to retrieve [Chl α] for different South African water types with a single product over a range spanning almost four orders of magnitude. The eleven OWTs are utilized in the classification and algorithm blending framework and applied to the full archive of MERIS Level 2 reflectance between the years 2002 and 2012 over South Africa's coastal waters. The persistence of the OWTs is presented and linked to the prominent environmental and physical drivers, whilst regions with low total class membership sums are discussed in terms of satellite data coverage and data quality. A time series of the blended [Chl α] product displays improved capability to capture the ranges of variability observed in the coastal, shelf and offshore environment compared to currently available regional and standard MERIS Level 2 products.

Includes bibliographical references (p. 172-180). / Capitalising on the vast knowledge gained in the tropical Pacific, tropical Atlantic variability is viewed from an energetics perspective and contrasted against the Pacific at both seasonal and inter-annual timescales. The character of remotely forced thermocline depth variability is evaluated using available potential energy which succinctly quantifies the response of the upper equatorial ocean to large scale wind forcing. To facilitate the Atlantic-Pacific comparison, available potential energy values are derived from the temperature and salinity fields of two global ocean models.

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.

Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) records in combination with outputs from an Ocean Linear Model (OLM) and altimetric data are used to investigate the link between the equatorial Atlantic Ocean dynamics and the variability in the coastal region of Angola-Namibia at interannual timescales over 1998 to 2012. The PIRATA records help to define an index of equatorial Kelvin wave activities in the Equatorial Atlantic. There is a good agreement between PIRATA monthly dynamic height anomalies, altimetric monthly sea surface height anomalies (SSHA), and sea level anomalies calculated with an OLM at interannual time scales. This allows the interpretation of PIRATA records in terms of equatorial Kelvin wave propagations. Extreme warm or cold events in the Angola – Namibia area lag strong anomalous eastward equatorial propagations by 1–2 months. Remote equatorial forcing via equatorial Kelvin waves which propagate poleward along the west African coast as coastal trapped waves is at the origin of their developments. Results show a seasonal phasing, with significantly higher correlations between the equatorial index and coastal sea surface temperature anomalies (SSTA) off Angola-Namibia in October - April season. Then, a systematic study of all the Benguela Niño and Benguela Niña events before 1982 is done using an Ocean general circulation model in combination with the OLM outputs from 1958 to 2015. 26 anomalous strong coastal events (16 warm and 10 cold) are identified. The analysis of their evolution confirms the remote equatorial origin of most of these coastal anomalous strong events. Modelled meridional transport anomalies across the Angola Benguela Front (ABF) contribute to the development of these anomalous coastal warm events. Across the ABF, the results obtain with the net temperature transport are similar to the ones with net mass transport. Most anomalous events peak in October - April season. Lagged composites of surface temperature and wind stress anomalies in the equatorial and southeastern Atlantic reveal that both local and remote forcings develop simultaneously 1-2 months before the peak of Benguela Niño or Niña. At the monthly scale, local atmospheric forcing is more correlated with anomalous coastal events occurring in Southern Angola which is a non-wind-upwelling driven region. The results from this thesis open the possibility to predict Benguela Niño and Benguela Niña events using an index depicting the equatorial interannual variability associated with Interannual Equatorial Kelvin Wave propagation, especially from October to April when the coastal stratification is favourable to the imprint of coastal trapped waves in the surface layer.

Ocean turbulence is responsible for stirring and spreading ocean tracers, and contributes to the mean circulation as eddy bolus fluxes. The influence of the eddies on the mean circulation becomes particularly important in regions where mean geostrophic flows are weak, such as the meridional flow across the Antarctic Circumpolar Current. However, high resolution observations of eddies and their influence on the circulation are generally lacking, particularly in the deep ocean that cannot be observed via satellites. The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was designed to observe the transport and stirring associated with the eddies in the Southern Ocean, using RAFOS floats and a passive tracer. In the first half of the thesis, the imprint of the eddies on the large scale circulation ([greater than] 100 km) is assumed to be diffusive, and the corresponding eddy diffusivities are quantified using the long term behavior of the RAFOS floats. The eddy diffusivities are found to be suppressed in the presence of mean flows. These eddy diffusivity estimates from DIMES, along with estimates from a couple of other diffusivity studies, are then used to quantify the eddy bolus fluxes in the Southern Ocean, which were found to vary in response to the bottom bathymetry. The second part of this work, addresses the flow at the length scales of the submesoscale and mesoscale ([less than] 100 km). Here, in addition to the DIMES RAFOS floats, we also used surface drifter observations from an experiment, Grand Lagrangian Deployment (GLAD), conducted in the Gulf of Mexico. The goal was to observe the kinematic stirring properties at these smaller scales, and also to characterize the dynamics of the turbulence that is active by investigating the energy spectrum. At the surface ocean in the Gulf of Mexico, we characterized the scale dependent energy distribution over 5 orders of length scales (10 m - 1000 km) using second order velocity structure functions. Divergent motions were found to be dominant, over non-divergent motions, at length scales smaller than 5km, where the Rossby number was greater than one and the third order velocity structure functions indicated the presence of a forward energy cascade. These methods were also used to explain subsurface turbulence in the Southern Ocean with DIMES RAFOS floats. The RAFOS floats showed that divergent flows are also present in the deep ocean at length scales smaller than 30 km, and become comparable in magnitude to the non-divergent flows near 5 km. The observed dispersion of the floats was used to address the question - is the mixing at small scales due mainly to large scale shear (non-local) or small scale eddies (local)? The associated stirring was found to be local at depth. / A Dissertation submitted to the Geophysical Fluid Dynamics Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2016. / November 10, 2016. / Eddy diffusivity, Lagrangian Observations, Meridional overturning circulation, Ocean Turbulence, Relative Dispersion, Structure Functions / Includes bibliographical references. / Kevin Speer, Professor Directing Dissertation; William Landing, University Representative; William Dewar, Committee Member; Phillip Sura, Committee Member.

The wave energy flux along the southern African coastline regularly reaches extreme levels, seriously impacting coastal communities, infrastructure, as well as near-coast and offshore marine operations. Understanding the drivers behind past high wave energy events and their frequency is key to forecasting future events. Using both the in-situ wave buoy data recorded by the Council for Scientific and Industrial Research (CSIR) and satellite altimeter data, 2 global wave hindcast products are evaluated and the best-performing is chosen to assess long-term variability and trends around the coastline between 1979 and 2020. Seasonal trend analysis revealed significant increasing trends in offshore flux for all seasons, with spring having the strongest coastal trends. The role of the Southern Annular Mode (SAM), El Nino Southern Oscillation (ENSO), and the semi-annual oscillation (SAO) on the interannual monthly wave energy flux and direction variations were assessed. Individually each mode showed significant anomalies for at least one season, however often there are multiple active modes making things more complex. SAM has the strongest control on the flux anomalies with the negative (positive) SAM associated with above (below) average anomalies in both flux and westerly (easterly) direction anomalies. ENSO directly impacts the summer wave climate, and the SAO indirectly impacts the wave energy flux over spring and winter. All the in-situ wave height correlations showed changes when compared to partial correlations controlling for the other 2 modes. The SAO relationships showed the largest differences when accounting for the SAM and ENSO phases, generally reducing in strength. Constructive modal interference has led to both strong positive and negative anomalies in the past and will continue to do so in the future. The largest near-coast positive anomalies occurred under concurrent negative SAM and negative SAO, with more intense offshore anomalies under El Niño whereas the strongest negative anomalies occurred under a combination of La Niña with positive SAM and SAO phases.

The KwaZulu-Natal Bight is a small, coastal region along South Africa's north-east coast. It stretches from Richards Bay to Durban and has a wide shelf compared to the surrounding coastline. As a result, the Agulhas Current is forced offshore, allowing the formation of complex circulation features on the KwaZulu-Natal Bight's shelf that assist with recruitment and retention of marine organisms in this region. This study aims to gain a deeper understanding of the surface circulation within the KwaZulu-Natal Bight and its impact on the connectivity between several surrounding Marine Protected Areas. These include iSimangaliso, uThukela Banks, Aliwal Shoal and Protea Banks and the information about their connectivity contributes to the CAPTOR (Connectivity And disPersal beTween prOtected aReas) project. The aim of this study is met by using high-resolution CROCO model output over a 10-year period, in combination with particle tracking tools, wind and surface drifter data. According to the model's mean circulation, the KwaZulu-Natal Bight's surface currents have a strong south-westward flow on the continental shelf slope where the effects of the Agulhas Current are strongly felt, butare weak and variable on the shelf. Observed variabilities of the mean flow have no distinct seasonal pattern and include a north- eastward current that repeatedly dominates the shelf. It is referred to as the Natal Bight Coastal Counter Current, which originates within the semi-permanent Durban Eddy in the southern KwaZulu- Natal Bight, where it extends throughout the water column. The Natal Bight Coastal Counter Current stretches along the mid-shelf into the northern KwaZulu-Natal Bight, gradually becoming shallower, weaker and narrower. When anticyclonic eddies offshore of the Agulhas Current pass this region, they occasionally replace the Durban Eddy and its associated Natal Bight Coastal Counter Current with a southward flow on the KwaZulu-Natal Bight's shelf. Therefore, the circulation in the KwaZulu-Natal Bight appears to be primarily driven by perturbations at the Agulhas Current front. However, there is also some indication of a direct wind-driven influence in coastal waters inshore of the 50 m isobath. To investigate the impact of the KwaZulu-Natal Bight's circulation on the connectivity between the above-mentioned Marine Protected Areas, particle tracking tools are used. Virtual particles are released in each Marine Protected Area within the model, during multiple northward and southward KwaZulu-Natal Bight surface circulation events. Their pathways are tracked for 30 days and reveal an overall strong southward Marine Protected Area connectivity, which is driven by the Agulhas Current, while a northward connection is less commonly observed. The northward flow of the Natal Bight Coastal Counter Current increases the water retention within uThukela Banks, but it does not extend into iSimangaliso to establish a northward Marine Protected Area connection. However, when the Natal Bight Coastal Counter Current originates within Aliwal Shoal, it may result in a northward Marine Protected Area connection between Aliwal Shoal and uThukela Banks. In this study, the virtual particles represent passively drifting larvae that are buoyant. To make these simulations more realistic, the virtual particles should be able to sink and appropriate swimming behaviours could be considered. However, swimming abilities will likely be overpowered by the surrounding circulation and observations on these behaviours are difficult to make. Therefore, the passive dispersion used in this study to mimic their trajectories may be sufficient and provides valuable insight into the impact of the KwaZulu-Natal Bight's surface circulation on Marine Protected Area connectivity and larval dispersion. The virtual particle tracking tools used in this study are not limited to biological applications. Future studies could use them to investigate the path and accumulation regions of virtual pollutants, such as microplastics, to determine the regions in which clean-ups would be most effective.