The research was carried out with a view to developing a procedure for the computation of sea surface advection velocities from pairs of NOAA AVHRR infrared images. The procedure was designed for application to the oceanic regions around South Africa and cognisance had to be taken of restrictions imposed by the specific oceanographic conditions, availability of satellite data, as well as the capabilities of the image processing system used. As a first step, a set of image navigation algorithms was developed, based on elliptical orbit and ellipsoidal earth models. Orbit parameters were obtained from TBUS-bulletins and one or more ground reference points had to be identified on each. The navigation algorithms were then used to develop a procedure for the geometric transformation of images to a Mercator map projection. The transformation procedure was evaluated through use of test-images and the results indicated that the maximum errors which could be expected in the computation of advection vectors were 4-5 cm/sin the north/south velocity component and 6-7 cm/sin the east/west component if two images, 12 hours apart in time, were used for the vector computation. An automatic feature tracking method was tested as a means for computing advection velocities but was found to be unsatisfactory. As a result, a 'semi-automated' procedure was developed. This process is essentially a manual (point-wise) feature tracking procedure into which the template matching technique which formed the basis of automated procedures, was incorporated as a labour saving device. Tests indicated a time saving of 20-40 % on the manual procedure and more rapid computation than with the automated procedure. The feature tracking procedure was applied to three sets of AVHRR images of the South East Atlantic. To assess the precision of the vector computation procedure, two independent vector sets were computed. A comparison of the two sets indicated that the rootmean- square deviation in vector magnitude (speed) was about 6-8 cm/sand in the vector direction, about 31° (12° if very small vectors ≤ 6 cm/s are excluded). The computed vectors compared very well with reported results from conventional methods. The derived vector fields also provide the first really detailed description of surface currents in the sea off South Africa: eg. on the flow field in the southern Benguela Current, the circulation associated with Agulhas Current rings, and advective influences on the transport of fish eggs and larvae from the spawning grounds on the Agulhas Bank to the favoured recruitment area off the West Coast.

The aim of this study was to quantitatively investigate the mechanisms which drive carbon fluxes in an eastern boundary coastal upwelling system of which the Benguela is one of four comparable examples in the world. Three hypotheses describe the way the key aspects of the problem: ■ The Benguela upwelling system is, by virtue of its high primary production and sediment organic carbon accumulation rates, an important CO₂ sink. ■ The carbon export flux and the magnitude of the CO₂ sink in the Benguela System can be predicted from the C:N stoichiometry provided by the Redfield Ratio. ■ The inorganic carbon pump through coccolithophore production plays a minimal role in driving changes to the magnitude of both the carbon export flux and the air-sea CO₂ flux in the Benguela System. Carbon and nitrogen bulk water concentrations together with relevant physical parameters were measured along three transects which spanned the Benguela System defining the physical and biogeochemical characteristics of waters at the key stages of the upwelling cycle.

Along the SW tip of Africa (30-35° S, 17-20° E) topographic irregularities shear the wind stress field giving rise to coastal upwelling with an alongshore variability. The relationship between wind shear and differential upwelling is established using a blend of oceanographic and meteorological investigations. Mesoscale aerial survey case studies form the observational basis from which the spatial variations in winds and upwelling are compared. Coastal winds, controlled by the pressure gradient between the South Atlantic Anticyclone and a summertime interior low, become modulated through interactions with the circumpolar jet stream. Low level winds accelerate over portions of the southern Benguela current region through deflections forced by three pronounced capes, two with mountain ridges exceeding 1000 m. Selected case studies are utilized to contrast the characteristics of deep and shallow wind flow, and the exposed and sheltered offshore regions. The spatial variability of the low level wind and sea surface temperature fields is correlated by means of aerial survey techniques applied at alongshore spacings of 10 to 50 km. Vertical transect and profile data collected at altitudes up to 1 km illustrate the variable depth of wind flow. Vertical wind shear controls the interaction of topography and winds. Field results show that vertical shears of -2 (10⁻²)s⁻¹ produce horizontal wind vorticities of -6 (10⁻⁴)s⁻¹ and alongshore sea surface temperature gradients of 1°C (10km)⁻¹ , a characteristic of summertime upwelling in the southern Benguela region.

Includes abstract. / Includes bibliographical references. / The main objective of this thesis was to investigate the circulation in the ComorosBasin using observed and model datasets. These data were used to establish whether or not a Comoros Gyre exists and to investigate the nature of the eddy variability in the basin.The water masses in the Comoros Basin emulate those found further south in the Mozambique Channel. The presence of AAIW north of Madagascar confirmed that this water mass enters the Comoros Basin from the east while the presence of North Atlantic Deep Water showed that this water mass is capable of spreading northward over the Davie Ridge. The main currents in the Comoros Basin, the westward flowing NEMC and a poleward current along the western boundary, are under the influence of the monsoon winds. The NEMC intensifies during the Southwest monsoon in response to a strong wind jet which develops off the northern tip of Madagascar, whereas the poleward current weakens due to the opposing force imposed by the southwesterlies. Additionally, the circulation in the basin consist of meso-scale eddies of both polarities. Anti-cyclonic eddies, with lifespans of

During a cruise of the R.V. Meiring Naude in August 1975, anomalous values for temperature, salinity and nutrients were recorded over a deep-sea region of the Southwestern Indian Ocean. It was thought that this oceanographic anomaly may represent part of a cyclonic mesoscale vortex of unknown origin. The scant information available at the time on the circulation in this area precluded any of the known, steady currents from being possible generators. Only once before, in 1962, had a similar observation been made, and its significance had not been recognised. In the period 1976 to 1982, several hydrographic cruises were executed on the R.V. Meiring Naude in the region 27 - 33°S, 32 - 43°E, to locate similar features and to find answers to the following questions: Was the 1975 anomaly really a vortex (i.e. a rotating body of water)? What are the physical and dynamic characteristics (i.e. temperature, salinity, density, velocity, volume transport, energy) and distribution of such vortices? How and where are the vortices generated, and what are their lifetime and eventual fate? In all, more than 500 routine hydrographic stations were occupied to collect data on water properties. Most of these stations extended to a depth of 1 000 m, while about 20% went to at least 1 800 m. Initially, hydrosondes designed and built by the Council for Scientific and Industrial Research were employed, but a Neil Brown Instrument Systems' CTD (conductivity-temperature-depth) microprofiler was used from 1979 onwards. Satellite-tracked buoys and infrared imagery were used to derive information on the circulation patterns, thus extending the coverage of the small research vessel. The drift rate of the ship and current measurements from a drifting array of current meters augmented the calculations of geostrophic velocity, volume transport and energy, and provided insight into the flow dynamics of the water.

Phytoplankton blooms are ecological hotspots in the ocean, and are fundamental to the biogeochemical cycling of elements, the storage of carbon and the ability to regulate the atmospheric carbon dioxide; and the life in the ocean. The South-East Madagascar Bloom, one of the largest blooms in the global ocean, coexists with the poleward flowing South-East Madagascar Current (SEMC), the eastward flowing South Indian Ocean Countercurrent (SICC) as well as westward-propagating surface and subsurface-intensified eddies. This austral summer bloom extends largely towards the open ocean, from the Madagascan coasts up to ~65°E and it exhibits an intriguing interannual variability. A variety of observational datasets as well as a high resolution coupled physical-biogeochemical model, based on CROCOPISCES, are used to explore the biophysical processes associated with the bloom and these westward-propagating eddies. Based on historical observational data, the bloom is shown to occur in a region of shallow mixed layer, with the surface layer exhibiting lower salinity, a possible signature of the coastal poleward flowing SEMC waters. The testing of various hypotheses revealed a dampening of the coastal current-driven upwelling south-east of Madagascar during bloom months. A dipole mesoscale feature is also prevalent close to the Madagascan coast during the bloom, from which a new hypothesis emerges. This new hypothesis states that the region south/south-east of Madagascar, influenced by local mesoscale turbulence, acts as a gate for the SEMC to flow either towards the African continent, or into the bloom region through an early retroflection, hence fertilizing the bloom. The model produces a sporadic enhancement of chlorophyll-a in the subsurface levels, associated with a low-salinity surface signature. The mean local circulation associated with the simulated bloom also reveals a dipole structure, as in observed datasets. Nitrate from subsurface levels (upwelling) as well as from the Madagascan coast (advection) is shown to influence the simulated bloom. A Lagrangian experiment shows dispersion of higher percentages of particles in the bloom region during bloom years and south of Madagascar during non-bloom years. Mesoscale eddies, originating close to Australia and which propagate westward towards southern Africa, can potentially impact the South-East Madagascar Bloom. In this study, a vast majority of these features have been shown to be subsurface-intensified eddies. A co-located eddy tracking dataset with Argo profiling floats are used to devise a subsurface-eddy identification method, which is based on the steric dynamic height anomaly of a specific eddy. Adding to the `eddy-zoo', these eddies are termed `SIDDIES' (South Indian ocean eDDIES), occurring as surface (surfSIDDIES) and subsurface (subSIDDIES) features. They travel along the latitudinal band range of 15°S to 35°S which we name the ‘SIDDIES corridor’. Advecting warm and fresh water during their propagation, cyclonic (anticyclonic) subSIDDIES contribute about 58% (32%) of the total eddy-heat flux in the South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole, high-saline water eddy-conveyor towards the western South Indian Ocean. These eddies could also possibly transport nutrients throughout their journey, impacting the biogeochemistry of the ocean near Madagascar.

The southern Benguela upwelling system (SBUS), located off the southwest coast of Africa, supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fuelled not only by upwelled nutrients, but also by nutrients regenerated on the broad and shallow continental shelf. We present the first nitrate nitrogen (N) and oxygen (O) isotope data (δ15N and δ18O, respectively) from the SBUS, generated for samples collected along four hydrographic lines in February (summer) and May (early winter) of 2017. During summer upwelling, a decrease in nitrate δ 18O on the shelf reveals that on average, 30% of the subsurface nutrients derive from in situ remineralization of sinking phytoplankton biomass. In the more quiescence winter, an average of 35% of the on-shelf nitrate is regenerated, with the signal propagating further westward along the mid-shelf region such that the total regenerated nitrate burden is greater during this season. In both seasons, a shoreward increase in subsurface nitrate δ 15N and decrease in N* (i.e., total dissolved nitrogen - 16 x phosphate + 2.9) suggests N loss to benthic denitrification coincident with the on-shelf remineralization, which implies that an even higher quantity of nitrate is regenerated than we calculate. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer and early winter, enhancing the nutrient pool that can be upwelled to support surface productivity and decreasing bottom water oxygen concentrations. The proposed mechanism for this “nutrient trapping” involves upwelled nutrients being removed from surface waters and converted into organic biomass that is sequestered and remineralized on the shelf while the now nutrient-deplete surface waters are advected offshore by Ekman transport. This process is aided by a number of equatorward-flowing fronts that impede the lateral exchange of waters in the upper 200 m of the water column, increasing their residence time on the shelf. The extent to which remineralized nutrients are trapped on the SBUS shelf has implications for bottom water hypoxia. Trapped nutrients will be supplied to the surface during upwelling, supporting high rates of primary productivity and a large sinking biomass flux. The subsequent on-shelf remineralization of this organic matter has the potential to further decrease already-low bottom water oxygen concentrations.

Knowledge of sea ice variability, which contributes to the detection of climate change trends, stems primarily from remote sensing information. However, sea ice in the Southern Ocean is characterised by large variability that remains unresolved and limits our confidence on the remotely sensed products. Although one of the biggest seasonal changes on Earth is the annual advance and retreat of the Antarctic sea ice cover, relatively little attention has been given to the processes by which the marginal ice zone (MIZ) edge forms and responds to synoptic events. This study aimed to assess the seasonal sea ice extent (SIE) of the MIZ by comparing sea ice observations estimated from aboard ship to high resolution passive microwave (PM) satellite imagery when transecting the MIZ. To achieve this, sea ice concentration (SIC) was derived from two AMSR (Advanced Microwave Scanning Radiometer ) products; the ARTIST (Arctic Radiation and Turbulence Interaction STudy) Sea Ice (ASI-AMSR ) and the bootstrap (BST-AMSR ). Theice concentration estimated from these PM satellite products was assessed against SIC observations collected from the S.A. Agulhas II (using the Antarctic Sea Ice Processes and Climate (ASPeCt) protocol). This assessment took place over summer and winter for the years 2016 and 2017. After evaluating how well these PM-SIC estimates compared against the ASPeCt SIC observations, we found that there was good correlation over summer MIZ conditions, while over winter MIZ conditions the correlation was relatively poor. This highlighted winter limitations inherent in PM SIC estimates. Therefore, from these comparison results, an analysis of the seasonal SIE was accomplished while being aware of the winter limitations linked to the PM products. We inferred that the MIZ acts as an indicator for what the evolution of winter SIE might look like over the following months. In addition to winter limitations associated with PM-SIC retrievals, the ASPeCt SIC estimates, based on human interpretation of the sea ice conditions, was limited because of subjective bias. This resulted in the development of an algorithm to automatically acquire SIC from image stills and videos. This method can be used to obtain quantitative seaice data from vessels of opportunity without the need to have trained personnel on-board. In summary, this study assesses seasonal MIZ SIE within the Atlantic sector after highlighting the limitations associated with various SIC-retrieval methods.

The Agulhas Current is the strongest Western Boundary Current of the Southern Hemisphere and it plays a significant role in the circulation of the shelf and coastal waters, whereby mesoscale (50- 500 km) and submesoscale (1 -10 km) instabilities in the Agulhas Current impact the local oceanography of the shelf region. The main objective of this study is to evaluate the ability of a gap-free and merged gridded satellite ocean current dataset, GlobCurrent, to resolve and monitor the variability of the Agulhas Current’s cross-shelf dynamics. In this study, GlobCurrent is compared to in-situ observations collected from underwater gliders through mapping and correlation analysis to assess the product’s accuracy in different subdomains and water depths of the Agulhas Current’s main area domain. We also investigate the value of using a higher resolution satellite and gap-free Sea Surface Temperature (SST) dataset to complement the GlobCurrent dataset in observing the Agulhas Current’s flow processes and features. The results show that GlobCurrent is adequate for describing large mesoscale features and deep water flows but the product has limitations in capturing fast-evolving and small mesoscale features, particularly the Durban Eddy in the KZN bight region. GlobCurrent also exhibits, at times, directional errors in addition to the current speed discrepancies. This research study demonstrates the limitation of the GlobCurrent product for monitoring ocean current variability in shallow, coastal waters and regions dominated by small mesoscale variability. This study also provides new insights on the joint use of other merged satellite products i.e. merged ODYSSEA SST, which may compensate for some of the GlobCurrent product’s shortfalls. Future studies should consider complementing altimetry-based satellite products like GlobCurrent with other merged satellite observation products such as ODYSSEA SST for better imaging of small mesoscale processes and features in shallow coastal waters.

Several papers have linked climate variability in the Southern Ocean (SO) with the Southern Annular Mode (SAM), which has seen an increase in the positive phase since the mid-1960s, due to the Antarctic ozone depletion and emissions of greenhouse gases. The SAM is recognized as the main mode of atmospheric variability in the SO. The SAM index allows an understanding of the latitudinal movement (south-north) of the westerly wind belt circling Antarctica and has significant impacts on Antarctic surface temperatures, ocean circulation, and many other aspects of Southern Hemisphere climate and thus the global ocean. During negative phases of the SAM Index, westerlies intensify and move north, bringing about more (or stronger) storms, and low pressure systems over southern Australia. The changes associated with SAM forcing may have impacts on carbon uptake and storage in the SO directly through upwelling and outgassing, and indirectly, by influencing nutrient cycles and phytoplankton activity. Understanding the variability of the wind field in the SO and how it affects ocean circulation, climatic and oceanic variables is important. Thus, this thesis presents the relationship of the SAM index and the upper ocean, specifically analysing sea surface salinity (SSS), sea surface temperature (SST) and the mixed layer depth (MLD), in the Southern Atlantic sector of the SO as presented in numerical ocean models. Two resolutions of NEMO ocean model are compared: a) eddy-permitting (SATLANTIC05), b) eddy-resolving (SATLANTIC12) models, with horizontal resolutions of ½ and 1/12 °, respectively. In situ data from 2013 World Ocean Atlas is used as a benchmark for the analysis. Our model‐based analysis confirms previous studies done on the influence of the SAM on the SO, that a strong relationship exists. The SAM index is positively correlated with wind speed in the Antarctic Zone (AZ) and negatively correlated in the Subantarctic Zone (SAZ). The impacts of this is clear in the upper ocean. These correlations between SAM index and the selected variables at these selected locations confirms that the SAM index corresponds with cool surface temperatures at higher latitudes and a weak cooling at midlatitudes during positive phase, which differs regionally.