Widespread flooding over parts of Mozambique, Zimbabwe, Malawi, Botswana as well as north-eastern South Africa was experienced in February 2017. The flooding was associated with Tropical Cyclone Dineo that was generated in the Mozambique Channel on 12 February 2017 and made landfall over the south-central coast of Mozambique on 15 February. This study investigates the atmospheric circulation and potential mechanisms responsible for the heavy rainfall that occurred during the passage of ex-Tropical Cyclone Dineo inland from the Mozambican coast with focus on the rainfall patterns over north-eastern South Africa. Output from the Weather Research and Forecasting (WRF) model, the Climate Forecast System Reanalyses version 2 (CFSv2) atmospheric reanalysis, satellite derived rainfall and wind data, and station rainfall data are used for this purpose. Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, WRF model rainfall and rainfall station data indicated that many parts of north-eastern South Africa experienced large amounts of rainfall during the final stages of Dineo (16-17 February 2017) while Mozambique experienced heavy rainfall soon after the cyclone made landfall. An inland trough ahead of Dineo led to substantial rainfall at this time over Malawi and Botswana. Furthermore, analysis of the station data revealed that in north-eastern South Africa some stations recorded about 80 % of their total monthly rainfall from this event. The WRF model run indicated low level monsoonal north-easterly moisture fluxes feeding into Dineo in the Mozambique Channel. Subsequent convergence over south-eastern Africa between this flow and the south-easterly cyclonic flux associated with Dineo led to substantial rainfall over Mozambique, Zimbabwe, Botswana and north-eastern South Africa. Although the 2016/17 tropical cyclone season recorded below average numbers of storms, it is suggested that the conditions prior to the storm iv formation were favourable for the track of Tropical Cyclone Dineo and landfall on the southcentral Mozambican coast.

Bibliography: leaves 33-34. / An ADCP and water-column temperature study was conducted to determine the circulation aspect of subtidal-scale, density-driven exchange in Saldanha Bay, South Africa. Density-driven exchange conditions develop in response to synoptic-scale wind events in the southern Benguela region, even under light (<5m s-') wind conditions. During a density-driven exchange event, directionally opposing bi-level flow, similar to an estuarine system, develops in response to remote upwelling-favourable winds. The bi-level flow component occurs in two distinct bands, bayward at 0-9m height from bottom and seaward 15-20m height off bottom, and is very sensitive to changes in wind forcing. Observations of current behaviour are added to the four-phase conceptual model of density-driven exchange developed by Monteiro and Largier (1999). In addition, estimates of bay flushing based on ADCP current velocities and the four-phase conceptual model are calculated and implications of shelf water influx into Saldanha Bay are discussed.

Bibliography: leaves 71-79. / The aim of this project was to investigate the effect that the Cape Flats Aquifer, situated on the north shore of False Bay, is having on the water quality of the bay. Following increased development on the Cape Flats, the risk of the aquifer becoming contaminated has increased as it is situated below the informal settlement of Khayelitsha (where there is poor sanitation) and the Philippi agricultural area (where fertilisers are widely used).

Wind waves in the vicinity of Cape Town, South Africa, were measured by means of an N.I.O. (England) ship-borne wave recorder, in water depths varying from 12 - 140 metres. One dimensional frequency spectra were computed from the records, by power spectrum analysis, based on the method of Blackman and Tukey, using an I.B.M. 1130 Computer. Assumptions of normality and stationarity were tested. The Gaussian assumption of the waves was found to be acceptable for the waters off Cape Town. At the 5% critical level, using the chi-square test, 3 out of 23 records tested were found to be non-Gaussian. No significant difference between the Gaussian properties of the deep and the shallow stations was found. Tests for stationarity applied to 3 selected records showed 1 record as clearly non-stationary. However, this recording was obtained under fluctuating wind conditions. Comparison of the total variances of the power spectra obtained in shoaling water showed a systematic decrease of variance with depth. Normalised spectra did not show a systematic selective attenuation of the variances with frequency. Factors which might have caused the systematic reduction of the total variances with depth have been examined. The bottom friction factor for this coast was estimated. The mean value of the bottom friction is 0.22. This is higher than found by other workers.

Bibliography: pages 209-221. / The presence of subinertial (2 to 20 day) oscillations in sea levels and currents over the continental shelf surrounding southern Africa is well-documented, however presently available observational data alone are largely ~nsuitable or insufficient to adequately characterise the shelf circulation in terms of coastal-trapped wave (CTW) theory. In this dissertation, an analysis of atmospheric pressure and sea level data from 1980 to 1990, a scale analysis of the equations of motion and numerical modelling are used to characterise the subinertial coastal-trapped wave motions off the west coast and south coasts of southern Africa. Appropriate scaling of the equations of motion and numerical analyses of the subinertial CTW response at a number of locations along the west and south coasts of southern Africa indicate that the large scale response of shelf waters to synoptic scale or "weather band" atmospheric forcing is predominantly barotropic. This permits the use of the linear, vertically integrated, shallow water wave equations on a J-plane to model the response of the West Coast shelf waters to "weather band" atmospheric forcing. The CTW dispersion curves have been calculated for a number of shelf profiles along the West and South Coast and are used in the interpretation of the sea level analyses and the numerical modelling results, and further to investigate the effect of increasing water column stratification on the subinertial CTW response

Bibliography: pages 132-140. / Exudation patterns of organically bound nitrogen and polyphenol substances, as well as the uptake of nitrate, ammonium and phosphate were studied in the kelp Ecklonia maxima (Phaeophyceae) in ex-situ experiments. It was found that exudation of organic materials only rarely followed a straight accumulatory trend ; fluctuations of kelp-derived substances occurred as a net result of pulsating production patterns coupled with differential rates of utilization by marine heterotrophic organisms. Pulsing took place at irregular intervals in different plants, precluding statistical treatment of exudation. The observed initial pulsing liberation of DON and polyphenols, affected the quantification of exudation rates which could only be tentatively determined. Antibiotic agents used in one experiment did not succeed in controlling free-living bacteria in the water column without affecting the kelp DOM production was diminished in the presence of antibiotics, leading to the conclusion that they affected the physiology of the epidermal cells. This provides indirect evidence that exudation is an energy-dependent, active process. Measurements of uptake velocities of ambient and enriched nitrogen established that there was a higher affinity for ammonium as a nitrogen source than for nitrate at enriched N levels. At ambient nutrient levels, nitrate uptake velocities were 1,5 times higher than those for ammonium, making nitrate the primary nitrogen resource of E. maxima. However, the uptake potential for either source was not competitively inhibited by the other. Ammonium and nitrate uptake velocities still increased from enriched levels of 30-40 uM N to approximately 100 uM N. Both exudation and uptake phenomena were subject to seasonality: During summer, the rates for exudation and uptake were found to be higher than in winter, which is known to be a dormant phase in the growth of E. maxima. A comparative analysis of dissolved free amino acids in the water surrounding the kelp and a tissue analysis of free amino acids obtained under enriched conditions showed that alanine was prominent b6th in the thallus and in the water. Glutamic acid was detected in the water only after the kelp had been introduced during a nitrogen feeding experiment and appeared to be GS/GOGAT derived. Aspartine was al so detected at high relative concentrations. Polyphenolic substances (phlorotannins) accumulated in exudates of E. maxima more readily due to the low biodegradability of these compounds and contributed to the yellow colouration and uvabsorption characteristics (Gelbstoff) of the surrounding water.

The Agulhas Current is the strongest western boundary current in the Southern Hemisphere and plays an important role in the exchange of heat and salt between the Indian and South Atlantic Ocean basins, thereby affecting global climate. The variability in the northern Agulhas Current is influenced by both cyclonic and anticyclonic mesoscale eddies, originating from the Mozambique Channel and south of Madagascar (known as source region eddies) and which propagate toward the offshore edge of the Agulhas Current. Using a combination of an eddy-tracking data set with in-situ surface drifter observations and altimetry-derived geostrophic currents, it is shown that source region eddies dissipate upon approaching the Agulhas Current. Their entrainment into the Agulhas Current affects its mean velocity and offshore position through a transfer of momentum, with anti-cyclonic eddies consistently increasing the Agulhas Current’s velocity by 0.16 ± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 ± 0.16 m.s-1 and shifting the current up to 144 ± 85 km offshore. These velocity anomalies propagate downstream at rates of 44 km.d-1 (anti-cyclonic eddies) and 23 km.d-1 (cyclonic eddies). Whilst existing numerical models are successfully able to capture many aspects of the Agulhas Current, many models are unable to accurately represent the observed eddy dissipation and interaction processes, affecting our understanding of mesoscale variability within in the current. In this study, we compare two simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy tracking algorithm assess the local effect of the changed wind stress on source region eddies and their interaction with the northern Agulhas Current. There is an overall reduction in eddy kinetic energy (EKE) of 33% over the Agulhas Current domain. Changes in eddy pathways, properties and energy conversion terms, resulting from the change in forcing from absolute to relative winds (the wind speed relative to the current speed) have resulted in significantly different mesoscale eddies in the regional HYCOM. The effects of the change in wind forcing on the variability within the Agulhas Current were examined and the differences between the two simulations were found to be very small. Finally, the evolution of meanders in the Agulhas Current, including the properties and dissipation of smaller meanders as well as mesoscale Natal Pulses type meanders, were assessed using both HYCOM experiments and compared to satellite observations. The representation of smaller meanders (under 50km in size) improved with the changed in wind forcing. However, larger Agulhas Current meanders (greater than or equal to 50km) which previously occurred too frequently in the regional HYCOM, are now too infrequent in the regional HYCOM, with an average of 1.1 meanders occurring each year. A decrease in the frequency of larger meanders was observed from the location offshore of Port Edward (30.22° E, 31.05° S) to the region of the ACT array (27.48° E, 33.35° S), in the satellite data as well as both model experiments, indicating that some of the meanders have dissipated and that both regional HYCOM models are able to resolve this.

Bibliography: leaves 228-237. / Tomales Bay, California, situated along the west coast of the United States of America, in the mid-latitudes at 38° 13' 50" N, 122° 58' 30" W, was chosen as the study site for the "Biogeochemical Reactions In Estuaries" (BRIE) project of the "land Margins Ecosystems Research" (LMER) program of the National Science Foundation in the U.S.A. Research on terrigenous inputs, air-sea exchange, sediment-water exchange, ocena-bay exchange and internal cycling in estuaries was conducted. This thesis addresses the ocean-bay exchange section in identifying and describing the estuarine water circulation and its variability, both temporally and spatially, that drives the observed water column structure in Tomales Bay and the effect of wind driven coastal upwelling on the estuary.

The evolution of cyclonic eddies across the Southern Mozambique Chanel and the downstream evolution of the Agulhas Current was investigated using Argo floats, in combination with output from ocean general circulation reanalysis models. Two dedicated experiments were undertaken in April and July 2013, whereby eight floats were deployed within two separate cyclonic eddies. Floats were set to either daily and five-daily profiling from 1000 db to the surface, with park depths ranging from 300 db to 1000 db. The two cyclonic eddies propagated southwestward across the Mozambique Channel from southwest Madagascar to the KwaZulu-Natal Bight, a distance of approximately 1300 km, in approximately 130 days at a mean speed of 0.13 m s−1 . Estimates indicate the April (July) eddy showed mean trapped depths of 595 ± 294 m (914 ± 107 m), volume transport of 13.4 ± 5.2 Sv (21.2 ± 9.1 Sv), heat flux of -0.07 ± 0.06 PW (-0.2 ± 0.09 PW) and freshwater flux of 0.04 ± 0.04 Sv (0.09 ± 0.05 Sv). These results highlight the role of Madagascar cyclonic eddies as transporters of cooled and freshened source waters into the Agulhas Current. During a third experiment, six floats were deployed in the Agulhas Current, and exited the current within 9 - 12 days at mean speeds of 0.51 – 0.76 m s−1 . An evolution of properties was shown from north to south for both Argo data and model output; for volume transport (16.76 – 38.18 Sv; 17.70 – 32.51 Sv), heat fluxes (0.85 – 1.79 PW; 0.99 – 1.91 PW) and salt fluxes (0.60 – 1.37 x 1012 kg s−1 ; 0.63 – 1.17 x 1012 kg s−1 ). This study illustrates the first near-real time survey of the Agulhas Current, and a potential method of quasi-synoptic surveys using Argo float technology. These experiments highlight alternative methods of studying regions of turbulence by altering the mission parameters of Argo floats. Increased observations of eddies and Western Boundary Currents are critical to our understanding of the global oceans and impacts on the earths climate. Even more so for the understudied Indian Ocean.

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