The relationship between the volume transport of the Agulhas Current at 34°S (the position of the Agulhas Current Time-series array) and the gradient of sea surface height across the current was investigated using a regional Hybrid Coordinate Ocean Model. Previous studies have suggested a high correlation between SSH slope and Agulhas Current transport and, based on 3 years of in situ measurements, a transport proxy between along-track satellite data and in situ data was developed. The purpose of this modelling study was to re-create the Agulhas Current transport proxy in a virtual modelling environment, to test the validity of the underlying assumption on which the satellite-altimeter proxy was based. The Agulhas transport proxy assumed nine, constant linear relationships between SSH slope and integrated transport per unit distance over the 22-year transport time-series, based on the 3-year sampling period and a constant vertical stratification. The 34-year regional-hindcast from HYCOM provided the means to test the sensitivity of the transport proxy to vertical changes in the current and the length scale of observations used to build a constant, linear relationship between transport and SSH slope. During the investigation it was found that HYCOM contained exaggerated levels of offshore variability. This resulted in stronger correlations for the inshore linear regression models with a decreasing trend moving offshore. Based on the overall performance of the 34-year transport proxies it was concluded that the proxy was more capable of estimating the net transport of the Agulhas Current across the array instead of only the southwest transport component. Therefore, transport estimates inshore were more accurate than the transport estimates offshore, when the current is in a meandering state, and the poorer performance of the southwest transport proxy, specifically developed to capture the transport during offshore meander events, was less capable of estimating an accurate transport estimate. Results showed that calculating the proxy over longer time periods did not significantly improve the skill of the Agulhas transport proxy, suggesting the 3-years was a sufficient time-period used to develop the transport proxy in HYCOM. This study motivates the need to improve long-term monitoring methods, where the usage of numerical ocean models could help understand the sensitivities and limitations involved in the development of transport proxies in future.

Bibliography: leaves 98-104. / The 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.

Includes abstract. / Includes bibliographical references (leaves 64-72). / The region surrounding the Tristan da Cunha Archipelago has received little attention to date due to its remote location. An extensive revision of previous literature covers the majority of oceanographic research undertaken in the region of interest. New satellite derived oceanographic data sets and the SODA Reanalysed model are used to overcome the sparse extent of in-situ data in this region. Using latitudinal temperature gradients to track the surface expression of the Subtropical Front from AMSR and OISST satellite derived sea surface temperature data reveals consistencies with previous literature.

Multi satellite data for surface chlorophyll‐a (Chl‐a), sea surface temperature (SST), sea surface wind (SSW) and sea level anomalies (SLA) have been obtained and analysed over the Delagoa Bight (24‐28°S, 32‐36°E), southern Mozambique for the period 2003‐2012 at monthly time scales. Both descriptive and quantitative analysis using wavelets have been used to obtain a better understanding of the nature of the interannual, seasonal and intra- seasonal variability of the data. Strong seasonal structure and interannual modulation were observed in the area averaged Chl‐a concentration and SST. The lowest maximum in monthly Chl-a was in December (0.127 mg.m--‐3) and the highest in August (0.541 mg.m‐3). The lowest maximum in monthly SST was in August (21.8°C) and the maximum in February (27.9°C). The Chl‐a and SST were strongly anti-correlated and both exhibited a well- defined seasonal cycle, contrasting with the SSW and SLA. The daily observations of temperature at 17 meters depth, from the northern Delagoa Bight at Ponta Zavora (24.48°S- 35.24°E) for the period 2006‐2011, have confirmed a seasonal signal with amplitude of about 6.5°C. Cool coastal water events were found mostly in summer and spring, with maximum amplitude of 6°C. Further analysis of this daily data did not reveal the timing of such events to be regular.

Located off the west coast of southern Africa, the southeast Cape Basin is characterised by a unique combination of strong coastal upwelling and vigorous offshore mesoscale variability. The juxtaposition of offshore mesoscale variability and coastal upwelling results in a complex and dynamic environment. In this study a combination of in situ Acoustic Doppler Current Profiler (ADCP) data and satellite observations were used to identify, describe and characterise the features driving the local circulation within the southeast Cape Basin. The ADCP data was obtained from the South Atlantic MOC Basin-wide Array (SAMBA), which included four deep sea moorings located along ~34.5°S on the 1000, 2000, 3000 and 4500m isobaths respectively. There was a distinct difference in the circulation observed at the mooring located on the shelf edge (1000m) and the moorings further offshore (2000m, 3000m and 45000m). The offshore circulation, observed by the moorings located on the 2000, 3000, 4500m isobaths, were driven by large mesoscale eddies, both cyclonic and anticyclonic, originating at the Agulhas retroflection and within the Cape Basin itself. The mesoscale eddies induced high speed baroclinic transport events which impacted the upper water column to a depth of at least 400m. The in situ observations were used to show the precise characteristics of two cyclonic shelf eddies and one anticyclonic eddy through the upper water column (~50 to 500m). The analysis of these features was important as the physical characteristics of both the cyclonic shelf eddies and anticyclonic eddies through depth are not well known. Considering the short period of observations (18th September 2014 to 1st December 2015), the analysis was not used to assume the prevailing physical characteristics of cyclonic shelf eddies and anticyclonic eddies. Instead the analysis showed the potential for future long term studies to use the sustained in situ observations from the SAMBA mooring array and similar analysis to define the precise characteristics of mesoscale eddies through depth. This will greatly improve the understanding of how these features influence the interocean exchange between the Indian and Atlantic Oceans. The circulation at the shelf edge, observed by the mooring located on the 1000m isobath, was shown to be driven by a combination of offshore mesoscale eddies, the position of the upwelling front and warm filaments formed at the reflection of the Agulhas Current. Identifying the influence of both the offshore mesoscale eddies and coastal upwelling at the shelf edge showed connectivity between the upwelling system and the offshore mesoscale variability. An example of the connectivity between the offshore mesoscale variability and the upwelling front was identified and presented. A dipole formed by two mesoscale eddies was observed to induced cross shelf transport advecting productive shelf waters offshore. The in situ observations were used to calculate the amount water transported from the shelf to the open ocean during this cross shelf transport event. The filament representing the cross shelf transport event was calculated to have a total volume of ~2 x 10¹² m³ with a volume transport of ~1Sv suggesting the event would have had a substantial impact on the local biology. The detailed analysis and quantification of the cross shelf transport event aimed to improve the current understanding of how mesoscale features interact with the upwelling system. In situ observations of cross shelf transport are rare, therefore the quantification of the amount of transported in the cross shelf transport event can serve as a baseline for future studies attempting to assess the impact of comparable cross shelf transport events on the local biology.

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