Browsing by Author "Rae, Christopher Duncombe"

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    Characteristics of near surface circulation patterns in the Benguela as derived from the ADCP(Acoustic Doppler Current Profiler).
    (2002) Iita, Aina T; Shillington, Frank; Rae, Christopher Duncombe
    This study forms part of one of the Benguela Environment Fisheries Interaction and Training (BENEFIT) program projects, whose main objective is to implement effective and quality ADCP data collection in the Benguela region. The thesis has two main components, firstly it undertook to investigate and assess the data quality, processing methods and software of ADCP data that are used to collect and process the ADCP data available in the Benguela region. An inter-comparison was made between the two different formats for data collection, i.e. raw and RDI proprietary "TRANSECT"processed data (which is more readily available in South Africa), to evaluate their respective accuracy in depicting current flow. The raw data were validated (edited and calibrated) using the CODAS software package before the current vectors could be drawn. TRANSECT -processed data could not be validated therefore vectors were drawn from the un-validated data. Data used here was collected during a monthly monitoring survey on 06-08 August 2001 on board one of the South African research vessel Algoa. The comparison results showed that TRANSECT -processed data includes unedited errors, noise and biases, which are already averaged into the data by the program and that the raw data presented a more realistic current flow. Secondly, the study undertook to apply the ADCP data collected to describe the state of oceanography of the Benguela region during the survey periods. Data from monitoring survey in 06-08 August 2001 was used to describe the southern Benguela while the second data set from a cruise conducted off Namibia in October 2000 was used to describe the oceanography of the central and northern Benguela. Most of the structures observed were in support of the literature and confirming previous studies of the region. In the southern region, the equator ward shelf jet off Cape Peninsula was observed to be forced offshore and entrained in an anticyclonic-like feature, which appeared to be part of a wann Agulhas filament. In northern Nambia, a strong poleward movement of the wann Angolan water was measured.
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    A descriptive analysis of the genesis and translation of a dipole vortex from the Agulhas retroflection region
    (2000) Whittle, Christo Peter; Shillington, Frank; Rae, Christopher Duncombe
    An anomalous leakage of Agulhas Current water into the south-east Atlantic Ocean, exhibiting a mushroom-like shape, was observed during routine observations of A VHRR satellite imagery in early December 1996. The development of this anomaly was followed on the sea surface temperature (SST) imagery and it was tentatively identified as a consequence of filament interaction between the Agulhas retroflection and an occluding Agulhas ring. This interpretation prompted a cruise onboard the FR.S Africana with the objective of conducting a hydrographic survey of the Agulhas ring and the associated filament near Cape Town. A descriptive analysis, gleaned from A VHRR satellite imagery and in situ data, of the hydrographic characteristics of a vortex dipole, surveyed during this cruise, is presented in this thesis. An analysis of water mass properties and geostrophic flow patterns determined that an Agulhas ring and a cyclonic eddy, containing Benguela Current water in its core, constituted a dipole vortex in the south-eastern Atlantic Ocean. During the period of the hydrographic survey, the secondary vortex exhibited an anticlockwise rotation of 8. 6°/day around the Agulhas ring A warm filament, originating from the western Agulhas Bank, was entrained between the two counter-rotating eddies, thus resulting in the mixing of Agulhas Bank water into the South Atlantic Ocean. Satellite altimetry and A VHRR imagery were used to "backtrack" the vortex dipole to its origin at the Agulhas retroflectiOn. By combining interpretations from the altimetry and A VHRR imagery, it was possible to describe the complex interactions the dipole displayed with the retroflection and the Agulhas Bank as it franslated in a north-westerly direction. The mushroom configuration, identified earlier on SST imagery, betrayed the presence of an adjacent pair of circulatory features of opposing spm. As the dipole translated northward, it interacted with the Agulhas Bank and the cyclone was strained, becoming a filament as it was forced between the Agulhas ring and the Agulhas Bank. West of Cape Town the dipole was re-established when the cyclone redeveloped, changing the orientation of the dipole so that a filament was drawn directly from the Agulhas Bank.