Typescript. / Bibliography: leaves 74-76. / xii, 76 l charts, graphs, maps, tables

Thesis (M.S. in Physical Oceanography)--Naval Postgraduate School, June 2008. / Thesis Advisor(s): Chu, Peter C. "June 2008." Description based on title screen as viewed on August 26, 2008. Includes bibliographical references (p. 67-71). Also available in print.

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Typescript. / Bibliography: leaves 131-133. / xi, 133 l charts, graphs, maps, tables

Thesis submitted in fulfilment of the requirements for the degree Master of Technology: Oceanography in the Faculty of Applied Sciences at the Cape Peninsula University of Technology / In order to validate remote sensing products and to provide data for model assimilation, a real-time monitoring line consisting of three moorings was deployed across the Agulhas Current off Port Edward, South Africa. This deployment formed part of a Technology and Human Resource for Industry Programme (THRIP) funded initiative to develop a real-time mooring system capable of measuring ocean parameters in the Agulhas Current during 2011. The slope and offshore moorings displayed a distinct stratified regime within the Agulhas Current, a northeastward flowing Agulhas Undercurrent and the southwestward flowing Agulhas Current. Three major reversal events, with northeastward currents occurred on 23 July, 02 September and on 11 October 2011. All current reversals caused a decrease in current velocity. The Agulhas Undercurrent was a persistent feature and average velocities between the line of moorings ranged between 13.38 cm/s and 15.52 cm/s. The results obtained from the mooring systems were consistent in terms of velocity, direction and hydrographic properties of the Agulhas Current as described in previous literature. The low directional variability in the surface layers at the offshore mooring and dominant southwestward flow, except during reversal events indicate the strong influence of the Agulhas Current in this region. The inshore mooring showed less occurrences of the Agulhas Undercurrent if northward flow in the bottom layers was to be considered as signs of the Agulhas Undercurrent. General current characteristics as well as the characterisation of the mesoscale features affecting the coast off Port Edward was accomplished through the use of the in situ moorings. All current reversals encountered were associated with the process of vortex shedding from the Natal Bight. These events may be related to the shedding of the Durban Cyclonic Eddy from its origin in the Natal Bight. Data from the offshore mooring suggested that for monitoring Agulhas Current core dynamics, it was ideally placed as highest surface velocities were measured by this mooring system. The slope mooring recorded highest velocities within the Agulhas Undercurrent and was thus ideally placed to measure the Agulhas Undercurrent’s core. Shelf dynamics were under the influence of the Agulhas Current and northerly current reversals and were aptly recorded by the inshore mooring which was placed on the continental shelf, close to the shelf break.

The circulation along the south coast of Western Australia was examined using field data and numerical modelling. Physical processes in this region, particularly along the continental shelf and slope regions, were poorly understood due to a paucity of field measurements. Data were collected during a research cruise on RV Southern Surveyor (04/2006) during April 2006 consisting of 18 CTD transects from Twilight Cove (126oE) to Cape Leeuwin (115oE) and was augmented by shipborne ADCP data. The field data set provided a detailed understanding of three major current systems: Leeuwin Current (LC), Leeuwin Undercurrent (LU) and Flinders Current (FC). The LC along the south coast exhibits different characteristics when compared to that along the west coast. The LC flows into the colder and lower salinity subantarctic environment of the south coast. This is evident in a strong geopotential gradient off the south-west corner of Australia (Cape Leeuwin) resulting in rapid acceleration of the LC as it reaches a maximum velocity in this region. Numerical modelling studies, using the Regional Ocean Modelling System (ROMS) indicated that wind stress is an important component of the dynamics in this region. This was identified when comparing summer and winter conditions when the winds act in opposite directions, from north-westerly to southeasterly respectively. Along the shelf break and slope, the Flinders Current (FC) interacts with LC. As the dominant current, the FC serves both as a surface and as an undercurrent, transporting sub Antarctic mode water (SAMW). This interconnection the FC and LU can be seen clearly from the salinity, temperature and velocities within the depth range 200-700m postulating a connection between subsurface waters off Tasmania (origin of the Flinders Current) and the tropical Indian Ocean through the Flinders and Leeuwin Undercurrents.

[Truncated abstract] This work was the first large-scale biological oceanographic study to be undertaken in the coastal eastern Indian Ocean adjacent to Western Australia, and covered both northwest (Exmouth Peninsula to the Abrolhos Islands) and southwest (Cape Naturaliste to Cape Leeuwin) regions. The study area was dominated by the Leeuwin Current (LC), an anomalous eastern boundary current that transports tropical water poleward and prevents deep nutrients from reaching the surface by creating large-scale downwelling. Indeed, LC and offshore waters were consistently associated with low nitrate concentrations and low phytoplankton biomass and production (< 200 mg C m-2 d-1). However, the physical forcing of the LC was offset, during the summer months, by upwelling associated with wind-driven inshore countercurrents (Ningaloo and Capes Currents), which provided a mechanism to access high nutrient concentrations normally confined to the base of the LC. ... Limited seasonal investigations off the Capes region of southwestern Australia showed that the winter production scenario can be very different than summer conditions, with strong Leeuwin Current flow that meanders onto the continental shelf and entrains seasonally nutrient-enriched shelf waters. However, production in the LC was still low (≤450 mg C m-2 d-1) due to light limitation resulting from both increased light attenuation and reduced surface irradiance characteristic of the winter months. This investigation provides fundamental knowledge on physical-biological coupling off Western Australia, with implications for fisheries management in view of seasonal and inter-annual variability in the strength of both the Leeuwin Current and inshore countercurrents.

The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21° and 28°S. This study presents CTD and ADCP data together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the continental margin, and the hydrography present in the upper 1km of ocean, between latitudes 21° and 35°S. It also discusses the outcome of a numerical modelling study that examined the physical factors contributing to a bifurcation event persistently observed in satellite imagery at Point Cloates. The region comprises a complex system of four surface water types and current systems. The Leeuwin Current dominated the surface flow, transporting lower salinity, warmer water poleward along the shelf-break, and causing downwelling. Its signature ‘aged’ from a warm (24.7°C), lower salinity (34.6) water in the north to a cooler (21.9°C), more saline (35.2) water in the south, as a result of 2-4Sv geostrophic inflow of offshore waters. The structure and strength of the current altered with changing bottom topographies. The Ningaloo Current flowed along the northernmost inner coast of the Gascoyne shelf, carrying upwelled water and re-circulated Leeuwin Current water from the south. Bifurcation of the Ningaloo Current was seen south of the coastal promontory at Point Cloates. Numerical modelling demonstrated a combination of southerly winds and coastal and bottom topography off Point Cloates to be responsible for the recirculation, and indicated that the strength of southerly winds affect recirculation. Hypersaline Shark Bay outflow influenced shelf waters at the Bay’s mouth and to the south of the Bay. The Capes Current, a wind-driven current from south of the study region was identified as a cooler, more saline water mass flowing northward. Results of the hydrography study show five different water masses present in the upper-ocean. Their orientations were affected by the geopotential gradient driven Leeuwin Current/Undercurrent system at the continental margin. The Leeuwin Undercurrent was found at the shelf-slope, carrying (>252 μM/L) Subantarctic Mode Water at a depth of 400m

[Truncated abstract] The study of eastern ocean boundary currents has been principally restricted to the Pacific and Atlantic ocean regions. The traditional view of the circulation near eastern ocean boundaries is that upwelling-favourable winds force surface waters offshore, leading to upwelling of cold, nutrient-rich subsurface water at the coast, the formation and offshore advection of a coastal front, and the generation of alongshore currents, generally having an equatorward surface flow and a poleward undercurrent. The eastern ocean boundary system of the southern Indian Ocean, off the west coast of Australia, is unique compared with these regions because a warm, poleward surface flow, known as the Leeuwin Current, dominates the dynamics over the continental shelf. Satellite imagery has shown the Leeuwin Current consists of a complex system of meanders, jet-like streams, and eddies, and has a seasonal and interannual variability. The oceanic circulation of the region between Carnarvon (latitude 25°S) and Jurien Bay (latitude 31°S) was examined using observational and remotely sensed data in conjunction with a detailed numerical modelling study. The model was validated using in situ ADCP and CTD data, and the horizontal eddy viscosity parameterization was tested against field observations. ... The resulting offshore meander grew laterally, shallowed, and closed to form an anticlockwise eddy to the original clockwise eddy’s south, forming a characteristic LC eddy pair (dipole). The model demonstrated the LC and Leeuwin Undercurrent (LUC) coupling played an important role in the onset of eddies at both sites. When an energy diagnostic scheme was used, the dominant instability process linked to the anticlockwise eddy’s development at site 1 was a mixed mode barotropic and baroclinic instability. The baroclinic instability’s source was the available potential energy stored within the mean lateral density gradient. The LC’s meandering southerly flow interacting with the LUC’s northerly subsurface flow generated the horizontal shear that sourced the barotropic instability. The dominant instability process at site 2 was baroclinic in origin. Possible links between the eddy field dynamics and the shelfslope region’s alongshore topographic variability were considered. The results of a suite of five model runs, differing only in the specification of bottom topography, were contrasted to investigate the effects. Except for the expected alongshore variability, delay in the onset of instabilities, varying growth rates, and some differences in the dominant wavebands’ mesoscale patterns, the overall impression was the response was similar.

D.Litt. et Phil. / Please refer to full text to view abstract