Entrainment in Saldanha Bay
Master Thesis
1996
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University of Cape Town
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Abstract
Saldanha Bay is located lOOkm north of Cape Town, along the south western coast of South Africa, at a latitude of approximately 33° S. In 1975 major harbour works, including the construction of an iron ore jetty, divided Saldanha Bay into two distinct bays. The bay to the north of the jetty has become known as Small Bay, while the bay to the south is commonly referred to as Big Bay. Big Bay is connected at its southern end to the shallow Langebaan Lagoon system, and to the west with the Benguela upwelling system (see figure 1.1). The oceanography of the shelf outside the bay is dominated by the coastal upwelling system (Shannon 1985). The equatorward winds which predominate for much of the year, drive an offshore flux of surface water, which is replaced near the coast by nutrient-rich water from deeper layers. These winds are the result of an interplay between the South Atlantic High Pressure Cell, a thermal low that forms over southern Africa in summer, coastal lows, and eastward moving extra tropical cyclones (Shannon 1985). In summer, the band of extra tropical cyclones associated with the jetstream moves to the south of its winter position, while the South Atlantic High intensifies and moves approximately 6° to the south (Preston-Whyte and Tyson 1988), creating a pressure gradient over the Benguela region. The presence of the thermal low over the adjacent subcontinent increases the existing pressure gradient, and enhances the equatorward air flow. This seasonal modulation of upwelling favourable winds results in an upwelling season that extends from about September to March (Shannon 1985). During this period, there is also synoptic modulation, provided by wind relaxation or reversal events. These events are either associated with the passage of a cyclone to the south of the continent, or with a coastal low passing along the coast (Shannon 1985).
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Bibliography: pages 64-67.
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Spolander, B. 1996. Entrainment in Saldanha Bay. University of Cape Town.