Numerical model investigation of near-surface circulation features of the Angola Basin
Master Thesis
2004
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University of Cape Town
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Abstract
The primary objective of this thesis is to identify and investigate the most prominent circulation features of the Angola Basin from the output parameters of the OPAITOTEM Ocean Parallelisel Trois Oceans Tropicaux) tropical circulation model, focusing particularly on the so-called Angola Gyre and the Angola Dome. Analyses of the effect of windstress, windstress curl and Ekman pumping, all computed from ERS satellite-derived wind speeds, were conducted. The OPAITOTEM model does not resolve the 'Angola Gyre', but it does resolve a large-scale (1000-2000km) dome-like feature, which has been defined as the 'Model Dome' within this study. The most conspicuous feature of the Angola Basin discerned in the thermohaline output of the OPAITOTEM model is a ridge-like structure of the thermocline (the Model Dome), which results in a cool feature that 'outcrops' most distinctly at a depth of 45m. At greater depths, a permanent upward doming of the isotherms beneath the Angola Benguela Frontal Zone (ABFZ) is observed. In January, February, September, October and November the 'outcropping' of the thermocline-ridge is deepest and appears as a distinctly isolated cool feature at 4Sm. No clear cydonic circulation is associated with the Model Dome. However, the northern limb is unequivocally coincident with the South Equatorial Undercurrent (SEUC) at 2-5"S and the South Equatorial Countercurrent (SECC) at 10·S. Upon reaching the African coast, the SEUC and SECC bend poleward to form the southward Angola Current, which constitutes the eastern limb of the Model Dome. The southern and eastern limbs of the dome are not as dearly defined and are associated with a weak westward flow regime. The shallow portion of the Model Dome has a distinct semi-annual signal whereby it migrates southward between September-November and again between January-April. This signal is in accordance with the magnitude and southward displacement of the core of the SEUC. The SEUC is most intense in January, February, September and October reaching velocities of about O.14m.s·1 and is weakest in June and July (O.02-O.06m.s-) The deep portion of the Model Dome shows little seasonal variability, other than a slight northward tilt of its vertical axis when the thermocline-ridge is furthest south. Similarly, the SECC, which constitutes the northern limb of the deeper cool feature, is fairly consistent throughout the year, in both position and magnitude (-0.01 m.s-)
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Reference:
Veitch, J. 2004. Numerical model investigation of near-surface circulation features of the Angola Basin. University of Cape Town.