Retention processes in the Southern Benguela upwelling system
The Benguela Upwelling System is one of four major Eastern Boundary Upwelling Systems (EBUS) in the global ocean. Here, equatorward winds drive the upwelling of cold, deep, nutrient-rich waters along the coast, which drives high levels of biological productivity. A consequence of this high productivity is the prevalence of hypoxic conditions that co-occur with elevated nearshore and shelf bound retention of nutrients. This thesis focuses on the Southern Benguela Upwelling System (SBUS) and explores the role that oceanic fronts play at the seasonal, interannual and event-scales in restricting the offshore advection of material and elevating the residence times of matter along the shelf. Furthermore, the role of variable, near-coastal, summer alongshore winds on retention is also studied. Focus is given to how these winds impact the nearshore current velocities and the offshore positions of fronts. A physical ocean model configuration of the SBUS in conjunction with a gradient-based edge detection algorithm and a Lagrangian tracking code are used and evaluated against a combination of in situ and satellite data products. The model is able to resolve the low- frequency seasonal and interannual variability in sea surface temperature (SST) and surface circulation features. The results show alongshore fronts in summer to be effective barriers to the offshore advection of matter as they are defined by strong SST gradients and in general, are long and cohesive structures. In winter, fronts tend to be short and filamentous in nature, which limits their ability to restrict the offshore movement of material. Furthermore, fronts in summer tend to be found closer to the coast than those in winter. Transport of material on the SBUS shelf is governed by the surface equatorward jets and poleward undercurrents along the shelf bottom. Interannual variability in residence times for material on the shelf is driven by natural modes of variability in the alongshore winds that impact the shelf SST patterns which in-turn shape the spatial patterns and intensities of fronts. At the event-scale, variability in the alongshore wind profiles generate unique responses in the nearshore current velocities such that enhanced retention is associated with wind relaxation events. The transport dynamics, cross-shore mixing regimes, spatial patterns of fronts, especial coastal topography and bathymetry as well as the influence of the Agulhas Current create a unique oceanographic environment in the SBUS. Drivers of retention in other EBUS are important to understand, especially when considering biogeochemical processes or how these systems might be impacted by climate change in the future.
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