The Benguela: A laboratory for comparative modeling studies

Abstract Equilibrium dynamics of the Benguela system is investigated using the holistic nature of the spatially and temporally cohesive output of a numerical model. The Regional Ocean Modeling System (ROMS) is used to simulate the Benguela system in its entirety. It successfully simulates the cool coastal upwelling regime and its division into seven distinctly separate cells, as well as the large-scale offshore regime and the respective seasonal fluctuations. It does however, present a cool bias at the coast due to an underestimation of the coastal wind drop-off as well as a warm bias offshore in the southern Benguela due to the overestimation of Agulhas Current input. The Benguela can be divided into northern and southern regimes, based on dynamic as well as topographic differences. Topographically, the division between the northern and southern regimes coincides with an abrupt narrowing of the continental shelf toward the north at 28°S. The large-scale depth-integrated flow to the north of this feature is weak but distinctly poleward, while to the south the flow regime is governed by the meandering nature of the equatorward Benguela Current and is the pathway for eddies that originate at the Agulhas retroflection. The poleward flowing regime of the northern Benguela is tied to the Sverdrup relation, which links meridional transport with wind stress curl. The Luderitz upwelling cell at 27°S experiences the most vigorous upwelling throughout the year and, as a result, offshore volume fluxes in this region are extremely large. This upwelling cell divides the northern and southern Benguela coastal upwelling systems into separate regimes, based on the fact that their seasonal signals are out of phase. The offshore gradient of eddy kinetic energy (EKE) is generally strong in the Benguela system and exceptionally so in the southern Benguela due to vigorous mesoscale activity offshore of the shelf-edge, originating from the Agulhas retroflection area. The juxtaposition between the steep offshore EKE gradients in the south and much weaker offshore gradients of EKE in the northern Benguela has different implications for cross-shore exchanges.

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