Browsing by Subject "Benguela Current"

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    Cross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia
    (2007) Van der Plas, A K; Monteiro, P M S; Pascall, A
    Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.
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    Evaluation of the operational Mercator Global Ocean analysis and forecast system modelled current vector product with in-situ ADCP data in the Southern Benguela upwelling system
    (2025) Gammon, Emily; Clark; Barry; Schmidt, Kevin; Vichi, Marcello
    Global ocean current vector datasets are frequently used to force, or are assimilated into, local and regional particle tracking and pollutant dispersal models, metocean forecasting platforms, climate analyses, and natural resource management applications. The Operational Mercator global ocean analysis and forecast system (Mercator data product), made publicly available in 2022, provides near-real-time vector components of global ocean currents at 50 vertical levels (depths) in the water column with a horizontal resolution of 1/12 degree. Validation of ocean forecasting and simulation models is required to assess a modelled product's ability to represent the local environment. Validation is done using in-situ data such as those gathered by Acoustic Doppler Current Profilers (ADCP). In this study, an upward-facing Nortek Signature 250 ADCP was deployed at a depth of 123 m off the Southern Namibian coast for ten months. The resulting current profile data were used to validate the Mercator data product for the same area. Data collected by the ADCP and simulated data from the NEMO model were matched spatially (nearest-neighbour) and temporally (rolling average), and the similarity of current vector components was statistically investigated. Spearman Rank-Order Correlation, root-mean-square-error and bias were calculated for the full time series. The seasonal percent occurrence of current vector magnitude and direction were subsequently derived to assess the Mercator data product's ability to represent seasonal variability. The Mercator data product captured the seasonal variation in the magnitude of the nearshore southern Benguela Upwelling System for both the zonal and meridional components well. The Mercator data product represented much lower complexity in the direction of bulk flow GAMMON iii within the system but did contain the daily oscillation pattern measured by the ADCP. The Benguela Current is a physically complex eastern boundary current spanning three countries, used extensively for fishing, mining, maritime transit, and oil extraction. The ocean current outputs from the Mercator data product could allow for improved pollution modelling (oil dispersion or plume modelling), improved fuel optimisation models for maritime transit, and productivity prediction to support fisheries management without the cost of deploying devices to measure ocean currents in situ.