Browsing by Author "Mashifane, Thulwaneng"

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    A numerical modelling study of Port Alfred upwelling along the inshore edge of Agulhas current
    (2024) Sunnassee, Taukoor Sheveenah; Penven, Pierrick; Ansorge, Isabelle J; Mashifane, Thulwaneng
    Port Alfred upwelling, located on the southeast African shelf, lies on the inshore edge of a western boundary current, the Agulhas Current. This study addresses the atmospheric and oceanographic forcing mechanisms responsible for these upwelling events through the daily simulations of a CROCO model of a horizontal spatial resolution of ~2.5km from 1993 to 2014. We tested several coastal upwelling indices and we identified 56 upwelling events from the residuals of sea surface temperature and 49 upwelling events through the decomposition of the vertical velocity. To assess the influence of the wind, we measured the alongshore wind stress, wind stress curl and frictional velocity at different temporal scales. Our analysis showed that upwelling during summer was primarily driven by northeasterlies inducing offshore Ekman transport and divergence. In contrast, stronger southwesterlies during winter could cause vertical mixing. To determine the Port Alfred upwelling’s driving mechanism during the mean state, we computed the terms of the generalized Ekman pumping equation and found that the advection of momentum contributed to 4.22 m/day of vertical velocity while the viscous flux term contributed partially to 0.5 m/day of vertical velocity. To determine whether the upwelling was driven by oceanographic mesoscale features (Agulhas Current, Natal pulses, Durban eddies, shear edge eddies and coastal trapped waves), we measured the sea surface height, geostrophic velocity, bottom Ekman transport and identified 1.9 large meander events annually from the LACCE current tracker algorithm, 2.2 based on eddy amplitude, 1.5 based on eddy area, and 1.3 based on eddy radius from the PY eddy tracker algorithm. We found that the Agulhas Current was the primary upwelling driver during the mean state, but some individual upwelling events were influenced by cyclonic eddies and coastal trapped waves. Finally, we conducted 2 combined Empirical Orthogonal Function analysis from (1) sea surface temperature and (2) vertical velocity and identified surface divergence as the most dominant upwelling driver in both combined EOFs while Ekman transport, the presence of cyclonic eddies, the Agulhas Current and coastal trapped waves also counted as contributing factors in stronger and weaker upwelling events. As an overall this study confirms that the Agulhas Current remains the primary upwelling driver during the mean state but through the lens of a different temporal scale (individual case studies and combined EOFs), it is likely that a combination of one or more forcing mechanisms (upwelling favourable winds, cyclonic eddy, Agulhas Current, coastal trapped waves) will trigger an upwelling event. Shedding more light on this topic and its main drivers allows oceanographers to focus more attention on this upwelling in the future and this could reinforce policymakers to consider Port Alfred upwelling region as a future Marine Protected Area.
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    Shelf biogeochemical interactions and feedback processes in the Benguela upwelling system
    (2017) Mashifane, Thulwaneng; Vichi, Marcello; Waldron, Howard
    Two coupled physical-biogeochemical models namely, (Regional Ocean Modelling System and Biogeochemical of Eastern Boundary Upwelling Systems) ROMS-BioEBUS (3D) and (Nucleus for European Modelling of the Ocean and Biogeochemical Flux Model) NEMO-BFM (1D) are applied in the Benguela upwelling system to understand biogeochemical interactions and their related feedback processes. The models are formulated differently but achieve similar objectives with respect to the physics and biogeochemistry. The BioEBUS model is used to simulate nitrogen processes under oxic and suboxic conditions in upwelling systems with no option for other cycles. Intermediary nitrogen processes, nitrous oxide production and nitrogen loss mechanisms are studied using this model. Physical and advection processes that drive the oceanic nitrogen cycle in the region are also studied with BioEBUS. The BFM is used to understand the implications of the nitrogen loss and suboxic-anoxic conditions on related biogeochemical cycles. The 1D model was selected for its low computational costs and flexibility for addition of new code. BFM includes the carbon, nitrogen, phosphorus, silicate, iron cycles and hydrogen sulphide production, which is a known occurrence in the Namibian shelf waters. New variables, nitrite and nitrous oxide production, are added in BFM to complete the nitrogen cycle. The nitrification process in BFM is also formulated in two stages as in BioEBUS to obtain comparative results in both models. Both models are compared and validated with data from the Maria S. Merian (MSM) 19/1b cruise and available products respectively. Simulated results from BioEBUS show primary and secondary nitrite maxima in the Benguela shelf waters. The primary nitrite maxima are attributed to nitrification and nitrate assimilation. Secondary nitrite maxima accumulate in the Angola-Benguela Front (ABF) oxygen minimum zone (OMZ) and are attributed to denitrification. Off Walvis Bay, these secondary nitrite maxima and ammonium are thought to be consumed by high rates of anaerobic ammonium oxidation (anammox). The nitrite maxima are restricted to the shelf off Walvis Bay and advected offshore in the ABF region. Interchanges between the poleward South Atlantic Central Water (SACW) and the equatorward, well-oxygenated Eastern South Atlantic Central Water (ESACW) drive the seasonality of nitrogen processes in the Benguela. Nitrous oxide concentrations are high in the ABF as a result of nitrification and accelerated production under suboxic conditions. Off Walvis Bay, nitrous oxide production is low when compared to the ABF. Nitrous oxide production in the ABF occurs in thermocline, intermediate and deeper water masses. Off Walvis Bay, nitrous oxide production in deeper water masses is missing because of the shallow coast. High fixed nitrogen fluxes in the Benguela are attributed to nitrification rather than anammox and denitrification. Simulated results show denitrification to be the dominant nitrogen loss mechanism in the Benguela shelf waters. Simulated results from BFM show higher nitrogen uptake rates than phosphate in shelf and offshore stations. The uptake rates are high on the shallow shelf due to luxury nutrient uptake. High N:P ratios occur at the stations at 21ᵒS than off Walvis Bay and are attributed to the presence of nutrient-rich, oxygen depleted SACW and denitrification respectively. Increased fixed nitrogen deficits (N*) occur in surface and subsurface waters at shallow stations as opposed to offshore. The positive N* anomalies off Walvis Bay are attributed to organic matter remineralization in deep, offshore stations. In contrast, increased phosphate (P*) concentrations occur in surface and subsurface waters. Phosphate is regenerated in subsurface waters and released under suboxic-anoxic conditions increasing P* concentrations. Nitrogen loss coupled with hydrogen sulphide production accelerate phosphate release in suboxic-anoxic bottom waters. The N:P stoichiometry, uptake rates, N* and P* concentrations appear to have considerable implications on potential estimated nitrogen fixation in the Benguela. BFM results suggest that the Benguela is a system characterized by excess nitrate in relation to silicate. This has been drawn from the low Si:N ratios observed at the simulated stations. A secondary Si:N peak is shown on the shallow coast due to high denitrification rates in suboxic waters. Note that high silicate concentrations occur in suboxic conditions and can be attributed to organic matter remineralization. The high silicate concentrations in the well-oxygenated offshore station are linked to sinking particles in deep waters. Increased silicate dissolution occurs in warm, surface waters and the particles that pass through the water column undissolved settle at the bottom where dissolution continues. From these results, it can be assumed that increased warming in the Benguela coastal waters should result in silicate being a limiting nutrient. This could affect carbon export as it has been shown that increased POC export is high in coastal waters due to ballasting of diatom biomass. The models used in this study simulated biogeochemical interactions in the Benguela fairly-well and can be applied in other regions.