Browsing by Subject "Southern Benguela"

Now showing 1 - 6 of 6
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    Assessment of a predictability index for humpback whales' supergroups occurrences in the Southern Benguela
    (2023) De Souza, Cayla; Vichi, Marcello
    The highly productive Benguela upwelling system in the south-eastern Atlantic Ocean plays a crucial role in ocean productivity and nutrient cycling. Its termination at the tip of Africa creates a distinct ecological niche, fostering a diverse marine ecosystem. Since 2011, unprecedented aggregations of humpback whales (Megaptera novaeangliae) have been observed in the Southern Benguela. It has been proposed that positive chlorophyll anomalies and reduced outward transport serve as potential indices for predicting the formation of humpback whale super-groups with a lead time of at least one month (Dey, S.P., Vichi, M., Fearon, G., Seyboth, E., Findlay, K.P., Meynecke, J.O., de Bie, J., Lee, S.B., Samanta, S., Barraqueta, J.L.M. and Roychoudhury, A.N., 2021. Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela. Scientific Reports, 11(1), p.20896.). By utilizing an extended time series of satellite data and a different ocean model output (CMEMS) until 2022, the study aims to validate the proposed indices and deepen our understanding of the oceanographic preconditioning and patterns associated with super-group formations. The Dey et al. (2021) index has been compared with new sightings from various formal and informal sources. There are notable discrepancies between the two Ocean Models; the CMEMS simulation demonstrates a more zonal orientation of the Good Hope Jet. Additionally, the CMEMS simulation presents a greater mean outward transport during 2011. Consistent observations of humpback whale super-groups well into the summer season despite negative chlorophyll and retention indices suggest a more intricate change in oceanographic conditions within the focus region. Notable changes in eddy kinetic energy and outward transport since 2017 suggest an increased variability in resource availability within the environment. The aggregations of humpback whale super- groups since 2017 occurred at smaller spatial scales near the Cape Peninsula upwelling cell. The rising trend of humpback whale super-groups within the Southern Benguela, particularly in the Cape Peninsula and Cape Columbine region, emphasizes the importance of these regions as critical feeding grounds.
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    Environmental changes and regime shift analysis in the southern Benguela using output from earth system models
    (2025) Wilcox, Maxine; Ortega-Cisneros, Kelly; Shannon, Lynne
    The accelerating pace of anthropogenic carbon emissions has triggered profound changes in Earth's climate, affecting marine ecosystems and fisheries globally. This study focuses on the southern Benguela, an integral region of the southern African Eastern Boundary Upwelling System (EBUS), investigating the potential impacts of climate change on this biologically productive region. Utilising data from the latest Coupled Model Intercomparison Project 6 (CMIP6), this research explores the implications of future extreme events, such as marine heatwaves, hypoxia, and acidification, on the southern Benguela ecosystem. Output from two earth system models (ESMs): the Geophysical Fluid Dynamics Laboratory (GFDL) and the Institut Pierre Simon Laplace (IPSL) was integrated with shared socioeconomic pathways (SSPs) to simulate environmental changes over time under historical scenarios (1970 – 2014), as well as low fossil fuel emission scenarios (SSP1-2.6) and high fossil fuel emission scenarios (SSP5-8.5). Trends in sea temperature, pH, chlorophyll-a concentration, integrated primary production, and oxygen concentration were plotted over the three different scenarios on the west and south coasts of the southern Benguela, over three depth levels. In addition, these time series were analysed using the sequential t-test algorithm for regime shift detection (STARS), in an attempt to detect any abrupt, long-term shifts. Projections under SSP1-2.6 tended to follow their historical trajectories, and displayed gradual changes over time. Under SSP5-8.5, however, future projections showed considerable temperature increases, conflicting trends in chlorophyll-a concentrations and integrated primary production, and concerning declines in pH and oxygen concentrations. Regime shifts were detected on both coasts for both ESMs, and tended to coincide temporally with extreme events observed in the environmental trends. In future scenarios on the west coast, clusters of regime shifts tended to be detected shortly after extreme events. Through a comparison of historical observations and hindcasts, the study also evaluated the reliability of the two ESMs, and it was found that GFDL generally exhibited better agreement with historical observations. The kind of ESM model-to-historic observation evaluations as carried out in this study play a crucial role in enhancing models' consistency and ought to be prioritised in the progression of CMIP7. Ultimately, this study contributes novel insights into the environmental dynamics of the southern Benguela. This research serves as a foundation for understanding future changes that may impact the region, providing valuable information for decision-makers, policymakers, and resource managers. In the face of climate change, these findings emphasise the need for adaptive and sustainable fisheries management strategies to ensure the conservation of the southern Benguela ecosystem and the millions of livelihoods it sustains.
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    Evaluating multi-satellite chlorophyll-a datasets an ocean colour case study within the Southern Benguela
    (2025) Oehley, Craig; CSIR, Smith; CSIR, Whittle
    Chlorophyll-a (chl-a), a photosynthetic pigment that can be derived from satellite ocean colour, is often used as a proxy for phytoplankton biomass and to derive primary productivity. Initiatives such as the Ocean Colour Climate Change Initiative (OC-CCI) produce merged multi-satellite products to create consistent long-term time-series datasets for climate studies at a global scale. Their ability to handle variable in-water conditions is critical for their mission parameters. This study compares the performance of two European multi-satellite chl-a products, from the OC-CCI and GlobColour projects against a regionally tuned Sentinel-3 product within the Southern Benguela region. The three products were assessed against a collated database of coincident in situ chl-a matchups to derive a series of performance metrics. The regionally tuned Sentinel-3 product outperformed the two global products in terms of Mean Absolute Error but showed a slight consistent overestimation bias. Analysis of match-ups showed an underestimation of high chl-a concentrations and overestimation of lower chl-a concentrations by both global products. An application of the products within St.Helena bay during high biomass bloom events showed that the Sentinel-3 product's ability to capture extreme chl-a concentrations was far higher than both global products. Spatial mismatch between zones of high chl-a concentration also indicate differences in processing chains and flagging techniques.
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    Is the southern Benguela a significant regional sink of CO2?
    (2013) Gregor, Luke; Monteiro, Pedro M S
    This study was undertaken to characterise the seasonal cycle of air-sea fluxes of carbon dioxide (CO2) in the southern Benguela upwelling system off the South African west coast. Samples were collected from six monthly cross-shelf cruises in the St. Helena Bay region during 2010. CO2 fluxes were calculated from pCO2 derived from total alkalinity and dissolved inorganic carbon and scatterometer-based winds. Notwithstanding that it is one of the most biologically productive eastern boundary upwelling systems in the global ocean, the southern Benguela was found to be a very small net annual CO2 sink of -1.4 ± 0.6 mol C/m2 per year (1.7 Mt C/year). Regional primary productivity was offset by nearly equal rates of sediment and sub-thermocline remineralisation flux of CO2, which is recirculated to surface waters by upwelling. The juxtaposition of the strong, narrow near-shore out-gassing region and the larger, weaker offshore sink resulted in the shelf area being a weak CO2 sink in all seasons but autumn (-5.8, 1.4 and -3.4 mmol C/m2 per day for summer, autumn and winter, respectively).
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    Marine heatwaves and warm events in the Cape Peninsula upwelling cell, Southern Benguela
    (2023) Petzer, Kirstin Robyn; Lamont, Tarron; Rouault, Mathieu
    Due to global warming, Marine Heatwaves (MHWs) are considered to be one of the emerging threats to marine ecosystems globally. MHWs are prolonged periods of extreme warm Sea Surface Temperature (SST) anomalies which can cause severe ecological impacts by decreasing biodiversity, negatively affecting cold water species and increasing ocean stratification. Using the Cape Point CSIR half-hourly in situ SST, CCI, REMSS, ERA5 wind time series, over 17-years the occurrence from January 2003 to March 2020, duration and maximum SST values as well as the influence of the wind on the formation and end on marine heatwaves and warm events (WEs) at a single location in the Cape Peninsula Cell, in the Southern Benguela, was examined. The MHW events were identified using Hobday et al. (2016), when the SST exceeds the climatological 90th percentile for at least five days. The WE events, defined similarly to a MHW but the SST must exceed the climatological 90th percentile for at least three days, are also studied due to the high variability of the Southern Benguela. In the half-hourly CSIR time series 14 MHWs and 21 WEs occurred over the 17 years. The average duration is between 7 to 8 days but the longest events occurred during periods of decreased upwelling but the highest maximum SSTs occur during the periods of upwelling dominance. The daily CSIR, CCI and REMSS time series all identified double the number of MHWs and WEs events than the half-hourly time series, raising the concern of applying the Hobday et al. (2016) definition to sub-daily time series and the ability of satellites to be used for MHW identification in the Southern Benguela close to the coast. The dominant wind at the formation of MHWs and WEs is a north-westerly wind, indicating the main driver of events at the CSIR Cape Point mooring is the movement of warm water masses to the mooring location. The dominant wind direction at the end of the MHWs and WEs is a south-easterly wind indicating that coastal upwelling limits the duration of warm water events at the Cape Point mooring. Marine heatwaves are expected to worsen globally with climate change by lasting longer with high temperature increases but the projected increase in southeasterly winds could further limit the duration of MHWs in the Southern Benguela upwelling system.
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    Retention processes in the Southern Benguela upwelling system
    (2025) Rogerson, Jonathan; Fawcett, Sarah
    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.