Browsing by Author "Chang, Nicolette"

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    Modelling transport of chokka squid (Loligo reynaudii) paralarvae off South Africa: reviewing, testing and extending the ‘Westward Transport Hypothesis'
    (John Wiley & Sons Ltd, 2013-08) Martins, RS; Roberts, MJ; Lett, C; Chang, Nicolette; Moloney, CL; Camargo, MG; Vidal, EAG
    Annual landings of chokka squid (Loligo reynaudii), an important fishing resource for South Africa, fluctuate greatly, and are believed to be related to recruitment success. The ‘Westward Transport Hypothesis’ (WTH) attributes recruitment strength to variability in transport of newly hatched paralarvae from spawning grounds to the ‘cold ridge’ nursery region some 100–200 km to the west, where oceanographic conditions sustain high productivity. We used an individual-based model (IBM) coupled with a 3-D hydrodynamic model (ROMS) to test the WTH and assessed four factors that might influence successful transport – Release Area, Month, Specific Gravity (body density) and Diel Vertical Migration (DVM) – in numerical experiments that estimated successful transport of squid paralarvae to the cold ridge. A multifactor ANOVA was used to identify the primary determinants of transport success in the various experimental simulations. Among these, release area was found to be the most important, implying that adult spawning behaviour (i.e., birth site fidelity) may be more important than paralarval behaviour in determining paralarval transport variability. However, specific gravity and DVM were found to play a role by retaining paralarvae on the shelf and optimizing early transport, respectively. Upwelling events seem to facilitate transport by moving paralarvae higher in the water column and thus exposing them to faster surface currents.
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    Ocean variability over the Agulhas Bank and its dynamical connection with the southern Benguela upwelling system
    (American Geophysical Union, 2009-12) Blanke, B; Penven, P; Roy, C; Chang, Nicolette; Kokoszka, F
    This study analyzes the oceanic pathway connecting the Agulhas Bank to the southern Benguela upwelling system by means of a quantitative Lagrangian interpretation of the velocity field calculated by a high-resolution numerical simulation of the ocean around the southwestern tip of Africa. The regional ocean model is forced with National Centers for Environmental Prediction surface winds over 1993–2006 and offers a relevant numerical platform for the investigation of the variability of the water transferred between both regions, both on seasonal and intraseasonal time scales. We show that the intensity of the connection fluctuates in response to seasonal wind variability in the west coast upwelling system, whereas intraseasonal anomalies are mostly related to the organization of the eddy field along the southwestern edge of the Agulhas Bank. Though the study only considers passive advection processes, it may provide useful clues about the strategy adopted by anchovies in their selection of successful spawning location and period. The pathway under investigation is of major interest for the ecology of the southern Benguela upwelling system because it connects the spawning grounds on the Agulhas Bank with the nursery grounds located on the productive upwelling off the west coast.
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    The Southern Ocean Carbon-Climate Observatory (SOCCO): A South African-Led Ocean-Climate Science and Innovation Programme that Links Fine Scale Dynamics to Large Scale Carbon-Climate Feedbacks
    (2024-10) Thomalla, Sandy J; Chang, Nicolette; Mongwe, Ndunisani P; Nicholson, Sarah-Anne; Ryan-Keogh, Thomas J; Monteiro, PM
    The Southern Ocean Carbon-Climate Observatory (SOCCO) is a South African-led programme, based at the Council for Scientific and Industrial Research (CSIR), whose strategy links ocean-climate-ecosystem science to society through technological innovation, human-capital development and evidenced-based policy support. SOCCO was established in 2010 in response to national and international priorities to understand the sensitivity of the Southern Ocean to climate change for improved global projections of the ocean carbon-climate system. SOCCO postulated that the influence of the Southern Ocean in global climate would be mediated by the characteristics of the seasonal cycle through adjustments in upperocean physics and biogeochemical response. Addressing this hypothesis required long-term investment in high-resolution models, integrated physical-biogeochemical observations and capacity building of a new generation of ocean-climateecosystem scientists. SOCCO pioneered the use of ocean robotics in the Southern Ocean through multiplatform seasonal cycle experiments that advance our understanding of the physical and biological drivers of the seasonal cycle. Innovation in observations, modelling, machine learning and sensor development remains key to the modus operandi of SOCCO as it continues on its mission to reduce uncertainties in key physical-biogeochemical processes of the Southern Ocean towards strengthening confidence in climate projections from Earth System Models (ESMs).