Browsing by Author "Malan, Neil"

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    An altimetry based examination of the path and variability of the Agulhas Return Current
    (2018) Fadida, Yotam; Hermes, Juliet; Malan, Neil
    As a result of climate change, partially driven by anthropogenic activity, strengthening wind stress over the worlds oceans is yielding a western boundary intensification and pole-ward shift of oceanic currents (Yang, 2016). Shifting wind regimes and perturbations in neighboring currents have been associated with variations in the flow path of western boundary currents (WBC) and their extensions (Talley, 2011; Combes and Matano, 2014; Nakamura and Kazmin 2003). WBCs, typically characterised by fast flowing and warm currents, are key regions of heat and salt transport as well as for oceanic carbon uptake, air-sea heat flux and nutrient transport, thus positional shifts may have far-reaching implications (Gray and Palter, 2017). While meridional trends (latitudinal migrations) have been observed in analogous WBCs and their extensions, no long-term investigation (decadal) into the flow path and position of the Agulhas Return Current (ARC) has been carried out. Now, with over 25 years of altimetric, satellite remote observation data available, a better understanding of the flow path, seasonal cycle, meridional trends and eddy kinetic energy (EKE) of the ARC is possible. Here we provide a detailed description of the flow path of the ARC together with several mechanisms that may be contributing to the current’s apparent stability. In addition we demonstrate that the documented western boundary intensification and pole-ward shift may be manifesting itself in two ways in the ARC, an increase in regional EKE and a southward trend found mainly in the flow-path of the eastern section of the ARC. Our results show a strong link between the EKE of the region and the Subtropical Indian Ocean Dipole, both of which are associated with the Southern Annular Mode (SAM), illustrating the effects that southward shifts in the subtropical high pressure system have on regional and mesoscale climate modes. The combination of a stable, topographically forced flow path with observed EKE increases, demonstrate the effects of wind stress intensification on a region highly influenced by its bathymetry. Further research into the effects of enhanced eddy activity is necessary, as it has been shown to affect regional primary production (Falkowski, 1991; Oschlies, 1998) and air-sea interactions, thus having potential ramifications for regional aquaculture, weather and fisheries, as well as calculations/models concerned with heat-flux and carbon exchange.
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    Driving mechanisms of the Port Alfred upwelling cell inshore of the Agulhas Current
    (2013) Malan, Neil; Ansorge, Isabelle Jane; Roberts, Mike
    The presence of a semi-permanent upwelling cell, with a surface expression more than 40 % of the time has previously been described inshore of the Agulhas Current in the vicinity of Port Alfred, South Africa. This study employs a combination of in-situ mooring data, hydrographic cruises and satellite remote sensing in order to investigate the nature and variability of this upwelling cell, as well as to investigate possible driving mechanisms. The study takes place over a period of 11 months. Special focus is given to the subsurface variability due to its possible implications for the greater Agulhas Bank environment. Upwelled water was found to be present on the shelf 85% of the time, highlighting the importance of subsurface variability in this area. The main timescales of variability were observed at 50-70 days, 8-12 days and 4-6 days. Upwelling was found to be maintained by continuous processes, driven by the interaction of the Agulhas Current with the changing bathymetry at Port Alfred. This upwelling is modulated by the effects of mesoscale features on the inshore edge of the current. While not a primary driver of upwelling, wind events were observed to have an effect on inshore bottom temperatures as well as the surface expression of the upwelling cell. A high degree of variability in was observed, with bottom temperatures at three mooring site fluctuating through a range of approximately 10° C. Future directions include further theoretical and idealised modeling studies to separate out the exact mechanisms of topographically driven, site-specific upwelling. The range of mesoscale interactions of the Agulhas Current with the shelf circulation also require further observational study.