Browsing by Author "Ansorge, Isabelle J"

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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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    Decay of eddies at the South-West Indian Ridge
    (2011) Durgadoo, Jonathan V; Ansorge, Isabelle J; de Cuevas, Beverly A; Lutjeharms, Johann R E; Coward, Andrew C
    The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a highresolution ocean model. The model's representation of the dynamic nature of this region is fully characteristic of observations. On average, 3-4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5-2.5 months after genesis and have depths of 2000 m - 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature-salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour.
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    Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
    (CrossMark, 2017-06) Ansorge, Isabelle J; Skelton, Paul; Bekker, Annie; de Bruyn, P J Nico; Butterworth, Doug S; Cilliers, Pierre; Cooper, John; Cowan, Don A; Dorrington, Rosemary; Fawcett, Sarah; Fietz, Susanne; Findlay, Ken P; Froneman, P William; Grantham, Geoff H; Greve, Michelle; Hedding, David; Hofmeyr, G J Greg; Kosch, Michael; le Roux, Peter; Lucas, Mike; MacHutcho, Keith; Meiklejohn, Ian; Nel, Werner; Pistorius, Pierre; Ryan, Peter; Stander, Johan; Swart, Sebastiaan; Treasure, Anne; Vichi, Marcello; Jansen van Vuuren, Bettine
    Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.
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    The variability and dynamics of the Antartic Circumpolar Current south of Africa using proxy techniques
    (2009) Swart, Sebastiaan; Ansorge, Isabelle J; Speich, Sabrina; Lutjeharms, Johann R. E.
    The general circulation of the Southern Ocean is dominated by the eastward flowing Antarctic Circumpolar Current (ACC). This is a continuous feature linking the three major ocean basins and thus forms a vital link in the transport of heat and salt on a global scale. These exchanges provide a vital mechanism for the global thermohaline circulation (THC), which regulates the Earth's climate. In the high latitudes, where conditions are hostile, routine hydrographic observations are scarce resulting in a poor understanding of the physical and dynamic processes controlling the variability of the ACC and its influence on the THC. The GoodHope program launched in early 2004 aimed to establish an intensive monitoring platform that would provide detailed information on the physical structure and volume flux of water masses south of Africa. Sustained observations along the GoodHope cruise track provide the means to monitor the vertical structure and variability of the ACC and its associated fronts south of Africa. Such intense monitoring has been under way in the Drake Passage and south of Australia since the 1970s. A major objective of this thesis is to provide sound estimates of ACC transport and variability using both in situ measurements and remote sensing techniques. These estimates are crucial in understanding the role the ACC plays in the global thermohaline circulation (THC) and how the region south of Africa acts as a major conveyor of heat and salt to the higher latitudes. Baroclinic transports of the ACC, relative to 2500 dbar, are calculated from altimetry data alone. These transports agree with simultaneous observed estimates (rms difference in net transport is 5.2 Sv). These observations suggest that sea level anomalies largely reflect baroclinic transport variations above 2500 dbar. The transports contribution per ACC front shows that the SAF is responsible for the highest variability signals (>50%) even though its net transport contribution to the ACC was less (9%) than the APF. Furthermore, direct measurements of heat and salt content in the Southern Ocean are based on the few synoptic transects, the majority of which are restricted in the austral summer. To overcome the poor temporal and spatial resolution of measurements in the south African sector of the Southern Ocean, this thesis makes use of the gravest empirical mode (GEM) method and applies this technique to weekly composites of satellite altimetry data. The GEM method makes use of all available hydrographic casts from the south-east Atlantic Ocean and projects the data into a baroclinic stream function space parameterised by pressure and dynamic height. The GEM fields were shown to compare closely with independent in situ observations of the water column, capturing more than 97% of the total temperature and density variance in the ACC domain. The GEM-derived heat and salt content estimates attempt to determine the variability signals of the ACC due to external influences, such as topographical obstacles and oceanic features originating from subtropical regions. The exploitation of such proxy methods is useful in improving our understanding of the subsurface properties of the Southern Ocean and more importantly the influences temporal changes in the system have on the structure and transport of the ACC. With time, these methods will be refined with the input of new observations, thereby enhancing their ability to determine the dynamic nature of the ACC and its impact on the Earth's system.