Browsing by Author "Swart, Sebastiaan"

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    Characterization of a warm-cored agulhas ring travelling south of the subtropical front
    (2010) Vianello, Patrick; Ansorge, Isabelle; Swart, Sebastiaan
    The south-eastern sector of the Atlantic Ocean near the South African coast is a region of high eddy activity in the form of Agulhas rings. These Agulhas rings play a vital role in the Inda-Atlantic inter-ocean exchange, and are critical for the global thermohaline circulation since these rings, spawned from the Agulhas retroflection, are typically of Indian Ocean origin. The pathway and behaviour of these Agulhas rings are affected by the underlying topography of the region with special reference to the Agulhas ridge and the Erica and Schmitt-Ott seamounts. This study investigates an Agulhas ring, which was identified during the Bonus-Goodhope cruise in February 2008. In addition to the hydrographic data obtained during this cruise, satellite data such as altimetry and Sea Surface Temperature (SST) are also employed to analyse and determine the origins of the feature, its development and lifespan. The majority of Agulhas rings remain within the Subtropical domain, however during the 2008 Goodhope cruise a positive anomaly was observed south of the Subtropical Front in the Subantarctic zone. Studying these features which drift into the Subantarctic domain may be important in understanding the global heat and salt budget since these Agulhas rings have positive heat and salt anomalies with respect to the surrounding region. Overall, it can be concluded that the Subantarctic zone had a major impact on the core properties of the ring such as its mixed layer depth and that the underlying topography such as the Agulhas ridge had a major effect on the behaviour and life span of the ring.
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    Dynamics and variability of the Subantarctic mixed-layer as determined from a high resolution glider dataset
    (2015) Du Plessis, Marcel David; Ansorge, Isabelle Jane; Jackson-Veitch, Jennifer; Swart, Sebastiaan; Mahadevan, Amala
    Traditional understanding of mixed-layer (ML) dynamics in the African sector of the Southern Ocean suggests that seasonal summer stratification and subsequent reduction in ML depth (MLD) is determined by the onset of a positive net heat ux. The impact of physical forcing mechanisms on the intra-seasonal variability of the ML is still relatively unknown. Recent research in the North Atlantic has highlighted the role that sub-mesoscale ML eddy dynamics has on ML stratification. It is now understood that large horizontal density gradients drive sub-mesoscale eddy formation which have been shown to result in the early onset of spring phytoplankton blooms at high latitudes. To date these ML eddies have been researched primarily in models with few observational studies available. To test the ML eddy hypothesis in the Subantarctic Zone (SAZ) we use high-resolution (~3km, 4-hourly) glider measurements between austral spring to late summer.
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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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    Intra-seasonal variability of Southern Ocean primary production: the role of storms and mesoscale turbulence
    (2016) Nicholson, Sarah-Anne; Lévy, Marina; Monteiro, Pedro; Swart, Sebastiaan; Vichi, Marcello
    The Southern Ocean is one of the stormiest places on earth; here strong mid-latitude storms frequently traverse large distances of this ocean. Underlying these passing storms, the Southern Ocean is characterized by having some of the highest eddy kinetic energy ever measured (eddies occupying the meso to sub-mesoscale). The presence of the passage of intense storms and meso to sub-mesoscale eddy variability has the potential to strongly impact the intraseasonal variability of the upper ocean environment where phytoplankton live. Yet, exactly how phytoplankton growth rates and its variability are impacted by the dominance of such features is not clear. Herein, lies the problem addressed by the core of this thesis, which seeks to advance the understanding of intra-seasonal variability of Southern Ocean primary production. The drivers of this intra-seasonal variability have been explored from two points of view: the local-scale and the remote-scale perspectives, with a suite of physicalbiogeochemical (NEMO-PISCES) numerical models of varying complexity. At the local-scale, these model experiments have suggested that intra-seasonal stormlinked physical supplies of dissolved iron (DFe) during the summer played a considerably more active and influential role in explaining the sustained summer productivity in the surface waters of the Southern Ocean than what was thought previously. This was through two important insights: 1. Storm-eddy interactions may strongly enhance the magnitude and extent of upperocean vertical mixing in both the surface mixed layer as traditionally understood as well as in the subsurface ocean. These two mixing regimes have different dynamics but act in concert to amplify the DFe fluxes to the surface ocean. 2. Storm initiated inertial motions may, through interaction with eddies, greatly reinforce w and thus, enhance the vertical advection of DFe to the surface ocean, an effect that may last several days after the storm. At the local-scale, such storm-eddy dynamics may greatly increase the intra-seasonal variability of primary production, a step towards helping to explain why this variability is so strong in large regions of the Southern Ocean. At the remote-scale, the cumulative impact of these short-term storm-eddy interactions have unexpected implications in respect of the larger-scale mean flow and its influence on the effectiveness of intra-seasonal forcing of DFe fluxes. This counter intuitive feedback is a reduced strength of the intra-seasonal variability in primary production despite what was shown at the local-scale. Moreover, the addition of storms intensified the main clockwise cell of the meridional overturning circulation particularly the downward branch thus, reducing DFe inventory from the upper-ocean. Such an impact could potentially be enhanced with increasing storm intensities as suggested by climate projections. Understanding these remote-scale and local-scale responses of primary productivity to storms and their interaction with the underlying ocean mesoscale turbulence may be key to better understanding the sensitivities of the carbon cycle to short-term variability and long-term trends in atmospheric forcing.
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    Quantifying spatial and temporal scales of phytoplankton variability in the Sub-Antarctic Ocean using a high-resolution glider dataset
    (2016) Little, Hazel Jean; Thomalla, Sandy; Swart, Sebastiaan; Vichi, Marcello
    Phytoplankton in the Sub-Antarctic Southern Ocean have a distinct seasonal cycle, which is highly variable in both space and time. The seasonal and spatial distribution of chlorophyll can be attributed to the complex nature of the physical and biogeochemical factors controlling phytoplankton production. Studies show that high-resolution sampling is required to understand variability in phytoplankton distribution and primary production. In this study, high-resolution glider data sampled in the Atlantic Sub-Antarctic Zone are used to characterise the scales of phytoplankton variability. Continuous glider data provide a novel way to assess phytoplankton variability at small time and space scales (meso- to submesoscale), especially in an area that has a lack of continuous measurements, which are necessary for addressing climate related questions. Temporal variability of phytoplankton was investigated using Empirical Mode Decomposition of surface chlorophyll-a concentrations collected from a Seaglider over a period of 5.5 months (25 September 2012 to 15 February 2013). This study found that during spring, chlorophyll-a concentrations were dominated by small scale daily fluctuations as well as by the rising seasonal ramp due to seasonal stratification. The removal of these signals revealed that the chlorophylla variability was dominated by submesoscales. In spring, phytoplankton blooms occurred as a result of features that shoaled the mixed layer depth when the wind stress weakened, elevating light conditions for short periods and allowing increased growth. In summer phytoplankton blooms were found to occur at submesoscales periods as well. This variability was found to be driven by synoptic storms varying the strength of the wind stress and consequently the mixed layer depth (that alters the nutrient and light environment). Additionally, through reconstructing the time series through subsampling at the dominant signals, this study found that in order to accurately resolve and characterise the multiseasonal variability of phytoplankton, chlorophyll needs to be sampled at high frequencies (<10 days). Spatial variability was investigated using daily MODIS ocean colour and sea surface temperature images coincident with the glider track. Spatial variability was characterised by the variance calculated at different length scales. Spatial analysis found that phytoplankton were patchier in both spring and summer when compared to sea surface temperature, at all length scales and that a greater variance was contained at small scales. There was also a greater variance in summer chlorophyll-a compared to spring due to higher maximum biomass. Further spatial analysis compared satellite spatial variance with glider measurements at the same length scale (70 km). This study found that a third of the variability found by the glider was caused by spatial patchiness, while the remainder could be contributed by local growth. These dominant meso- and submesoscale changes in chlorophyll-a at both temporal and spatial scales, highlights the need to resolve for both meso- to submesoscales in order to accurately reflect phytoplankton seasonal variability and ultimately to understand the impact of phytoplankton variability on carbon flux.
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    The impact of submesoscales on the stratification dynamics in the Southern Ocean
    (2018) du Plessis, Marcel David; Swart, Sebastiaan; Ansorge, Isabel; Mahadevan, Amala
    Submesoscale dynamics O(1-10 km, hours to days) are considered to strongly affect the stratification of the upper ocean. In the Southern Ocean, studies of submesoscale dynamics are biased to regions preconditioned for strong frontal activity and topographical influence. This dissertation considers the role of submesoscales on the evolution of mixed layer depth and upper ocean stratification in the open-ocean Subantarctic Ocean. First, we present autonomous ocean glider measurements from spring to late-summer to show that transient increases in stratification within the mixed layer during spring result in rapid mixed layer shoaling events. A realistically-forced simulation using a one-dimensional mixed layer model fails to explain these observed stratification events. We show that during this time, baroclinic mixed layer instabilities periodically induce a restratification flux of over 1000 W. m2, suggesting that the unexplained restratification is likely a result of submesoscale flows. Second, we study four separate years of seasonal-length (mid-winter to latesummer) glider experiments to define how submesoscale flows may induce interannual variations in the onset of spring/summer mixed layer restratification. Sustained temporal increases of stratification above the winter mixed layer, which defines the onset of seasonal restratification, can differ by up to 28 days between the four years studied. To explain this discrepancy, equivalent heat fluxes of baroclinic mixed layer instabilities (restratification) and Ekman buoyancy flux (restratification or mixing) are parameterized into a one-dimensional mixed layer model. Simulations including the parameterizations reveal a seasonal evolution of mixed layer stratification which is significantly more comparable to the glider observations than model simulations using heat and freshwater fluxes alone. Furthermore, the parameterization dramatically improves the sub-seasonal variability of mixed layer stratification, particularly during the onset of seasonal restratification when the mixed layer remains deep despite a positive surface heat flux. Following this, we characterize the full seasonal cycle of submesoscale flows using a realistically-forced 1/36 NEMO simulation of the Atlantic Southern Ocean. We show that deep winter mixed layers enhance the upper ocean available potential energy, which through the release of baroclinic mixed layer instabilities drive increased vertical buoyancy flux and potential to kinetic energy. These processes are associated with strong vertical velocities within the mixed layer characterized by large instantaneous upwelling and downwelling fluxes at the location of fronts. The insights from the glider observations propose that baroclinic mixed layer instabilities lead to increased near surface restratification in winter to spring, but are regulated by the synoptic-scale increases in Ekman buoyancy flux, which can keep the mixed layer deep for up to a month after surface warming. We propose the balance between restratification by baroclinic mixed layer instabilities and strong Ekman buoyancy flux driven by the passing of Southern Ocean storms is key in setting the large inter-annual variations of seasonal mixed layer restratification in the Subantarctic Ocean. Finally, we constrain the ability of gliders to represent regional submesoscale dynamics to provide context to current observations and inform future field work operations. Virtual gliders simulated within the 1/36 simulation show that horizontal buoyancy gradients in the Subantarctic are largely isotropic. We show that increasing the number of gliders sampling simultaneously over one month from one to a swarm of six results in improving the representation of the total distribution of horizontal buoyancy gradients across the Subantarctic from 10% to 42%. Similarly, by having a single glider sampling for six consecutive months, the distribution of horizontal buoyancy gradients observed increases to 47% of the total distribution. The insights presented in this dissertation enhance our understanding of submesoscale flows in the open-ocean Southern Ocean. These results are likely to have direct implications for physical and biological processes related to the ocean’s role on climate.
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    Thermohaline variability of AAIW in the Atlantic sector of the Southern Ocean investigated using an Altimetry Gravest Empirical Mode
    (2013) Hutchinson, Katherine Alessandra; Speich, Sabrina; Swart, Sebastiaan; Ansorge, Isabelle Jane
    The southeast Atlantic sector of the Southern Ocean connects the Atlantic with the Indian Ocean and the Antarctic Circumpolar Current, thereby acting as a major conduit within global ocean circulation. Thermohaline transports in this region are widely thought to have a critical influence on global climate. Yet magnitudes of the associated heat and salt content variations are poorly understood due to a lack of hydrographic observations and model limitations. An improved Gravest Empirical Mode (GEM) is set up for the Southern Ocean south of Africa using the updated store of hydrographic measurements obtained from CTD transects for the area, combined with the available Argo profiles sampled in the region. Satellite altimetry is combined with the GEM relationships to create an Altimetry GEM (AGEM), thereby generating 20 years of temperature and salinity fields. These thermohaline sections for the region of the ocean south of Africa are found to be proficient at reproducing observations, with associated RMS errors being two orders of magnitude smaller than those reported by other comparable Southern Ocean GEM studies. Confident in the accuracy of the AGEM produced fields, an examination of the temporal evolution of Antarctic Intermediate Water (AAIW) is undertaken. The fluctuation and trends in heat and salt content anomalies and budgets is presented for each Southern Ocean frontal zone, along with the examination of the change in position of the isopycnal limits and resultant water mass thickness. So as to better understand one of the factors that may be influencing some of the changes detected within AAIW, property alterations of eddies identified in the region from 1992 to 2010 are investigated. A general decrease in magnitude and frequency of cyclones, coupled with an increase in absolute dynamic topography (ADT) of anticyclones, designates elevated injection of warm, saline water into the area. The connection identified between eddy property variations and AAIW modification in the region of the ocean south of Africa indicates that the water mass experiences ventilation with the mixed layer at latitudes further north than previously thought to occur. Obtaining an improved image of the magnitudes and variability of AAIW thermohaline properties in the Atlantic sector of the Southern Ocean greatly improves our understanding of its role in the ocean-climate system.
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    Turbulence in the sea ice impacted Southern Ocean and its implications for primary production and carbon export
    (2023) Nunes, Da Costa Isabelle; Ansorge, Isabel; Swart, Sebastiaan; Nicholson, Sarah-Anne
    The sea-ice impacted Southern Ocean, south of the Antarctic Circumpolar Current, is one of the most important regions on earth for the cycling of carbon and distribution of heat and freshwater around the globe. Here, along-isopycnal upwelling of warm, carbon-rich circumpolar deep water coincides with the annual growth and melt of Antarctic sea ice that represents one of the world's largest surface water transformations. The air-sea-ice buoyancy exchanges and biological processes that change the surface water properties therefore have global consequences, as they set the properties of downwelling intermediate waters that enter the upper branch of the global thermohaline circulation. The region hosts some of the largest uncertainties in global climate models. The reason for this stems from two sources. Firstly, the spatio-temporal resolution of global climate models is limited by computational constraints such that smaller scale processes need to be parameterized. Secondly, the challenges associated with making observations in or near sea ice and in the harsh and remote conditions of the Southern Ocean means that the region is sparsely sampled, and as such, the parameterizations of the small scale and turbulent terms in global climate models are validated based only on a few in situ samples. This thesis concerns the observation and interpretation of (sub)meso- to micro scale turbulence and its implications in the sea ice impacted Southern Ocean. I aimed to understand the 0.01-1 km scale physical and biological processes that drive changes in the properties of the upper ocean following sea ice melt, using groundbreaking sustained high temporal and spatial resolution observations made by gliders. There are three main findings. Firstly, we find that sea ice melt by introducing a lateral freshwater gradient enhances stirring of submesoscale flows (0.1-10 km) and therefore lateral variability in the upper ocean, but simultaneously constrains vertical fluxes between the ocean interior and surface by enhancing stratification. Secondly, turbulent diapycnal mixing and double diffusive convection (0.1-1 m scales) drive the warming of the subsurface winter water, therefore mediating fluxes between the ocean interior and surface. Finally, phytoplankton respond favourably to larger volume sea ice that enhances winter mixing of nutrients from the deep reservoir and to upper ocean stratification in the summer. The preliminary evidence from this study suggests that the resultant higher intensity phytoplankton bloom translates to enhanced short term carbon export but not necessarily long term export. Overall, we show, using observations, that the variability and transport of heat and freshwater flux in the sea ice impacted Southern Ocean are sensitive to sea ice, with downstream impacts on phytoplankton, the biological carbon pump and ultimately the upper cell of the meridional overturning circulation.