Browsing by Author "Ansorge, Isabelle Jane"

Now showing 1 - 19 of 19
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    Characterization of the Carbonate System across the Agulhas and Agulhas Return Currents
    (2015) Melato, Lebohang Innocentia; Ansorge, Isabelle Jane; Monteiro, Pedro M S
    In this study, we investigate the role that the solubility and biological pumps have on CO₂ variability across the Agulhas Current system ( Agulhas Current and the Agulhas Return Current). The Agulhas Current system transports heat and salt from the Indian Ocean into the South Atlantic Ocean via the Agulhas leakage, which influences the Atlantic Meridional Overturning Circulation (AMOC). This study presents for the first time a characterization of the role the Agulhas Current system (Agulhas and Agulhas Return Currents) has on the uptake of anthropogenic CO₂. Fugacity of carbon dioxide (fCO₂ ) values were obtained from a ship-based underway pCO₂ (partial pressure of carbon dioxide) system and the air-sea CO₂ fluxes were computed using 6-hourly wind speeds from the NOAA Blended Sea Winds. An experiment was conducted during the Crossroads scientific monitoring line in May 2013, where surface dissolved inorganic carbon, total alkalinity and CO₂ flux were compared between the Agulhas and Agulhas Return Currents and the region directly south over the Agulhas Plateau. Our findings highlighted that the solubility and biological pumps played minimal to no role in the drawdown of carbon across the sub-Tropical zone and the Agulhas Current system (Agulhas and Agulhas Return Currents), due to opposing effect between chlorophyll and temperature on pCO₂ that explained why although there was carbon drawdown by primary production in the Agulhas and Agulhas Return Current regions, this does not play a role in enhancing the air-sea exchange of CO₂. The solubility pump was responsible for CO₂ in the sub-Antarctic zone. The biological and solubility pumps were responsible for CO₂ sink in the Agulhas Plateau eddy. The highest CO₂ flux in the study was observed in the Agulhas Plateau eddy at a flux value of -8.12 mmolC.m-².day-¹ due to the cooler mean sea surface temperature of ~16.5 °C. This is the first time that such as study has been undertaken and aims to provide a better understanding of the role of Western Boundary Currents such as the Agulhas Current has in the uptake of CO₂.
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    Circulation at the South-West Indian Ridge in a high-resolution global ocean model
    (2008) Durgadoo, Jonathan V; Ansorge, Isabelle Jane
    This study explores the use of the 1/4° and the 1/12° Ocean Circulation and Climate Advanced Modelling (OCCAM) project. The model's representation of the dynamic nature of this region is assessed. On average 2 - 3 intense and well-defined eddies are generated per year within the model; having mean longevities of 4.89 ± 2.20 months with average advection speeds of 5.51 ± 1.57 km day¯¹
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    A description of the hydrography between Cape Town and Antarctica along the GoodHope Transect between 2004-2012
    (2015) Jacobs, Christopher Richard; Ansorge, Isabelle Jane
    The Antarctic Circumpolar Current (ACC) within the Southern Ocean (SO) is the primary driver for global interoceanic exchanges. These exchanges form the foundation for the Meridional Overturning Circulation (MOC), a deep density driven circulation, which extends throughout the global oceans. The fronts of the ACC, consisting of several branches, separate zones of distinct water masses, thus identifying the location of the fronts and understanding their dynamics is of global importance. The GoodHope programme, is a repeat annual transect between South Africa and Antarctica, monitoring the exchanges within the Southeast Atlantic sector of the Southern Ocean. This is achieved through high resolution Expendable Bathythermograph (XBT) sampling. In this study XBT data over an eight year period (2004 – 2012) were investigated and analysed. One aim is to illustrate the variability of the fronts associated with the ACC using in-situ data from 21 transects during this eight year period. The Sub-tropical Front (STF) and Northern branch of the Sub-Antarctic Front (SAF-N) are seen to be the most variable with frontal latitudinal shifts ranging from 2-4° and 1-2° respectively. One cause of this high variability is the interaction of mesoscale features, particularly in the form of eddies and Agulhas rings. The southernmost fronts of the ACC, namely the Southern Antarctic Circumpolar front(sACCf), consisting of a Northern and Southern branch of the sACCf (sACCf-N and sACCf-S) and Southern Boundary (SBdy), also display high variability due to seasonality brought on by ice melt. The central fronts of the ACC, the Middle and Southern branches of the SAF (M-SAF and S-SAF)and Northern, Middle and Southern branches of the Polar Front (PF-N, PF-M and PF-S) remain throughout the eight year observations fairly constant, with shifts observed to be less than 1° of latitude. Grouping the Good Hope transects into austral summer periods, illustrates the di-pentadal nature of the STF, whereby the shifts are in response to mesoscale interactions. Comparing the mean position of the fronts and their position observed during a single winter cruise in July 2012, an overall Northward shift was observed with most of the fronts with significant shifts occurring in the SAF-N and PF-N. This is due to the outcrop of different water masses occurring further South ward and Northward respectively.
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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.
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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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    Dynamics, interactions and ecosystem implications of mesoscale eddies formed in the southern region of Madagascar
    (2014) Braby, Laura; Ansorge, Isabelle Jane; Backeberg, Bjorn Christoph; Roberts, Michael J
    Several species of marine organisms occurring off the southern African coast have been found to be identical to those occurring in the Madagascan coastal water although the reason for this is unknown. It has been proposed that eddies act as a vector of transport for planktonic larvae from the Madagascar island to the southern African east coast. In this study it is shown that eddies spawned off southern Madagascar entrain chlorophyll-a rich coastal waters into their periphery. This is indicative of the mechanism whereby organisms could become entrained in eddies. Approximately one eddy per year, usually cyclonic, interacts with the southern Madagascan coast, then from its origin crosses the southern Mozambique Channel and arrives at the African coast where it dissipates. By tracking eddies and combining their trajectories with drifter data and satellite remote sensing observations of ocean colour, it is shown that chlorophyll-a rich waters are entrained within the eddies, and these waters are mostly conserved during their passage across the channel. This study suggests that biota may be transported from Madagascar to Africa in eddies, providing further evidence that eddies are potentially a viable mechanism for the transport of organisms across the southern Mozambique Channel.
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    Evaluation of an eddy resolving HYCOM simulation in the South Atlantic Ocean
    (2014) Giddy, Isabelle Sindiswa; Backeberg, Bjorn Christoph; Ansorge, Isabelle Jane; Reason, Chris; Campos, Edmo
    The South Atlantic Ocean acts as a key region of ocean teleconnections and water mass transformation, directly transporting waters from the Southern, Pacific and Indian Oceans towards the North Atlantic and feeding the upper branch of the Atlantic Meridional Overturning Circulation. To date, this region remains under sampled over long time periods. Models form useful platforms for the wholistic study of such regions where long term datasets are not available. However, it is important to first gauge to what extent the model deviates from available observations. In a detailed analysis, this study evaluates an eddy resolving 1/12° HYbrid Coordinate Ocean Model simulation in the South Atlantic and Agulhas regions with reference to available observations and literature. The outcomes of this research contribute to improving the configurations of future ocean models by identifying the limitations of the model analysed. In addition, analysis of the 50 year (1960-2009) simulation reveals that, in this model simulation, mesoscale activity and sea surface temperature trends in the Cape Basin are not related to Agulhas Current transport changes. These findings highlights the need for long term observational databases and further research into climate related trends and interannual variability of the ocean, in particular around Southern Africa.
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    The hydrography and dynamics of the general oceanic environment of the Prince Edward Islands
    (2000) Ansorge, Isabelle Jane
    The Southern Ocean is the name given to the oceanic region that surrounds the entire Antarctic continent. It comprises the southern extents of the Atlantic, Indian and Pacific Oceans. The Antarctic continent forms the southern boundary of the Southern Ocean, while the northern border is not physiographically fixed, but is usually considered to coincide with the geographic location of the Subtropical Convergence (STC) (Lutjeharms 1985). The unique geography of the Southern Ocean makes it the only place where ocean currents run completely around the globe.
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    The impact of increased grid resolution on the mixed layer depth variability in the South Atlantic Ocean and Southern Ocean
    (2016) Williams, Tania Carol; Ansorge, Isabelle Jane
    The Southern Ocean plays a major role in global climate system. An understanding of Southern Ocean dynamics allows for a better understanding of the carbon cycle and possible future climate conditions. Earth System Models are used to study Southern Ocean dynamics and are currently producing reliable global annual carbon uptake but have limiting seasonal abilities. These models produce dependable results on a global scale, with more conflicting results on a basin scale. Here we study the impact of mesoscale variability on the Mixed Layer Depth in the Sub-Tropical and Sub-Antarctic Zone of the South Atlantic. The region is hugely impacted by the mesoscale variability as a result of the South African boundary currents. We use two regional simulations both at 1/4o resolution, with one model containing online nested child domain over the South African boundary currents (1/12o resolution). The inter-annual simulations both use the same forcing which allow for a comparison study between the two models. Both the nested and standalone model are able to capture the large scale oceanographic features in the domain. The biggest difference is seen in the Agulhas Current region, where the nested model simulates better mesoscale features, resulting in a fairly accurate position of the Agulhas retroflection and return current. The standalone model contains a high temperature and salinity bias which influences the vertical structure of the water column. Both models are able to simulate the seasonality of the MLD in the Sub-Tropical and Sub-Antarctic Zone in the Atlantic sector. The models overestimate MLD in regions closer to the boundary currents. In the nested model the presence of increased mesoscale features promotes stratification of the water column. The differences seen in the MLD of the two models are linked to the temperature and salinity bias in the standalone model as well as the increased mesoscale variability in the nested model.
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    The influence of ocean ridges on the circulation to the south of the Mozambique channel and Madagascar
    (2008) Halo, Issufo Ferrão Mário; Ansorge, Isabelle Jane; Penven, Pierrick; Lutjeharms, Johann R E
    The Mozambique Channel and the region south of Madagascar are dominated by high variability of the oceanic flow due to the ubiquitous presence of mesoscale eddies. The bottom topography of this region has several shallow ridges. The water flowing through the Mozambique Channel propagates southwards predominantly as a train of anti-cyclonic eddies, moving towards the Agulhas Current. South of Madagascar, dipolar vortices regularly propagate in a south-westward direction. Their deep extent favours interaction with the shallow bathymetry of the Davie, Mozambique and Madagascar Ridges. The role of the Madagascar Ridge on the mesoscale circulation is investigated using altimetric observations and model simulations.
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    Investigating the local circulation of the southeast Cape Basin
    (2018) Carr, Matthew David; Lamont, Tarron; Ansorge, Isabelle Jane
    Located off the west coast of southern Africa, the southeast Cape Basin is characterised by a unique combination of strong coastal upwelling and vigorous offshore mesoscale variability. The juxtaposition of offshore mesoscale variability and coastal upwelling results in a complex and dynamic environment. In this study a combination of in situ Acoustic Doppler Current Profiler (ADCP) data and satellite observations were used to identify, describe and characterise the features driving the local circulation within the southeast Cape Basin. The ADCP data was obtained from the South Atlantic MOC Basin-wide Array (SAMBA), which included four deep sea moorings located along ~34.5°S on the 1000, 2000, 3000 and 4500m isobaths respectively. There was a distinct difference in the circulation observed at the mooring located on the shelf edge (1000m) and the moorings further offshore (2000m, 3000m and 45000m). The offshore circulation, observed by the moorings located on the 2000, 3000, 4500m isobaths, were driven by large mesoscale eddies, both cyclonic and anticyclonic, originating at the Agulhas retroflection and within the Cape Basin itself. The mesoscale eddies induced high speed baroclinic transport events which impacted the upper water column to a depth of at least 400m. The in situ observations were used to show the precise characteristics of two cyclonic shelf eddies and one anticyclonic eddy through the upper water column (~50 to 500m). The analysis of these features was important as the physical characteristics of both the cyclonic shelf eddies and anticyclonic eddies through depth are not well known. Considering the short period of observations (18th September 2014 to 1st December 2015), the analysis was not used to assume the prevailing physical characteristics of cyclonic shelf eddies and anticyclonic eddies. Instead the analysis showed the potential for future long term studies to use the sustained in situ observations from the SAMBA mooring array and similar analysis to define the precise characteristics of mesoscale eddies through depth. This will greatly improve the understanding of how these features influence the interocean exchange between the Indian and Atlantic Oceans. The circulation at the shelf edge, observed by the mooring located on the 1000m isobath, was shown to be driven by a combination of offshore mesoscale eddies, the position of the upwelling front and warm filaments formed at the reflection of the Agulhas Current. Identifying the influence of both the offshore mesoscale eddies and coastal upwelling at the shelf edge showed connectivity between the upwelling system and the offshore mesoscale variability. An example of the connectivity between the offshore mesoscale variability and the upwelling front was identified and presented. A dipole formed by two mesoscale eddies was observed to induced cross shelf transport advecting productive shelf waters offshore. The in situ observations were used to calculate the amount water transported from the shelf to the open ocean during this cross shelf transport event. The filament representing the cross shelf transport event was calculated to have a total volume of ~2 x 10¹² m³ with a volume transport of ~1Sv suggesting the event would have had a substantial impact on the local biology. The detailed analysis and quantification of the cross shelf transport event aimed to improve the current understanding of how mesoscale features interact with the upwelling system. In situ observations of cross shelf transport are rare, therefore the quantification of the amount of transported in the cross shelf transport event can serve as a baseline for future studies attempting to assess the impact of comparable cross shelf transport events on the local biology.
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    The Mozambique Channel eddies : characteristics and mechanisms of formation
    (2012) Halo, Issufo F M; Penven, Pierrick; Ansorge, Isabelle Jane
    The model outputs and the satellite data have shown a number of cyclonic and anticyclonic eddies being generated within the Mozambique Channel. The eddies were identified using a newly designed eddy detection algorithm. The algorithm has shown that cyclonic eddies appeared to be more ubiquitous than the anticyclonic and had a single mode sizedistribution. On the other hand, the anticyclonic eddies seemed to have a favorite site for their formation, and have shown a bi-modal size-distribution.
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    A qualitative and physical analysis of processes around the Mascarene plateau
    (2015) Vianello, Patrick Angus; Rouault, Mathieu; Ansorge, Isabelle Jane
    The Mascarene Plateau is a submerged volcanic plateau to the east of Madagascar which extends over 2200 km between the Seychelles and Mauritius. It is a complex feature, which is composed of 4 banks separated by 3 channels - namely the Seychelles, Saya de Malha, Nazareth and Cargados-Carajos Banks. The main objective of this thesis is to analyze cruise data obtained during the October/November 2008 ASCLME (Agulhas Somali Current Large Marine Ecosystem) cruise on board the Dr Fridtjof Nansen which surveyed the region around the Mascarene Plateau. Due to the paucity of data in the region, the cruise was a routine cruise with no specific scientific questions to be answered. A comparison is also made between sea surface temperature (SST) and Acoustic Doppler Current Profiler (ADCP) derived currents on board to satellite estimate to possibly extend cruise results in space and time. Although the resolution of satellite estimate is low compared to cruise measurements, satellite estimate of geostrophic velocities and sea surface temperatures compare well with cruise data and can therefore be used in the region with confidence. This is invaluable since the Mascarene Plateau is relatively poorly understood and it allows us to link cruise data with Rossby waves and currents impacting the region. This is the second cruise in the region as there was a research cruise on board the RRS Charles Darwin during June/July 2002. However, the June/July 2002 cruise did not sample the banks of the Mascarene Plateau.
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    The relationship between coastal oceanographic variability and the spatial distribution of Argyrosomus coronus on the west coast of southern Africa
    (2012) Munnik, Kate Caroline; Potts, Warren; Ansorge, Isabelle Jane
    The northern Benguela, under the influence of the opposing forces of the poleward Angola Current and the eastern boundary Benguela upwelling regime is a highly variable environment. The impact of climate change, especially in warming water temperatures and the shifting position of the Angola Benguela Frontal Zone (ABFZ), may be compounding this variability. Biological responses such as shifts in temperate species away from the warming mid latitudes, towards the cooler poles are becoming evident. This study aimed to quantify the effects of these changes (in terms of temperature, dissolved oxygen and salinity) on the spatial distribution of an important inshore commercial, artisanal and recreational fishery species in the northern Benguela, Argyrosomus coronus (west coast dusky kob).
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    A statistical investigation of the behavioural responses of a deep diving predator to mesoscale oceanographic features
    (2014) Massie, Philip; Ansorge, Isabelle Jane; Mcintyre, Trevor; Ryan, Peter G
    Marion Island lies directly in the path of the ACC and to the south-west of the island is an extensive region of high sea level anomaly variability (eddy field). Previous research has shown how southern elephant seals from Kerguelen Island and Peninsula Valdés have targeted eddies for foraging. These findings combined with the observed post-moult migrations of tagged female southern elephant seals from Marion Island which traverse the nearby eddy field suggest that they may exploit the eddy field for foraging. Overall the seals undertook significantly more of their dives (18.5) within the eddy field than predicted (p < 0.01). Eleven of the 16 seals dived within the eddy field region significantly more often than predicted. Dives were generally shallower and shorter within the eddy field. Mixed models estimated that a maximum depth reduction of 53.03 ± 5.30 m (LME: t=10.00, DF=24986, p < 0.01) and a dive duration reduction of 6.17 ± 0.58 min (LME: t=10.70, DF=24986, p < 0.01) could be attributed to diving within the eddy field. Within the eddy field U-dives increased by 6.97, W dives decreased by 7.18 and O-dives increased by 0.21. Seven individuals’ maximum dive depths were shallower and nine individuals dive durations were shorter as a result of diving within the eddy field. Ten individuals showed significant changes in the types of dives which they undertook inside of the eddy field with eight seals undertaking more Uand fewer W-dives and two seals undertaking more W- and fewer U-dives. Overall the effect of diving within the eddy field appeared to be biologically important to ten seals. While dive durations were significantly affected by changing sea level anomaly (SLA) (LME: 0.15 ± 0.01 min, t=11.72, DF=12372, p < 0.01), maximum depths were not. The log-odds of switching from U- to W-dives within the eddy field were significantly affected by changing SLA (Multinomial: 0.00 ± 0.00, z=2.97, p < 0.01). However all significant effects of SLA on dive parameters within the eddy field were very small in comparison to those of day-stage. The low magnitudes of the effects of SLA were similar for all individuals.
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    The structure and transport of the Agulhas Return Current
    (1996) Ansorge, Isabelle Jane; Lutjeharms, Johann R E
    The Agulhas Current flows along the eastern coast of southern Africa as one of the largest western boundary currents in the world's ocean. On overshooting the southern tip of Africa at approximately 20°E, a retroflection loop is formed that causes the current to double back on itself and to form the easterly flowing Agulhas Return Current. No focussed investigation to establish the hydrography of this important component of global ocean circulation has to date been carried out.
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    A survey of anticyclonic mesoscale eddies, within the Southern Ocean, and their propagation south from the South West Indian Ridge
    (2016) Reid, Kirrin Gail; Ansorge, Isabelle Jane
    Eddies within oceans act as vehicles, transporting smaller bodies of water, with certain oceanographic characteristics, from one place to another within a larger body of water. The South West Indian Ridge [SWIR] is a topographically complex bathymetric feature which amplifies the production of mesoscale eddies in and around the Antarctic Circumpolar Current [ACC]. Within the Southern Ocean [SO], a section of this ridge - the Andrew Bain Fracture Zone [ABFZ] - has been found to be the starting line of an eastward extending eddy corridor. Earlier research shows an area of diminishing mesoscale variability within this corridor which extends down from 45°S to approximately 60S. A recent study focused on a southward extending anticyclonic eddy corridor and proved its existence. The anticyclonic [warm core] eddies which are propagating south, not previously investigated through in situ means, were observed during the 2014 Marion Island Relief Cruise [MIRC2014] aboard the SA Agulhas II. Two anticyclonic mesoscale eddies [one juvenile and one mature] were bisected with transects of conductivity, temperature and depth stations and expendable bathythermograph deployments. This paper used the in situ data captured during the MIRC2014 to study the internal structure of the two eddies. The objectives of this study were also to examine both the recent and the historical trajectory characteristics of the southward advecting anticyclonic eddies, to confirm the origin of the two sampled eddies, and to assess the structural differences between the two anticyclonic eddies. This paper plots the behaviour of the anticyclonic mesoscale eddies found within the area of the southward eddy corridor, firstly using website available data collected over a two year period [May 2012 - May 2014] and then utilizing a previously compiled data set to plot the historical dynamics [October 1992 - April 2012]. The trajectories of the southward anticyclones during that time period were found to be predominantly southward, typically following the south west slope of the SWIR. The two MIRC2014 eddies were confirmed to originate from the ABFZ section of the SWIR. Each eddy had a similar grouping of water masses; Antarctic Bottom Water, Circumpolar Deep Water, Antarctic Intermediate Water, Winter Water and Sub-Antarctic Surface Water: water masses characteristic of the Antarctic Polar Frontal Zone [APFZ]. The in situ measurement and analysis of these eddies allowed the first comparison between a juvenile and a mature anticyclonic eddy in the recently discovered southward extending eddy corridor. Thermal section comparisons of these two sampled anticyclonic eddies showed that, over time, these anticyclonic eddies appear to shrink in surface diameter, deepen and lose heat to host waters. This loss of heat occurs due to the degradation of water mass boundary integrity over time and is theorised to accelerate as time passes. This study shows that the southward extending eddy corridor is a means of shifting heat and salt further south within the SO, large sections of which are sink areas for atmospheric CO₂. This poleward heat transport influences the capability of the SO to absorb atmospheric CO₂, since higher temperatures negatively affect the ocean's CO₂ uptake capability. The results of this study are proposed to be a catalyst for future in situ sampling across eddies in this area, in order that heat and salt transport, through this southward anticyclonic eddy corridor, can be monitored for fluctuations. As this carbon sink is vitally important with regards to climate change, the quantification of the heat and salt sources of the SO, which alter the SO's ability to absorb CO₂, is imperative.
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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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    Understanding the physical, chemical and biological processes across the subtropical convergence during austral automn
    (2009) George, Caren; Ansorge, Isabelle Jane; Froneman, William
    A research survey was conducted, consisting of six transects between 38°- 46°S and 38 - 41°45'E, during the austral autumn of 2007. The aim of the survey was to investigate the physical, chemical and biological dynamics of the Subtropical Convergence (STC), in the SW Indian sector of the Southern Ocean. Satellite data was obtained and in-situ data were collected. Mixed layer depth (MLD), geostrophic velocities, density and Brunt Vaisala frequencies were calculated. The STC meandered across the survey area between 41 ° - 42°15'S. The total integrated Chl-a ranged from 12.8 to 40.1 mg Chl-a/m2. The most significant correlation between biological and physical data was that of Chl-a and MLD (r=-0.374, n=45, p=0.013) over the entire survey region. Phosphate and nitrate (r=0.8779, n=45, p=0.001) measured over the survey region were strongly correlated, as expected. The surface currents showed cyclonic motion between 38° - 39°E and 38 - 42°S, with the exception of an eddy-like feature between 39.5°and 40.5"S and generally anti-cyclonic motion to the east of 39°E.