Browsing by Author "Ansorge, Isabel"

Now showing 1 - 13 of 13
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    An assessment of four decades of wave power variability - a critical requirement for coastal resilience
    (2022) Hall, Candice; Ansorge, Isabel; Jensen, Robert E; Wang, David W
    Wave power estimates and trend assessments are crucial for coastal management and resilience, as increases in wave power introduces significant risks of flooding and shoreline erosion. This study evaluates wave power trends at 29 National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) moored buoy sites with associated U.S. Army Corps of Engineers (USACE) Wave Information Study model estimates within the North Pacific Ocean, Hawaiian Islands, Gulf of Mexico and North Atlantic Ocean. This work is the first conclusive study to show spatially and temporally comparative observational and model wave power results, providing new information on the accuracy of model estimates using wave power as a proxy. Wave power data were interpolated to augment missing values and detrended for seasonality to facilitate testing of interannual and interdecadal trends in wave power. Results show that the majority of the eastern Pacific Ocean and Hawaii wave power trends are downward, with mixed slope wave power trends apparent within the Atlantic Ocean and Gulf of Mexico. Observational and model results show that wave power peaks in long term interannual trends are similar with respect to timing, but not magnitude. Variability in the wave power trend direction within each region suggests that site specific wave power trends should not be generalised to represent a large region, with regionally grouped annual maximum 90th percentiles obscuring the variability of individual site results. Prior to the calculation of these wave power estimates, a thorough interrogation of the quality of the observational wave data was conducted. Three tasks achieved confidence in these observational datasets: a) an evaluation of the effects of changing NDBC instrumentation technologies on data quality; b) the development of an independent, self describing, archive that mitigates for historical data storage issues; and c) the subsequent removal of identified discontinuities within the time series datasets. Instrumented buoy intercomparisons within the Pacific Ocean and U.S. Great Lakes prove that the recently deployed NDBC 2.1-m hulls show an increased wave data accuracy when compared to the legacy NDBC 3-m hulls for significant wave height, average wave period, and spectral signal-to-noise ratio, which allows for an increase in swell energy retention in the lower frequency spectral range. With confidence in the newly deployed NDBC platforms, this work then addressed NDBC data accessibility issues, as data are stored in multiple archives with unique storage, metadata, and quality control protocols. Known storage and quality control inconsistencies were removed and the validated data for all NDBC moored buoy stations are stored within a USACE Quality Controlled, Consistent (QCC) Measurement Archive, which is now a public database of best available historical NDBC data with verified metadata. Spectral wave data from this USACE QCC Archive were interpolated for frequency equivalency and used to recalculate the required wave power input parameters to ensure consistency through the historical datasets, successfully removing a number of previously identified time series discontinuations. With these data discontinuity corrections, uncertainties and inaccuracies are removed from the estimated wave power trends. Overall, this study highlights the undeniable need for accurate and consistent observational data that are essential for a realistic estimation of local wave climate studies, a vital requirement for all coastal risk management considerations. Although these observational and model wave power trends are U.S. specific, the methodologies developed within this work are applicable to datasets in any region.
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    Building A Mean-state Of Oceanographic Properties (Temperature And Salinity) For The Kwazulu-natal Bight Using The Roms Model: A Contribution Towards Marine Protected Areas Analysis
    (2018) Malange, Mathabo Noxolo; Morris, Tamaryn; Collins, Charine; Lamont, Tarron; Ansorge, Isabel
    The KwaZulu-Natal Bight, located along the east coast of South Africa, is an important recruitment and nursery area for various marine species. In an effort to conserve a number of threatened species, two Marine Protected Areas (MPA) have been established in the Bight. The African Coelacanth Ecosystem Programme is conducting MPA analyses along the Bight through a series of biological and oceanographic studies and this study forms part of the oceanographic research component that will assist in the decision-making process of MPAs in the region. This study uses a 30-year, high-resolution, regional ROMS simulation to build a climatology representative of the mean-state of the Bight. The model is also used to investigate the seasonal and annual variability as well as the influence of the Agulhas Current on the shelf. The Bight was cooler and less saline than the surrounding waters and seasonal variation was limited to the upper 50 m of the water column. The depth of the Bight ranges from 50 m in the inner shelf to 100 m at the shelf edge in the central region of the Bight. In the northern and southern region of the Bight, the depth of the water column extends down to about 150 m at the shelf edge. In summer, surface temperatures were on average 4.8°C and 4.3°C warmer than in winter over the uThukela Banks and Aliwal Shoal respectively. Bottom temperatures at both MPAs had a mean seasonal variation of about 3°C. Salinity, a more conservative variable, showed little variability over the year throughout the water column except for at 50 m where lower salinities were observed in the winter months. Wavelet analysis showed that a strong annual (12 month) signal was dominant at the surface (10 m). Bottom temperatures displayed a weaker annual signal than the surface in addition to a slight semi-annual cycle. Further investigations indicated that the Agulhas Current influenced the Aliwal Shoal MPA more than the uThukela Banks MPA as they shared similar temperature values (at the surface and bottom) throughout the 30-year period. In contrast, the uThukela was cooler than the Agulhas Current by 0.5 to 1.5°C at the surface and 1 to 2.5°C at the bottom. These time series also enabled us to identify anomalous features such as the Natal Pulse that could have important implications for temperature-sensitive species in the area.
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    Climate change impact on ecosystems of Prince Edward Islands: role of oceanic mesoscale processes
    (2019) Asdar, Sarah; Ansorge, Isabel; Penven, Pierrick; Deshayes, Julie
    The subantarctic Prince Edward Islands (PEIs, 47◦S-38◦E) are classified as isolated and hostile regions, in which the terrestrial and marine ecosystems are relatively simple and extremely sensitive to perturbations. The island’s location, between the Subantarctic Front (SAF) and the Polar Front (PF), bordering the Antarctic Circumpolar Current (ACC) provides an ideal natural laboratory for studying how organisms, ecological processes and ecosystems respond to a changing climate in the Southern Ocean. Recent studies have proposed that climate changes reported at the islands may correspond in time to a southward shift of the ACC and in particular of the SAF. This southward migration in the geographic position of major ocean fronts is likely to coincide with dramatic changes in the distribution of species and total productivity of this region. However, there are other sources of variability in the hydrodynamic conditions around the PEIs: upstream of the islands, at the South West Indian Ridge (SWIR), a region of high eddy kinetic activity produces mesoscale features that directly irrigate the PEIs and may impact their marine environment. Based on satellite altimetry in that region, the positions of the SAF and PF were found to be highly variable at interannual and monthly time scales. They also revealed a significant long-term southward trend which was highlighted at the Southern Ocean scale. The mesoscale activity also showed an interannual and intra-annual variability and a decrease in eddy kinetic energy over 24 years was observed in the region. At a more local scale, we highlighted that the archipelago’s environment was impacted by the mesoscale features produced at the SWIR. The temperature, the mixed layer and velocities recorded between the islands were clearly affected by the eddies passing in the vicinity of the PEIs. Moreover, a large signal dominating the main current time series appeared to be a tidal signal, another important driver of variability of the circulation in between the two islands. On a second hand, an idealised model configuration was designed for the PEIs region to study the mesoscale eddy properties and the physical mechanisms of their formation at the ridge. The Eddy Available Potential Energy revealed a maximum of energy around 800 m depth, confirming the deep reaching characteristic of the eddy originated in the region and suggested the presence of a local energy source at this depth. This eddies activity was shown to be the result of a combination of barotropic and baroclinic instabilities occurring at the ridge. Finally, we investigated on the potential consequences of a southward shift of the SAF in the region of the islands. Because the model was idealised, it allowed us to simulate an SAF southward shift by shifting the initial and boundary conditions. The main result was the clear decrease of mesoscale activity in the region which could potentially impact the ecosystems of the PEIs.
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    Deriving a policy document towards an early warning system for estuaries in South Africa: case study Great Brak estuary, Eden District, Southern Cape
    (2020) Stander, Johan; Ansorge, Isabel; Hermes, Juliet
    South Africa's estuaries and their surrounding communities are becoming increasingly vulnerable to storm surges and accompanied estuary flooding. These events are largely due to increasing severity of storm surges combined with growing housing and commercial developments. A particularly severe weather event in 2007/2008 highlighted the pressing need to understand the processes involved and the urgency to develop proactive response and management actions to mitigate the effects of future storm events on these coastal areas. Scientific research on estuarine flooding is limited not only for South Africa but within the international community as well and only recently has received committed attention from policy makers. It is clear that our current knowledge of South African estuary flooding events remains rudimentary; while necessary action to mitigate such events are poorly understood and planned. The aim of this PhD thesis is to devise and implement an Estuary Early Warning – Emergency Preparedness and Response Guide for stakeholders and government policymakers. This guide will target South Africa's coastal region by analysing past information on storm surges and estuary flooding, particularly in the low-lying southern coast region of the Western Cape, South Africa. The key objective of this thesis is to assess the best processesfor the issuing of estuary alerts and to better standardise them so that the response remains in line with multi-hazard early warning standard procedures and practices within South Africa. A further aim is to provide a comprehensive national guideline on how best to effectively disseminate and communicate such information and to establish an Estuary Early Warning (EEW) – Emergency Preparedness and Response Guide (EPRG), which forms part of the South African Multi-Hazard Early Warning System (MHEWS). It is critical that this EEW meets general principles accepted internationally for an effective Early Warning System. This thesis addresses the following key elements namely: (1) Risk identification, (2) Key drivers and contributions to estuary flooding, (3) Monitoring and alert early warning system, (4) Alert dissemination and (5) Response actions. Such pioneering work is an essential tool to translate science into policy, a crossover field, which remains poorly implemented.
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    Downstream evolution of ocean properties and associated fluxes in the Greater Agulhas Current System: Ad hoc Argo experiments and modeling
    (2020) Morris, Tamaryn; Ansorge, Isabel; Hermes, Juliet; González, Borja Aguiar; Lamont, Tarron
    The evolution of cyclonic eddies across the Southern Mozambique Chanel and the downstream evolution of the Agulhas Current was investigated using Argo floats, in combination with output from ocean general circulation reanalysis models. Two dedicated experiments were undertaken in April and July 2013, whereby eight floats were deployed within two separate cyclonic eddies. Floats were set to either daily and five-daily profiling from 1000 db to the surface, with park depths ranging from 300 db to 1000 db. The two cyclonic eddies propagated southwestward across the Mozambique Channel from southwest Madagascar to the KwaZulu-Natal Bight, a distance of approximately 1300 km, in approximately 130 days at a mean speed of 0.13 m s−1 . Estimates indicate the April (July) eddy showed mean trapped depths of 595 ± 294 m (914 ± 107 m), volume transport of 13.4 ± 5.2 Sv (21.2 ± 9.1 Sv), heat flux of -0.07 ± 0.06 PW (-0.2 ± 0.09 PW) and freshwater flux of 0.04 ± 0.04 Sv (0.09 ± 0.05 Sv). These results highlight the role of Madagascar cyclonic eddies as transporters of cooled and freshened source waters into the Agulhas Current. During a third experiment, six floats were deployed in the Agulhas Current, and exited the current within 9 - 12 days at mean speeds of 0.51 – 0.76 m s−1 . An evolution of properties was shown from north to south for both Argo data and model output; for volume transport (16.76 – 38.18 Sv; 17.70 – 32.51 Sv), heat fluxes (0.85 – 1.79 PW; 0.99 – 1.91 PW) and salt fluxes (0.60 – 1.37 x 1012 kg s−1 ; 0.63 – 1.17 x 1012 kg s−1 ). This study illustrates the first near-real time survey of the Agulhas Current, and a potential method of quasi-synoptic surveys using Argo float technology. These experiments highlight alternative methods of studying regions of turbulence by altering the mission parameters of Argo floats. Increased observations of eddies and Western Boundary Currents are critical to our understanding of the global oceans and impacts on the earths climate. Even more so for the understudied Indian Ocean.
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    Long-term climate variability at the Prince Edward Islands in the Southern Ocean
    (2021) Shangheta, Anna Liisa Penelao Tulimevava; Lamont, Tarron; Ansorge, Isabel; Rouault, Mathieu
    A warming Southern Ocean (SO), due to climate change and global warming, has many implications on the sub-Antarctic Islands in the SO. Due to the distance away from continental land these islands experience an oceanic climate, making them the perfect sentinels to climate change in this sector of the Southern Ocean. Studies have proposed that climate changes reported at the Prince Edward Islands (PEIs) correspond in time to a southward shift of the Antarctic Circumpolar Current (ACC) particularly the Subantarctic Front (SAF). While other studies have shown distinctive trends in ocean and atmospheric parameters such as sea surface temperature (SST), air temperature, sunshine, rainfall, air sea level pressure and wind speed and direction from the 1950s to the early 2000s, the aim of this study is to update those studies to a more recent time with updated time series. Among the changes recorded is an increase in SST and air temperature, which is a strong indication of the changing local and global climate. Using linear regression, this study showed that the rates of increase from 1949 to 2018 of the SST (0.022°C/year), minimum (0.0072°C/year) and maximum air temperatures (0.016°C/year) are smaller than estimated in previous studies. The increasing trend in SST and air temperature reported by previous papers has actually stopped since the 2000s, which reduces the formerly reported trend (0.028°C/year). Although the in-situ measured SST data had gaps, a good correlation with in-situ SST and large scale satellite derived Reynolds SST help to corroborate the covariation between SST, in-situ SST and air temperature giving weight to the hypothesis of a reversal of the positive temperature trends reported by others. The change in decadal variability a decrease in air pressure of 4 hPa since the late 1990s to late 2000s, which coincided with a decrease in minimum and maximum air temperatures of 1°C over the same period; decrease in westerly wind and an increase in the northerly component of the wind, which would explain the decrease of inshore sea surface temperature a while thereafter. This study further corroborates previous findings of a continued decrease in rainfall, while the sunshine has largely remained the same. The seasonal cycle of the air pressure is significantly associated with that of rainfall, showing that the bimodal high air pressure signature resulting from the Semi-annual Oscillation (SAO) is associated with a decrease in rainfall. The Southern Annual Mode (SAM) was significantly yet weakly correlated with the SST (0.24), rainfall (-0.25) and air pressure (0.16), indicating that it does have an impact at the PEIs but not as strong as previously speculated. The El Niño Southern Oscillation (ENSO) has very weak and insignificant relationships with the parameters examined except for a weak relationship with in-situ SST, sunshine and air pressure. These new insights, especially at the decadal timescale, could further our insight on how subAntarctic islands have responded to climatic changes.
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    Multiple scenario analyses forecasting the impacts of sea level rise in Cape Town, South Africa
    (2018) Taukoor, Sheveenah Sunnassee; Ansorge, Isabel; Sithole, George
    Sea level rise is highly interdisciplinary and its study entails not only oceanography, but other fields such as geomatics, climatology and geology. In this study we relied on the tools from geomatics to produce sea level rise maps in order to assess the vulnerability of the coastline of Cape Town, South Africa. After generating a DEM of a spatial resolution of 2 m from LiDAR point cloud data, we made use of GIS to design 4 sea level rise scenarios based on the RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5 scenarios from the IPCC. Among the findings, it was found that 2.16 – 3.09 km² of land would be potentially inundated by 2100. The main receptors which were identified were sandy beaches, rocky shores and built-up land. Permanent inundation would possibly change the appeal and the nature of the beaches and affect the tourism industry. Hence the coastline requires immediate attention as it is one of the most valuable assets in the tourism industry. Tidal effect and storm surge effect were also identified as additional factors which brought temporary changes to the sea level in Cape Town. These impacts were further investigated in 8 coastal suburbs (Tableview, Woodbridge Island, Paarden Eiland, Foreshore, Sea Point, Glencairn, Fish Hoek and Strand.) Suitable adaptation strategies including hard protection measures (e.g groynes, sea walls, barriers) and soft protection measures (e.g beach nourishment) were also proposed for these 8 suburbs.
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    Seasonality of the Agulhas Current with respect to near- and far-field winds
    (2018) Hutchinson, Katherine; Penven, Pierrick; Beal, Lisa; Ansorge, Isabel; Hermes, Juliet
    The Agulhas Current plays a critical role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain poorly understood. Model studies predict an austral winter-spring maximum in poleward volume transport, whilst observations reveal an austral summertime (February-March) maximum. Here, the role of winds on Agulhas Current seasonality is investigated using shallow water models, satellite measurements, and a 23-year transport proxy based on observations. A one-and-a-half layer reduced gravity model is shown to successfully reproduce the seasonal phasing of the current. This seasonality is found to be highly sensitive to the propagation speed of Rossby waves, which determines the arrival time of the wind stress signal at the western boundary. By matching Rossby wave speeds to those observed using altimetry, an Agulhas Current with a maximum flow in February and a minimum flow in July is simulated, agreeing well with observations. Near-field winds, to the west of 35◦E, dominate this seasonality, as signals from more remote wind forcing dissipate due to destructive interference while crossing the basin. Local winds driving coastal upwelling/downwelling directly over the Agulhas cannot, alone, account for the observed seasonal phasing, as they force a NovemberDecember maximum and June minimum in flow. The seasonal response to Indian Ocean winds is also investigated using a barotropic (single layer) model with realistic topography. A barotropic adjustment cannot explain the observed Agulhas Current seasonality, predicting a wintertime maximum in transport. The results from the barotropic simulation are similar to previous model studies, where seasonality is dominated by a southward propagation of signals via the Mozambique Channel, suggesting that these models are too barotopic in their response to the winds. Findings from this study elucidate the role of near-field winds and baroclinic processes in determining the seasonality of the Agulhas Current.
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    Surface and sub-surface hydrographic variability at the Prince Edward Islands: perspectives from the high resolution GLORYS model
    (2023) Soares, Bianca; Ansorge, Isabel
    The Prince Edward Islands (PEI), comprising of Marion and Prince Edward Island, are located in the direct path of the eastward flowing Antarctic Circumpolar Current, nestled within the Antarctic Polar Frontal Zone of the Southern Ocean. The islands are home to a multitude of species which are supported by a sensitive and complex oceanic environment. Understanding the mechanisms at work, which sustain this rich ecosystem is therefore imperative for both the ecological management of the PEIs, and for the possible prediction of future climate changedriven environmental impacts. The presence of a possible Taylor column has been suggested as a main driver in supporting and maintaining the PEI ecosystem. However, due to the remote and hostile environment in which the islands are located, in situ data collection has proven to be a challenging task, which is necessary to study the Taylor column and to understand the island's general hydrographic variability. This study thus makes use of available in situ, satellite, reanalysis and modelled bathymetry data, to compare against the GLORYSV12 model output. This was to determine how accurately the model could reproduce surface and subsurface variability of temperature, salinity, Sea Surface Height (SSH) and surface circulation at the PEIs, and to identify whether in situ, satellite and model conditions are suitable for the existence and evolution of a possible Taylor column at the islands. A clear overestimation of the geostrophic currents (up to 0.2 m s -1 ) and underestimation of SSH (up to 0.6 m) by GLORYS was observed. The spatial and temporal variability of temperature and salinity was captured by GLORYS throughout the entire water column, despite the differences between temperature (biases from -2 to 2 oC) and salinity (biases from -0.4 to 0.4 PSU). Additionally, GLORYS was also able to simulate all five water masses, known to occur within the PEI region, throughout time. When GLORYS was compared to single point in situ SST time series data, a seasonal bias was observed with GLORYS overestimating SST in late summer (January to March) and underestimating SST for the remainder of the year (May to December). However, statistically significant strong, positive correlations (r > 0.80, p < 0.001) and relatively low biases ( -0.50 to 0.10 oC) were still observed between GLORYS and the this in situ SST time series. Overall, this suggested that GLORYS reasonably captures temperature and salinity variability on a climatological-scale. However, when it comes to event-scale, the model fails to accurately reproduce specific mesoscale events, as observed in 2013, 2014 and 2015 when cyclonic and anticyclonic eddies, simulated by the model were not of the same size, intensity nor in the same location as observed by in situ CTD data. GLORYS and satellite data both successfully proved that conditions are suitable for the formation and persistence of a possible Taylor column/Taylor cone at the PEIs. This was concluded with the relatively low Rossby numbers (< 0.07), high Reynolds numbers (> 2000), Blocking parameters which did not exceed 0.2, appropriate Rossby radius of deformation values (< 1000 km for a barotropic ocean and < 24 km for a baroclinic ocean) and an anticyclonic flow pattern around the PEI plateau, all indicative of the fact that conditions, which are typical for the formation of Taylor columns/Taylor cones, occur at the PEIs
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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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    The physical oceanographic processes on the southeast African shelf and slope
    (2018) Russo, Cristina Serena; Lamont,Tarron; Ansorge, Isabel; Barlow, Ray
    Situated between the southeast coast of South Africa and the western edge of the Agulhas Current, is the southeast African shelf. The Agulhas Current, like other western boundary currents of the world, strongly influences the oceanographic conditions of its adjacent shelf system. Limited societal drivers have resulted in the southeast African shelf being one of the least studied and under-sampled shelf systems in southern Africa. The in situ sample deficit has led to the majority of studies conducted in this region to be carried out using satellite data and modelling. In order to identify the physical oceanographic processes occurring on the southeast African shelf and slope, as well as the influence that the Agulhas Current has on them, this study analysed satellite data combined with high resolution in situ data, acquired during two hydrographic surveys of the southeast African shelf and slope, during January/February (austral summer) and July/August (austral winter) 2017. The Agulhas Current as well as a number of physical processes were observed to influence the shelf region. The snapshot of the two seasons given by the January/February and July/August in situ data presented indications of seasonality of temperature and salinity within the upper water masses but not within the deeper water masses. The same five water masses, Tropical Surface Water (TSW), Subtropical Surface Water (STSW), South Indian Central Water (SICW), Red Sea Water (RSW) as well as Antarctic Intermediate Water (AAIW), were observed during each cruise. As a result of the westward widening shelf from just east of East London, divergence-induced upwelling was observed between the coast and the inshore edge of the Agulhas Current, during both January/February and July/August. Ekman veering in the bottom boundary layer was observed between Port Alfred and East London during January/February as a result of the close proximity of the Agulhas Current to the slope. Altimetry data indicated the presence of a cyclonic eddy during both the January/February and July/August 2017 surveys. The presence of cold nutrient-rich SICW in the bottom layers on the shelf during January/February and July/August was as a result of the uplift caused by Ekman pumping associated with the cyclonic eddies. AAIW, usually observed along the offshore edge of the Agulhas Current, was found on the slope along the inshore edge of the Agulhas Current, as a result of the presence of cyclonic eddies. Wind-driven upwelling, induced by offshore Ekman transport and vertical mixing, resulted in the surfacing of cooler waters from below at several locations. Chlorophyll-a concentrations were found to be higher during January/February than during July/August 2017. The dissolved oxygen levels observed on the shelf during both surveys were found to be in the range where the biological responses would be insignificant, suggesting that oxygen availability, on the southeast African shelf, adequately facilitates the survival of the existing shelf biology and that low oxygen conditions do not seem to impact the shelf. This study provides the description of the first high resolution shelf wide hydrographic surveys of the region, during the austral summer and winter (even though they are considered as snapshots) which sets a baseline of oceanographic conditions on the shelf as well as the processes affecting it. The results of this study can be used by the scientific community and governmental departments to better implement conservation regulations regarding marine protected areas in the region.
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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.
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    Understanding variability across the Crossroad transect from 3 years (2013 to 2015) of hydrographic data
    (2018) Sejeng, Manare C; Ansorge, Isabel; Lamont, Tarron; Maes, Christophe
    The southwest continental shelf of Africa is characterized by a strong western boundary current with three interdependent components, namely the Agulhas Current, Agulhas Retroflection and Agulhas Return Current. This system plays a key role in setting oceanic conditions south of Africa. The Crossroad transect intersects both the Agulhas Current and Agulhas Return Current; a monitoring line established in 2013 to sample both the currents and determine inter-ocean fluxes, as well as the influence of the Agulhas Current on the Agulhas Bank shelf. The objective of the study was to examine both mesoscale and submesoscale features that influence the dynamic and variant nature of the Agulhas system. In this study we make use of Ship board Acoustic Doppler Current Profiler (SADCP), Conductivity Temperature and Depth (CTD), Thermosalinograph (TSG) and satellite Sea Surface Height data as main observations for analysis. The study also examines both the spatial and temporal characteristics of water properties across the Crossroad transect. The fundamental findings of the study include the abundance of both the mesoscale and submesoscale features observed in the Agulhas system, which are often overlooked. In addition, a noticeable variability in current measurements was observed, where velocity ranging from 2 to 2.5 m/s represented the Agulhas Current and 1.4 to 1.7 m/s, Agulhas Return Current. The position of the Agulhas Current and Agulhas Return Current displayed variation from 2013 to 2015, with the Agulhas Return Current exhibiting a meandering pattern in 2014 along the transect. Furthermore, an intrusion of cool (8 to 13 °C), lower salinity (34.8 to 35 psu) South Indian Central Water masses were also observed along the Agulhas Bank. The ability to combine altimetry and in situ data also contributed to the analysis of the results. Therefore, given the inherent advantage of satellite and in situ measurements, an overview of the variability across the Crossroad transect was determined.