Browsing by Subject "Southern Ocean"

Now showing 1 - 7 of 7
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    Biological survey of the Prince Edward Islands, December 2008
    (2009) Cooper, J; Bester, M N; Chown, S L; Crawford, R J M; Daly, R; Heynse, E; Lamont, T; Ryan, P G; Shaw, J D
    A biological survey of the Prince Edward Islands took place in December 2008. The survey repeated an earlier survey of the populations of surface-nesting seabirds on both islands and of fur seals (Arctocephalus spp.) and alien plants on Prince Edward Island in December 2001. Observations on burrowing seabirds, macro-invertebrates and plant communities on Prince Edward Island and an oceanographic survey of surrounding waters were also included. The survey confirmed many of the observations made on the earlier survey and permitted an assessment of trends in the abundance and distribution of biota since 2001.
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    Decay of eddies at the South-West Indian Ridge
    (2011) Durgadoo, Jonathan V; Ansorge, Isabelle J; de Cuevas, Beverly A; Lutjeharms, Johann R E; Coward, Andrew C
    The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a highresolution ocean model. The model's representation of the dynamic nature of this region is fully characteristic of observations. On average, 3-4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5-2.5 months after genesis and have depths of 2000 m - 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature-salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour.
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    Detection and identification of mechanisms and trends of sea ice drift variability in the Southern Ocean
    (2025) De Jager, Wayne; Vichi, Marcello
    Antarctic sea ice regulates global climate processes by mediating exchanges of heat, momentum, and mass between the Southern Ocean and atmosphere. However, the logistical constraints of acquiring in situ sea-ice observations is challenging. Inferring sea-ice properties, such as concentration and motion, from satellite observations helps to address these data gaps, but the retrieval algorithms carry some limitations that complicate data acquisition. Temporal resolution limits are particularly pronounced when studying the effect of transient weather phenomena, such as polar and extra-tropical cyclones, which are major drivers of short-term sea-ice variability. This thesis presents a systematic investigation into the weather-induced rotation dynamics in Antarctic sea ice, with a focus on the feasibility of quantifying hourly-to-daily sea-ice concentration and drift variability. A new algorithmic approach that is sensitive to the detection of rotational sea-ice drift features from satellite data at spatial and temporal scales comparable to that of polar weather is proposed, yielding a novel index for assessing interannual dynamical trends. Comparing selected 48-hour satellite ice motion products in the Atlantic Sector between 2013–2020 indicates good agreement in detecting anticyclonic rotation but discrepancies for cyclonic rotation. Cyclonic rotation is shown to be more intense than anticyclonic rotation, with the mean intensity of the 95th percentile of cyclonic features shown to be 1.5–2.0 times larger than their anticyclonic counterparts. Cyclonic rotation is also associated with greater variability and observational uncertainties, suggesting that the cyclones engender more heterogenous motion compared to high-pressure systems but also reduce the capacity of satellite products to accurately detect this motion. The analysis was expanded from 1991–2020 over the entire Southern Ocean using the latest generation 24-hour satellite sea-ice drift product. Results demonstrate that sea-ice rotation has become more correlated with atmospheric rotation at daily timescales, suggesting an increasing responsiveness of sea ice to weather forcing over the past three decades. Notably, the increased correlation between ice and atmospheric rotational dynamics is evidenced despite no discernible trends in the atmospheric forcing, pointing to a plausible role of the ocean in modifying the rheology of pack-ice. While the Southern Annular Mode was considered, it does not directly explain the strengthened coupling. Uniquely, sea ice in the Weddell Sea experienced a sharp decline in the intensity of its cyclonic and anticyclonic components from 2002 onwards, which may instantiate that large synoptic systems are not the primary driver of ice rotation in this region. To further address the temporal resolution limitations associated with daily-averaged satellite products, this thesis also explored a novel method for deriving sub-daily sea-ice type and motion observation from passive microwave swath data. The ECICE algorithm was directly applied to AMSR-2 Level-1B footprint data to produce sub-daily young (YI), first year (FYI) and multiyear (MYI) sea-ice concentration (SIC) observations. Results indicate ice-type classification was inconsistent, primarily because the radiometric signature of YI overlaps considerably with that of FYI and MYI. The use of motion vectors derived from the same swath data revealed a significant sea-ice response to weather forcing in just ~9 hours, which cannot be resolved by traditional daily-averaged products. The role of ice advection, melt, and growth contributions to SIC variability was calculated, but two major challenges arose: (i) coverage limitations associated with the drift retrieval algorithm caused unrealistic melt and growth estimates in the marginal ice zone (MIZ); and (ii) SIC saturation in the mobilized ice interior yielded minimal SIC changes despite a significant and widespread drift response. Ultimately, an assessment of drift products against dedicated observational experiments is recommended to evaluate product performance, especially in the MIZ, before swath-based retrieval techniques can be improved and systematically implemented.
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    Ice - ocean - atmosphere interactions in the Southern Ocean and implications for phytoplankton phenology
    (2021) Hague, Mark; Vichi, Marcello
    The annual advance and retreat of sea ice in the Southern Ocean is recognised as one of the largest seasonal events on Earth. Such considerable physical changes have profound effects on the vertical structure of the water column, and hence controls the availability of both light and nutrients to phytoplankton. This means that in the region seasonally covered by sea ice (the SSIZ), the timing of the growth and decline (phenology) of phytoplankton is determined to a large degree by the dynamic interactions between ice, ocean and atmosphere. However, this region is simultaneously one of the most poorly observed in the global ocean, and one of the most complex. This has led to significant gaps in our understanding of how sea ice modulates the exchanges of heat and momentum between atmosphere and ocean, as well as the implications this has for phytoplankton phenology in the SSIZ. This study seeks to address these gaps by combining both model and observationallybased methods. The lack of observational data are directly tackled through an analysis of BGC-Argo float data sampling under ice. Such data reveal high growth rates in the presence of near full ice cover and deep mixed layers, conditions previously thought to prevent growth. These results suggest a revision of our current understanding of the drivers of under ice phytoplankton phenology, which should take into account the unique character of Antarctic sea ice and its effect on the under ice light environment. In addition, results obtained from several numerical process studies indicates that phytoplankton may have a higher affinity for low light conditions than previously thought. From a modelling perspective, an analysis and intercomparison of 11 Earth System Models (ESMs) and their representation of vertical mixing and phenology is presented. This revealed that misrepresentations in phenology where driven by model biases in sea ice cover and vertical mixing. That is, only models with either too much or too little ice cover were able to simulate phenology close to observations. Furthermore, a strong correlation between the location of the ice edge and the extent of vertical mixing suggested that ESMs overly dampen ocean-atmosphere fluxes as mediated by sea ice. This led to the development of a regional ocean-sea ice model of the Atlantic sector of the Southern Ocean, from which experiments enhancing both heat and momentum fluxes could be conducted. It was found that the model responded more uniformly to enhanced heat flux, generally deepening the mixed layer closer to observations in winter. On the other hand, the effects of enhanced momentum flux (implemented by increased air-ice drag) where more complex and spatially heterogeneous, with contrasting responses depending on the initial vertical density structure of the water column. Overall, the argument is made that the unique features of Antarctic sea ice should be included in models if we are to improve the representation of the SSIZ mixed layer, and hence phenology
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    Sampling scale sensitivities in surface ocean pCO2 reconstructions in the Southern Ocean
    (2023) Djeutchouang, Laique Merlin; Vichi, Marcello; Monteiro Pedro
    The Southern Ocean plays a pre-eminent role in the global carbon-climate system. Model studies show that since the start of the preindustrial era, the region has absorbed about 75% of excess heat and 50% of the oceanic uptake and storage (42±5 PgC) of anthropogenic CO2 emissions. However, due to the spatial and seasonal sparseness of the Southern Ocean CO2 observations (biased toward summer), this role is poorly understood. The seasonal sampling biases have hampered observation-based reconstructions of partial pressure of CO2 at the surface ocean (pCO2) using machine learning (ML) and contributed to the convergence of the root mean squared errors (RMSEs) of ML methods to a common limit known in the literature as the “wall”. The hypothesis here is that addressing the critical missing sampling scale will get the community reconstructions of pCO2 “over the wall”. In this study, I explore the sensitivity of pCO2 reconstructions to these observational scale gaps. Using a scale-sensitive sampling strategy means adopting a sampling strategy which addresses these observational limitations including intra-seasonal as well as seasonal sampling aliases in high eddy kinetic energy and mesoscale-intensive regions. In increasing CO2 sampling efforts in the Southern Ocean using autonomous sampling platforms such as floats, Wave Gliders and Saildrones, the community has tried to answer this problem, but the effectiveness of these efforts has not yet been tested. This study aims to do this evaluation and advance our understanding of the sampling scale sensitivities of surface ocean pCO2 reconstructions from machine-learning techniques and contribute – through a scale-sensitive sampling strategy of observing platforms in the Southern Ocean – to breaking through the proverbial “wall”. This aim was achieved through a series of observing system simulation experiments (OSSEs) applied to a forced mesoscale-resolving (±10km) ocean NEMO-PISCES physics-biogeochemistry model with daily output. In addition to underway ships, the sampling scales of the autonomous sampling platforms such as Floats, WaveGliders and Saildrones, on pCO2 reconstructions were investigated in this series of OSSEs. The primary results showed that two sampling scales, which Saildrones are able to address, are required to improve the RMSE scores of machine-learning techniques and then reduce uncertainties and biases in pCO2 reconstructions. The two sampling scales include (1) the seasonal cycle of the meridional gradients and (2) the intra-seasonal variability. Based on the impacts of these two sampling scales on the RMSE scores and biases, it wasfound that resolving the seasonal cycle of the meridional gradient is the first-order requirement while resolving the intra-seasonal variability is the second. Applying the second-order requirement in the whole Southern Ocean to explore the sensitivity of the clustering choice to the two-step pCO2 reconstruction (clustering- regression). It was found that using an ensemble of clustering methods in this two-step reconstruction performs far much better than using a clustering method. Using these findings, I proposed an observational strategy that is viable and strengthens the limitations in existing underway SOCAT ship- and SOCCOM float-based reconstructions of surface ocean pCO2. More specifically, I proposed a hybrid scale-sensitive sampling strategy for the whole Southern Ocean by integrating underway ships with Saildrones on winter lines. The analysis of these multiple OSSEs indicates that improving the pCO2 reconstructions requires scalesensitive data to supplement the underway ship-based observations gridded in the SOCAT product. It was also found that scale-sensitive data consisting of high-resolution observations ( 1 day) extending over the seasonal cycle and capturing the pCO2 meridional gradients results in breaking through the proverbial “wall”. These findings will contribute to an accurate mean annual global carbon budget which is critical for the trend of the ocean sink feedback on global warming as well as ocean acidification.
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    Seasonal variability of phytoplankton photophysiology in the Southern Ocean: an analysis of uncertainties and the impact of assumptions
    (2025) Ruiters, Lillina; Ryan-Keogh, Thomas; Thomalla, Sandy; Vichi, Marcello
    Understanding and accurately quantifying primary production in the Southern Ocean is increasingly important due to its central role in global carbon cycling and climate regulation. However, this region remains undersampled, and its sensitivity to environmental change is not yet fully understood. One approach for estimating phytoplankton productivity is through active single-turnover chlorophyll-a fluorescence (ST-ChlF) techniques, such as Fast Repetition Rate fluorometry (FRRf), which infer photosynthetic capacity by measuring the transport of electrons during photosynthesis. While promising, this technique requires the derivation of several photophysiological parameters, and uncertainties in how these are calculated can affect the accuracy and comparability of results. Additionally, environmental forcing plays a direct role in shaping phytoplankton productivity, but disentangling the drivers of variability in phytoplankton photophysiology remains challenging due to the complex and dynamic nature of the Southern Ocean environment. This two-part thesis addresses both methodological and ecological uncertainties using a multi-seasonal dataset collected in the Atlantic sector of the Southern Ocean. The first part evaluates the impacts of key model assumptions and corrections steps used to derive photophysiological parameters from FRRf measurements. Results highlight the importance of using a fitted connectivity coefficient (ρ) when applying the Kolber-Prasil-Falkowski biophysical model to estimate primary photophysiological parameters from fluorescence transients. Furthermore, the dark-regulated σPSII and the light independent model were shown to be the best performing when deriving the secondary photophysiological parameters of electron transport rates. The application of blank and spectral corrections was also essential, particularly in winter when phytoplankton biomass is low. These corrections had a substantial influence on derived photophysiological parameters. The second part of this thesis investigates how seasonal changes in light availability impact phytoplankton photophysiology and pigment allocation. Under low light conditions during autumn and winter, phytoplankton had lower maximal electron transport rate (ETRmax) and showed increased energetic investment in photosynthetic pigments to maximise photosynthetic capacity. In contrast, higher light availability in spring and summer were associated with enhanced photosynthetic capacity driven by increased σPSII and ETRmax. Spring was identified as a transitional season due to the considerable variability in Ek due to the rapid increase in available light between winter and spring. The high ETRmax in summer resulted from an increase in αETR which was in turn driven by higher σPSII as a photoacclimation response to iron limitation in this season. This two-part thesis demonstrates the best approaches for processing single-turnover chlorophyll fluorescence data to minimise errors. In addition, it highlights how the application of active ST-ChlF techniques can be used to interrogate seasonal and regional variability in phytoplankton photophysiology, and how these differences are reflected in diverse phytoplankton photoacclimation mechanisms, which are in turn driven by changes in the availability of light. This contributes to improving estimates of primary production and understanding its variability in the Southern Ocean.
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    SHARC Buoy: Robust firmware design for a novel, low-cost autonomous platform for the Antarctic Marginal Ice Zone in the Southern Ocean
    (2021) Jacobson, Jamie Nicholas; Verrinder, Robyn; Mishra, Amit; Vichi, Marcello
    Sea ice in the Antarctic Marginal Ice Zone (MIZ) plays a pivotal role in regulating heat and energy exchange between oceanic and atmospheric systems, which drive global climate. Current understanding of Southern Ocean sea ice dynamics is poor with temporal and spatial gaps in critical seasonal data-sets. The lack of in situ environmental and wave data from the MIZ in the Antarctic region drove the development of UCT's first generation of in situ ice-tethered measurement platform as part of a larger UCT and NRF SANAP project on realistic modelling of the Marginal Ice Zone in the changing Southern Ocean (MISO). This thesis focuses on the firmware development for the device and the design process taken to obtain key measurements for understanding sea ice dynamics and increasing sensing capabilities in the Southern Ocean. The buoy was required to survive the Antarctic climate and contained a global positioning system, temperature sensor, digital barometer and inertial measurement unit to measure waves-in-ice. Power was supplied to the device by a power supply unit consisting of commercial-grade batteries in series with a temperature-resistant low dropout regulator, and a power sensor to monitor the module. A satellite modem transmitted data through the Iridium satellite network. Finally, Flash chips provided permanent data storage. Firmware and peripheral driver files were written in C for an STMicroelectronics STM32L4 Arm-based microcontroller. To optimise the firmware for low power consumption, inactive sensors were placed in power-saving mode and the processor was put to sleep during periods of no sampling activity. The first device deployment took place during the SCALE winter expedition in July 2019. Two devices were deployed on ice floes to test their performance in remote conditions. However, due to mechanical and power errors, the devices failed shortly after deployment. A third device was placed on the deck of SA Aghulas II during the expedition and successfully survived for one week while continuously transmitting GPS coordinates and ambient temperature. The second generation featured subsequent improvements to the mechanical robustness and sensing capabilities of the device. However, due to the 2020 COVID-19 pandemic, subsequent Antarctic expeditions were cancelled resulting in the final platform evaluation taking place on land. The device demonstrates a proof of concept for a low-cost, ice-tethered autonomous sensing device. However, additional improvements are required to overcome severe bandwidth and power constraints.