Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The ionosphere is of practical importance for satellite-based communication and navigation systems due to its variable refractive nature which affects the propagation of trans-ionospheric radio signals. This thesis reports on the first attempt to investigate the mechanisms responsible for the generation of positive ionospheric storm effects over mid-latitude South Africa. The storm response on 15 May 2005 was associated with equatorward neutral winds and the passage of travelling ionospheric disturbances (TIDs). The two TIDs reported in this thesis propagated with average velocities of ∼438 m/s and ∼515 m/s respectively. The velocity of the first TID (i.e. 438 m/s) is consistent with the velocities calculated in other studies for the same storm event. In a second case study, the positive storm enhancement on both 25 and 27 July 2004 lasted for more than 7 hours, and were classified as long-duration positive ionospheric storm effects. It has been suggested that the long-duration positive storm effects could have been caused by large-scale thermospheric wind circulation and enhanced equatorward neutral winds. These processes were in turn most likely to have been driven by enhanced and sustained energy input in the high-latitude ionosphere due to Joule heating and particle energy injection. This is evident by the prolonged high-level geomagnetic activity on both 25 and 27 July. This thesis also reports on the phase scintillation investigation at the South African Antarctic polar research station during solar minimum conditions. The multi-instrument approach that was used shows that the scintillation events were associated with auroral electron precipitation and that substorms play an essential role in the production of scintillation in the high latitudes. Furthermore, the investigation reveals that external energy injection into the ionosphere is necessary for the development of high-latitude irregularities which produce scintillation. Finally, this thesis highlights inadequate data resources as one of the major shortcomings to be addressed in order to fully understand and distinguish between the various ionospheric storm drivers over the Southern Africa mid-latitude region. The results presented in this thesis on the ionospheric response during geomagnetic storms provide essential information to direct further investigation aimed at developing this emerging field of study in South Africa.

Studying the diet of marine top predators is important for understanding the trophic ecology of the predators but also the distribution, abundance and seasonal variation of prey species. Dietary studies on subantarctic fur seals (Arctocephalus tropicalis) and Antarctic fur seals (A. gazella) is important for assessing their role as predators in the Southern Ocean. Seven hundred and sixty four scats were collected from an A. tropicalis colony at Cape Davis while 659 scats were collected from an A. gazella colony at Watertunnel stream from April 2006 to March 2010. Hard remains such as fish otoliths and cephalopod beaks were used for identification and quantification of prey. A. tropicalis diet comprised of three animal groups (fish, cephalopod and crustaceans) while A. gazella diet was made up of four animal groups (fish, cephalopods, crustaceans and seabirds). Fish contributed substantially more than other prey items in the diet of both fur seal species over the study period. Both species fed predominantly on fish of the family Myctophidae which constituted a numerical abundance of 98.9 percent in scats for both species combined. A. tropicalis fed on fish prey species from eight families, namely: Myctophidae, Notosudidae, Paralepididae, Nototheniidae, Bathylagidae, Chlorophthalmidae, Sternoptychidae and Photichthyidae while A. gazella fed on fish from six of these families, with Chlorophthalmidae and Photichthyidae not represented in its diet. Cephalopods and crustaceans were found in low numbers in the diet of both seal species and seabird remains were only found in the diet of A. gazella. Numerically, the most dominant species were from the genus Gymnoscopelus followed by Protomyctophum and then Electrona. In the diet of A. tropicalis, Gymnoscopelus contributed 66 percent, Protomyctophum 18 percent and Electrona 5.9 percent while in the diet of A. gazella Gymnoscopelus, Protomyctophum and Electrona contributed 59.43 percent, 20.24 percent and 14.24 percent respectively. Gymnoscopelus species comprised G. piabilis, G. bolini, G. braueri, G. fraseri, and G. nicholsi; Protomyctophum comprised Protomyctophum bolini, P. choriodon and P. tenisoni while Electrona comprised Electrona antarctica, E. carlsbergi and E. subaspera. Gymnoscopelus piabilis was the most dominant species from this genus while Electrona carlsbergi and Protomyctophum tenisoni dominated the genus Electrona and Protomyctophum respectively. The total number of fish species utilised by A. tropicalis was twenty four while twenty three fish species were identified in A. gazella diet. Bathysauropsis gracilis only featured in the diet of A. tropicalis while Lampanyctus ater and Lepidonotothen larseni were only found in the diet of A. gazella. To compare the diet of the seal species between seasons, a year was divided into three seasons, early summer, late summer and winter. Their diet was also compared between years from 2006 to 2010. Myctophid fish species showed substantial seasonal and annual fluctuations in the diet of the two fur seal species. E. carlsbergi, G. nicholsi and G. piabilis increased in numerical abundance in winter while Melectrona ventralis increased in late summer. The numerical abundance of G. fraseri and P. tenisoni increased in early summer. Generally, the two fur seal species fed on the same prey species. However, there was a significant difference when the diet of both fur seals was compared seasonally. There was no significant difference when the diet of A. gazella was compared annually, although such a difference was apparent for A. tropicalis.

In the McMurdo Dry Valleys, Victoria Land, East Antarctica, melting of glacial ice is the primary source of water to streams, lakes, and associated ecosystems. To better understand meltwater production, three hypotheses are tested: 1) that small changes in the surface energy balance on these glaciers will result in large changes in melt, 2) that subsurface melt does not contribute significantly to runoff, and 3) that melt from 25-m high terminal cliffs is the dominant source of baseflow during cold periods. These hypotheses were investigated using a surface energy balance model applied to the glaciers of Taylor Valley using 14 years of meteorological data and calibrated to ablation measurements. Inclusion of transmission of solar radiation into the ice through a source term in a one-dimensional heat transfer equation was necessary to accurately model summer ablation and ice temperatures. Results showed good correspondence between calculated and measured ablation and ice temperatures over the 14 years using both daily and hourly time steps, but an hourly time step allowed resolution of short duration melt events and melt within the upper 15 cm of the ice. Resolution of short duration melt events was not important for properly resolving seasonal ablation totals. Across the smooth surfaces of the glaciers, ablation was dominated by sublimation and melting was rare. Above freezing air temperatures did not necessarily result in melt, and low wind speed was important for melt initiation. According to the model, subsurface melt between 5 and 15 cm depth was extensive and lasted for up to six weeks in some summers. The model was better able to predict ablation if some subsurface melt was assumed to drain, lowering ice density, consistent with observations of a low density weathering crust that forms over the course of the summer on Dry Valley glaciers. In extreme summers, drainage of subsurface melt may have contributed up to half of the observed surface lowering through reduction of ice density and possibly through collapse of highly weathered ice. When applied spatially, the model successfully predicted proglacial streamflow at seasonal and daily time scales. This was despite omitting a routing scheme, and instead assuming that all melt generated exits the glacier on the same day, suggesting refreezing is not substantial. Including subsurface melt as runoff improved predictions of runoff volume and timing, particularly for the recession of large flood peaks. Because overland flow was rarely observed over much of these glaciers, these model results suggest that runoff may be predominantly transported beneath the surface in a partially melted permeable layer of weathered ice. According to the model, topographic basins, particularly the low albedo basin floors, played a prominent role in runoff production. Smooth glacier surfaces exhibited low melt rates, but were important during high melt conditions due to their large surface area. Estimated runoff contributions from cliffs and cryoconite holes was somewhat smaller than suggested in previous studies. Spatial and temporal variability in albedo due to snow and debris played a dominant role in flow variations between streams and seasons. In general, the model supported the existing assumption that snowmelt is insignificant, but in extreme melt years snowmelt in the accumulation area may contribute significantly to runoff in some locations.

No description available.

Reconstructed snow accumulation rate observations from five new firn cores show a statistically significant negative trend in accumulation rates over the past four decades across the central West Antarctic ice sheet. A negative temporal trend in accumulation is unexpected in light of rising surface temperatures and simulations run by GCMs. Both the magnitude of the mean accumulation rates and the range of interannual variability observed in the new records compares favorably to older records, suggesting that the new accumulation rate records may serve as a regional proxy for recent temporal trends in West Antarctic accumulation rates. The observed negative trend is likely the result of Southern Hemisphere high-latitude internal atmospheric dynamics, dominated by changes in the austral fall season. The well-documented positive trend in the Southern Annular Mode causes a low pressure center to form over the Amundsen Sea, which in turn produces lower accumulation rates across the western portion of the West Antarctic ice sheet. The new accumulation rate records are compared to several models/reanalyses to test the skill of simulated accumulation rate predictions. While the models/reanalyses and the new observations agree well in both mean and variability, the simulated records do not capture the full negative trend observed in the reconstructed records.

Widely spaced tidal gravity records have been used to determine the spatial and temporal variation of the ocean tide beneath the Ross Ice Shelf. Cotidal-coamplitude maps have been drawn for the six greatest harmonic constituents of the tide. These are K₁, P₁, O₁, M₂, S₂, and N₂. The tide is principally diurnal, the diurnal amplitudes being roughly 3 times longer than the semidiurnals. The range of the tropic tide is about 1 m at the northern extremity of the ice shelf, and can be as great as 2 m in the southeastern part of the region. The diurnal constituents can each be viewed as a wave that propagates towards the southwest across the sea, having an amplitude that is closely related to the thickness of the water-layer beneath the ice. For each of the semidiurnal constituents there is an amphidromic region located within the Ross 5ea near 80° S latitude, 1900 W longitude, and having a clockwise sense of rotation. Theoretical calculations of the tidal current indicate that the semidiurnal and diurnal current constituents have roughly the same amplitude. The semidiurnal current is magnified by near resonance with the inertia current due to the high latitude of the sea. Because of the resonance, calculations of the semidiurnal components of the tidal current are sensitive to the treatment of the retarding effects of the ice shelf and sea floor. Waves having periods shorter than 20 min were observed in the ice shelf. These have been identified as flexural waves that are generated by the action of the ocean swell on the northern edge of the shelf. The observed speed of these waves was predicted within the uncertainty of the measurement by the classical flexural wave theory. / Ph. D.

Permafrost is a variable in Antarctic terrestrial ecosystems, and the role it plays in the cryosphere is not well understood. There is much still to be learnt about the thermal state, physical properties, thickness and age of permafrost in Western Dronning Maud Land (WDML). Active layer dynamics and observed change over time have the potential to improve our knowledge of climate change. Understanding the effects of a warming climate on permafrost can also be of benefit to infrastructure, especially in areas with a large amount of frozen ground such as Scandinavia, Canada and Russia. Active layer and permafrost dynamics of WDML, Antarctica, are presented and discussed using data from six study sites, namely the Robertskollen, Vesleskarvet, Flarjuven, Grunehogna, Slettjfell nunataks and the Troll research station in the Jutulsessen area. Ground and ambient air temperature, as well as ground moisture data were collected for each site. An inventory of active layer and permafrost landforms was compiled, as were the frequency of cycles over the zero-degree isotherm, and the depth of the active layer. Furthermore, 3D models, geo-referenced maps and Digital Elevation Models were created of study areas with the use of an Unmanned Aerial Vehicle (UAV). Polygonal features are the most common landscape feature and are common to most of the study sites. Robertskollen has the deepest active layer at over 66cm and Slettfjell the shallowest at 9cm. A maximum recorded air temperature of 8.76°C (10/11/2014) occurred at Troll with the second highest maximum of 6.77°C (22/12/2010) recorded at Vesleskarvet. Robertskollen has the highest observable biological growth and a maximum recorded ground temperature of 22.84°C (10/01/2014). Troll and Valterkulten, registered the second and third highest ground temperatures respectively. The high ground Temperature observed for Robertskollen may be ascribed to it being the lowest altitude site. The highest number of cycles over the zero-degree isotherm was observed at Troll (11.01%), followed by Robertskollen (10.99%). For relatively warm areas, such as Robertskollen it is recommended that two metre borehole loggers are installed in order to capture a detailed understanding of the active layer. The UAV proved to be a beneficial tool for capturing aerial photographs for post fieldwork analysis and 3D modelling.

Feedbacks between abiotic variables and community structure in Antarctica are poorly understood. Research is, therefore, required to elucidate the patterns of biodiversity that exist and the factors that influence them, particularly under changing climates. Landscape processes affect environmental heterogeneity, which in turn affect patterns of biodiversity. Two inland Antarctic nunataks, Robertskollen and the Northern Buttress of Vesleskarvet, were selected for investigation to determine the potential impact of selected environmental factors on lichen distribution and abundance, at the intra- and inter-nunatak scales. Lichens were found to prefer rock faces with dips between 1° and 45°, and northern/southern aspects. Lichen colonisation was mostly in microtopographical features that result from rock weathering. The distribution of lichens was found to be regular at the intra- and inter-nunatak scale, whereas lichen abundance was found to be mostly influenced by temperature. On the Northern Buttress, rock hardness displays a similar pattern to lichen abundance, both of which are suggested to be a function of exposure time, which is dependent on deglaciation. The two nunataks serve as excellent laboratories that can potentially be used as proxies for investigating the possible impacts of climate change