School of Earth Sciences - Theses

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    Antarctic sea ice and its interactions with high latitude weather and climate
    Watkins, Andrew Bruce ( 1998)
    Antarctic sea ice plays a major role in the earth system by greatly influencing the high latitude exchanges of heat, moisture and momentum between the ocean and atmosphere, as well as profoundly effecting the salt budget of the ocean, and thus the production of Antarctic Bottom Water, one of the driving mechanisms of worldwide oceanic circulation. With such considerable and far reaching impact, it is important to document its climatology, understand its variability and quantify its influence. Climatologies and trends of the Southern Ocean sea ice pack are presented using the most recent satellite observations available from the Defense Meteorological Program’s (DMSP) Special Sensor Microwave Imager (SSM/I). The analysis of these data show that Antarctic sea ice is highly variable in both time and space. Statistically significant increases in the sea ice extent, open water and ice areas have been determined from the SSM/I data for the 9 year period 1987 to 1996, a result which differs from the Scanning Multichannel Microwave Radiometer (SMMR) observations (1978-1987). The increasing trend in the SSM/I observations can be attributed to the large increases in sea ice observed in 1994-1995, as confirmed by an analysis of data from the ERS-1 satellite. The mean season length during these years has remained relatively unchanged. Regional trends, both in the sea ice concentration and in season length, showed vast spatial inhomogeneity. SSM/I data displayed increasing season length in the central Weddell Sea, Bellingshausen Sea and Balleny Islands regions, with decreasing length in the Amundsen Sea, eastern Ross Sea and in the coastal areas off Wilkes Land. Similar trends are observed in the seasonal sea ice concentration. In most cases, these trends are opposite to those observed in the SMMR data, which may be linked to the shift observed in the Amundsen Sea low after 1990. Comparisons with historical data would suggest that no large scale anomalous change has occurred in the Antarctic sea ice limits over the course of human observation. Furthermore, the degree of variability suggests great care is needed in interpreting large scale changes in sea ice conditions, and hence atmospheric or oceanic change, from locally observed anomalies. Case studies of the effect of individual cyclones upon the sea ice concentration show small but definite modification of the ice conditions. To further diagnose aspects of the thermodynamic and dynamic forcing upon the Antarctic pack, detailed analysis of the sea ice concentration variability has been conducted using spectral techniques, and the spectra have been compared to those of the European Centre for Medium Range Weather Forecasts (ECMWF) temperature and wind data. In all cases, and with the seasonal cycle removed, the sea ice concentration shows a bias towards longer timescales of variability than either the wind stress or surface air temperature. This “red shift” in its frequency spectrum is strongest with the wind stress, and weakest with the temperature. For longer period waves, this may be due to the formation of new ice by surface cooling or the moderation of melting by the cold surface water, whereas for shorter period waves, where wind stress dominates temperature and ice concentration respectively, time is required for winds to draw in warmer or cooler air, as well as to overcome the ice masses inertia and keel friction to open or close leads. Strong intraseasonal variability of the sea ice concentration is observed in the 20-25 day period, reflecting similar timescales of the temperature variability, as well as that of the energetic eddies of the Antarctic circumpolar current. Examination of the latitudinal variation of the sea ice concentration, temperature and wind stress spectra showed not only the importance of the north-south temperature gradient in influencing the variability, but also the seasonal changes in the semi annual oscillation of the circumpolar trough. Regional spectra showed clear differences between location, and reflected the influences of the atmosphere and ocean upon the sea ice pack. This is clearly shown in the Weddell Polynya region and off East Antarctica, with high variability in the synoptic timescales, and in the western Ross Sea where changes occur in timescales of greater than 20 days. In order to determine if satellite derived, real time sea ice concentration and distribution would be of benefit to operational numerical weather prediction (NWP) schemes, the effect of sea ice concentration change upon the atmosphere in synoptic timescales was examined using a general circulation model in conjunction with the Australian Bureau of Meteorology’s GASP analyses. Experiments were conducted with a typical July sea ice concentration and distribution, as well as slab concentrations of 0, 10, 25, 50, 80 and 100%. Results from 5-day numerical weather forecasts show that the central pressure, structure and tracks of individual cyclones are sensitive to the ‘switch on’ of different sea ice conditions. Composites of all forecasts made with each concentration showed considerable, and mostly statistically significant, anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations except for the typical July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were induced over the ice in all cases, as were reductions in mean sea level pressure. The July sea ice runs displayed a distribution of the mean sea level pressure anomaly different from all others, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. The results suggest that sea ice concentration does induce anomalies in the atmospheric parameters in timescales of less than five days. Further, the use of a realistic distribution of sea ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real time Antarctic sea ice data may be of considerable benefit to numerical weather prediction models.
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    Modelling the atmospheric influence of coral reef-derived dimethyl sulfide
    Fiddes, Sonya Louise ( 2020)
    Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a significant amount of DMS, which is currently unaccounted for in modelling studies. It has further been hypothesised that the coral reef source of DMS may modulate the climate. In this thesis, two atmospheric models coupled to online chemistry and aerosol schemes were used for the first time to explore the influence of coral reef-derived DMS on atmospheric composition and meteorology across temporal and spatial scales. A simple non-varying representation of coral reef-derived DMS was developed and added to a common DMS surface water climatology. By comparing the differences between simulations with and without coral reef-derived DMS, the role of coral reef-derived DMS was quantified. The Australian Community Climate Earth System Simulator coupled to the United Kingdom Chemistry and Aerosol model (ACCESS-UKCA) was used to quantify the influence of coral reefs at the global scale. ACCESS-UKCA was evaluated against satellite observations and other global climate models and the sensitivity of aerosol, clouds and radiation to large scale perturbations of DMS was tested. ACCESS-UKCA was found to have similar biases and DMS sensitivity compared to other models and it was estimated that marine DMS contributes 0.45K cooling to the present climate. The influence of coral reef-derived DMS on global to regional scale climate was then investigated. In the Maritime Continent-Australian region, where the highest density of coral reefs exist, a small decrease in nucleation and Aitken mode aerosol was found when coral reefs were removed from the system. However, these small responses were found to have no robust effect on global or regional climate. The Weather Research Forecast model coupled to the CBMZ-MOSAIC (Carbon Bond Mechanism Z - Model for Simulating Aerosol Interactions and Chemistry) chemistry-aerosol scheme (WRF-Chem) was then used to study the same question at higher spatial and temporal scales. WRF-Chem was run to coincide with an October 2016 field campaign over the Great Barrier Reef, Australia, against which the model was evaluated. After halving the DMS surface water climatology, the model performed well for DMS and sulfur processes, though aerosol number was overestimated. The inclusion of coral reef-derived DMS resulted in no compositional change in sulfate aerosol mass or total aerosol number. No direct or indirect aerosol effects were detected. Throughout this work, the complexities of the aerosol-climate system have been emphasised and the limitations of current modelling capabilities highlighted. In conclusion, while total marine DMS was found to have an important climatic influence, this thesis has found no robust link between coral reef-derived DMS and climate or weather. Thus, these results do not support hypotheses around the ability of coral reefs to modulate global or regional climate.
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    Characterising the zonally asymmetric features of the Southern Hemisphere extratropical circulation and their influence on regional climate variability
    Irving, Damien Brent ( 2016)
    The major zonally asymmetric features of the Southern Hemisphere (SH) extratropical circulation are the zonal wavenumber one (ZW1), zonal wavenumber three (ZW3) and the Pacific-South American (PSA) pattern. These tropospheric waveforms play a critical role in the meridional transport of heat and moisture and in the development of blocked flow, causing the regional surface climate to vary strongly depending on the strength, frequency and phase of their activity. The PSA pattern is widely regarded as the primary mechanism by which the El Nino-Southern Oscillation (ENSO) influences the high southern latitudes, and in recent years it has been suggested as a mechanism by which longer-term tropical sea surface temperature trends have influenced the Antarctic climate. This thesis presents novel approaches to identifying both the zonal waves and PSA pattern in reanalysis and model output. In comparison to existing wave identification methods, the approaches more fully exploit the information available from Fourier analysis. For the zonal wave analysis, this was achieved by adapting the wave envelope construct recently used in the identification of synoptic-scale Rossby wave packets. In order to apply similar methods to the non-zonal PSA pattern, a grid rotation method traditionally used in ocean modelling was used to orient the equator along the approximate great circle path of the pattern. These new wave identification methods were applied to ERA-Interim reanalysis data in order to analyse the climatological characteristics of the waveforms and their influence on regional climate variability. The results reveal that both the zonal waves and PSA pattern are important drivers of temperature, precipitation and sea ice variability in the mid-to-high southern latitudes. While ZW1 and ZW3 are both prominent features of the climatological circulation, the defining feature of highly meridional hemispheric states is an enhancement of the ZW3 component. Identified seasonal trends towards the negative phase of the PSA pattern were largely inconsistent with recent high latitude temperature and sea ice trends. Only a weak relationship was identified between the PSA pattern and ENSO, suggesting that the pattern might be better conceptualised as preferred regional atmospheric response to various external (and internal) forcings. The analysis of large datasets such as ERA-Interim typically requires extensive use of various software tools and packages, to the point where coding/programming is a major component of the research methodology. Despite this strong reliance on computation, traditional academic publishing formats and conventions do not allow for the documentation of computer software and code, which means it is impossible to replicate and verify much of today's academic literature. In an attempt to provide a practical solution to this so-called reproducibility crisis, the zonal wave and PSA pattern results have been presented in a reproducible manner. The procedure used to document the computational aspects of the research was developed to be consistent with recommended best practices in scientific computing and seeks to minimise the time burden on authors. It should provide a starting point for weather and climate scientists looking to publish reproducible research, and it is proposed that relevant academic journals could adopt the procedure as a formal minimum standard.
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    Investigating stratospheric ozone change and associated impacts on circulation and climate
    Stone, Kane ( 2015)
    With stratospheric ozone on the path to recovery, understanding its future role in modulating Southern Hemisphere circulation and climate is essential. This work contributes to answering this question through both observational and modelling studies. First, a new Dobson Umkehr retrieval technique optimised for resolution was developed, with the retrievals contributing in the evaluation of the Australian Community Climate and Earth-System Simulator-Chemistry Climate Model (ACCESS- CCM). This model was used to investigate future Southern Hemisphere stratospheric ozone changes and associated dynamical and climate responses, particularly in the Southern Annular Mode (SAM), the tropopause height and quasi-stationary waves. For this purpose, four simulations were completed comprising of historical, future projection and sensitivity simulations with fixed ozone depleting substances and greenhouse gases (GHGs) at 1960 levels. These simulations also act as Australia’s contribution to the international Chemistry Climate Model Initiative. The Dobson Umkehr retrieval technique developed here uses all available data, unlike other algorithms, which use designated solar zenith angles (SZAs) from a single wavelength pair (C), out of three (A, C and D). Investigating a test case from Melbourne, the degrees of freedom for signal increased from 3.1 to 3.4 when using all C-pair SZAs, and up to 6.5 when using all available SZAs and wavelength pairs; a significant improvement over current operational methods. ACCESS-CCM evaluation shows excessive ozone but an accurate distribution, and a temporally persistent ozone hole. Comparison with the Dobson Umkehr retrievals, total column ozone observations, ERA-Interim reanalysis and past modelling studies shows ACCESS-CCM produces excess ozone at altitudes above 25 km for Melbourne, but with substantial improvements in Antarctic total column ozone over it’s precursors (CCMVal-2 UMUKCA models). Comparisons with Davis and South Pole ozonesondes display a large disparity in the vertical location of perturbed ozone. Maximum depletion is seen between 100–50 hPa in ozonesondes, compared to above 50 hPa in ACCESS-CCM. This difference is likely caused by cold model biases enhancing polar stratospheric cloud formation and subsequent chlorine release at high altitudes. The lack of supercooled ternary solution may be a cause of less depletion between 100–50 hPa. Despite these inadequacies, ACCESS-CCM is simulating the amount of historical Antarctic October ozone depletion, the SAM and 50 hPa zonal wind anomalies well compared to ERA-Interim and past modelling studies. This gives confidence that the model simulates reasonable ozone-induced circulation responses. The model shows that October averaged Antarctic ozone is returning to 1980 levels just after 2060. Increasing GHG and ozone concentrations act to delay and advance the breakup of the polar vortex respectively. Regression analysis shows that in the future, increasing GHGs and ozone concentrations drive an increase and decrease in the SAM index respectively, effectively cancelling each other out. Contrary to the SAM, the high latitude tropopause changes, while influenced heavily by ozone changes in the past, is dominated by increasing GHGs in the future. The phase of spring and summer wave 1 in TCO, 50hPa temperature and 10hPa zonal wind undergo an eastward shift due to both ozone depletion and GHG increases. The wave 1 phase influence from GHGs is seen to originate from the troposphere, and therefore is influenced heavily by Andes orography. This is not the case for ozone concentration changes, indicating that the main influence is through modulation of the stratospheric polar vortex. A decrease in the amplitude is also seen due to GHG increases, primarily due to a decrease in the amplitude of the tropospheric wave 1.
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    The weather and climate of Australia at the Last Glacial Maximum
    Hope, Pandora ( 2005)
    The global climate has experienced four glacial cycles in the last 420,000 years, with each cycle characterised by a prolonged period of cooling culminating in maximal glaciation followed by a brief warm period. The most recent period of maximal glaciation is termed the Last Glacial Maximum (LGM) and occurred about 21,000 years ago. We currently live in one of the warm periods. The global climate is changing, and it is becoming more important to understand the extremes of the climate system and how well our modelling capability can capture those extremes. There has been a modelling intercomparison project established to examine how global general circulation models compare in simulating past climates, including the LGM. Analysis and comparison of these model results has been presented for many parts of the globe, but there has not been a comparison of the different model results over the Australian region. This thesis aims to fill that gap and explore the simulated LGM weather and climate of Australia and its drivers in more detail. Comparison with proxy evidence is also undertaken, and inconsistencies seen in the literature addressed. The Australian climate at the LGM was believed to be generally cooler, drier and possibly windier from proxy evidence in the literature. In the comparison done here the mean temperature and precipitation fields from most models show cooler and drier conditions, with some seasonal variability, but there are some strong outliers. It was found that the differences were not dependent on model resolution, but that the surface parameterisations were highly important for these fields. The shifts in the circulation were examined both in the model results and with a study of the non-linear link between the wind, surface moisture and dunes, which are a proxy for past winds. All the models simulate a southward shift in the westerlies in the Australian region. This is strongly driven byte prescribed sea-surface temperatures. Australia's current wind regime is conducive to dune building. However, the binding effect of soil moisture (or vegetation) is strong enough to limit present day movement, whereas in the drier climate at the LGM there was a capacity for sand movement. The analysis of dune orientations did not produce conclusive evidence for how the westerlies might have shifted at the LGM. An apparent enigma in the proxy evidence at the LGM is the high lake levels in Australia’s south east, while most inland lakes were dry. Previous authors believed that the precipitation was still low, but the high lake levels were driven by lowered potential evaporation. The hydrological cycle was generally depressed in the LGM simulations, but the potential for evaporation remained high. Thus an alternative hypothesis is posed based on increased run off due to a known shift in the vegetation types and a lag in the timing of the run off due to snowmelt. The analysis here shows that our capacity to simulate climates quite different from the present is still developing, but that model results can help explain apparent inconsistencies in the reconstruction of past climates from proxies.
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    Recent glacier and climate change in the New Zealand Alps
    Ruddell, Andrew Reginald ( 1995-07)
    The sensitivity of glaciers in the Southern Alps of New Zealand is evaluated to identify the nature of recent climate change. Past glaciological observations are compiled and to these are added 4 summer field seasons on the Tasman (including Hochstetter), Dart, Fox and Franz Josef Glaciers. The field data are an important aspect in the calibration and verification of glacier modelling. The detailed studies of these glaciers provides the basis for assessing the glacier and climatic changes over the whole glacierized region. (For complete abstract open document)
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    Experiencing, understanding and adapting to climate in south-eastern Australia, 1788-1860
    Fenby, Claire Dimity ( 2012)
    This thesis explores a somewhat overlooked theme in Australian history - climate. Phenomena like drought, flood and bushfire continue to place a strain on Australian society, as we have clearly seen in the past few years alone. The Black Saturday bushfires of 2009, the 2010-2011 Queensland floods and the ‘Big Dry’ drought that stretched from 1997-2009 have had an enormous impact on modern Australian society. It is obvious that climate extremes frequently affect Australia today and this thesis asks: • How were societies in south-eastern Australia affected by weather and climate between 1788 and 1860? • Did European Australians adapt to the problems posed by weather and climate during this period? • Did the impact of rainfall variation in south-eastern Australia differ between and within colonies? An interdisciplinary approach has been taken to answer these research questions. Climate is ordinarily analysed and understood using scientific data like meteorological observations and palaeoclimate records derived from tree-rings, coral growth, ice cores and cave deposits. However, meteorological observations were not routinely kept prior to 1860, leaving gaps in our knowledge of early climate. This thesis fills these gaps by examining historical documents including letters, diaries, newspapers and government records. The information uncovered in these sources was then compared to available historical meteorological records and palaeoclimate data. This unique mix of historic and scientific data sheds light on Australia’s past climate. Australian history includes literature on a range of environmental themes but there remains very little on climate and weather. This thesis makes a substantial contribution to Australian climate history, deepening our understanding of colonial history and broadening our knowledge of historical weather and climate events in the southern hemisphere. This analysis reveals the way European settlers adapted to weather and climate challenges and also reveals the combined impact of climate extremes and colonisation on indigenous people in south-eastern Australia.
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    High resolution solar exposure estimates from geosynchronous satellite observations for climate and near real-time applications
    Weymouth, Gary Trevor ( 1998)
    This thesis describes research and development work undertaken to produce a satellite based near-real time high resolution (6 to 24 km) surface solar exposure estimation system. Physical models of radiative transfer within the atmosphere have been developed to produce the estimates of exposure for the entire Australian continent from full resolution hourly visible Geostationary Meteorological Satellite (GMS) Stretched-Visible and Infrared Spin Scan Radiometer (S-VISSR) data. This thesis describes the exposure estimation system, including details of the physical processes modelled. The accuracy of the exposure data is presented. The first high resolution climatology of exposure across Australia is also presented and discussed. Detailed charts of mean daily exposure for each month and annual mean daily exposure form part of the climatology, based on the period November 1990 to June 1994 inclusive. Annual and four-monthly charts are compared to the available Australian Bureau of Meteorology (BoM) National Climate Centre (NCC) Solar Radiation Atlas (1975) charts based on cloud and sunshine records for the period 1968 to 1974 inclusive. Generally the agreement is good, with the satellite system providing greater spatial and temporal (all months) detail, some significant differences and higher accuracy. The satellite climatology shows that minimum exposure in the far north occurs in February due to the monsoon even though the sub-solar point is at a similar latitude. During the monsoon, the exposure minimum over Cape York is seen to shift from the east to the west side. Considerable detail of coastal and orographic exposure gradients about the east and southeast coasts is available. Other features seen for the first time are also presented. For exposure estimation, the model of Diak and Gautier (1983) has been developed further and carefully tuned for use with GMS-4 data (1990 to 1994). Extensive changes have been made to this model to use data from GMS-5, which replaced GMS- 4 in May 1995. GMS-5 has a sensor response extending from the visible to the near-infrared. The GMS-5 based model now runs operationally within the BoM, using total precipitable water estimates from the BoM regional numerical weather prediction (NWP) system, and real-time ozone estimates from the local readout of the National Oceanographic and Atmospheric Administration (NOAA) satellites. The models perform best in clear-sky conditions, with the average deviation of spatially-averaged daily model estimates from surface-based point pyranometer data being less than 5% (less than 4% against available high quality pyranometer data). In cloudy conditions, the average percentage deviation is larger. Australia-wide estimates of the accuracy of satellite-based exposure estimates have been developed. Over most of the continent, typical cloud conditions lead to daily estimates being within 8% of collocated point pyranometer measurements. No other high-resolution data set is available for direct comparison. However, results achieved here are comparable to or better than those reported for other locations. In clear-sky conditions, results presented here are as accurate as measurements from well-maintained good-quality pyranometers. The spatial and temporal variability of the exposure data has also been examined. From this, it has been estimated that over typical Australian agricultural areas, daily satellite exposure estimates are more accurate than extrapolation from a high-accuracy pyranometer more than 20 to 50 km distant. The exposure data have already been used for crop modelling purposes, as an aid to siting of high-quality ground-based measurements for a solar-thermal power station feasibility study, and for hydrological modelling. Such applications, while briefly discussed, are outside the focus of this thesis.