School of Earth Sciences - Theses
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ItemModelling the atmospheric influence of coral reef-derived dimethyl sulfide( 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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ItemCharacterising the zonally asymmetric features of the Southern Hemisphere extratropical circulation and their influence on regional climate variability( 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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ItemInvestigating stratospheric ozone change and associated impacts on circulation and climate( 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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ItemExperiencing, understanding and adapting to climate in south-eastern Australia, 1788-1860( 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.