School of Earth Sciences - Theses

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    Australian rainfall and El Niño diversity: past variability and context for recent changes
    Freund, Mandy Barbara ( 2018)
    The climate system integrates internally and externally induced variability at various time scales as a result of interactions between the ocean and atmosphere. The influence of external forcing on the climate system and with it the structural changes of climate variability, in particular on seasonal and longer time-scales, is difficult to examine due to high natural variability and short observational records. The interplay between high and low-frequency variability restricts our understanding of the full range of climate variability and our ability to contextualise changes. This thesis explores and evaluates the potential to use seasonal paleoclimate information to advance our knowledge of natural climate variability and the multi-century context of recent changes in the Australasian and tropical Pacific region. Climate modes of variability including the El Niño -Southern Oscillation influence Australian rainfall and make Australian rainfall highly variable at interannual timescales. Multi-century reconstructions of past climate variability are developed for Australian rainfall at bi-seasonal resolution. The rainfall reconstruction is based on local paleoclimate proxies and teleconnected links between remote paleoclimate proxies, climate modes of variability and Australian rainfall. In a multi-century context, the recent drying trends in parts of southern Australia, as well as the tendency towards wetter conditions in northern Australia, are found to be unusual. The cool and warm season rainfall reconstructions allow the documentation of distinct characteristics of past major droughts in terms of their spatial extent, duration, intensity, and seasonality. Using coral data at seasonal resolution, two El Niño index reconstructions illustrate the sequence of diversity of past eastern and central Pacific El Niño events for the last 400 years. The distinct spatio-temporal signatures of both types of El Niño are exploited, and together with a novel machine learning approach, the diversity of past El Niño events is reconstructed and compared to recent changes. The recent increase in the frequency of central Pacific El Niño events relative to eastern Pacific El Niño events during the late 20th century appears unusual. The most recent 30-year period includes more intense eastern Pacific events compared to the past four centuries. To further investigate the changes and interactions between Australian rainfall and El Niño diversity, observations and climate model simulations are compared to the multi-century reconstructions. A number of climate models taking part in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are identified that simulate spatially distinct El Niño behaviour. Identification of El Niño events reveals a lack of model agreement about projected changes of El Niño diversity. The probability of infrequent El Niño characteristics is evaluated and point towards an under-representation of central Pacific events that are followed by eastern Pacific events in the observational records. Future simulations in climate models indicate that this El Niño transition as observed most recently in 2014-2016, could become less common. Based on the rainfall and El Niño reconstructions, the general drying impacts of El Niño is consistent for both types. Despite the strength asymmetry between eastern and central Pacific El Niño events, the impact on Australian rainfall is of a similar order of magnitude but also highlights a strong variable nature of the different types of El Niño and Australian hydroclimate. The context of recent changes provided by the reconstructions in this thesis advances our knowledge of natural climate variability in the Australasian and tropical Pacific region and offers new insights into the future climate of the region.
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    Extending the instrumental climate record of southeastern Australia
    Ashcroft, Linden Claire ( 2013)
    Southeastern Australia (SEA) is the most highly populated and agriculturally productive area of the Australian continent. The region also experiences one of the most variable climates in the world, particularly in terms of interannual rainfall. To understand the influence that anthropogenic climate change will have on future SEA climate, it is important to identify the full range of natural climate variability in the region. However, the current instrumental datasets used for SEA climate research only begin in the early 20th century, restricting efforts to identify long-term climate changes and low-frequency variability. This thesis describes the first instrumental climate record for SEA to extend from European settlement in 1788 to the end of 2012. Historical instrumental observations of air temperature, atmospheric pressure and rainfall over SEA have been located and homogenised for the period 1788 to 1909. The historical observations have then been combined with existing high-quality SEA climate data for 1910 to 2012 to examine interannual SEA climate variability over the past 225 years. The extended climate record includes a number of previously unpublished observations during 1788–1859, providing new insight into the climate experienced by early European settlers in Australia. The interannual climate variability identified using the extended record is in good agreement with SEA documentary records, palaeoclimate reconstructions and a historical reanalysis product, verifying the quality of the homogenised pre-1910 SEA climate data. Assessing rainfall variability during 1788–2012 identifies the most recent droughts in 1935–1942 and 1997–2009 as the longest periods of rainfall deficit in SEA since at least 1832, when continuous rainfall observations begin. Conversely, prolonged wet periods during the 1870s and 1890s appear to be more extreme than more recent wet conditions experienced in the 1950s and 1970s. Analysis of the extended SEA temperature record reveals that the current positive temperature trend seen in the region is the strongest and most significant since at least 1860. Long-term stability of teleconnections associated with climate variations in the SEA region are also examined, with a focus on the influence of El Niño–Southern Oscillation (ENSO) on SEA rainfall. Breakdowns in the ENSO–SEA rainfall relationship are identified during 1835–1850 and 1920–1959, in agreement with previous observational and palaeoclimate studies. The decrease in ENSO–SEA rainfall correlations appear to be associated with changes in the Southern Hemisphere mid-latitude meridional pressure gradient, possibly linked to prolonged negative phases of the Southern Annular Mode. The extended instrumental climate record developed in this thesis makes a significant contribution to the emerging field of historical climatology in Australia. It offers valuable new data for historians, climatologists and palaeoclimatologists exploring SEA’s past and present climate. As the regional impacts of anthropogenic climate change become an increasing reality, improved understanding of past climate variability is vital for future climate research.