School of Earth Sciences - Theses
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ItemAustralian rainfall and El Niño diversity: past variability and context for recent changes( 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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ItemFire weather in two regions of the Southern Hemisphere( 2017)This thesis investigated fire weather in Victoria, Australia and the Ecuadorian Andes. The selection of these areas considered several criteria. First of all, bushfires cause significant impacts in these two regions. Victoria has endured some of the most catastrophic bushfire events in Australian history (e.g. “Black Friday” (1939), “Ash Wednesday” (1983), “Black Saturday” (2009)). On the other hand, bushfires in Ecuador destroy every year large areas of national parks in one of the most biodiverse countries in the world. Secondly, the El Niño- Southern Oscillation (ENSO) is a strong climate driver in the two study areas. Finally, Victoria and Ecuador share the Eucalyptus as the dominant bushfire-prone species. The aim of this thesis is to better understand the drivers and evolution of fire weather in these two regions of the Southern Hemisphere. Specifically, it examined three aspects. First of all, it investigated fire weather spatial patterns in Victoria and their relationship with associated events like heatwaves. Subsequently, the study explored long-term fire weather variability and changes. Finally, the investigation evaluated the influence of ENSO and other climate drivers over fire weather. The analyses used three groups of data: bushfire records, meteorological and climate indices data. Consistent bushfire records were available only for Victoria during the period 1961-2010. Additionally, the investigation required observations from weather stations in Victoria and the Ecuadorian Andes. This research also analysed reanalysis data from the Twentieth Century Reanalysis Project (20CR) and the European Reanalysis of Global Climate Observations ERA-Clim project (ERA-20C). The study had a stronger emphasis on ENSO since it affects both regions. This research used two indices to represent fire weather. The first index was the McArthur Forest Fire Danger Index (FFDI). This Australian metric was designed for an Eucalyptus environment. Therefore, this investigation applied the FFDI for Victoria and Ecuador. Additionally, this thesis proposes an alternative fire weather index for Victoria: the “Victorian Seasonal Bushfire Index” (VSBI). The VSBI combines local meteorological variables and sea surface temperature in ENSO regions to represent—and predict—extreme fire weather. The investigation of fire weather in Victoria and the Ecuadorian Andes yielded several findings. First of all, bushfire and heatwave weather patterns display differences from one another in Victoria. These comparisons used synoptic climatologies with reanalysis data during the period 1961-2010. Additionally, the investigation showed that Victoria experienced an increase in fire danger during the period 1974-2010. There is also weaker evidence suggesting an increasing trend since 1920. “El Niño” events are the leading remote driver of fire activity in Victoria. In fact, the incorporation of ENSO indicators in a simple index (VSBI) shows skill to forecast extreme fire weather in this region. For the Ecuadorian Andes, this research indicates that its fire danger season (July-September) is longer than reported. October and November also display “high” fire danger during the period 1997-2012. Finally, “El Niño” events increase fire risk in the Ecuadorian Andes.