School of Earth Sciences - Theses
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ItemNo Preview AvailableAntarctic sea ice and its interactions with high latitude weather and climate( 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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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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ItemThe role of health co-benefits in the development of climate change mitigation policies in Australia, the European Union, China and the United States( 2019)Governments are yet to commit to action on climate change commensurate with the likelihood and severity of predicted impacts. The human health consequences of a changing climate are substantial, already being felt and will be exacerbated without ambitious and urgent action. Acting to mitigate climate change can result in ancillary benefits to health outcomes, also known as health co-benefits. Numerous studies over the past two decades have estimated the monetised value of a range of health co-benefits that may result from the implementation of mitigation measures. These studies conclude that accounting for health co-benefits can partially, if not fully, offset abatement costs. Despite this economic rationale for climate action, numerous climate change and health scholars have questioned the influence of health co-benefits on final policies. To date, there has been limited research investigating the political traction of health co-benefits. To begin to address this knowledge gap, this thesis examines the role of health co-benefits in climate change mitigation policy-making in four Parties to the United Nations Framework Convention on Climate Change. To do so, I first review literature on i) the political economy of health and climate change; ii) the science-policy interface; and iii) power in policy-making in order to identify areas where barriers for the consideration of health co-benefits in climate change mitigation policies may exist. Next, I outline the methods and analytical approach used. I then examine the role of health co-benefits in climate change mitigation policies through the development of case studies for Australia and the European Union. Next, I present results of my analysis of select Chinese and American climate change policy documents published between 2007 and 2017. The key finding of this research is that while health co-benefits are often a driver of air pollution mitigation policies, their consideration in the development of climate change mitigation policies is context- and policy-dependent. In considering the implications of this finding, I discuss key factors influencing the political traction of health co-benefits in the context of existing literature and possible policy implications. This thesis concludes by outlining contributions of this research to the literature and suggesting future research opportunities. The significance of this research is its extension of the burgeoning literature on health co-benefits and climate change mitigation policy-making from a social science perspective. Further, this thesis articulates implications for policy and identifies potential opportunities to enhance the political traction of health co-benefits in climate change mitigation policies at a time when strong climate action is so desperately needed.
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ItemThe energy business system: transformation, social innovation & disruptive smart grid business models( 2018)A radical shift from a high- to a low-carbon energy system is not occurring at the speed required to address climate change. One reason for this, is that conventional energy firms are locked into producing shareholder profit with fossil-intensive business models that are still operable in current markets. This PhD thesis employs systems thinking to analyse lock-in of the energy business system (EBS) and then adopts design thinking to propose disruptive innovations that can accelerate low-carbon transitions. Dynamics of the EBS transition are evaluated in an interdisciplinary way and across systems scales, from the Earth system to the distribution edge of the electrical power system. Conceptual frameworks that combine complex system and transition theory are developed to evaluate the global EBS in an Earth system context and to analyse the role of business models in the decarbonisation of the electricity sector. Previous research suggests that strategies for escaping EBS lock-in include incorporation of environmental liability to shareholders, carbon taxation and other economic measurements to alter market conditions. The systems analysis presented here suggests that to ensure environmental resilience the EBS’s fundamental purpose and governance need revision. I theorise that more social innovation in business models could influence business trajectory in the energy sector. Instead of maximising shareholder profit, social innovations can shift business purpose towards long-term social and environmental value-creation using emerging market-based tools. A business model analysis of the electricity sector is used to identify opportunities for EBS disruption from social innovations. I find that, in general, social and environmentally driven low-carbon projects often struggle to achieve scale and commercial advantage. However, recent business model innovations in smart grids can provide such projects with the required competitive position. Specifically, Virtual Power Plant technology has emerged as an energy management system that allows aggregation and coordination of multiple distributed energy resources. Aggregation can include diverse resources such as photovoltaics, batteries, electric vehicles and windfarms. Coordination can achieve improved physical and market performance as a functional unit within the electricity market. Using theoretical model development and illustrative examples, I highlight how emerging opportunities such as peer-to-peer Internet platforms and blockchain technology, also have significant potential as tools to enable disruptive business models, through decentralised value creation using assets from online participants. Building from the systems thinking analysis of the EBS lock-in, the second part of this PhD thesis uses design thinking to propose and further develop a new business model termed a ‘social virtual energy network’ (SVEN). As an urban social electricity-trading network, a SVEN is designed to help accelerate the decarbonisation of the power system and influence paradigm shifts in EBS governance. Two iterations in the design of the SVEN concept are presented and critiqued based on insights derived from the first part of the thesis. The first iteration focuses on the role of virtual power plants and tariff design for business feasibility, and the second on blockchain and user interfaces for mainstream market adoption. Through systems analysis, this study argues that an adequate response to climate change requires a paradigm shift in the EBS. Using a systems design approach, the thesis provides a vision for the architecture of a democratic open energy economy where users and their distributed energy resources have an active role in the value chain of the EBS. The findings and proposals of this work are relevant to debates about the most effective ways of accelerating targeted innovations to achieve a low-carbon energy system.
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ItemGoverning geoengineering sustainably: a scenario exercise to inform Australian geoengineering policy development( 2018)Geoengineering, the technofix to climate change, is a complex, contentious and high-stakes proposal. Yet in the absence of credible long-term global emissions reduction, the idea cannot be overlooked. To ensure that geoengineering contributes to sustainable environmental and social outcomes, a forward-looking, inclusive and reflexive decision-making framework is needed. Instead, Australia’s history on climate policy reflects a short- sighted, impulsive and polarised approach. The motivation of this project is therefore to inform sustainable geoengineering governance in Australia. Within a sustainable governance context, scenarios are often used in the management of long-term, complex, and uncertain issues. This thesis investigates how a scenario exercise can inform sustainable geoengineering governance in Australia. The sustainability focus justifies an interdisciplinary mixed-methods approach. The thesis begins by surveying the present before interrogating the future and is presented as a compilation of five papers. First, the thesis explores from an Earth System Governance perspective what global geoengineering governance exists and how this might evolve. The analysis characterises geoengineering governance as governance-by-default where there is no purposive regulation—decisions are guided by existing norms and driven by the motivations of engaged academics. Given the influence of these actors, the research examines, through a systematic quantitative review, the types of geoengineering governance frameworks proposed in academic and non-academic literature. The study finds that the challenges of geoengineering governance can be likened to issues in other policy domains but suggests that normalising the debate thusly could obscure major threats and novel opportunities. Next, a meta-analysis of geoengineering scenarios is undertaken. It finds that the treatment of geoengineering within these scenarios does not align with sustainability concepts. An emphasis on technological solutions ignores the interdependence of nature and society and conceals alternative options; a focus on global effects and actions disregards local or regional issues; and scenarios portray only a narrow range of perspectives. Finally, an inductive and deductive scenario design method is proposed and demonstrated, producing four scenarios that are analysed in several ways: their key determinants are compared to those of scenarios in the geoengineering literature; they are studied individually and collectively to identify causal relationships and early warning signals; shared learning throughout scenario process is explored; and finally, they are used to stress-test climate policies and inform robust strategies. Proposed Australian climate strategies are found not to be robust. Policies are based on the expectation of enduring government legitimacy and that technological solutions obviate the need for behavioural change. The geoengineering strategy proposed for Australia is engagement nationally and internationally on geoengineering issues in a technologically and ideologically neutral manner and investment in transparent and inclusive research. The contributions of this thesis are several. It establishes that geoengineering governance is not tracking on a sustainable trajectory globally. It suggests that the role of scenarios, already central to geoengineering scholarship and governance design, can be expanded. It proposes and demonstrates a successful scenario development and analysis method. It begins a cross-sectoral Australian geoengineering conversation. It makes specific policy recommendations; and in doing so, it opens up the scope of policy options.
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ItemClimate justice: can we agree to disagree? Operationalising competing equity principles to mitigate global warming( 2017)With the Paris Agreement, the international community has agreed to limit global warming to well below 2 °C and to pursue efforts to stay below 1.5 °C (UNFCCC 2015a) to avoid dangerous climate impacts. Staying within these boundaries requires important emissions mitigation efforts from all countries (Rogelj et al 2015). Equitable distribution across countries of mitigation efforts, or equivalently of emissions rights, consistent with global mitigation objectives is a contentious issue that involves divergent interpretations of distributive justice (Winkler and Rajamani 2014a). The latest Intergovernmental Panel on Climate Change (IPCC) report categorises equity approaches from the scientific literature in five groups (Clarke et al 2014). At climate negotiations, most countries tend to support the approach that requires the least efforts on their behalf (Fleurbaey et al 2014, Lange et al 2010). With the absence of consensus on an effort-sharing approach, current negotiations under the United Nations Framework Convention on Climate Change (UNFCCC) follow a self-interested, or ‘bottom-up’, approach to target setting (Andresen 2015, Bodansky 2016) where each country decides its own effort following its understanding of fairness. As a result, the sum of all parties’ announced contributions is not consistent with limiting global warming to 2 °C, let alone 1.5 °C (Rogelj et al 2016a). Under the Paris Agreement, countries committed to increase the ambition of their post-Kyoto climate pledges through a ratcheting-up process that begins in 2023. With the disagreement on effort-sharing approaches, the international community relies on diverging metrics to evaluate the adequacy of national pledges with the global warming thresholds. Since the beginning of climate negotiations under the United Nations, a rich literature has modelled allocations of emissions rights to countries using various effort-sharing approaches with uncoordinated parameterisation. At the start of this PhD work, no study modelled the effort-sharing categories presented in the last IPCC report under a common parameterisation. Additionally, the literature on the combination of effort-sharing approaches remained thin and consisted of averaging the emissions allocations of multiple effort-sharing approaches. This PhD thesis addresses these gaps with the modelling of a new emissions allocation framework, the ‘PRIMAP-Equity’ framework, and with the suggestion of a new combination of effort-sharing approaches. Firstly, this thesis quantifies allocations of emissions rights to countries in a manner that reflects the existing literature on distributive justice. An emissions allocation framework is developed to derive national emissions allocations that reflect the five equity categories of the fifth IPCC report. This modelling framework is applied to derive emissions allocations, under each of the five equity categories, consistent with the emissions mitigation goals of the G7 Elmau agreement signed in June 2015. The allocation framework is then used to derive national emissions trajectories aligned with the recent Paris Agreement goals of both well below 2 °C and 1.5 °C, consistently with the five equity categories . This work represents the first quantification of equitable national trajectories to achieve 1.5 °C goal and informs scientists and government experts in the preparation of the IPCC Special Report on 1.5 °C (IPCC 2017). The Nationally Determined Contributions (NDCs), countries’ national pledges, of 171 Parties are then evaluated in order to determine which, if any, categories of equity they are consistent with. As well, the thesis highlights the consistency of G20 countries’ pledges with equity allocations. This is discussed in the context of the statement on fairness contained in each pledge. This PhD thesis then addresses the apparent incompatibility between the global warming thresholds and countries’ self-interested visions of effort-sharing by suggesting a new quantitative approach. Doing so, this PhD thesis provides a new metric, inclusive of all international positions, to assess the ambition of the NDCs under the Paris Agreement. This new ‘hybrid’ allocation method reconciles the ‘bottom-up’ approach of equity with the ‘top-down’ climate threshold that they commonly agreed. Under this ‘hybrid’ approach, each country follows the least stringent effort-sharing approach – out of the five that reflect the equity categories presented in the last IPCC report – to achieve the Paris Agreement. The aggregation of current national pledges is found to align with such a ‘bottom-up’ combination of approaches and lead to a warming of up to 2.3 °C in 2100 (with a 50% chance). Conversely, an enhanced ‘bottom-up’ approach – ‘hybrid’ – of global emissions scenarios leading to 1.1 °C and 1.3 °C warmings results in the achievement of the Paris Agreement mitigation goals of 1.5 °C and well below 2 °C, respectively. Ultimately, this study quantifies a compromise where each country can choose an equity approach to determine its effort, but does directly use that approach to assess other countries’ pledges. Finally, the application of this ‘hybrid’ approach provides a temperature assessment for all countries’ climate pledges, indicating the consistency of countries’ ambition in light of the global temperature goals. The NDCs of India, the EU, the USA and China are in line with global ‘bottom-up’ situations leading to warmings of 2.6 °C, 3.2 °C, 4 °C and over 5.1 °C, respectively. The results of this thesis can inform public opinions and decision makers through the ratcheting-up process on what constitutes fair and ambitious pledges to achieve the Paris Agreement following a range or combination of equity approaches. Additionally, the assessments of the adequacy of countries’ pledges with international agreements can inform courts when ruling ‘climate cases’ where governments are sued for their lack of ambition in mitigating emissions (Sabin Center for Climate Change Law 2018).
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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.
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ItemAn attribution study of southeast Australian wildfire risk( 2016)Extreme weather and climate-related events often have a serious impact on our economy, environment and society. This is particularly true in Australia where recurring heat waves, floods, droughts and wildfires have resulted in the loss of life, property and livelihoods. The 2009 'Black Saturday' wildfires in southeast Australia provides a tragic example, having resulted in the death of 173 people and the destruction of over 2000 homes. While there are a number of recorded attribution studies for Australian temperature and precipitation-related events, no such study exists for fire weather. This thesis presents a new climate modelling system for regional climate simulation and the attribution of weather and climate extremes over Australia and New Zealand. The system, known as weather@home Australia-New Zealand, uses public volunteers' home computers to run a moderate-resolution global atmospheric model with a nested regional model over the Australasian region. By harnessing the aggregated computing power of home computers, weather@home is able to generate an unprecedented number of simulations of possible weather under various climate scenarios. This combination of large ensemble size with high spatial resolution allows a range of extreme events to be examined over Australia and New Zealand with well-constrained estimates of sampling uncertainty. The model is seen to be capable of resolving many climate features that are important for the Australian and New Zealand regions, including the influence of El Nino-Southern Oscillation (ENSO) on driving natural climate variability. Using the new weather@home modelling framework, this thesis presents the first known attribution study of southeast Australian fire weather. By applying the McArthur Forest Fire Danger Index to large ensembles of regional climate model simulations generated for factual and counterfactual climate scenarios, this thesis reveals that anthropogenic climate change increased the likelihood of elevated wildfire risk over southeast Australia during the 2008-2009 fire season. Furthermore, the influence of anthropogenic climate change on wildfire risk is found to be greater in spring than summer. Through a series of further modelling experiments, this thesis also demonstrates a novel approach for separating the influence of ENSO and anthropogenic climate change within the context of an attribution study. Across southeast Australia, the increase in wildfire risk due to a change in ENSO phase (from La Nina to El Nino conditions) was identified to be much greater than the increase attributed to anthropogenic climate change. This was largely due to the strong increase in drought factor, and decrease in relative humidity, from La Nina to El Nino conditions.
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ItemThe response of nonlinear feedbacks associated with the Indian Ocean Dipole to global warming( 2015)The Indian Ocean Dipole (IOD) is an interannual mode of variability in the tropical Indian Ocean which has the potential to severely impact the surrounding countries. A prominent feature of the IOD is its positive skewness, where positive IOD events tend to be larger than negative events resulting in stronger impacts. Models from the Coupled Model Intercomparison Project phase 5 (CMIP5) show a threefold increase in the frequency of extreme events by 2100. However, the contribution of feedbacks associated with the IOD to this skewness and how they respond to increasing greenhouse gases are not well understood. The nonlinearity of four feedbacks (the Bjerknes feedback, sea surface temperature (SST)-cloud-radiation feedback, wind-evaporation-SST feedback, and nonlinear dynamic heating) was investigated and their responses to a warmer climate examined using CMIP5 models. A single model was first used to examine the nonlinearity of the four feedbacks, allowing the cause and effect of each feedback process to be determined. Following this, the analysis was applied to a multi-model ensemble to determine the cause of positive IOD skewness. The role of nonlinear dynamic heating in increasing the frequency of extreme events was then examined. This provides a better understanding of IOD dynamics and future behaviour. The positive Bjerknes feedback controls IOD skewness through the thermocline feedback. In a warmer climate, models show that the skewness of the IOD weakens and the asymmetry of the thermocline feedback displays a significant relationship with this reduced skewness. This decreased asymmetry occurs due to the mean state change of the tropical Indian Ocean where the Walker circulation exhibits weakening. The weaker mean westerly winds allow the climatological thermocline to shoal in the east, reducing the asymmetry of the thermocline feedback and thereby IOD skewness. The other feedbacks do not show as strong a relationship with IOD skewness. The negative SST-cloud-radiation feedback shows stronger damping of positive IODs and this is detrimental for IOD skewness. Under greenhouse warming there appears to be a shift amongst most models, with weaker damping of positive IODs and stronger damping of negative IODs. This is unfavourable for the reduction in skewness that occurs under increasing greenhouse gases. The positive wind-evaporation-SST feedback is not well simulated in coupled models. Therefore the relationship between IOD skewness and the wind-evaporation-SST feedback is uncertain and there does not appear to be a significant change in the asymmetry of this feedback in a warmer climate. Nonlinear dynamic heating involves advection of heat by oceanic currents, reinforcing positive IODs but damping negative IOD events. Amongst the models analysed, this process does not appear to contribute significantly to IOD skewness; however it does play an important role in the generation of extreme events and the projected increase. Nonlinear zonal and vertical advection during moderate events becomes more extreme-like due to the mean state change of the tropical Indian Ocean. This means that conditions during future moderate positive IOD events are similar to present-day extreme events, facilitating the projected increase of extreme positive IODs in a warmer climate.
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ItemHydrological extremes and consequences of climate change( 2013)In the design of infrastructure, risk has been – and often still is – assessed on the basis of long-term averages. Information on variation in hydrological extremes is required as the basis for informed decision-making, preparedness and possible adaptation. Long-term trends are fairly well understood for temperature but less well for precipitation. While climate models have become sophisticated tools for projecting future changes in our climate, their ability to replicate observed variations in precipitation is limited and it is therefore prudent to complement climate models through analysis of historical observations. Design rainfall is used as one of the required inputs for hydrological models in the design of structures such as dams and bridges. Design rainfall estimates are supplied in form of intensity-frequency-duration curves. Rainfall frequency analysis is almost invariably based on the assumption of a stationary climate. Sub-daily durations are of particular interest for urban applications. This thesis was strongly driven by the motivation to provide guidance to decision makers who have to account for non-stationarity in rainfall extremes. Non-stationarity in rainfall extremes comes about as a conflation of climate change and climate variability. Unlike for temperature extremes, rainfall extremes for Australia as a whole exhibit no clear increase or decrease in intensity over time but strong association with the El Niño-Southern Oscillation (ENSO). This has implications for the choice of suitable analysis techniques, e.g. sophisticated non-parametric techniques. Depending on the planning horizons both climate change and climate variability may have to be accounted for. The association of rainfall extremes with ENSO leads to an opportunity to develop statistical models to support decision-making on shorter time scales. Analysis of seasonality in frequency and magnitude of rainfall extremes revealed considerable variation across a set of sites in the southeast of Australia, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. The strongest signal for an increase in extreme precipitation is found for short durations. Changes in rainfall extremes come about through a combination of changes in thermodynamical and dynamical variables. To assess large-scale changes in circulation, a classification technique (self-organising maps, SOM) was applied and synoptic types were identified. Rainfall extremes were then related to the synoptic type under which they occurred, to assess observed changes in the frequency of rainfall extremes. Rainfall extremes are typically preceded by conditions that are much wetter (both in absolute and relative terms) and warmer than the climatological average. These anomalies tend to be larger for shorter durations, and for rarer events. Given that increase in humidity exhibits strong regional variability and that it may be counteracted by changes in dynamics, it appears simplistic to state categorically that climate change will lead to an increase in extreme rainfall events and observed trends in rainfall extremes show a picture that is more complex. In summary, the combination of changes in thermodynamic and dynamic variables will define the change in frequency and intensity of rainfall extremes. The factors that are most relevant for the effect of climate change on rainfall extremes depend on geographical location.