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(2023)ThesisThe global ocean plays a major role in moderating atmospheric temperature rise, thereby buffering climate change. Amongst the various oceanic regions undergoing warming, the Southern Ocean is a primary heat sink in the climate system. Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are the dominant water masses in the upper Southern Ocean, and play a fundamental role in ocean ventilation and the uptake of heat and carbon into the ocean interior. This thesis focuses on understanding the geographic and seasonal variability in the formation of SAMW and AAIW, as well as the role of SAMW, AAIW, and other mode and intermediate waters in recent global ocean warming, using observationally based hydrography and estimates of mixing strength. Firstly, the mechanisms controlling the volumetric change of SAMW within the mixed layer and in the ocean interior are investigated separately. We find that the seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August) and entrainment (35%), while the interior SAMW formation is controlled by subduction during August-October. The annual mean subduction estimate shows strong regional variability with hotspots of large SAMW subduction, consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Secondly, a volume budget analysis is performed to identify the mechanisms governing the spatial and seasonal variability of AAIW. Firstly, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter variety AAIW and SAMW. The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. Secondly, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing. Lastly, part of AAIW is exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the Antarctic Circumpolar Current (ACC) and northward into the subtropical gyres. Finally, the ocean absorbs >90% of anthropogenic heat in the Earth system. However, it remains unclear how this heat uptake is distributed across water masses. Here we show that ocean heat accumulation during 2010–2020 has more than doubled relative to 1990–2000. Of the total ocean heat uptake, 94% is found in global mode and intermediate water layers that have subsequently warmed and increased in volume. After factoring out volumetric changes, warming of mode and intermediate waters explains ~40% of net global ocean warming, despite occupying just ~16% of the total ocean volume. These water masses in the subtropical Pacific and Atlantic Oceans, as well as in the Southern Ocean, are responsible for a large fraction of total heat uptake, with important implications for ongoing ocean warming, sea-level rise, and climate impacts.
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(2023)ThesisClimate change is typically modelled using sophisticated mathematical models (climate models) of physical processes that range in temporal and spatial scales. Multi-model ensemble means of climate models show better correlation with the observations than any of the models separately. Currently, an open research question is how climate models can be combined to create an ensemble mean in an optimal way. We present a novel stochastic approach based on Markov chains to estimate model weights in order to obtain ensemble means and uncertainty estimations on spatially explicit climate data. The method was compared to existing alternatives by measuring its performance in cross-validation and model-as-truth experiments on a diverse set of public climate datasets. The Markov chain method showed improved performance over those methods when measured by a set of metrics: root mean squared error, climatological monthly root mean squared error, monthly trend bias, interannual variability, uncertainty error etc. The results of this comparative analysis should serve to motivate further studies in applications of Markov chain and other nonlinear methods that address the issues of finding optimal model weight for constructing weighted ensemble means and uncertainty estimations.
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(2023)ThesisTerrestrial ecosystems sequester about one third of anthropogenic greenhouse gas emissions every year, and strongly influence the interannual variability in the growth rate of atmospheric CO2. Ecosystems in semi-arid regions of the Southern Hemisphere have a disproportionately large impact on the year-to-year variability and trend in the net global carbon sink. In these regions, the carbon balance is linked to circulation-driven variations in both precipitation and temperature that in turn are influenced by climate modes of variability, such as the El Nino-Southern Oscillation. Typically, future carbon cycle predictions depend on terrestrial biosphere models (TBMs), and on climate predictions based on simulations by Global Circulation Models (GCMs). However, GCM simulations are associated with large biases in both the representation of climate modes of variability, and in the averages of climate variables, such as temperature and precipitation. Studies have also shown significant uncertainties in the representation of the terrestrial carbon cycle across different TBMs. This thesis explores the degree to which uncertainty in i) climate modes of variability, ii) climate simulations based on GCMs, and iii) terrestrial biosphere models represent a source of uncertainty in simulations of the terrestrial carbon cycle. The overarching goal is to achieve a constrained estimate of the future carbon cycle over Australia. This thesis first investigates whether the expression (or flavour) of El Nino (as distinct from the El Nino-La Nina cycle) affects the interannual carbon cycle variability. Using the dynamic global vegetation model LPJ-GUESS within a synthetic experimental framework, the results show that different expressions of El Nino affect interannual variability in the terrestrial carbon cycle, but the effect on longer timescales is small. This suggests that capturing the characteristics specific to the expression of El Nino may not be critical for robust simulations of the terrestrial carbon cycle on multidecadal timescales. Known as a hotspot for terrestrial carbon cycle variability, and strongly influenced by climate modes of variability, the remainder of this thesis then focuses on Australia as a testbed to study areas of uncertainty in regional carbon cycle projections. At regional scales, climate projections display large biases, which hamper predictive capacity in impact studies. Many methods exist to either remove biases in the climate forcing, or to achieve informed ensemble averages, but it is not obvious whether some methods are preferable to others. Simulations using LPJ-GUESS and climate output from the Coupled Model Intercomparison Project Phase 6 (CMIP6) show that all bias correction methods reduce the bias in simulated carbon cycle to similar degrees but can lead to different vegetation distributions in the individual simulations. Bias corrections do not influence the ensemble average, but do reduce the ensemble uncertainty significantly. Choosing an informed ensemble averaging method, such as a weighted or random forest approach, is preferential to calculating a simple arithmetic ensemble average. However, suitable target datasets for carbon cycle variables covering both the spatial and temporal scales necessary are sparse, limiting the applicability of these methods for future studies. In addition, the representation of the Australian carbon cycle in TBMs, namely those part of the TRENDY v8 ensemble, was analysed. Land-use change is the main driver for discrepancies in the simulated long-term accumulated net carbon balance across TBMs. The TBMs also have different sensitivities to atmospheric carbon dioxide (CO2) concentration, but climate drives the year-to-year variability in the net carbon sink rather than the trend. Further, differences in the timing of simulated phenology and fire dynamics, as well as simulated vegetation carbon, and apparent carbon residence time are associated with differences in simulated or prescribed vegetation cover and process representation. These results highlight the need to evaluate parameter assumptions and the key processes that drive vegetation dynamics, such as phenology, mortality, and fire, in an Australian context to reduce uncertainty across models. Since none of the TBMs investigated clearly outperforms the others, LPJ-GUESS was then taken as the model with which to constrain the Australian carbon cycle. Observed plant traits were prescribed to achieve an improved representation of the vegetation cover in LPJ-GUESS. A comparison between the model and satellite-derived datasets showed reasonable agreement for gross primary productivity and leaf area index. LPJ-GUESS further captured the woody and non-woody cover over Australia. This allowed the model to be used to explore the future terrestrial carbon cycle over Australia. Based on the above findings, this thesis then explores the future Australian terrestrial carbon cycle using the CMIP6 ensemble together with the regionally parametrised LPJ-GUESS. The uncertainty in Australia’s future carbon cycle is strongly linked to biases in the meteorological forcing, and can be significantly reduced via bias correction. However, implementing bias correction methods still leads to an unresolved uncertainty in carbon storage in the vegetation at the end of the century. Variations in carbon residence time, and model sensitivities to CO2, temperature, and precipitation are the key drivers for the discrepancy in simulated carbon stored in vegetation. Reducing this uncertainty will require improved terrestrial biosphere models, but also major improvements in the simulation of regional precipitation by global circulation models. The thesis concludes with suggestions of future work that should help to resolve the large uncertainties in the future carbon stored in vegetation over Australia.
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(2023)ThesisConservation translocations are becoming an increasingly necessary tool to stem the decline of threatened species globally. The greater bilby (Macrotis lagotis) is a nationally threatened species in Australia. While bilby translocations are expected to contribute to the species’ persistence, the scarcity of information on their behaviour and ecology prevents informed-management. By intensively studying a population of bilbies both prior to, and following reintroduction, and subsequent reinforcements to a fenced sanctuary, I aimed to (1) advance knowledge of bilby behaviour and examine behaviours potentially relevant to fitness (i.e. survival and breeding success), (2) improve ecological knowledge of bilbies within understudied (temperate) climates, and (3) use this knowledge to suggest and develop effective tools for their conservation. Chapter 1 describes the current state of research in applied conservation research, and how increased behavioural data could help address some of the current knowledge gaps for bilby conservation. In Chapter 2, I examined patterns in bilby resource selection, finding that selection changed between seasons and years due to the environmental conditions and resources available. I also found that resource requirements are likely to be behavioural-state dependent and sex-specific. In Chapter 3, I constructed social networks to examine nocturnal proximity of bilbies and concurrent burrow sharing and found that associations were non-random. Expanding on this, in Chapter 4, I found that burrow sharing was likely to help describe breeding strategies, as males strongly avoided other males, and mixed-sex dyads exhibited kin-avoidance when mate choice was more limited. In Chapter 5, I developed a test to screen personality traits in bilbies, finding links between male response to handling and relative breeding success post-release. Lastly, in Chapter 6, I described a method to collect detailed movement data on the bilby, and discussed some of the practical and animal welfare constraints for its application. My thesis provides new insights into the behavioural ecology of the bilby with potential implications for future management of the species. With further translocations necessary for long-term persistence of the bilby, this research is highly relevant to current and future management of this ecologically important species, with potential applications to other similarly at-risk species.
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(2023)ThesisMonsoon Asia (MA) is the world’s most populous continent with high vulnerability to extreme weather, notably precipitation extremes. Due to sparse observations and limited modelling, past trends in extreme precipitation and future projections over many parts of the region are not well known. This thesis investigates regional precipitation (e.g., distribution, seasonality, variability, extremes, and past and future changes) over different sub-regions of MA using observations and climate models. The intercomparison of multiple observational precipitation products reveals the high temporal and spatial consistency in precipitation extremes in high-station density areas (e.g., Japan, India) and the large inter-product spread over limited-station regions (e.g., Southeast Asia - SEA). Products with high consistency in trends and variability for individual sub-regions of MA are selected to evaluate the performance of an ensemble of high-resolution regional climate models (RCMs) from the Coordinated Regional Downscaling Experiment (CORDEX). Rainfall patterns are investigated using various aspects of the precipitation distribution in CORDEX-SEA RCMs and compared with their forcing global climate models (GCMs). We find that RCMs are wetter and generally not as close to observations as their forcing GCMs. The more intense precipitation in RCMs is associated with 1) an increased supply of moisture from both local and large-scale sources and 2) a widespread increase in convective precipitation across the region. Our findings suggest that the RCM setup (e.g., parameterization scheme) is more important than the choice of GCM. Given the range of RCM performance, two sub-ensembles representing “better” and “worse” performing models are selected and their respective projections are compared to assess how past model performance can affect future projections. The thesis results highlight that careful model evaluation is needed and could lead to more well-informed future projections at the regional and seasonal scales relevant to the complex region of SEA. The framework and method developed in this thesis enable many avenues of research, such as understanding biases in regional and global models and how these could impact future projections. Ultimately, our understanding of regional rainfall patterns is improved, which in turn helps to better inform modelling strategies and the risks associated with future changes in precipitation under a warmer climate.
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(2023)ThesisRising atmospheric CO2 concentration is one of the major drivers of climate change. To provide effective mitigation policies to curb these emissions, a thorough understanding of past changes in the carbon cycle is required. Decades of research on understanding carbon cycle changes during the last glacial cycle have put forward several processes impacting the concentration of atmospheric CO2. One of these processes is changes in aeolian iron flux into the Southern Ocean. Marine plankton fix dissolved inorganic carbon (DIC) during photosynthesis and transfer the fixed carbon to the deep ocean. DIC removal from the surface lowers the surface ocean partial pressure of CO2, which leads to carbon drawdown from the atmosphere. As the Southern Ocean is a high-nutrient-low-chlorophyll region, the increase in iron input can impact Southern Ocean marine ecosystems, by increasing export production, and therefore decreasing surface DIC. This thesis aims to investigate the responses of Southern Ocean marine ecosystems to changes in iron flux, and their impact on ocean biogeochemistry and atmospheric CO2 during the last glacial period. For this, I use a recently developed complex ecosystem model, which includes four different classes of phytoplankton functional types. Chapter 2 of this thesis is the first study to use this complex ecosystem model and document the competitive dynamics between different plankton species for light and nutrient availability under Last Glacial Maximum (LGM) climate boundary conditions (∼21 thousand years ago, 21 ka). Chapter 2 further assesses the impact of enhanced aeolian iron input on ecosystems. This study shows that lower sea surface temperatures and greater sea ice cover during the LGM causes a 2.4% reduction in Southern Ocean export production. However, a 78% increase in iron supply with a weaker ventilation in the Weddell Sea, increases diatoms and coccolithophores in the Southern Ocean, leading to a 4.4% higher carbon export at the LGM compared to pre-industrial (PI). Proxy records indicate a ∼32 ppm decrease in CO2 around ∼70 ka. Previous modelling studies have indicated a possible decline of 5 to 28 ppm in atmospheric CO2 driven by enhanced iron fertilization under PI and LGM boundary conditions. I constrain this contribution in chapter 3, by performing a series of sensitivity experiments under 70 ka climate boundary conditions taking into account the uncertainty associated with iron solubility in the ocean. I find that the CO2 change follows an exponential decay relationship with increasing iron flux due to saturation of biological pump at high iron values. Based on this, I suggest that enhanced iron input at 70 ka most likely led to a 9 to 11 ppm CO2 decrease with a maximum decrease of 21 ppm. Iron fertilisation could thus provide a 28 to 34% contribution to the total observed CO2 decline at 70 ka. Finally, in chapter 4, I include a unique approach to understand the processes leading to the abrupt 15 to 20 ppm increase in atmospheric CO2 during Heinrich Stadials, which are associated with a near collapse of the Atlantic Meridional Overturning Circulation (AMOC), a sudden decrease in Greenland temperature and warming in the Southern Ocean. Previous modelling studies have investigated the role of the ocean circulation in driving changes in atmospheric CO2 concentration during these abrupt events, while the role of reduced aeolian iron input during Heinrich stadials remained poorly constrained. I find that reduced iron fertilization combined with an AMOC shutdown could lead to a 7 ppm CO2 increase, 6 ppm of which is due to iron fertilisation. The research presented in this thesis improves our understanding of the impact of iron fertilization on Southern Ocean ecosystems, and on the global carbon cycle, particularly in the context of the last glacial period. This work also elucidates the importance of including changes in iron input to the ocean when investigating changes in atmospheric CO2 during abrupt climate change.
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(2023)ThesisBiodiversity loss is occurring globally with intensifying human-driven land-use change. Effective conservation planning with increasing anthropogenic pressure requires knowledge of: (1) species’ responses to habitat modification, including their ability to persist in, and their relative susceptibility to, human modified environments, and (2) species’ traits that facilitate persistence in these landscapes. This information is critical for predicting extinction risk and mitigating species declines. As one of the most threatened yet understudied vertebrate taxa, amphibians are promising candidates to broaden understanding of biodiversity responses to habitat change and identify conservation options. This thesis examines species’ responses to anthropogenic habitat modification and explores how species’ ecology, behaviour and life history may influence persistence in these environments, with a focus on frogs. I use a multi-scale approach, combining taxonomically broad analyses of citizen science data (landscape scale) with targeted species-specific fieldwork (local and regional scale). Chapter 1 reviews current knowledge of species’ responses to habitat modification, highlights research priorities and outlines my research approach. Chapter 2 integrates continental citizen science data with a global human modification index to quantify frog species’ tolerance of habitat modification and to identify broad trait-based associations. Chapter 3 uses this data to assess if and how habitat modification influences frog breeding phenology and call acoustics. Chapter 4 explores underexamined genetic threats to species in modified landscapes, with evidence from two sympatric frog species, the threatened Booroolong frog (Litoria booroolongensis) and the non-declining eastern stony creek frog (L. wilcoxii). Chapter 5 considers how species’ behaviours may influence vulnerability to habitat modification, drawing on movement and habitat use data gained from radiotracking these two species. Chapter 6 synthesises these findings, discusses implications for conservation management and outlines future research avenues. Overall, this thesis offers insight into why some species persist whilst others decline in modified landscapes, and the capacity of frogs to cope with habitat modification. I show how integrating big data with field studies can improve knowledge of species’ traits and species-environment relationships at multiple scales, with broad conservation implications.
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(2023)ThesisWhen two or more extreme events co-occur, we call them compound events. There is growing interest in such events because many types of extremes are becoming more frequent - giving rise to more compound events - and because compound events may have more impact than the sum of parts. The overall aim of this thesis is to provide a statistical characterisation and physical understanding of the mechanisms driving compound terrestrial and marine heat extremes. First, the relationship between adjacent coastal marine and THWs around Australia is quantified using observation and reanalysis data. A significant increase in the number of THW days is found in the presence of adjacent co-occurring marine heatwaves along the coastal belt of Australia. Moreover, synoptic conditions driving THWs, at three locations around Australia, are conducive to warming the ocean, which would increase the likelihood of a marine heatwave. However, the prior ocean state must also be conducive to reach marine heatwave conditions. These findings suggest that co-occurring terrestrial and marine heatwaves co-occur more frequently than chance would dictate, and that large scale synoptics may be favourable for both coastal terrestrial and marine heatwaves. Following this, the focus was shifted to a specific region to isolate the influence of a single marine heatwave event on terrestrial warming. The ocean off south-eastern Australia has recently experienced a string of major marine heatwaves including the 2015/16 Tasman Sea marine heatwave. This was an unprecedented event that lasted for over eight months. It is possible that marine heatwaves can enhance temperatures of adjacent coastal areas through mechanisms such as advection and local radiative changes. This hypothesis was tested by carrying out a large ensemble of simulations forced by the 2015/16 Tasman Sea sea surface temperature anomalies using a regional atmosphere model, and examining how this influenced land temperatures and local circulation. It was found that sea surface temperature anomalies drive seasonally dependent significant warming (up to 1°C), primarily in coastal southeast Australia, Tasmania, and New Zealand, from September to February. Most of the coastal warming is consistent with the advection of anomalous heat from the marine heatwave regions as opposed to local radiative effects or adiabatic heating. Terrestrial heat extremes are most dangerous to human health when combined with high relative humidity levels. Extreme heat stress from high heat and humidity, is increasingly becoming an issue in South Asia, the Middle East, and coastal southwest North America. These three regions are near high sea surface temperatures. While the initial components of the thesis focused on extreme temperature events, this final study investigated the combined effect of extreme heat during marine heatwaves. By using observation and reanalyses data for regions surrounding Mumbai, Karachi, Kuwait, Doha, Miami and New Orleans, a higher number of extreme heat stress days was found during marine heatwaves, compared to normal ocean conditions. Typically, onshore winds advect hot and humid air from marine heatwave regions to the adjacent coastal cities which can exacerbate the heat stress at these locations. It was also found that temperature contributes most to the heat stress metric over land while relative humidity tends to play a larger role over the ocean. These results show that regions which are already susceptible to high heat stress, are likely to experience more extreme heat stress events during a marine heatwave. This thesis has helped improve our understanding of compound extreme heat events including co-occurring marine/ THWs and marine heatwaves with extreme heat stress conditions. Compound extreme events are much less common than extremes in single variables and so have been harder to study. With anthropogenic global warming, these events will become more frequent making the study of compound events more relevant.
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(2023)ThesisAustralia’s climate is highly variable, which poses considerable challenges for climate assessments over regional and local scales. This thesis focuses on the effects of internal variability on temperature extremes over Australia, including historical evaluation, future projections, and attribution. Conclusions are mainly based on state-of-the-art climate models in Phase 6 of the Coupled Model Intercomparison Project (CMIP6), in which several initial-condition large ensembles (LEs) are analysed to assess the effects of internal variability. This thesis investigates whether model estimates of internal variability are robust and how this can affect future risk assessments. First, evaluation of temperature extremes indicates modest improvement in CMIP6 compared to previous generation models (CMIP5). Model ranges for some extreme indices in CMIP6 tend to be narrower, implying a reduction in model uncertainty. For different LEs, model differences in internal variability indicate that the metrics used to examine model performance in simulating climatology are not suitable to examine intrinsic variability. Second, in addition to projected changes, the time of emergence (TOE) concept is introduced which indicates when the trend in a climate variable emerges above the “noise” of natural climate variability. A “warm-gets-warmer” pattern exists for some extremes over Australia and tropical regions usually show the largest warming. In comparison with CMIP5, partly due to the higher climate sensitivity in some models, CMIP6 shows stronger warming and exhibits earlier TOE. The results also signify the role of internal variability in the noise, further influencing the spread of TOE. Third, compared to hot extremes, it is found that the long-term changes of cold extremes are less evident to changes in anthropogenic aerosols. Moreover, based on a non-stationary generalised extreme value distribution, results indicate that greenhouse gases (anthropogenic aerosols) can increase (decrease) the occurrence probability of hot extremes while the converse is true for cold extremes. Uncertainty in attribution statements can be further reduced by better representing internal variability. To enable more robust climate assessments, associated physical mechanisms and observationally-based Large Ensembles, in which internal variability from observations is sampled to create surrogate realizations, need to be further developed and understood.
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(2023)ThesisSince the 1970s the ocean has absorbed over 90% of the excess heat trapped in the Earth system due to increasing greenhouse gases. However, sparse observations limit our understanding of the processes driving this heat uptake and its regional patterns. In this thesis, three numerical modelling projects demonstrate how ocean warming has played out over the last 50 years, including how it is affected by El Niño-Southern Oscillation (ENSO), the Earth's dominant mode of interannual climate variability. Part 1 of this thesis investigates recent multi-decadal ocean heat content trends basin-by-basin, including what proportion of the total trend is forced by atmospheric surface warming, surface wind changes or both. The analysis reveals that Southern Ocean heat uptake accounts for almost all the planet’s ocean warming since the 1970s, thereby controlling the rate of climate change. This heat uptake is facilitated in almost equal parts by both warming of the atmosphere and changes in the surface winds. An integral part of forecasting ENSO is the analysis of the Pacific warm water volume (WWV), the volume of water above 20°C between 5°S and 5°N of the equator. This is because WWV variations lead ENSO events by 6-8 months. WWV variability is thought to be dominated by adiabatic advection of warm water into and out of the equatorial latitude band. Part 2 uses a complete heat budget to illustrate that WWV changes associated with diabatic processes (surface heat fluxes and vertical mixing) are also important. ENSO impacts remote regions around the globe, including West Antarctica through its atmospheric teleconnections to the Amundsen Sea. Subsurface warming associated with ENSO in this region has the potential to affect basal melting of West Antarctic ice shelves, yet our knowledge of the oceanic ENSO response here remains limited. Part 3 reveals that during El Niño, the Amundsen Sea Low and coastal easterlies in West Antarctica weaken and reduce the poleward Ekman transport of cold waters across the shelf break. Consequently, warm Circumpolar Deep Water (CDW) flows onto the continental shelf to balance this mass deficit. The La Niña shelf circulation response is largely opposite and inhibits cross-shelf upwelling of CDW. This has implications for global sea level rise as basal melting can reduce the buttressing of the ice sheets behind the West Antarctic ice shelves.