School of Earth Sciences - Theses
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ItemThe effect of statistical wind corrections on global wave forecastsDurrant, Thomas Hawkins ( 2011)The ability to forecast ocean waves relies to a large extent on numerical models. Current third generation wave models have been found by many studies to produce highly accurate forecasts several days in advance. The skill of these models is such that the quality of the wave forecast is highly dependent on errors in the forcing wind field. On global scales, a lack of wind and wave observations has historically hampered efforts to separate large scale systematic error due to inherent wave model deficiencies from that imparted by the forcing winds. The advent of remotely sensed observations from altimetry, and more recently scatterometry, provide high quality observations on the open ocean, allowing the spatial structure of the systematic error in both modelled fields to be quantified. In this study, surface winds from the Australian Community Climate Earth System Simulator (ACCESS), the recently implemented operational atmospheric model at the Australian Bureau of Meteorology, are used as forcing for the WAVE-WATCH III® wave model. A number of global wave hindcasts are performed over a four month period from July to October 2008. The geographical variation of systematic error in the surface winds and resulting modelled Significant Wave Height (Hs) are then assessed using QuikSCAT scatterometer data and Jason-1 and Envisat altimeter data respectively. A negative bias in the modelled Hs is identified over most of the globe. The cause of this bias is determined to be largely due to a negative bias in the ACCESS winds. Subsequent to this finding, a number of means of statistically correcting the winds are explored. A simple correction over the entire domain is found to inadequately account for geographical variation in the wind bias. This is addressed by considering corrections that vary in space. Finally, these spatially varying corrections are extended to vary in time. In an operational environment, the error characteristics of the wind forcing can be expecting to change over time with the evolution of the atmospheric model. This in turn requires any applied correction to be monitored and maintained. Motivated by a desire to avoid this manual maintenance, a self learning correction method is proposed whereby spatially and temporally varying corrections are calculated in real time from a moving window of historical comparisons between observations and preceding forecasts. This technique is shown to effectively remove both global, and regionally varying wind speed biases. Finally, the effect of these wind corrections on the modelled wave field is assessed. Large improvement is demonstrated in the Northern Hemisphere Hs, however, the applied corrections produce a positive bias in the Southern Hemisphere. Overall, it is clear that by correcting the winds, their contribution to the modelled Hs error is reduced, allowing inherent wave model deficiencies to be more confidently isolated.
ItemEstimating uncertainties in future global warming using a simple climate modelBodman, Roger William ( 2011)This research has investigated the sources of uncertainty that apply to global–mean temperature change projections. Uncertainties in climate system processes have led to a wide range of projections for future temperature changes, which are compounded by the range of possible future greenhouse–gas emissions. For example, the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report reported that, by 2100, the global–mean temperature increase relative to 1990 is likely to be in the range 1.1°C to 6.4°C, a result that reflects uncertainties in both future emissions and the response of the climate system. However, such a wide range is not particularly helpful for policy and planning purposes, especially in the absence of probabilities. This research has investigated the reasons for this wide range, assessed the sources of uncertainty and developed a method for producing probabilistic temperature change projections. A simple climate model was selected as the research tool for this investigation, instead of a complex three–dimensional model. The model chosen was the latest version of MAGICC (Model for the Assessment of Greenhouse–gas Induced Climate Change), which represents many of the important processes that determine variations of the Earth’s climate, including radiative forcing, heat uptake in the ocean and the carbon cycle, albeit highly simplified and only for temperature changes at the global scale. One of the features of this research is the development of alternative approaches to constraining the model’s primary climate system and carbon cycle parameters. It was found that indices using land minus ocean and Northern Hemisphere minus Southern Hemisphere temperature anomalies are only weakly correlated with global–mean temperatures, and therefore provide additional independent information that can assist in better estimating some model parameters. A ratio of sea–surface temperature to ocean heat content was also found to have a low correlation to the sea– surface temperatures, creating an alternate measure for constraining ocean parameters. This ratio, as well as the vertical ocean temperature change profile, led to revised estimates for the ocean vertical diffusivity parameter, resulting in a new estimate that is nearly a quarter of the previously standard setting used with the Third and Fourth IPCC assessment report versions of MAGICC. In addition to constraining individual model parameters with targeted observational information, a Bayesian statistical technique, the Monte Carlo Metropolis–Hastings algorithm (MCMH), was applied to constraining groups of model parameters using historical observations. One advantage of the MCMH technique is that it addresses uncertainty that arises from observations, model structure and climate system response. This resulted in probability distributions for the key model parameters which then allowed the production of probabilistic temperature change projections. The carbon cycle was included in the MCMH process, leading to a successful calibration of MAGICC’s key carbon cycle parameters with observations for the first time. The MCMH technique was applied to a number of emissions scenarios, enabling probabilistic estimates to be made of global–mean temperature changes to the end of this century. These show reduced uncertainty ranges for future warming projections, with higher lower bounds for warming due to business–as–usual emissions as compared to the results reported in the IPCC’s Fourth Assessment Report. The upper bound for the likely range is also considerably reduced. For the highest emissions scenario, the SRES A1FI, there is a 50% probability of exceeding 2°C by 2042, with a 73% probability of exceeding 4°C by 2100. Analysis of stabilisation scenarios shows that limiting further increases in global–mean temperature to 2°C above pre-industrial requires massive reductions in anthropogenic greenhouse–gas emissions, to the extent that almost zero CO2 emissions are required by the end of this century. Even then, the temperature increase will peak around mid-century, with the upper bound of the likely range temperature change exceeding 2°C, which then entails the risk of irreversible changes to the climate system.
ItemStratigraphy and sedimentology of Cryogenian carbonates, Flinders Ranges, South AustraliaFromhold, Thomas Alexander ( 2011)The Adelaide Geosyncline of South Australia contains a Neoproterozoic-aged sedimentary succession consisting of a complex accumulation of sedimentary formations and units recording a diverse and unique depositional record. A detailed stratigraphic and sedimentological investigation of the interglacial period within the Cryogenian-aged Umberatana Group of the Northern and Central Flinders Ranges reveals a complex array of sedimentary successions lying between the Sturtian and Marinoan glacial deposits. In the Northern Flinders Ranges a major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a sub-aerial erosion surface with terrestrial and marginal marine sediments directly above the Angepena and Balcanoona formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with the previously defined Sturtian-Marinoan time series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta Reef Complex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation. In the Northern Flinders Ranges the Angepena Formation occurs as a marginal marine red-bed succession consisting of supratidal mudstones which are interbedded with subtidal and intertidal carbonates. The Angepena Formation is interpreted as a coastal mudflat succession that formed as a shoreward, laterally equivalent facies of the extensive carbonate platforms (reefs) of the Balcanoona Formation. Sedimentological and geochemical investigation of the Angepena Formation reveal that the unit contains a diverse accumulation of shallow marine carbonates including ooidal sands, tepee buckled algal mats, intraformational breccia (palaeo-caliche) and fenestral-bearing microbial deposits. The stratigraphic and sedimentological relationship within the interglacial successions of the Umberatana Group of the Northern Flinders Ranges are found to extend well over a hundred kilometres southwards into regions of the Central Flinders Ranges. The post-glacial Sturtian-aged Tapley Hill Formation records a near-identical depositional record to the Tapley Hill Formation of the Northern Flinders Ranges. In the Central regions, the Tapley Hill Formation is overlain by deep-marine carbonates and calcareous shales of the Wockerawirra Dolomite and Sunderland Formations respectively. The base of the Wockerawirra Dolomite is defined by an erosional surface, which is directly correlated to the unconformity found overlying the Tapley Hill Formation in the Northern Flinders Ranges (Sturtian-Marinoan series boundary). This stratigraphic relationship indicates the Wockerawirra Dolomite and Sunderland Formations of the Central Flinders Ranges are direct correlatives of the Yankaninna Formation of the Northern Flinders Ranges. The regionally widespread carbonate platform complexes of the Balcanoona Formation in the Northern Flinders Ranges preserve a unique history of the depositional record within the middle Umberatana Group of the Adelaide Geosyncline. Cessation of reef development coincides with a major regression event situated immediately below the Sturtian-Marinoan boundary. The regional consistency of the stratigraphic features found at the Sturtian-Marinoan boundary (i.e. unconformities) suggests that regional scale mechanisms, such as glacio-eustasy, were probably active during this otherwise ‘interglacial’ succession of the Cryogenian-aged Umberatana Group.
ItemMesoscale circulation and variability of the Indian OceanDivakaran, Prasanth ( 2011)The introduction of satellite altimetry in the 1990's and the increasing sophistication of ocean modelling have seen mesoscale oceanography become an increasingly prominent area of research over the last few decades. Evidence is continuing to emerge that the mesoscale variability does not average to zero over extended periods (i.e. the available observational record which is now multi-decadal) and plays an important role in the general circulation of the ocean, with implications for models from short-range through to climate-scales. This study examines mesoscale variability of the Indian Ocean based on a range of observational and model products including a new type of product, which is based on the merging of observations and high-resolution models in the form of a multi-year ocean reanalysis (BlueLink ReANalysis – BRAN2.1). Due to the computational limitations and the grid resolution of this product, we give special emphasis to the southeast Indian Ocean where the model is eddy-resolving. This study has identified quasi-zonal alignment of the mesoscale variability for surface salinity and temperature in the eddy-resolving model domain. Specifically quasi-zonal alignments are found in the orientation of spatial correlation ellipses. The statistical properties have a 2-3° meridional width that are consistently zonal in the tropics, but in the subtropics tend to incline towards the equator. Similar quasi-zonal alignments are also found in spatial anomalies of mean surface prognostic variables for salinity, temperature, as well as surface density. The statistical properties of the reanalysis for surface temperature are verified using a merged product from multiple satellites of high-resolution surface temperature observations from AVHRR. The seasonal variations of the time-mean surface quasi-zonal alignment in physical properties and its link to the coastal current systems of the southeast Indian Ocean are detailed in this study. Consistent quasi-zonal or “arterial” ocean currents of the southeast Indian Ocean are revealed using a colorwheel visualisation tool. In particular, we have identified five eastward jets in the broad surface geostrophic flow regime of the southeast Indian Ocean. Mean meridional sections of ocean currents show that the zonally coherent mean features seen at the surface extend into the abyssal ocean though decline in magnitude with depth; and are similarly quasi-zonal. The seasonality of the surface layer eastward transports in the southeast Indian Ocean is found to be positively correlated with the phase of the Leeuwin Current. Mean westward flows are seen at mid-depth with greater arterial structure. Mesoscale eddies are found to follow regularly along the seasonally persistent mean westward feature connecting the Naturaliste and Broken Plateau. Time-varying properties of quasi-zonal structures in the southeast Indian Ocean show equatorward meridional drift between 25°S-15°S and poleward meridional movements are noted between the equator and 14°S. Meridional displacements of quasi-zonal features in the southeast Indian Ocean are found to correspond with the sign and strength of the mean meridional velocity. Quasi-zonal signatures are found to be also present in the 10-year mean spatial anomalies of the overlying wind-stress field observed from the QuikSCAT satellite observations. Statistical relationships of quasi-zonal features in wind-stress and observed sea surface temperature are derived.
ItemTropical cyclone characteristics in the Fiji regionChand, Savin Suvnesh ( 2011)Tropical cyclones (TCs) are among the most extreme meteorological events, often having devastating impacts on life and property in the southwestern tropical Pacific islands such as Fiji, Samoa and Tonga (the FST region). However, the FST region has garnered little attention in the scientific literature regarding many aspects of TC behaviour when compared with other cyclone basins of the world. In this thesis, we first examine the spatio-temporal variability of TC genesis position, track and intensity due to the two major modes of natural climate variability in this region – the El Niño Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO) phenomena. Statistical forecast models are also formulated using sophisticated methodologies to provide forewarning of TC strike in the FST region. At interannual timescales, TC genesis position, track and intensity are strongly modulated over the FST region by ENSO. During the El Niño phase, for example, TC genesis is enhanced east of the dateline, extending from north of Fiji to over Samoa. The TCs formed during El Niño years take three characteristic paths depending on their mean genesis locations. In the La Niña phase, fewer TCs are observed compared to the El Niño phase and genesis is more common in the west of the region to about 170°E. The TCs formed during La Niña years are often steered over the Fiji islands and Tonga from the Coral Sea region with relatively little or no threat to Samoa. In addition, TC intensity is also considerably influenced by the ENSO signal. It is found that TCs entering the region poleward of 15°S are able to sustain their intensity for a longer period of time in La Niña years as opposed to TCs entering the region in El Niño years, when they decay more rapidly. Equatorward of 15°S, TCs are more intense during El Niño years than in La Niña years. The intraseasonal modulation of TC activity in the FST region by the MJO is also investigated. Results suggest strong MJO-TC relationships. TC genesis patterns are significantly altered over the FST region with approximately five times more cyclones forming in the active phase than in the inactive phase of the MJO. Because the FST region exhibits a marked ENSO-TC relationship, which is identifiable prior to a cyclone season through the use of appropriate ENSO indices, it possible to predict TC strike well in advance. Two models are developed here to make probabilistics forecasts of TC strike in the FST region up to three months in advance: (i) a model to predict TC counts, and (ii) a model to predict the overall TC activity. While both models have a substantial skill to make predictions during El Niño and La Niña years, the latter is more appropriate to make prediction during ENSO-neutral years. Finally, a new methodology is evaluated here to diagnose the probabilistic potential of any tropical depression in the FST region to develop further into a tropical cyclone within up to 72 hours in advance. Evaluation of this methodology suggests its future application in forecasting probabilistic potential for TC formation in the FST region.