School of Earth Sciences - Theses
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ItemThe weather and climate of Australia at the Last Glacial Maximum( 2005)The global climate has experienced four glacial cycles in the last 420,000 years, with each cycle characterised by a prolonged period of cooling culminating in maximal glaciation followed by a brief warm period. The most recent period of maximal glaciation is termed the Last Glacial Maximum (LGM) and occurred about 21,000 years ago. We currently live in one of the warm periods. The global climate is changing, and it is becoming more important to understand the extremes of the climate system and how well our modelling capability can capture those extremes. There has been a modelling intercomparison project established to examine how global general circulation models compare in simulating past climates, including the LGM. Analysis and comparison of these model results has been presented for many parts of the globe, but there has not been a comparison of the different model results over the Australian region. This thesis aims to fill that gap and explore the simulated LGM weather and climate of Australia and its drivers in more detail. Comparison with proxy evidence is also undertaken, and inconsistencies seen in the literature addressed. The Australian climate at the LGM was believed to be generally cooler, drier and possibly windier from proxy evidence in the literature. In the comparison done here the mean temperature and precipitation fields from most models show cooler and drier conditions, with some seasonal variability, but there are some strong outliers. It was found that the differences were not dependent on model resolution, but that the surface parameterisations were highly important for these fields. The shifts in the circulation were examined both in the model results and with a study of the non-linear link between the wind, surface moisture and dunes, which are a proxy for past winds. All the models simulate a southward shift in the westerlies in the Australian region. This is strongly driven byte prescribed sea-surface temperatures. Australia's current wind regime is conducive to dune building. However, the binding effect of soil moisture (or vegetation) is strong enough to limit present day movement, whereas in the drier climate at the LGM there was a capacity for sand movement. The analysis of dune orientations did not produce conclusive evidence for how the westerlies might have shifted at the LGM. An apparent enigma in the proxy evidence at the LGM is the high lake levels in Australia’s south east, while most inland lakes were dry. Previous authors believed that the precipitation was still low, but the high lake levels were driven by lowered potential evaporation. The hydrological cycle was generally depressed in the LGM simulations, but the potential for evaporation remained high. Thus an alternative hypothesis is posed based on increased run off due to a known shift in the vegetation types and a lag in the timing of the run off due to snowmelt. The analysis here shows that our capacity to simulate climates quite different from the present is still developing, but that model results can help explain apparent inconsistencies in the reconstruction of past climates from proxies.
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ItemGlobal changes in synoptic activity with increasing atmospheric CO2( 2005-11)Over the last century, increases in anthropogenic greenhouse gases and global temperature in the atmosphere has drawn our attention to changes in extra tropical cyclones which influence daily weather patterns in the mid and high latitudes and redistribute energy, momentum and moisture across the globe. This study is aimed at examining changes in extra tropical cyclones: observed over the past two decades using the NCEP-DOE reanalysis II data (NCEP2); and simulated in the CSIRO Mark2 atmosphere-ocean coupled general circulation model (GCM) with increasingCO2. Furthermore, we attempt to explore the physical mechanisms driving such changes by modelling idealised experiments with the Melbourne University atmospheric GCM. The Melbourne University cyclone finding and tracking scheme is utilised to detect and track cyclones observed in NCEP2 and simulated in the two models. The study demonstrates significant changes in Southern Hemisphere (SH) cyclone features from 1979-2000. SH cyclones have decreased in their number at the surface but increased at the 500 hPa level. On the other hand, SH cyclone physical features such as intensity, radius and depth have significantly increased over the two decades at the mean sea level and 500 hPa level. Moreover, cyclones became vertically better organized in both hemispheres, and particularly in the SH. The changes in the characteristics of Northern Hemisphere (NH) cyclones were statistically less significant than their SH counterparts in the period of 1979-2000. Results from the coupled climate model simulation with enhanced CO2 suggest general reductions in cyclone frequency and intensity throughout the troposphere between the surface and500 hPa level but increases in cyclone radius and organization of vertical structure. These changes are persistent throughout the entire transient run with increasing CO2 and during a 100 year stabilisation period. It is found in the CSIRO simulation with enhanced CO2 that the geographical changes of cyclone features are similar in both hemispheres and between the surface and 500 hPa level. Furthermore, we conclude that some observed changes in extra tropical cyclone features seem to follow the patterns of simulated changes with increasing CO2 from 1xCO2 to 2xCO2 particularly in the SH. Modelling latitudinal temperature gradient at different levels of the troposphere has revealed that the warming over the tropics at the upper troposphere causes cyclone frequency and depth to increase in the high latitudes but decrease in the mid latitudes. By contrast, the warming over the high latitudes at the lower troposphere results in decreases in the cyclone features in the high latitudes but increases in them in the mid latitudes. Therefore, the warming over the tropics seems to play an important role in the changes in SH summer cyclone frequency and depth appearing in the simulation with enhanced CO2, whereas the warming over both tropics and high latitudes affects the changes in SH winter cyclone features. In the NH, the change in latitudinal temperature gradient seems less influential in the changes of cyclone features than it does in the SH.