Year

2008

Degree Name

Doctor of Philosophy

Department

School of Earth and Environment Sciences - Faculty of Science

Abstract

This thesis classifies and describes the synoptic weather patterns resulting in floods, investigates the spatial properties of dryland rainfall, and identifies the links between flooding and flow patterns with the Southern Oscillation Index (SOI) and Sea Surface Temperatures (SSTs) in Australian dryland rivers catchments.

Flooding of dryland rivers in the monsoon dominated northern areas, such as in Timor Sea and Gulf of Carpentaria drainage divisions, is exclusively the result of tropical trough/lows, deep tropical lows (monsoon depressions) and tropical cyclones. The Timor Sea division, however, has a larger proportion of floods caused by tropical cyclones (32% of the total) compared to the Gulf of Carpentaria division (7% of the total). In the latter division 80% of the floods are the result of tropical trough/lows and deep tropical lows whereas in the Timor Sea division only 48% of floods are the result of these two synoptic classes. They also cause a high proportion of floods in the Indian Ocean division together with the Gascoyne-Murchison-Greenough region of the central west of Western Australia. Tropical cyclones tend to produce the largest floods in all these divisions.

In the more centrally located regions such as the Lake Eyre division, the northeast Murray-Darling Rivers, the Bulloo, Paroo and Warrego Rivers, and the Greenough, Murchison and Gascoyne Rivers, there are many more types of flood-producing weather patterns compared to the monsoon dominated drainage divisions further north. Flooding can result from at least six different synoptic classes individually, including tropical trough/lows, deep tropical lows, tropical cyclones, frontal systems and easterly dips (continental and offshore). Twenty-nine percent of floods result from combined synoptic classes with 15 different combinations involving every synoptic class. Tropical trough/lows and easterly dips (continental) are dominant. Easterly dips (continental) are significant flood producing weather patterns in these centrally located regions and tend to occur in the winter months in association with favourable upper atmosphere conditions and strengthening high-pressure systems.

In southern areas, such as in the southwest coastal division of Western Australia and the southern Murray-Darling basin, frontal systems and cut-off lows are the dominant flood-producing weather patterns. However, in the latter a larger proportion of floods result from cut-off lows (18%) relative to frontal systems (8%), whilst in the southwest coastal division frontal systems produce a larger proportion of floods (29%) relative to cut-off lows (26%). Cut-off lows were also found to result in a significant number of floods in the Greenough, Murchison and Gascoyne Rivers in central Western Australia. These can often combine with northwest cloudbands in this area, the largest flood on record for the Gascoyne River being the result of such a combination.

In contrast to many other dryland regions of the world, rainfall in dryland Australia (both flooding and storm rainfall totals) is relatively widespread and uniform rather than localized and convective. However, rainfall in the arid Todd River region around Alice Springs is slightly less widespread than rainfall in the semi-arid Thomson River region, particularly for smaller storm-rainfall events. It appears that the smaller rainfall events in more arid areas of Australia are the result of more convective, smaller scale weather patterns that lead to more discrete, localized rainfall totals. In comparison, larger rainfall events that produce flooding are the result of synoptic scale weather patterns such as tropical trough/lows, deep tropical lows or ex-tropical cyclones, and tend to result in relatively widespread and uniform rainfall across the region.

Only north and northeast Australia show a clear, consistent and predictable relationship between the magnitude of partial series flood events and the Southern Oscillation Index (SOI) and Sea Surface Temperature (SST) indices, with correlation coefficients modest but reaching 0.40-0.50. The SST1 anomaly (The SST1 anomaly represents the sea surface temperatures in the eastern equatorial Pacific Ocean) and the SOI appear to be the most reliable indicators. For seasonal correlations in north and northeast Australia, spring, summer and autumn monthly flows are more highly correlated with the SOI and SSTs, whereas in southern Australia winter and spring monthly flows are more highly correlated. This demonstrates that the specific synoptic weather patterns that cause high seasonal monthly flows are related to the SOI and SSTs. In Western Australia autumn and winter seasonal flows are strongly negatively correlated with Indian Ocean SSTs. These correlations are strongest in the northwest and are related to the Indian Ocean dipole.

Over the last 300Ka it has been inferred that the monsoon played a pivotal role during warmer interglacial periods, generating greater runoff throughout much of dryland Australia. Under climate change, where temperatures would be higher, it thought that the monsoon, with its characteristic wet and dry seasons, would affect areas further to the south. Monsoon lows and ex-tropical cyclones would more frequently affect southern areas, whilst mid-latitude weather patterns, such as frontal systems and cut-off lows, would only seriously affect the very southern areas. Further to this the more central dryland regions, such as the northern Murray-Darling Rivers, that currently receive flooding through a large range of weather patterns, would probably become dominated by tropical systems.

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