Climatic Background to Past and Future Floods in Australia
Section snippets
Summary
Rainfall variability in Australia is generally among the highest in the world, largely due to the dominant influence of the El Niño–Southern Oscillation. Australian climate has been characterized by wet and dry periods with sometimes sudden transitions from one mode to the other. Synoptic explanations and teleconnections are discussed, with an emphasis on the Murray–Darling Basin (MDB). Floods in Australia are generally of two types: local “flash floods” and widespread basin flooding. The
Geographic and Climatic Setting
Humans tend to see floods as water that is out of place, inundating normally dry land used or potentially used by humans. However, floods can also be seen as part of the natural process that deposits silt and nourishment on flood plains and periodically maintains vegetation and wildlife, especially water birds. Floods as a hazard are largely conditioned by the degree of human adaptation to floods. In Australia's early years of European colonization, flood hazards loomed large because of lack of
Interannual Variability of Rainfall
The large variability of Australian rainfall is due to variations in large‐scale influences including the El Niño–Southern Oscillation (ENSO), the latitude of the westerlies and variations in standing wave patterns associated with blocking phenomena (Fig. 2).
The pattern of rainfall variability associated with ENSO is a dominant influence on the frequency and intensity of floods in the MDB, with the center of its influence situated right over the Basin, where it accounts for up to a third of the
The Paleoclimatic Record
At the time of the last glacial maximum, some 20,000 years ago, the MDB region was in general wetter, but most of this was due to the much colder conditions, and rainfall was probably less, with fewer intense rain events (Hesse et al., 2004). There is evidence that the Willandra Lakes and others in the region were full and fresh, with Aboriginal occupation and remains nearby (Bowler 1998, Smith 2005).
However, by the time of the postglacial maximum warming around 8000–6000 years ago, the
The Instrumental Record
Instrumental records of Australian rainfall began in the mid‐nineteenth century. As records became longer, fluctuations in the rainfall record were analyzed (Deacon 1953, Kraus 1954, Gentilli 1971). They all noted a wetter period in the MDB in the late nineteenth century then a drier period in the first half of the twentieth century. Pittock (1975) analyzed twentieth century rainfall records, examining the influence of the high‐pressure belt (and by implication, the latitude of the midlatitude
Recent Trends and Observations
Observational data indicate that the climate of Australia and the MDB has changed considerably since 1950. Both Smith (2004) and Collins and Jones (personal communication, 2005) have used data from the Bureau of Meteorology high‐quality station network to analyze trends in rainfall and temperature for Australia.
Strong contrasts in rainfall trends (Fig. 3A) are evident throughout the country and seasons since 1950. The eastern part of the country, including the MDB, has experienced strong
Climate Change Projections
Regional climate change scenarios, that is, projections of possible or plausible climate changes as a result of the enhanced greenhouse effect, were first mooted in the late 1970s, and the first for the Australian region in the early 1980s (Pittock and Salinger, 1982). These were based on primitive general circulation models of the climate, with grossly simplified oceans, crude analogies with past warm epochs and ensembles of individual warm versus cool years, and elementary theoretical
Conclusions
Floods in the MDB can be due to local severe storms leading to flash flooding, or to heavy rains from much larger‐scale systems leading to basin‐wide great floods which can take months to travel down stream. The Basin is subject to large‐scale rain events from diverse synoptic origins, mainly tropical lows in the summer half year in the northern region and fronts and cutoff lows in the southern sector during the winter half year. Great year‐to‐year variability is associated with fluctuations in
Acknowledgments
Thanks are due to fellow members of the Climate Impacts, Adaptation and Risk group in CSIRO Marine and Atmospheric Research, and in particular to Tony Rafter for help with graphics and Ian Smith for valuable comments. Thanks are also due to Dean Collins and David Jones of the Australian Bureau of Meteorology in Melbourne for data and Figures, Martin Thoms of the University of Canberra for help with Table 1, and Mike Smith of the National Museum of Australia for additional comments.
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