Atmosphere Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
Climate Variability and Change (continued)
Extreme rainfall events [A Indicators 1.6 and 1.17]
Extreme rainfall events have become more common in Australia during the 20th century. Increasing trends in extreme rainfall (95th percentile) and total rainfall have occurred for the period November through April. For the months May to October, increases in extreme rainfall have also occurred, except in south-west Western Australia (Suppiah and Hennessy 1998). Nicholls et al. (2000) suggested using the 95th and 90th percentiles as extreme rainfall indices. However, this Report presents results of the changes in the 99th percentile as a measure of extreme daily rainfall, as the authors of this section consider the 99th percentile to be a better representation of extreme rainfall than 95th and 90th percentiles. There are variations in trends among states and territories (Figure 19). Significant increases in extreme rainfall, 20 to 30%, have occurred in New South Wales in autumn and summer, in the Northern Territory in autumn, and in Western Australia in summer. South-west Western Australia has experienced a 15% decrease in heavy rainfall in winter. Annual extreme rainfall events in tropical Australia are closely linked to ENSO variations.
Figure 19: Percentage change in extreme daily rainfall from 1910 to 1995, as defined by the 99th percentile or the top 1%
Source: CSIRO Atmospheric Research
The number of summer days with rainfall totals over 50.8 mm has increased by 1.1% per decade relative to the 1910 to 1996 mean, and a significant increase of 4.6% per decade occurred in coastal regions of New South Wales and Victoria.
Trends in seasonal rainfall intensity are derived from daily rainfall values. A linear trend fitted to extreme values of areal-averaged Australian daily rainfall shows a very weak and insignificant increasing trend (Figure 19), but the actual and smoothed values indicate increasing and decreasing trends on a decadal time scale (Figure 20). In particular, recent decades (from the mid-1970s) indicate a decreasing trend in extremes (Figure 20). However,some extreme events have resulted in floods recently in eastern Australia. There are regional and seasonal variations in the trend. Greatest increases in rainfall intensity have been reported in northern and eastern Australia, while decreases in rainfall intensity have been found in winter in south-west Western Australia, where reduced winter rainfall has occurred in recent decades. In terms of state autumn rainfall intensity, New South Wales (Figure 21) shows the strongest increase over the continent.
Figure 20: Interannual variations in the annual extreme rainfall (99th percentile of daily rainfall) for Australia.
Source: CSIRO Atmospheric Research
Figure 21: Interannual variations in the autumn extreme rainfall (99th percentile of daily rainfall) for New South Wales.
Source: CSIRO Atmospheric Research
Most of the areas that have increases in rainfall intensity also have increases in the frequency of intense rainfall. The frequency is the number of days above the long-term mean of a rainfall intensity threshold (e.g. the 95th percentile). Interannual variations in rainfall intensity and its frequency are associated with ENSO variations. Increased amount and number of days with heavier rainfall have been linked to La Nia events, while reduced rainfall intensity and frequency have been found in El Nio events.
The percentage of Australia experiencing extreme wet conditions shows a slight increase during the 1900s, while the percentage of dry conditions indicates a slight decrease (Figure 22). Long-term trends in these time series are superimposed on strong interannual and interdecadal variations (Plummer et al. 1999). Neither of the trends is statistically significant at the 95% confidence level.
Figure 22: Percentage of Australia experiencing extreme wet conditions (red line) (above 90th percentile of annual total) and extreme dry conditions (teal line) (below 10th percentile) in each year from 1900 to 1999.
Source: BoM
Implications
Floods associated with an increase in heavy rainfall events could have a significant effect on agriculture, industries and infrastructure if this trend continues. Floods associated with the increasing trend in extreme rainfall events have serious implications for coastal management including protecting of coral reefs, mangroves and coastal habitat.