Extreme rainfall

There is recent evidence that extremes of intense precipitation, over various time intervals, are increasing in more places around the globe than not.

Consistent with global studies, an increase in the proportion of heavy rainfall has been detected over Australia. The fraction of Australia receiving a high proportion (greater than the 90th percentile) of annual rainfall from extreme rain days (greater than the 90th percentile for 24 hour rainfall) has been increasing since the 1970s. Significant regional variability exists, with the east coast region experiencing a significant decrease in extreme rain events since 1950. There is also an increase in the fraction of Australia receiving summer (December to February, accumulated) rainfall that is above the 90th percentile.

Detection of changes in heavy rainfall in Australia tends to be sensitive to the indices and thresholds chosen to monitor change over time. The period 2010 through to 2013 has also seen widespread, individual very-heavy rainfall events, particularly through the warmer months of the year. Based on the linear relationship between Southern Oscillation Index (SOI) values and Australian rainfall, the El Niño Southern Oscillation (ENSO) remains the dominant driver of changes in rainfall extremes in Australia. However the extent to which record rainfall totals during the period 2010 to 2011 are due to natural variability is difficult to determine, since global warming can be expected to influence ENSO itself. The La Niña event of 2010-2011 was record breaking as measured by the SOI.

Attribution studies have also found that the warming trend in sea surface temperatures (SSTs) to the north of Australia may have contributed to the magnitude of recent heavy rainfall in 2010-11 in eastern Australia — contributing around 10 to 20 percent of the heavy rainfall anomalies. Another study found that the warm SSTs increased the chances of above average rainfall in eastern Australia in March 2012 by 5-15 %.

For more detail, including the context of other variables and climatic influences, see the Technical Report (Section 4.2.2).