Issued 4 February 2009
Long-term rainfall deficiencies persist in southeastern Australia
Although rainfall was below to very much below average across most of southeastern Australia during January 2009, above to very much above average rainfall over most of Australia during November and December 2008 has resulted in a significant reduction in the total area of short-term rainfall deficiencies. Drought periods of less than 12 months generally show only small and isolated areas with rainfall totals in the lowest 10% of historical records.
13-month rainfall deficiencies
For the 13-month period from January 2008 to January 2009, above average rainfall was recorded across most of the tropics, along with the west and southwest of WA. Rainfall was generally average to below average across the remainder of the country. For this period, areas of serious to severe rainfall deficiencies are evident in southern and western Victoria, eastern Tasmania, near Hobart, and in areas of southeast SA and central Australia.
The 2007/08 La Niña resulted in a wet start to the 13-month period across the Top End and most of eastern Australia. However, during autumn 2008 the La Niña weakened, coinciding with the failure of autumn rains in many areas. When averaged across the southern half of the country it was the second driest autumn since records began in 1900. Winter rains were mixed, while spring started with well below average rains across most of the southeast. The dry start to spring severley stressed farming activity and water supplies in the region.
20-month rainfall deficiencies
The rainfall deficiencies map for the 20-month period from June 2007 to January 2009 shows areas of serious to severe deficiencies spread across southeast and central Australia. A small area of lowest on record is evident on the Eyre Peninsula in SA. Over the 20-month period the Top End and much of eastern Queensland and northeast NSW had some benefit from above average rainfall associated with the 2007/08 La Niña. Tropical rainfall has also been generally above average through the 2008/09 wet season. However, in southeastern and central Australia, apart from wet November and December periods in both 2007 and 2008, rainfall was generally below to very much below average.
The Indian Ocean Dipole (IOD) is a pattern of sea surface variability across the Indian Ocean that is associated with lower than normal rainfall over central and southern Australia when it is in a positive phase (see IOD). Both 2007 and 2008 were consecutive years of sustained positive IOD, which partly explains rainfall deficits through the 20-month period in these areas. The IOD usually has its greatest impacts between June and November.
The deficiencies discussed above have occurred against a backdrop of decade-long rainfall deficits and record high temperatures that have severely stressed water supplies in the east and southwest of the country. The combination of record heat and widespread drought during the past five to ten years over large parts of southern and eastern Australia is without historical precedent and is, at least partly, a result of climate change.