A record dry and hot autumn for regions
GroundCover™ Issue: 57
By Dr Andrew Watkins and Neil Plummer, National Climate Centre - Bureau of Meteorology
The first five months of 2005 were extremely dry and hot for much of Australia, leading many to label it a truly exceptional short-term drought.
From January to May 2005, Australia received only 167 millimetres of rainfall, the second lowest January to May total (after 1965) since Australia-wide records began in 1900. For the crucial autumn planting season (Figure 1) the national area-average rainfall was 65.7mm, 48 per cent below normal, with record low totals over central and western Victoria, the southern agricultural areas of South Australia and parts of western New South Wales. Overall, Victoria and SA experienced their driest autumn on record. While the GRDC northern zone (69.4mm average -18th driest autumn) bettered the national average, the southern zone was hard hit, with an autumn total of only 23.6mm. This was well below the previous driest autumn, (1912: 38.8mm) and 1902, when 40.6mm fell.
The 106-year average autumn rainfall for the GRDC southern cropping zone is 101.7mm, with a median rainfall of 95.5mm. Wet conditions in the western zone (132.1mm - 17th wettest autumn) compensated, to some degree, for the dry southern area. However, overall the Australian cropping regions experienced their 10th driest autumn, averaging only 68.3mm.
Rainfall deficits were compounded by mean Australian temperatures for January to May which were 1.75°C above normal, 0.57°C higher than the previous January-May record. For autumn, despite cloudless and hence ordinarily cold nights, mean temperatures were 1.6°C above average, making this the hottest autumn since these records began in 1950.
Maximum temperatures (Figure 2), averaged 2.2°C above average, 0.4°C hotter than the previous record in 2002. Daytime temperatures in the GRDC cropping regions were 1.8°C above the mean (previous record +1.6°C; 2002), while the dry southern zone saw record temperatures, 2.1°C above average.
While hot temperatures increased evaporation rates, they may have elevated soil temperatures enough to take advantage of June rains. For much of eastern Australia, June brought one to four times the monthly mean rainfall, removing serious deficiencies in southern Queensland, western NSW and eastern SA.
However many parts of western NSW and southern Queensland remained below 50 per cent of their year-to-June average.
GRDC cropping regions faired well, averaging 82.6mm for the month - the fifth wettest. The parched southern zone recorded 75.4mm, its eighth wettest June, and well above its median of 44mm.
The extremely dry start to 2005 resulted from unusually high pressure over Australia. Not only did this suppress rainfall in the interior, but it also kept fronts south of the continent and hindered significant inland penetration of the Australian monsoon during the summer and early autumn.
The 2005 dry and hot conditions were further hampered by the long-term dry: the 39 months from March 2002 (the start of the 2002-03 El Niño-related drought) to May 2005 was eastern Australia"s second driest such period after the "Federation" drought of 1900-1903.