Rainfall across Australia was 43% below average in July, continuing a long-term drying trend consistent with rising atmospheric greenhouse gas emissions, the weather bureau says.
The Bureau of Meteorology’s climate statement for July says the continent was drier than average across most of its south, from Western Australia to the south-west of New South Wales and including most of Victoria. Tasmania recorded its second-driest July on record behind 1957. South Australia had its lowest rainfall for the month since 1997.
Victoria’s east and the south-coast of NSW were exceptions. Both received heavy dumps in the last week of the month as they were hit by an intense low pressure system. Several places in NSW and south-east Queensland had their wettest July day on record between 26-28, and the highest rainfall for the month in more than 20 years.
Despite this, the country as a whole registered its 19th driest July since records began in 1900. Rainfall over southern Australia between April and October has dropped by 15-20% since the 1970s.
Blair Trewin, a senior climatologist, said nine of the past 10 Julys had rain below the long-term average. “We definitely see a long-term downward trend in rainfall in much of southern Australia,” he said.
Trewin said it was associated with a strengthening of the subtropical ridge – a belt of high pressure systems circling the southern hemisphere at midlatitudes – that has increasingly pushed westerlies that used to hit Australia further south.
“There is evidence that a strengthening of the subtropical ridge is something you’d expect to see with the atmospheric forcing [due to greenhouse gases] that’s going on,” Trewin said.
The dry July followed a wetter start to the year that brought much-needed rain after the hottest and driest year on record in 2019 and the horrific 2019-20 bushfire season. Trewin said it meant 2020 was still tracking close to average for total rainfall.
Temperatures in July were high compared to the historical record on average. Maximum daytime temperatures across the continent were 1.54C above the average for the years 1961-1990. It made it the seventh-hottest July on record.
Nights were not quite as warm relative to recent years. Average temperatures across day and night were 0.99C above the long-term, making the month the 15th hottest on record by that measure.
As always, the picture varied across the country. Much of Western Australia, eastern Victoria and NSW and the Cape York Peninsula were hotter than the long-term average, but some places in South Australia and Victoria recorded their lowest July average daily temperature since modern records began.
The hottest temperature of the month was recorded at Noonamah, south-east of Darwin, which hit 36.2C on the 11th. The coldest day was recorded at the snowfields of Thredbo, NSW, which reached only -2.9C on the 3rd. The night was dark and deep at Liawenee, on the banks of Great Lake in central Tasmania, on the 17th, when it fell to a national low of -9.1C.