How 'unprecedented' early season heat baked Australia

How 'unprecedented' early season heat baked Australia


Sydney is experiencing severe dry weather following months of very little rain. Photo of Gladesville park in Sydney. Photo: Steven Siewert Oct, 3, 2017

Sydney is experiencing severe dry weather following months of very little rain. Photo of Gladesville park in Sydney. Photo: Steven Siewert Oct, 3, 2017

Aa

Last month's early-season bursts of heat sent temperatures of some parts of eastern Australia a full 16 degrees above average, new analysis by the Bureau of Meteorology shows.

Aa
NSW RFS crews make the most of cooler temperatures tonight and tidy up within containment lines at an old hazard reduction in Bowen Mountain. 13th September 2017, Photo: Wolter Peeters, The Sydney Morning Herald.

NSW RFS crews make the most of cooler temperatures tonight and tidy up within containment lines at an old hazard reduction in Bowen Mountain. 13th September 2017, Photo: Wolter Peeters, The Sydney Morning Herald.

Last month's early-season bursts of heat sent temperatures of some parts of eastern Australia a full 16 degrees above average, new analysis by the Bureau of Meteorology shows.

In a Special Climate Statement examining two pulses of extreme warmth that seared large areas of inland Australia from Victoria to Queensland, the bureau identified the many national and statewide September records that tumbled.

During the first of those events, which affected NSW on September 23-24, all but three inland sites set record hot days for September.

On September 23 alone, 62 per cent of NSW had its hottest September day on record. A second heat burst set records in some northern parts of the state that had only been set a few days earlier.

"Hot, dry northerly winds ahead of [a] trough and cold front contributed to New South Wales, southern Queensland, and areas in neighbouring States experiencing unprecedented hot weather for this time of the year," the bureau said in its report.

While associated with large high pressure systems, the bureau noted global warming had pushed up background temperatures by about a degree over the past century for Australia and the planet as a whole, making the occurrence of such extreme events more likely.

"Studies undertaken by the Bureau and other scientific institutions have shown that climate change has contributed to the severity and frequency of recent heat events, including spring warmth," it said.

The special report comes a day after new research showed Australian cities such as Melbourne and Sydney need to prepare for summer days when the temperature climbs to 50 degrees in decades to come - even if nations kept to Paris climate pledges to keep global warming to less than 2 degrees.

Big dry impact

The bureau's report also noted that unusually dry conditions contributed to the recent abnormal heat.

Both Sydney and NSW, for instance, posted their driest Septembers on record last month. Statewide, rainfall was just one-seventh of the usual amount for the month, extending a worsening dry spell that has lasted about three months.

Sydney, meanwhile, is on track to break the record for the most days in a row with less than 2 millimetres of rain.

The current record, spanning August-October 1989, stands at 64 days, a record likely to be match on Saturday and broken on Sunday, if current forecasts are correct, Blair Trewin, the bureau's senior climatologist, said.

Hot times

The recent significant heat emerged on September 21, with almost all the country recording above-average temperatures, the bureau said.

Australia as a whole recorded its warmest September day on record the following day, with a mean maximum temperature on 33.47 degrees. That was more than 6 degrees warmer than the September average, and eclipsed the previous September record of 33.39 degrees set on September 30, 1998.

For NSW, though, the biggest heat spike came on September 23, with a large inland area reaching 16 degrees above average during the day. (See chart below.)

Apart from setting NSW's first September day with a maximum of 40 degrees, the statewide average maximum of 35.81 degrees smashed the previous record set in 2003 by 1.61 degrees.

An odd quirk in the heat was the range during the day, with some areas warming 30 degrees from their overnight minimums.

Canberra Airport, for instance, ranged from 1.3-30.2 degrees during the day day, exceeding the previous record for any month of the year for the diurnal range.

Victoria posted its hottest September reading on September 23, with Mildura clocking a 37.7 reading.

For Queensland, the peak of the heat hit on September 27 when 36 per cent of the state had its hottest September day.

The state's mean maximum temperature that day reached was 37.79 degrees, eclipsing the previous September record of 37.43 degrees set in 1988.

Drought widens

The ongoing lack of rain in many regions has meant less cloud cover and available soil moisture to be evaporated, helping to drive daytime temperatures higher.

The bureau's latest drought statement, out on Wednesday, showed rainfall deficiencies are intensifying in inland regions, particularly in NSW. (See bureau chart below.)

There is some hope for better rainfall in the weeks ahead, including signs that the Pacific is tilting towards La Nina conditions.

During La Nina events, eastern Australia tends to have above-average rainfall although cyclones also tend to increase in frequency in the Australian region.

Before the change, though, farmers and fire fighters are in for an anxious few weeks if not longer.

As the bureau's special report noted, dangerous fire conditions emerged during the recent heatwaves, including during the shorter second wave at the end of September.

According to the McArthur Forest Fire Danger Index, values exceeded extreme levels at many sites across the country from September 22-27, the bureau said.

As the map below shows, most of the nation had fire danger index ratings very much above average at the end of September.

The story How 'unprecedented' early season heat baked Australia first appeared on The Sydney Morning Herald.

Aa

From the front page

Sponsored by