Question Aus summer 2019/2020 fire weather (attribution)


Ski Pass
Jun 19, 2019
I've been turning my mind to some of the climate drivers behind the recent bushfires we've experienced in south eastern Australia. Here is a bit of a synopsis from me:

-As is usual there has been a gradual southwards shift of the fire affected areas, starting in south east Queensland in late Sept 2019, moving through northern NSW in Oct, north of Sydney prior to Xmas then south of Sydney/Snowy Mtns around New Years and now getting into prime Victorian bushfire season.
- Strongest +ve Indian Ocean Dipole in decades (60 yrs, or on record if I remember correctly) leading to dramatically delayed monsoon and a very severe drought in south eastern Australia.
- Sept 2019 saw the occurrence of a Sudden Stratospheric Warming over Antarctica. Stratospheric temps increased from roughly -60 deg C to about zero and the polar vortex weakened. This was an unusual occurrence without too much knowledge about how it would play out I believe. But general expectation of these SSW events was they allowed cold fronts embedded within the Antarctic Circumpolar Trough to "spin out" polewards.
- Unusually snowy Nov-Dec 2019. Three alpine snow falls in November and one in December. Western Tasmania about the only place with +ve rainfall anomalies for last three months of 2019, driven by this frontal activity.
- Summer 18/19 saw record breaking heat across south eastern Australia but very little bushfire activity.
- Summer 19/20 again smashed temp records in NSW/ACT but with much greater fire weather problems.
- Ongoing anthropogenic global warming driven by increasing greenhouse gas emissions.
- 2019 was the hottest and driest year on record for Australia as a whole.

The current summer and the previous one were a useful study in contrasts to my mind. Last summer saw an unrelenting and prolonged but gentle wafting of hot continental air over south eastern Australia, setting temperature records. However summer of 18/19, despite the high temperatures, lacked the "pulsing" associated with the frontal activity of this summer and consequently saw negligible fire activity. Spring and summer of 19/20 saw reasonably consistent levels of this frontal pulsing - causing widespread westerly winds over south eastern Australia and a general failure of spring and early summer rainfall. As the frontal activity has continued into December it has brought about the bad fire weather conditions we have seen (New Years Eve 2019, 04 Jan 2020, 10 Jan 2020).

A few interesting retrospective GCM modelling studies could be done here to try and unpack the relative roles of the +ve IOD/SSW/global warming influences. I'm quite curious to know whether the SSW is still judged to be "playing out" in terms of affecting the frontal activity over south eastern Australia.

And of course we've got the general global "+1 to +2 degrees C" overlay of anthropogenic greenhouse gas emissions on top of everything. One important question for me though is is whether the "+1 to +2 degrees C" general global increase is actually manifesting in something larger over Australia?

Thoughts on this from the crowd?
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One of Us
Ski Pass
Jun 21, 2003
Yackandandah NE Victoria
Just a weather observer but definitely the difference in the last 2 years was the negative SAM this year being so strong and persistent as you said bringing hot windy frontal changes and fire danger.
Its a wonderful thing in winter/ spring for skiing but not in summer.
The hottest 2 summers I have experienced in NE Vic thanks to a cloudless interior +positive IOD and it didnt produce much winter rain. Cant comment about SSW.
Statistics seem to show that Jan 1939 was hotter on average at Rutherglen.
So we were in a bad cycle and maybe climate change is slowly aggravating the problem.
The earth in terms of thousands of years is in a warm period between ice ages but it seems runaway cc could change things in 10 to 100 years making life in Australia very unpleasant.
i would think Australia has the most to lose from cc [ maybe California , Kiribati too] due to our dependence on agriculture and tourism and most of the population concentrated in a few low lying coastal cities .
This is causing those of us living in bushy environments to question our commitment , I think for peace of mind and the greater good , lots of gum trees near buildings will be lopped severely + kept that way with growth retardants if practical.
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A disciple of the blessed avi giraffe
Ski Pass
Jul 13, 1998
Middle Oz
I don’t think it’s about the details.

It’s about how long it’s been hot & dry for.

As the more direct causes of long hot & dry there are some good & interesting ideas above.
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