The 2008-2009 fire season was one of three since 1970 where more than 2,000 homes were destroyed.
Before each season, the worst affected areas had a prolonged period of below-average rainfall lasting at least 12 months.
Of Australia’s nine most destructive fire seasons, the worst-affected regions had a rainfall deficit across at least six months.
And on most occasions, it was severe and over a prolonged period of at least 18 months.
While drought and dry vegetation are key contributors, bushfires are not inevitable unless ignition takes place, normally from lightning, arson, or negligence.
Whether or not a fire then becomes large and uncontrollable is dependent on the weather.
For fire to spread quickly, a combination of hot temperatures, low humidity and strong winds is normally required.
The graph below shows how the worst bushfire seasons have corresponded with the presence of several influences occurring simultaneously.
Our worst fire seasons have occurred with a background of at least two contributors which enhance fire activity.
Black Summer followed a positive IOD, multi-year drought, negative SAM and decades of climate change.
Black Summer was ‘perfect storm’
The lead-up to the 2019-2020 fire season was unprecedented.
Other than El Niño, all contributors that enhance fire behaviour were present and active at extreme levels.
That included the worst multi-year drought on record, one of the strongest positive IODs on record, and a rare Sudden Stratospheric Warming (SSW) event over Antarctica which triggered a period of negative SAM.
The warming over Antarctica weakened the polar vortex (fast-moving air spinning around the South Pole) in spring 2019, causing westerly winds to expand north towards Australia.
Twelve months’ rain in the 12 months to July leading up to our three most severe forest fire seasons shows 1982 and 2019 had widespread severe rainfall deficits while 1982 saw emerging dry areas. The past 12 months has seen well above average rain across most of Australia. (ABC News)
Considering this was only the second major SSW since 1960, it was a true worst-case scenario and brought numerous days of strong, dry westerly winds to south-east Australia, fanning hundreds of bushfires across several states well into the summer.
Without the initial rare warming over Antarctica, Black Summer may not have happened, even after a record drought.
The only other Sudden Stratospheric Warming was in spring 2002. That also led to a horror fire season.
Outlook for the season ahead
So, what could enhance this year’s fire season?
The short answer: While it’s impossible to know for certain, we can rule out a multi-year severe drought taking place beforehand.
The country has experienced three years of good rain. (Supplied: Zak Insch)
Australia is coming off three years of good rains, and while a few months of hot and dry weather will no doubt dry out grasslands and some forested areas, the amount of combustible fuel available to burn will not match previous severe summers in most forested areas until dry weather has dominated for an extended period.
What about El Niño?
The weather bureau has not yet declared El Niño because there has been no shift in weather over the Pacific this winter, however, it is still the most likely scenario this spring and summer, but to what extent it will affect Australia’s weather is uncertain.
In relation to a Positive Indian Ocean Dipole forming, modelling has consistently forecast one to develop this winter, however, it hasn’t happened.
Similar to El Niño, there is still time for a dry signal to develop in the Indian Ocean as we transition into spring, however, it remains another unknown.
The least likely to occur would be a negative SAM (increased westerly winds).
That gives us only one guaranteed fire enhancer for the season ahead, which is climate change.
Even if the number of contributors climbs to three or four, the absence of a multi-year drought should be enough to prevent fires on the scale seen through our most severe seasons.