Insights & News

Australia Bushfires: Chances of a 2019-20 Repeat in 2020-21?

12th December 2020

The Previous Bushfire Season: A Searing Tragedy

Spring-Summer 2019-20 was a memorable one for Australia – but sadly, not for a positive reason.

Even before the peak summer heat, approx. 4 million acres were burned between the 1st Jul to 31st Dec, with some 9,000 structures destroyed. My opening blog of 2020 examined the driving forces behind this disaster. What I found was a dangerous combination of above-normal temperatures and well below normal rainfall during Sep-Dec. Incredibly, these conditions affected nearly all of Australia.

Map showing observed maximum temperature anomalies across Australia in Sep-Dec 2019.Map showing observed total rainfall anomalies across Australia in Sep-Dec 2019.

Dry terrain heated to exceptional levels – for example, 17th-18th Dec broke Australia’s country-average max temperature record twice in succession.

The intense bushfire season continued through early 2020. New South Wales was one of the worst affected, with over 5 million hectares burned by 31st January. More than 2,000 houses were destroyed and many more damaged.

Recent research has found that the combined scale of the 2019-20 bushfires was so vast (5.8 million hectares burned!) that it had an unprecedentedly strong impact on the large-scale atmosphere. A huge pall of smoke travelled 66,000 km during a 13 week lifespan and likely influenced weather patterns to some degree.

In the blog piece I mentioned earlier, I pointed an accusing finger at a phenomenon known as the Indian Ocean Dipole (IOD). This was in a strongly warm phase, during which waters in the Indian Ocean are unusually warm in the west and cool in the east.

During Sep-Dec, the IOD is negatively correlated with rainfall and positively with temperature. That means a warmer IOD promotes drier, hotter weather. Strong preconditioning for an intense bushfire season.

Did Climate Change Contribute?
Depends Where You Are…

Alongside the IOD, I noted that climate change may be a contributing factor. Most obviously due to higher temperatures meaning more very hot days potentially leading to tinder-dry vegetation. Less obviously and more locally, due to a drying trend that’s stronger in spring than winter. Put simply, a wet winter means more vegetation cover to dry out in the spring. This combination of trends is most apparent in the southeast of the country – where last season’s bushfires hit hardest.

Shown below is a comparison of full-year averages of the output from MetSwift’s estimated wildfire (i.e. bushfire) risk model, between three different periods.

This model doesn’t take into account changes of land cover, so what you see is entirely down to changes in climate.

Map showing the average weekly maximum wildfire risk across all weeks of the year, for 2000-2019 compared to 1950-2019.Map showing the average weekly maximum wildfire risk across all weeks of the year, for 2000-2019 compared to 1950-2019.

Two key characteristics to note: The direction of change is not uniform across the country and it also varies with time.

These shifts are mainly driven by the relationship between atmospheric temperatures and moisture content. Altered wind patterns also contribute locally.

You may be wondering about the northern reaches of the country – less human populated but containing valuable wildlife habitats. Well, except for the Cape York peninsula, most of northern Australia has seen increased rainfall since the 1970s. But in many places, this has been outweighed by increases to temperatures, wind speeds and typical duration of dry weather. The individual changes are slight in this region, but combined, they have a stronger effect.

The Outlook for Early 2021

I’m pleased to open this section with a positive: A strongly positive IOD is not occurring, or expected, for this season.

This would be more positive, were it not for forecasts suggesting a weakly positive IOD in early 2021. Ideally, we’d like it at least weakly negative.

One thing that is negative can be found in the central and eastern tropical Pacific Ocean. Currently, sea temperatures are widely below normal there. The BOM’s outlook is for a gradual easing toward neutral during early to mid-2021.

La Niña In Play

The current state of play is part of what’s known as a La Niña event. It’s at moderate strength as of Dec 2020. As shown below, this tends to mean a shortfall of rainfall across much of the country in January, then central and south-eastern parts in February while it turns wetter than usual in most other areas.

Map showing the January weather tendency in Australia during Moderate La Nina conditions.Map showing the February weather tendency in Australia during Moderate La Nina conditions.

It would be foolhardy, though, to base expectations on this one driving force behind weather patterns.

Especially when there are others which aren’t behaving as we usually expect in tandem with a La Niña…

Disruptive Discrepancies

Shown below-left is an observed weekly sea surface temperature (SST) anomaly plot for 29th Nov to 5th Dec (via psl.noaa.gov). Next to that are ‘typical SST anomaly’ plots for positive (top) and negative (bottom) phases of the Pacific Decadal Oscillation (PDO). This is a cyclical variation in SSTs that occurs on a large enough scale to impact weather patterns far and wide.

Left: Observed weekly sea surface temperature (SST) anomaly plot for 29th Nov to 5th Dec (via psl.noaa.gov). Right: ‘Typical SST anomaly’ plots for positive (top) and negative (bottom) phases of the Pacific Decadal Oscillation (PDO).

Now here’s the problem – we technically have a ‘typical’ combination of La Niña (cool equatorial waters in Pacific) and a negative PDO, but if you compare the observations with the negative PDO typical SST anomaly plot, there is a key discrepancy. The cold anomalies are less widespread than usual, while the warm anomalies cover a larger area of the Pacific.

This is the impact of climate change – an overall warming of the oceans. It’s likely that this is having some impact on how weather patterns across the world respond to the La Niña and negative PDO combination. Arguably, the PDO should be considered less negative than the measurements suggest.

Another key discrepancy the lack of a negative IOD being observed or expected (as referenced earlier). Usually, it’s negative during a La Niña event, but it appears 2020 has other ideas!

What Do the Most Representative
Historical Years Suggest?

Alongside the moderate La Niña, the weakened PDO signal and lack of negative IOD are factored in by the MetSwift analogues.

They have a profound impact on what historical precedent suggests will occur in January 2021 (below-left) compared to considering La Niña alone.

Map showing the weather tendency across Australia during the MetSwift analogue years for January

The MetSwift Analogues suggest January will be wetter or cooler than average, or both, across about half the country. This is generally good news for dampening the bushfire activity this month. However, its effect may be limited away from the north, as even above-average rainfall here can still be quite small amounts.

Elsewhere, there’s mostly no clear signal, except it looks hotter than average around Adelaide and Mount Gambler. It’s also looking hotter than usual in New Zealand’s South Island, for any interests there.

February, by contrast, is much less modified from the La Niña-only view.  We still see the wetter north and west (cooler southwest), drier and hotter southeast pattern.

Attention is drawn to the latter, where we could see the additional rainfall in January turn from friend to foe. That being due to increased vegetation cover providing additional potential fuel to bushfires that initiate in hot, dry conditions. This possibility is exacerbated by the plentiful rains of spring 2020 across central and south-eastern areas, which led to strong grass growth. Cropland also looks at risk in this last month of summer.

In Summary: Concerns for the Southeast

Overall, while the outlook is encouraging for bushfires being less widespread than in summer 2020, there are concerning signs that the hardest-hit southeast could be at high risk again in the later stages of summer 2021.

I should stress, though, that the MetSwift analogues and wildfire model are only intended for use as guidance. They’re probability-based, showing a most likely outcome. Remember, even a 99% chance wouldn’t be a guarantee.

 

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Unknown Author is licensed under CC BY-NC-ND

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