Climate change made recent fire conditions over NSW less likely?
There is even the possibility that climate change made the extreme fire conditions experienced over NSW this spring and early summer less likely despite increasing temperatures and an increasing trend in fire weather danger over many parts of Australia and the world with global warming.
This is because a key feature of this spring and summer was a prolonged period of negative Southern Annular Mode(SAM).
The SAM is actually trending more positive because of climate change.
A more negative SAM like that experienced this bushfire season increases fire conditions because it leads the more frequent occurrence of strong dry westerly winds over New South Wales whereas more positive SAM leads to more frequent moist southeasterly winds over New South Wales.
See also "No statistically significant increasing trend in summer fire weather conditions over eastern NSW" .
BOM schematic on the influence of different modes of SAM on Australia:
This is because a key feature of this spring and summer was a prolonged period of negative Southern Annular Mode(SAM).
The SAM is actually trending more positive because of climate change.
A more negative SAM like that experienced this bushfire season increases fire conditions because it leads the more frequent occurrence of strong dry westerly winds over New South Wales whereas more positive SAM leads to more frequent moist southeasterly winds over New South Wales.
See also "No statistically significant increasing trend in summer fire weather conditions over eastern NSW" .
BOM schematic on the influence of different modes of SAM on Australia:
![sam-summer-negative.png](http://weatherandclimateblog.weebly.com/uploads/8/0/3/8/8038112/sam-summer-negative_orig.png)
![sam-summer-negative.png](http://weatherandclimateblog.weebly.com/uploads/8/0/3/8/8038112/sam-summer-positive_orig.png)
SAM this year:
![aao.obs (2).gif](http://weatherandclimateblog.weebly.com/uploads/8/0/3/8/8038112/aao-obs-2_orig.gif)
Image from NOAA.
The Southern Annular Mode(SAM) is becoming more positive particularly during summer. We would have to see whether the climate change effect of the Southern Annular Mode(SAM) becoming more positive and thus making such fire conditions less likely or the effect of climate change leading to higher temperatures and thus increasing such fire conditions more likely is dominant and if so by how much is each factor contributing with uncertainty quantified.
But it is also worth noting that 2019's prolonged period of negative SAM was largely due to a Sudden Stratospheric Warming(SSW) event and we need to see how such events in the Southern Hemisphere are affected by climate change.
Negative SAM is becoming less common:
The Southern Annular Mode(SAM) is becoming more positive particularly during summer. We would have to see whether the climate change effect of the Southern Annular Mode(SAM) becoming more positive and thus making such fire conditions less likely or the effect of climate change leading to higher temperatures and thus increasing such fire conditions more likely is dominant and if so by how much is each factor contributing with uncertainty quantified.
But it is also worth noting that 2019's prolonged period of negative SAM was largely due to a Sudden Stratospheric Warming(SSW) event and we need to see how such events in the Southern Hemisphere are affected by climate change.
Negative SAM is becoming less common:
Seasonal values of the observation-based SAM index. The smooth black curve shows decadal variations. This figure is updated from Figure 3.32 in Trenberth et al. (2007). (contributed by G Marshall).
The rainfall patterns seen in December 2019 were that of negative SAM, decreased rainfall over New South Wales and increased rainfall over Western Tasmania.
The wind patterns were also that of negative SAM with westerlies penetrating into New South Wales with a westerly anomaly, the December 2019 BOM 850 hPa wind charts below show.
December 2019 rainfall anomalies:
The rainfall patterns seen in December 2019 were that of negative SAM, decreased rainfall over New South Wales and increased rainfall over Western Tasmania.
The wind patterns were also that of negative SAM with westerlies penetrating into New South Wales with a westerly anomaly, the December 2019 BOM 850 hPa wind charts below show.
December 2019 rainfall anomalies:
850 hPa winds December 2019:
![2019120120191231.hres.gif](http://weatherandclimateblog.weebly.com/uploads/8/0/3/8/8038112/2019120120191231-hres_orig.gif)
850 hPa winds anomaly December 2019:
![2019120120191231.hres.gif](http://weatherandclimateblog.weebly.com/uploads/8/0/3/8/8038112/2019120120191231-hres-1_orig.gif)
This analysis shows that it would be wrong to assume without prior study and analysis that a single or every single extreme fire event was made worse by climate change just because statistically on average, fires are expected to become more frequent and more intense over many parts of the world. This is because any individual event occurs due to a unique set of circumstances which led to it and these circumstances may be affected by different ways by global warming.
Hope, P. et al., (2018), a study on record fire weather across central-eastern Australia in February 2017 did not find a clear signal as to the influence of increasing atmospheric CO2 on fire weather.
It would even be wrong to assume that all extreme heat events were made more likely or all extreme cold events were made less likely by climate change just because that is the projected global pattern.
For example, climate change could have made frost over Southwestern Australia more likely because of the drying trend as well as changes in atmospheric circulation, location and behaviour of high pressure systems and ridges.etc.
Nevertheless regardless of the influence of climate change on the bushfires in late spring and early summer recently, it is likely that fire conditions are expected to become more common and intense over large parts of Australia and the world as temperatures increase with parts of Southern Australia also drying with climate change.
Irrelevant of whether or not climate change were to blame for the bushfire crisis in New South Wales the case for sensible climate action is extremely strong and has been made long before this intense bushfire season.
It would be missing the forests for the trees to use these bushfires as an argument for or against sensible climate action.
Some charts above are from the Australian Bureau of Meteorology(BOM).