What’s Causing Australia’s Bushfire Crisis?  A meteorological perspective.

This spring, we’ve experienced the worst bushfire season in recorded history for the state of Queensland and the northeast quarter of NSW.  We look at why this bushfire season is so severe and the ‘perfect storm’ of climatic patterns that have all culminated to generate such significant bushfires.

 

Firstly – bushfires aren’t rare in these areas of Australia, but the ferocity of these bushfires has been unusual.  The main bushfire season for Queensland and northeast NSW typically occurs in late winter and early spring.  This coincides with the driest time of year.  It may seem more logical that mid-summer would be the worst time since it’s hotter (and bushfires certainly do occur in summer), but summer is typically wetter which means the vegetation is a little more resilient to burning.  Summer in this region is normally less windy than spring – and if it is windy, those winds typically come from the ocean where humidity levels are higher.  It’s the southern states (Victoria, South Australia, Tasmania & southern NSW/ACT) that experience their peak bushfire season in late summer because this is the driest period of the year for these areas (and also the hottest).  This can be seen with the average rainfall charts for Australia that shows significant rainfall differences between the cooler and warmer months across Australia.

One of the most significant factors contributing to the bushfires has been a prolonged drought over the region.  Not one, but multiple wet seasons have failed to materialise over the region meaning that thick forests (and rainforests) have become extremely dry.  This significantly reduces the resilience of the vegetation causing it to burn much easier than if it was full of moisture which can contribute to increasing the speed of the fire.

For many locations, it’s not just 12 month anomalies that are at record levels – but even two and three year rainfall anomalies are at their lowest on record for large parts of northern NSW and southern Queensland (but the worst anomalies have occurred this year and many locations will receive their driest year on record).

This time of year is the peak of the thunderstorm season – but there’s been only a handful of storms across Queensland and northeast NSW so far.  The big culprit here are the unusual ocean temperature patterns at play.

Firstly, the Indian Ocean continues to have very warm water sit towards India and Africa, with continuous cool sea surface temperature anomalies sitting close to the West Australian coastline.  Very simply, most of the moisture, rain and storm activity continues to remain favoured over the western half of the Indian Ocean.  In a typical spring, when a cold front sweeps across the continent it generates a strong Jetstream that taps into the moisture across the northern tropics and in northern Western Australia and drags it southwards.  This is an important source of moisture and has been virtually completely absent so far this spring.

Secondly, while the Pacific Ocean is not in a classic El Nino nor La Nina pattern, it’s not exactly “neutral” either.  Instead, the warm water has been sitting more so across the central Pacific and this means there’s limited moisture coming across from the Coral Sea/Pacific Ocean.  While it’s not officially an El Nino, the patterns and results are somewhat “El Nino like.”

This means we have two major climatic indicators that have not been supportive for rainfall to occur.  But the icing on the cake has been the general negative Southern Annular Mode since August (which has been highly negative since the start of November).  This causes the westerly wind belt around Antarctica to extend northwards and increases the number of cold fronts that sweep across southeastern Australia.  The result is a strengthening of westerly winds across eastern Australia.  These westerly winds often generate showers and even snow to alpine areas in the southern states as these winds come directly off the ocean, but further north these winds need to travel across thousands of kilometres of land meaning they’re much drier.  To top it all off, the combination of high temperatures across northern Australia and cold temperatures over the southern half of the country helps create tight temperature and pressure gradients.  This accelerates the westerly winds, meaning that it’s not only hot from the westerly winds pulling in heat from northern to central Australia, but windy.

This provides all the factors required for a horror bushfire season:

  1. A prolonged drought with record low rainfall for not just one year, but multiple years.
  2. Hot and dry prevailing conditions
  3. And then finally, unusually strong winds to help fan the bushfires (these winds normally occur in August and September, not November)

Unfortunately this is a pattern that doesn’t look like breaking anytime soon – though there does remain some signs it may weaken later this month and into December.  However the most likely scenario will be that it could finally bring some of the shower and storm activity the area typically gets this time of year.  We will need these climatic patterns to break down further to generate a higher chance of rainfall.

2019 has almost been the complete opposite of 2010 (when significant floods occurred over large parts of Queensland).  In 2010, every climatic indicator was favouring rainfall, while in 2019 every climatic indicator is favouring dry conditions – though this should finally begin to break down early next week, but whether it will be sufficient to generate drought breaking rainfall is too early to determine.