Tasmania's bushfires may be an indication of what's to come this summer

Tasmania just shivered through its coldest spring in 22 years and its wettest in 37 years, leading to a favourable seasonal fire outlook for the state that gave no indication of increased activity.

But days into summer bushfires have sparked up along the east coast, with twin blazes threatening properties and burning through hundreds of hectares of coastal land.

The presence of major fires, despite the lack of heat and drought, can be linked back to a rare spike in temperatures above Antarctica back in September.

Three months later and this polar warming continues to impact Southern Ocean weather patterns and will potentially result in further bushfire outbreaks during the coming weeks and months.

A tale of two springs, west vs east

While Tasmania's average rainfall through spring was 139 millimetres above normal, the discrepancy between east and west was notable.

For most areas in the western half of the state, rain totals were in the top decile, meaning they were in the top 10 per cent of years.

But for the east coast rainfall was actually below average, including just 131mm at St Helens, comfortably below the town's 21st-century average of 199mm.

As a result, the amount of moisture in Tasmania's soils heading into summer was variable.

The critical measure used to assess the fire risk — along with the prevailing weather — is the root zone soil moisture (RZSM), which is the amount of water in the top metre of the ground.

When RZSM is high, the vegetation — along with leaf litter and twigs — can absorb moisture and are defended against rapid combustion, while lower levels allow an accelerated rate of burning.

According to the Bureau of Meteorology's Australian Water Outlook website, Tasmania's east coast currently has average to slightly-below-average RZSM, confirming spring rainfall did not reduce the fire threat heading into summer.

Polar warming unleashes westerlies

Major bushfires in Australia are synonymous with heat and drought, but Tasmania is not seeing either at the moment.

However, analysing weather charts from the past few months reveals one glaring anomaly — abnormally persistent strong westerly winds have hammered the state.

While westerlies and their embedded fronts bring copious rain to western Tasmania, they are wrung out over the inland mountains and arrive on the east coast as a dry, offshore airstream ideal for fanning fires.

The map below shows the deviation from average since the beginning of September. 

Remarkably, westerly winds are blowing harder by up to 5 metres per second.

That is a substantial 18 km/h, easily the highest anomaly for any land area across the entire globe during the past 3 months — and, averaged across an entire season, represents a major shift in the weather.

So where are these westerlies coming from? The source of the high wind speeds can be traced back to a weakening of the polar vortex through spring.

The vortex is a semi-permanent belt of westerly winds that spin around Antarctica, but a sudden warming of the atmosphere above the South Pole in September set off a chain reaction that resulted in a weaker vortex and a northward migration of the westerlies towards Australia.

Critically, for summer weather in Tasmania, the trend is continuing well after Antarctic temperatures stabilised.

The most common measure of the prominence of Southern Ocean westerlies is called the Southern Annular Mode (SAM).

A negative phase of SAM indicates westerlies have shifted north, and a positive value signifies the westerlies have contracted south away from Tasmania.

The latest daily calculation of the metric is below minus 2, indicating a deeply negative state, and the BOM's modelling predicts values below zero are likely to continue until at least January.

Dry, windy summer to fan flames

With the pattern of above-average westerlies favoured to persist, it is not surprising that the next burst of gales is only days away.

Modelling shows a cold front will sweep over Tasmania on Sunday and generate westerly gusts from about 80 to 100 km/h.

Although temperatures will be mild relative to last Thursday when the Dolphin Sands fire flared up, the fire danger rating is still forecast to reach "high" — an indication fires could be fast moving and difficult to control.

The longer-term outlook also offers little hope of reprieve — not only is Tasmania likely to remain windy, but the BOM's modelling also favours above-average temperatures and below-average rain.

The three-month outlook for rainfall shows a 60 to 65-per-cent chance of below-median rain for eastern regions this summer, while the probability of daytime temperatures being warmer than normal stands at above 80 per cent.

Essentially, the state of the weather across Tasmania is pointing towards the very real possibility that bushfire activity could be more prominent this summer than first anticipated.