Hot, dry, and windy: Australia’s wildfires explode
Atmosphere Daily Earth Science Environment Repost

Hot, dry, and windy: Australia’s wildfires explode

The fire season still has a month to go in Australia: military assets deployed, 24 people dead, mass evacuations underway, 12 million acres burned, and half a billion animals killed. So, why are Australia’s wildfires so bad? The conditions for devastating brush and forest fires are the same around the world. The three magic ingredients are heat, drought, and high winds.

Penrith, a suburb of Sydney, registered 48.9 degrees Celsius or 120 degrees Fahrenheit on a recent Saturday.  This extreme heat follows the driest spring on record for the continent since records began 120 years ago. Additionally, these arid conditions come on top of a string of previous dry seasons. New South Wales and Queensland have been short of rain since 2017.

The dry weather is not merely a random event. Instead, it connects to larger climatic drivers. In this case, the Indian Ocean Dipole (IOD) is part of the problem.

Indian Ocean Dipole (IOD)

The IOD refers to oscillations in the sea-surface temperatures over the Indian Ocean. Usually, these oscillations are described as positive, neutral, or negative. A positive phase indicates warmer than average temperatures in the western Indian Ocean off the coast of Africa, and cooler waters off the north coast of Australia. 

During a positive IOD period, the ocean between the north coast of Australia and Indonesia turns colder due to the upwelling of deep ocean waters. The usual west to east air currents, which carry moisture to Australia, may weaken or reverse. Thus, these climatic conditions bring dryer weather to the Australian continent. 

Weak winds and colder water translate into less moisture in the air, and the resulting weather system is unable to deliver the normal amount of rain to Australia. So, the heat and dryness prime the continent for wildfires. Even without the wind, fires could break out. The wind, however, is the difference between a controllable breakout and an uncontrolled wildfire.

Southern Annular Mode (SAM)

The SAM is another climate driver. In the southern hemisphere, a belt of high-pressure circles the globe at about the latitude of Australia. This belt is called the subtropical ridge. South of the ridge, strong westerly winds (the Roaring Forties) move air from the west to the east. 

Under certain conditions, these westerly air currents shift northwards. So the air currents that normally flow to the south of Australia then blow directly across the Australian continent. They bring with them lots of heat and very little moisture.

So together these natural circulation systems create the perfect storm. Record-breaking heat combines with dry weather from a positive IOD, creating an environmental tinderbox. Then, as the fires start, strong westerly winds from a negative SAM phase help them intensify and spread. 

The 2019–2020 Australian fires will not be easily forgotten. As Andrew Constance, the transport minister, told the news, “This is not a bush fire. It’s an atomic bomb.”


ArcheanWeb:

Polar Vortex: Cold weather meets global warming (Source: ArcheanWeb) – https://archeanweb.com/2019/12/04/polar-vortex-cold-weather-meets-global-warming/ Also:

Wildfires and Mudslides (Source: ArchesnWeb) – https://archeanweb.com/2019/12/06/wildfires-and-mudslides/ Also:


Sources:

‘It’s an Atomic Bomb’: Australia Deploys Military as Fires Spread (New York Times) – https://www.nytimes.com/2020/01/04/world/australia/fires-military.html?&te=1&nl=morning-briefing&emc=edit_nn_20200105?campaign_id=9&instance_id=14976&segment_id=20062&user_id=4465f4a7cb96c405090de4e78f6ff9a9&regi_id=79313334_nn_20200105 Also:

Australia fires: How do we know how many animals have died? (BBC) – https://www.bbc.com/news/50986293 Also:

Why the Fires in Australia Are So Bad (Andy Parsons and Russell Goldman – New York Times) – https://www.nytimes.com/2020/01/01/world/australia/fires.html Also:

Bureau of Meteorology declares spring 2019 the driest on record (Kate Doyle – ABC News) – https://www.abc.net.au/news/2019-12-02/bureau-of-meteorology-declares-spring-2019-the-driest-on-record/11755848 Also:

Summer outlook from Bureau of Meteorology suggests hot, dry times to continue (Kate Doyle – ABC News) – https://www.abc.net.au/news/2019-11-28/bom-summer-outlook/11734828

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.

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