The quite horrendous series of windy August and September days across SE Australia was felt in Sydney under bright blue skies. Further south and in Tasmania however the wind gusted up to category 3 cyclone strength with torrential rain and widespread flooding. We are getting weird and wonderful and often not so wonderful weather outbreaks, usually fires and floods, which are often attributed to or amplified by climate change, but what exactly led to this one?

On 3 September, Caitlin Fitzsimmons, the Sydney Morning Herald’s environment and climate reporter, explained that it’s largely the fault of the jet streams. Two jet streams affect Australia, the subtropical and polar. They both blow fiercely from the west at altitudes of ±10,000 metres but are normally separate at this time of year. However, they have merged across the south of the continent and have amplified each other. The subtropical tends to stay aloft, however the polar often drives weather at lower levels and is linked to the Roaring Forties wind belt of the Southern Ocean within which this dangerous weather resides.

What has climate change done to cause this? Climate change in the Northern Hemisphere has already been blamed for amplified bends and deviations in jet stream paths leading to bottled-up heat domes and devastating fires, or southward surges of the Arctic vortex feeding intense chill across continental Eurasia and North America. Our recent experience is more complex! Warming is apparently nudging the subtropical stream southwards away from the equator while warming and ice melt in Antarctica may be easing the polar one northwards.

John Martyn