The intensifying climate driver behind the coastal deluges and inland drought
From Queensland to the South Coast of NSW, rainfall has been unusually high. That started with Ex-Tropical Cyclone Alfred in March, which a rapid attribution study by ClimaMeter found was slower and wetter because of climate change. Climate scientists expect fewer cyclones in Australia as the planet heats up, but warn they are moving further south and carrying more rain.
This week the higher rainfall has been evident with floods on the Mid North Coast of NSW. McDowell said Taree on the Mid North Coast had recorded its highest ever two-day rainfall on record in the two days to 9am Wednesday – about 400 millimetres or about a third of its typical annual rainfall.
East Coast Lows – which can be influenced by a positive SAM – have also caused widespread coastal erosion, with beaches on the Central Coast still denuded of sand after high swells in April.
Climate change is increasing the amount of moisture the atmosphere can hold by about 7 percentage points of humidity for every degree of warming. Already, global humidity has increased 7 percentage points over the past few decades alongside one degree of warming.
Scientia Professor Matthew England, a climate expert at the University of NSW, said the marine heatwave in the oceans around Australia would also contribute to the intensity of rainfall.
'We know that onshore winds pick up moisture from the ocean, and that warmer oceans drive increased atmospheric moisture and humidity, so we can expect that as our oceans warm, this effect will be enhanced,' England said.
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'The temperatures in the Tasman Sea have been well above average for the past few years, so some of this increased coastal rainfall could be due to those warmer ocean temperatures.'
At the same time, McDowell also said the showers and rain-bearing systems blowing in from the east don't make it very far across the land mass, meaning it is abnormally dry inland.
'If you look at April, or the whole of March and April, you've got parts of Victoria, NSW, South Australia and Tasmania that have the lowest rainfall on record, so it's pretty unusual,' McDowell said.
'In the past 30 years or so, we've seen [the Southern Annular Mode in positive] becoming more frequent … and there's potential that this is a climate change signal.'
England said the recent trends for a positive SAM 'have been driven by both increasing greenhouse gases and ozone depletion'.
'Both act to alter atmospheric pressure fields in such a way that the jet stream intensifies and shifts poleward, making the SAM more positive,' England said. (The jet stream is a band of powerful high-altitude winds that encircle the globe).
'While the ozone hole is recovering, we expect ongoing positive trends in the SAM due to increasing greenhouse gases.'
Dr Chiara Holgate in the ARC Centre of Excellence for Weather of the 21st Century at the Australian National University, said Australia had a highly variable climate, and without an attribution analysis, it was impossible to say that climate change was the cause of any particular drought or floods.
At the same time, Holgate said people currently experiencing drought should expect more of it.
'The area of southern Australia that is at the moment drought-affected pretty closely resembles the parts of Australia that are projected to experience more drought in the future with climate change,' Holgate said.
'Unless emissions are cut drastically, the current science is saying that this part of Australia is expected to see more frequent and longer droughts in the future.'
Heading into winter, rain could be on the way. The weather bureau's long-term outlook suggests the east coast and inland Australia have a strong likelihood of higher rainfall than usual, especially in August.
McDowell said the main reason was that global climate models, including the one used by BoM, predicted the Indian Ocean Dipole to become negative in winter, meaning more rainfall was likely for inland Australia.
The uncertainties of the long-range outlook meant it should be taken with 'a pinch of salt', he added.
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