As Australia’s north and east brace for a very wet winter, the weather bureau’s seasonal outlook shows that WA’s most populated region looks drastically different from the rest of the country.
- WA’s Southwest Land Division Expected to Have a Drier Winter
- Warmer ocean temperatures are part of the reason
- Climate change models suggest drying will continue
Instead of being covered in blues and greens, the precipitation forecast map is marked by hues of orange and red – a sign that a dry winter is likely.
Chief of long-range forecasting at the Bureau of Meteorology (BOM), Andrew Watkins, said much of the South West Land Division, which included Perth, was likely to see below-average rainfall this winter .
For Perth, that means less than 395 millimeters of rain.
Maximum temperatures in Perth and the southwest of the state are expected to be above average.
Meanwhile, northern regions of the state, including the Pilbara and the Kimberley, have been warned for the next three months wetter than average.
So why the difference?
Warm oceans play a role
The BOM outlook incorporates the influence of climate change and natural climatic factors.
Dr Watkins said it was a “probabilistic forecast”, which essentially looked at how the dice were loaded for the upcoming season.
He said a major driver of above-average rainfall for the north and east of the country this winter was warmer ocean temperatures.
This included a climatic factor known as the Indian Ocean Dipole (IOD), which was expected to enter a “negative” phase in the coming months.
A negative IOD occurs when sea surface temperatures in the Indian Ocean near the east coast of Africa are cooler than usual, while waters northwest of Australia are warmer. .
Every event is different, but a negative IOD typically brings wet conditions to regions from Gascoyne in Western Australia to Tasmania.
“So the negative IOD favors those northwest cloud bands, especially at this time of year as you enter winter,” he said.
“But the other thing is that we have a lot of hot water in northern Australia, so that’s going to provide more humidity.”
WA’s Pilbara has been experiencing these types of systems for the past few weeks, with moisture-laden back-to-back cloud bands bringing a deluge of rain to the area.
But Dr Watkins said these factors did not have a big influence on rainfall in south-west Western Australia.
He said while a negative IOD could increase rainfall in the southwest at the right time, such as July 2021, it wouldn’t tip the scales for the region anyway.
One climatic factor that influences the region, however, is known as “SAM” or Southern Annular Mode.
A positive SAM has a negative impact
Most cool season precipitation for south-west Western Australia comes from thunderstorm systems, such as cold fronts and low pressure systems.
Helping to carry these storms is a constant band of strong westerly winds south of Australia.
The SAM refers to the distance north to Australia, or south to Antarctica, this wind belt is positioned.
Dr Watkins said models this winter indicated the wind belt would be in a “positive” phase, meaning it would head further south than normal.
This would mean that it is more difficult for storm systems to reach the region.
He said there were also likely more large, persistent high pressure systems in the bay, which bring easterly winds to WA and favor clear skies.
The east coast, on the other hand, tends to experience drizzier conditions with high pressure systems due to onshore winds.
Climate change indicates a drying trend
Finally, there is climate change.
“The trends we’ve seen since the 1970s certainly suggest more dry spells than wet spells in the south-west parts of Western Australia,” he said.
“And certainly the outlook reflects that.”
Southwest WA was one of the first places on the planet to see a trend of reduced rainfall, and the latest report from the Intergovernmental Panel on Climate Change (IPCC) names it as the ‘one of the few regions in the world where the majority of models agree that drying will continue
Since the late 1960s, rainfall in the region has decreased by up to 20% overall.
Figures show in recent decades, the downward trend has gained momentum.
Good perspective accuracy
The data shows that past winter outlook accuracy has been reasonably good for WA.
Most of South West Western Australia had an accuracy rate of 65-75%, while in the North West region it was between 70-75%.
“So much better than having a guess, anyway,” Dr Watkins said.