As northern and eastern Australia prepares for a very wet winter, the weather forecast for the weather office shows that WA’s most populous region looks totally different from the rest of the country.
Key points:
- The land southwest division of WA is expected to have a drier winter
- Warmer ocean temperatures are part of the reason
- Climate change models suggest that drying will continue
Instead of being covered in blue and green, the rain forecast map is marked in shades of orange and red, a sign that there is likely to be a dry winter.
Bureau of Meteorology (BOM) long-term forecast chief Andrew Watkins said much of the southwestern land division, which included Perth, could see below-average rainfall this winter.
“The odds are around 70 per cent and they are drier than average, so quite the opposite of anywhere else in Australia,” he said.
For Perth, this means less than 395 millimeters of rain.
The Bureau of Meteorology is forecasting a wet winter for most of the country, but the most populous region of WA is set aside. (Supplied by: BOM)
Maximum temperatures in Perth and the southwestern part of the state are expected to be above average.
Meanwhile, the northern regions of the state, including Pilbara and Kimberley, have been leaning for the next three months wetter than average.
So why the difference?
Warm oceans play a role
The BOM perspective incorporates the influence of climate change and natural climate factors.
Dr. Watkins said it was a “probabilistic forecast,” which basically looked at how the dice were loaded for next season.
The Pilbara and Kimberley regions of WA are likely to have a wetter winter. (Provided by: Nicky Macnee)
He said one of the key factors in above-average rainfall in the north and east of the country this winter was warmer ocean temperatures.
This included a climate engine 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 colder than usual, while the waters of northwestern Australia are warmer.
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Every event is different, but a negative IOD usually brings wet conditions to regions from Gascony in Western Australia to Tasmania.
“So negative IOD promotes these northwestern cloud bands, especially at this time of year when you get into winter,” he said.
“He sees large cloud currents coming mainly from Indonesia to southeastern Australia.
“But the other thing is we have a lot of hot water in North Australia, and that will provide extra moisture.”
La Pilbara has experienced heavy rainfall in recent weeks. (Provided by: Melisha Leggett)
WA Pilbara has experienced such systems in recent weeks, with bands of moisture-laden clouds attached to them that caused a torrential downpour across the region.
But Dr. Watkins said these factors did not have a big influence on rainfall in southwestern WA.
He said that while a negative IOD could increase rainfall in the southwest when properly scheduled, such as July 2021, it did not tip the odds in any way for the region.
One climatic factor that does influence the region, however, is something known as the “SAM” or southern annular mode.
Positive SAM has a negative impact
Most of the cool season rain in southwestern WA comes from storm systems such as cold fronts and low pressure systems.
Helping with these storms is a steady band of strong westerly winds in South Australia.
The positive SAM in winter is when high pressure systems and storm tracks move further south, but when highs are further north in the negative phase, the fronts reach the south of the continent. (ABC Weather: Kate Doyle)
The SAM refers to the extent of this wind belt to the north to Australia, or to the south to Antarctica.
Dr. Watkins said the models indicated this winter that the wind belt would be in a “positive” phase, meaning it would tend further south than normal.
This would mean that it is more difficult for storm systems to reach the region.
“Weather systems can move a little further south than normal and therefore tend to affect parts of South Australia facing west,” he said.
Heavy rain and drizzle are expected on the east coast of the country. (ABC News: Emma Pollard)
He said there were also likely to be larger, more 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 see rainier conditions with high pressure systems due to land winds.
Climate change points to a drying trend
Finally, there is climate change.
“The trends we have observed since the 1970s certainly suggest more dry periods than wet periods in the south-western parts of Western Australia,” he said.
“And the prospects are certainly taking that into account.”
As the trend for less winter and more summer rains becomes commonplace, farmers are finding new ways to conserve summer rains for the winter growing season (Tara De Landgrafft: ABC)
Southwestern WA was one of the first places on the planet to see a downward trend in rainfall, and the latest report from the Intergovernmental Panel on Climate Change (IPCC) points to it as one of the few regions. of the world where most Models agree that drying will continue
Since the late 1960s, rainfall in the region has declined by as much as 20% overall.
The figures show that in recent decades, the downward trend has gained momentum.
Good perspective accuracy
The data show that the past accuracy of the winter outlook has been reasonably good for WA.
Most of southwest WA has had an accuracy rate of 65 to 75 percent, while in the northwest region it has been between 70 and 75 percent.
“So, anyway, it’s so much better than making an assumption,” Dr. Watkins said.