THE CLIMATE KELPIE BLOG: Western climate drivers take the road less traveled

Posted by BCG on 6th April 2021

In the words of that well-known armchair forecaster, Bob Dylan, ‘the times (and climate) they are a changing’, and this is just as true on the west coast as it is on the east.

Over the last 50 years changes in Western Australia’s climate have increased average annual temperatures by one degree Celsius. While rain in the north and interior has increased, rain in the agriculturally productive regions in the south west and west coast have declined by close to 20 per cent over the same period.

As a result, historical rainfall zones have shifted westward by up to 100 kilometers in some areas, creating real challenges, particularly for broadacre croppers who are heavily reliant on increasingly unreliable growing-season rainfall.

Driving WA

So, what is driving this concerning trend?

Ian Foster, Research Scientist and climatologist at the WA Department of Industry and Regional Development (DPIRD) says that the main climate drivers influencing WA are the Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD).

“Unlike the rest of Australia, the El Nino Southern Oscillation (ENSO) doesn’t have much of an impact on WA. It is there but the effect is subtle. The Indian Ocean is more important as it generates moisture that, under the right conditions, can propagate across the state,” Dr Foster says.

Even though SAM is a short-term climate driver, it has played a big influence in the WA climate, particularly between April and December.

“SAM impacts mainly on the south-west region and, working in tandem with the subtropical ridge, has a big impact in winter. For those who follow the Climatedogs, I’d suggest that SAM and Ridgy are a couple of mates who like to get together and cause a bit of havoc,” he said.

In a normal winter, the subtropical ridge sits over the middle of Australia and cold fronts pass over southern Australia delivering rain fronts where they are needed.

“Unfortunately, we’ve had a trend to a more positive SAM, which means that the subtropical ridge is pushed further south, and we end up with fewer and weaker cold fronts across the land in the winter.”

Dr Foster says that, as a short-term driver, we can only predict SAM’s behaviour a few weeks ahead. The other challenge is that our measure of SAM is averaged across all latitudes – in other words, we have a single value for SAM regardless of whether the wave is over South Africa, South America or Australia. These two factors combined make accurate, long-range forecasts challenging to say the least.

The Indian Ocean Dipole (IOD) is the second key driver of rainfall across WA as well as other countries surrounding the Indian Ocean Basin. Temperature gradients across the Indian Ocean result in warm, moisture laden air rising on one side of the Indian Ocean or the other, which then descends over cooler areas on the opposite side.

In an IOD negative year, this system acts as a conveyor belt, bringing moisture from the ocean around Indonesia to Australia. When this moist air meets up with the SAM, it can deliver significant rainfall across the southern parts of the country.

In an IOD positive year, the conveyor belt reverses moving moisture in the opposite direction, often resulting in a drier spring for the southern parts of the country. Ocean temperatures around Indonesia are cooler than normal in these years. Unfortunately, the Western Indian Ocean appears to be warming faster than the waters off Australia, potentially leading to stronger, and more frequent IOD positive events if global temperatures continue to rise.

The combination of the Southern Annular Mode and Indian Ocean Dipole have a significant influence on rainfall over Western Australia. Source: Bureau of Meteorology.

Clearly, this has implications for WA farmers, who are highly reliant on in-season rainfall, due to soil type, especially the prevalence of sandy soils.

WA farmers among the most water efficient anywhere in the world

Australian farmers are among the most water efficient in the world, capturing and utilising every bit of moisture that falls during the growing season. Tools like controlled traffic farming (CTF), time of sowing, fallowing, rotation and variety selection, and early and deep sowing with long-coleoptile varieties have resulted in significant gains in water use efficiency.

Projects like the GRDC’s Water Use Efficiency Initiative, which bring together growers, farming systems groups and researchers across Australia, have helped to increase growers’ knowledge of the tools to understand and improve water use efficiency in dryland cropping systems.

“Given the reductions we’ve seen in winter rain, I’d have to say that the fact that our grain growers are able to produce a decent crop, even in an IOD positive year like 2019, is a real success story,” Ian said. “And the big WA crop in 2020 showed what can be achieved with these technologies, even in another below-average rainfall season.”

The ability to innovate, adapt and overcome has long been a strength of Australian farmers, who have managed to create a multi-billion-dollar industry exporting food to the world, all the while farming the driest inhabited content on earth.

More information

Dr Ian Foster, 0428 944 478, ian.foster@dpird.wa.gov.au

Climatedogs

DPIRD Climate trends in Western Australia

GRDC Water use efficiency of grain crops in Australia: principles, benchmarks and management

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