Our southern farmers could really use a break, literally.
The first good rainfall that traditionally triggers the southern cropping season is known as the autumn break.
We usually expect it by around Anzac Day, but it doesn't always arrive.
The autumn break is a complicated beast, so what causes it, is it changing, and what will happen to it in the future?
Defining the break isn't easy
Dr Michael Pook and Dr James Risbey from the CSIRO have studied the break, concentrating on the Mallee region of north-west Victoria where they asked farmers what they regarded as an autumn break.
"Naturally enough, we got a pretty wide divergence of opinion," Dr Pook said.
"But most people generally agree that if you had a fall of about, in the old terms, one inch or 25 millimetres of rain in a day, that was generally regarded as the basic definition of an autumn break.
"But clearly that rarely happens, particularly in those drier areas, so we came up with an alternative definition for what we'd call an ideal break."
The scientists defined the break as 25mm or more in a day, or 30mm or more over about seven days. Having asked around, most would go with a similar sort of definition.
It also matters that it happens in autumn (obviously), because it's the time of year when grain crops, particularly wheat and canola, are planted in Australia, according to Dr Pook.
"The crops are grown during the winter season, so they have to be put into the ground early enough to get established and then grow on slowly during winter and then really come to life in spring," he said.
Generally, if it hasn't rained by June, the chances of getting a good crop out are slim.
"But if you plant too early, your emerging crop is susceptible to frost damage, so there's a lot of concern about when to plant and how to plant," Dr Pook said.
When and how much it rains after the break matters too of course, but that is a whole other story.
The break is also important for livestock to ensure there is pasture to carry over into winter.
What causes it?
The break generally applies to the southern pasture and cropping areas mainly in New South Wales, Victoria, South Australia, Western Australia and Tasmania — and occasionally parts of southern Queensland.
"Part of what's implicit in the notion of an autumn break is that southern Australia tends towards what would be called a Mediterranean climate, meaning it tends to have a more winter-dominated rainfall regime," Dr Risbey said.
To explain why that is requires an explanation of how the world works.
Basically, because Earth's axis is on a tilt, where the Sun's energy is focused shifts up and down thereby dictating the season.
As the Sun's energy moves further north in our winter, it allows the rain-bearing westerly wind belt to move up over southern Australia.
"So even though the sunshine is not at a maximum at that time of year, that's when a lot of the rain falls, and that's part of the reason why we have a winter crop here and why an autumn break is essential, because it's the lead-up to the winter and the rainy season," Dr Risbey said.
The main systems that bring rain in this westerly wind belt are frontal systems that steadily move from west to east.
"But other systems that break away from the westerlies and move more slowly are called cut-off lows," Dr Pook said.
"They tend to produce more rainfall generally because they're slower-moving, and they also bring quite a lot of the moisture down from the tropics and subtropics."
In their study of the Mallee region over a 40-to-50-year period, they found most ideal autumn breaks came from cut-off lows.
"Sometimes [cut-off lows] give us periods with more autumn breaks, and sometimes they drop away and we become as equally dependent on the fronts," Dr Risbey said.
Is the break broken?
This is a tricky question. Even back in the golden days, breaks were inconsistent, and now climate change has further stirred the global pot.
While climate change is expected to influence rainfall contributions from frontal systems, there are more question marks when it comes to the cut-off lows.
"Frontal systems are basically embedded in what we call the storm tracks — the belt of westerlies around the hemisphere — and there are reasonable expectations that climate change is tending to make that belt contract more poleward," Dr Risbey said.
"As that belt contracts more poleward, it tends to drag the fronts down more poleward with it and so we can expect a reduction in rain from frontal systems so that it goes with our climate change expectations."
But the cut-off lows are related to blocking highs in the atmosphere which go through multidecadal variations.
"We do have some decades or even multiple decades when we have blocking and cut-offs are more prevalent in our region, and other decades when they are less prevalent in our region," Dr Risbey said.
"If we draw a line today, we might say that [autumn breaks] have been less frequent in recent decades, but we may enter a period of a couple of decades where blocking becomes more favourable for cut-offs and autumn breaks in our region and we may have an increase in them again.
"All we can really say is that we can expect more long-period variations in future, so that they will be periods where we have more favourable conditions for autumn breaks and less favourable conditions, at least due to the changes in cut-off low contribution."
That's for the Mallee region where doctors Pook and Risbey carried out their research, but because cut-off lows are small and relatively localised, they say their impact is likely to be different in different parts of southern Australia.
Dr Risbey said what to expect in future depended on where you were located; the expectations for Western Australia in particular are different from the south-east.
"South-west WA is an area where frontal rainfall dominates over cut-off rainfall because of the preference for a strong trough that brings the front preferentially into that region," he said.
"Because the break there is more strongly associated with frontal systems than cut-off lows, then that would be more sensitive to any change in frontal systems and any contraction of the storm tracks.
"So you might expect a reduction in autumn break conditions in the west, although the west tends to have much better rainfall from the frontal systems anyway and so it's a different regime for grain than in the south-east."
What's driving the highs that drive the lows that drive the break in (at least parts of) the south-east?
Strap in climate nerds, this is about to get technical.
Dr Risbey said the east/west location of the blocking highs was a lot more variable than simply the north/south shift of the storm track.
Even small displacements in the preferred position of the blocks change where those cut-offs form and where they move, he said.
"The cut-off low itself is a very small-scale system that can change the rainfall regime quite a bit over small distances, even within south-east Australia itself.
"So you have all of the ingredients for a highly variable rainfall system, which is one of the things that makes grain farming in south-east Australia so challenging.
"The factors that control their preferred east/west locations are not well-enough understood and not strongly enough linked to climate change for us to really have an expectation about how they're going to change."
So the jury might still be deliberating, but there is plenty of evidence and proposed culprits for them to wade through.
"We know that there are some thermal links," Dr Pook said.
"The sea surface temperature can have an effect on exactly where [the blocking highs] set up, but they are really formed by a combination of the topographic features in the Southern Hemisphere and in the temperature gradients across the ocean and also across the land."
Then of course it's Australian climate we are talking about, so El Niño gets a guernsey — although in this case it isn't a very clear link.
"Whether we have an El Niño or a La Niña changes the tropical temperatures, so that has an effect on this pole-to-equator temperature difference," Dr Risbey said.
"That has an effect on our jet streams and our storm track because they feed off this temperature gradient, and when you change the strength of the jet stream, you change the way in which the waves that form in that jet stream set themselves up, and that changes the positions of the blocks."
He said the difference between the tropical and polar temperatures, or "meridional temperature gradient", across the Indian Ocean was important too.
"I know that's quite a complicated explanation, but it comes down to understanding what changes the jet stream, and then the jet stream changes how it interacts to give you different blocking positions.
"The reasons why it varies on such long timescales are not completely clear."
This whole thing is starting to scream potential PhD, so let's leave it there.
Farmers fighting back
Farmer and agronomist Melissa Rebbeck said that over the past 20 years, the frequency of good breaks had been reducing but that there had been a few good years recently.
"I think there is generally a little bit of a panic or worry if a break hasn't come by Anzac Day," she said.
"But if we look historically, the actual probability of a break happening by Anzac Day right across south Australia, it's only happened about 25 per cent of years historically anyway.
"Fifty per cent of the years you get a break by mid-May."
Ms Rebbeck said farming methods were evolving in response to the variable break.
"We are actually getting better and better at water-use efficiency by choosing the right crops, but also maybe dry sowing a certain amount of that crop and spreading our risks and then going more when the break occurs. We can also get our crops in faster.
"If you are a livestock producer, I think what people are doing is basically having more flexibility to adjust animal numbers as the season progresses and respond to the break when it occurs and have trigger points for managing their livestock numbers over time.
"People are involving more trading and less breeding — just having flexibility in the way that people can move animals around and more confinement feeding."
Ms Rebbeck said the problem with planning going into autumn was that the break was difficult to predict.
"At this time of year the relationship of the ocean and the atmosphere to autumn rainfall is low, making it difficult to forecast the opening break accurately," she said.
"We've really just got to rely on our stored soil moisture and the 10-day outlook. The 10-day outlooks are exceptional now; it's getting more and more accurate."
But so far this season, that 10-day outlook has remained determinedly dry.ABC