Australia’s Weather – a Two Year Summary
Annabelle Honner, Agfarm Freight Coordinator
Over the last 24 months, all growing regions of Australia have experienced at least below average rainfall, with parts of north western NSW, the SA Mallee, WA Esperance and northern Adelaide areas receiving their lowest rainfall on record. In fact, the BOM has defined parts of the Murray-Darling basin as being in drought since January 2017. There is no question this has been a prolonged and intense drought, but the question is, why have we been in a drought for so long?
There have been a series of complicated and inter-connecting atmospheric weather events, particularly in the last 12 months, that have resulted in continued low rainfall across Australia, particularly in the eastern states. Throughout 2017, there were no strong climatic influences that were affecting Australia’s climate either way until the end of 2017 and into the beginning of 2018, when we experienced a short-lived La Niña. Unfortunately, this event did not have a huge effect on rainfall in Australia and decayed quickly after its initial formation. Following this decay, certain climatic indicators of El Niño began forming, however these indicators were not long lived enough to constitute an El Niño event. Around October 2018, a positive Indian Ocean Dipole (IOD) formed, which lasted until mid-way through December. At the beginning of this year there were once again climatic signs of El Niño forming, but by June this year, the ENSO had returned to neutral and instead, another positive IOD formed, which is still active.
The IOD is influenced by the difference in ocean temperatures between the West and East Indian Ocean. In a positive IOD year, these interactions generally result in moisture shifting away from Australia, instead being pulled towards the east coast of Africa and the Middle East. The IOD can impact anytime from May to December, after which time the onset of the northern monsoon results in an IOD being unable to form. Generally, the northern rainfall onset, which signals the beginning of the monsoon season in northern QLD, NT and WA, occurs anytime from November to early December. This year, predictions are indicating any chances of rainfall onset occurring before November/December are unlikely, meaning the effects of the positive IOD may well be felt right up to the end of this year.
Image source: http://www.bom.gov.au/climate/rainfall-onset/
Recently, the positive IOD has been the main climate driver in Australia, however over the last month or so, a negative Southern Annular Mode (SAM) has developed. The SAM describes the shift of westerly winds and rain-bearing weather systems in the Southern Ocean from their normal position. These shifts move in north-south movements. In a positive SAM, the systems are pushed further south which generally results in increased rainfall over the eastern states. However, in negative SAM events, the systems are pushed north, which results in less rainfall in northern NSW and QLD, but increased rainfall in south-western WA and in Victoria. Generally, these phases are short-lived, lasting anywhere from between a week to a couple of months. The current negative SAM has been predicted by the BOM to be an unusually long-lasting event. Despite this outlook, SAM systems can fluctuate in strength, and there have been indications that the current negative system may be weakening, and even has the potential of developing into a positive SAM event as we come into spring, with some significant rainfall events for QLD and NSW starting to appear on forecasts for the next seven to 10 days.
Despite the short-term rainfall predictions, the long-term outlook for the next three to four months isn’t offering any more reprieve from the dry weather. In the next three months to December, the majority of Australia won’t meet a 50% chance of exceeding their median rainfalls, nearly everywhere is hovering between a 20-30% chance. In fact, the only part of Australia that has a 50% chance or above is the Kimberley region of WA. From November to January, the outlook improves slightly, with most areas looking at a 25-40% chance of receiving above average rainfall. These outlooks do not do much to boost confidence, and expectations of a late rainfall event to pull struggling winter crops through to seed may not be met. If we don’t see a significant fall in the next month or so, we will likely (and some areas are already seeing) an increase in hay production this year as it becomes apparent that some crops may not make it through to harvest. Right now, production estimates are sitting at around 17.5 MMT wheat, 8.5 MMT for barley, and 2.2 MMT for Canola, however these estimates could easily change, depending on what happens weather-wise over the next three to four weeks.
These long-range rainfall outlooks also lay shaky foundations for the upcoming summer crop plant in QLD and NSW. Significant soil moisture deficits persist through NSW and QLD, and this combined with the lack of forecast rainfall may well be enough to convince many growers to not sow a summer crop. Instead, they may choose to fallow their paddocks in hope for decent rain at the end of summer and into autumn to set them up for a 2020/21 winter crop. Furthermore, water availability for irrigation is becoming increasingly tighter, with ABARES forecasting water prices for the 2019/20 season to reach as high as $650/ML in Southern NSW, VIC and SA, making irrigation an expensive option for most. In its most recent production report, ABARES has forecasted the area planted to sorghum to decrease by 28% from 2018/19 plant to 391,000 ha, 30% below the ten-year average. Area planted to cotton is forecast to reduce by 58% to 145,000ha. Despite a reduction in planting, lint yields are forecast to be average to higher, as nearly the whole crop planted will be irrigated.
The BOM defines ending rainfall deficiency as the rainfall in the last month already exceeding the 30th percentile for the three-month period commencing that month, or if the rainfall for the past three months is above the 70th percentile for that period. Basically, we need to receive an above-average rainfall for consecutive months for a drought to break. Even though the long-range forecasts are not looking positive, the forecast rainfall being triggered by the formation of a positive SAM may well be enough to get some moisture stressed crops through to harvest, and once the positive IOD breaks down and the northern monsoon begins, hopefully we begin to see a change in the rainfall patterns.
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