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Can SAM find the mojo to improve summer rain prospects?

by Neil Lyon, 15 November 2018

CottonInfo climate technical lead, Jon Welsh, discusses the weather outlook with AgEcon partner, Janine Powell, and Auscott’s Bill Back, Narrabri, NSW. (Photo: Mel Jensen)

WHILE climate forecasters point to the recent development of an El Niño as a sign of dry times ahead, summer crop producers throughout the eastern farming zone may take heart from other climate indicators that are more optimistic about the likelihood of summer rains.

The development of a positive Southern Annular Mode (SAM) and the chance of the Madden-Julian Oscillation (MJO) holding strength as it passes over Australia raise the possibility of increased moisture patterns over summer for cotton, sorghum, sunflower and mung bean producers.

That’s the view of CottonInfo climate technical lead, Jon Welsh, who said the often-quoted El Niño/Southern Oscillation (ENSO) was not well-correlated with climate going into the summer monsoon period. Instead, the SAM and the MJO were key drivers of climate in the coming months, particularly in the mid-latitude summer cropping areas.

“…it is encouraging that the SAM appears to be moving into a positive phase at the same time as the MJO passes through the Australian region.”

“The SAM is the leading mode of variability through the extensive western areas of the cotton growing regions of New South Wales. Whilst there are many factors at play, it is good to see it is pushing positive. I’m quite buoyed by the predictions that it could swing into the positive phase by the next fortnight,” he said.

Mr Welsh described SAM as a ‘big steering wheel’ for moisture circulation that can direct moisture from the warm, tropical seas north and east of Australia into trough and frontal systems affecting cropping areas.

“It is the contracting and expanding of winds and the frontal systems originating from Antarctica and it’s the strength and positioning of these systems that influence the spring and summer climate where most of the nation’s cotton crops are grown,” he said.

“The SAM is much like the SOI (Southern Oscillation Index) in that the sea-level air pressure differences between two latitudes at the South Pole creates an index. From that measurement they can tell whether the frontal systems are contracting or expanding.

“The SAM is a high-frequency indicator and is inherently difficult to predict on long-lead times out past 3-4 weeks.”

Figure 1: SAM chart showing moisture-in-feed into Australia (three-month blocks). (Click on chart to enlarge)

Mr Welsh said the SAM moved between negative and positive phases, also impacting extreme events such as heat-waves, particularly over summer, throughout vast parts of the inland eastern cropping belt.

“When it is in negative phase, western Victoria will often receive rain and NSW can experience dry winds coming off the desert in Central Australia. That is not favourable for rain in the cotton areas, whereas in a positive phase the circulation comes more anti-clockwise and moisture can feed-in to trough systems into cotton areas,” he said.

“Obviously, you need activation mechanisms to make it rain as well, such as frontal systems coming through to push high pressure systems out of the road and combine with the SAM to make rain happen.”

Figure 2: SAM +- index. (Click on graph to enlarge)

Mr Welsh said the other climatic indicator overlaying rainfall prospects for summer was the MJO which was a large-scale pulse of low air pressure and tropical convection which rotated around the Earth’s equator, roughly every 30 to 60 days.

“It is something that, once we get into the monsoon period, we keep a close eye on,” he said.

“Analysis over the last six summers has shown that the passing of an active MJO has been reasonably well-aligned with rain events through eastern Australia. As the summer season progresses into January and February the rain events can often occur 1-2 weeks later following an active MJO. In the record hot, dry summer of 2013-14 the MJO was benign, so it is definitely something we like to see come to life early in the wet season.”

Mr Welsh said modelling indicated the MJO had been at weak to moderate strength over the last two weeks.

“It is still maintaining strength as it propagates eastward across the Indian Ocean at the moment and is due to pass over Australia in the next 7-10 days” he said.

“It is likely to weaken as it gets to Australia, possibly due to the fact there are some very cool waters from a positive Indian Ocean Dipole (IOD) off Sumatra and waters to the north west of Australia. That’s a real shame, but it may instigate something none-the-less if it can maintain some strength.”

Mr Welsh said it was encouraging that the SAM appeared to be moving into a positive phase at the same time as the MJO passed through the Australian region.

He said history had taught farmers they needed to take ENSO commentary “with a grain of salt” coming into the summer cropping season and focus more on other influences such as SAM and MJO.

It was also useful for growers to follow the multi-week and weather forecast models, but less so the longer term, three-monthly models.

“When we reach the monsoon period the air-sea interaction is quite dynamic and can change quite quickly. The climate models going out three months aren’t going to be as helpful as they are during winter and spring,” he said.

 

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