Grains Research and Development

Date: 01.09.1998

What does this means for weather across the globe?

Probability of exceeding Median Rainfall

Air pressure at Darwin and Tahiti has been recorded to calculate the SOI back to 1851.

When the SOI was compared with climatic records for the last 140 years it was found that changes in the Southern Oscillation often preceded corresponding changes in cloud cover, temperature, humidity and evaporation, rainfall, number of wet days and the frequency of cyclones.

These historic rainfall records have helped determine a relationship between phases of the SOI and rainfall patterns in many parts of the world. Australia is among the countries most strongly affected by this phenomenon.

On this rainfall probability map for September-November, based on a consistently positive SOI phase in July-August, the blue areas show above 60 per cent probability of above-median rainfall (see key on map).

So, what's an SOI phase?

Research at the University of Queensland first applied mathematical and statistical methods to certain patterns of the Southern Oscillation Index (SOI). The outcome since the 1980s has been a system of 'SOI phases', a robust means of estimating future rainfall probabilities.

The climatologists at the Queensland Centre for Climate Applications say the SOI phase system has been extensively tested for about 10 years in different types of farming operations in several countries.

The five SOI phases

  • a 'consistently negative' pattern of the SOI over twc months
  • a 'consistently positive SOI' over two months
  • a 'rapidly falling' SOI pattern over two months
  • a 'rapidly rising' SOI pattern over two months
  • an SOI pattern 'near zero' over two months

The phase patterns have strict boundaries. It is always known what the SOI phase is at the end of any month.

Autumn lock-in

It is worth knowing that the SOI locks into its preferred pattern from about the end of the southern hemisphere autumn of one year to the autumn of the following year.

The SOI phases tend to demonstrate the most 'predictive skill' from the end of May onwards (as the pattern locks in).

Looking two months ahead

From there, climatologists can map the lag-relationships of rainfall in any rain gauge around the world with the SOI 'phases'.

That means, for example, if the SOI is consistently rising (e.g. SOI +15) in April and May, there is a high likelihood in July and August in many districts for average to above-average rainfall.

And if the SOI stays consistently high in July and August, which frequently happens due to the 'lock-in', September to November are also likely to produce above-average rainfall. The opposite pattern occurs with consistently falling and low (minus number) SOI figures.

However, growers should be aware that the effect of the SOI varies from season to season and according to location. A rising SOI in spring predicts something different from a rising SOI in autumn/winter for many districts.

A particular SOI pattern does not guarantee a certain amount of rainfall, but it considerably shifts the odds of receiving a greater or lesser amount.

Other systems influence the outcome

The SOI and the related El Niño weather patterns are not the only means of climate prediction for all of Australia, but the system can help growers understand at least part of the observed, long-term climatic variability.

Other factors, such as Indian Ocean sea temperatures and the latitude and movement of high-pressure and low-pressure systems can either strengthen or counteract any SOI signal.

Climate forecasting systems based on sea-surface temperature changes have shown good predictive value in parts of Australia. For example, Indian Ocean sea-surface temperatures in summer and autumn influence the probability of rainfall in early winter over parts of southern and eastern Australia, through the formation of north-west cloudbands.(See page 15 for the Ian Holton system.)

What phase maps tell you

The shades of blue on the 1998 phase map show the higher-than-median probabilities of September-November rainfall across much of Australia based on a consistently positive SOI phase pattern in July-August.

In stark contrast to the 1998 map, the rapidly falling SOI during July-August 1997 resulted in a patchy outlook with some parts of northern NSW and southern Queensland having a fair chance of median or above rainfall (blue areas), while many more areas (yellow and orange) only had a 20-40 per cent chance of exceeding the median.

Provability of exceeding median rainfall
Growers can obtain two-month phase maps by contacting Karen Taylor at the Queensland Centre for Climate Applications 07 4688 1348. The computer softwareAustralian Rainman also provides detailed climate information for specific locations related to the rise and fall of SOI phases — see 'Want to Know More?'

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