The Southern Oscillation Index (SOI) is a climatic index that has proven reasonably useful in the prediction of future weather patterns across Australia and the Southern Pacific Ocean.
The SOI measures the difference in air pressure between Darwin and Tahiti and is calculated daily. The air pressure difference is compared with the long-term normal difference, and is expressed as an index number, which can range from about –30 to +30.
The pattern of air circulation across the Pacific Ocean is known as the Walker Circulation Pattern. The diagram below shows the difference between typical air circulation over the Pacific, and the air circulation during El Nino periods (periods of below average rainfall).
The SOI gives an indication of what is happening with atmospheric circulation patterns across the Pacific.
When the SOI is positive, the trade winds typically blow strongly across the warm western Pacific Ocean and pick up plenty of moisture; this can then lead to rain over eastern Australia.
When the SOI is strongly positive, rainfall is usually above average. When the SOI is negative the trade winds are usually weakened, and the rainfall in eastern Australia will often be below average.
During an El Nino episode, as shown above, a sustained warming occurs over the central and eastern tropical Pacific Ocean, and the strength of the Pacific Trade Winds decreases. This results in a reduction in rainfall over eastern and northern Australia.
La Nina episodes, periods of above average rainfall, on the other hand, are associated with stronger Pacific Trade Winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern Pacific Ocean become cooler during this time. When these patterns occur together, they give an increased probability that eastern and northern Australia will be wetter than average.
In some parts of Australia, there is a close correlation between the SOI and the rainfall received. Around Cairns in Queensland and in the central Murray -Darling Basin in NSW are two areas with the best correlation. The worst correlations occur in the central deserts of Western Australia.
If you farm somewhere in Australia, look closely at this map to see how closely the rainfall patterns follow the SOI in your area. The SOI provides a useful tool for predicting rainfall patterns in many parts of Australia.
Similar global patterns and Indices have been identified for other countries and these are usually explained on the websites of the Meteorological Service in your country.
Hampton lies in a region with a relatively close correlation to the SOI of 5-6, making the SOI a valuable reference for farmers in the district. Rainfall records from Hampton, Qld, for the period 1983-2002, were compared to the Southern Oscillation Index data for the corresponding period in Topoclimate Farm plans for the area.
This shows SOI values, which are overlaid with actual Hampton rainfall for the period 1983-2002. This graph indicates a strong relationship between low SOI values and lower than average rainfall during 1987-88 and 1994-49. In the 90’s however, the relationship between SOI and rainfall has been less clearly defined.
Despite the variables, it would seem that the general relationships between SOI and rainfall can be relatively useful when making management decisions. The SOI can be used to help establish the chance of events such as drought or summer rainfall.
This knowledge of likely rainfall events can assist when making decisions on planting times for crops, or the timing of fertiliser applications. For example, SOI predictions for above-average rainfall may indicate years where the yield potential is higher, allowing farmers to increase plant densities, grow different varieties, or increase stocking rates.
It would be beneficial for farmers to keep informed of SOI values and predictions as part of their decision-making processes. These are available on the Bureau of Meteorology (BOM) website.
The SOI should be used in conjunction with other sources of management information to help manage climatic risk.
