NSW Arbovirus Surveillance & Vector Monitoring Program

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Graph of SOI values

Data obtained from the Bureau of Meteorology web site <http://www.bom.gov.au>

What is the SOI?

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. (Note that the SO is measured via air pressure at Darwin only).

Negative values of the SOI indicate El Niņo episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in rainfall over eastern and northern Australia.  

Positive values of the SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia, popularly known as a La Niņa episode. Together these give a high probability that eastern and northern Australia will be wetter than normal. Waters in the central and eastern tropical Pacific Ocean become cooler during this time.

Positive SOI values suggest that rainfall will be above average across eastern Australia, while negative values suggests that rainfall will be below average. As mosquito population tend to fluctuate according to rainfall patterns, the SOI can give a very broad indication of likely mosquito activity. On the basis of past trends in the SO (as opposed to the SOI) and MVE activity, Nicholls (1986) developed a theory to predict MVE activity in southeastern Australia. The Medical Entomology Department, Arbovirus Surveillance Unit, maintains a database of SO values in order to utilise this predicitve tool developed by Nicholls.

Nicholls N. 1986. A method for predicting Murray Valley encephalitis in southeast Australia using the Southern Oscillation. Aust.J.Exp.Biol.Med.Sci. 64:587-94.


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