The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure differences between Tahiti and Darwin, Australia.
The SOI is a leading measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes.
In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific.
The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin.
The positive phase of the SOI represents above-normal air pressure at Tahiti and below-normal air pressure at Darwin.
Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes.
In contrast, prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.
Sustained negative values of the SOI below −8 often indicate El Niño 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.
Sustained positive values of the SOI above +8 are typical of a La Niña episode.
They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time.
The graph below ( from the Bureau of Meteorology in Australia) shows monthly values of the SOI in recent years updated to mid April 2017.
The El Niño-Southern Oscillation (ENSO) is neutral. However, model outlooks and recent warming in the Pacific Ocean mean there is an increased chance of El Niño forming later this year. The Australian Bureau of Meteorology ENSO Outlook is currently at El Niño WATCH, which means the likelihood of El Niño forming this year is around double the average chance at 50%.
Most atmospheric and oceanic indicators of ENSO are currently neutral. However, sea surface temperatures (SSTs) in the eastern Pacific Ocean have warmed since the start of the year, and the Southern Oscillation Index (SOI) has been trending downwards. While these are fairly typical changes in the lead up to El Niño, trade winds and cloudiness have not shown any significant shift away from neutral.
All eight international models surveyed by the Bureau show steady warming of the central tropical Pacific Ocean over the next six months. Six models suggest El Niño thresholds may be reached by July 2017. However, some caution must be taken, as models have lower accuracy when forecasting through the autumn months than at other times of the year.
El Nino and La Nina weather affects over New Zealand (source Niwa)
During El Niño, New Zealand tends to experience stronger or more frequent winds from the west in summer, typically leading to drought in east coast areas and more rain in the west.
In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean.
In spring and autumn south–westerly winds are more common.
La Niña events have different impacts on New Zealand's climate. More north–easterly winds are characteristic, which tend to bring moist, rainy conditions to the north–east of the North Island, and reduced rainfall to the south and south–west of the South Island.
Therefore, some areas, such as central Otago and South Canterbury, can experience drought in both El Niño and La Niña.
Warmer than normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions.
Although ENSO events have an important influence on New Zealand's climate, it accounts for less than 25 per cent of the year to year variance in seasonal rainfall and temperature at most New Zealand measurement sites.
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