 |
Weather Eye with |
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 (El Nino) (represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. The positive phase of the SOI (La Nina) 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 Australian Bureau of Meteorology, BOM) shows monthly values of the SOI from January 2021 to 14 August 2023.
El Niño Alert continues—El Niño remains likely
The Bureau's El Niño Alert continues, with El Niño development likely during spring. When El Niño Alert criteria have been met in the past, an El Niño event has developed around 70% of the time.
Sea surface temperatures (SSTs) in the tropical Pacific are exceeding El Niño thresholds, with climate models indicating this is likely to continue at least through to early 2024. The past fortnight has seen a decrease in Southern Oscillation Index (SOI) values to moderate negative values (more El Niño-like). However, this strong swing has been due to increased pressure over Australia, with Tahiti close to normal. El Niño is associated with higher than average pressure over Australia and lower than average pressure over Tahiti. Overall, atmospheric indicators suggest the Pacific Ocean and atmosphere are not yet consistently reinforcing each other, as occurs during El Niño events. El Niño typically suppresses winter–spring rainfall in eastern Australia.
The Indian Ocean Dipole (IOD) is currently neutral. Climate model forecasts suggest a positive IOD is likely to develop during spring. A positive IOD typically decreases winter–spring rainfall for much of Australia and can increase the drying influence of El Niño.
The Madden–Julian Oscillation (MJO) is in the western hemisphere and currently weak. Some climate models are forecasting a moderately strong MJO pulse over the Pacific Ocean in the coming week; this could assist development towards El Niño conditions.
The Southern Annular Mode (SAM) index is currently neutral and is expected to remain neutral for the coming fortnight. A neutral SAM is associated with typical climate conditions for Australia.
The Bureau's El Niño Alert continues, with El Niño development likely during spring. When El Niño Alert criteria have been met in the past, an El Niño event has developed around 70% of the time.
For further Infomation about a wide range of weather/climate matters see my new book "Climate Change ... A Realistic Perspective The fall of the weather dice and the butterfly effect". Google "Amazon"
