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 7 November 2023.
Monthly ENSO Outlook values 1980 - 2023
El Niño Watch
El Niño Alert
La Niña Watch
La Niña Alert
El Niño and La Niña years
Shading of text in the year column refers to years in which El Niño (red) or La Niña (blue) events began.
Past ENSO events
Historical values of the ENSO Outlook status prior to 2014 are based on the set criteria alone. Values from the beginning of 2014 include expert analysis by climatologists at the Bureau of Meteorology to make the final assessment on a status level having been reached.
El Niño and positive Indian Ocean Dipole continue
El Niño continues in the tropical Pacific. Warmer than average sea surface temperatures (SSTs) in the tropical Pacific persist above El Niño thresholds, with warmer water beneath the surface to support that at the surface. In the atmosphere, cloud, wind and pressure patterns are consistent with El Niño conditions. Climate model forecasts indicate some further warming of the central to eastern Pacific is likely, with SSTs remaining above El Niño thresholds into the early southern hemisphere autumn 2024.
The positive Indian Ocean Dipole (IOD) event continues. All models indicate that this positive IOD will likely persist into early December. A positive IOD typically leads to reduced spring rainfall for central and south-east Australia.
The Southern Annular Mode (SAM) index is currently positive with forecasts indicating it will return to neutral in the coming days.
The Madden–Julian Oscillation (MJO) is currently weak. Approximately half of international climate models suggest the MJO will strengthen and move eastwards across the western Pacific later this week. When the MJO is in the western Pacific, there is an increased chance of showers and rain over northern parts of the NT and Queensland.
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