About the ENSO and IOD indices

The El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are patterns of ocean and atmosphere in the tropics. They shift rainfall and temperature in many parts of the world. This dashboard uses three of the Bureau of Meteorology's measures of them.

Where the regions are

El Niño and La Niña: the ocean

During El Niño the central and eastern tropical Pacific is warmer than usual and the trade winds weaken. During La Niña the same waters are cooler than usual and the trade winds strengthen. The Niño indices give the sea surface temperature anomaly averaged over boxes along the equator:

Relative indices. Since September 2025 the Bureau has used Relative Niño indices. The whole tropical ocean has warmed, and that warming makes an ordinary Niño index read warmer. That would make El Niño look more common and La Niña less common. A relative index removes it by subtracting the tropical mean anomaly (20°S–20°N) from the Niño-region anomaly:

S is a scaling factor that gives the relative index the same variance as the traditional one, so the familiar thresholds still apply. The Bureau treats sustained monthly values above +0.8 °C as typical of El Niño and below −0.8 °C as typical of La Niña. That is about one standard deviation: roughly 70% of months fall between the two. The method follows L'Heureux et al. (2024, J. Climate 37, 1197–1211).

The weekly values here come from the Bureau's GAMSSA sea surface temperature analysis, as anomalies from the 1991–2020 average. Each value is for a Monday–Sunday week. The weekly record starts in July 2008.

The Southern Oscillation Index: the atmosphere

The SOI tracks the air-pressure see-saw across the Pacific that goes with El Niño and La Niña. The Bureau uses the Troup SOI, the standardised difference in mean sea-level pressure between Tahiti and Darwin:

Pdiffav and SD(Pdiff) are the long-term mean and standard deviation for the same time of year, over the 1933–1992 base period. Values usually run from about −35 to +35. Sustained values below −7 often mean El Niño is active, and sustained values above +7 are typical of La Niña. The scale is flipped compared with the Niño indices: negative SOI goes with El Niño.

The dashboard uses two SOI files. The 30-day SOI is updated daily; each value covers the 30 days ending on that date, and the file starts in December 2023. The monthly SOI goes back to 1876, so it gives the long view. The Bureau does not issue single-day or weekly SOI values, because day-to-day weather swamps them.

When both the ocean and the SOI pass their thresholds together, the ocean and atmosphere are said to be coupled. Coupling is what lets an El Niño or La Niña sustain itself and affect weather far away.

The Indian Ocean Dipole

The IOD compares sea surface temperatures across the tropical Indian Ocean. Its index (the Dipole Mode Index, DMI) is the west-box anomaly minus the east-box anomaly:

In a positive IOD the east (off Sumatra and Java) is cooler than usual and the west is warmer. A negative IOD is the reverse. Australian climatologists treat sustained values above +0.4 °C as typical of a positive IOD and below −0.4 °C as typical of a negative IOD. IOD events usually form in the southern winter, peak in spring, and break down by early summer, when the monsoon reaches northern Australia.

Reading the dashboard

Thresholds are not declarations. One week past a threshold is not an El Niño or a positive IOD. The Bureau's status also draws on sub-surface ocean temperatures, trade winds, cloud near the date line and model outlooks; see its Climate Driver Update. The “Periods beyond the thresholds” tables are a simple screen: runs of consecutive values past ±threshold (8 weeks for the SST indices, 30 days for the 30-day SOI, 3 months for the monthly SOI).

“Same time of year”. ENSO and the IOD have strong seasonal cycles. A value is therefore ranked against the same point in every other year: the week ending nearest the same date, the same date for the 30-day SOI, or the same calendar month for the monthly SOI. The weekly records are short (from 2008), so ranks there cover about 18 years. The monthly SOI covers about 150.

Year on year. Each line in this chart is one year. You can run years January–December, or July–June, which keeps a whole ENSO event on one line, as events usually grow through the southern winter and peak around December.

Data files

More background: Bureau of Meteorology climate driver graphs and ENSO outlook. Data © Commonwealth of Australia (Bureau of Meteorology).